Binh Tinh Tran

Threading Dislocation Blocking in Metamorphic InGaAs/GaAs for Growing High-Quality In0.5Ga0.5As and In0.3Ga0.7As on GaAs Substrate by Using Metal Organic Chemical Vapor Deposition

High quality In0.3Ga0.7As and In0.51Ga0.49As epilayers have been successfully grown on the GaAs substrate by MOCVD. A cross-sectional study by transmission electron microscopy showed that the threading dislocations (TDs) have been successfully contained and limited within the buffer layers designed to stop the elongation of TDs into the In0.3Ga0.7As and In0.51Ga0.49As epilayers. A TD density of 1?106 cm-2 in a fully relaxed In0.51Ga0.49As epilayer was achieved. The measurement of lifetimes of n- and p-type In0.51Ga0.49As has been done by using time-resolved photoluminescence. A great reduction in the number of recombination centers in the InGaAs epilayer has been shown.

Growth of lattice-matched InAlN/GaN on Si (111) substrate for ultraviolet photodiode application

In this paper, we report on the growth of high quality InxAl1−xN/GaN hetero-structures on Si substrate by metal organic chemical vapor deposition with various indium compositions (x = 10.2, 16.2 and 17.6%). The lattice-matched In0.176Al0.838N/GaN structure shows a smooth surface with good crystalline quality. In addition, the ultraviolet photodiode device shows excellent device characteristics with a low leakage current of 0.12 µA, and a high spectral response. It has good quantum efficiency of 94 mA/W and 44% at 265 nm which is comparable to that of the InAlN photodiode grown on sapphire substrate.

Performance Improvements of AlGaN/GaN HEMTs by Strain Modification and Unintentional Carbon Incorporation

An advanced AlGaN/GaN HEMT structure, grown on a sapphire substrate by MOCVD utilizing a high temperature (HT) AlN interlayer (IL) and a multilayer high-low-high temperature (HLH) AlN buffer layer, demonstrates a superior performance both in breakdown voltage (>200 V) and maximum drain current (IDSS = 667 mA/mm). The HT AlN IL produces an additional compressive strain into the above GaN layer. Accordingly, an AlGaN barrier, grown on the more compressive GaN, introduces less tensile strain leading to an improvement in surface morphology (RMS = 0.19 nm in 2 × 2 μm2), a remarkable increase in 2DEG mobility by 46% (μs = 1900 cm2/Vs) and a decrease in densities of defects acting as paths for the leakage current through the AlGaN barrier. A high semi-insulating buffer is achieved by eliminating leakage paths both through the buffer layer and the buffer-substrate interfacial layer. These result from an increase in unintentional carbon introduced by AlN layers, especially by a low temperature AlN layer; which are grown under low pressure (50 Torr). Lastly, the decrease in AlGaN barrier tensile strain and low leakage current in the advanced HEMTs structure using an HT AlN IL and an HLH AlN buffer are promising for an improvement in AlGaN/GaN HEMTs’ reliability.

Effect of Multiple AlN Layers on Quality of GaN Films Grown on Si Substrates

AbstractIn this paper, we present the effect of multiple thin high-low-high-temperature AlN (HLHT AlN) nucleation layers on GaN film quality. A 1.9-μm-thick GaN film grown on Si (111) substrate shows that the multiple HLHT AlN nucleation layers have a significant effect on GaN film quality. This process also plays a very important role in the growth of GaN films on Si (111) substrates. A high quality GaN film with a uniformly faceted surface with very low dislocation densities is obtained at the optimized multiple HLHT AlN nucleation layers 50-nm thick at a temperature of 1010-800-1010°C and a growth pressure of 50 Torr.

Broadband antireflection sub-wavelength structure of InGaP/InGaAs/Ge triple junction solar cell with composition-graded SiNx

This work reports a fabrication strategy to improve the antireflective ability of a InGaP/GaAs/Ge triple-junction solar cell, by combining a nano-templating technique and a chemical-synthesis approach. SiH4 and N2 were used as ammonia-free reaction gases in a plasma-enhanced chemical vapor deposition (PECVD) to prepare Si3N4 as an original antireflective coating (ARC) layer with better chemical stability. Composition-graded SiNx was successfully integrated with sub-wavelength structure by modulating SiH4/N2 ratio during PECVD deposition, and followed by a controllable gold-nanoparticle masking technique on top of the solar cell. Finite-difference time-domain solution was employed to simulate and optimize the aspect-ratio of the ARC, under the condition of variable refractive index over a broad wavelength window, and followed by the masking technique to obtain the desired ARC dimension. This enabled a low light reflectance (<10%) over a broad spectral bandwidth (300–1800 nm) for the solar cell with excellent stability, because of the triple advantages of structural optimization, better chemical stability and graded refractive index of the ARC. The solar cells performance was tested and showed great competitiveness to those of forefront studies, suggesting the feasibility of the proposed technology.

The effect of CdS QDs structure on the InGaP/GaAs/Ge triple junction solar cell efficiency

This work describes optical and electrical characteristics of InGaP/GaAs/Ge triple-junction (T-J) solar cells with CdS quantum dots (QDs) fabricated by a novel chemical solution. With the anti-reflective feature at long wavelength and down-conversion at UV regime, the CdS quantum dot effectively enhance the overall power conversion efficiency more than that of a traditional GaAs-based device. Experimental results indicate that CdS quantum dot can enhance the short-circuit current by 0.33 mA/cm2, which is observed for the triple-junction solar cells with CdS QDs of about 3.5 nm in diameter. Moreover, the solar cell conversion efficiency is improved from 28.3% to 29.0% under one-sun AM 1.5 global illumination I–V measurement.

InGaP/GaAs dual-junction solar cell with AlGaAs/GaAs tunnel diode grown on 10° off misoriented GaAs substrate

þþ -AlGaAs/N þþ -GaAs TDs grown on 10 � off GaAs substrates not only show a higher external quantum efficiency (EQE) but also generate a higher peak current density (Jpeak) at higher concentration ratios (185� ) than the solar cells with P þþ -GaAs/ N þþ -InGaP TDs grown on 6 � off GaAs substrates. Furthermore, the cell design with P þþ -AlGaAs/N þþ -GaAs TDs grown on 10 � off GaAs substrates

2H-silicon carbide epitaxial growth on c-plane sapphire substrate using an AlN buffer layer and effects of surface pre-treatments

The effects of surface pre-treatments and the role of an AlN buffer layer for 2H-SiC growth on c-plane sapphire substrates by thermal CVD are investigated. While the crystallinity of SiC directly grown on sapphire substrate always degrades with a hydrogen pre-treatment but improves by optimizing carbonization, the crystallinity of SiC grown on sapphire substrate using an AlN buffer grown by MOCVD improves with sufficient time of exposure to the H pre-treatment but always deteriorates with carbonization. Detailed microstructural analysis by phi-scan x-ray diffraction reveals that SiC film grown on sapphire substrate consists of crystalline domains with two different crystallographic orientations which are rotated relative to each other along the [111] axis by 60°. A highly oriented hexagonal 2H-SiC film is obtained on low-cost c-plane sapphire substrate by using an AlN buffer. 2H-SiC is unambiguously determined not only by phi-scan x-ray diffraction but also by high-resolution transmission electron microscopy. The growth relationship between 2HSiC and 2H-AlN are coherent due to the favorable bonding of C and Al between SiC and AlN.

Effect of the circle-grid electrodes on concentrated GaAs solar cell efficiency

In this study, we investigate the effect of the shading factor of the front grid pattern on concentrated solar cell efficiency, taking the trade-off between the series resistance of the electrodes and the amount of incident light into consideration. We examine the thermal effect with regard to five different circle-grid electrode patterns of the front contact. The front contacts with different grid patterns affect the characteristics of light-concentrated-type GaAs single-junction solar cells. The device parameters analyzed include the open-circuit voltage (Voc), short-circuit current (Isc), fill factor (FF) and conversion efficiency (η). The results of our study show that for a concentration ratio greater than 60x with AM1.5G, the device with a shading factor of 7.1% has the best cell efficiency of 27.05%, due to the smaller current crowding at the center spot. The results indicate that the conversion efficiency of solar cells can be improved by establishing a compromise between the shading effect and the series resistance effect.