Chenhui Yu

Tunability of intersubband transition wavelength in the atmospheric window in AlGaN/GaN multi-quantum wells grown on different AlGaN templates by metalorganic chemical vapor deposition

The properties of intersubband transition in AlGaN/GaN multi-quantum wells (MQWs) grown on different AlGaN templates by metalorganic chemical vapor deposition are investigated. The strain states of GaN wells are studied by Raman spectra and reciprocal space mappings, which shows that the GaN wells are compressively strained and the compressive strain is increased when the Al mole composition is varied from 0 to 0.3. The Fourier transform infrared spectrometer results show that the intersubband transition wavelength in the AlGaN/GaN MQWs can be tuned from 5.14 μm to 4.65 μm when the Al mole composition of the AlGaN template is increased. The results can be attributed to the quantum confined Stark effect.

Nonpolar a-plane ZnO films grown on GaN/sapphire templates with SiNx interlayer by pulsed laser deposition

Nonpolar a-plane ZnO films were deposited for the first time on nearly lattice-matched a-plane GaN templates using an in situ SiNx interlayer by the pulsed laser deposition method. The symmetric and asymmetric reciprocal space mappings reveal the broadening effects of the reciprocal lattice points and the residual biaxial in-plane tensile strain of a-plane ZnO to be 0.335% and 0.055% along the [0 0 0 1] c-axis and p-axis, respectively. The photoluminescence spectrum at 85 K is dominated by neutral donor-bound excitons and free-electron-to-bound (e-A0) emissions; relatively intense LO-phonon replicas of (e-A0) have also been observed in a-plane ZnO. Temperature-dependent PL spectra have also been discussed to identify the origin of the emission peaks. Up to fourth A1(LO) phonon mode can be observed to be enhanced significantly from the resonant Raman spectrum.

The effects of growth temperature of AlN buffer layers on a-plane GaN grown on r-plane sapphire by MOCVD

To grow high quality a-GaN films on r-sapphire substrates, it is essential to optimize the buffer layers (BLs) growth parameters such as temperature, thickness, and V/III ratio. In this work, we investigated the effects of growth temperature for AlN BLs on the surface morphology and the crystal quality of a-GaN films. The films studied here were grown epitaxially by metalorganic chemical-vapor deposition (MOCVD) with different AlN BLs grown at 690 °C, 720 °C, 750 °C, respectively. The properties of a-GaN films were comprehensively studied via high resolution x-ray diffraction (HRXRD), optical microscope (OM) and atomic force microscope (AFM). It is found that the crystal quality and the surface morphology of a-GaN films have been remarkably affected by the growth temperature of the AlN BLs. At the optimum AlN BLs growth temperature of 720 °C, the a-GaN films have the best crystal quality – and the smoothest surface morphology with the (1120) HRXRD FWHM of 864 arcsec and the root mean square (RMS) surface roughness of 1.2 nm, respectively.

Influence of the base layer thickness and the graded buffer layer thickness on the conversion efficiency of a metamorphic triple-junction solar cell

Metamorphic multiple-junction soalr cells based on III-V compound semiconductor have advantages of more degree of freedom in selection of bandgaps of subcells. Recently, Spectrolab, Inc. reported a high conversion efficiency of 40.7% in a triple-junction Ga0.44In0.56P/Ga0.92In0.08As/Ge solar cell where the top and middle cells are lattice-mismatched to Ge substrate. Optimization of device structure of such metamorphic Ga0.44In0.56P/Ga0.92In0.08As/Ge solar cell is important to increase its efficiency. In this work, two-dimensional simulation has been performed on the metamorphic Ga0.44In0.56P/Ga0.92In0.08As/Ge solar cell. Efficiency dependence of the Ga0.44In0.56P/Ga0.92In0.08As/Ge triple-junction solar cell on the base layer thickness and the InGaAs graded buffer layer thickness has been investigated. It has been found that the efficiency depends significantly on the thickness of the GaInP base layer. The metamorphic triple junction solar cell has hightest efficiency when the thicknesses of the GaInP base layer, the GaInAs base layer and the GaInAs graded buffer layer connecting the Ge substrate and GaInAs middle-cell are 0.35 μm, 2.5 μm and 0.15 μm, respectively.

The design of back surface field layer for a single GaAs solar cell

The inclusion of a back surface field (BSF) can significantly improve the efficiency of GaAs solar cells due to a better collection of photogenerated minority carriers. In this work, the efficiency of a single GaAs solar cell has been optimized by varying the material properties of AlGaAs back surface field layer using an advanced commercial software, Crosslight APSYS simulator. By optimizing the Al composition and width of the back surface field layer, a maximum efficiency 25.660% is obtained when Al composition is 0.25 and the width is 0.3 μm. The band diagrams, 2-dimensional relative energy density profile and I-V curve of the device structure with the optimal back surface field layer are also obtained by APSYS.

Epitaxial growth a-plane ZnO films on a-GaN/r-Al2O3 templates

In this work, we have grown a-plane ZnO films on a-plane GaN/r-sapphire templates by pulsed laser deposition. The aplane GaN of the templates is aimed to mitigate the large lattice mismatch between ZnO and sapphire, and was grown by metal organic chemical vapor deposition. The grown a-plane ZnO films have been analyzed by various techniques such as high resolution X-ray diffraction, photoluminescence. It shows that high quality a-plane ZnO films have been achieved by our growth method.

High quality a-plane GaN layers grown by pulsed atomic-layer epitaxy on r-plane sapphire substrates

In this work, pulsed atomic-layer epitaxy (PALE) technique has been used to grow a-plane GaN films on r-plane sapphire substrates. During growth, the supply of N atoms was alternatively turned on and off while Ga atoms were continuously supplied with a constant flow rate. By optimizing the on/off periods of N source at 20/10 secs, pit-free GaN films have been obtained with significantly reduced full width at half maximum of X-ray rocking curve and brighter characteristic emission of stacking faults, as compared to the case using conventional metal-organic chemicalvapor deposition. The improved epitaxy quality and optical properties in PALE is due to the enhanced the Ga-adatom surface migration and improved the lateral-growth rate, which results in larger grain size and longer stacking faults.