PeiQiang Xu

Analyses of 2-DEG characteristics in GaN HEMT with AlN/GaN super-lattice as barrier layer grown by MOCVD

GaN-based high-electron mobility transistors (HEMTs) with AlN/GaN super-lattices (SLs) (4 to 10 periods) as barriers were prepared on (0001) sapphire substrates. An innovative method of calculating the concentration of two-dimensional electron gas (2-DEG) was brought up when AlN/GaN SLs were used as barriers. With this method, the energy band structure of AlN/GaN SLs was analyzed, and it was found that the concentration of 2-DEG is related to the thickness of AlN barrier and the thickness of the period; however, it is independent of the total thickness of the AlN/GaN SLs. In addition, we consider that the sheet carrier concentration in every SL period is equivalent and the 2-DEG concentration measured by Hall effect is the average value in one SL period. The calculation result fitted well with the experimental data. So, we proposed that our method can be conveniently applied to calculate the 2-DEG concentration of HEMT with the AlN/GaN SL barrier.

Improvement of Breakdown Characteristics of a GaN HEMT with AlGaN Buffer Layer

High-electron-mobility transistors (HEMTs) with a highly resistive two-layer buffer layer (AlGaN/GaN) were grown on 6H-SiC substrates by metalorganic chemical vapor deposition. The characteristics were compared with those of conventional HEMTs utilizing GaN as the high-resistivity buffer. The results of x-ray diffraction and atomic force microscopy indicate that the crystal quality of the HEMT heterostructure is not deteriorated by the AlGaN buffer layer. The direct-current (DC) characteristics of the HEMTs with the two different structures are similar, while the off-state breakdown voltage is enhanced and the mobility of the two-dimensional electron gas is improved by the AlGaN buffer layer. The reasons for the effects of the AlGaN buffer layer are discussed systematically.

Tunable emission wavelength of InGaN/GaN multiquantum wells employing strain-accommodative structures

In this paper, we focused on tuning the emission wavelength of InGaN/GaN multi-quantum wells (MQW) employing strain-accommodative structures. Generally, the adjustment of emitting wavelength is realized by controlling the quantum well (QW) thickness and the QW growth temperature, which decides the indium concentration. It needs large thickness and low temperature to emit long wavelength photons. However, the material quality, electrical and optical properties will degrade with low growth temperature or wide QW. Meanwhile, the growth of long wavelength LEDs based on the InGaN material still faces severe difficulties because of the large (11%) lattice mismatch between InN and GaN and the strong piezoelectric field-induced quantum-confined Stark effect (QCSE) induced by the high strain due to lattice mismatch. Compared to the conventional LEDs, LEDs with proper strain-accommodative structures not only increase the emitting wavelength but also reduce the strain in InGaN well. It provides an alternative approach to tune the wavelength. Two types of strain-accommodative structures are inserted between n-GaN and the multi-quantum wells: one is short period super lattices (SPSL) consisted of 15 period of the 1-nm-thick InGaN well and the 2-nm-thick GaN barrier , and the other is 45nm InxGa1-xN (x=0.07-0.09). The samples with strain-accommodative structures demonstrate that: firstly the two structures would efficiently increase the wavelength, which should be attributed to the relief of strain in the InGaN/GaN MQWs. The wavelengths of the two structures in the electroluminescence measurement were 561.6nm and 531nm, respectively. It is longer than that of the control sample (511.8nm). Secondly; the structures can weaken the QSCE. When the current increased from 3mA to 20mA during the electroluminescence measurement, the peak wavelength blue-shift were 5.1nm and 3.1nm, respectively. It is smaller than that of the control sample (7.4nm).