The influence of applied electric fields on the absorption coefficient and subband distances in asymmetrical A1N/GaN coupled double quantum wells （CDQWs） has been investigated by solving SchrSdinger 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 （lodd - 2even） can be equal to zero when the electric fields are applied in asymmetrical A1N/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 A1N/GaN CDQWs.
This paper calculates the wavelengths of the interband transitions as a function of the Al mole fraction of AlxGa1-xN bulk materml. It is finds that when the Al mole fraction is between 0.456 and 0.639, the wavelengths correspond to the solar-blind （250 nm to 280 nm）. The influence of the structure parameters of AlyGa1-yN/GaN quantum wells on the wavelength and absorption coefficient of intersubband transitions has been investigated by solving the SchrSdinger and Poisson equations self-consistently. The Al mole fraction of the AlyGa1-yN barrier changes from 0.30 to 0.46, meanwhile the w;dth of the well changes from 2.9 nm to 2.2 am, for maximal intersubband absorption in the window of the air （3μm 〈 A 〈 5μm）. The absorption coefficient of the intersubband transition between the ground state and the first excited state decreases with the increase of the wavelength. The results are finally used to discuss the prospects of GaN-based bulk material and quantum wells for a solar-blind and middle infrared two-colour photodetector.
A novel wet etching method for AlGaN/GaN heterojunction structures is proposed using thermal oxidation f ollowed by wet etching in KOH solution.It is found that an AlGaN/GaN heterostructure after high temperature oxidation above 700℃could be etched off in a homothermal（70℃） KOH solution while the KOH solution had no etching effects on the region of the AlGaN/GaN heterostructure protected by a SiO_2 layer during the oxidation process.A groove structure with 150 nm step depth on an AlGaN/GaN heterostructure was formed after 8 h thermal oxidation at 900℃followed by 30 min treatment in 70℃KOH solution.As the oxidation time increases,the etching depth approaches saturation and the roughness of the etched surface becomes much better.The physical mechanism of this phenomenon is also discussed.
By using temperature-dependent Hall, variable-frequency capacitance-voltage and cathodoluminescence （CL） measurements, the identification of inductively coupled plasma （ICP）-induced defect states around the AlxGa1-xN/GaN heterointerface and their elimination by subsequent annealing in AlxGa1-xN/GaN heterostructures are systematically investigated. The energy levels of interface states with activation energies in a range from 0.211 to 0.253 eV below the conduction band of GaN are observed. The interface state density after the ICP-etching process is as high as 2.75× 10^12 cm^-2.eV^-1. The ICP-induced interface states could be reduced by two orders of magnitude by subsequent annealing in N2 ambient. The CL studies indicate that the ICP-induced defects should be Ga-vacancy related.
In this article, a detailed analysis of the wet- etching technique for AIGaN/GaN heterostructure using dry thermal oxidation followed by a wet alkali etching was performed. The experimental results show that the oxida- tion plays a key role in the wet-etching method and the etching depth is mainly determined by the oxidation tem- perature and time. The correlation of etching roughness with oxidation time and temperature was investigated. It is found that there exists a critical oxidation temperature in the oxidation process. Finally, a physical explanation of the oxidation procedure for A1GaN layer was given.
Yang LiuJin-Yan WangZhe XuJin-Bao CaiMao-Jun WangMin YuBing XieWen-Gang
In contrast with Au/Ni/Al0.25Ga0.75N/GaN Schottky contacts, this paper systematically investigates the effect of thermal annealing of Au/Pt/Alo.25Ga0.75N/GaN structures on electrical properties of the two-dimensional electron gas in Alo.25Ga0.75N/CaN heterostructures by means of temperature-dependent Hall and temperature-dependent current-voltage measurements. The two-dimensional electron gas density of the samples with Pt cap layer increases after annealing in N2 ambience at 600℃ while the annealing treatment has little effect on the two-dimensional electron gas mobility in comparison with the samples with Ni cap layer. The experimental results indicate that the Au/Pt/Al0.25Ga0.75N/GaN Schottky contacts reduce the reverse leakage current density at high annealing temperatures of 400-600 ℃. As a conclusion, the better thermal stability of the Au/Pt/Alo.25Gao.75N/GaN Schottky contacts than the Au/Ni/Al0.25Ga0.75N/GaN Schottky contacts at high temperatures can be attributed to the inertness of the interface between Pt and AlxGa1-xN.
The temperature dependence of carrier transport properties of Alx Gal-xN/InyGal-yN/CaN and AlzGal-xN/GaN heterostructures has been investigated. It is shown that the Hall mobility in Alo.25Gao.75N/Ino.03Gao.97N/GaN heterostructures is higher than that in Alo.25Gao.75N/GaN heterostructures at temperatures above 500 K, even the mobility in the former is much lower than that in the latter at 300 K. More importantly, the electron sheet density in Alo.25Gao.75N/Ino.03Gao.97N/GaN heterostructures decreases slightly, whereas the electron sheet density in Al0.25Gao.75N/CaN heterostructures gradually increases with increasing temperature above 500 K. It is believed that an electron depletion layer is formed due to the negative polarization charges at the Iny Can-yN/GaN heterointerface induced by the compressive strain in the InyCal-yN channel, which effectively suppresses the parallel conductivity originating from the thermal excitation in the underlying GaN layer at high temperatures.
Semi-on DC stress experiments were conducted on A1GaN/GaN high electron mobility transistors （HEMTs） to find the degradation mechanisms during stress. A positive shift in threshold voltage （VT） and an increase in drain series resistance （RD） were observed after semi-on DC stress on the tested HEMTs. It was found that there exists a close correlation between the degree of drain current degradation and the variation in VT and RD. Our analysis shows that the variation in Vx is the main factor leading to the degradation of saturation drain current （IDs）, while the increase in RD results in the initial degradation of Ios in linear region in the initial several hours stress time and then the degradation of VT plays more important role. Based on brief analysis, the electron trapping effect induced by gate leakage and the hot electron effect are ascribed to the degradation of drain current during semi-on DC stress. We suggest that electrons in the gate current captured by the traps in the A1GaN layer under the gate metal result in the positive shift in VT and the trapping effect in the gate-drain access region induced by the hot electron effect accounts for the increase in RD.
