Qin Zhi-Xin

AlGaN-Based Deep-Ultraviolet Light Emitting Diodes Fabricated on AlN/sapphire Template

We report on the growth and fabrication of deep ultraviolet (DUV) light emitting diodes (LEDs) on an AlN template which was grown on a pulsed atomic-layer epitaxial buffer layer. Threading dislocation densities in the AlN layer are greatly decreased with the introduction of this buffer layer. The crystalline quality of the AlGaN epilayer is further improved by using a low-temperature GaN interlayer between AlGaN and AlN. Electroluminescences of different DUV-LED devices at a wavelength of between 262 and 317 nm are demonstrated. To improve the hole concentration of p-type AlGaN, Mg-doping with trimethylindium assistance approach is performed. It is found that the serial resistance of DUV-LED decreases and the performance of DUV-LED such as EL properties is improved.

Effect of Indium Ambient on Electrical Properties of Mg-Doped AlxGa1?xN

Mg-doped AlxGa1-xN epilayers were grown on AlN/sapphire templates by metal organic chemical vapor deposition (MOCVD) using an indium-assisted growth method. At room temperature, the resistivity of Mg-doped Al0.43Ga0.57N epilayer grown under indium (In) ambient is of the order of 104 Ω-cm, while the resistivity of Mg-doped Al 0.43Ga0.57N grown without In assistance is of the order of 106 Ω-cm. The ultraviolet light-emitting diodes (UV-LEDs) using the In-assisted Mg-doped Al0.43Ga0.57N as the p-type layers were fabricated to verify the function of indium ambient. It is found that there are a lower turn-on voltage and a lower diode series resistance in the UV-LEDs fabricated with p-type Al0.43Ga0.57N layers grown under In-ambient.

Effects of Dislocation on High Temperature Transport Characteristics of Unintentionally Doped GaN

High temperature transport characteristics of unintentionally doped GaN have been investigated by means of high temperature Hall measurements from room temperature to 500°C. The increment of electron concentration from room temperature to 500°C is found to vary largely for different samples. The dispersion of temperature dependence of electron concentration is found to be directly proportional to the density of dislocations in GaN layers calculated by fitting the FWHM of the rocking curves in x-ray diffraction measurements (XRD). The buildup levels in persistent photoconductivity (PPC) are also shown to be directly proportional to the density of dislocations. The correlation of XRD, Hall and PPC results indicate that the high temperature dependence of electron density in unintentional doped GaN is directly dislocation related.

Barrier Enhancement Effect of Postannealing in Oxygen Ambient on Ni/AlGaN Schottky Contacts

Al0.2Ga0.8N/GaN samples are grown by metalorganic chemical vapour deposition (MOCVD) method on (0001) sapphire substrates. A 10 nm-thick Ni layer is deposited on AlGaN as the transparent Schottky contact. The effect of postannealing in oxygen ambient on the electrical properties of Ni/AlGaN is studied by current–voltage–temperature (I–V–T) measurement. The annealing at a relatively low temperature of 300°C for 90 s results in a decrease of the ideality factor from 2.03 to 1.30 and an increase of the Schottky barrier height from 0.77 eV to 0.954 eV. The I–V–T analysis confirms the improvement originated from the formation of NiO, a layer with higher resistance, which could passivate the surface states of AlGaN and suppress the tunnelling current. Furthermore, the annealing also leads to an increase of the transmittance of the contacts from 57.5% to 78.2%, which would be favourable for AlGaN-based photodetectors.

AlGaN-Based Solar-Blind Schottky Photodetectors Fabricated on AlN/Sapphire Template

We report AlGaN-based back-illuminated solar-blind Schottky-type ultraviolet photodetectors with the cutoff-wavelength from 280nm to 292nm without bias. The devices show low dark current of 2.1 × 10−6A/cm2 at the reverse bias of 5 V. The specific detectivity D* is estimated to be 3.3 × 1012cmHz1/2 W−1. To guarantee the performance of the photodetectors, the optimization of AlGaN growth and annealing condition for Schottky contacts were performed. The results show that high-temperature annealing method for Ni/Pt Schottky contacts is effective for the reduction of leakage current.

Strain Effect on Photoluminescences from InGaN MQWs with Different Barriers Grown by MOCVD

InGaN/GaN MQWs, InGaN/AlGaN MQWs and InGaN/AlInGaN MQWs are grown on (0001) sapphire substrates by MOCVD. Membrane samples are fabricated by laser lift-off technology. The photoluminescence spectra of membranes show a blue shift of peak positions in InGaN/GaN MQWs, a red shift of peak positions in InGaN/AlGaN MQWs and no shift of peak positions in InGaN/AlInGaN MQWs from those of samples with substrates. Different changes in Raman scattering spectra and HR-XRD (0002) profile of InGaN/AlInGaN MQWs, from those of InGaN/GaN MQWs and InGaN/AlGaN MQWs, are observed. The fact that the strain changes differently among InGaN MQWs with different barriers is confirmed. The AlInGaN barrier could adjust the residual stress for the least strain-induced electric field in InGaN/AlInGaN quantum wells.

Influence of applied electric field on the absorption coefficient and subband distances in asymmetrical AlN/GaN coupled double quantum wells

The influence of applied electric fields on the absorption coefficient and subband distances in asymmetrical AlN/GaN coupled double quantum wells (CDQWs) has been investigated by solving Schrodinger and Poisson equations self-consistently. It is found that the absorption coefficient of the intersubband transition (ISBT) between the ground state and the third excited state (1odd − 2even) can be equal to zero when the electric fields are applied in asymmetrical AlN/GaN CDQWs, which is related to applied electric fields induced symmetry recovery of these states. Meanwhile, the energy distances between 1odd − 2even and 1even − 2even subbands have different relationships from each other with the increase of applied electric fields due to the different polarization-induced potential drops between the left and the right wells. The results indicate that an electrical-optical modulator operated within the opto-communication wavelength range can be realized in spite of the strong polarization-induced electric fields in asymmetrical AlN/GaN CDQWs.

Strain effects on the polarized optical properties of InGaN with different In compositions

Strain effects on the polarized optical properties of c-plane and m-plane InxGa1−xN were discussed for different In compositions (x = 0, 0.05, 0.10, 0.15) by analyzing the relative oscillator strength (ROS) and energy level splitting of the three transitions related to the top three valence bands (VBs). The ROS was calculated by applying the effective-mass Hamiltonian based on k · p perturbation theory. For c-plane InxGa1−xN, it was found that the ROS of |X) and |Y)-like states were superposed with each other. Especially, under compressive strain, they dominated in the top VB whose energy level also went up with strain, while the ROS of the |Z)-like state decreased in the second band. For m-plane InxGa1−xN under compressive strain, the top three VBs were dominated by |X), |Z), and |Y)-like states, respectively, which led to nearly linearly-polarized light emissions. For the top VB, ROS difference between |X) and |Z)-like states became larger with compressive strain. It was also found that such tendencies were more evident in layers with higher In compositions. As a result, there would be more TE modes in total emissions from both c-plane and m-plane InGaN with compressive strain and In content, leading to a larger polarization degree. Experimental results of luminescence from InGaN/GaN quantum wells (QWs) showed good coincidence with our calculations.

Electrical Characteristics of InGaN/AlGaN and InGaN/GaN MQW Near UV-LEDs

Electrical characteristics of In0.05Ga0.95N/Al0.07Ga0.93N and In0.05Ga0.95N/GaN multiple quantum well (MQW) ultraviolet light-emitting diodes (UV-LEDs) at 400 nm wavelength are measured. It is found that for InGaN/AlGaN MQW LEDs, both ideality factor and parallel resistance are similar to those of InGaN/GaN MQW LEDs, while series resistance is two times larger. It is suggested that the Al0.07Ga0.93N barrier layer did not change crystal quality and electrical characteristic of p-n junction either, but brought larger series resistance. As a result, InGaN/AlGaN MQW LEDs suffer more serious thermal dissipation problem although they show higher light output efficiency.

Microstructures of GaN films laterally overgrown on Si(111) by hydride vapour phase epitaxy

We have investigated the microstructures of GaN films laterally epitaxially overgrown (LEO) on Si(111) substrates using hydride vapour phase epitaxy. The threading dislocation density in the LEO GaN is reduced by about two orders. Different etching angles of the two sidewalls of SiO2 masks (66o and 90o) lead to the asymmetry of the LEO and cause the particular microstructures of LEO GaN. In micro-Raman spectra, the intensities vary weakly periodically about 5 µm perpendicular to the mask stripes. The indistinct selective growth in the top surface is discussed.