Andrew Melton

Design and Realization of Wide-Band-Gap (

The design of coherently strained InGaN epilayers for use in InGaN p-n junction solar cells is presented in this letter. The X-ray diffraction of the epitaxially grown device structure indicates two InGaN epilayers with indium compositions of 14.8% and 16.8%, which are confirmed by photoluminescence peaks observed at 2.72 and 2.67 eV, respectively. An open-circuit voltage of 1.73 V and a short-circuit current density of 0.91 mA/cm2 are observed under concentrated AM 0 illumination from the fabricated solar cell. The photovoltaic response from the InGaN p-n junction is confirmed by using an ultraviolet filter. The solar cell performance is shown to be related to the crystalline defects in the device structure.

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In this article several kinetic effects are proposed that induce compositional instabilities in thick InGaN heteroepitaxial layers on GaN templates grown by metalorganic chemical vapor deposition. It was found that by reducing the growth temperature, or increasing the growth rate, or introducing Mg doping, the epitaxial layer changes from a pseudomorphic InGaN with a low indium mole fraction to a relaxed InGaN with a high indium mole fraction. In certain circumstances, both phases can be present in a single layer. The composition and strain inhomogeneity was correlated to the surface morphology and crystalline quality, governed by the growth conditions. It is believed that the compositional instability in InGaN originates from the coupled effects of compressive strain and surface morphology. A smooth surface allows for the growth of pseudomorphic low-indium InGaN, whereas a rough surface promotes the formation of a relaxed high-indium InGaN layer.

2.67 eV) InGaN p-n Junction Solar Cell

In this work, effects of interstitial oxygen on the ferromagnetism in Gd-doped GaN system were investigated via first principles, including both total energy and band structure calculation. The results of these calculations indicate that splitting interstitial sites oxygen is a plausible candidate for defect-induced magnetism in n-type GaN:Gd due to the p-d hybridization. The total energy difference was found to be more than 100 meV in a wide electron concentration range. Furthermore, the formation energies of various interstitial oxygen atomic configurations are also discussed.

Compositional instability in strained InGaN epitaxial layers induced by kinetic effects

This paper presents the first report on Gd doping (0%–4%) of GaN thin films by metal organic chemical vapor deposition. The Ga1−xGdxN films grown in this study were found to be of good crystalline quality, single-phase, and unstrained, with a high saturation magnetization strength of 20 emu/cm3 being obtained for GaN films doped with 2% Gd at room temperature. Furthermore, these films were found to be conductive with an enhanced n-type behavior suggesting that unintentional donors are responsible for stabilizing the ferromagnetic phase in as-grown Ga1−xGdxN. Additionally, it was found that this magnetization can be enhanced by n-(Si: 1018 cm−3) and p-(Mg: 1019 cm−3) doping to 110 emu/cm3 and ∼500 emu/cm3, respectively. This paper shows empirically that holes are more efficient in stabilizing the ferromagnetic phase as compared to electrons. Overall, this research has resulted in a room temperature ferromagnetic dilute magnetic semiconductor that is conductive and whose magnetic properties can be tuned by ...

Ferromagnetism and its stability in n-type Gd-doped GaN: First-principles calculation

Crack-free GaN light emitting diodes (LEDs) have been grown by metal organic chemical vapor deposition on Si(111) substrates using an atomic layer deposition-grown Al2O3 interlayer. Devices on Si show a longer emission wavelength compared to those on sapphire. This is attributed to tensile strain in the layers on Si, which may increase indium incorporation. Internal quantum efficiency is similar on both substrates. Luminescence intensity versus current density measurements show higher efficiency for the LEDs on Si relative to sapphire at high drive currents. These results show comparable performance characteristics for GaN-based devices on Si and sapphire substrates.

Electrical and magnetic properties of Ga1−xGdxN grown by metal organic chemical vapor deposition

p-type, n-type and semi-insulating ZnO:N thin films were successfully grown by metal organic chemical vapor deposition on c-plane sapphire using diethyl zinc and O2 precursors, N2 carrier gas, and NH3 as dopant. NH3 flow rates were varied from 0.2% to 4% in the growth runs. The resulting films were characterized for their structural, optical, and electrical properties by scanning electron microscopy, x-ray diffraction (XRD), Raman spectroscopy, photoluminescence (PL), and Hall effect measurements. XRD show a single ZnO (002) peak; Raman data show the presence of ZnO:N modes at 275, 510, 575, and 645cm−1; and PL results show broad peaks at 480 and 600nm corresponding to deep N incorporation for all the samples. Hall effect show n-type films with carrier concentrations of 6.57×1018cm−3, p-type with carrier concentrations of 4.24×1014cm−3, and semi-insulating with resistivity on the order of 1.5×105Ωcm.

Metal organic chemical vapor deposition of crack-free GaN-based light emitting diodes on Si (111) using a thin Al2O3 interlayer

We report on the structural, morphological, and optical qualities of thick InxGa1−xN heteroepitaxial layers grown by metalorganic chemical vapor deposition with various growth conditions for applications in wide-band gap solar cells. The indium incorporation depending on the growth temperature and indium precursor flow rate and the crystalline and optical qualities of InGaN layers depending on indium mole fraction were investigated. The InGaN layers with high structural and optical qualities were obtained for indium mole fractions, xIn < 0.18, whereas significant degradation of material qualities was observed for xIn < 0.18, which is associateWe report on the structural, morphological, and optical qualities of thick InxGa1−xN heteroepitaxial layers grown by metalorganic chemical vapor deposition with various growth conditions for applications in wide-bandgap solar cells. The indium incorporation depending on the growth temperature and indium precursor flow rate and the crystalline and optical qualities of InGaN layers depending on indium mole fraction were investigated. The InGaN layers with high structural and optical qualities were obtained for indium mole fractions, xIn < 0.18, whereas significant degradation of material qualities was observed for xIn < 0.18, which is associated with the change of growth mode induced by reduced growth temperature. Stokes shift and microscopic and macroscopic phase separations were also studied. Two types of additional phases besides InGaN matrix, i.e., indium-rich InGaN microstructures and macroscopic InGaN domains, were demonstrated to be suppressed by controlling surface adatom mobility and growth rates.d with the change of growth mode induced by reduced growth temperature. Stokes shift and microscopic and macroscopic phase separations were also studied. Two types of additional phases besides InGaN matrix, i.e., indium-rich InGaN microstructures and macroscopic InGaN domains, were demonstrated to be suppressed by controlling surface adatom mobility and growth rates.

n-type, p-type and semi-insulating ZnO:N thin film growth by metal organic chemical vapor deposition with NH3 doping

In this report we present recent results for MOCVD growth of high indium content InGaN films on ZnO substrates. Growth was attempted on both bulk ZnO as well as ZnO epilayers grown on sapphire by MOCVD. ZnO is an attractive alternative substrate for III-Nitrides because of its superior lattice match: specifically ZnO is perfectly matched with In0.18Ga0.82N and low cost of substrates. Stable InGaN films with ≫18% indium were achieved on the bulk substrates and were characterized by HRXRD, PL, and optical transmission. Varying the growth parameters - primarily growth temperature and In/(In + Ga) flow ratio - was found to affect the optical and structural properties of the films. By growing on a better matched substrate the high indium composition InGaN epitaxial films experience less strain and can therefore be grown thicker without creating relaxation-induced extended crystal defects. This is important, as high indium content InGaN films cannot be grown on GaN thick enough for full above-bandgap absorption without introducing detrimental extended crystal defects. This limitation is thought to be a limiting factor in the achievable ISC in InGaN solar cells.

Growth and characterization of InxGa1−xN alloys by metalorganic chemical vapor deposition for solar cell applications

The influence of super-atmospheric reactor pressures (2.5‐18.5bar) on the electrical and structural properties of InN epilayers deposited on GaN/sapphire (0001) templates by high-pressure chemical vapor deposition has been studied. The epilayers were analyzed by Raman, x-ray diffraction (XRD), and Fourier transform infrared reflectance spectrometry to determine the structural properties as well as the phonon frequencies, dielectric function, plasma frequency, layer thickness and damping parameters of the epilayers. For the studied process parameter space, best material properties were achieved at a reactor pressure of 12.5bar and a group-V/III ratio of 2500 with a free carrier concentration of 1.5 � 10 18 cm � 3 , a mobility of the bulk InN layer of 270 cm 2 V � 1 s � 1 , and a Raman (E2 high) FWHM value of 10.3cm � 1 . This study shows that the crystalline layer properties—probed by XRD 2h‐x scans—improve with increasing reactor pressure. V C 2012

High indium composition (≫20%) InGaN epi-layers on ZnO substrates for very high efficiency solar cells

GaN is a promising material for neutron detection applications, with advantages over Si and GaAs. GaN films doped with Gd have been grown by MOCVD and investigated for their feasibility for neutron detection. The films were structurally and electrically characterized through HRXRD and Hall effect measurements. Alpha particle luminescence of both doped and undoped films was used to investigate gamma discrimination properties.