Transport Studies on Two-subband-populated AlGaN/GaN Heterostructures
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L11.47.1
Transport Studies on Two-subband-populated AlGaN/GaN Heterostructures D. R. Hang, C. F. Huang, Y. F. Chen, and B. Shen1 Department of Physics, National Taiwan University, Taipei, Taiwan, 106 Republic of China 1 National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing, 210093, China. ABSTRACT We report an investigation of electronic properties of two-dimensional electron gas (2DEG) confined at AlGaN/GaN heterostructures by magnetotransport measurements. The second-subband population is manifested by the multi-frequency in the Shubnikov-de Haas (SdH) oscillations. The modulated patterns of SdH oscillations which are due to the two-subband occupancy can be drastically enhanced by employing the microwave modulation technique. This unique advantage enables us to provide direct experimental evidence that the 2DEG in the second subband has a higher mobility than that in the first subband in the modulation-doped Al0.22Ga0.78N/GaN heterostructures by means of microwave-modulated magnetotransport measurements. The carrier concentrations and 2DEG Fermi energy for each subband were determined. It was found that the second-subband population ratio increases with spacer thickness up to 5 nm, while the subband separation decreases.
INTRODUCTION In the past several years, fundamental properties of AlXGa1-XN/GaN heterostructures have received a great amount of attention for the application of heterostructure-field-effect transistors (HFETs), which are capable of working at high frequencies in high-power and high-temperature environments [1-4]. They are very promising for applications in microwave and optoelectronic devices. Compared with other III-V material based HFETs, a larger amount of two-dimensional electron gas (2DEG) can be easily accumulated in GaN-based ones. The lattice mismatch of 2.5 % between AlN and GaN and the lack of inversion symmetry in the wurtzite structure result in large induced and spontaneous polarizations [3]. Therefore the better carrier confinement at AlXGa1-XN/GaN interface than that at AlXGa1-XAs/GaAs interface arising from the large conduction-band offset [4] and strong piezoelectric polarization of the AlXGa1-XN barrier layer naturally leads to a recent research interest in high-carrier-density AlXGa1-XN/GaN heterostructures in which multiple subbands were occupied [5-9]. Because of the different subband wave function distribution profile, the 2DEG mobility is expected to be enhanced once the second subband becomes more populated, and hence eases the design limitations and allows higher frequency performance for microwave and optoelectronic devices. In order to optimize the performance of HFETs with high electron densities, it is important to understand the electronic properties of 2DEG of high densities at AlXGa1-XN/GaN heterointerface. In this paper, we report on an investigation of electronic properties of 2DEG confined in spacer-dependent Al0.22Ga0.78N/GaN heterostructures by magnetotransport measurements.
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