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 popularity of III-nitride materials has taken up the semiconductor industry to newer applications because of their remarkable properties. In addition to having a direct and wide band gap of 3.4 eV, a very fascinating property of GaN is the band gap tuning from 0.7 to 6.2 eV by alloying with Al or In. The most common orientation to grow optoelectronic devices out of these materials are the polar c-plane which are strongly affected by the intrinsic spontaneous and piezoelectric polarization fields. Devices grown in no polar orientation such as (1 0 –1 0) m-plane or (1 1 –2 0) a-plane have no polarization in the growth direction and are receiving a lot of focus due to enhanced behaviour. The first part of this thesis deals with the development of non-polar epimGaN films of usable quality, on an m-plane sapphire by plasma assisted molecular beam epitaxy. Growth conditions such as growth temperature and Ga/N flux ratio were tuned to obtain a reasonably good crystalline quality film. MSM photodetectors were fabricated from (1 0 -1 0) m-GaN, (1 1 -2 0) a-GaN and semipolar (1 1 -2 2) GaN films and were compared with the polar (0 0 0 2) c-GaN epilayer. Later part of the thesis investigated (1 0 -1 0) InN/ (1 0 -1 0) GaN heterostructures. Further, we could successfully grow single composition nonpolar a-plane InxGa1-xN epilayers on (1 1 -2 0) GaN / (1 -1 0 2) sapphire substrate. This thesis focuses on the growth and characterisation of nonpolar GaN, InxGa1-xN and InN by plasma assisted molecular beam epitaxy and on their photodetection potential. Chapter 1 explains the motivation of this thesis work with an introduction to the III-nitride material and the choice of the substrate made. Polarization effect in the polar, nonpolar and semipolar oriented growth is discussed. Fabrication of semiconductor photodetectors and its principle is explained in details. Chapter 2 discusses the various experimental tools used for the growth and characterisation of the film. Molecular beam epitaxy technique is elaborately explained along with details of the calibration for the BEP of various effusion cells along with growth temperature at the substrate. Chapter 3 discusses the consequence of nitridation on bare m-sapphire substrate. Impact of nitridation step prior to the growth of GaN film over (1 0 -1 0) m-sapphire substrate was also studied. The films grown on the nitridated surface resulted in a nonpolar (1 0 -1 0) orientation while without nitridation caused a semipolar (1 1 -2 2) orientation. Room temperature photoluminescence study showed that nonpolar GaN films have higher value of compressive strain as compared to semipolar GaN films, which was further confirmed by room temperature Raman spectroscopy. The room temperature UV photodetection of both films was investigated by measuring the I-V characteristics under UV light illumination. UV photodetectors fabricated on nonpolar GaN showed better characteristics, including higher external quantum efficiency, compared to photodetectors fabricated on semipolar GaN. Chapter 4 focuses on the optimization and characterisation of nonpolar (1 0 -1 0) m-GaN on m-sapphire by molecular beam epitaxy. A brief introduction to the challenges in growing a pure single phase nonpolar (1 0 -1 0) GaN on (1 0 -1 0) sapphire without any other semipolar GaN growth is followed by our results achieving the same. Effect of the growth temperature and Ga/N ratio on the structural and optical properties of m-GaN epilayers was studied and the best condition was obtained for the growth temperature of 7600C and nitrogen flow of 1 sccm. Strain in the film was quantitatively measured using Raman spectroscopy and qualitatively analyzed by RSM. Au/ nonpolar GaN schottky diode was fabricated and temperature dependent I-V characteristics showed rectifying nature. Chapter 5 demonstrates the growth of (1 0 -1 0) m-InN / (1 0 -1 0) m-GaN / (1 0 -1 0) m-sapphire substrate. Nonpolar InN layer was grown at growth temperature ranging from 3900C to 440C to obtain a good quality film at 4000C. An in-plane relationship was established for the hetrostructures using phi-scan and a perfect alignment was found for the epilayers. RSM images on the asymmetric plane revealed highly strained layers. InN band gap was found to be around 0.8 eV from absorption spectra. The valance band offset value is calculated to be 0.93 eV for nonpolar m-plane InN/GaN heterojunctions. The heterojunctions form in the type-I straddling configuration with a conduction band offsets of 1.82 eV. Chapter 6 focuses on the optimization of nonpolar (1 1 -2 0) a-GaN on (1 -1 0 2) r-sapphire by molecular beam epitaxy. Effect of the growth temperature and Ga/N ratio on the structural and optical properties of a-GaN epilayers was studied and the best condition was obtained for the growth temperature of 7600C and nitrogen flow of 1 sccm. An in-plane orientation relationship is found to be [0 0 0 1] GaN || [-1 1 0 1] sapphire and [-1 1 0 0] GaN || [1 1 -2 0] sapphire for nonpolar GaN on r-sapphire substrate. Strain in the film was quantitatively measured using Raman spectroscopy and qualitatively analyzed by RSM. UV photo response of a-GaN film was measured after fabricating an MSM structure over the film with Au. EQE of the photodetectors fabricated in the (0 0 0 2) polar and (1 1 -2 0) nonpolar growth directions were compared in terms of responsively, nonpolar a-GaN showed the best sensitivity at the cost of comparatively slow response time. Chapter 7 demonstrates the growth of non-polar (1 1 -2 0) a-plane InGaN epilayers on a-plane (1 1 -2 0) GaN/ (1 -1 0 2) r-plane sapphire substrate using PAMBE. The high resolution X-ray diffraction (HRXRD) studies confirmed the orientation of the films and the compositions to be In0.19Ga0.81N, In0.21Ga0.79N and In0.23Ga0.77N. The compositions of the films were controlled by the growth parameters such as growth temperature and indium flux. Effect of variation of Indium composition on the strain of the epilayers was analyzed from the asymmetric RSM images. Further, we report the growth of self-assembled non-polar high indium clusters of In0.55Ga0.45N over non-polar (1 1 -2 0) a-plane In0.17Ga0.83N epilayer grown on a-plane (1 1 -2 0) GaN / (1 -1 0 2) r-plane sapphire substrate. The structure hence grown when investigated for photo-detecting properties, showed sensitivity to both infrared and ultraviolet radiations due to the different composition of InGaN region. Chapter 8 concludes with the summary of present investigations and the scope for future work.

<p dir="ltr">Rapid evolution in wireless technology and the increasing demand for high bandwidth communication for 5G/6G and the Internet of Things (IoT) have necessitated a growing number of components in radio front-end modules in an increasingly overcrowded radio frequency (RF) spectrum. Low-cost ad-hoc radios have drawn consumer interest, enabling new devices like microelectromechanical (MEMS) resonators for on-chip clocking, frequency-selective filters, RF signal processing, and spectral sensing for their small footprint and low power consumption. Gallium nitride (GaN) is an attractive electromechanical material due to its high coupling coefficient, acoustic velocity, and low viscoelastic losses. These benefits enable high-Q MEMS resonators in GaN monolithic microwave integrated circuits (MMICs) with scaling capability up to mm-wave frequencies, making this technology platform a contender for high-performance programmable radios in RF/mm-wave, sensors for harsh environments, and information processing in quantum systems.</p><p dir="ltr">The bias-dependent control mechanism of the 2D electron gas (2DEG) in GaN heterostructures can be exploited to design different switchable and tunable devices for reconfigurable MEMS components. This work presents, for the first time, a comprehensive study of the electromechanical performances of different transduction mechanisms in switchable GaN MEMS resonators. A unique OFF-state shunt design, where the 2DEG in an AlN/GaN heterostructure is utilized to control electromechanical transduction in Lamb mode resonators, is also experimentally demonstrated in this work. To make a valid comparison among switchable transducers, equivalent circuit models are developed to extract key parameters from the measurements by fitting them in both ON and OFF states. The switchable transducer with Ohmic interdigitated transducers (IDTs) and Schottky control gate shows superior performance among the designs under consideration with complete suppression of the mechanical mode in the OFF state and a maximum frequency-quality factor product of 5x10¹²s^-1 and a figure-of-merit of 5.18 at 1GHz in the ON state.</p><p dir="ltr">Over the past few years, there have been numerous efforts to scale the frequencies of MEMS devices in the GaN platform towards mm-wave frequencies. However, challenges remain due to the multi-layer thick buffer, typical in the growth of GaN epilayer on a substrate. This work presents the investigation of SweGaN QuanFINEbuffer-free and ultrathin GaN-on-SiC for the performance of surface acoustic wave (SAW) devices beyond 10GHz. Finite element analysis (FEA) is performed to find the range of frequencies for the Sezawa mode in the structure. Transmission lines and resonators are designed, fabricated, and characterized. Modified Mason circuit models are developed for each class of devices to extract critical performance metrics and benchmark with the state-of-the-art and theoretical limits for GaN. Sezawa modes are observed at frequencies up to 14.3GHz, achieving a record high in GaN MEMS to the best of our knowledge. A maximum piezoelectric coupling of 0.61% and frequency-quality factor product of 6x10¹²s^-1 are achieved for Sezawa resonators at 11GHz, with a minimum propagation loss of 0.26dB/λ for the two-port devices. The devices also exhibit high linearity with input third-order intercept points (IIP3) of 65dBm at 9GHz.</p><p dir="ltr">This work also investigates tunable acoustoelectric (AE) devices in the QuanFINE platform, leveraging its inherent 2DEG in the AlGaN/GaN heterostructure. Using 9.7GHz Sezawa mode acoustic delay lines, we report the highest frequency of AE in GaN to date. Active and passive AE devices are designed for voltage-dependent non-reciprocity and propagation loss without modification to the standard process for the High Electron Mobility Transistors (HEMTs) in MMICs. Drain/source Ohmic contacts control the drift velocity of the 2DEG, and the Schottky gate modulates 2DEG carrier concentration, resulting in a 30dB/cm separation between forward and reverse acoustic waves for a 2.56kV/cm lateral DC electric field and a maximum change in propagation loss of 50dB/cm for -5V DC at the control gate, respectively. The QuanFINEtechnology with AlGaN/GaN heterostructure enables a platform for switchable MEMS resonators and tunable acoustoelectric devices in MMICs for reconfigurable front end approaching mm-wave frequencies.</p>

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.