Dissertations / Theses on the topic 'III-NITRIDE DEVICE'

Recently, group III-nitride nanowire heterostructures have been extensively investigated. Due to the effective lateral stress relaxation, such nanoscale heterostructures can be epitaxially grown on silicon or other foreign substrates and can exhibit drastically reduced dislocations and polarization fields, compared to their planar counterparts. This dissertation reports on the achievement of a new class of III-nitride nanoscale heterostructures, including InGaN/GaN dot-in-a-wires and nearly defect-free InN nanowires on a silicon platform. We have further developed a new generation of nanowire devices, including ultrahigh-efficiency full-color light emitting diodes (LEDs) and solar cells on a silicon platform.We have identified two major mechanisms, including poor hole transport and electron overflow, that severely limit the performance of GaN-based nanowire LEDs. With the incorporation of the special techniques of p-type modulation doping and AlGaN electron blocking layer in the dot-in-a-wire LED active region, we have demonstrated phosphor-free white-light LEDs that can exhibit, for the first time, internal quantum efficiency of > 50%, negligible efficiency droop up to ~ 2,000A/cm2, and extremely high stable emission characteristics at room temperature, which are ideally suited for future smart lighting and full-color display applications.We have also studied the epitaxial growth, fabrication and characterization of InN:Mg/i-InN/InN:Si nanowire axial structures on n-type Si(111) substrates and demonstrated the first InN nanowire solar cells. Under one-sun (AM 1.5G) illumination, the devices exhibit a short-circuit current density of ~ 14.4 mA/cm2, open circuit voltage of 0.14 V , fill factor of 34.0%, and energy conversion efficiency of 0.68%. This work opens up exciting possibilities for InGaN nanowire-based, full solar-spectrum third-generation solar cells.
Récemment, les hétérostructures à base de nitride et de groupe III ont fait l'objet de recherches intensives. Grâce à la relaxation latérale effective du stress, de telles hétérostructures d'échelle nanométrique peuvent être déposés sur du Silicium ou d'autres substrats. Celles-ci démontrent une réduction dramatique des dislocations et des champs de polarisations comparativement à leurs contreparties planes. Cette dissertation rapporte l'accomplissement d'une nouvelle classe de matériau nanométrique, soit des hétérostructures III-nitride incluant InGaN/GaN point dans fils ainsi que des nanofils d'InN presque sans défauts sur du Silicium. De plus, nous avons développé une nouvelle génération de dispositifs à base de nanofils, incluant des diodes émettrices de lumière (LEDs) à efficacité ultra haute et spectre visible complet ainsi que des cellules solaires sur une gaufre de Silicium. Nous avons identifié 2 mécanismes majeurs, incluant le faible transport des trous et le surplus d'électrons, qui limitent sérieusement la performance des LEDs à base de nanofils de GaN. Avec l'ajout de certaines techniques spéciales de modulation de type p, et une couche bloquante d'électrons faite de AlGaN dans la région active de la LED point dans fil. Par ailleurs, nous avons démontré des LEDs blanche sans phosphore qui démontrent, pour la première fois, une efficacité quantique supérieure à 50% ainsi qu'une baisse d'efficacité négligeable jusqu'à ~ 2,000A/cm2 et des caractéristiques d'émissions très hautes et stables à température pièce. Celles-ci sont donc toutes désignées pour des applications d'illumination intelligentes et des écrans pleines couleurs. La croissance par épitaxie, la fabrication et la caractérisation des nanofils d'InN:Mg/i-InN/InN:Si axiaux sur des substrats de Si(111) de type n et démontré la première cellule solaire à base d'InN. Sous l'illumination d'un soleil (AM 1.5G), les dispositifs démontrent une densité de courant de ~ 14.4 mA/cm2 en court-circuit, un voltage de circuit ouvert de 0.14V, un facteur de remplissage de 34.0% et une efficacité de conversion d'énergie de 0.68%. Ce travail ouvre des portes excitantes pour des cellules solaires plein spectre de troisième génération à base de nanofils d'InGaN.

Thesis (Ph. D)--Ohio State University, 2002.
Title from first page of PDF file. Document formatted into pages; contains xxx,198 p.: ill. (some col.). Includes abstract and vita. Advisor: Leonard J. Brillson, Dept. of Electrical Engineering. Includes bibliographical references (p. 188-198).

III-nitride optoelectronic devices have become ubiquitous due to their ability to emit light efficiently in the blue and green spectral ranges. Specifically, III-nitride light emitting diodes (LEDs) have become widespread due to their high brightness and efficiency. However, III-nitride devices such as single photon sources are also the subject of research and are promising for various applications. In order to improve design efficient devices and improve current ones, the relationship between the structure of the constituent materials and their optical properties must be studied. The optical properties of materials are often examined by photoluminescence or cathodoluminescence, whilst traditional microscopy techniques such a transmission electron microscopy and scanning electron microscopy are used to elucidate their structure and composition. This thesis describes the use of a dual-beam focussed ion beam/scanning electron microscope (FIB/SEM) in bridging the gap between these two types of techniques and providing a platform on which to perform correlative studies between the optical and structural properties of III-nitride materials. The heteroepitaxial growth of III-nitrides has been known to produce high defect densities, which can harm device performance. We used this correlative approach to identify hexagonal defects as the source of inhomogeneous electroluminescence (EL) in LEDs. Hyperspectral EL mapping was used to show the local changes in the emission induced by the defects. Following this the FIB/SEM was used to prepare TEM samples from the apex of the defects, revealing the presence of p-doped material in the active region caused by the defect. APSYS simulations confirmed that the presence of p-doped material can enhance local EL. The deleterious effects of defects on the photoelectrochemical etching of cavities were also studied. We performed TEM analysis of an edge-defect contained in unetched material on the underside of a microdisk using FIB/SEM sample preparation methods. The roughness and morphology of microdisk and nanobeam cavities was studied using FIB-tomography (FIBT), demonstrating how the dual-beam instrument may be used to access the 3D morphology of cavities down to the resolution of the SEM and the slicing thickness of the FIB. This tomography approach was further extended with electron tomography studies of the nanobeam cavities, a technique which provided fewer issues in terms of image series alignment but also the presence of reconstruction artefacts which must be taken into account when quantitatively analysing the data. The use of correlative techniques was also used to establish the link between high Si content in an interlayer running along the length of microrods with changes in the optical emission of these rods. The combination of CL, FIB/SEM and TEM-based techniques has made it possible to gain a thorough understanding of the link between the structural and optical properties in a wide variety of III-nitride materials and devices.

Thesis (Ph. D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2008.
William Doolittle, Committee Member ; Joy Laskar, Committee Member ; Russell Dupuis, Committee Chair ; David Citrin, Committee Member ; Srinivas Garimella, Committee Member.

Thesis (Ph.D.)--Boston University PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.
The terahertz (THz) spectral region, commonly defined as the frequency (wavelength) range between 0.3 and 10 THz (1 mm and 30 µm) has many important applications in the industrial, biomedical, and military sectors. However, due to a lack of practical semiconductor materials with adequately small bandgap energy, the development of THz light sources and photodetectors has so far been limited. In recent years, devices based on intersubband transitions between discrete energy states in quantum heterostructures have been under intense research and development to address this issue. Of particular promise in the THz range are quantum cascade lasers (QCLs) and quantum well infrared photodetectors (QWIPs), which utilize intersubband transitions in specially designed quantum well (QW) structures to emit light and generate photocurrent, respectively. This research work has focused on the development of THz light sources and photodetectors using intersubband transitions in GaN/A1GaN QvVs, whose basic materials properties allow for improved spectral coverage and high-temperature operation compared to existing semiconductor devices. To design the active region of QCLs and QWIPs based on inter-conduction-subband transitions in these materials, the necessary numerical tools have first been developed. Sequential tunneling, the key electronic transport mechanism ofintersubband light emitters, has then been demonstrated in GaN/A1GaN QC structures. Furthermore, we have measured promising THz electroluminescence spectra from the same devices through the use of lock-in step-scan Fourier transform infrared spectroscopy. In the area of photodetectors, we have developed a novel double-step QW design in order to overcome the material limitations presented by the intrinsic internal electric fields of GaN/A1GaN QWs. With this design approach, we have experimentally demonstrated the operation of a far infrared QWIP with a peak detection wavelength of 23 µm (13 THz frequency), which is the longest wavelength reported for this materials system.

In this work, fabrication and characterisation of nanostructure devices has been performed on InGaN/GaN multiple quantum wells (MQW) grown on either c-plane sapphire or (111) silicon substrates. A cost effective nanosphere lithography technique has been employed for the fabrication of a number of nano structures such as nanorod arrays, nanoholes arrays; and single micro-disk lasers. Photonic crystal structures based on nanohole arrays have been designed and then fabricated on InGaN/GaN MQWs with an emission wavelength of 500 nm grown on c-plane sapphire by means of a nanosphere lithography technique, demonstrating a clear photonic crystal effect. Significant suppression of spontaneous emission has been observed when the emission is within the photonic bandgap. Angular dependent measurements show a change in the far-field pattern when the emission lies outside the photonic bandgap compared with the emission which lies inside the photonic bandgap. A coherent nanocavity a two-dimensional (2D) periodic array of nanodisks, was designed and fabricated on an InGaN/GaN MQW structure with an emission wavelength at 510 nm, leading to a significant enhancement in the internal quantum efficiency (IQE) as a result of enhanced spontaneous emission rate. Finite-difference time-domain (FDTD) analysis has performed for the structure design. The coherent nanocavity effect has been confirmed using means of time-resolved photoluminescence measurements, showing a clear enhancement in spontaneous emission rate. Finally, an improvement in IQE of 88 times at 510 nm has been achieved. Optically pumped green lasing has been achieved with thresholds as low as 1 kW/cm2, using an InGaN/GaN based micro-disk with an undercut structure on silicon substrates. The micro-disks with a diameter of around 1 μm were fabricated by means of a combination of a cost-effective silica micro-sphere approach, dry-etching and subsequent a wet-etching. The combination of these techniques both minimises the roughness of the sidewalls of the micro-disks and also produces excellent circular geometry. Utilizing this fabrication process, lasing has been achieved at room temperature under optical pumping from a continuous-wave laser diode. Time–resolved micro-photoluminescence (PL) and confocal PL measurements have been performed in order to further confirm the lasing action in whispering gallery modes and also investigate the excitonic recombination dynamics of the lasing.

Degradation processes in AIGaN/GaN high electron mobility transistors were investigated by optical methods. Temperatures within device channels, as well as electric fields are important for on-state degradation. Raman and photoluminescence (PL) thermography were used to investigate these temperatures and the thermal conductivity of GaN channels in both conventional GaN-on-SiC structures and novel AIGaN/GaN/AIGaNon- Si double heterostructure field effect transistors (DHFETs). The thin (150 nm) GaN channel layer in the DHFET had a lower thermal conductivity, at - 60 W m-I K- 1 than typical epilayers, which at - 2 pm thick have more than twice this value. This reduced thermal conductivity has implications for the design of devices employing thin GaN layers, especially when combined with the thick strain relief layers common on Si substrates, as the resulting high temperatures will affect their reliability by on- state thermal degradation processes. The depth resolution of Raman thermography on devices with typical GaN buffers usually limits results to one temperature, averaged through the buffer thickness. A method was developed to improve the depth resolution using a spatial filter and azimuthal polarisation; when combined with offset focal planes it was possible to obtain temperatures of the top and bottom of the GaN epilayer separately. Off-state degradation processes are more closely related to electric fields than self-heating; the generation of leakage current paths from the gate to the channel is particularly important. This leakage-path generation and associated localised electroluminescence (EL) emission was studied using EL imaging and spectroscopy combined with deep UV PL spectroscopy. The PL from the AIGaN barrier was reduced in regions associated with localised EL, indicating the formation of non-radiative recombination centres in the form of defects in the AIGaN. These non-radiative recombination centres were found to be generated over a larger area than the location of the gate leakage currents - these currents only start to flow when sufficient defects form to constitute a path.

This dissertation describes the development of III-nitride (III-N) bipolar devices for optoelectronic and electronic applications. Research mainly involves device design, fabrication process development, and device characterization for Geiger-mode gallium nitride (GaN) deep-UV (DUV) p-i-n avalanche photodiodes (APDs), indium gallium nitride (InGaN)/GaN-based violet/blue laser diodes (LDs), and GaN/InGaN-based npn radio-frequency (RF) double-heterojunction bipolar transistors (DHBTs). All the epitaxial materials of these devices were grown in the Advanced Materials and Devices Group (AMDG) led by Prof. Russell D. Dupuis at the Georgia Institute of Technology using the metalorganic chemical vapor deposition (MOCVD) technique. Geiger-mode GaN p-i-n APDs have important applications in DUV and UV single-photon detections. In the fabrication of GaN p-i-n APDs, the major technical challenge is the sidewall leakage current. To address this issue, two surface leakage reduction schemes have been developed: a wet-etching surface treatment technique to recover the dry-etching-induced surface damage, and a ledged structure to form a surface depletion layer to partially passivate the sidewall. The first Geiger-mode DUV GaN p-i-n APD on a free-standing (FS) c-plane GaN substrate has been demonstrated. InGaN/GaN-based violet/blue/green LDs are the coherent light sources for high-density optical storage systems and the next-generation full-color LD display systems. The design of InGaN/GaN LDs has several challenges, such as the quantum-confined stark effect (QCSE), the efficiency droop issue, and the optical confinement design optimization. In this dissertation, a step-graded electron-blocking layer (EBL) is studied to address the efficiency droop issue. Enhanced internal quantum efficiency (ɳi) has been observed on 420-nm InGaN/GaN-based LDs. Moreover, an InGaN waveguide design is implemented, and the continuous-wave (CW)-mode operation on 460-nm InGaN/GaN-based LDs is achieved at room temperature (RT). III-N HBTs are promising devices for the next-generation RF and power electronics because of their advantages of high breakdown voltages, high power handling capability, and high-temperature and harsh-environment operation stability. One of the major technical challenges to fabricate high-performance RF III-N HBTs is to suppress the base surface recombination current on the extrinsic base region. The wet-etching surface treatment has also been employed to lower the surface recombination current. As a result, a record small-signal current gain (hfe) > 100 is achieved on GaN/InGaN-based npn DHBTs on sapphire substrates. A cut-off frequency (fT) > 5.3 GHz and a maximum oscillation frequency (fmax) > 1.3 GHz are also demonstrated for the first time. Furthermore, A FS c-plane GaN substrate with low epitaxial defect density and good thermal dissipation ability is used for reduced base bulk recombination current. The hfe > 115, collector current density (JC) > 141 kA/cm², and power density > 3.05 MW/cm² are achieved at RT, which are all the highest values reported ever on III-N HBTs.

Efficiency droop is a critical issue for the Group III-nitride based light-emitting diodes (LEDs) to be competitive in the general lighting application. Carrier spill-over have been suggested as an origin of the efficiency droop, and an InAlN electron-blocking layer (EBL) is suggested as a replacement of the conventional AlGaN EBL for improved performance of LED. Optimum growth condition of InAlN layer was developed, and high quality InAlN layer was grown by using metalorganic chemical vapor deposition (MOCVD). A LED structure employing an InAlN EBL was grown and its efficiency droop performance was compared with a LED with an AlGaN EBL. Characterization results suggested that the InAlN EBL delivers more effective electron blocking over AlGaN EBL. Hole-injection performance of the InAlN EBL was examined by growing and testing a series of LEDs with different InAlN EBL thickness. Analysis results by using extended quantum efficiency model shows that further improvement in the performance of LED requires better hole-injection performance of the InAlN EBL. Advanced EBL structures such as strain-engineered InAlN EBL and compositionally-graded InAlN EBLs for the delivery of higher hole-injection efficiency were also grown and tested.

Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2010.
Committee Chair: Yoder, Douglas; Committee Member: Graham, Samuel; Committee Member: Allen, Janet; Committee Member: Klein, Benjamin; Committee Member: Voss, Paul. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Through the history of mankind, novel materials have played a key role in techno- logical progress. As we approach the limits of scaling it becomes difficult to squeeze out any more extensions to Moore’s law by just reducing device feature sizes. It is important to look for an alternate semiconductor to silicon in order to continue making the progress predicted by Moore’s law. Among the various semiconductor options being explored world-wide, the III-nitride semiconductor material system has certain unique characteristics that make it one of the leading contenders. We explore the III-nitride semiconductor material system for the unique advantages that it offers over the other alternatives available to us. This thesis studies the device applications of epitaxial III-nitride films and nanos- tructures grown using plasma assisted molecular beam epitaxy (PAMBE) The material characterisation of the PAMBE grown epitaxial III-nitrides was car- ried out using techniques like high resolution X-ray diffraction (HR-XRD), field emis- sion scanning electron microscopy (FESEM), room temperature photoluminescence (PL) and transmission electron microscopy (TEM). The epitaxial III-nitrides were then further processed to fabricate devices like Schottky diodes, photodetectors and surface acoustic wave (SAW) devices. The electrical charcterisation of the fabricated devices was carried out using techniques like Hall measurement, IV and CV measure- ments on a DC probe station and S-parameter measurements on a vector network analyser connected to an RF probe station. We begin our work on Schottky diodes by explaining the motivation for adding an interfacial layer in a metal-semiconductor Schottky contact and how high-k di- electrics like HfO2 have been relatively unexplored in this application. We report the work carried out on the Pt/n-GaN metal-semiconductor (MS) Schottky and the Pt/HfO2/n-GaN metal-insulator-semiconductor (MIS) Schottky diode. We report an improvement in the diode parameters like barrier height (0.52 eV to 0.63 eV), ideality factor (2.1 to 1.3) and rectification ratio (35.9 to 98.9 @2V bias) after the introduction of 5 nm of HfO2 as the interfacial layer. Temperature dependent I-V measurements were done to gain a further understanding of the interface. We observe that the barrier height and ideality factor exhibit a temperature dependence. This was attributed to inhomogeneities at the interface and by assuming a Gaussian distribution of barrier heights. UV and IR photodetectors using III-nitrides are then studied. Our work on UV photodetectors describes the growth of epitaxial GaN films. Au nanoparticles were fabricated on these films using thermal evaporation and annealing. Al nanostruc- tures were fabricated using nanosphere lithography. Plasmonic enhancement using these metallic nanostructures was explored by fabricating metal-semiconductor-metal (MSM) photodetectors. We observed plasmonic enhancement of photocurrent in both cases. To obtain greater improvement, we etched down on the GaN film using reac tive ion etching (RIE). This resulted in further increase in photocurrent along with a reduction in dark current which was attributed to creation of new trap states. IR photodetectors studied in this thesis are InN quantum dots whose density can be controlled by varying the indium flux during growth. We observe that increase in InN quantum dot density results in increase in photocurrent and decrease in dark current in the fabricated IR photodetectors. We then explore the advantages that InGaN offers as a material that supports surface acoustic waves and fabricate InGaN based surface acoustic wave devices. We describe the growth of epitaxial In0.23 Ga0.77 N films on GaN template using molecular beam epitaxy. Material characterisation was carried out using HR-XRD, FESEM, PL and TEM. The composition was determined from HR-XRD and PL measurements and both results matched each other. This was followed by the fabrication of interdigited electrodes with finger spacing of 10 µm. S-parameter results showed a transmission peak at 104 MHz with an insertion loss of 19 dB. To the best of our knowledge, this is the first demonstration of an InGaN based SAW device. In summary, this thesis demonstrates the practical advantages of epitaxially grown film and nanostructured III-nitride materials such as GaN, InN and InGaN using plasma assisted molecular beam epitaxy for Schottky diodes, UV and IR photodetec- tors and surface acoustic wave devices.

Through the history of mankind, novel materials have played a key role in techno- logical progress. As we approach the limits of scaling it becomes difficult to squeeze out any more extensions to Moore’s law by just reducing device feature sizes. It is important to look for an alternate semiconductor to silicon in order to continue making the progress predicted by Moore’s law. Among the various semiconductor options being explored world-wide, the III-nitride semiconductor material system has certain unique characteristics that make it one of the leading contenders. We explore the III-nitride semiconductor material system for the unique advantages that it offers over the other alternatives available to us. This thesis studies the device applications of epitaxial III-nitride films and nanos- tructures grown using plasma assisted molecular beam epitaxy (PAMBE) The material characterisation of the PAMBE grown epitaxial III-nitrides was car- ried out using techniques like high resolution X-ray diffraction (HR-XRD), field emis- sion scanning electron microscopy (FESEM), room temperature photoluminescence (PL) and transmission electron microscopy (TEM). The epitaxial III-nitrides were then further processed to fabricate devices like Schottky diodes, photodetectors and surface acoustic wave (SAW) devices. The electrical charcterisation of the fabricated devices was carried out using techniques like Hall measurement, IV and CV measure- ments on a DC probe station and S-parameter measurements on a vector network analyser connected to an RF probe station. We begin our work on Schottky diodes by explaining the motivation for adding an interfacial layer in a metal-semiconductor Schottky contact and how high-k di- electrics like HfO2 have been relatively unexplored in this application. We report the work carried out on the Pt/n-GaN metal-semiconductor (MS) Schottky and the Pt/HfO2/n-GaN metal-insulator-semiconductor (MIS) Schottky diode. We report an improvement in the diode parameters like barrier height (0.52 eV to 0.63 eV), ideality factor (2.1 to 1.3) and rectification ratio (35.9 to 98.9 @2V bias) after the introduction of 5 nm of HfO2 as the interfacial layer. Temperature dependent I-V measurements were done to gain a further understanding of the interface. We observe that the barrier height and ideality factor exhibit a temperature dependence. This was attributed to inhomogeneities at the interface and by assuming a Gaussian distribution of barrier heights. UV and IR photodetectors using III-nitrides are then studied. Our work on UV photodetectors describes the growth of epitaxial GaN films. Au nanoparticles were fabricated on these films using thermal evaporation and annealing. Al nanostruc- tures were fabricated using nanosphere lithography. Plasmonic enhancement using these metallic nanostructures was explored by fabricating metal-semiconductor-metal (MSM) photodetectors. We observed plasmonic enhancement of photocurrent in both cases. To obtain greater improvement, we etched down on the GaN film using reac tive ion etching (RIE). This resulted in further increase in photocurrent along with a reduction in dark current which was attributed to creation of new trap states. IR photodetectors studied in this thesis are InN quantum dots whose density can be controlled by varying the indium flux during growth. We observe that increase in InN quantum dot density results in increase in photocurrent and decrease in dark current in the fabricated IR photodetectors. We then explore the advantages that InGaN offers as a material that supports surface acoustic waves and fabricate InGaN based surface acoustic wave devices. We describe the growth of epitaxial In0.23 Ga0.77 N films on GaN template using molecular beam epitaxy. Material characterisation was carried out using HR-XRD, FESEM, PL and TEM. The composition was determined from HR-XRD and PL measurements and both results matched each other. This was followed by the fabrication of interdigited electrodes with finger spacing of 10 µm. S-parameter results showed a transmission peak at 104 MHz with an insertion loss of 19 dB. To the best of our knowledge, this is the first demonstration of an InGaN based SAW device. In summary, this thesis demonstrates the practical advantages of epitaxially grown film and nanostructured III-nitride materials such as GaN, InN and InGaN using plasma assisted molecular beam epitaxy for Schottky diodes, UV and IR photodetec- tors and surface acoustic wave devices.

abstract: Group III-nitride semiconductors have been commercially used in the fabrication of light-emitting diodes and laser diodes, covering the ultraviolet-visible-infrared spectral range and exhibit unique properties suitable for modern optoelectronic applications. InGaN ternary alloys have energy band gaps ranging from 0.7 to 3.4 eV. It has a great potential in the application for high efficient solar cells. AlGaN ternary alloys have energy band gaps ranging from 3.4 to 6.2 eV. These alloys have a great potential in the application of deep ultra violet laser diodes. However, there are still many issues with these materials that remain to be solved. In this dissertation, several issues concerning structural, electronic, and optical properties of III-nitrides have been investigated using transmission electron microscopy. First, the microstructure of In_xGa_1-xN (x = 0.22, 0.46, 0.60, and 0.67) films grown by metal-modulated epitaxy on GaN buffer /sapphire substrates is studied. The effect of indium composition on the structure of InGaN films and strain relaxation is carefully analyzed. High luminescence intensity, low defect density, and uniform full misfit strain relaxation are observed for x = 0.67. Second, the properties of high-indium-content InGaN thin films using a new molecular beam epitaxy method have been studied for applications in solar cell technologies. This method uses a high quality AlN buffer with large lattice mismatch that results in a critical thickness below one lattice parameter. Finally, the effect of different substrates and number of gallium sources on the microstructure of AlGaN-based deep ultraviolet laser has been studied. It is found that defects in epitaxial layer are greatly reduced when the structure is deposited on a single crystal AlN substrate. Two gallium sources in the growth of multiple quantum wells active region are found to cause a significant improvement in the quality of quantum well structures.
Dissertation/Thesis
Doctoral Dissertation Physics 2014

碩士
長庚大學
光電工程研究所
94
This thesis concerned with the studies on the optical properties of AlGaN/GaN heterostructure and InGaN/InGaN multiple quantum wells, the silicon doping effect on InGaN/InGaN quantum wells. The content had three major parts. (1) Optical properties in AlGaN/GaN heterostructure We used Williamson-Hall plot to obtain the threading dislocation densities for different aluminum fraction of AlxGa1-xN epitaxial layers. As the aluminum fraction increased, the threading dislocation densities expanded. We found that higher aluminum fraction would lead to smaller Hall mobility. As the polarization induced sheet charge was stronger, the PL intensity of two dimensional electron gases (2DEG) peak was weaker. The PL energy separation between the 2DEG peak and the GaN FE emission decreased with increasing temperature. The result was attributed to the screening effect of electrons on the bending of the conduction band at the heterointerface. (2) Optical properties in InGaN/InGaN multiple quantum wells(MQW) We found that the degree of blue shift in temperature-depend photoluminescence (TDPL) was greater in higher indium composition samples. From T-x diagram, the higher indium composition samples had higher probability to prevail phase separation. Using Arrhenius plot in TDPL, we could confirm that higher indium samples had stronger carrier localization effect. Based on the two conjectures, the quantum dots-like might be formed in the in-rich regions. Thus, the higher indium composition samples had greater polarization field, which caused the variation of energy gap which was induced by screening effect greater as temperature increased. The reason of the obviously blue shift of the TDPL in higher indium composition could be obtained by the carrier localization effect or the polarization effect. (3) Current and optical properties in Si-doped InGaN/InGaN MQW Silicon doping in barriers reduced the mismatch between barriers and wells, and silicon doping in barriers could fill the defects or change the dislocation mobility. Therefore, Si-doped samples had reduced threading dislocation densities. By the calculated results of polarization field and PL spectra, we found that the enhancement of PL intensity and the blue shift of the PL spectra with increasing silicon doping concentrations were explained by a decrease in potential fluctuations and/or screening effect of the internal piezoelectric field. By comparing T-PL and current-depend electroluminescence, we found that the quantum well related signals in PL spectra weren’t the band-to-band emission, but the indium localized states induced by the potential fluctuations. According to the C-V measurement, we found that carriers were accumulating in the wells which were near surface because the width of depletion had changed by silicon doing in the barriers near substrate. The current properties showed that silicon doping could improve samples’ quality, reduce leakage current and turn-on voltage, and increase light efficiency.

III-Nitride semiconductor devices reserve their utmost place in the ubiquitous class of technology due to its applicability in a wide range of applications, which include solar cells, light-emitting diodes (LEDs), laser diodes (LDs), photodetectors (PDs), etc. Consequently, the extensive range of applications and projected gigantic market size generates continuous demand for more efficient, durable, and quality device products. The researchers have world-wide taken up this challenge of the never-ending requirement of technological improvisation and devoting their rigorous efforts to achieve better materials quality and device design. Among numerous domestic, commercial, and defence applications of III-nitride semiconductor-based devices, the detection of harmful ultraviolet (UV) radiation (<400 nm) found to be an extreme necessity. Universally, technologist finds potential needs to detect UV for plume detection, flame detection, chemical as well as biological analyses, secure space to space communications, environmental monitoring, astronomical research, and antimissile technology. Thus, the recommended ideal photodetector (PD) device should be adverse environment friendly with excellent sensitivity, high responsivity, high signal to noise ratio, and high spectral selectivity. Henceforth, as compared to conventional Sibased UV PD devices, III-nitride is proven to be a promising candidate due to their remarkable credentials such as wide-direct bandgap, high thermal conductivity, superb radiation hardness, which makes them operable in harsh environmental conditions. Besides, the detectors with Si-based technology severely loses signal to noise ratio as well as efficiency as a function of increasing dark current with elevated temperature. The solution to these problems has been provided by usage of the bulky cooling arrangement, expensive optical filters, and their operational instrumentation setups. Therefore, the movement of the semiconductor market from the Si-based to III-nitride based can be well understood. However, a key challenge is the presence of a large density of defects and dislocations due to lack of suitable lattice-matched substrates, and low resistance Ohmic contacts. Thus, particularly in the case of energy-efficient UV PD fabrication, the curiosity to grow good quality epitaxial III-nitride films has been mainly directed towards the study on GaN nanostructures (NSs). Thereby, the xiv growth of high surface to volume ratio based GaN NSs increases the incident photon absorption sites without sacrificing the device nano dimensionality. Besides, the possibility of integration of GaN with existing Si-based device technology leads us towards growth & fabrication on Si (111) substrates. Till date, numerous methodologies have been adopted to grow and optimize stress relaxed, higher aspect ratio, high crystalline quality as well as an excellent opto-electrical transport based GaN NSs by using plasma-assisted molecular beam epitaxy (PAMBE). Despite improvisation in crystalline quality, grown NSs as well as device geometry, recent advancement for enhancing the performance of PD devices emphasized on other exciting and potential approaches such as hybridization of III-nitride materials with other UV sensitive semiconductors, functionalization by novel metal nanoparticle’s nanoplasmonics and sensitization by highly conductive quantum dots. Thus, the thesis aims to explore and meticulously investigate the precise control epitaxial growth of high surface to volume ratio oriented GaN NSs on Si (111) substrate using the PAMBE system and their utilization as energy-efficient UV PDs. Besides, the effort made in this work excavates and execute the new prospects for emerging next-generation highly efficient photodetectors via. ZnO/GaN heterojunction hybridization, Au-NPs functionalization, and GQDs sensitization. The thesis consists of seven chapters, briefly described below: Chapter 1 gives a brief overview of the photodetection devices which are sensitive towards the UV range. Further, the significant contribution of III-nitrides in the fabrication of energy-efficient and durable UV PDs has been discussed. Moreover, the introduction of III-nitrides semiconductor’s inherent physical, chemical, optical properties with their bandgap engineering has been elaborated, which were found responsible for the fabrication of narrow to broadband PDs for harsh environment. Additionally, to enhance the performance of existing III-nitride UV PD technology, other potential approaches such as nitride’s surface functionalization, hybridization and sensitization have also been suggested. Chapter 2 describes the detailed mechanism of the technique used for the growth of III-nitride semiconductors along with various in-situ as well as ex-situ analytical tools xv and methods utilized for probing the structural, morphological, optical and electrical properties of the grown structures. This chapter illustrates a brief description of the steps involved in devices fabrication process and evaluation of their performance parameters as well. Chapter 3 elucidates the growth of nanoisland shaped, lower stress, and strain facilitated GaN-NS on Si (111) substrate via PAMBE and fabrication of GaN-NS based UV photodetection device even with NS’s tiny dimensionality. The threedimension (3D) growth of GaN-NS in real-time was observed by the in-situ RHEED technique, which displays transformation from streaky to the spotty pattern. A microRaman technique has been employed to elaborate on NS’s crystallinity and lower stress value, which is found to be in good agreement with related lower strain as evaluated by HR-XRD spectra. An observed sharp near band edge emission at 363.2 nm by room temperature photoluminescence measurement signifies the presence of GaN. After that, the as-grown ultra-thin GaN NSs were utilized to fabricate energyefficient self-powered UV PD, wherein non-homogeneous GaN nanoislands were perceived on the Si surface with a thickness of ~30 nm and an average distribution density of 2 ×1010 cm -2 . Despite nano dimensional GaN NSs film, the capability of UV detection of fabricated PD added novelty to this work, where performance parameters such as photosensitivity (~102 ), detectivity (~109 Jones), responsivity (1.76 mA/W) and NEP: noise equivalent power (3.5 × 10-11 WHz-1/2) under selfpowered mode were observed. The transient photo-response measurement revealed a rapid rise and decay time constants of ~18 ms and ~27 ms, respectively. Under varying optical power (1 mW to 13 mW), the GaN PD displayed significant enhancement in photocurrent with increasing optical power. The performance of the fabricated detector has also been analyzed under the photoconductive mode of operation, where it revealed significantly enhanced responsivity (23 fold) and detectivity (~1000 fold). Such nanostructured self-powered GaN-based UV PD paves the way towards the fabrication of energy-efficient optoelectronic devices. Chapter 4 presented the nanoplasmonic impact of chemically synthesized Au nanoparticles (NPs) on the performance of GaN NS based UV PD is analyzed. The devices with uniformly distributed Au NPs on GaN NSs (nanoislands) prominently xvi respond toward UV illumination (325 nm) in both self-powered as well as photoconductive modes of operation and have shown fast and stable time-correlated response with significant enhancement in the performance parameters. A comprehensive analysis of the device design, laser power, and bias-dependent responsivity and response time is presented. The fabricated Au NP/GaN nanoislandbased device yields the highest responsivity of ∼ 216 mA/W, detectivity of ∼ 109 jones, reduced NEP of ∼ 1.8 × 10−12 W Hz−1/2, External quantum efficiency (EQE) of ∼ 82%, and fast response/recovery time of ∼ 40 ms. Moreover, the study also illustrates the mechanism where light interacts with the chemically synthesized NPs guided by the surface plasmon to enhance the device performance effectively. Further, the decoration of low dimensional Au NPs on GaN NSs acts as a detection enhancer with fast recovery time. It paves the way toward the realization of energyefficient optoelectronic device applications. Chapter 5 elaborates on the fabrication of GaN-Nanotowers (GaN-NTs) based on highly efficient UV PD with distinct AlN buffer layer thickness and NTs lengths. The unique nanotower (tapered ended) morphology of an epitaxially grown hexagonal stacked nanocolumnar structure with truncated strain and high surface-to-volume ratio contributes to the significantly enhanced performance of the fabricated detector towards UV illumination. The fabricated GaN-NT UV PDs displays very low dark current (~12nA) & very high ILight /IDark ratio (>104 ) along with the highest responsivity of 485 A/W. The device exhibits very high EQE ~105 %, fast time-correlated transient response (~430 µs), very low NEP (~10-13 WHz-1/2), and excellent UV/Vis rejection ratio. Therefore, the utilization of such GaN-NT structures can be advantageous towards the fabrication of energy-efficient ultraviolet photodetector. Chapter 6 introduce the concept of performance augmentation of existing III-nitride (GaN) UV PDs technology employing hybridization using UV sensitive & compatible semiconductors (ZnO), surface functionalization by novel metal Gold (Au) NPs and sensitization by highly conductive graphene quantum dots (GQDs). Thereby, as grown (as discussed in chapter 5) vertically aligned longer GaN NTs with higher AlN thickness oriented highly responsive UV demonstrated fast response with excellent stability when functionalized with Au Nanoparticles, GQDs, and ZnO Nanorods. xvii Initially, GaN-NTs structure is hybridized by ZnO Nanorods (ZnO NRs), wherein, to capture maximum incident photons, a strategically formulated model of NSs over NSs has been proposed as an enhanced device active surface area. Consequently, the fabricated GaN-NTs based device with ZnO NRs hybridization, GQDs sensitization, and Au NPs functionalization significantly accelerate the performance of the device where a prominent three order reduction in dark current is observed along with gigantic R, lower NEP and enormously enhanced EQE of 7042 A/W, 1.84×10-14 W.Hz-1/2 and 2.7×106 % respectively. Mechanism elaborating the enhanced device performance with an appropriate energy band diagram has been discussed in detail. The fabricated highly sensitive device can lead a path towards future optoelectronic applications of integrated III-Nitride technology. Chapter 7 enlightens the major conclusions derived from the thesis work and the scope of future work.

博士
國立清華大學
物理學系
99
本論文討論以在矽基板上利用分子束磊晶成長法應用於三族氮化物(氮化鎵、氮化銦鎵)奈米柱材料之成長。並對其結構以及光學性質做詳細的分析與討論，並以實際元件來說明此材料之發展潛力。 矽基板相較於其它基板(例如： 氧化鋁、碳化矽)擁有與價格便宜之優勢，適合大面積成長，且易與IC製程整合。利用分子束磊晶所成長出的三族氮化物奈米柱呈垂直於基板的排列，且每一奈米柱皆為無應力之單晶結構，依據螢光光譜量測，氮化鎵奈米柱具有激子的發光特性且有很好的發光效率。確定了氮化鎵奈米柱之優良特性後，我們利用此結構當作模板，並且成功地在上面成長出涵蓋全可見光(400 -700 nm)波段之氮化銦鎵奈米碟。相較於氮化銦鎵薄膜，奈米碟擁有發光參數容易調控，且在長波長範圍之發光強度減弱不如薄膜嚴重。利用這些特性，我們試著將各種不同發光波段之奈米碟依不同厚度與層數疊在一起，利用此概念，成長出不需要利用任何螢光粉輔助之白光發光二極體結構，對白光照明，提供了一個新的解決方法。

博士
國立交通大學
電子物理系
87
We have carried out systematic studies on the epitaxial growth of AlAs1-xSbx, GaN and InxGa1-xN compounds using metalorganic vapor phase epitaxy technique. Experimental data indicate that the solid composition of AlAsSb depends strongly on the input reactant flow rates and the growth temperature. A high Sb concentration of AlAsSb alloy can only be obtained at a V/III ratio close to 1, whereas too high the Sb flow rates and too low the V/III ratio will lead to the formation of Sb droplets and Al metal platelets, respectively. For AlAsSb prepared at high growth temperatures, the side reaction of TBAs, b-elimination, is believed to response for the result in a decrease of the As solid concentration. By employing a thermodynamic analysis, a novel phase diagram for AlAsSb with simpler solid-vapor distribution relationship was obtained, according to which the As solid concentration can be directly determined by the input As/Al mole flow rate ratio. The AlAs1-xSbx alloy was also used to fabricate two novel diodes, the enhanced InP Schottky diode and the In0.53Ga0.47As/AlAs0.44Sb0.56/In0.53Ga0.47As single barrier tunneling diode. By introducing AlAsSb into the conventional Schottky structure, the InP Schottky barrier height was improved greatly from 0.45eV to 0.76eV. For single-barrier tunneling diode, a negative differential resistance characteristic was successfully observed at 100 and 300K. A high peak-to-valley current ratio of 4.2 is obtained at 100K, which is the best value ever reported for such type of device. For GaN, the film quality appears to be very sensitive to the buffer layer property and the growth temperature. The optimized buffer layer thickness, temperature ramping rate and growth temperature are found to be around 100~300A, 75~100℃/min, and 1,000~1,050℃, respectively. A phase transition from hexagonal to cubic structure for GaN has been evidenced at a growth temperature around 750℃. The best quality of our GaN films in terms of FWHMs of x-ray and 300K-PL are as narrow as only 160 arcsec and 28meV, respectively. The corresponding electron mobility and carrier concentration also exhibit superior values of 330 cm^2/V and 1.1x10^17 cm^-3, respectively. Regarding to the InGaN growth, our experimental results indicate that the solid composition and characteristic of InGaN are determined not only by the growth temperature, but also by the TMGa and TMIn flow rates. The films with the good structural and optical properties can only be obtained at temperatures above 750℃. For the solid distribution, we found that too high the TMIn flow rate and too low the TMGa flow rate will both bring a decrease of In concentration solid, unfavorable to the high-In content InGaN growth. Besides, the thermodynamic analysis was also performed in our InGaN study. By introducing an empirical high-temperature factor in our modified InGaN growth model, we can successfully predict the solid-vapor distribution in InGaN and the appearance of In-droplets during growth. Based on thermodynamic arguments, the maximum allowed In solid concentration for a single phase InGaN is constrained primarily by the high temperature effect, such as In desorption, and the In saturation vapor pressure. By optimizing the growth conditions, we can obtain high quality InGaN epilayers with the narrow FWHMs of 150 arcsec and 92 meV for (0002) x-ray diffraction and 300K-PL peaks, respectively.

Group III-A nitrides (GaN, AlN, InN and alloys) are materials of considerable contemporary interest and currently enable a wide variety of optoelectronic and high-power, high-frequency electronic applications. All of these applications utilize device structures that employ a single or multiple hetero-junctions, with material compositions varying across the interface. For example, the workhorse of GaN based electronic devices is the high electron mobility transistor (HEMT) which is usually composed of an AlGaN/GaN hetero-junction, where a two-dimensional electron gas (2DEG) is formed due to differences in polarization between the two layers. In addition to such hetero-junctions in the same material family, formation of hetero-interfaces in nitrides begins right from the epitaxy of the very first layer due to the lack of native substrates for their growth. The consequences of such "dissimilar" hetero-junctions typically manifest as large defect densities at this interface which in turn gives rise to defective films. Additionally, if the substrate is also a semiconductor, the electrical properties at such dissimilar semiconductor-nitride hetero-junctions are particularly important in terms of their influence on the performance of nitride devices. Nevertheless, the large defect densities at such dissimilar 3D-3D semiconductor interfaces, which translate into more trap states, also prevents them from being used as active device layers to say nothing of reliability considerations arising because of these defects. Recently, the advent of 2D materials such as graphene and MoS2 has opened up avenues for Van der Waal’s epitaxy of these layered films with practically any other material. Such defect-free integration enables dissimilar semiconductor hetero-junctions to be used as active device layers with carrier transport across the 2D-3D hetero-interface. This thesis deals with hetero-epitaxial growth platforms for reducing defect densities, and the material and electrical properties of dissimilar hetero-junctions with the group III-A nitride material system.

Group III-A nitrides (GaN, AlN, InN and alloys) are materials of considerable contemporary interest and currently enable a wide variety of optoelectronic and high-power, high-frequency electronic applications. All of these applications utilize device structures that employ a single or multiple hetero-junctions, with material compositions varying across the interface. For example, the workhorse of GaN based electronic devices is the high electron mobility transistor (HEMT) which is usually composed of an AlGaN/GaN hetero-junction, where a two-dimensional electron gas (2DEG) is formed due to differences in polarization between the two layers. In addition to such hetero-junctions in the same material family, formation of hetero-interfaces in nitrides begins right from the epitaxy of the very first layer due to the lack of native substrates for their growth. The consequences of such "dissimilar" hetero-junctions typically manifest as large defect densities at this interface which in turn gives rise to defective films. Additionally, if the substrate is also a semiconductor, the electrical properties at such dissimilar semiconductor-nitride hetero-junctions are particularly important in terms of their influence on the performance of nitride devices. Nevertheless, the large defect densities at such dissimilar 3D-3D semiconductor interfaces, which translate into more trap states, also prevents them from being used as active device layers to say nothing of reliability considerations arising because of these defects. Recently, the advent of 2D materials such as graphene and MoS2 has opened up avenues for Van der Waal’s epitaxy of these layered films with practically any other material. Such defect-free integration enables dissimilar semiconductor hetero-junctions to be used as active device layers with carrier transport across the 2D-3D hetero-interface. This thesis deals with hetero-epitaxial growth platforms for reducing defect densities, and the material and electrical properties of dissimilar hetero-junctions with the group III-A nitride material system.

博士
國立成功大學
微電子工程研究所
103
Growth and fabrication of high efficiency GaN-based optoelectronic devices were successfully demonstrated in this dissertation. This dissertation proposed two directions to enhance the efficiency for fabrication of InGaN-based LEDs, include the methods of Mg-doped AlGaN/InGaN superlattice electron blocking layer and digital InN/GaN growth in InGaN well. We also proposed two methods to improve the efficiency of InGaN-based solar cells by introducing ex-situ AlN nucleation layer and Si-doped AlGaN barriers. Firstly, the operating voltage, light output power, and efficiency droops of GaN-based light emitting diodes (LEDs) were improved by introducing Mg-doped AlGaN/InGaN superlattice (SL) electron blocking layer (EBL). The thicker InGaN layers of AlGaN/InGaN SL EBL could have a larger effective electron potential height and lower effective hole potential height than that of AlGaN EBL. This thicker InGaN layer could prevent electron leakage into the p-region of LEDs and improve hole injection efficiency to achieve a higher light output power and less efficiency droops with the injection current. The low lateral resistivity of Mg-doped AlGaN/InGaN SL would have superior current spreading at high current injection. Secondly, thick InGaN wells of LEDs with high crystal quality can be prepared by using digital InN/GaN growth for the InGaN wells. The thickness of high crystal quality InGaN wells can be sustained with In% of 17.6% more than 7 nm. The 60 mA output power of LEDs with thick InN/GaN alternative growth InGaN wells indicate an enhancement of at least 28.9% compared with that of LEDs with conventional InGaN wells. However, LEDs with thick InN/GaN alternative growth InGaN wells have larger efficiency droops than LEDs with conventional InGaN wells. Compared with conventionally grown thin InGaN wells, thick InGaN wells with digitally grown InN/GaN exhibit superior optical properties. The activation energy (48 meV) of thick InGaN wells (generated by digital InN/GaN growth from temperature-dependent integrated photoluminescence intensity) is larger than the activation energy (25 meV) of conventionally grown thin InGaN wells. Moreover, thick InGaN wells with digitally grown InN/GaN exhibit a smaller  value (the degree of localization effects) of 19 meV than that of conventionally grown thin InGaN wells (23 meV). Thirdly, GaN solar cells (SCs) with ex-situ AlN nucleation layer are examined in this study. GaN with sputtered ex-situ AlN nucleation layer has mixed-type dislocation density at approximately one order less than that of GaN with in-situ GaN nucleation layer. The reduction of dislocation density by the sputtered AlN nucleation layer could suppress the reverse leakage current and the recombination forward current in low forward voltage range of SCs, and then can increase Jsc and Voc of the SCs. 1-sun η% of SCs with ex-situ AlN nucleation (1.92%) showed an enhancement of 27.2% compared with that of conventional SC at 1.51%. Furthermore, the 100-sun η% of SCs with ex-situ AlN nucleation (1.99%) showed 18.5% improvement compared with that of conventional SC (1.68%). Finnaly, the Voc, FF%, and η% of GaN-based SCs can be improved by replacing an initial 2-pair GaN/InGaN with 2-pair AlGaN/InGaN multilayer. The IPCE of SCs with 2-pair AlGaN/InGaN multilayer is higher than that with GaN/InGaN at 2 V for wavelengths in range of 365 nm to 420 nm. From the results of the numerical simulation, we found that the FF% improvement of the SCs with the 2-pair AlGaN/InGaN multilayer should be attributed to the photo-generated carrier recombination rate suppression near the bias of the Voc. The 1-sun η of SCs with Si-doped Al0.10GaN barriers (1.86%) showed 27.4% enhancement compared with that with Si-doped GaN barriers (1.46%).

This thesis describes a number of advancements in the III-V nitride material system achieved at Caltech. Major improvements in optimized growth of III-V nitride materials by radio frequency molecular beam epitaxy are presented. The first GaN based MEMs devices are reported, and their fabrication and characterization are detailed. The growth of InN is discussed, and the morphology and crystal quality are presented. In addition, the switching behavior of the first GaN thyristor is reported. The surface chemistry and band offsets of the nitrides are also investigated using x-ray photoelectron spectroscopy. These investigations are presented in two parts. Part I describes the growth and characterization of III-V nitride materials and includes a discussion of piezoelectric effects in this material system. Part II describes the device related successes mentioned above. The focus of the second chapter is the description and calculation of piezoelectric phenomena in nitride heterojunctions. The magnitude of the piezoelectric effects in nitride materials is such that entirely new device concepts can be developed. On the other hand, traditional devices are impacted by these polarization fields, and proper accounting for them must be made for device design. Some of these effects are calculated and new devices employing these fields proposed. The major issues pertaining to MBE growth are discussed in the third chapter. Substrates, nitridation, film nucleation, and the dependence of film quality on growth parameters are investigated by means of AFM, cathodoluminescence, x-ray diffraction, and reflection high energy electron diffraction. It is found that the buffer layer deposition parameters and V/III ratio during subsequent growth are the major factors in determining the resulting films crystal quality. In addition to growth, x-ray photoelectron spectroscopy has been used to investigate the surface chemistry of column-III nitrides. It was found that the oxidation of GaN was self limiting and did not proceed significantly farther than the first mono layer. Many of the core level and valence band maximum positions were determined, as well as the valence band offsets for the AIN/AlGaN system. Part II describes characterization of nitride devices. The switching behavior of a GaN thyristor is described and possible explanations for its behavior investigated. GaN pri and pin diodes are characterized electrically and via electroluminescence. It is found that less than ideal electrical properties of these devices may be related to a high level of deep trap states that produce high levels of leakage current as well as suppress the light emission of these diodes. Finally, a new method of preferentially etching n type GaN is presented. This technique is extended to the fabrication of GaN MEMs devices, arid piezoelectric strain sensors and micropumps fabricated with this technique are characterized.

Thesis (Ph.D.)--State University of New York at Buffalo, 2005.
Title from PDF title page (viewed on Feb. 24, 2006) Available through UMI ProQuest Digital Dissertations. Thesis adviser: Alexander N. Cartwright.

abstract: Group III-nitride semiconductors have wide application in optoelectronic devices. Spontaneous and piezoelectric polarization effects have been found to be critical for electric and optical properties of group III-nitrides. In this dissertation, firstly, the crystal orientation dependence of the polarization is calculated and in-plane polarization is revealed. The in-plane polarization is sensitive to the lateral characteristic dimension determined by the microstructure. Specific semi-polar plane growth is suggested for reducing quantum-confined Stark effect. The macroscopic electrostatic field from the polarization discontinuity in the heterostructures is discussed, b ased on that, the band diagram of InGaN/GaN quantum well/barrier and AlGaN/GaN heterojunction is obtained from the self-consistent solution of Schrodinger and Poisson equations. New device design such as triangular quantum well with the quenched polarization field is proposed. Electron holography in the transmission electron microscopy is used to examine the electrostatic potential under polarization effects. The measured potential energy profiles of heterostructure are compared with the band simulation, and evidences of two-dimensional hole gas (2DHG) in a wurtzite AlGaN/ AlN/ GaN superlattice, as well as quasi two-dimensional electron gas (2DEG) in a zinc-blende AlGaN/GaN are found. The large polarization discontinuity of AlN/GaN is the main source of the 2DHG of wurtzite nitrides, while the impurity introduced during the growth of AlGaN layer provides the donor states that to a great extent balance the free electrons in zinc-blende nitrides. It is also found that the quasi-2DEG concentration in zinc-blende AlGaN/GaN is about one order of magnitude lower than the wurtzite AlGaN/GaN, due to the absence of polarization. Finally, the InAlN/GaN lattice-matched epitaxy, which ideally has a zero piezoelectric polarization and strong spontaneous polarization, is experimentally studied. The breakdown in compositional homogeneity is triggered by threading dislocations with a screw component propagating from the GaN underlayer, which tend to open up into V-grooves at a certain thickness of the InxAl1-xN layer. The V-grooves coalesce at 200 nm and are filled with material that exhibits a significant drop in indium content and a broad luminescence peak. The structural breakdown is due to heterogeneous nucleation and growth at the facets of the V-grooves.
Dissertation/Thesis
Ph.D. Physics 2012

III-Nitride materials, which consist of AlN, GaN, InN, and their alloys have become the cornerstone of the third generation compound semiconductor. Planar IIINitride materials are commonly grown on sapphire substrates which impose several limitations such as challenging scalability, rigid substrate, and thermal and lattice mismatch between substrate and material. Semiconductor nanowires can help circumvent this problem because of their inherent capability to relieve strain and grow threading dislocation-free without strict lattice matching requirements, enabling growth on unconventional substrates. This thesis aims to investigate the microscopic characteristics of the nanowires and expand on the possibility of using transparent amorphous substrate for III-nitride nanowire devices. In this work, we performed material growth, characterization, and device fabrication of III-nitride nanowires grown using molecular beam epitaxy on unconventional substrates. We first studied the structural imperfections within quantum-disks-in-nanowire structure grown on silicon and discovered how growth condition could affect the macroscopic photoluminescence behavior of nanowires ensemble. To expand our work on unconventional substrates, we also used an amorphous silica-based substrate as a more economical substrate for our nanowire growth. One of the limitations of growing nanowires on an insulating substrate is the added fabrication complexity required to fabricate a working device. Therefore, we attempted to overcome this limitation by 5 investigating various possible GaN nanowire nucleation layers, which exhibits both transparency and conductivity. We employed various nucleation layers, including a thin TiN/Ti layer, indium tin oxide (ITO), and Ti3C2 MXene. The structural, electrical, and optical characterizations of nanowires grown on different nucleation layers are discussed. From our work, we have established several key processes for transparent nanowire device applications. A nanowire LED emitting at ∼590 nm utilizing TiN/Ti interlayer is presented. We have also established the growth process for n-doped GaN nanowires grown on ITO and Ti3C2 MXene with transmittance above 40 % in the visible wavelength, which is useful for practical applications. This work paves the way for future devices utilizing low-cost substrates, enabling further cost reduction in III-nitride device fabrication.