Dissertations / Theses on the topic 'Semiconductor Nanostructures - Growth'

Semiconductors and related low-dimensional nanostructures are extremely important in the modern world. They have been extensively studied and applied in industry/military areas such as ultraviolet optoelectronics, light emitting diodes, quantum-dot photodetectors and lasers. The knowledge of growth dynamics of semiconductor nanostructures by metalorganic chemical vapour deposition (MOCVD) is very important then. MOCVD, which is widely applied in industry, is a kind of chemical vapour deposition method of epitaxial growth for compound semiconductors. In this method, one or several of the precursors are metalorganics which contain the required elements for the deposit materials. Theoretical studies of growth mechanism by MOCVD from a realistic reactor dimension down to atomic dimensions can give fundamental guidelines to the experiment, optimize the growth conditions and improve the quality of the semiconductor-nanostructure-based devices. Two main types of study methods are applied in the present thesis in order to understand the growth dynamics of semiconductor nanostructures at the atomic level: (1) Kinetic Monte Carlo method which was adopted to simulate film growths such as diamond, Si, GaAs and InP using the chemical vapor deposition method; (2) Computational fluid dynamics method to study the distribution of species and temperature in the reactor dimension. The strain energy is introduced by short-range valence-force-field method in order to study the growth process of the hetero epitaxy. The Monte Carlo studies show that the GaN film grows on GaN substrate in a two-dimensional step mode because there is no strain over the surface during homoepitaxial growth. However, the growth of self-assembled GaSb quantum dots (QDs) on GaAs substrate follows strain-induced Stranski-Krastanov mode. The formation of GaSb nanostructures such as nanostrips and nanorings could be determined by the geometries of the initial seeds on the surface. Furthermore, the growth rate and aspect ratio of the GaSb QD are largely determined by the strain field distribution on the growth surface.
QC 20100713

This thesis describes the growth and characterisation of III-V semiconductor materials and nanostructures. The material was grown by molecular beam epitaxy (MBE) and characterised using a range of techniques including atomic force microscopy (AFM), cross-sectional scanning tunnelling microscopy (XSTM) and x-ray diffraction (XRD).

En este trabajo hemos investigado distintas posibilidades para aprovechar las tensiones almacenadas en los materiales nanoestructurados para obtener estructuras 3D auto-organizadas. En particular hemos estudiado el crecimiento epitaxial de puntos cuánticos auto-organizados de Ge sobre Si depositando una submonocapa de carbono antes del crecimiento de las islas de Ge. Empleando la microscopía de fuerza atómica combinada con difracción RHEED y técnicas ópticas como la dispersión Raman y la elipsometría, hemos llevado a cabo un estudio sistemático de la influencia de la interdifusión de Si y de la composición de la capa de mojado en la densidad y la morfología de las islas. Los resultados aportan evidencia experimental de un mecanismo de crecimiento cinéticamente limitado donde la movilidad de los adátomos de Ge se ve afectada por la interacción química entre C, Si, y Ge. Como resultado, presentamos un protocolo de crecimiento en dos etapas para manipular la topografía de las islas (densidad, forma y tamaño), útil para posibles aplicaciones en optoelectrónica. Hemos investigado el fenómeno de relajación de las tensiones elásticas cuando recubrimos las islas, un proceso necesario para la ingeniería de dispositivos que constan de multicapas de puntos cuánticos. También hemos analizado la evolución de nanoestructuras de Ge preparadas combinando el uso de nanoplantillas (nanostencils) con la técnica PLD, una estrategia que tiene mucho potencial para producir patrones de nanoestructuras semiconductoras para optoelectrónica. Además del crecimiento de islas 3D, hemos aplicado la ingeniería de capas tensadas para fabricar microtubos que se enrollan espontáneamente a partir de heteroestructuras tensadas de semiconductor. Mediante la espectroscopía Raman con resolución microscópica hemos conseguido medir las tensiones residuales, que se manifiestan en un cambio de la frecuencia de los fonones, comparando la señal colectada en la pared del tubo con el valor de referencia del material sin tensiones. Hemos desarrollado un modelo elástico para describir dicho cambio de frecuencia, lo que nos permite caracterizar la distribución de tensiones en el microtubo. Los resultados demuestran que la espectroscopía Raman es una potente técnica de diagnóstico del estado de tensión en dispositivos tipo MEMS. Hemos aplicado la tecnología de fabricación de microtubos enrollados para obtener un sensor bioquímico “lab-in-a-tube” óptico, donde se emplea la luz como sonda. Hemos fabricado microtubos de Si/SiOx integrados en un chip de Silicio y hemos evaluado sus propiedades como sensor refractométrico. Introduciendo una solución azucarada en el microtubo, se produce un cambio en el índice de refracción, que se manifiesta en un desplazamiento de las frecuencias de los modos ópticos de “whispering gallery”. Este prototipo demuestra que la integración de microtubos enrollados es un proceso de fabricación con mucho potencial para diseñar canales optofluídicos en dispositivos “lab-on-a-chip”.
In this work we explored different pathways to exploit the strain stored into nanoscale layers of materials as a driving force to self-assemble 3D structures. In particular, we have studied the epitaxial growth of self-assembled Ge quantum dots when a submonolayer of carbon is deposited prior to the growth of the dots. Using atomic-force microscopy combined with RHEED and optical techniques like Raman scattering and ellipsometry, we performed a systematic study of the role played by thermally activated Si interdiffusion and the composition of the wetting layer on dot density and morphology. The results give experimental evidence of a kinetically limited growth mechanism in which Ge adatom mobility is determined by chemical interactions among C, Si, and Ge. We suggest a two-stage growth procedure for fine-tuning the dot topography (density, shape and size), useful for possible optoelectronic applications. Moreover we investigated the dynamics of strain relaxation during the capping of islands, which is useful for engineering devices based on multistacks of quantum dots. We also analysed the evolution of Ge nanostructures grown by combining nanostenciling and pulsed laser deposition, as a promising approach for the parallel patterning of semiconductor nanostructures for optoelectronics. Apart from the growth of 3D islands, we applied strain-driven engineering to release rolled-up microtubes, obtained from strained semiconductor heterostructures. Through micro-Raman spectroscopy we were able to determine the residual strain, which results in a frequency shift of phonon modes measured on the tube as compared with reference unstrained material. We developed a simple elastic model to describe the measured phonon-frequency shifts, from which we estimate the strain status of the microtube. Results demonstrate the power of Raman spectroscopy as a diagnostic tool for engineering of strain-driven self-positioning microelectromechanical systems. We tested the potential application of this rolled-up nanotechnology to obtain a lab-in-a-tube device where light is used as a biochemical sensor. We fabricated rolled up microtubes consisting of Si/SiOx integrated on a Si chip and we analysed their properties to use them as a refractometric sensor. An aqueous sugar solution was inserted into the microtube, which leads to a change in refractive index and, as a result, to a detectable spectral shift of the whispering gallery modes. This prototype proved that the monolithic on-chip integration of strain-engineered microtubes is a promising approach to design optofluidic channels for lab-on-a-chip applications.

Colloidal semiconductor nanoparticles (NPs) contain hundreds to thousands of atoms in a roughly spherical shape with diameters in the range of 1-10 nm. The extremely small particle size confines electron transitions and creates size tunable bandgaps, giving rise to the name quantum dots (QDs). The unique optoelectronic properties of QDs enable a broad range of applications in optical and biological sensors, solar cells, and light emitting diodes. The most common compound semiconductor combination is chalcogenide II-VI materials, such as ZnSe, CdSe, and CdTe. But III-V and group IV as well as more complicated ternary materials have been demonstrated. Coordinating organic ligands are used to cap the NP surface during the synthesis, as a mean of protecting, confining, and separating individual particles. This study investigated the impact of the ligand on particle growth and self-assembly into hierarchical structures. ZnSe QDs were synthesized using an aqueous route with four different thiol ligands, including 3-mercaptopropionic acid (MPA), thioglycolic acid (TGA), methyl thioglycolate (MTG), and thiolactic acid (TLA). The particle growth was monitored as a function of reaction time by converting the band gaps measured using UV-vis spectroscopy into particle sizes. A kinetic model based on a diffusion-reaction mechanism was developed to simulate the growth process. The growth data were fit to this model, yielding the binding strength in the order TLA < MTG ≈ TGA < MPA. This result showed the relationship between the QD growth rates and the chemical structures of the ligands. Ligands containing electron-withdrawing groups closer to the anchoring S atom and branching promoted growth, whereas longer, possibly bidendate, ligands retarded it. Removing TGA ligands from the surface of CdTe QDs in a controlled manner yielded new superstructures that were composed of either intact or fused particles. Purifying as-synthesized QDs by precipitating them using an anti-solvent removed most of the free ligand in solution. Aging this purified QD suspension for a week caused self-assembly of QDs into nanoribbons. The long time needed for self-assembly was due to the slow equilibrium between the ligands on QD surface and in solution. Accelerating the approach to equilibrium by diluting purifed CdTe QDs with organic solvents triggered rapid self-assembly of superstructures within a day, forming various nanostructures from nanoribbons to nanoflowers. The type of nanostructures that formed was determined by the solvation of TGA in the trigger solvent. Extracting the smallest portion of TGA in methanol promoted vectorial growth into ribbons consistent with dipole-dipole attractive and charge-charge repulsive interactions. Removing more of the TGA layer in IPA caused the dots to fuse into webs containing clustered ribbons and branches, and the directional nature of the superstructure was lost. Completely deprotecting the surface in acetone promoted photochemical etching and dissolved the QDs, yielding ower-like structures composed of CdS. Nanocrystal (NC) growth mediated by a ligand was also studied in the organic synthesis of FeS₂ nanocubes. Oleylamine was used not only as the ligand but also the solvent and reductant during the reaction. A one hour reaction between iron (II) chloride and elemental sulfur in oleylamine at 200 ℃ and a S to Fe ratio of 6 yielded phase pure pyrite cubes with dimensions of 87.9±14.1 nm. X-ray diffraction (XRD) spectra and Raman peaks for pyrite at 340, 375, and 426 cm⁻¹ confirmed phase purity. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) results showed that the oleylamine remained on the FeS₂ surface as a ligand. The reaction mechanism includes the production of pyrrhotite Fe₁₋ᵪS (0≤x<0.5) via reduction of S⁰ to S²⁻ by oleylamine and the oxidation of pyrrhotite to pyrite with remaining S⁰.

One-dimensional ZnO nanostructures have great potential applications in the fields of optoelectronic and sensor devices.  Therefore, it is very important to realize the controllable growth of one-dimensional ZnO nanostructures and investigate their properties. The main points for this thesis are not only to successfully realize the controllable growth of ZnO nanorods (ZNRs), ZnO nanotubes (ZNTs) and ZnMgO/ZnO heterostructures, but also investigate the structure and optical properties in detail by means of scanning electron microscope (SEM), transmission electron microscope (TEM), resonant Raman spectroscopy (RRS), photoluminescence (PL), time resolved PL (TRPL), X-ray photoelectron spectroscopy (XPS) and Secondary ion mass spectrometry (SIMS). For ZNRs, on one hand, ZNRs have been successfully synthesized by a two-step chemical bath deposition method on Si substrates. The diameter of ZNRs can be well controlled from 150 nm to 40 nm through adjusting the diameter and density of the ZnO nanoparticles pretreated on the Si substrates. The experimental results indicated that both diameter and density of ZnO nanoparticles on the substrates determined the diameter of ZNRs. But when the density is higher than the critical value of 2.3×108cm-2, the density will become the dominant factor to determine the diameter of ZNRs. One the other hand, the surface recombination of ZNRs has been investigated in detail. Raman, RRS and PL results help us reveal that the surface defects play a significant role in the as-grown sample. It is the first time to the best of our knowledge that the Raman measurements can be used to monitor the change of surface defects and deep level defects in the CBD grown ZNRs. Then we utilized TRPL technique, for the first time, to investigate the CBD grown ZNRs with different diameters. The results show that the decay time of the excitons in ZNRs strongly depends on the diameter. The altered decay time is mainly due to the surface recombination process. A thermal treatment under 500°C can strongly suppress the surface recombination channel. A simple carrier and exciton diffusion equation is also used to determine the surface recombination velocity, which results in a value between 1.5 and 4.5 nm/ps. Subsequently, we utilized XPS technique to investigate the surface composition of as-grown and annealed ZNRs so that we can identify the surface recombination centers. The experimental results indicated that the OH and H bonds play the dominant role in facilitating surface recombination but specific chemisorbed oxygen also likely affect the surface recombination. Finally, on the basis of results above, we explored an effective way, i.e. sealing the beaker during the growth process, to effectively suppress the surface recombination of ZNRs and the suppression effect is even better than a 500oC post-thermal treatment. For ZNTs, the structural and optical properties have been studied in detail. ZNTs have been successfully evolved from ZNRs by a simple chemical etching process. Both temperature-dependent PL and TRPL results not only further testify the coexistence of spatially indirect and direct transitions due to the surface band bending, but also reveal that less nonradiative contribution to the emission process in ZNTs finally causes their strong enhancement of luminescence intensity. For ZnMgO/ZnO heterostructures, the Zn0.94Mg0.06O/ZnO heterostructures have been deposited on 2 inch sapphire wafer by metal organic chemical vapor deposition (MOCVD) equipment. PL mapping demonstrates that Mg distribution in the entire wafer is quite uniform with average concentration of ~6%. The annealing effects on the Mg diffusion behaviors in Zn0.94Mg0.06O/ZnO heterostructures have been investigated by SIMS in detail. All the SIMS depth profiles of Mg element have been fitted by three Gaussian distribution functions. The Mg diffusion coefficient in the as-grown Zn0.94Mg0.06O layer deposited at 700 oC is two orders of magnitude lower than that of annealing samples, which clearly testifies that the deposited temperature of 700 oC is much more beneficial to grow ZnMgO/ZnO heterostructures or quantum wells. This thesis not only provides the effective way to fabricate ZNRs, ZNTs and ZnMgO/ZnO heterostructures, but also obtains some beneficial results in aspects of their optical properties, which builds theoretical and experimental foundation for much better understanding fundamental physics and broader applications of low-dimensional ZnO and related structures.
Endimensionella nanostrukturer av ZnO har stora potentiella tillämpningar för optoelektroniska komponenter och sensorer. Huvudresultaten för denna avhandling är inte bara att vi framgångsrikt har realiserat med en kontrollerbar metod ZnO nanotrådar (ZNRs), ZnO nanotuber (ZNTs) och ZnMgO/ZnO heterostrukturer, utan vi har också undersökt deras struktur och optiska egenskaper i detalj. För ZNRs har diametern blivit välkontrollerad från 150 nm  ner till 40 nm. Den storlekskontrollerande mekanismen är i huvudsak relaterad till tätheten av ZnO partiklarna som är fördeponerade på substratet. De optiska mätningarna ger upplysning om att ytrekombinationsprocessen spelar en betydande roll för tillväxten av ZNR. En värmebehandling i efterhand  vid 500 grader Celsius eller användande av en förseglad glasbägare under tillväxtprocessen kan starkt hålla nere kanalerna för ytrekombinationen.För ZNT, dokumenterar vi inte bara samexistensen av rumsliga indirekta och direkta  övergångar på grund av bandböjning, men vi konstaterar också att vi har mindre icke-strålande bidrag till den optiska emissionsprocessen i ZNT. För ZnMgO/ZnO heterostrukturer konstaterar vi med hjälp av analys av Mg diffusionen i den växta och den i efterhand uppvärmda Zn(0.94)Mg(0.06)O filmen, att en tillväxt vid 700 grader Celsius är den mest lämpliga för att växa ZnMgO/ZnO heterostrukturer eller kvantbrunnar.   Denna avhandling ger en teoretisk och experimentell grund för bättre förståelse av grundläggande fysik och för tillämpningar av lågdimensionella strukturer.
SSF, VR

Dans ce mémoire sont abordées quelques études sur la croissance, l'électromigration et la thermomigration de la surface des semiconducteurs tels que le Ge(111), le Si(100) et le Si(111). Sur le plan expérimental, la Microscopie à Electrons Lents (LEEM) nous a permis d'accéder à la dynamiques des phénomènes in situ et en temps réel. Nous étudions l'électromigration et la thermomigration sur la surface de Si(100) qui présente deux reconstructions de surfaces (2x1) et (1x2) selon l'orientation des dimères. Nous montrons que l'anisotropie de diffusion peut affecter le sens de mouvement des nanostructures (trous et îlots). Nous étudions aussi l'électromigration et la thermomigration sur la surface de Si(111). Nous montrons que les trous (1x1) dans la phase (7x7) bougent dans le sens opposé au courant électrique, et dans le même sens du gradient thermique. Nous avons obtenu la charge effective et le coefficient de Soret des atomes de Si en présence d'un courant électrique et d'un gradient thermique. Enfin est abordée l'étude de la nucléation, la croissance et la coalescence dynamique de gouttelettes d'Au sur la surface d'Au/Ge(111), ainsi que l'électromigration des domaines 2D d'Au/Ge(111)-(√3x√3) dans la phase (1x1)
This thesis is focused on the study of the growth, electromigration and thermomigration of nanostructures on the surface of semiconductors such as Si(100), Si(111) and Ge(111). On an experimental viewpoint, Low Energy Electron Microscopy (LEEM) allows us to access to the dynamics of the phenomena in situ and in real time. We have studied under electromigration and thermomigration the motions of 2D monoatomic holes and islands on the Si (100) surface. We have shown that diffusion anisotropy due to (2x1) and (1x2) surface reconstructions can affect the direction of motion of nanostructures. We have also studied electromigration and thermomigration of Si (111) surface. We show that 2D-(1x1) holes in the (7x7) phase move in the direction opposite to the electric current, while in the direction of the thermal gradient. We have obtained the effective charge and the Soret coefficient of Si atoms in presence of an electric current and a thermal gradient. At last, the nucleation, growth and dynamic coalescence of Au droplets on Au/Ge(111) surface is studied, and the electromigration of 2D Au/Ge(111)-( √3x√3) domains on Au/Ge(111)-(1x1) surface

Metallic nano-structures provide for new and exciting domains to investigate light-matter interactions. The coupling of these metallic nano-structures to semiconductor emitters allows for the observation of cavity QED effects including Purcell enhancement and Vacuum Rabi splitting. The focus of this dissertation will be to present an introduction and background to semiconductor optics, and metallic metamaterial systems. This will be followed by the presentation of the spectroscopy systems designed and constructed during my tenure as graduate student and the experimental data obtained with these systems. Some of the results have been published, while some of the presented material is still actively being pursued for publication. More specifically, the dissertation will cover the research at hand, experimental techniques, and results.

El propósito principal de esta tesis ha sido desarrollar un nuevo proceso con el fin de lograr el control reproducible de las dimensiones y la ubicación espacial de nanoestructuras (islas y nano hilos) de Ge sobre sustratos de Si. En este contexto, nuestro objetivo básico era formar matrices bidimensionales de nanocúmulos de Au utilizando haces de iones focalizados (FIB) de Au2+ con filtro de masas y posteriormente utilizarlos como patrón para la nucleación de nanoestructuras de Ge. El primer capítulo está dedicado a la revisión de: 1) las propiedades fundamentales de Si y Ge, 2) generalidades sobre las capas de aleación de SiGe y 3) el crecimiento de islas y nano hilos de Ge auto-ensamblados. Se han expuesto los detalles sobre el modo de crecimiento de las nanoestructuras. El mecanismo de crecimiento para los nano hilos - vapor líquido sólido (VLS) - se describe junto con el comportamiento del Au como catalizador. El segundo capítulo ofrece una visión general de las tecnologías de grabado (haces de iones focalizados) y crecimiento (epitaxia por haces moleculares). Además, se describen los métodos para la caracterización de las muestras, tanto morfológica como desde un punto de vista óptico. El tercer capítulo está dedicado a investigar la influencia de la formación de nanocúmulos de Ge sobre la determinación de la composición y de la tensión de capas de aleaciones de Si1-xGex/Si. La causa de la gran dispersión de los valores de las frecuencias Raman de fonones medidos en capas de aleación relajadas que se encuentra en los datos de la literatura se reveló que estaba relacionada con la existencia de los nanocúmulos de Ge. Su presencia dentro de las capas de aleación de SiGe es el resultado de haber empleado métodos de crecimiento epitaxial de no equilibrio, tales como la epitaxia por haces moleculares. Las capas de aleación obtenidas en este trabajo que contenían nanocúmulos de Ge se trataron térmicamente y el efecto de dichos recocidos acumulativos para conseguir una disposición aleatoria de los átomos de Ge dentro de la capa de aleación fue demostrado. Adicionalmente, hemos demostrado que la tensión de las capas de SiGe y su composición pueden ser evaluadas y determinadas mediante la realización de una sola medición Raman. Se elaboró también un método gráfico para estimar la composición en Ge y el estado de tensión dentro de las capas de aleación de SiGe independiente de su estado de tensión. El crecimiento de las nanoestructuras de Ge en substratos de Si grabados se ha presentado en el cuarto capítulo. Se ha investigado y desarrollado un proceso en tres pasos basado principalmente en: i) el uso de haces de iones focalizados (FIB) para obtener los patrones, ii) recocido y iii) el crecimiento de nanoestructuras mediante epitaxia por haces moleculares (MBE). Se grabaron sustratos de silicio con distintas orientaciones cristalográficas (Si (001) y Si (111)) usando un equipo de haces de iones de Au2+ focalizados. Se examinó la variación de los patrones en función de los parámetros FIB empleados, y se exploró su evolución al realizar tratamientos térmicos utilizando una amplia gama de temperaturas y tiempos. Durante los recocidos se logró la formación de cúmulos de AuSi dentro de las áreas grabadas con el FIB. Después de la deposición de Ge se formaron islas y/o nano hilos dependiendo de la dosis de iones utilizada para grabar los sustratos. También se investigó el efecto de la cantidad de Au y de las dimensiones de los hoyos sobre el modo de crecimiento y la morfología de las nanoestructuras formadas.
The main purpose of this thesis has been to develop a new process in order to achieve reproducible control of the dimensions and spatial location of Ge nanostructures (islands and nanowires) formed on Si substrates. In this context, our primary objective was to form bi-dimensional arrays of Au nanoclusters using mass-filtered focused ion beam (FIB) with Au2+ ions and to use them as patterns to nucleate the Ge nanostructures. The first chapter is dedicated to a review of: 1) Si and Ge fundamental properties, 2) generalities about the SiGe alloy layers, and 3) growth of the self assembled Ge islands and nanowires. Details about the nanostructure growth mode are exposed. The nanowire growth mechanism – vapor liquid solid (VLS) – is described along with the Au behavior as a catalyst. Chapter 2 gives an overview of the patterning (focused ion beam) and growth (molecular beam epitaxy) techniques. Besides them, methods for sample characterization from a morphological and optical point of view are described. The third chapter is devoted to investigate the influence of Ge nanoclustering on the composition and strain determination of Si1-xGex/Si alloys. The cause of the large scatter of the phonon frequency values measured by Raman for relaxed alloy layers that is found in the literature was revealed as due to Ge nanoclustering effects. This phenomenon occurs in SiGe alloy layers as a result of employing nonequilibrium epitaxial growth methods such as molecular beam epitaxy. The obtained alloy layers presenting Ge nanoclusters were thermally treated and the effect of the cumulative annealings of randomizing the Ge atom distribution within the alloy layer was demonstrated. Additionally, we proved that the strain and composition of the SiGe alloy layers can be evaluated and determined by performing a single Raman measurement. An analytical/graphical method to estimate the Ge composition and strain status within the SiGe alloy layers independent of their strain status was elaborated. The growth of the Ge nanostructures on patterned Si substrates was investigated and is presented in chapter 4. For this purpose a three step process was developed, based mainly on: i) focused ion beam (FIB) patterning, ii) annealing and iii) molecular beam epitaxy growth. Si substrates with different crystallographic orientations [(001) and (111)] were patterned using a FIB equipment with Au2+ ions. Pattern evolution as a consequence of the employed FIB parameters was examined, and their progress with the thermal treatments performed using a large range of temperatures and times was explored. The formation of AuSi clusters inside the FIB patterned areas during the annealings was achieved. Depending on the ion dose used to pattern the substrates, after Ge deposition, both islands and/or nanowires were formed. The effect of the Au amount and hole dimensions on the growth mode, morphology, and two-dimensional ordering of the nanostructures was investigated.

Ce travail de thèse porte sur la croissance et sur la caractérisation optique de nanostructures à base d’antimoine sur substrats InP, en vue d’applications dans le domaine des télécommunications optiques. La transition inter-sous-bande est un processus ultrarapide qui permet la modulation de la lumière dans les réseaux de télécommunication optique. Durant cette thèse, une absorption inter-sous-bande dans le proche-infrarouge provenant de puits quantiques Ga0.47In0.53As/AlAs0.56Sb0.44 a été observée pour la première fois au laboratoire. Les analyses par microscopie électronique à effet tunnel sur la face clivée montrent cependant de nombreux déviations à l’idéalité de nos structures : mélange à l’échelle atomique aux interfaces entre GaInAs et AlAsSb, inhomogénéité de l’alliage GaInAs, incorporation non-intentionnel d’antimoine dans le GaInAs. Les puits quantiques InAs/AlAs0.56Sb0.44 sont potentiellement des objets de choix pour la réalisation de composants intersous- bande travaillant à 1,55 μm. Des puits quantiques InAs/AlAs0.56Sb0.44 contraint, exempt de défauts ont été obtenus par croissance assistée par effet surfactant de Sb. En symétrisant la contrainte induite par le dépôt d’InAs par l’insertion de couches nanométriques de AlAs dans les barrières, des multi-puits InAs/AlAs0.56Sb0.44 sans contrainte macroscopique ont été réalisés. L’effet de l’antimoine en surface sur la croissance de structure InAs/GaAs0.51Sb0.49 a également été étudié. En présence d’antimoine sur substrats InP d’orientation (001), le dépôt d’InAs conduit à la formation de puits quantiques. Par contre sur ceux orientés suivant (113)B des boites quantiques sont formées suivant le mode de croissance Volmer-Weber. Ces résultats sont discutés en termes d’effets cinétiques ou énergétiques de l’antimoine en surface. La modification de l’anisotropie de l’énergie de surface induite par l’antimoine permet d’interpréter nos résultats sur substrats (100) et (113) B
This PhD work presents molecular beam epitaxy growth and optical studies on several Sb-nanostructures on InP substrate, for their potential use in optical telecommunication. Inter-subband transition in Ga0.47In0.53As/AlAs0.56Sb0.44 quantum well is a useful physical process for implementing ultrafast fulloptical modulations. Near-infrared inter-subband transition in this material was achieved and microscopic studies on this structure has revealed that the intermixing at GaInAs/AlAsSb interface, unintentional Sb incorporation in GaInAs layer and the inhomogeneity within GaInAs layer could prevent Ga0.47In0.53As/ AlAs0.56Sb0.44 multiple quantum wells from achieving intersubband transition in 1.55 μm optical telecommunication band. The strained InAs/AlAs0.56Sb0.44 quantum well is another material that has potential use in 1.55 μm full-optical modulation. 2 nm-thick defect-free InAs/AlAs0.56Sb0.44 was obtained under Sb surfactant-mediated growth, and by using strain compensation techniques, InAs/AlAs0.56Sb0.44 multiple quantum wells with zero net-strain were realized. The study of Sb-mediated growth is also carried on to InAs/GaAs0.51Sb0.49 nanostructures. The growths of such structures on InP (001) substrate has led to the formation of flat InAs layer, while high-density InAs/GaAs0.51Sb0.49 quantum dots were obtained on InP (113)B substrates under Volmer-Weber growth mode. We attribute such phenomena to the surfaceorientation dependent surfactant effect of Sb. Emission wavelength close to 2 μm was achieved with only 5 ML of InAs deposition, which makes these quantum dots attractive to InPbased mid-wave applications

The objective of this project is to establish a new technology to grow high quality GaN based material by nano selective area growth (NSAG). The motivation is to overcome the limit of the conventional growth method, which yield a high density of dislocation in the epitaxial layer. A low dislocation density in the epitaxial layer is crucial for high performance and high efficiency devices. This project focuses on growth and material characterization of GaN based nanostructures (nanodots and nanostripes) grown using the NSAG method that we developed. NSAG, with a precise control of diameter and position of nanostructures opens the door to new applications such as: 1) single photon source, 2) photonic crystal, 3) coalescence of high quality GaN template, and 4) novel nanodevices.

Orientadores: Lisandro Pavie Cardoso, Marco Sacilotti
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Os nitretos (Ga, Al, In)N assim como os compostos GaInP, GaCuO2, representam um sistema de materiais muito importante para as aplicações em opto-eletrônica e dispositivos tais como os diodos emissores de luz (LEDs), lasers e nanosensores. Entretanto, o requisito essencial para as aplicações industriais desses materiais é a redução em seus tamanhos. Neste trabalho foram crescidos materiais metálicos formados por nitretos de gálio e também de semicondutores do tipo GaInP, GaCuO2 na forma de estruturas 3D, pela técnica de deposição química de organometálicos em fase vapor (MOCVD). Foi utilizado como precursor organometálico (OM) o trimetil gálio Ga(CH3)3e o nitrogênio N2 como gás portador. A temperatura e a pressão foram controladas durante o crescimento variando entre 500 e 750 o C e 100 a 760 Torr, respectivamente. Duas classes de estruturas 3D foram obtidas a partir da decomposição total ou parcial do gás pre-cursor, devido a interação entre o OM e o substrato que gera diferentes morfologias: i) as ligas metálicas (Ga, Al, In) formando estruturas semelhantes a balões, cetros (hastes com terminações esféricas) e neurônios, todos apresentando uma fina membrana de carbono amorfo que reveste a estrutura. Após o crescimento, estas estruturas foram submetidas ao processo de nitretação sob atmosfera de NH3 para transformá-las em micro/nanocristais de GaN; ii) os fios semicondutores micro/nanométricos com uma esfera metálica em sua terminação (bambus e cetros) . Na formação de ambas as estruturas, os precursores OM são como moléculas catalisadoras do crescimento. Este crescimento é considerado como um método alternativo e original para se obter estruturas 3D. Uma possível associação com o modelo apresentado pelo mecanismo de crescimento Vapor-Líquido-Sólido (VLS), que utiliza uma partícula metálica para promover os nanotubos de carbono e os nanofios semicondutores, ainda está em discussão. Informações estruturais e ópticas dessas novas estruturas crescidas sobre substratos de Cu (grade de difração), Si (001), InP (policristalino) e Al/SiO2/Si (fotolitografia) foram obtidas através da caracterização por difração de raios-X, microscopia eletrônica de varredura e de transmissão em alta resolução, espectroscopia por energia disper-siva, catodoluminescência e a espectroscopia de excitação por dois fótons. Nas amostras nitretadas, micro/nano cristais de GaN obtidos da liga de Ga aparecem impregnados no carbono turbostrático (folhas de carbono sem orientação obtidas do amorfo) que revestem as estruturas, e emitem na região do espectro l £ 365 nm, devido às suas dimensões quânticas. As hastes das estruturas do tipo bambus apresentam nódulos formados por discos monocristalinos de GaInP rotacionados de 60 o um em relação ao outro. Óxidos CuGaO2 e CuGa2O4compondo nanofios, denominados cetros, também foram obtidos
Abstract: Nitride (Ga, Al, In)N as well as GaInP, GaCu O2 compounds represent a very important class of materials to be used in the opto-electronic and devices applications such as light emission diodes (LEDs) lasers and nanosensors. However, the essential requirement to the industrial applications of these materials is the reduction in theirs sizes. In this work 3D structures based on gallium nitride and also GaInP, GaCuO2 semiconductors were grown by metalorganic chemical vapor deposition (MOCVD) technique. Trimethyl-gallium Ga(CH3) was used as the metal-organic (MO) precursor and nitrogen N2as carrier gas. During the growth to the temperature and pressure intervals of 500 - 700 oC and 100 - 760 Torr, respectively. Two 3D material classes were obtained from the total or partial precursor gas decomposition, since the interaction between the MO compound and the substrate gives rise to different morphologies: i) (Ga,In,Al) metallic alloys form ballons, scepters (wires with spherical ends) and neurons like structures, all involved by a thin carbon amorphous membrane. After growth, these structures were turned into GaN micro/nanocrystals by nitridation process under NH3 atmosphere; ii) micro/nanometer semiconductor wires with a metallic sphere at its end (bamboos and scepters). In order to form both structures, the MO precursors are taken as a catalyst molecule of the growth process. This is an alternative and original method to obtain 3D structures and a possible association to the model used in the vapour-liquid-solid (VLS) growth mechanism, in which a metallic particle promotes the carbon nanotubes and semiconductors nanowires is still under discussion. Structural and optical informations on these new structures grown on Cu (diffraction grid), Si(001), InP (polycrystalline) and Si/Al (photolithography) substrates were obtained through the characterization by X-ray diffraction, scanning electron microscope, high resolution transmission electron microscopy, en-ergy dispersive x-rays, cathodoluminescence and two photon excitation. In the nitrided samples, GaN micro/nanocrystals obtained from Ga alloy appear embedded in the turbostratic carbon (C sheets at random obtained from the amorphous) which involves the structures and, they emit in the l £ 365 nm region specter, due to their quantum dimensions. The bamboo rods present nodes consisting of GaInP single crystal discs turned by 60o one with respect to the other. The CuGaO2 and CuGa2O4 oxides compounding nanowires, called scepters, also were obtained.
Doutorado
Física
Doutor em Ciências

L’alliage germanium-étain est un semiconducteur qui suscite une grande attention en raison de ses propriétés électriques et optiques. L’incorporation de Sn dans le germanium permet d’ajuster la largeur de bande interdite (gap) et d’améliorer la mobilité des électrons et des trous, et pour une quantité suffisante d’étain, le matériau passe d’un gap indirect à direct. Cet alliage est versatile parce qu’il peut être intégré d’une façon monolithique sur le Si, c’est ce qui en fait un matériau idéal dans les domaines de l'optoélectronique à base de silicium. Cette thèse est sur la fabrication et la caractérisation de nanofils cristallins Ge1-xSnx à haute concentration en Sn. Des nouvelles stratégies ont été employées pour fabriquer de nombreux types de nanofils GeSn. Les résultats ont été expliqués en fonction des modèles cinétiques existants. Un nouveau mécanisme de croissance y est décrit: le mécanisme solide-solide-solide – SSS. Il consiste à faire croître des nanofils de GeSn dans le plan du substrat à l’aide de catalyseurs d’étain à une température inférieure au point de fusion de Sn. Quatre modèles de transport de masse sont proposés pour le mécanisme de croissance du SSS. Diverses caractérisations (par exemple TEM et APT) ont été effectuées pour étudier les propriétés physiques, et chimiques des nanofils
Germanium-Tin alloy is a unique class semiconductor gaining a strong attention because of its significant electrical and optical properties. Sn incorporation in Ge allows straightforward band-gap engineering enabling to enhance the electron and hole mobilities, and for a sufficient Sn amount an indirect-to-direct band-gap transition occurs. Its versatility rises due the possible monolithic integration on Si-platforms making it an ideal material in domains of optoelectronics, and high speed electronic devices. This thesis has focused on the fabrication and characterization of crystalline Ge1-xSnx nanowires with high Sn concentrations. New strategies were designed to fabricate many types of GeSn nanowires. The results have been explained as function of the existing kinetic models. A new growth mechanism was reported (i.e. Solid-Solid-Solid mechanism – SSS), it consists of growing in-plane GeSn nanowires using Sn catalysts below the melting point of Sn. Four mass transport models were proposed for the SSS growth mechanism. Various characterizations (e.g. TEM and APT) were done to investigate the physical and chemical properties of the obtained nanowires

Ce travail a porte sur la croissance epitaxiale des nitrures d'elements iii gan, aln, et inn, en utilisant l'epitaxie par jets moleculaires assistee par plasma d'azote. Nous avons optimise les premiers stades de la croissance de gan ou aln sur substrat al#2o#3 (0001). Le processus utilise consiste a nitrurer la surface du substrat a l'aide du plasma d'azote, afin de la transformer en aln, puis a faire croitre une couche tampon d'aln ou de gan a basse temperature, avant de reprendre la croissance de gan ou aln a haute temperature (680 a 750c). Nous avons en particulier etudie les proprietes d'une couche de gan en fonction de la temperature a laquelle est realisee l'etape de nitruration. Lorsque les conditions de demarrage de la croissance sont optimisees, nous avons pu observer des oscillations de rheed pendant la croissance de la couche de gan. Nous avons etudie l'effet du rapport v/iii sur la morphologie de surface et les proprietes optiques et structurales de cette couche. Nous avons propose l'utilisation de l'indium en tant que surfactant pour ameliorer ces proprietes. Nous avons ensuite aborde la realisation de superreseaux gan/aln dont nous avons optimise les interfaces. Les mecanismes de relaxation des contraintes de aln sur gan et gan sur aln ont ete etudies. Nous avons egalement elabore les alliages algan et ingan, comme barrieres quantiques dans les heterostructures. Nous avons montre que la relaxation elastique des contraintes de gan en epitaxie sur aln donne lieu a la formation d'ilots de tailles nanometriques, qui se comportent comme des boites quantiques. Leur densite et leur taille dependent de la temperature de croissance, et des conditions de murissement apres croissance. Les proprietes optiques de ces ilots sont gouvernees a la fois par les effets de confinement quantique et par le fort champ piezo-electrique induit par la contrainte dans les ilots.

博士
國立清華大學
材料科學工程學系
93
One-dimensional nanostructures, such as nanowires and nanotubes, have attracted great attention because of their peculiar optical, electrical and mechanical properties. 1D nanostructures illustrate the smallest dimension structure that can be efficiently transport electrical carriers, and thus are ideally suited to the critical and ubiquitous task of moving charges in integrated nanoscaled systems. Second, 1D nanostructures can also exhibit device function, and thus can be exploited as both the wiring and device elements in architectures for functional nanosystems. III-V nitrides (GaN, InGaN, AlGaN) are promising wide bandgap materials for photonic and electronic device applications such as LEDs in blue and ultraviolet regions. Gallium nitride is a direct and wide-band-gap (3.4 eV) semiconductor possessing very strong Ga-N bonds. These and some other features make this compound almost an ideal material for green/blue/ultraviolet optoelectronics. The present thesis focused on the synthesis and characterization of nanostructures of the Ga-related components. The thesis includes the following topics: (1) growth processes of GaN nanowires synthesized by metalorganic chemical vapor deposition, (2) template-free synthesis of tubular nanostructures by metalorganic chemical vapor deposition, (3) synthesis of GaN/CNT/Si/SiOx nanocables by metalorganic chemical vapor deposition, (4) high yield synthesis of Ga-containing oxide/CNT nanocables, (5) thermal properties of Ga-containing oxide/CNT nanocables; and (6) one-step growth of zinc blend GaN@carbon nanotube nanocables.

In recent years, there has been growing interests on II-VI semiconductor nanostructures, which are suitable for applications in electronics and optoelectronic devices such as solar cells, UV lasers, sensors, light emitting diodes and field emission displays. II-VI semiconductor nanostructures with different morphologies such as wires, belts, rods, tubes, needles, springs, tetrapods, plates, hierarchical structures and so on, have been widely grown by vapor transport methods. However the process conditions used for the growth of nanostructures still remains incompatible for device fabrication. The realization of practical nanoscale devices using nanostructured film depends mainly on the availability of low cost and lower processing temperatures to manufacture high purity nanostructures on a variety of substrates including glass and polymer. In this thesis work, studies have been made on the growth and characterization of II-VI semiconductor nanostructures prepared at room temperature, under high vacuum, without employing catalysts or templates. (i) ZnO nanostructured films with different morphology such as flowers, needles and shrubs were deposited at room temperature on glass and polymer substrates by plasma assisted reactive process. (ii) Zn/ZnO core/shell nanowires were grown on Si substrates under optimized oxygen partial pressure. Annealing of this core shell nanowire in high vacuum resulted in the formation of ZnO nanocanals. (iii) ZnS and ZnSe nano and microstructures were grown on Si substrates under high vacuum by thermal evaporation. The morphology, structural, optical properties and composition of these nano and microstructures were investigated by XRD, SEM, TEM, Raman, PL and XPS. The growth mechanism behind the formation of the different nanostructures has been explained on the basis of vapour-solid (VS) mechanism.

In recent years, there has been growing interests on II-VI semiconductor nanostructures, which are suitable for applications in electronics and optoelectronic devices such as solar cells, UV lasers, sensors, light emitting diodes and field emission displays. II-VI semiconductor nanostructures with different morphologies such as wires, belts, rods, tubes, needles, springs, tetrapods, plates, hierarchical structures and so on, have been widely grown by vapor transport methods. However the process conditions used for the growth of nanostructures still remains incompatible for device fabrication. The realization of practical nanoscale devices using nanostructured film depends mainly on the availability of low cost and lower processing temperatures to manufacture high purity nanostructures on a variety of substrates including glass and polymer. In this thesis work, studies have been made on the growth and characterization of II-VI semiconductor nanostructures prepared at room temperature, under high vacuum, without employing catalysts or templates. (i) ZnO nanostructured films with different morphology such as flowers, needles and shrubs were deposited at room temperature on glass and polymer substrates by plasma assisted reactive process. (ii) Zn/ZnO core/shell nanowires were grown on Si substrates under optimized oxygen partial pressure. Annealing of this core shell nanowire in high vacuum resulted in the formation of ZnO nanocanals. (iii) ZnS and ZnSe nano and microstructures were grown on Si substrates under high vacuum by thermal evaporation. The morphology, structural, optical properties and composition of these nano and microstructures were investigated by XRD, SEM, TEM, Raman, PL and XPS. The growth mechanism behind the formation of the different nanostructures has been explained on the basis of vapour-solid (VS) mechanism.

博士
國立中興大學
材料科學與工程學系所
99
The syntheses of metal-oxide semiconductor nanostructures have stimulated intensive research activities because of their contribution to the understanding of basic concepts and potential technological applications. A broad range of metal-oxide semiconductor nanostructure materials, such as In2O3, SnO2, TiO2, MgO, ZnO, and Ga2O3, have been successfully synthesized by various methods including laser ablation, template-assisted growth, arc discharge, vapor-phase transportation, and hydrothermal process growth. Tin oxide (SnO2) and Indium oxide (In2O3) were important n-type wide band gap semiconductor and exhibit unique optical, electrical, and catalytic properties. Therefore, to study the growth mechanism and performances of SnO2 and In2O3 are very important. 1D SnO2 nanostructure in pure form is rarely used and usually required to increase its electrical conductivity by incorporation of dopant ions, such as Sb, F, and Ta. Up to now, only few investigations on the synthesis of Sb-doped SnO2 nanostructures were reported and none of them discussed their photoluminescence (PL) properties. Single-crystalline Sb-additivated SnO2 nanorods, beaklike nanorods, and nanoribbons were synthesized by a catalyst-assisted thermal evaporation process on single-crystallin Si substrates. As the Sb:Sn weight ratios were increased, the morphologies of Sb-additivated SnO2 nanostructures would progressively transform from nanorods to beaklike nanorods and to the mixture of nanowires and nanoribbons. The SnO2 nanorods grow along the [0 0] direction and have lateral facets defining a square column consisting of {100} and {001} planes. The Sb-additivated SnO2 beaklike nanorods initially grow along the [0 ] direction and then switch to the [03 ] direction to form the beak, while the nanoribbons grow along the [110] direction. The Sb atoms were found to uniformly distribute over the whole Sb-additivated SnO2 nanostructures and the addition of Sb atoms would not affect the single crystallinity of SnO2 nanostructures. The photoluminescence spectra of the nonadditivated and Sb-additivated SnO2 nanostructures exhibited multipeaks with peak positions centered at 403, 453, 485, 557, and 622 nm. When Sb atoms were additivated into SnO2 nanostructures, the luminescence intensities would significantly decrease and photoluminescence at 557 and 622 nm would almost disappear. These can be explained by the replacements of the six- and five-fold coordinated Sn atoms on low-index facets by five- and four-fold coordinated Sb atoms, respectively, leading to the cancellation of 100° tin coordinated on-plane oxygen bridging vacancies and 130° tin coordinated in-plane oxygen vacancies. In addition, many simple In2O3 nanostructures, such as nanotubes, nanowires, nanorods, and nanobelts, have been successfully synthesized. In contrast, the investigations on the complex In2O3 nanostructures such as nanotowers and nanocubes are limited. It is well-known that the properties of nanostructures strongly depend on their morphologies so that nanostructures with different morphologies have special applications. Accordingly, it is necessary to clarify which process really dominates the growth mechanism of In(OH)3 nanocubes and nanotower. To elucidate the growth mechanism of In2O3 nanotowers synthesized via a Au-catalyzed vapor transport process, the structural evolution of In2O3 nanotowers was carefully examined during the synthesis process. It was found that Au catalysts only play a role at the initial stage, where they facilitate the formation of In2O3 nanoparticles and nanorods. After the Au atoms are consumed by the formation of Au-In compound(s), the liquid In droplets will form on the tips of In2O3 nanoparticles or nanorods, and the self-catalytic vapor-liquid-solid (VLS) growth mechanism will dominate the subsequent one-dimensional (1D) growth of In2O3 nanopillars. Since the supply of In2O may not be sufficient for the continuous 1D growth, the lateral growth of In2O3 nanopillars governed by the vapor-solid (VS) mechanism will occur. The periodical axial and continuous lateral growth leads to the formation of In2O3 nanotowers with a truncated octahedron structure of 4-fold symmetry {111} accumulated planes along the [100] direction. The photoluminescence (PL) spectrum of In2O3 nanotowers exhibited an intense green-yellow luminescence at the wavelength of 580 nm, which can be ascribed to the possible recombination of electrons on singly ionized oxygen vacancies and holes on the valence band or doubly ionized oxygen vacancies. In addition, single-crystalline In(OH)3 nanocubes were synthesized in a simple aqueous solution without using surfactant at a temperature as low as 90 oC. To elucidate the growth mechanism, the structural evolution of In(OH)3 nanocubes during the synthesis process were carefully examined. The experimental results showed that the formation of In(OH)3 nanocubes is primarily guided by the oriented attachment mechanism following a zero-dimensional (0D) → one-dimensional (1D) → three-dimensional (3D) mode. The 0D In(OH)3 nanoparticles will first assemble to 1D nanorods, then the nanorods would orientedly attach to form 3D nanorod bundles, and finally the In(OH)3 nanorod bundles will fuse into strip-like or square nanocubes. Small strip-like or square nanocubes can further orientedly attach and fuse into big single-crystalline strip-like or square nanocube. However, the growth of strip-like and square nanocubes may also occur based on the Ostwald ripening. The cathodoluminescence (CL) spectra at room temperature of the as-synthesized In(OH)3 nanocubes exhibited a weak ultraviolet luminescence at 350 nm (3.54 eV) and a strong blue luminescence at 450 nm (2.75 eV), which can be attributed to the hydroxy ion defects generated by the incomplete reaction of In3+ ions with OH- radicals during the synthesis process.

Właściwości układów wielowarstwowych, złożonych z cienkich filmów organicznych oraz nieorganicznych warstw i/lub podłoży, są tematem rosnącej liczby badań zapoczątkowanych przez wiele grup naukowych w okresie ostatnich 20 lat. Ze względu na ich rosnące znaczenie aplikacyjne, pokazane na przykładzie takich działających urządzeń jak dioda elektroluminescencyjna, tranzystor cienko-filmowy czy ogniwo słoneczne, układy te znacząco zwiększyły swoją konkurencyjność w stosunku do produktów opartych na technologii krzemowej, co zostało zademonstrowane np. w produkcji giętkich wyświetlaczy OLED. Ich ogromny potencjał tkwi w ilości dostępnych kombinacji związków organicznych i nieorganicznych. Jednakże, w celu głębszego zrozumienia tego typu układów, a przez to osiągnięcia nad nimi lepszej kontroli w nanoskali, potrzebne są dalsze badania nad podstawowymi mechanizmami rządzącymi formowaniem się interfejsu molekuła- podłoże oraz, generalnie rzecz biorąc, organizacją molekuł na powierzchni. W poniższej rozprawie doktorskiej zaprezentowano wyniki szeregu eksperymentów z zakresu fizyki powierzchni przeprowadzonych w ultra czystych warunkach próżniowych oraz poświęconych morfologicznej i strukturalnej stronie wzrostu warstw molekularnych o grubościach rzędu kilku nanometrów na podłożach półprzewodnikowych. Innowacyjność zaprezentowanych badań opiera się na zastosowaniu - w roli podłoża - powierzchni rutylu TiO2(110), modyfikowanej wiązką jonową w celu uzyskania różnego stopnia funkcjonalizacji powierzchni, a w rezultacie zmiany interfejsu/oddziaływania molekuła- podłoże. Odpowiedni dobór warunków preparatyki pozwala uzyskać powierzchnię TiO2(110) nie tylko w formie atomowo czystych tarasów, ale również taką o rosnącej ilości defektów powierzchniowych. Najwięcej uwagi poświęcono niestosowanym wcześniej podłożom o znacznie zmodyfikowanej powierzchni, która na skutek długotrwałego oddziaływania z wiązką jonową przekształca się w regularną strukturę przypominającą zmarszczki na wodzie. Modyfikacja oddziaływania molekuła-podłoże ma szczególne znaczenie dla struktur opartych na molekułach organicznych o wyraźnej anizotropii kształtu, takich jak parahexafenyl (6P) reprezentujący grupę pręto-podobnych oligofenyli. Ze względu na swoje interesujące właściwości optoelektroniczne, stosunkowo prostą budowę aromatyczną, oraz tendencję do organizowania się w wysoce uporządkowane struktury 2D i 3D, molekuła 6P jest idealnym kandydatem do budowy nanostruktur organicznych. Zaprezentowana praca przedstawia systematyczną charakterystykę różnych ścieżek wzrostu 6P na Ti02(110) skupiając się głównie na jej początkowych etapach, gdzie wpływ podłoża jest najlepiej widoczny. Pokazano, że już stosukowo niewielkie zmiany na powierzchni Ti02 mogą "przełączyć" cały układ 6P z orientacji "leżącej" do "stojącej", dając ostatecznie zupełnie odmienne struktury 3D, tzn. druty albo wyspy 6P. Ich powstanie poprzedza formująca się od samego początku depozycji warstwa 2D, której stabilność w warunkach powietrznych także została przebadana. Kontrola parametrów zmarszczek, a w szczególności zachowanie krystaliczności powierzchni TiO2(110) - w formie charakterystycznych rzędów atomowych - pozwala efektywnie wpływać na adsorpcję i dyfuzję nanoszonych molekuł organicznych. Dzięki temu możliwe stało się modyfikowanie stabilności, kształtu oraz typu nanostruktur 6P, przy jednoczesnym zachowaniu ich regularnej struktury wewnętrznej. Ich potencjał tkwi w anizotropii przewodnictwa prądu oraz właściwości optycznych, które są ściśle związane z orientacją struktury krystalicznej 6P względem podłoża.
During the last 20-30 years we have witnessed a growing interest in systems based on thin organic layers formed on inorganic substrates, which proved to be promising candidates for application in devices designed for energy conversion/storage, and particularly in high-tech optoelectronic appliances. It is due to a huge diversity of the available organic compounds and inorganic substrates. As demonstrated on the example of a fully-operational light emitting diode (OLED), thin film transistor (OTFT) or solar cell, organic-based devices have significantly increased their competitiveness in respect to the currently dominating silicon-based technology. This new trend can be already noticed in the production of consumer electronics where, e.g., flexible displays have been applied. However, a further development and miniaturisation of organic (opto)electronic devices can be reached only by studying the fundamental aspects of the growth of thin organic layers, especially concerning the formation of molecular-substrate (MS) interface. This dissertation presents a comprehensive study devoted to the morphological and structural side of the growth of very thin molecular layers on a semiconducting surface. To provide a proper insight into the formation of MS interface and to control the crucial conditions of molecular growth at atomic scale, most experiments were performed in an ultrahigh vacuum (UHv). Innovation of the presented research comes from the fact that a substrate - in this role a rutile Ti02(110) surface - was modified by ion beam sputtering (IBS). This method allows to change the substrate surface roughness/structure and thus, effectively influences the MS interface. By means of IBS method - in combination with thermal treatment - one can produce an atomically clean, crystalline surface with flat terraces as well as with a wide range of concentration of surface defects. However, most attention has been drawn to a strongly modified Ti02 surface, which develops upon a long-time IBS into a regular, wavelike structure called ripples. IBS-induced modification of MS interaction reveals a crucial impact on the growth of organic molecules with pronounced shape anisotropy, like para-hexaphenyl (6P) being a model/prototypical representative of rod-like, semiconducting oligo-phenyls. This relatively simple aromatic molecule stands out with its promising optoelectronic properties and stability in ambient conditions. Moreover, 6P molecules tend to organize into highly crystalline 2D and 3D structures which makes them an ideal candidate to construct a welldefined organic nanostructure. One of the key findings of this thesis is the identification and systematic characterisation of a few different growth pathways of 6P on Ti02(110), with particular emphasis on initial stages of growth where the influence of the substrate is the most prominent. It has been shown that already slight changes to Ti02 surface are enough to "switch" a whole growth mode from the flat-lying to the up-right standing orientation with respect to the substrate surface. As a result, completely different 3D nanostructures - like needles or islands - are observed. In addition, it has been found that the tuning of properties of a rippled substrate, in particular a crystallinity of Ti02(110) surface, is a very effective way not only to control the most energetically favored path of molecular growth, but also to change stability or shape of the final thin film structure. Finally, the thesis provides a molecular-scale insight into the formation of initial stages of 6P growth before 3D nucleation, where different forms of 2D molecular layer have been identified.

博士
國立清華大學
材料科學工程學系
96
We have successfully grown wurtzite-phase InN epitaxial layers on Si(111) substrates using atomically flat AlN intermediate layers. Both hetero-interfaces were found to be abrupt based on the field emission scanning electron microscopy and transmission electron microscopy observations. Compositional analysis and the crystal structures for the best InN grown on the AlN/Si(111) has been intensively studied using scanning electron microscopy, field emission electron probe microanalysis, high resolution x-ray diffractometer (HR-XRD), and transmission electron microscopy. Cs and O2 sputtering secondary ion mass spectroscopy have also been used to study the depth profile of the epitaxial InN in order to probe the oxygen content precisely. For the photoluminescence obtaining at 77 K, two peaks which the energy difference is around 82 meV has been detected. According to the transmission electron microscopy microanalysis, two distinct heterostructures, such as InN/AlN/Si(111) and InN/AlN/SiOx/Si(111), are found. It is believed that the structure induced energy shift is the origin of this abnormal phenomenon. This structure induced strain energy is estimated to be around 2.6 GPa based on the hydrostatic photoluminescence reported from the literature and the low temperature photoluminescence taken in this study. Meanwhile, this sample is possessed of high electron mobility even though the carrier concentration is as high as ~ 10^19 /cm^3. The room temperature result is higher than 1100 cm^2/(V．s) via the van der Pauw method. In the second part of this report, unidirectional self-formation and self-apex selective InN nanotips have been successfully fabricated by using the top-down technique on the molecular beam epitaxy (MBE) grown InN/AlN/Si(111) template. Field emission measurement shows that this novel material has very low turn-on field (0.90+/-0.34 V/um at 1 uA/cm^2 and 2.08+/-0.53V/um at 10 uA/cm^2) with very high current density (~180 mA/cm^2) under the field ~ 7 V/um. Possible formation mechanism is discussed based on the cross-sentional analysis using transmission electron microscopy after field emission characterization. The angle dependent experiment further confirmed the proposed mechanism. The extra low turn-on characteristics of electron emission is attributed to the double enhancement of (i) the geometrical factor of the spherical InN nanostructures with suitable tip density, and (ii) the inherently high carrier concentration of the degenerate InN semiconductor with surface electron accumulation layer induced downward band bending effect that significantly reducing the effective electron tunneling barrier.

Thesis (M.S.)--State University of New York at Buffalo, 2005.
Title from PDF title page (viewed on Apr. 13, 2005) Available through UMI ProQuest Digital Dissertations. Thesis adviser: Mountziaris, Triantafillos J. Includes bibliographical references.

博士
國立成功大學
材料科學及工程學系碩博士班
98
The aim of this research is to understand how the microstructure influences the magnetism and magneto-transport properties of nanomaterials and diluted magnetic semiconductors (DMS). Microstructural investigation by TEM is performed to characterize magnetic nanostrucutres in various dimensions. The materials covered in this thesis include iron and iron silicide nanoislands, Ni nanowires and Co-doped ZnO films. First, self-assembled Fe islands were successfully grown on Si(001) by ion-beam sputtering at room temperature. Nanometer-scale iron islands, ranging from 25 to 100 nm with narrow size distributions, can be achieved with a silicide interfacial layer analogous to the S-K growth mode. Furthermore, the perpendicular magnetic anisotropy is found to fall away with increasing island size. It is suggested that surface effects from the morphologies of 3D islands are mainly responsible for the spin reorientation. With increasing growth temperature to 200oC, the reactive interaction between Si and Fe leads to the formation of FeSi islands, the evolution of the growth of FeSi nanoislands on Si(001) is investigated. Under proper growth conditions, nanoislands spontaneously cluster into groups on rectangular FeSi terraces, depending on both substrate temperature and deposition coverage. This study discussed the self-clustering mechanism in the context of strain relaxation and mass transportation between nanoislands and terraces. As for Ni nanowires fabricated by electrodeposition on Anodic-aluminum-oxide (AAO) templates, their magnetic and magneto-transport properties have been investigated. The AAO pores have diameters ranging from 35 to 75 nm, while the crystallinity of Ni NW arrays could change from polycrystalline to single-crystalline with the [111] and [110] orientations based on electro-deposition potential. The crystalline orientation of Ni NW arrays significantly influenced the corresponding magnetic and magneto-transport properties. It s suggested that these magnetic behaviors are dominated by the interplay between magnetocrystalline and shape anisotropy. Finally, DMS, which is most commonly used for spintronics has also been studied. Co-doped ZnO films were synthesized by ion beam sputtering using multilayer (ZnO/Co) growth. Both the distribution and the chemical states of Co in ZnO can be well controlled by varying the ratio of the nominal layer thickness of ZnO to Co. Transmission electron microscopy indicated that all of the as-deposited Zn1-x(Co)xO films were polycrystalline with a (0002) preferred orientation. In ZnO (1.5 nm)/Co (0.1 nm), homogeneous Co-doped ZnO was observed to have been formed through inter-diffusion. However, decreasing or increasing the thickness of ZnO leads to the formation of Co clusters in the ZnO matrix or Zn1-x(Co)xO multilayers, respectively. For ZnO thickness≧1.5 nm, Co is substituted for Zn, and its valence state is 2+. All Co-doped ZnO films show room-temperature ferromagnetic behavior, which appears to depend strongly on the Co distribution.

In addition, we have studied the growth conditions and the properties of ZnSSe alloy nano-tetrapods grown by chemical vapor deposition. Different from the ZnSSe nanowires synthesized by MOCVD, the ZnSSe nano-tetrapods are of hexagonal structure. We observed a wavelength-tunable near band gap luminescence in the UV-blue region from this nanostructurally-designed system.
Recently, semiconductor nanostructures have attracted much attention because they are potentially useful as fundamental building blocks in nanodevices. As an important member of group II-VI semiconductors, ZnS and its alloys with ZnSe are particularly important for optical applications in the UV-blue region . Thus, we concentrated on the synthesis of ZnS, ZnSe and ZnSSe nanostructures and studied their optical properties.
Vertically-aligned ZnSe nanowires were also synthesized by MOCVD using Ag and Ga nanoparticles as catalysts. In the photoluminescence spectra from Ag or Ga catalyzed ZnSe nanowires, we observed recombination of excitons bound to substitutional Ag or Ga impurities respectively, which indicates that Ag and Ga have been doped into ZnSe nanowires in our experiments.
We are among the first group to grow vertically well-aligned ZnSSe alloy nanowires of controllable composition. Most of ZnSSe nanowires were found to have a cubic structure. We also found a compositional relationship between the nanowires and precursors, which is useful for predicting the lattice constant and band-gap emission energy of ZnSSe nanowires.
ZnS nanowire arrays were fabricated on the GaAs (100), (110) (311)A and (111)B substrates by metal organic chemical vapor deposition (MOCVD) using Ag, Au and Ga particles as catalysts. Their orientation was adjusted by changing the crystallographic orientation of the substrate. Moreover, Ga was doped into ZnS nanowires, when Ga nanoparticles serve as catalysts.
Liang, Yao = ZnS和ZnSSe納米結構的生長和光學性質 / 梁瑤.
Adviser: Hank Suikong.
Source: Dissertation Abstracts International, Volume: 72-11, Section: B, page: .
Thesis (Ph.D.)--Chinese University of Hong Kong, 2009.
Includes bibliographical references.
Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Abstract also in Chinese.
Liang, Yao = ZnS he ZnSSe na mi jie gou de sheng chang he guang xue xing zhi / Liang Yao.

碩士
中原大學
應用物理研究所
90
Cd1-xZnxTe crystals were grown by the temperature gradient solution growth (TGSG). Optical properties of the Cd1-xZnxTe crystals were investigated by the photoluminescence (PL) spectroscopy. The full width at half maximum (FWHM) of 11 meV for the near band edge photoluminescence was obtained. The temperature dependent broadening of the photoluminescence line-width was fitted by the acoustic phonon, longitudinal optical phonon, and impurity interaction parameters. The defect radiative density is as low as 0.02. In addition to the Cd1-xZnxTe crystals, self-assembled ZnTe quantum dot structures were grown on the GaAs substrates with the ZnSe buffer layer of 200 nm by the molecular beam epitaxy (MBE). Surface morphology was studied by the atomic force microscopy (AFM). A three-dimensional Volmer-Weber growth mode was identified. Two types of dots were observed. Strong photoluminescence observed at 1.9-2.2 eV was attributed to the emission from the type II ZnTe quantum dots with larger size. While, emission from the smaller ZnTe quantum dots is observed at an energy around 2.26 eV. The density of the larger and smaller dots was approximately 108/cm2 and 109/cm2, respectively.

博士
國立臺灣科技大學
光電工程研究所
106
Cuprous oxide (Cu2O) is a promising semiconductor material for photo-voltaic(PV), photoelectrochemical cells (PEC) and other optoelectronic applications. The aim of this thesis was to deposit and characterize high quality Cu2O nano-structures for these and other applications. In this study, well-structured Cu2O nano-rods, truncated nano-cubes, and nano-pyramids were successfully fabricated using ion beam sputter deposition (IBSD) in which the Cu2O samples were grown on quartz and silicon substrates with a substrate temperature of 450°C and a base pressure of 4.5 x 10-5 Torr using metallic copper as a target material. In the first part of this thesis, we studied the growth of different Cu2O nano-structures deposited on quartz substrates. The results show that by changing the argon/oxygen flow rates starting from 6:1 to 14:1, Cu2O thin film (Ar:O2=3.6:0.6), Cu2O nano-rods of length 1-2 µm (Ar:O2=4.5:0.5, 6:0.5, and 7:0.5) were grown on quartz substrates. In addition to the Cu2O thin film and Cu2O nano-rods grown on quartz substrates, triangular pyramids (Ar:O2=3.6:0.4), and Cu2O truncated nano-cubes (Ar:O2=4.4:0.4) were grown on Si/SiO2 substrates. The structural characterizations were done employing XRD and FE-SEM. The results show that all the samples are poly-crystalline structures Cu2O and the preferable growth direction was to the (111) crystal plane. All the samples were also investigated by Micro Raman spectra and the results indicate all the Raman bands are typical characteristic of the phonon modes of Cu2O phases. The temperature dependent PL measurements have been done for the samples and the results show Cu2O samples deposited with low oxygen flow rates (Ar:O2=4.5:0.5, 6.0:0.5, 4.4:0.4 and 7.0:0.5), have PL emission peak centered at ~720 nm which is attributed due to doubly oxygen vacancy induced luminescence at a temperature of 12 to 100 K. While the sample deposited with Ar:O2=3.6:0.6 shows no PL emissions which indicates that Cu2O nanostructures are prone to enhance the PL emissions. Among the different Cu2O nanostructures, Cu2O triangular nanopyramids interestingly shows a strong green exciton PL emission centered at 509 nm resulted due to radiative recombination of photogenerated electrons at room temperature. The PL measurements show that no PL emission as a result of copper vacancy defects showing the carrier types are of n-type. Furthermore, all the samples have been investigated using XPS and the results show the Cu2O Cu 2p core level binding energies centered at 932.4 eV, and 952.4 eV were observed. Moreover, the O 1s spectra at 530.5 eV shows additional confirmation of the surface composition and phase purity of Cu2O samples. Optical transmittance measurements also have been done and the results show that transmittance value is lower for Cu2O thin film and higher for Cu2O nanorods. The highest transmittance value is obtained for the sample with argon/oxygen flow rate of 9:1 which is ~79%. The Tauc's plots of all the samples show also that the band gap shifts from 2.5 eV to 2.3 eV as the oxygen flow rates drops. The photoelectrochemical characterizations also show all the samples deposited with low oxygen flow rates show n-type carrier and p-type for samples deposited at high oxygen flow rates. The stability of the photocurrent values generated by Cu2O photoelectrodes were excellent (~95%). The photocurrent measurements show that the sample with argon/oxygen flow rate of 9:1 shows a significantly enhanced anodic photocurrent density of ~2.2 mA/cm2. The Mott-Schottky plots of the samples except the one with higher oxygen flow rate (Ar:O2=6:1) (which shows a negative value of the slope), show positive values of the slopes which are additional confirmations of the n-type carriers. The calculations of the carrier densities from the Mott Schottky plots shows that the highest values is obtained for the sample with argon/oxygen flow rate of 9:1 which has a value of and the smaller value of the acceptor densities is for the p-type sample which is . We also demonstrated the application of the Cu2O photo-electrodes for water reduction and oxidation by studying the band edge positions of the semiconductors with respect to both normal hydrogen electrode and vacuum energy levels and the samples are promising for use as photo-electrode materials.

The present work has been focused on the growth of Group III-nitride epitaxial layers and nanostructures on Si (111) substrates by plasma-assisted molecular beam epitaxy. Silicon is regarded as a promising substrate for III-nitrides, since it is available in large quantity, at low cost and compatible to microelectronics device processing. However, three-dimensional island growth is unavoidable for the direct growth of GaN on Si (111) because of the extreme lattice and thermal expansion coefficient mismatch. To overcome these difficulties, by introducing β-Si3N4 buffer layer, the yellow luminescence free GaN can be grow on Si (111) substrate. The overall research work carried out in the present study comprises of five main parts. In the first part, high quality, crack free and smooth surface of GaN and InN epilayers were grown on Si(111) substrate using the substrate nitridation process. Crystalline quality and surface roughness of the GaN and InN layers are extremely sensitive to nitridation conditions such as nitridation temperature and time. Raman and PL studies indicate that the GaN film obtained by the nitridation sequences has less tensile stress and optically good. The optical band gaps of InN are obtained between ~0.73 to 0.78 eV and the blueshift of absorption edge can be induced by background electron concentration. The higher electron concentration brings in the larger blueshift, due to a possible Burstein–Moss effect. InN epilayers were also grown on GaN/Si(111) substrate by varying the growth parameters such as indium flux, substrate temperature and RF power. In the second part, InGaN/Si, GaN/Si3N4/n-Si and InN/Si3N4/n-Si heterostructures were fabricated and temperature dependent electrical transport behaviors were studied. Current density-voltage plots (J-V-T) of InGaN/Si heterostructure revealed that the ideality factor and Schottky barrier height are temperature dependent and the incorrect values of the Richardson’s constant produced, suggests an inhomogeneous barrier at the heterostructure interface. The higher value of the ideality factor compared to the ideal value and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission rather than thermionic emission. The valence band offset of GaN/β-Si3N4/Si and InGaN/Si heterojunctions were determined by X-ray photoemission spectroscopy. InN QDs on Si(111) substrate by droplet epitaxy and S-K growth method were grown in the third part. Single-crystalline structure of InN QDs (droplet epitaxy) was verified by TEM and the chemical bonding configurations of InN QDs were examined by XPS. The interdigitated electrode pattern was created and (I-V) characteristics of InN QDs were studied in a metal–semiconductor–metal configuration in the temperature range of 80–300 K. The I-V characteristics of lateral grown InN QDs were explained by using the trap model. A systematic manipulation of the morphology, optical emission and structural properties of InN/Si (111) QDs (S-K method) is demonstrated by changing the growth kinetics parameters such as flux rate and growth time. The growth kinetics of the QDs has been studied through the scaling method and observed that the distribution of dot sizes, for samples grown under varying conditions, has followed the scaling function. In the fourth part, InN nanorods (NRs) were grown on Si(111) and current transport properties of NRs/Si heterojunctions were studied. The rapid rise and decay of infrared on/off characteristics of InN NRs/Si heterojunction indicate that the device is highly sensitive to the IR light. Self-aligned GaN nanodots were grown on semi-insulating Si(111) substrate. The interdigitated electrode pattern was created on nanodots using photolithography and dark as well as UV photocurrent were studied. Surface band gaps of InN QDs were estimated from scanning tunneling spectroscopy (STS) I-V curves in the last part. It is found that band gap is strongly dependent on the size of InN QDs. The observed size-dependent STS band gap energy blueshifts as the QD’s diameter or height was reduced.

The present work has been focused on the growth of Group III-nitride epitaxial layers and nanostructures on Si (111) substrates by plasma-assisted molecular beam epitaxy. Silicon is regarded as a promising substrate for III-nitrides, since it is available in large quantity, at low cost and compatible to microelectronics device processing. However, three-dimensional island growth is unavoidable for the direct growth of GaN on Si (111) because of the extreme lattice and thermal expansion coefficient mismatch. To overcome these difficulties, by introducing β-Si3N4 buffer layer, the yellow luminescence free GaN can be grow on Si (111) substrate. The overall research work carried out in the present study comprises of five main parts. In the first part, high quality, crack free and smooth surface of GaN and InN epilayers were grown on Si(111) substrate using the substrate nitridation process. Crystalline quality and surface roughness of the GaN and InN layers are extremely sensitive to nitridation conditions such as nitridation temperature and time. Raman and PL studies indicate that the GaN film obtained by the nitridation sequences has less tensile stress and optically good. The optical band gaps of InN are obtained between ~0.73 to 0.78 eV and the blueshift of absorption edge can be induced by background electron concentration. The higher electron concentration brings in the larger blueshift, due to a possible Burstein–Moss effect. InN epilayers were also grown on GaN/Si(111) substrate by varying the growth parameters such as indium flux, substrate temperature and RF power. In the second part, InGaN/Si, GaN/Si3N4/n-Si and InN/Si3N4/n-Si heterostructures were fabricated and temperature dependent electrical transport behaviors were studied. Current density-voltage plots (J-V-T) of InGaN/Si heterostructure revealed that the ideality factor and Schottky barrier height are temperature dependent and the incorrect values of the Richardson’s constant produced, suggests an inhomogeneous barrier at the heterostructure interface. The higher value of the ideality factor compared to the ideal value and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission rather than thermionic emission. The valence band offset of GaN/β-Si3N4/Si and InGaN/Si heterojunctions were determined by X-ray photoemission spectroscopy. InN QDs on Si(111) substrate by droplet epitaxy and S-K growth method were grown in the third part. Single-crystalline structure of InN QDs (droplet epitaxy) was verified by TEM and the chemical bonding configurations of InN QDs were examined by XPS. The interdigitated electrode pattern was created and (I-V) characteristics of InN QDs were studied in a metal–semiconductor–metal configuration in the temperature range of 80–300 K. The I-V characteristics of lateral grown InN QDs were explained by using the trap model. A systematic manipulation of the morphology, optical emission and structural properties of InN/Si (111) QDs (S-K method) is demonstrated by changing the growth kinetics parameters such as flux rate and growth time. The growth kinetics of the QDs has been studied through the scaling method and observed that the distribution of dot sizes, for samples grown under varying conditions, has followed the scaling function. In the fourth part, InN nanorods (NRs) were grown on Si(111) and current transport properties of NRs/Si heterojunctions were studied. The rapid rise and decay of infrared on/off characteristics of InN NRs/Si heterojunction indicate that the device is highly sensitive to the IR light. Self-aligned GaN nanodots were grown on semi-insulating Si(111) substrate. The interdigitated electrode pattern was created on nanodots using photolithography and dark as well as UV photocurrent were studied. Surface band gaps of InN QDs were estimated from scanning tunneling spectroscopy (STS) I-V curves in the last part. It is found that band gap is strongly dependent on the size of InN QDs. The observed size-dependent STS band gap energy blueshifts as the QD’s diameter or height was reduced.

Group III-nitride semiconductors are well suited for the fabrication of devices including visible-ultraviolet light emitting diodes, high-temperature and high-frequency devices. The wurtzite III-nitride based heterostructures grown along polar c-direction have large internal electric fields due to discontinuities in spontaneous and piezoelectric polarizations. For optoelectronic devices, such as light-emitting diodes and laser diodes, the internal electric field is deleterious as it causes a spatial separation of electron and hole wave functions in the quantum wells, which decreases emission efficiency. Growth of GaN-based heterostructures in alternative orientations, which have reduced (semipolar) or no polarization (nonpolar) in the growth direction, has been a major area of research in the last few years. The correlation between structural, optical and transport properties of semipolar and nonpolar III-nitride would be extremely useful. The thesis focuses on the growth and characterizations of semipolar and nonpolar III-nitride heterostructures by plasma-assisted molecular beam epitaxy. Chapter 1 provides a brief introduction to the III-nitride semiconductors. The importance of semipolar and nonpolar III-nitride heterostructures over conventional polar heterostructures has been discussed. Chapter 2 deals with the descriptions of molecular beam epitaxy system and working principles of different characterization tools used in the present work. Chapter 3 addresses the molecular beam epitaxial growth of nonpolar (1 1 -2 0) and semipolar (1 1 -2 2) GaN on sapphire substrates. An in-plane orientation relationship is found to be [0 0 0 1] GaN || [-1 1 0 1] sapphire and [-1 1 0 0] GaN || [1 1 -2 0] sapphire for nonpolar GaN on r-sapphire substrates. Effect of growth temperature on structural, morphological and optical properties of nonpolar GaN has been studied. The growth temperature plays a major role in controlling crystal quality, morphology and emission properties of nonpolar a-plane GaN. The a-plane GaN shows crystalline anisotropy nature and it has reduced with increase in the growth temperature. The surface roughness was found to decrease with increase in growth temperature and film grown at 760°C shows reasonably smooth surface with roughness 3.05 nm. Room temperature photoluminescence spectra show near band emission peak at 3.434 -3.442 eV. The film grown at 800 ºC shows broad yellow luminescence peak at 2.2 eV. Low temperature photoluminescence spectra show near band emission at 3.483 eV along with defect related emissions. Raman spectra exhibit blue shift due to compressive strain in the film. An in-plane orientation relationship is found to be [1 -1 00] GaN || [1 2-1 0] sapphire and [-1 -1 2 3] GaN || [0 0 0 1] sapphire for semipolar GaN on m-plane sapphire substrates. The surface morphology of semipolar GaN film is found to be reasonably smooth with pits on the surface. Room temperature photoluminescence shows the near band emission (NBE) at 3.432 eV, which is slightly blue shifted compared to the bulk GaN. The Raman E2 (high) peak position observed at 569.1 cm1. Chapter 4 deals with the fabrication and characterizations of Au/nonpolar and Au/semipolar GaN schottky diodes. The temperature-dependent current–voltage measurements have been used to determine the current mechanisms in Schottky diodes fabricated on nonpolar a-plane GaN and semipolar GaN epilayers. The barrier height (φb) and ideally factor (η) estimated from the thermionic emission model are found to be temperature dependent in nature indicate the deviations from the thermionic emission (TE) transport mechanism. Low temperature I-V characteristics of Au/ GaN Schottky diode show temperature independent tunnelling parameter. Barrier heights calculated from XPS are found to be 0.96 eV and 1.13 eV for Au/nonpolar GaN and Au/semipolar GaN respectively. Chapter 5 demonstrates the growth of InN on r-sapphire substrates with and without GaN buffer layer. InN film and nanostructures are grown on r-sapphire without GaN buffer layer and they are highly oriented along (0002) direction. The electron microscopy study confirms the nanostructures are vertically aligned and highly oriented along the (0001) direction. The Raman studies of InN nanostructures show the SO modes along with the other possible Raman modes. The band gap of InN nanostructures is found to be 0.82 eV. InN grown with a-plane GaN buffer shows nonpolar orientated growth. Growth temperature dependent studies of nonpolar a-plane InN epilayers are carried out. The valence band offset value is calculated to be 1.31 eV for nonpolar a-plane InN/GaN heterojunctions. The heterojunctions form in the type-I straddling configuration with a conduction band offsets of 1.41 eV. Chapter 6 deals with the temperature dependent I-V characteristics of the nonpolar a-plane (1 1 -2 0) InN/GaN heterostructures. The measured values of barrier height and ideality factor from the TE model show the temperature dependent variation. The double Gaussian distribution has mean barrier height values ( ϕb ) of 1.17 and 0.69 eV with standard deviation (σs ) of 0.17 and 0.098 V, respectively. The modified Richardson plot ln (Is/T2)-q2σ2/2k2T2 ) versus q/kT in the temperature range of 350 – 500 K, yielded the Richardson constant of 19.5 A/cm2 K2 which is very close to the theoretical value of 24 A/cm2 K2 for n-type GaN. The tunneling parameters E0 found to be temperature independent at low temperature range (150 –300 K). Chapter 7 concludes with the summary of present investigations and the scope for future work.

Group III-nitride semiconductors are well suited for the fabrication of devices including visible-ultraviolet light emitting diodes, high-temperature and high-frequency devices. The wurtzite III-nitride based heterostructures grown along polar c-direction have large internal electric fields due to discontinuities in spontaneous and piezoelectric polarizations. For optoelectronic devices, such as light-emitting diodes and laser diodes, the internal electric field is deleterious as it causes a spatial separation of electron and hole wave functions in the quantum wells, which decreases emission efficiency. Growth of GaN-based heterostructures in alternative orientations, which have reduced (semipolar) or no polarization (nonpolar) in the growth direction, has been a major area of research in the last few years. The correlation between structural, optical and transport properties of semipolar and nonpolar III-nitride would be extremely useful. The thesis focuses on the growth and characterizations of semipolar and nonpolar III-nitride heterostructures by plasma-assisted molecular beam epitaxy. Chapter 1 provides a brief introduction to the III-nitride semiconductors. The importance of semipolar and nonpolar III-nitride heterostructures over conventional polar heterostructures has been discussed. Chapter 2 deals with the descriptions of molecular beam epitaxy system and working principles of different characterization tools used in the present work. Chapter 3 addresses the molecular beam epitaxial growth of nonpolar (1 1 -2 0) and semipolar (1 1 -2 2) GaN on sapphire substrates. An in-plane orientation relationship is found to be [0 0 0 1] GaN || [-1 1 0 1] sapphire and [-1 1 0 0] GaN || [1 1 -2 0] sapphire for nonpolar GaN on r-sapphire substrates. Effect of growth temperature on structural, morphological and optical properties of nonpolar GaN has been studied. The growth temperature plays a major role in controlling crystal quality, morphology and emission properties of nonpolar a-plane GaN. The a-plane GaN shows crystalline anisotropy nature and it has reduced with increase in the growth temperature. The surface roughness was found to decrease with increase in growth temperature and film grown at 760°C shows reasonably smooth surface with roughness 3.05 nm. Room temperature photoluminescence spectra show near band emission peak at 3.434 -3.442 eV. The film grown at 800 ºC shows broad yellow luminescence peak at 2.2 eV. Low temperature photoluminescence spectra show near band emission at 3.483 eV along with defect related emissions. Raman spectra exhibit blue shift due to compressive strain in the film. An in-plane orientation relationship is found to be [1 -1 00] GaN || [1 2-1 0] sapphire and [-1 -1 2 3] GaN || [0 0 0 1] sapphire for semipolar GaN on m-plane sapphire substrates. The surface morphology of semipolar GaN film is found to be reasonably smooth with pits on the surface. Room temperature photoluminescence shows the near band emission (NBE) at 3.432 eV, which is slightly blue shifted compared to the bulk GaN. The Raman E2 (high) peak position observed at 569.1 cm1. Chapter 4 deals with the fabrication and characterizations of Au/nonpolar and Au/semipolar GaN schottky diodes. The temperature-dependent current–voltage measurements have been used to determine the current mechanisms in Schottky diodes fabricated on nonpolar a-plane GaN and semipolar GaN epilayers. The barrier height (φb) and ideally factor (η) estimated from the thermionic emission model are found to be temperature dependent in nature indicate the deviations from the thermionic emission (TE) transport mechanism. Low temperature I-V characteristics of Au/ GaN Schottky diode show temperature independent tunnelling parameter. Barrier heights calculated from XPS are found to be 0.96 eV and 1.13 eV for Au/nonpolar GaN and Au/semipolar GaN respectively. Chapter 5 demonstrates the growth of InN on r-sapphire substrates with and without GaN buffer layer. InN film and nanostructures are grown on r-sapphire without GaN buffer layer and they are highly oriented along (0002) direction. The electron microscopy study confirms the nanostructures are vertically aligned and highly oriented along the (0001) direction. The Raman studies of InN nanostructures show the SO modes along with the other possible Raman modes. The band gap of InN nanostructures is found to be 0.82 eV. InN grown with a-plane GaN buffer shows nonpolar orientated growth. Growth temperature dependent studies of nonpolar a-plane InN epilayers are carried out. The valence band offset value is calculated to be 1.31 eV for nonpolar a-plane InN/GaN heterojunctions. The heterojunctions form in the type-I straddling configuration with a conduction band offsets of 1.41 eV. Chapter 6 deals with the temperature dependent I-V characteristics of the nonpolar a-plane (1 1 -2 0) InN/GaN heterostructures. The measured values of barrier height and ideality factor from the TE model show the temperature dependent variation. The double Gaussian distribution has mean barrier height values ( ϕb ) of 1.17 and 0.69 eV with standard deviation (σs ) of 0.17 and 0.098 V, respectively. The modified Richardson plot ln (Is/T2)-q2σ2/2k2T2 ) versus q/kT in the temperature range of 350 – 500 K, yielded the Richardson constant of 19.5 A/cm2 K2 which is very close to the theoretical value of 24 A/cm2 K2 for n-type GaN. The tunneling parameters E0 found to be temperature independent at low temperature range (150 –300 K). Chapter 7 concludes with the summary of present investigations and the scope for future work.

Group III-nitride semiconductors have received much research attention and witnessed a significant development due to their ample applications in solid-state lighting and high-power/high-frequency electronics. Numerous growth methods were explored to achieve device quality epitaxial III-nitride semiconductors. Among the growth methods for III-nitride semiconductors, molecular beam epitaxy provides advantages such as formation of abrupt interfaces and in-situ monitoring of growth. The present research work focuses on the growth and characterizations of III-nitride based epitaxial films, nanostructures and heterostructures on c-sapphire substrate using plasma-assisted molecular beam epitaxy system. The correlation between structural, optical and electrical properties of III-nitride semiconductors would be extremely useful. The interfaces of the metal/semiconductor and semiconductor heterostructures are very important in the performance of semiconductor devices. In this regard, the electrical transport studies of metal/semiconductor and semiconductor heterostructures have been carried out. Besides, studies involved with the defect induced room temperature ferromagnetism of GaN films and InN nano-structures have also been carried out. The thesis is organized in eight different chapters and a brief overview of each chapter is given below. Chapter 1 provides a brief introduction on physical properties of group III-nitride semiconductors. It also describes the importance of III-nitride heterostructures in the operation of optoelectronic devices. In addition, it also includes the current strategy of the emergence of room temperature ferromagnetism in III-nitride semiconductors. Chapter 2 deals with the basic working principles of molecular beam epitaxy system and different characterization tools employed in the present work. Chapter 3 describes the growth of GaN films on c-sapphire by plasma-assisted molecular beam epitaxy. The effects of N/Ga flux ratio on structural, morphological and optical properties have been studied. The flux ratio plays a major role in controlling crystal quality, morphology and emission properties of GaN films. The dislocation density is found to increase with increase in N/Ga flux ratio. The surface morphologies of the films as seen by scanning electron microscopy show pits on the surface and found that the pit density on the surface increases with flux ratio. The room temperature photoluminescence study reveals the shift in band-edge emission towards the lower energy with increase in N/Ga flux ratio. This is believed to arise from the reduction in compressive stress in the GaN films as it is evidenced by room temperature Raman study. The transport studies on the Pt/GaN Schottky diodes showed a significant increase in leakage current with an increase in N/Ga ratio and is found to be caused by the increase in dislocation density in the GaN films. Chapter 4 deals with the fabrication and characterization of Au/GaN Schottky diodes. The temperature dependent current–voltage measurements have been used to determine the current transport mechanism in Schottky diodes. The barrier height (φb) and the ideality factor (η) are estimated from the thermionic emission model and are found to be temperature dependent in nature, indicating the existence of barrier height inhomogeneities at the Au/GaN interface. The conventional Richardson plot of ln(Is/T2) versus 1/kT gives Richardson constant value of 3.23×10-5 Acm-2 K-2, which is much lower than the known value of 26.4 Acm-2 K-2 for GaN. Such discrepancy of Richardson constant value was attributed to the existence of barrier height inhomogeneities at the Au/GaN interface. The modified Richardson plot of ln(Is/T2)-q2σs2/2k2T2 versus q/kT, by assuming a Gaussian distribution of barrier heights at the Au/GaN interface, provides the Schottky barrier height of 1.47 eV and Richardson constant value of 38.8 Acm-2 K-2 which is very close to the theatrical value of Richardson constant. The temperature dependence of barrier height is interpreted on the basis of existence of the Gaussian distribution of the barrier heights due to the barrier height inhomogeneities at the Au/GaN interface. Chapter 5 addresses on the influence of GaN underlayer thickness on structural, electrical and optical properties of InN thin films grown using plasma-assisted molecular beam epitaxy. The high resolution X-ray diffraction study reveals superior crystalline quality for the InN film grown on thicker GaN film. The electronic and optical properties seem to be greatly influenced by the structural quality of the films, as can be evidenced from Hall measurement and optical absorption spectroscopy. Also, we present the studies involving the dependence of structural, electrical and optical properties of InN films, grown on thicker GaN films, on growth temperature. The optical absorption edge of InN film is found to be strongly dependent on carrier concentration. Kane’s k.p model is used to describe the dependence of optical absorption edge on carrier concentration by considering the non-parabolic dispersion relation for carrier in the conduction band. Chapter 6 deals with the analysis of the temperature dependent current transport mechanisms in InN/GaN heterostructure based Schottky junctions. The barrier height (φb) and the ideality factor (η) of the InN/GaN Schottky junctions are found to be temperature dependent. The temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. The higher value of the ideality factor and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission (TFE) other than thermionic emission (TE). The room temperature barrier height and the ideality factor obtained by TFE model are 1.43 eV and 1.21, respectively. Chapter 7 focuses on the defect induced room temperature ferromagnetism in Ga deficient GaN epitaxial films and InN nano-structures grown on c-sapphire substrate by using plasma-assisted molecular beam epitaxy. The observed yellow emission peak in room temperature photoluminescence spectra and the peak positioning at 300 cm-1 in Raman spectra confirms the existence of Ga vacancies in GaN films. The ferromagnetism in Ga deficient GaN films is believed to originate from the polarization of the unpaired 2p electrons of nitrogen surrounding the Ga vacancy. The InN nano-structures of different size are grown on sapphire substrate, the structural and magnetic properties are studied. The room temperature magnetization measurement of InN nano-structures exhibits the ferromagnetic behavior. The saturation magnetization is found to be strongly dependent on the size of the nano-structures. Finally, Chapter 8 gives the summary of the present work and the scope for future work in this area of research.

Group III-nitride semiconductors have received much research attention and witnessed a significant development due to their ample applications in solid-state lighting and high-power/high-frequency electronics. Numerous growth methods were explored to achieve device quality epitaxial III-nitride semiconductors. Among the growth methods for III-nitride semiconductors, molecular beam epitaxy provides advantages such as formation of abrupt interfaces and in-situ monitoring of growth. The present research work focuses on the growth and characterizations of III-nitride based epitaxial films, nanostructures and heterostructures on c-sapphire substrate using plasma-assisted molecular beam epitaxy system. The correlation between structural, optical and electrical properties of III-nitride semiconductors would be extremely useful. The interfaces of the metal/semiconductor and semiconductor heterostructures are very important in the performance of semiconductor devices. In this regard, the electrical transport studies of metal/semiconductor and semiconductor heterostructures have been carried out. Besides, studies involved with the defect induced room temperature ferromagnetism of GaN films and InN nano-structures have also been carried out. The thesis is organized in eight different chapters and a brief overview of each chapter is given below. Chapter 1 provides a brief introduction on physical properties of group III-nitride semiconductors. It also describes the importance of III-nitride heterostructures in the operation of optoelectronic devices. In addition, it also includes the current strategy of the emergence of room temperature ferromagnetism in III-nitride semiconductors. Chapter 2 deals with the basic working principles of molecular beam epitaxy system and different characterization tools employed in the present work. Chapter 3 describes the growth of GaN films on c-sapphire by plasma-assisted molecular beam epitaxy. The effects of N/Ga flux ratio on structural, morphological and optical properties have been studied. The flux ratio plays a major role in controlling crystal quality, morphology and emission properties of GaN films. The dislocation density is found to increase with increase in N/Ga flux ratio. The surface morphologies of the films as seen by scanning electron microscopy show pits on the surface and found that the pit density on the surface increases with flux ratio. The room temperature photoluminescence study reveals the shift in band-edge emission towards the lower energy with increase in N/Ga flux ratio. This is believed to arise from the reduction in compressive stress in the GaN films as it is evidenced by room temperature Raman study. The transport studies on the Pt/GaN Schottky diodes showed a significant increase in leakage current with an increase in N/Ga ratio and is found to be caused by the increase in dislocation density in the GaN films. Chapter 4 deals with the fabrication and characterization of Au/GaN Schottky diodes. The temperature dependent current–voltage measurements have been used to determine the current transport mechanism in Schottky diodes. The barrier height (φb) and the ideality factor (η) are estimated from the thermionic emission model and are found to be temperature dependent in nature, indicating the existence of barrier height inhomogeneities at the Au/GaN interface. The conventional Richardson plot of ln(Is/T2) versus 1/kT gives Richardson constant value of 3.23×10-5 Acm-2 K-2, which is much lower than the known value of 26.4 Acm-2 K-2 for GaN. Such discrepancy of Richardson constant value was attributed to the existence of barrier height inhomogeneities at the Au/GaN interface. The modified Richardson plot of ln(Is/T2)-q2σs2/2k2T2 versus q/kT, by assuming a Gaussian distribution of barrier heights at the Au/GaN interface, provides the Schottky barrier height of 1.47 eV and Richardson constant value of 38.8 Acm-2 K-2 which is very close to the theatrical value of Richardson constant. The temperature dependence of barrier height is interpreted on the basis of existence of the Gaussian distribution of the barrier heights due to the barrier height inhomogeneities at the Au/GaN interface. Chapter 5 addresses on the influence of GaN underlayer thickness on structural, electrical and optical properties of InN thin films grown using plasma-assisted molecular beam epitaxy. The high resolution X-ray diffraction study reveals superior crystalline quality for the InN film grown on thicker GaN film. The electronic and optical properties seem to be greatly influenced by the structural quality of the films, as can be evidenced from Hall measurement and optical absorption spectroscopy. Also, we present the studies involving the dependence of structural, electrical and optical properties of InN films, grown on thicker GaN films, on growth temperature. The optical absorption edge of InN film is found to be strongly dependent on carrier concentration. Kane’s k.p model is used to describe the dependence of optical absorption edge on carrier concentration by considering the non-parabolic dispersion relation for carrier in the conduction band. Chapter 6 deals with the analysis of the temperature dependent current transport mechanisms in InN/GaN heterostructure based Schottky junctions. The barrier height (φb) and the ideality factor (η) of the InN/GaN Schottky junctions are found to be temperature dependent. The temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. The higher value of the ideality factor and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission (TFE) other than thermionic emission (TE). The room temperature barrier height and the ideality factor obtained by TFE model are 1.43 eV and 1.21, respectively. Chapter 7 focuses on the defect induced room temperature ferromagnetism in Ga deficient GaN epitaxial films and InN nano-structures grown on c-sapphire substrate by using plasma-assisted molecular beam epitaxy. The observed yellow emission peak in room temperature photoluminescence spectra and the peak positioning at 300 cm-1 in Raman spectra confirms the existence of Ga vacancies in GaN films. The ferromagnetism in Ga deficient GaN films is believed to originate from the polarization of the unpaired 2p electrons of nitrogen surrounding the Ga vacancy. The InN nano-structures of different size are grown on sapphire substrate, the structural and magnetic properties are studied. The room temperature magnetization measurement of InN nano-structures exhibits the ferromagnetic behavior. The saturation magnetization is found to be strongly dependent on the size of the nano-structures. Finally, Chapter 8 gives the summary of the present work and the scope for future work in this area of research.