Baozhu Wang

Effects of rapid thermal annealing on self-assembled InGaAs/GaAs quantum dots superlattice

Postgrowth rapid thermal annealing was used to study the relaxation mechanism and optical properties of InGaAs/GaAs self-assembled quantum dots superlattice grown by molecular beam epitaxy. It is found that a significant narrowing of the luminescence linewidth (from 80 to 42 meV) occurs together with about 86 meV blue shift at annealing temperature up to 950°C. Double crystal X-ray diffraction measurements show that the intensity of the satellite diffraction peak, which corresponds to the quantum dots superlattice, decreased with the increasing annealing temperature and disappeared at 750°C, but recovered and increased again at higher annealing temperatures. This behavior can be explained by two competing relaxation mechanisms; interdiffusion and favored migration. The study indicates that a suitable annealing treatment can improve the structural properties of the quantum dots superlattice.

Hydrogen sensors based on Pt-AlGaN/AIN/GaN Schottky diode

Pt/AlGaN/AlN/GaN Schottky diodes have been fabricated and characterized for H2 sensing. Platinum (Pt) with a thickness of 20nm was evaporated on the sample to form the Schottky contact. The ohmic contact, formed by evaporated Ti/Al/Ni/Au metals, was subsequently annealed by a rapid thermal treatment at 860°C for 30 s in N2 ambience. Both the forward and reverse current of the device increased greatly when exposed to H2 gas. The sensors responses under different hydrogen concentrations from 500ppm to 10% H2 in N2 at 300K were investigated. A shift of 0.45V at 297K is obtained at a fixed forward current for switching from N2 to 10% H2 in N2. Time response of the sensor at a fixed bias of 0.5 V was also measured. The turn-on response of the device was rapid, while the recovery of the sensor at N2 atmosphere was rather slow. But it recovered quickly when the device was exposed to the air. The decrease in the barrier height of the diode was calculated to be about 160meV upon introduction of 10% H2 into the ambient. The sensitivity of the sensor is also calculated. Some thermodynamics analyses have been done according to the Langmuir isotherm equation.

MOCVD grown AlGaN/AlN/GaN HEMT structure with compositionally step-graded AlGaN barrier layer

Unintentionally doped AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with compositionally step-graded AlGaN barrier layer were grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD). The HEMT structure exhibited typical two-dimensional electron gas (2DEG) mobility of 1600cm2/Vs at room temperature and 6412cm2/Vs at 79K with almost equal 2DEG concentration of 1.0times1013/cm2. The 50mm HEMT wafer exhibited an average sheet resistance of 318.0Omega/square, with a good resistance uniformity of 0.89%. Atomic force microscopy (AFM) measurements revealed a smooth AlGaN surface with root-mean-square roughness (RMS) of 0.199nm and 0.295nm for scan area of 2mumtimes2mum and 5mumtimes5mum, respectively. A combined using of compositionally step-graded AlGaN barrier structure and AlN interlayer results in the high electrical performance and smooth surface of this heterostructure

Growth and Structural Properties of InAlGaN Quaternary Alloys on Sapphire Substrates by RF-MBE

InAlGaN quaternary alloys were grown on nitrided sapphire, AlN nuclear layer and GaN layer by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE). Reflection high-energy electron diffraction (RHEED), Rutherford back-scatter spectrometry (RBS), atomic-force microscopy (AFM) and high resolution X-ray diffraction (HRXRD) were used to characterize the InAlGaN alloys. Different buffer layers strongly affect the growth mode, indium content and depth distribution, surface morphology and crystal quality of InAlGaN quaternary alloys. The experimental results show that the InAlGaN film grown on GaN layer can acquire layer-by-layer growth mode, uniform indium deep distribution, flat surface with RMS of InAlGaN to be 2.2nm and smallest full-width at half-maximum (FWHM) of InAlGaN (0002) peak to be 4.8 arcmin