Narrow Results

III-nitride materials have attracted wide interests for optoelectronic and electronic applications due to their unique and tuneable semiconducting and optical properties. Gallium nitride (GaN) exhibits wide bandgap of 3.42eV, high thermal and chemical stability, with a strong potential to act as an electron selective contact (i.e., emitter) in GaN/Si heterojunction solar cells. However, despite all the advantages, to date, there is no published research that has which utilized GaN as an emitter in the GaN/Si heterojunction solar cells. In this work, SCAPS-1D simulation is utilized to investigate the electrical properties of GaN emitter on 150$\mu$m-thick monocrystalline silicon (mono c-Si) absorber layer in GaN/Si heterojunction solar cell architecture. GaN emitter with the thickness of 30–120nm are studied and the effects toward open-circuit voltage ($V_{\mathrm{\text{OC}}}$), short-circuit current density ($J_{\mathrm{\text{SC}}})$, fill factor (FF), power conversion efficiency (PCE) and external quantum efficiency (EQE) of the solar cell are analyzed. The optimum thickness of the GaN emitter is found to be 60nm with $J_{\mathrm{\text{SC}}}$ of 37.53mA/cm², $V_{\mathrm{\text{OC}}}$ of 628.73mV and PCE of 19.43%. The findings show the potential of the GaN/Si heterojunction solar cell for the future of the PV industry, especially for applications in harsh and extreme conditions.

We analyze steady-state and transient electron transport in the group III-nitride materials at high and ultra-high electric fields for different electron concentration regimes. At high electron concentrations where the electron distribution function assumes a shifted Maxwellian, we investigate different time-dependent transient transport regimes through the phase-plane anyalysis. Unexpected electron heating pattern is observed during the velocity overshoot process with a moderate electron temperature near the peak velocity followed by rapid increase in the deceleration period. For short nitride diodes, space-charge limited transport is considered by taking into account the self-consistent field. In this case, the overshoot is weaker and the electron heating in the region of the peak velocity is greater than that found for time-dependent problem. The transient processes are extended to sufficiently larger distances as well. When the electron concentration is small, we propose a model which accounts the main features of injected electrons in a short device with high fields. The electron velocity distribution over the device is found as a function of the field. It is demonstrated that in high fields the electrons are characterized by the extreme distribution function with the population inversion.

AlGaN thin films and Schottky barrier Al_0.4Ga_0.6N diodes exhibit generation-recombination (GR) noise with activation energies of 0.8 - 1 eV. GR noise in AlGaN/GaN Heterostructure Field Effect transistors (HFETs) corresponds to activation energies in the range from 1 - 3 meV to 1 eV. No GR noise is observed in thin doped GaN films and GaN MESFETs. GR noise with the largest reported activation energy of 1.6 eV was measured in AlGaN/InGaN/GaN Double Heterostructure Field Effect Transistors (DHFETs). Local levels responsible for the GR noise in HFETs and DHFETs might be located in AlGaN barrier layers.

A III-Nitride/SiC separate absorption and multiplication avalanche photodiode (SAM-APD) offers a novel approach for fabricating high gain photodetectors with tunable absorption over a wide spectrum from the visible to deep ulltraviolet. However, unlike conventional heterojunction SAM APDs, the formation of polarization-induced charge at the hetero-interface arising from spontaneous and piezoelectric polarization can dramatically affect the performance of this detector. In this paper we report on the role of this interface charge on the performance of GaN/SiC SAM APDs. Simulations of the electric field profile within this device structure while biased near avalanche breakdown indicate that the density of positive interface charge may be sufficient to confine the electric field within the SiC multiplication region with negligible punch-through into the GaN absorption region, a distribution that is likely undesirable for efficient collection of photo-generated holes due to the presence of defects at the hetero-interface. Simulations further show that the incorporation of a p-type doped interface charge control layer at the hetero-interface can modify the total density of charge at the interface and allow for the tailoring of the electric field profile within this device. Experimental results are provided that correlate well with the simulation results.

Very little information is available about the structural properties of III-nitride binary compounds in the rock-salt phase. We report/review a comprehensive theoretical study of structural properties of these compounds in rock-salt, zinc-blende and wurtzite phases. Calculations have been made using full-potential linearized augmented plane wave plus local orbitals (FP-L(APW+lo)) method as embodied in WIEN2k code framed within density functional theory (DFT). In this approach of calculations, local density approximation (LDA) [J. P. Perdew and Y. Wang, Phys. Rev. B45 (1992) 13244] and generalized gradient approximation (GGA) [J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett.72 (1996) 3865] have been used for exchange-correlation energy and corresponding potential. Calculated results for lattice constants, bulk modulus, its pressure derivative and cohesive energy of these compounds are consistent with the experimental results. Following these calculations, besides many new results for the rock-salt and other phases, a comprehensive review of the structural properties emerges. We also list some peculiar features of these compounds.

The application of thermal annealing at various annealing temperatures (473–1073 K) has been shown to significantly modify surface morphology of platinum (Pt) metal contacts on AlGaN/GaN/AlN heterostructure grown on silicon by plasma-assisted molecular beam epitaxy (PA-MBE). Structural analysis of the AlGaN/GaN samples used for the Pt Schottky contacts fabrication were performed by using high resolution X-ray diffraction (HR-XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Pt metal contacts were then deposited on the samples followed by current–voltage (I–V) characterization. Thermally-treated samples showed significant decrease in current compared with untreated samples. From the I–V measurements, the Schottky barrier height (SBH) and ideality factor (n) were calculated. We found that the lowest value of SBH obtained was 0.526 eV at 873 K annealing temperature. Unfortunately, there are no values for the SBH and ideality factor at 1073 K annealing temperature. The SEM analysis has shown some island formation at high annealing temperature due to the difference of surface energies between thin metal films and AlGaN that causes dewetting. We suggest that the reason for the barrier height reduction is due to the metal island formation on the samples.

In this paper, the growth and characterization of epitaxial Al_0.29Ga_0.71N grown on Si(111) by RF-plasma assisted molecular beam epitaxy (MBE) are described. The Al mole fraction was derived from the HR-XRD symmetric rocking curve (RC) ω/2θ scans of (0002) plane as x = 0.29. PL spectrum of sample has shown sharp and intense band edge emission of GaN without the existence of yellow emission band, showing that it is comparable in crystal quality of the sample when compared with previous reports. From the Raman measurement of as-grown Al_0.29Ga_0.71N layer on GaN/AlN/Si sample. We found that the dominant E₂ (high) phonon mode of GaN appears at 572.7 cm^-1. The E₂ (high) mode of AlN appears at 656.7 cm^-1 and deviates from the standard value of 655 cm^-1 for unstrained AlN. Finally, AlGaN Schottky photodiode have been fabricated and analyzed by mean of electrical characterization, using current–voltage (I–V) measurement to evaluate the performance of this device.

In this paper, we investigated growth of GaN pn-homo junction layers on silicon (111) by plasma-assisted molecular beam epitaxy (PA-MBE) system and its application for photo-sensors. Silicon (Si) and magnesium (Mg) were used in this study as n- and p-dopants, respectively. Structural and optical analysis of the GaN homo-junction samples were performed by using reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), Raman spectroscopy and photoluminescence (PL) to analyze the crystalline quality of the samples. Surprisingly, the Raman analysis has revealed that there is no quenching of the A₁ (LO) peak, with the presence of Si- and Mg-dopants in sample. The pn-junctions sample has good optical quality as measured by the PL system. Electrical characterization of the photo-sensors was carried out by using current–voltage (I–V) measurements.

We analyze steady-state and transient electron transport in the group III-nitride materials at high and ultra-high electric fields for different electron concentration regimes. At high electron concentrations where the electron distribution function assumes a shifted Maxwellian, we investigate different time-dependent transient transport regimes through the phase-plane anyalysis. Unexpected electron heating pattern is observed during the velocity overshoot process with a moderate electron temperature near the peak velocity followed by rapid increase in the deceleration period. For short nitride diodes, spacecharge limited transport is considered by taking into account the self-consistent field. In this case, the overshoot is weaker and the electron heating in the region of the peak velocity is greater than that found for time-dependent problem. The transient processes are extended to sufficiently larger distances as well. When the electron concentration is small, we propose a model which accounts the main features of injected electrons in a short device with high fields. The electron velocity distribution over the device is found as a function of the field. It is demonstrated that in high fields the electrons are characterized by the extreme distribution function with the population inversion.

AlGaN thin films and Schottky barrier Al_0.4Ga_0.6N diodes exhibit generation-recombination (GR) noise with activation energies of 0.8 - 1 eV. GR noise in AlGaN/GaN Heterostructure Field Effect transistors (HFETs) corresponds to activation energies in the range from 1 - 3 meV to 1 eV. No GR noise is observed in thin doped GaN films and GaN MESFETs. GR noise with the largest reported activation energy of 1.6 eV was measured in AlGaN/InGaN/GaN Double Heterostmcture Field Effect Transistors (DHFETs). Local levels responsible for the GR noise in HFETs and DHFETs might be located in AlGaN barrier layers.