Electrical transport of an AlGaN/GaN two-dimensional electron gas
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phase epitaxy on a sapphire substrate. The AlxGa1-xN was approximately 50 nm thick. The details of the growth have been reported previously. 10 Hall effect measurements were performed over a temperature range of 10 – 300 K and a magnetic field range of 0 – 2 T. Both the Hall effect and the SdH measurements were taken in the four-probe van der Pauw configuration using annealed Ti/Al contacts placed at the four corners of a square. Measurements of SdH magnetoresistance oscillations were performed using a low ac bias current to avoid electron heating. A magnetic field of 0 - 9 T was used. The temperature was controlled from 1.2 – 4.2 K by immersing the sample in liquid helium and setting the pressure. A calibrated thermometer was then used to measure the temperature. RESULTS AND DISCUSSION As seen in Fig. 1, the sheet electron concentration and mobility measured by the Hall effect are nearly independent of temperature below about 100 K. This behavior is typical of two dimensional electron gas structures. At 10 K, the electron concentration, nH, was 5.06 x 1012 cm-2, and the Hall mobility, µH, was 1.91 x 104 cm2/Vs. The transport scattering time, τ c = µ H m * / e , where m* is the effective mass and e is the electron charge, was found to be 2.34 ps. 14
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(a) (b) Figure 1. (a) Hall sheet carrier concentration nH as a function of inverse temperature. (b) Hall mobility µH as a function of temperature. In order to determine if there was any significant parallel conduction at low temperature, magnetic field dependent Hall effect measurements were taken. The data taken at 10 K is plotted in Fig. 2 (a) along with the fit as in the analysis of Kim et al. 11 The reduced conductivities, X and Y, are σxx and 2σxy respectively divided by the zero field conductivity. When the reduced conductivities are plotted in this manner and only a single electron mobility µ is present, at a magnetic field B=1/µ, X passes through 0.5 and Y peaks with a magnitude of 1. It is clear from this data that there is only one singlecarrier conduction path at 10 K. The single carrier fit to this data gives an electron
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concentration of 5.08 x 1012 cm-2 and a mobility of 1.90 x 104 cm2/Vs, in very good agreement with the single magnetic field Hall taken at 0.5 T. The mobility spectrum shown in Fig. 2(b) was obtain
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