Wang Cuimei

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.

Growth and characterization of 0.8-μm gate length AlGaN/GaN HEMTs on sapphire substrates

AlGaN/GaN high electron mobility transistor (HEMT) structures were grown on 2 inch sapphire substrates by MOCVD, and 0.8-µm gate length devices were fabricated and measured. It is shown by resistance mapping that the HEMT structures have an average sheet resistance of approximately 380 Θ/sq with a uniformity of more than 96%. The 1-mm gate width devices using the materials yielded a pulsed drain current of 784 mA/mm atVgs=0.5 V andVds=7 V with an extrinsic transconductance of 200 mS/mm. A 20-GHz unity current gain cutoff frequency (fT) and a 28-GHz maximum oscillation frequency (fmax) were obtained. The device with a 0.6-mm gate width yielded a total output power of 2.0 W/mm (power density of 3.33 W/mm) with 41% power added efficiency (PAE) at 4 GHz.AlGaN/GaN high electron mobility transistor (HEMT) structures were grown on 2 inch sapphire substrates by MOCVD, and 0.8-μm gate length devices were fabricated and measured. It is shown by resistance mapping that the HEMT structures have an average sheet resistance of approximately 380 Ω/sq with a uniformity of more than 96%. The 1-mm gate width devices using the materials yielded a pulsed drain current of 784 mA/mm at Vgs=0.5 V and Vds=7 V with an extrinsic transconductance of 200 mS/mm. A 20-GHz unity current gain cutoff frequency (ƒT ) and a 28-GHz maximum oscillation frequency (ƒmax) were obtained. The device with a 0.6-mm gate width yielded a total output power of 2.0 W/mm (power density of 3.33 W/mm) with 41% power added efficiency (PAE) at 4 GHz.

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.

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.

Variation of Optical Quenching of Photoconductivity with Resistivity in Unintentional Doped GaN

The optical quenching of photoconductivity under dual illumination in GaN samples with different resistivity is investigated to reveal the variation of deep levels. The samples are grown by metal organic chemical vapour deposition without intentional doping. Quenching bands centered at 1.35 eV, 1.55 eV, 1.98 eV, and 2.60 eV are observed. It is found that the 1.98 eV quenching band is dominated in all the samples and the 2.60 eV band is observed only in the high-resistivity samples. The possible defect levels responsible for the quenching bands and the origin of different quenching behaviour at 2.60 eV are discussed. It is suggested that the defect level responsible for quenching at 2.60 eV plays an important role for the enhancement of resistivity.

The Valence Band Offset of an Al0.17Ga0.83N/GaN Heterojunction Determined by X-Ray Photoelectron Spectroscopy

The valence band offset (VBO) of an Al0.17Ga0.83N/GaN heterojunction is determined to be 0.13 ± 0.07 eV by x-ray photoelectron spectroscopy. From the obtained VBO value, the conduction band offset (CBO) of ~0.22 eV is obtained. The results indicate that the Al0.17Ga0.83N/GaN heterojunction exhibits a type-I band alignment.

Properties of AlyGa1–yN/AlxGa1–xN/AlN/GaN Double-Barrier High Electron Mobility Transistor Structure

Electrical properties of AlyGa1-yN/AlxGa1-xN/AlN/GaN structure are investigated by solving coupled Schrodinger and Poisson equation self-consistently. Our calculations show that the two-dimensional electron gas (2DEG) density will decrease with the thickness of the second barrier (AlyGa1-yN) once the AlN content of the second barrier is smaller than a critical value y(c), and will increase with the thickness of the second barrier (AlyGa1-yN) when the critical AlN content of the second barrier y(c) is exceeded. Our calculations also show that the critical AlN content of the second barrier y(c) will increase with the AlN content and the thickness of the first barrier layer (AlxGa1-xN).

Hydrogen Sensors Based on AlGaN/AlN/GaN Schottky Diodes

Pt/AlGaN/AlN/GaN Schottky diodes are fabricated and characterized for hydrogen sensing. The Pt Schottky contact and the Ti/Al/Ni/Au ohmic contact are formed by evaporation. Both the forward and reverse currents of the device increase greatly when exposed to hydrogen gas. A shift of 0.3V at 300K is obtained at a fixed forward current after switching from N2 to 10%H2+N2. The sensor responses under different concentrations from 50ppm H2 to 10%H2+N2 at 373K are investigated. Time dependences of the device forward current at 0.5V forward bias in N2 and air atmosphere at 300 and 373K are compared. Oxygen in air accelerates the desorption of the hydrogen and the recovery of the sensor. Finally, the decrease of the Schottky barrier height and sensitivity of the sensor are calculated.