Xiao Hongling

Computational Investigation of InxGa1-xN/InN Quantum-Dot Intermediate-Band Solar Cell

An InxGa1−xN/InN quantum-dot intermediate-band solar cell is calculated by means of solving the Schrodinger equation according to the Kronig—Penney model. Based on particular assumptions, the power conversion efficiency is worked out. The results reveal that the InxGa1−xN/InN quantum-dot intermediate-band solar cell manifests much larger power conversion efficiency than that of p-n junction solar cells, and the power conversion efficiency strongly depends on the size of the quantum dot and the interdot distance.

Structural and Magnetic Properties of Sm Implanted GaN

The structural and magnetic properties of Sm ion-implanted GaN with different Sm concentrations are investigated. XRD results do not show any peaks associated with second phase formation. Magnetic investigations performed by superconducting quantum interference device reveal ferromagnetic behavior with an ordering temperature above room temperature in all the implanted samples, while the effective magnetic moment per Sm obtained from saturation magnetization gives a much higher value than the atomic moment of Sm. These results could be explained by the phenomenological model proposed by Dhar et al. [Phys. Rev. Lett. 94(2005) 037205, Phys. Rev. B 72(2005) 245203] in terms of a long-range spin polarization of the GaN matrix by the Sm atoms.

Growth and Characterization of AlGaN/AlN/GaN HEMT Structures with a Compositionally Step-Graded AlGaN Barrier Layer

A new AlGaN/AlN/GaN high electron mobility transistor (HEMT) structure using a compositionally step-graded AlGaN barrier layer is grown on sapphire by metalorganic chemical vapour deposition (MOCVD). The structure demonstrates significant enhancement of two-dimensional electron gas (2DEG) mobility and smooth surface morphology compared with the conventional HEMT structure with high Al composition AlGaN barrier. The high 2DEG mobility of 1806 cm(2)/Vs at room temperature and low rms surface roughness of 0.220 nm for a scan area of 5 mu m x 5 mu m are attributed to the improvement of interfacial and crystal quality by employing the step-graded barrier to accommodate the large lattice mismatch stress. The 2DEG sheet density is independent of the measurement temperature, showing the excellent 2DEG confinement of the step-graded structure. A low average sheet resistance of 314.5 Omega/square, with a good resistance uniformity of 0.68%, is also obtained across the 50 mm epilayer wafer. HEMT devices are successfully fabricated using this material structure, which exhibits a maximum extrinsic transconductance of 218 mS/mm and a maximum drain current density of 800 mA/mm.

Theoretical investigation of efficiency of a p-a-SiC:H/i-a-Si:H/n-μc-Si solar cell

A solar cell with a novel structure is investigated by means of the analysis of microelectronic and photonic structure (AMPS). The power conversion efficiency is investigated with the variations in interface recombination velocity, thicknesses of p-type layer, intrinsic layer, n-type layer, and doping density. Results show that it is available and preferable in theory to employ a-SiC:H as a window layer in p-a-SiC:H/i-a-Si:H/n-μc-Si solar cells, and provide a new approach to improving the power conversion efficiency of amorphous silicon solar cells.

Neutron irradiation effect in two-dimensional electron gas of AlGaN/GaN heterostructures

AlGaN/GaN heterostructures have been irradiated by neutrons with different influences and characterized by means of temperature-dependent Hall measurements and Micro-Raman scattering techniques. It is found that the carrier mobility of two-dimensional electron gas (2DEG) is very sensitive to neutrons. At a low influence of 6.13 x 10(15) cm(-2), the carrier mobility drops sharply, while the sheet carrier density remains the same as that of an unirradiated sample. Moreover, even for a fluence of up to 3.66 x 10(16) cm(-2), the sheet carrier density shows only a slight drop. We attribute the degradation of the figure-of-merit (product of n(s) x mu) of 2DEG to the defects induced by neutron irradiation. Raman measurements show that neutron irradiation does not yield obvious change to the strain state of AlGaN/GaN heterostructures, which proves that degradation of sheet carrier density has no relation to strain relaxation in the present study. The increase of the product of n(s) x mu of 2DEG during rapid thermal annealing processes at relatively high temperature has been attributed to the activation of Ge-Ga transmuted from Ga and the recovery of displaced defects.

High-Voltage AlGaN/GaN-Based Lateral Schottky Barrier Diodes *

Lateral Schottky barrier diodes (SBDs) on AlGaN/GaN heterojunctions are fabricated and studied. The characteristics of the fabricated SBDs with different Schottky contact diameters and different Schottky-Ohmic contact spacings are investigated. The breakdown voltage can be increased by either increasing the Schottky-Ohmic contact spacing or increasing the Schottky contact diameter. However, the specific on-resistance is increased at the same time. A high breakdown voltage of 1400 V and low reverse leakage current below 20nA are achieved by the device with a Schottky contact diameter of 100 μm and a contact spacing of 40 μm, yielding a high V2BR/RON,sp value of 194 MW.cm−2.

InxGa1-xN/GaN Multiple Quantum Well Solar Cells with Conversion Efficiency of 3.77%

We report on fabrication and photovoltaic characteristics of InxGa1−xN/GaN multiple quantum well solar cells with different indium compositions and barrier thicknesses. The as-grown samples are characterized by high-resolution x-ray diffraction and reciprocal space mapping. The results show that the sample with a thick barrier thickness (10.0nm) and high indium composition (0.23) has better crystalline quality. In addition, the dark current density-voltage (J–V) measurement of this device shows a significant decrease of leakage current, which leads to high open-circuit voltage Voc. Through the J–V characteristics under an Air Mass 1.5 Global (AM 1.5 G) illumination, this device exhibits a Voc of 1.89 V, a short-circuit current density Jsc of 3.92 mA/cm2 and a fill factor of 50.96%. As a result, the conversion efficiency (η) is enhanced to be 3.77% in comparison with other devices.

Investigation of the current collapse induced in InGaN back barrier AlGaN/GaN high electron mobility transistors

Current collapses were studied, which were observed in AlGaN/GaN high electron mobility transistors (HEMTs) with and without InGaN back barrier (BB) as a result of short-term bias stress. More serious drain current collapses were observed in InGaN BB AlGaN/GaN HEMTs compared with the traditional HEMTs. The results indicate that the defects and surface states induced by the InGaN BB layer may enhance the current collapse. The surface states may be the primary mechanism of the origination of current collapse in AlGaN/GaN HEMTs for short-term direct current stress.

The Growth and Fabrication of InGaN/GaN Multi-Quantum Well Solar Cells on Si(111) Substrates

Metalorganic chemical vapor deposition of a crack-free mirror-like surface of InGaN/GaN MQWs on Si (111) substrate is demonstrated, and an InGaN/GaN MQWs solar cell device is fabricated. Photo response measurement of the solar cell devices shows that the fill factor FF = 49.4%, open circuit voltage Voc = 0.32 V, and short circuit current Jsc = 0.07 mA/cm2, under AM 1.5 G illumination. In order to analyze the influence of material quality on the performance of solar cells, XRD, SEM and Raman scattering experiments are carried out. It is found that insertion of a proper top AIN layer can effectively improve the material quality, and therefore enhance the photovoltaic performance of the fabricated device.

Cathodoluminescence of Yellow and Blue Luminescence in Undoped Semi-insulating GaN and n-GaN

Yellow and blue luminescence in undoped GaN layers with different resistivities are studied by cathodoluminescence. Intense yellow and blue luminescence bands are observed in semi-insulating GaN, while in n-GaN the yellow luminescence and blue luminescence bands are very weak. The stronger yellow and blue luminescences in semi-insulating GaN are correlated to the higher edge-type dislocation density. The scanning cathodoluminescence image reveals strong defect-related luminescence at the grain boundaries where the dislocations accumulate. It is found that the relative intensity of the blue luminescence band to the yellow luminescence band increases with the cathodoluminescence beam energies and is larger in n-GaN with a lower density of edge-type dislocations. An approximately 3.35 eV shoulder next to the near-band-edge peak is observed in n-GaN but not in semi-insulating GaN. A redshift of the near-band-edge peak with cathodoluminescence beam energy is observed in both samples and is explained by internal absorption.