Gallium Nitride (GaN) technology is capable of very high power levels but suffers from high non-linearity. With the advent of 5G technologies, high linearity is in greater demand due to complex modulation schemes and crowded RF (Radio Frequency) spectrum. Because of the non-linearity issue, GaN power amplifiers have to be operated at back-off input power levels. Operating at back-off reduces the efficiency of the power amplifier along-with the output power. This research presents a technique to linearize GaN amplifiers. The linearity can be improved by splitting a large device into multiple smaller devices and biasing them individually. This leads to the cancellation of the IMD3 (Third-order Intermodulation Distortion) components at the output of the FETs and hence higher linearity performance. This technique has been demonstrated in Silicon technology but has not been previously implemented in GaN. This research work presents for the first time the implementation of this technique in GaN Technology. By the application of this technique, improvement in IMD3 of 4 dBc has been shown for a 0.8-1.0 GHz PA (Power Amplifier), and 9.5 dBm in OIP3 (Third-order Intercept Point) for an S-Band GaN LNA, with linearity FOM (IP3/DC power) reaching up to 20. Large-signal simulation and analysis have been done to demonstrate linearity improvement for two parallel and four parallel FETs. A simulation methodology has been discussed in detail using commercial CAD software. A power sampler element is used to compute the IMD3 currents coming out of various FETs due to various bias currents. Simulation results show by biasing one device in Class AB and others in deep Class AB, IMD3 components of parallel FETs can be made out of phase of each other, leading to cancellation and improvement in linearity. Improvement up to 20 dBc in IMD3 has been reported through large-signal simulation when four parallel FETs with optimum bias were used. This technique has also been demonstrated in simulation for an X-Band MMIC PA from 8-10 GHz in GaN technology. Improvements up to 25-30 dBc were shown using the technique of biasing one device with Class AB and other with deep class AB/class B. The proposed amplifier achieves broadband linearization over the entire frequency compared to state-of-the-art PA's. The linearization technique demonstrated is simple, straight forward, and low cost to implement. No additional circuitry is needed. This technique finds its application in high dynamic range RF amplifier circuits for communications and sensing applications. / Doctor of Philosophy / Power amplifiers (PAs) and Low Noise Amplifiers (LNAs) form the front end of the Radio Frequency (RF) transceiver systems. With the advent of complex modulation schemes, it is becoming imperative to improve their linearity. Through this dissertation, we propose a technique for improving the linearity of amplifier circuits used for communication systems. Meanwhile, Gallium Nitride (GaN) is becoming a technology of choice for high-power amplifier circuits due to its higher power handling capability and higher breakdown voltage compared with Gallium Arsenide (GaAs), Silicon Germanium (SiGe) and Complementary Metal-Oxide-Semiconductor (CMOS) technologies. A circuit design technique of using multiple parallel GaN FETs is presented. In this technique, the multiple parallel FETs have independently controllable gate voltages. Compared to a large single FET, using multiple FETs and biasing them individually helps to improve the linearity through the cancellation of nonlinear distortion components. Experimental results show the highest linearity improvement compared with the other state-of-the-art linearization schemes. The technique demonstrated is the first time implementation in GaN technology. The technique is a simple and cost-effective solution for improving the linearity of the amplifier circuits. Applications include base station amplifiers, mobile handsets, radars, satellite communication, etc.

The wide-bandgap (WBG) devices, like gallium nitride (GaN) and silicon carbide (SiC) devices have proven to be a driving force of the development of the power conversion technology. Thanks to their distinct advantages over silicon (Si) devices including the faster switching speed and lower switching losses, WBG-based power converter can adopt a higher switching frequency and pursue higher power density and higher efficiency. As a trade-off of the advantages, there also exist the high-frequency-oriented challenges in the adoption of the GaN HEMT under research, including narrow safe gate operating area, increased switching overshoot, increased electromagnetic interference (EMI) in the gate loop and the power stages, the lack of the modules of packages for high current application, high gate oscillation under parallel operation. The dissertation is developed to addressed the all the challenges above to fully explore the potential of the GaN HEMTs. Due to the increased EMI emission in the gate loop, a small isolated capacitor in the gate driver power supply is needed to build a high-impedance barrier in the loop to protect the gate driver from interference. A 2 W dual-output gate driver power supply with ultra-low isolation capacitor for 650 V GaN-based half bridge is presented, featuring a PCB-embedded transformer substrate, achieving 85% efficiency, 1.6 pF isolation capacitor with 72 W/in3 power density. The effectiveness of the EMI reduction using the proposed power supply is demonstrated. The design consideration to build a compact 650 V GaN switching cell is presented then to address the challenges in the PCB layout and the thermal management. With the switching cell, a compact 1 kW 400 Vdc three-phase inverter is built and can operate with 500 kHz switching frequency. With the inverter, the high switching frequency effects on the inverter efficiency, volume, EMI emission and filter design are assessed to demonstrate the tradeoff of the adoption of high switching frequency in the motor drive application. In order to reduce the inverter CM EMI emission above 10 MHz, an active gate driver for 650 V GaN HEMT is proposed to control the dv/dt during turn-on and turn-off independently. With the control strategy, the penalty from the switching loss can be reduced. To build a high current power converter, paralleling devices is a normal approach. The dissertation comes up with the switching cell design using paralleled two and four 650 V GaN HEMTs with minimized and symmetric gate and power loop. The commutation between the paralleled HEMTs is analyzed, based on which the effects from the passive components on the gate oscillation are quantified. With the switching cell using paralleled GaN HEMTs, a 10 kW LLC resonant converter with the integrated litz-wire transformer is designed, achieving 97.9 % efficiency and 131 W/in3 power density. The design consideration to build the novel litz-wire transformer operated at 400 kHz switching frequency is also presented. In all, this work focuses on providing effective solutions or guidelines to adopt the 650 V GaN HEMT in the high frequency, high power density, high efficiency power conversion and demonstrates the advance of the GaN HEMTs in the hard-switched and soft-switched power converters. / Ph. D. / Silicon (Si) -based power semiconductor has developed several decades and achieved numerous outstanding performances, contributing a fast development of the power electronics. While the theatrical limit of the silicon semiconductor is almost reached limiting the progress speed to purse the high-efficiency, high-density high-reliability power conversion, the new material, including gallium-nitride (GaN) and silicon-carbide (SiC), based semiconductor, becomes the driven force to retain the development. Compared with Si-based device, GaN and SiC device own a faster switching speed and a lower on-resistance, enabling the adoption of high switching frequency and the possibility to increase the efficiency, power density and dynamic response. The GaN-based semiconductor is explored to be an even promising game changer than SiC device thanks to a higher theoretical ceiling. However, to adopt GaN-based semiconductors and fully utilize its benefits with high switching frequency, there are numerous high-frequency-oriented challenges, including high frequency oscillation at device termination, increased electromagnetic interference (EMI), the lack of the modules of packages for high current application, high frequency oscillation under parallel operation. The dissertation is developed to address the key high-frequency-oriented challenges to adopt GaN-based semiconductors in the power conversion and come up with the novel design strategy and analysis for high-switching-frequency power conversion using GaN devices. To the reduce the increased EMI emission in the gate loop, a novel PCB-embedded transformer structure is proposed to maintain a low isolation capacitor in the gate driver power supply for the GaN phase leg. With the proposed technique, the dual-output gate driver power supply can achieve high efficiency (85%), ultra-low isolation capacitor (1.6 pF) with high power density (72 W/in³ ). To reduce the high frequency oscillation at the GaN device termination, the strategy to layout GaN devices and its gate driver is proposed with corresponding thermal management. A compact structure for three-phase inverter is then presented, operating with a very high switching frequency (500 kHz). Within the inverter, the high switching frequency effects on the inverter performances are assessed to demonstrate the tradeoff and bottle neck to adopt high switching frequency in the motor drive application. In order to reduce the inverter EMI emission at high frequency ( >10 MHz), an active gate driver for GaN device is proposed for the active dv/dt control strategy. To build a high current power converter, the strategy to parallel GaN devices is proposed in the dissertation with the analysis on the commutation between the paralleled GaN devices. A high-frequency high-current litz-wire transformer structure for LLC resonant converter is presented with modeling and optimization. With the technique, a 10 kW LLC resonant converter achieves high efficiency (97.9 %) and high power density (131 W/in³).

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

This master’s thesis deals with the design and realization of a functional sample power inverter for an electric compressor, which is used in hybrid cars. The electric compressor powered by the inverter is E-compressor by Garrett Advancing Motion. An inverter will be using modern High Electron Mobility Transistors which are based on gallium nitride (GaN). The purpose of this thesis is to find if GaN transistors can be used in E-boosting application.

Isolated inverter can provide galvanic isolation which is necessary for some applications with safety regulations. Traditionally, a two-stage configuration is widely applied with isolated dc-dc stage and a sinusoidal pulse-width-modulated (SPWM) dc-ac stage. However, this two-stage configuration suffers from more components count, more complex control and tend to have lower efficiency and lower power density. Meanwhile, a large dc bus capacitor is needed to attenuate the double line frequency from SPWM for two-stage configuration. Therefore, the single-stage approach including an isolated dc-rectified sine stage and a line frequency unfolder is preferable. Since the unfolder circuit is at line frequency being almost lossless, the isolated dc-rectified sine stage becomes critical. However, the relevant research for the single-stage isolated inverter is limited. People either utilize PWM based converter as dc-rectified sine stage with duty cycle adjustment or apply SRC or LLC resonant converter for better soft switching characteristics. For PWM based converter, hard switching restricts the overall inverter efficiency, while for SRC/LLC, enough wide voltage gain range and full range ZVS are the major issues. This dissertation aims to provide solutions for a high-efficiency, high-frequency resonant converter based single-stage soft-switching isolated inverter design. The LLC and LCLCL resonant converters are applied as the isolated dc-rectified sine stage with variable frequency modulation (VFM). Therefore, the rectified sine wave generation consists of many dc-dc conversion with different switching frequencies and an efficient dc-rectified sine stage design needs each dc-dc conversion to be with high efficiency. This dissertation will first propose the optimization methods for LLC converter dc-dc conversion. ZVS models are derived to ensure fully ZVS performance for primary side GaN devices. As a large part in loss breakdown, the optimization for transformer is essential. The LLC converter can achieve above 99% efficiency with proposed optimization approach. Moreover, the channel turn-off energy model is presented for a more accurate loss analysis. With all the design and optimization considerations, a MHz LLC converter based isolated inverter is designed and a hybrid modulation method is proposed, which includes full bridge (FB) VFM for output high line region and half bridge (HB) VFM for output low line region. By changing from FB to HB, the output voltage gain is reduced to half to have a wider voltage gain range. However, the total harmonic distortion (THD) of output voltage at light load will be impacted since the voltage gain will be higher with lighter load at the maximum switching frequency. A MHz LCLCL converter based isolated inverter is proposed for a better output voltage THD at light load conditions. The paralleled LC inside the LCLCL resonant tank can naturally create a zero voltage gain point at their resonant frequency, which shows superior performance for rectified sine wave generation. Besides the better THD performance, the LCLCL converter based isolated inverter also features for easier control, better ZVS performance and narrower switching frequency range. Meanwhile, the LCLCL based inverter topology has bi-directional power flow capability as well. With variable frequency modulation for ac-dc, this topology is still a single-stage solution compared to the traditional two-stage solution including PFC + LLC configuration. / Doctor of Philosophy / Inverters can convert dc voltage to ac voltage and typically people use two-stage approach with isolated dc-dc stage and dc-ac stage. However, this two-stage configuration suffers from more components count, more complex control and tend to have lower efficiency and lower power density. Therefore, the single-stage solution with dc-rectified sine wave stage and a line frequency unfolder becomes appealing. The unfolder circuit is to unfold the rectifier sine wave to an ac sine wave at the output. Since the unfolder is at line frequency and can be considered lossless, the key design is for the dc-rectified sine stage. The resonant converter featured for soft switching seems to be a good candidate. However, the inverter needs soft switching for the whole range and an enough wide voltage gain, which makes the design difficult, especially the target is high efficiency for the overall inverter. This dissertation aims to provide solutions for a high-efficiency, high-frequency resonant converter based single-stage soft-switching isolated inverter design. The LLC and LCLCL resonant converters are applied as the isolated dc-rectified sine stage with variable frequency modulation (VFM). Therefore, the rectified sine wave generation consists of many dc-dc conversion with different switching frequencies and an efficient dc-rectified sine stage design needs each dc-dc conversion to be with high efficiency. The design considerations and optimization methods for the LLC dc-dc conversion are firstly investigated. Based on these approaches, a MHz LLC converter based isolated inverter is designed with proposed hybrid modulation method. To further improve the light load performance, a MHz LCLCL converter based isolated inverter topology is proposed. The paralleled LC inside the LCLCL resonant tank can naturally create a zero voltage gain point which shows superior characteristics for rectified sine wave generation. Moreover, the LCLCL resonant converter based topology has bi-directional capability as well so it can work well for ac voltage to dc voltage conversion.

The possibility of using methylamines instead of ammonia as a nitrogen precursor for the CVD of nitrides is studied using quantum chemical computations of reaction energies: reaction electronic energy (Δ𝑟𝐸𝑒𝑙𝑒𝑐) reaction enthalpy (Δ𝑟𝐻) and reaction free energy (Δ𝑟𝐺). The reaction energies were calculated for three types of reactions: Uni- and bimolecular decomposition to more reactive nitrogen species, adduct forming with trimethylgallium (TMG) and trimethylaluminum (TMA) followed by a release of methane or ethane and surface adsorption to gallium nitride for both the unreacted ammonia or methylamines or the decomposition products. The calculations for the reaction entropy and free energy were made at both STP and CVD conditions (300°C-1300°C and 50 mbar). The ab inito Gaussian 4 (G4) theory were used for the calculations of the decomposition and adduct reactions while the surface adsorptions were calculated using the Density Functional Theory method B3LYP. From the reactions energies it can be concluded that the decomposition was facilitated by the increasing number of methyl groups on the nitrogen. The adducts with mono- and dimethylamine were more favorable than ammonia and trimethylamine. 𝑁𝐻2 was found to be most readily to adsorb to 𝐺𝑎𝑁 while the undecomposed ammonia and methylamines was not willingly to adsorb.

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

The present work focuses on the growth and characterizations of GaN and InN layers and nanostructures on p-Si(100) and p-Si(111) substrates by plasma-assisted molecular beam epitaxy and the studies of GaN/p-Si and InN/p-Si heterojunctions properties. The thesis is divided in to seven different chapters. Chapter 1 gives a brief introduction on III-nitride materials, growth systems, substrates, possible device applications and technical background. Chapter 2 deals with experimental techniques including the details of PAMBE system used in the present work and characterization tools for III-nitride epitaxial layers as well as nanostructures. Chapter 3 involves the growth of GaN films on p-Si(100) and p-Si(111) substrates. Phase pure wurtzite GaN films are grown on Si (100) substrates by introducing a silicon nitride layer followed by low temperature GaN growth as buffer layers. GaN films grown directly on Si (100) are found to be phase mixtured, containing both cubic and hexagonal modifications. The x-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy studies reveal that the significant enhancement in the structural and optical properties of GaN films grown with silicon nitride buffer layer grown at 800 oC, when compared to the samples grown in the absence of silicon nitride buffer layer and with silicon nitride buffer layer grown at 600 oC. Core-level photoelectron spectroscopy of SixNy layers reveals the sources for superior qualities of GaN epilayers grown with the high temperature substrate nitridation process. The discussion has been carried out on the typical inverted rectification behavior exhibited by n-GaN/p-Si heterojunctions. Considerable modulation in the transport mechanism is observed with the nitridation conditions. The heterojunction fabricated with the sample of substrate nitridation at high temperature exhibites superior rectifying nature with reduced trap concentrations. Lowest ideality factors (~1.5) are observed in the heterojunctions grown with high temperature substrate nitridation which is attributed to the recombination tunneling at the space charge region transport mechanism at lower voltages and at higher voltages space charge limited current conduction is the dominating transport mechanism. Whereas, thermally generated carrier tunneling and recombination tunneling are the dominating transport mechanisms in the heterojunctions grown without substrate nitridation and low temperature substrate nitridation, respectively. A brief comparison of the structural, optical and heterojunction properties of GaN grown on Si(100) and Si(111) has been carried out. Chapter 4 involves the growth and characterizations of InN nanostructures and thinfilms on p-Si(100) and p-Si(111) substrates. InN QDs are grown on Si(100) at different densities. The PL characteristics of InN QDs are studied. A deterioration process of InN QDs, caused by the oxygen incorporation into the InN lattice and formation of In2O3/InN composite structures was established from the results of TEM, XPS and PL studies. The results confirm the partial oxidation of the outer shell of the InN QDs, while the inner core of the QDs remains unoxidized. InN nanorods are grown on p-Si(100), structural characterizations are carried out by SEM, and TEM. InN nanodots are grown on p-Si(100), structural characterizations are performed. InN films were grown on Si(100) and Si(111) substrates and structural characterizations are carried out. Chapter 5 deals with the the heterojunction properties of InN/p-Si(100) and InN/p-Si(111).The transport behavior of the InN NDs/p-Si(100) diodes is studied at various bias voltages and temperatures. The temperature dependent ZB BH and ideality factors of the forward I-V data are observed, while it is governed through the modified Richardson’s plot. The difference in FB BH and C-V BH and the deviation of ideality factor from unity indicate the presence of inhomogeneities at the interface. The band offsets derived from C-V measurements are found to be Δ EC=1.8 eV and Δ EV =1.3 eV, which are in close agreement with Anderson’s model. The band offsets of InN/p-Si heterojunctions are estimated using XPS data. A type-III band alignment with a valence band offset of Δ EV =1.39 eV and conduction band offset of ΔEC=1.81 eV is identified. The charge neutrality level model provides a reasonable description of the band alignment of the InN/p-Si interface. The interface dipole deduced by comparison with the electron affinity model is 0.06 eV. The transport studies of InN NR/p-Si(100) heterojunctions have been carried out by conductive atomic force microscopy (CAFM) as well as conventional large area contacts. Discussion of the electrical properties has been carried out based on local current-voltage (I-V) curves, as well as on the 2D conductance maps. The comparative studies on transport properties of diodes fabricated with InN NRs and NDs grown on p-Si(100) substrates and InN thin films grown on p-Si(111) substrates have also been carried out. Chapter 6 deals with the growth and characterizations of InN/GaN heterostructures on p-Si(100) and p-Si(111) substarets and also on the InN/GaN/p-Si heterojunction properties. The X-ray diffraction (XRD), scanning electron microscopy (SEM) studies reveal a considerable variation in crystalline quality of InN with grown parameters. Deterioration in the rectifying nature is observed in the case of InN/GaN/p-Si(100) heterojunction substrate when compared to InN/GaN/p-Si (111) due to the defect mediated tunneling effect, caused by the high defect concentration in the GaN and InN films grown on Si(100) and also due to the trap centers exist in the interfaces. Reduction in ideality factor is also observed in the case of n-InN/n-GaN/p–Si(111) when compared to n-InN/n-GaN/p–Si(100) heterojunction. The sum of the ideality factors of individual diodes is consistent with experimentally observed high ideality factors of n-InN/n-GaN/p–Si double heterojunctions due to double rectifying heterojunctions and metal semiconductor junctions. Variation of effective barrier heights and ideality factors with temperature are confirmed, which indicate the inhomogeneity in barrier height, might be due to various types of defects present at the GaN/Si and InN/GaN interfaces. The dependence of forward currents on both the voltage and temperatures are explained by multi step tunneling model and the activation energis were estimated to be 25meV and 100meV for n-InN/n-GaN/p–Si(100) and n-InN/n-GaN/p–Si(111) heterojunctions, respectively. Chapter 7 gives the summary of the present study and also discusses about future research directions in this area.

The objective of this master thesis was to provide the optimization of an ion-atom beam source for the improvement of its properties. The improvement of the parameters increases the efficiency of the source during the deposition of gallium nitride ultrathin films (GaN) being important in microeletronics and optoelectronics. After optimization, the depositions of GaN ultrathin films on Si(111) 7x7 at lower temperatures (

In this thesis I present a study of nanowires, and, in particular, I apply EBIC microscopy for investigation of their electro-optical properties. First, I describe details of the EBIC analytical technique together with a brief historical overview of the electron microscopy, the physical principles of the EBIC, its space resolution, parameters defining the signal amplitude, and the information we can acquire concerning defects, electric fields, etc. Then I focus on the characterization of LEDs based on GaN nanowires, which were analyzed in a cross-section and in a top view configurations. The EBIC measurements were correlated with micro-electroluminescence mapping. Further, I address the fabrication and measurement of nanowire-based InGaN/GaN LEDs detached from their original substrate. I present the EBIC measurements of individual nanowires either cut from their substrate and contacted in a planar geometry or kept standing on supphire substrate and cleaved to reveal the horizontal cross-section.The next part of this thesis is dedicated to an EBIC study of irregular Si nanowire array-based solar cells, and then of the regular nanowire array devices. The current generation was analyzed on a submicrometer scale. Finally, I discuss the fabrication and EBIC measurements of GaN nanowires grown on Si substrate. In particular, I show that the p-n junction was induced in the Si substrate by Al atom diffusion during the nanowire growth. / Dans cette thèse je me propose d’étudier des nano-fils, et en particulier d’utiliser la technique EBIC pour explorer leurs propriétés électro-optiques. Je décris d’abord les détails de la technique d’analyse EBIC avec un bref retour historique sur la microscopie électronique, le principe physique de l’EBIC, sa résolution spatiale, les paramètres conditionnant l’amplitude du signal, et les informations que l’on peut en tirer sur le matériau en termes de défauts, champ électrique, etc. Je m’intéresse ensuite à la caractérisation de LEDs à nano-fils à base de GaN, qui ont été observés par EBIC, soit en coupe soit en vue plane (depuis le haut des fils). Les mesures EBIC sont comparées à celles de micro-électroluminescence. Plus loin j’adresse la fabrication et la mesure de nano-fils à base de GaN séparés de leur substrat d’origine. Je présente les mesures EBIC de nano-fils uniques entiers, puis de nano-fils en coupe horizontale.La partie suivante de la thèse traite d’étude EBIC des cellules solaires à base de nano-fils Si ayant d’abord une géométrie aléatoire, puis une géométrie régulière. La génération de courant dans ces cellules solaires est analysée à l’échelle submicronique. A la fin du manuscrit je discute la fabrication et les mesures EBIC de fils GaN épitaxiés sur Si. Je montre en particulier qu’une jonction p-n est enduite dans le substrat Si par la diffusion d’Al lors de la croissance de nanofils.