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Self-aligned GaN nanorods of various densities are grown on an r-plane Al2O3 substrate with Stranski–Krastanov or layer-plus-island growth conditions by using a plasma-assisted molecular beam epitaxy system. These conditions result in the formation of a GaN nanorod matrix on an epitaxial GaN thin film. The orientation of the nanorods was found to be at an inclination of ∼60° from the substrate. As expected, the GaN thin film grows along the [11–20] direction, but interestingly the nanorods have a preferential growth direction along the [0002] axis. The overall structure mimics the Gaussian distribution of Schottky barriers at the metal–semiconductor interface. The GaN nanorod/thin-film matrix systematically causes the well-known Au/GaN Schottky metal–semiconductor interface to display an Ohmic type of behavior. A systematic reduction of the Schottky barrier is observed with an increase in the GaN nanorod density (from 5 to 65 nanorods micron−2). The overall configuration provides a tunable Gaussian distribution of Schottky barriers with nanorod density, which could be extremely useful for replacing conventional multi-level electrode stacking techniques. [ABSTRACT FROM AUTHOR]

Phase pure wurtzite GaN films were 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) were found to be phase mixtured, containing both cubic (β) and hexagonal (α) modifications. The x-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy studies reveal that the significant enhancement in the structural as well as in the optical properties of GaN films grown with silicon nitride buffer layer grown at 800 °C when compared to the samples grown in the absence of silicon nitride buffer layer and with silicon nitride buffer layer grown at 600 °C. 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 was observed with the nitridation conditions. The heterojunction fabricated with the sample of substrate nitridation at high temperature exhibited superior rectifying nature with reduced trap concentrations. Lowest ideality factors (∼1.5) were 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. [ABSTRACT FROM AUTHOR]

GaN films were grown on c-plane sapphire by plasma-assisted molecular beam epitaxy (PAMBE). The effect of N/Ga flux ratio on structural, morphological, and optical properties was studied. The dislocation density found to increase with increasing the N/Ga ratio. The surface morphology of the films as seen by scanning electron microscopy shows pits on the surface and found that the pit density on the surface increases with N/Ga ratio. The room temperature photoluminescence study reveals the shift in band-edge emission toward the lower energy with increase in N/Ga ratio. This is believed to arise from the reduction in compressive stress in the films as is evidenced by room temperature Raman study. The transport studied on the Pt/GaN Schottky diodes showed a significant increase in leakage current with an increase in N/Ga ratio and was found to be caused by the increase in pit density as well as increase in dislocation density in the GaN films. [ABSTRACT FROM AUTHOR]

Recently, various high quality factor photonic crystal microcavities have been demonstrated theoretically and experimentally with only one-dimensional periodicity. However, in most cases high-index materials such as silicon were chosen for easily achievable large photonic bandgap and elaborate refractive index modulation or taper structure is required for reducing radiation loss. Here, we present a design of high-Q microcavities in one-dimensional multilayer polystyrene photonic crystal slab structures with a low-index contrast of 1.59:1. Microcavities are introduced by simply decreasing the thickness of layers at the center region to form a double-heterostructure. A resonant mode with a quality factor up to 20 000 is obtained and found to originate from the modal gap confinement by comparing with a Fabry-Perot cavity. The dependence of the maximal quality factor on the cavity length further reveals that the small group velocity of light within the heterostructure cavity contributes significantly to the high-Q. In terms of the high quality factor, ease of fabrication, and large Kerr nonlinearity of polystyrene, our double-heterostructure microcavities will find potential application in realizing all-optical modulation devices. [ABSTRACT FROM AUTHOR]

Studies on the optical properties of InGaN alloy of relatively higher indium content are of potential interest to understand the effect of indium content on the optical band gap of epitaxial InGaN. We report the growth of self assembled non-polar high indium clusters of In0.55Ga0.45N over non-polar (11-20) a-plane In0.17Ga0.83N epilayer grown on a-plane (11-20)GaN/(1-102) r-plane sapphire substrate using plasma assisted molecular beam epitaxy (PAMBE). Such structures are potential candidates for high brightness LEDs emitting in longer wavelengths. The high resolution X-ray diffraction studies revealed the formation of two distinct compositions of InxGa1-xN alloys, which were further confirmed by photoluminescence studies. A possible mechanism for the formation of such structure was postulated which was supported with the results obtained by energy dispersive X-ray analysis. 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. [ABSTRACT FROM AUTHOR]

Nonpolar a-plane InN films were grown on r-plane sapphire substrate by plasma assisted molecular beam epitaxy with GaN underlayer. Effect of growth temperature on structural, morphological, and optical properties has been studied. The growth of nonpolar a-plane (1 1 -2 0) orientation was confirmed by high resolution X-ray diffraction study. The film grown at 500 °C shows better crystallinity with the rocking curve FWHM 0.67° and 0.85° along [0 0 0 1] and [1 -1 0 0] directions, respectively. Scanning electron micrograph shows formation of Indium droplets at higher growth temperature. Room tem-perature absorption spectra show growth temperature dependent band gap variation from 0.74-0.81 eV, consistent with the expected Burstein-Moss effect. The rectifying behaviour of the I-V curve indicates the existence of Schottky barrier at the InN and GaN interface. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

This report focuses on the structural and optical properties of the GaN films grown on p-Si (100) substrates along with photovoltaic characteristics of GaN/p-Si heterojunctions fabricated with substrate nitridation and in absence of substrate nitridation. The high resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), Raman and photoluminescence (PL) spectroscopic studies reveal that the significant enhancement in the structural as well as in the optical properties of GaN epifilms grown with silicon nitride buffer layer when compared with the sample grown without silicon nitride buffer layer. The low temperature PL shows a free excitonic (FX) emission peak at 3.51 eV at the temperature of 5 K with a very narrow line width of 35 meV. Temperature dependent PL spectra follow the Varshni equation well and peak energy blue shifts by ~63 meV from 300 to 5 K. Raman data confirms the strain free nature and reasonably good crystallinity of the films. The GaN/p-Si heterojunctions fabricated without substrate nitridation show a superior photovoltaic performance compared to the devices fabricated in presence of substrate nitridation. The discussions have been carried out on the junction properties. Such single junction devices exhibit a promising fill factor and conversion efficiency of 23.36 and 0.12 %, respectively, under concentrated AM1.5 illumination. [ABSTRACT FROM AUTHOR]

Abstract: We present the study involving the dependence of carrier concentration of InN films, grown on GaN templates using the plasma assisted molecular beam epitaxy system, on growth temperature. The influence of InN carrier concentration on the electrical transport behavior of InN/GaN heterostructure based Schottky junctions is also discussed. The optical absorption edge of InN film was found to be strongly dependent on carrier concentration, and was described by Kane''s k.p model, with non-parabolic dispersion relation for carrier in the conduction band. The position of the Fermi-level in InN films was modulated by the carrier concentration in the InN films. The barrier height of the heterojunctions as estimated from I–V characteristic was also found to be dependent on the carrier concentration of InN. [Copyright &y& Elsevier]

Abstract: We present an extensive study on the structural, electrical and optical properties of InN thin films grown on c-Al2O3, GaN(130nm)/Al2O3, GaN(200nm)/Al2O3 and GaN(4μm)/Al2O3 by using plasma-assisted molecular beam epitaxy. The high resolution X-ray diffraction study reveals better crystalline quality for the film grown on GaN(4μm)/Al2O3 as compared to others. 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. Kane''s k.p model was used to describe the dependence of optical absorption edge of InN films on carrier concentration by considering the non-parabolic dispersion relation for carrier in the conduction band. Room temperature Raman spectra for the InN films grown on GaN show the signature of residual tensile stress in contrast to the compressive stress observed for the films grown directly on c-Al2O3. [Copyright &y& Elsevier]

Abstract: Ultra thin films of pure β-Si3N4 (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using β-Si3N4 layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/β-Si3N4/Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the β-Si3N4 / Si interface was determined by valence-band photoelectron spectra to be 1.84eV. The valence band of GaN is found to be 0.41±0.05eV below that of β-Si3N4 and a type-II heterojunction. The conduction band offset was deduced to be ~2.36eV, and a change of the interface dipole of 1.29eV was observed for GaN/β-Si3N4 interface formation. [Copyright &y& Elsevier]

Abstract: The electrical transport properties of InN/GaN heterostructure based Schottky junctions were studied over a wide temperature range of 200–500 K. The barrier height and the ideality factor were calculated from current–voltage (–) characteristics based on thermionic emission (TE), and found to be temperature dependent. The barrier height was found to increase and the ideality factor to decrease with increasing temperature. The observed temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. Such inhomogeneous behavior was modeled by assuming the existence of a Gaussian distribution of barrier heights at the heterostructure interface. [Copyright &y& Elsevier]

We report the structural and optical properties of a-plane GaN film grown on r-plane sapphire substrate by plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction was used to determine the out-of-plane and in-plane epitaxial relation of a-plane GaN to r-plane sapphire. Low-temperature photoluminescence emission was found to be dominated by basal stacking faults along with near-band emission. Raman spectroscopy shows that the a-GaN film is of reasonably good quality and compressively strained. [Copyright &y& Elsevier]

Abstract: High-quality GaN epilayers were grown on Si (111) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth. [ABSTRACT FROM AUTHOR]

Abstract: The removal of native oxide from Si (111) surfaces was investigated by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectra (SIMS) depth profiles. Two different oxide removal methods, performed under ultrahigh-vacuum (UHV) conditions, were carried out and compared. The first cleaning method is thermal desorption of oxide at 900°C. The second method is the deposition of metallic gallium followed by redesorption. A significant decrease in oxygen was achieved by thermal desorption at 900°C under UHV conditions. By applying a subsequent Ga deposition/redesorption, a further reduction in oxygen could be achieved. We examine the merits of an alternative oxide desorption method via conversion of the stable SiO2 surface oxide into a volatile Ga2O oxide by a supply of Ga metals. Furthermore, ultra thin films of pure silicon nitride buffer layer were grown on a Si (111) surface by exposing the surface to radio-frequency (RF) nitrogen plasma followed by GaN growth. The SIMS depth profile shows that the oxygen impurity can be reduced at GaN/β-Si3N4/Si interfaces by applying a subsequent Ga deposition/redesorption. [Copyright &y& Elsevier]

Abstract: Photoluminescence (PL) of high quality GaN epitaxial layer grown on β-Si3N4/Si (111) substrate using nitridation–annealing–nitridation method by plasma-assisted molecular beam epitaxy (PA-MBE) was investigated in the range of 5–300K. Crystallinity of GaN epilayers was evaluated by high resolution X-ray diffraction (HRXRD) and surface morphology by Atomic Force Microscopy (AFM) and high resolution scanning electron microscopy (HRSEM). The temperature-dependent photoluminescence spectra showed an anomalous behaviour with an ‘S-like’ shape of free exciton (FX) emission peaks. Distant shallow donor–acceptor pair (DAP) line peak at approximately 3.285eV was also observed at 5K, followed by LO replica sidebands separated by 91meV. The activation energy of the free exciton for GaN epilayers was also evaluated to be ∼27.8±0.7meV from the temperature-dependent PL studies. Low carrier concentrations were observed ∼4.5±2×1017 cm−3 by measurements and it indicates the silicon nitride layer, which not only acts as a growth buffer layer, but also effectively prevents Si diffusion from the substrate to GaN epilayers. The absence of yellow band emission at around 2.2eV signifies the high quality of film. The tensile stress in GaN film calculated by the thermal stress model agrees very well with that derived from Raman spectroscopy. [Copyright &y& Elsevier]

Abstract: Non-polar a-plane GaN films were grown on an r-plane sapphire substrate by plasma assisted molecular beam epitaxy (PAMBE). The effect of growth temperature on structural, morphological and optical properties has been studied. The growth of non-polar a-plane (11−20) orientation of the GaN epilayers were confirmed by high resolution X-ray diffraction (HRXRD) study. The X-ray rocking curve (XRC) full width at half maximum of the (11−20) reflection shows in-plane anisotropic behavior and found to decrease with increase in growth temperature. The atomic force micrograph (AFM) shows island-like growth for the film grown at a lower temperature. Surface roughness has been decreased with increase in growth temperature. Room temperature photoluminescence shows near band edge emission at 3.434–3.442eV. The film grown at 800°C shows emission at 2.2eV, which is attributed to yellow luminescence along with near band edge emission. [ABSTRACT FROM AUTHOR]

Abstract: Ultra thin films of pure silicon nitride were grown on a Si (111) surface by exposing the surface to radio-frequency (RF) nitrogen plasma with a high content of nitrogen atoms. The effect of annealing of silicon nitride surface was investigated with core-level photoelectron spectroscopy. The Si 2p photoelectron spectra reveals a characteristic series of components for the Si species, not only in stoichiometric Si3N4 (Si4+) but also in the intermediate nitridation states with one (Si1+) or three (Si3+) nitrogen nearest neighbors. The Si 2p core-level shifts for the Si1+, Si3+, and Si4+ components are determined to be 0.64, 2.20, and 3.05eV, respectively. In annealed sample it has been observed that the Si4+ component in the Si 2p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and photoluminescence (PL) studies showed a significant improvement of the crystalline qualities and enhancement of the optical properties of GaN grown on the stoichiometric Si3N4 by molecular beam epitaxy (MBE). [Copyright &y& Elsevier]

Plasma-assisted molecular beam epitaxy growth of (10–10) m -InN/(10–10) m -GaN was carried out on bare (10–10) m -sapphire substrate. The high resolution X-ray diffraction studies confirmed the orientation of the as-grown films. Nonpolar InN layer was grown at different growth temperatures ranging from 390 °C to 440 °C and the FWHM of rocking curve revealed good quality film at low temperatures. An in-plane relationship was established for the hetrostructures using phi-scan and a perfect alignment was found for the epilayers. Change of morphology of the films grown at different temperatures was observed using an atomic force microscopy technique showing the smoothest film grown at 400 °C. InN optical band gap was found to be vary from 0.79–0.83 eV from absorption spectra. The blue-shift of absorption edge was found to be induced by excess background electron concentration. [ABSTRACT FROM AUTHOR]

Abstract: Negative differential capacitance (NDC) has been observed in n-GaN/p-Si heterojunctions grown by plasma assisted molecular beam epitaxy (PAMBE). The NDC is observed at low frequencies 1 and 10 kilohertz (kHz) and disappeared at a higher testing frequency of 100 kHz. The NDC is also studied with temperature and found that it has disappeared above 323 ∘C. Current–Voltage characteristics of n-GaN /p-Si heterojunction were measured at different temperatures and are attributed to the space-charge-limited current (SCLC). A simple model involving two quantum states is proposed to explain the observed NDC behavior. [Copyright &y& Elsevier]