Rozprawy doktorskie na temat „InGaN/Si”

Depuis une vingtaine d’année les nanofils des semiconducteurs suscitent un intérêt majeur pour des applications diverses grâce à leurs propriétés optoélectroniques particulières. Dans le domaine du photovoltaïque ils présentent aussi un atout majeur. La combinaison du fort coefficient d’absorption des semiconducteurs III-V et le faible coût des substrats de silicium permettraient la réalisation des cellules photovoltaïques à faible coût et à haut rendement. C’est dans ce contexte que s’est déroulé cette thèse qui visait le développement des dispositifs à base des nanofils III-V pour le photovoltaïque. Dans une première partie, les techniques de nanofabrication pour la réalisation des dispositifs à base d’ensemble de nanofils pour les cellules photovoltaïques sont présentées. Ensuite, la fabrication et la caractérisation de dispositifs à base d’ensembles de nanofils de GaN pour les applications photovoltaïque sont permis d’ouvrir la voie au développement des cellules solaires tandems d’InGaN⁄Si. Dans la suite des travaux on a étudié la croissance des nanofils de GaAs du type cœur-coquille sur Si ainsi que les étapes technologiques pour la fabrication des dispositifs à base d’ensemble de nanofils dans l’optique de préparer le terrain pour la réalisation d’une cellule tandem III-V sur Si. Enfin la croissance et la caractérisation électro-optique des nanofils contenant des jonctions axiales de GaAsP crus par la méthode VLS-EJM a permis de déterminer le type de dopage et l’optimisation de la structure en vue d’obtenir un effet photovoltaïque
Over the past twenty years, semiconductor nanowires have attracted major interest for various applications thanks to their particular optoelectronic properties. The combination of the high absorption coefficient of the III-V semiconductors and the low cost of the silicon substrates would allow the realization of photovoltaic cells at low cost and high efficiency. It is in this context that this thesis was developed which focused on the development of devices based on III-V nanowires for photovoltaics. In a first part, the nanofabrication techniques for the realization of devices based on set of nanowires for photovoltaic cells are presented. Next, the fabrication and characterization of devices based on GaN nanowire arrays for photovoltaic applications is paving the way for the development of InGaN / Si tandem solar cells. In the following, we studied the growth of core-shell GaAs nanowires on Si as well as the technological steps for the fabrication of nanowire-based devices in order to prepare the ground for the realization of a tandem III-V cell on Si. Finally, the growth and electro-optical characterization of the nanowires containing axial junctions of raw GaAsP by the VLS-EJM method made it possible to determine the type of doping and the optimization of the structure in order to obtain a photovoltaic effect

碩士
國立中興大學
材料科學與工程學系所
106
In this study, InGaN-based light-emitting diodes were separated from silicon substrates as a light-emitting membrane by electrochemical wet etching technique. High lateral wet etching rate on the sacrificial layer was achieved to the lift-off process. In the FE-SEM image and optical measurement, the thickness of the separated membrane which we calculated is about 5.4 micrometer. It matched the thickness which we grew by MOCVD. In the Raman spectra, the Raman peak of the ST-LED was measured at 568.9 cm-1 and the LEM was observed at 568.3 cm-1. The peak shifted about 0.6 cm-1. It indicated that the compressive strain was released from the silicon substrate. In the photoluminescence spectra, the PL intensity of the LEM stronger than the ST-LED . In the far field radiation pattern, the divergence angle of ST-LED is 105 degree, and the LEM is 142 degree, furthermore, the LEM (bend down) is 116 degree. So, the divergence angle of LEM can be tuned by changing the curvature under bending condition. In the electroluminescence spectrum, the peak wavelength of the ST-LED and LEM (flat) (bend down) (bend up) are almost the same. And the EL intensity of LEM is stronger than ST-LED. This result was consistent with the analysis of photoluminescence spectra. Using the methods of this paper to produce InGaN Light Emitting Membranes which can separate devices from the substrate rapidly and the Si substrate can be re-used. In this paper, the InGaN LEMs were separated from Si substrate by electrochemical etching system. It was found that the luminescence wavelength of the LEM was not affected by the bending test but the divergence angle of LEM could be changed by the bending test. Furthermore, after the separation process, the Si substrate can be re-used because of the good surface roughness. In the future, this process technology has considerable potential for the separation of the InGaN-based LEDs from the substrate.

碩士
國立成功大學
物理學系
104
In this study, InGaN nanorods were grown on pyramided Si substrate by plasma-assisted molecular beam epitaxy system (PA-MBE). We have grown white-light emitting InGaN nanorods on pyramid Si substrate with single flux ratio, thus causing different In and Ga contents and different emission colors on each face of pyramid Si substrate. From SEM images, the different morphologies of the nanorods are revealed on each face of pyramid Si substrate. When In flux and Ga flux impinges vertically on pyramid, the InGaN nanorods show large rod diameter. However, when In flux and Ga flux impinges with grazing incidence on pyramid result in nanorods with small diameter. The length of the nanorods is about 1 μm. In addition, the direction of N flux enables to control the growth direction of InGaN nanorods on pyramid substrate, which we obviously found nanorods to tilt toward the top of pyramid. PL spectrum measurement results show that the white light emission has been achieved successfully by InGaN nanorods, and the spectrum exhibits a continuous emission range. We have confirmed that each face of pyramid substrate has different composition and different light emission by using spatial resolved catholuminescence (CL) and electron energy loss spectroscopy (EDS) measurements. Finally, we apply the mechanical force on InGaN nanorods, and PL spectrum shows the emission peaks with blue shift of 7 nm due to the photo-piezoelectric effect of III-nitride semiconductors.

碩士
國立中央大學
光電科學與工程學系
102
this paper proposes the use of ZnO nanorods structure to overcome the mismatch of thermal expansion and the atomic lattice between Si and GaN. We have successfully grown diameter 150nm,and 400nm length of zinc oxide nano-rods array that can achieve GaN on Si , ZnO not only exhibits the lattice constant and thermal expansion coefficient similar to GaN, the oxide alloy can also be easily etched in chemical solutions, which greatly saves the subsequent processing cost. In order to increase internal quantum efficiency (IQE) of the emitter grown on Si, we grew semi-polar nano-pyramidal InGaN/GaN multiple quantum wells with uniquely developed conditions. Further, it is found that the GaN grown on ZnO/Si exhibits p-type behaviors, which is due to the diffusion of Zn into GaN. If confirmed, IQE of the semi-polar quantum wells can be further enhanced through a p-side-down structure, our simulation results show P-side down structure reduces Spillover current ratio of 0.1 or less, and can increase IQE to 0.7 to enhance light efficiency.

The present research work focuses on the growth and characterization of group III-Nitride (InGaN) epitaxial layers as well as nanostructures on Si(111) substrates. The growth system used in this study was a plasma-assisted molecular beam epitaxy (PAMBE) system equipped with a radio frequency (RF) plasma source. Device quality GaN epilayers were obtained and InGaN/Si(111) heterojunctions were studied. In- GaN based multi-quantum well LED structure has been realized for green emission. Further catalyst free ultra fine GaN nanorods were grown using a two step method and further InGaN nanostructures were embedded in the as-grown nanorods. InGaN quantum dots were grown using droplet epitaxy and were characterized by Scanning Tunnelling Microscopy and Spectroscopy. It gives a brief introduction about III-nitride materials, growth, substrate selection, significance of III-Nitrides and Si integration and role of dimensionality. It deals with experimental techniques including the details of PAMBE system used in this work, substrate preparation, and detailed characterization of III-nitride epitaxial layers as well as nanostructures. It deals with the optimization of GaN epilayers on AlN/Si (111) templates. AlN underlayer was chosen to minimize the concentration of defects and also acts as an insulating layer which is crucial when it comes to integration of many other devices. The growth temperature was optimized under nitrogen rich growth regime and with the use of a thinner and better quality AlN underlayer and Si doping we could achieve device quality epilayers ( 1500 arc sec) for a thickness of 150 nm. The electron concentration and mobility were found to be -1.374 _1019cm􀀀3 (indicating n-type) and 72 cm2/V.s. Current-voltage measurements were carried out in temperature range of 77K-400K and the current conduction mechanisms at room temperature were identified. An in-depth analysis of temperature dependent current-voltage measurements reveal that the barrier height at the interface is not uniform and is found to have a double Gaussian distribution of barrier heights. It deals with the growth of InGaN epilayers on Si (111) with various substrate treatments. Actual indium composition was determined considering the bi-axial strain present in the epilayers. The effect of substrate treatment on epilayers evolution and quality are discussed. We could observe room temperature photoluminescence from the as-grown epilayers indicating that the epilayers are of good optical quality. InGaN/Si heterojunctions were studied for UV-detection applications. It was found that the heterojunction behaved as a self-powered device, i.e., the device showed a sharp rise in the photocurrent under UV illumination at zero bias. The rise and decay times were found to be 20ms and 33 ms respectively. The bandgap of grown InGaN epilayers were tuned for emission in Green wavelength range. (500nm-550nm) It discusses the sequential process involved in the unition of individual layers to successfully achieve a multi- quantum well structure. In the previous chapter, InGaN epilayers with emission in the green (500nm) region were obtained and having identified the growth conditions for green emission, InGaN epilayers were further grown on GaN/Si (111) and we could tune the bandgap to obtain the emission in blue region. The effect of InGaN growth on thickness was studied which finally led us to develop a growth sequence for successfully obtaining a multi quantum well structure. It deals the growth, structural and optical characterization of InGaN nanostructures embedded in GaN nanorods. The first part deals with the spontaneous growth of very fine (20nm diameter) GaN nanorods on Si (111). Low temperature photoluminescence spectroscopy (LTPL) was used to determine the optical properties of the GaN nanorods. The second part discusses the growth conditions for embedding InGaN in the earlier formed GaN nanorods. The effect of substrate temperature on the evolution of InGaN structures is assessed. Scanning Transmission Electron Microscopy along with Energy Dispersive Spectroscopy (STEM/EDS) is used to determine the elemental distributions in the as-grown nanostructures. LTPL was carried out to determine the emission characteristics of the InGaN/GaN nanostructures. We could successfully obtain room temperature emission in blue region from the core-shell nanorods which happens to be rare achievement. It deals with the growth of high indium content InGaN QDs by droplet epitaxy has been attempted for the first time. The experimental conditions behind InGa droplet formation have been discussed. The influence of droplet formation temperature on the transition from nanoscale structures to quantum dots has been discussed. Room temperature scanning tunnelling microscopy and spectroscopy measurements were carried out. It was found that the QDs exhibited compositional variations even at nanoscale from STM/STS studies. It gives the summary and conclusions of the present study and also discusses about future research directions in this area.

碩士
國立中央大學
電機工程研究所
99
This dissertation describes an innovative method for selective epitaxial growth of semi-polar (1-101) GaN on V-grooved Si substrates. In addition to the SiO2 mask along the V grooves, SiO2 stripes perpendicular to the V grooves are introduced to overcome the issue of cracking caused by the large mismatch in the thermal expansion coefficients between GaN and Si. The structural and optical properties of the GaN films thus grown, particularly the reduction in dislocation density and the enhancement of their luminescence properties by the selective area epitaxial process, are investigated and elucidated. The growth of semi-polar (1-101) GaN films as thick as 1µm without cracks and InGaN/GaN multiple quantum wells (MQWs) on the resultant GaN have been successfully achieved on V-grooved (001) Si substrate in a dimension of 1x1 cm2. The transmission electron microscopy (TEM) measurements reveal that the dislocations bend toward the [1-100] and [11-20] directions as a consequence of the (1-101) and (11-22) facets that form during the initial stage of the lateral overgrowth upon the SiO2 stripes. This reduction in the dislocation density leads to an increase in the luminescence intensity as observed by photoluminescence (PL) and cathodoluminescence (CL) measurements at room temperature. Finally, the crack-free film was successfully fabricated to devices and showed low leakage current under the bias of -12 V while the turn-on voltage is about 4 V due to the inefficient ionization of Mg in p-GaN layer. Judging from the optical and electrical properties observed, the selective growth method holds promise for high quality free-standing semi-polar GaN substrates and III-nitride semiconductor devices once we further improve Magnesium (Mg) ionization in GaN layer. Secondly, the performance of semi-polar (1-101) light emitting diodes (LEDs) are simulated with variations in the thickness of the MQWs, the barrier doping concentration, and the Aluminum (Al) composition of the AlGaN Electron Blocking Layer (EBL). The highest efficiency is obtained when the thickness of the (1-101) semi-polar quantum wells (QWs) and quantum barriers (QBs) are 3 nm and 9 nm, respectively. It is also found that inserting an AlGaN EBL does not help much for blocking electrons possibly due to the inherently weak polarization, which suppresses the escape of the electrons out of the QW active region. Finally, an opposite IQE tendency was found when different doping concentrations in QBs were applied in (1-101) and (0001) LED structures. The lower polarization field in the semi-polar structure enables the QWs to accommodate more electrons and facilitates radiative recombination before spilling over to p-GaN region, thus light efficiency rises with increasing doping concentration to a specific degree; whereas the polarized (0001) LEDs exhibit an initial monotonic drop in efficiency due to the easy overflow of electrons from the MQWs. The higher the doping concentration in the barriers, the higher the spillover current is, suggesting that the Quantum Confined Stark Effect (QCSE) caused by the strong polarization field is the major factor in the observed efficiency degradation with barrier doping.

碩士
國立中央大學
光電科學與工程學系
107
In this study, the effect of lattice strain before and after substrate transfer on the optoelectrionic properties of nanostructured InGaN quantum wells (QWs) structures was investigated. The nanostructure QWs were grown on (100) Si substrates by metal-organic chemical vapor deposition (MOCVD), employing ZnO nanorods as the buffer layer to release the huge stratin between Si and the nitride epilayer. Using the small lattice mismatch between ZnO and GaN, we successfully grew the (10-11) semi-polar nanopyramidal QWs on the (100) Si substrate. The diffusion of Zn into GaN during the epitaxial growth also allows us to achieve the naturally formed p-type GaN, producing the desired p-side-down structure for QWs with enhanced interal quantum efficiency. During the growth, three different flow rates of Bis(cyclopentadienyl)-magnesium Mg(C5H5)2, i.e. 40, 80, and 120 sccm were adopted with the attempt to study the effect of p-type doping on the strain and the quantum efficiency of the QWs. Due to the large thermal mismatch between the GaN epilayer and the Si substrate, huge lattice strain is expected in the epilayer after the MOCVD growth. The strain decreased the internal quantum efficiency of the InGaN QWs via the quantum-confinement Stark effect. The semipolar nanostructured QWs produced in this study are expected to exhibit improved radiative recombination efficiency becoause of the alleived QCSE. In addition, we transferred the epitaxial layer from the Si substrate to a silver substrate using a wet-etching technique, releasing the stress on the samples and increasing reflectivity at the epilayer/substrate interface. The released stress and enhanced interface reflectivity should lead to improved external quantum efficiency of the nanopyramidal QWs. Scanning electron microscopy was used to observe the microstructure of the samples and a simulation software is used to analyze the relationship between the film thickness and the reflection wavelength. The samples were also characterized by x-ray diffraction (XRD) and Raman spectroscopy. According to these characterizations, it is found that the sample with less magnesium doping exhibits less tensile stress, and thus the higher internal quantum efficiency.

碩士
國立臺灣大學
光電工程學研究所
100
The difference of thermal expansion coefficient between GaN and Si results in a strong tensile stress on the GaN epitaxial layer during the cooling process. In our study, we create a compressive thermal stress by using an AlN buffer layer grown with graded temperatures to compensate the tensile thermal stress. The InGaN/Gan quantum well (QW) sample with the largest number of graded-temperature growth stages has the weakest residual tensile stress, shortest emission wavelength, and highest emission internal quantum efficiency. In this study, the variation trend of indium composition of the QW samples grown on Si and the control sample grown on sapphire based on strain state analysis is shown to be the same as that based on the X-ray diffraction measurement and fitting. However, the variation range becomes larger. Also, from the cross-sectional transmission electron microscopy study, it is found that the threading dislocation (TD) density decreases with decreasing residual stress. The TD density above the GaN/AlN superlattice inter-layer is lower than that below the inter-layer, indicating that the inter-layer can block the TDs.

碩士
國立高雄大學
應用物理學系碩士班
99
InxGa1-xN alloys feature a bandgap ranging from 0.7eV to 3.4eV, covering almost the entire solar spectrum. To optimize the efficiency and the best parameters of solar cells, numerical simulations of InGaN single junction and InGaN/Si double junction solar cells are conducted. The simulation modelling is important and indispensable for designing and fabricating InGaN single junction and InGaN/Si tandem solar cells. We changed the In composition and the thickness of the n- and p-InGaN to determine the short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), conversion efficiency (η), and power maximum (Pmax). First, for InGaN single junction solar cell, the Jsc, Voc, and FF have a strong dependence on the In composition. In composition is a critical parameter to determine Jsc, Voc, FF, and η of InGaN solar cells. In0.6Ga0.4N solar cell shows the maximum η ~ 22%. The band gap of In0.6Ga0.4N is 1.42 eV and is almost the same with GaAs. When the total layer thickness is greater than 500 nm, the absorption becomes saturated and the η increases smoothly. The simulation results are congruent with this trend. Second, the p- and n-junction thickness and In composition of InGaN junction are the key point to determine the characteristics of InGaN/Si double junction solar cell. The current matching should be considered in the InGaN/Si double junction solar cells. The smaller Jsc in each junction determines the total Jsc of InGaN/Si double junction solar cell. The total Voc is the sum of the Voc in each junction of InGaN/Si double junction solar cell. Because the current matching affects the Jsc, the curves of the FF have some turning points. The η increases with increasing In content and with dramatically drops with a turning point. With 100 nm p-type InGaN junction, the In0.6Ga0.4N/Si p-n double junction solar cell has the maximum η ~37%. The enhancement of the optimal η of In0.6Ga0.4N/Si p-n double junction solar cell is ~68% higher than that of In0.6Ga0.4N single junction solar cell. The total thickness of InGaN junction must be less than 500 nm, or the most light is absorbed in the InGaN junction and Si junction can not work. The simulation results could provide the clues for optimizing the device structures and process conditions of InGaN single junction and InGaN/Si tandem solar cells.

碩士
國立臺灣大學
電機工程學研究所
89
The rapidly development of GaN-based optoelectronic devices has yielded commercially available blue light-emitting diodes (LEDs) and more recently blue laser diodes (LDs). Due to the lack of a bulk large-area GaN substrate, GaN thin films are deposited on available dissimilar substrates such as sapphire or SiC. The most commonly used growth substrate, sapphire, imposes constraints on GaN film quality due to the lattice and thermal expansion coefficient mismatch between the sapphire and GaN. The disparate properties of these two materials introduce a high density of extended defects, such as dislocations and stacking faults, at the GaN thin film/substrate interface. Furthermore, the sapphire substrate inhibits LED, LD, and transistor device performance due to its poor thermal and electrical conductivity. The inherent sapphire constrains have spurred interest in integrating GaN-based optoelectronics with Si substrates by direct growth of GaN on Si. The Si substrate offers better thermal and electrical conductivity compared to sapphire at a substantially lower cost. The integration of GaN-based optoelectronics with Si-based integrated circuit technology by direct deposition has also allowed the development of color LED displays, although their performance is still typically poorer compared to those LEDs fabricated on sapphire. A thin—film lift-off technique, in conjunction with low temperature wafer bonding, may be used as a direct approach for eliminating the sapphire growth substrate and for integrating GaN with more thermally and electrically conductive substrate materials. Ideally, the bonding and lift-off process should not damage the final substrate after transfer, the microstructure and properties of the GaN thin film should be preserved or improved by process. We access the characteristics of the good electrical and thermal conductivity of Si. By using wafer bonding and thin film lift-off techniques, transferring GaN epilayer onto Si substrate. GaN and Si are combined together by wafer bonding and laser lift-off techniques. We have been transferred InGaN LED epilayer onto Si substrate, and subsequently made a blue InGaN LED on the bonded InGaN/Si wafer. The properties of InGaN LED made on the bonded wafer are not much different from that of the LED made on the as-grown InGaN/sapphire wafer.

碩士
國立中央大學
電機工程研究所
94
Structural and optical characterization of GaN grown on V-grooved (100) Si by metal organic vapor phase epitaxy is carried out. The resultant GaN, which has a semi-polar (10-11) surface plane, can be grown with thickness over 1 �慆 without any cracks in spite of the large tensile stress induced by the difference of thermal expansion coefficient between GaN and Si. Transmission electron microscopy (TEM) selected area diffraction of GaN indicates that partial strain is present around the GaN/Si interface region and strain is less at the surface region of GaN. Moreover, we propose a dislocation propagation model to explain the distribution of dislocations in the GaN epilayer. We have also investigated the optical properties of GaN/InGaN quantum wells grown on both the semi-polar (10-11) GaN and (0001) GaN. Power-dependent photoluminescence measurements show that the piezoelectric field in the quantum wells grown on the semi-polar plane is lower than its counterpart on the (0001) plan, but not significant.

碩士
國立中山大學
光電工程學系研究所
98
A high efficient packaging technique was proposed for power InGaN light emitting diodes( LEDs ).In this approach , sub-mounts based on Si bench technology were used to provide a fact heat conducting channel between the LEDs and the cases.Two different structures of the Si sub-mounts were used, namely, a conventional Si block and a Si block with a copper-filled V-groove. 　　The thermal resistance of the two different sub-mounts were measured and compared. For a 45mil power LED biased at 1W, thermal resistance of 12.77℃/W and 18.79℃/W were measured for the Si sub-mount and the Si sub-mount with copper-filled V-groove. We believe the better thermal resistance of the sub-mount with copper-filled V-groove is due to high thermal conductivity of the copper.

碩士
國立交通大學
加速器光源科技與應用碩士學位學程
99
In this study, we investigated the structural, optical and electrical characteristics of InGaN-based light emitting diodes (LED) grown on micro- and nano-patterned Si (111) substrates (MPLED and NPLED). The result of X-ray diffraction revealed that the GaN layer grown on the nano-patterned Si (NPSi) substrate had higher degree of mosaicity, indicating less strain in this sample. Moreover, the density of threading dislocation densities in the GaN film grown on NPSi was reduced as indicated by Williamson-Hall analysis results, that is also consistent with the transmission electron microscopy (TEM) results. The results elucidate that nanoscale epitaxial lateral overgrowth (NELOG) effectively reduces the density of threading dislocations in GaN film grown on NPSi. Power dependent photoluminescence (PL) measurements show that NPLED has a smaller peak blue shift with increasing pumping power as compared with that of the MPLED, revealing a reduced quantum confined Stark effect (QCSE). This phenomenon is attributed to the reduced strain in NPLED. Furthermore, NPLED shows better carrier confinement than MPLED as revealed by temperature dependent PL measurements. In terms of device performance, NPLED exhibits smaller electroluminescence (EL) peak wavelength blue shift, lower reverse leakage current and lower efficiency droop, compared with the MPLED. These results suggest that using NPSi as the substrate for InGaN based LED growth can effectively reduce the tensile strain and density of threading dislocations of the GaN layer and improve the optical and electrical performance of the LED.

碩士
國立臺南大學
材料科學系碩士班
100
We use micro-Raman spectra, scanning electron microscope, atomic force microscope, and photoluminescence spectra to study the properties of III-V nitride semiconductor materials grown on (111) Si substrates with different growth conditions of AlN buffer layers. The studies are divided into two parts. The first part is the growth of multiple AlN buffer layers with the decrease of growth temperature from 1000 to 700 oC. For the increase of the numbers of AlN buffer layer, it shows the blue shift of near band edge light emission energy and intensity, high energy shift of E2high and A1(LO) scattering modes of GaN from micro-Raman spectra, pronounced decrease of cracks density in scanning electron microscope images, and the reduced surface roughness in atomic force microscope images. These results indicate that such growth conditions of AlN buffer layers can help in decreasing tensile stress in GaN on (111) Si substrates. The second part of the researches follow the first part. We also prepared the samples with AlN buffer layers on (111) Si substrates to improve the quality of GaN thin film. Furthermore, the InGaN/GaN multiple quantum wells were deposited on high quality GaN thin films. The near-band edge emission energies and intensities of samples show blue shift and increase respectively with the increase of the numbers of AlN buffer layer. This is due to the reduction of piezoelectric fields as well as the quantum confined Stark effect in InGaN/GaN multiple quantum wells. Also, the shift of E2high and A1(LO) modes of GaN from micro-Raman spectra were observed. The tensile stress in GaN on (111) Si substrates was decreased effectively .

Group III-nitride semiconductors have enabled revolution in solid-state lighting and high-power/high-frequency electronics. Now-a-days, III-nitride based photodetectors are of great importance because of their various applications from everyday consumer electronics such as compact disc players, smoke detectors, remote control etc. to more elegant applications such as environmental monitoring, space research and in optical communications. Materials such as AlN and AlGaN have been used as solar blind photodetectors, whereas AlGaN and GaN based devices have been extensively used as UV photodetectors that depend on the concentration of Ga. On the other hand, InGaN and InN based devices are well-established for broad band and infrared photodetection applications, respectively. The key point of a broad band photodetector is that it occupies multiple wavelength region and therefore, allows much higher throughput over a single medium. Furthermore, along with broad band detection, the infrared detection in the optical telecommunication range (1550 nm) is also a demanding research area in the scientific community. Most of the photodetectors require an applied bias for appreciable detectivity, which needs a constant electrical power source. However, a self-powered photodetector can operate at zero bias without any external power source. The self-powered photodetectors such as p-n junction, heterojunction, Schottky junctions and organic/inorganic hybrid junctions can immediately separate the electron-hole pairs due to the built-in electric field, exhibiting faster photo response and higher responsivity at zero bias. Therefore, InN and InGaN based self-powered photodetectors in the present work will enthuse the scientific community considering the recent energy crisis. In our work, we have optimised InN epilayer on AlN/Si (111) template and achieved the self-powered infrared photoresponse with appreciable responsivity. Furthermore, InGaN epilayers were grown on AlN template to realize the UV and visible photodetection. We had grown three InGaN epilayers on AlN template with different Indium concentration. The point defects dominate on lower indium content sample due to hydrostatic strain and trench and sub-interfacial extended defects dominate on higher indium content sample due to in-plane biaxial strain. Moreover, the localised states dominate on higher indium content sample over electron-phonon interaction and therefore, we have chosen the InGaN sample with low In content for UV-Visible photodetection. The InGaN/AlN/n-Si (111) devices exhibit excellent self-powered photoresponse under UV-Visible (300-800 nm) light illumination. Furthermore, to cover the broad band range and as well as the infrared optical fibre communication range (1550 nm) with the help of binary (InN) and ternary (InGaN) compounds we have combined the InN binary layer with InGaN ternary layer in the form of nanorods and epilayer junction. The InN nanorods and InGaN epilayers were grown on AlN/n-Si (111) template by using plasma-assisted molecular beam epitaxy. The device structure displays outstanding self-powered photodetection from UV, visible to infrared (300 – 800 nm and 1550 nm) wavelength range. This work is thus believed to make a great impact in nanoelectronics, optoelectronics and in optical fibre communication due to its superior device structure.

This thesis focuses on studying heterostructures of GaN, Silicon and AlN. GaN nanostructures are grown on bare Si (111) with and without a GaN buffer layer and GaN film was grown on an AlN layer. Apart from the material characterization of the grown samples we have studied the carrier transport across GaN/Si and AlN/Si heterojunctions by means of the I-V-T curves from these junctions and we also studied the band alignment across GaN/AlN and AlN/Si heterojunctions by means of X-ray photoelectron spectroscopy. The thesis is divided in 7 chapters. The first chapter deals with general introduction of the field, choice of the substrate, different growth techniques and an overview of nanostructures. In the second chapter different experimental techniques used in the current study are briefly mentioned. These techniques include Growth by Plasma Assisted Molecular Beam Epitaxy (PAMBE), X-Ray Diffraction (XRD), Raman spectroscopy, Photoluminescence spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Then in the 3rd chapter the growth and characterization of GaN nanostructures on Silicon (111) is discussed. Attention has been paid to the effect of substrate temperature and V-III ratio on the morphology and optical quality of the grown structures when other growth parameters has been kept constant. However due to the complexity involved in forming proper electrical contacts from such rods and low yield of single nanowire-based devices, the need to grow compact nanorods was felt. Hence in the fourth chapter compact GaN nanorods were grown on n-Si with a buffer layer for improved quality and elimination of any possibility of electrical short with the substrate during metallization. The main focus in this chapter is the electrical characterization of GaN nanorods/Si (111) heterojunction. The temperature dependent current-voltage characteristics from GaN/n-Si (111) junctions are analyzed and explained as the result of a lateral inhomogeneity in barrier heights with Gaussian distribution and temperature dependent Gaussian parameters. The importance of AlN as a buffer layer for many III-Nitride based devices and as an active layer in many electromechanical devices drew our attention towards band off-set studies of the GaN/AlN/Si heterojunction and electrical transport across the AlN/n-Si junction, in 5th and 6th chapter respectively. The 5th chapter starts with the structural and optical characterization of AlN/Si (111) templates and overgrown GaN thin film. The rest of the 5th chapter is dedicated to the band off sets studies on GaN/AlN and AlN/Si (111) heterojunctions with X-ray photoelectron spectroscopy (XPS). A band diagram of GaN/AlN and AlN/Si is suggested based on our studies. In the 6th chapter, which happens to be the last work chapter, the temperature dependent electrical characterization of AlN/n-Si (111) heterojunction was carried out from 100K to 400K and the transport mechanism was explained with the help of the trap states at the interface. Finally, the thesis is concluded and insights for future work is presented in the seventh chapter.

碩士
國立臺南大學
材料科學系碩士班
107
There are two parts of investigations in this thesis. The first part is the study of plasmonic resonance of characteristics of GaxZn1-xO thin films grown on sapphire substrate with molecular beam epitaxy (MBE). Three series of samples are prepared for the variations of Ga and Zn effusion cell temperature and substrate temperature. The results exhibit that electron concentration of GaxZn1-xO thin films can reach 1020~1021 cm-3. Preferential orientation along (002) of X-ray diffraction (XRD) pattern is demonstrated in GaxZn1-xO thin films. For the GaxZn1-xO thin film with the x content of Ga 6.26 % and substrate temperature 250 °C, it shows the strongest peak intensity of (002), high electron concentration 8.18×1020 cm-3, high electron mobility 30.1 cm2/Vs, low electron resistivity 2.54×10-4 Ω-cm, lower strain of the film, and better micro-photoluminescence (micro-PL) and crystalline structure. Besides Hall effect measurements, spectroscopic ellipsometry (SE) provides an nondestructive and contactless method to obtain electrical properties of semiconductor material with plasmonic behaviors. The second part of the researches is the influence of AlN buffer layers on GaN films and InGaN/GaN quantum wells grown on Si substrates. Graded decrease of growth temperature from 1000 to 700 oC in depositing multiple AlN buffer layers can effectively reduce the cracks of GaN and hence increase crystalline quality and PL intensity. Two-photon excitation microscopy could help in-depth analysis of formation of GaN thin films.

碩士
國立臺南大學
材料科學系碩士班
103
There are two parts in this research. The first part is the study of effects of Mn content in Cd1-xMnxTe grown by molecular beam epitaxy (MBE) on silicon (Si) (111) substrate. Owing to different binding energy of atoms, Te-Te binding shows the lower forming energy than Cd-Te, results in the production of Te-Te binding defects. Incorporation of Mn atoms into CdTe demonstrates that the lower binding energy of Mn-Te than Te-Te can cause the reduction of Te-Te binding defects in CdTe:Mn. In this report, the ternary alloy of Cd1-xMnxTe with Mn content in 0, 2, 21 % were prepared. The integrated intensity of photoluminescence (PL) spectra exhibits three times larger than other samples in sample with Mn content 21 %. The Stokes shift (SS) characterized by peak energy of PL spectra and bandgap energy of photoreflectance (PR) spectra show the greater value in Mn 21 % than others. The results indicate that the larger the Mn content in Cd1-xMnxTe, the more the composition fluctuations of Mn content in Cd1-xMnxTe showing larger SS. Scan electron microscope (SEM) images give the direct behavior of materialmicrostructures. The results indicates that Mn 21 % sample have relatively flat surface of the film. Micro-Raman spectra also exhibit the diminished intensity of scattering modes of A1 and E that are related to Te-Te binding defects. The consequence is that the better condition of Mn content is 21 % in Cd1-xMnxTe for superior optical and material properties. The second part of the research is the study of optical behaviors of blue light emitting diodes (LEDs) with multiple quantum wells (MQs) containing part of Si doping. The Si doping layers are the first two, three, four, and five barriers of QWs in the growth sequence from sapphire substrate of four samples. The results indicate that the reduction of piezoelectric field in QWs were occurred in all samples for the blue shift in PL peak energy compared with the undoped Si sample. First four Si-doped barrier samples show larger PL spectra intensity with greater carrier localization in QWs and smaller quantum confined Stark effect (QCSE). Soft confinement potential of QWs was observed in first four Si-doped barrier samples due to the existence of strong absorption intensity in the bandgap energy between quantum wells and barriers. The uniform spreading of carriers in QWs were expected in this sample. Blue LED with first four Si-doped barrier, thus having better output power and external quantum efficiency (EQE) under high current injection. Therefore, first four Si-doped barrier is the favorite condition for light emission of blue LED having 8 QWs.