Evidence of Potential Fluctuations in Modulation Doped GaN/AlGaN Heterostructures
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[3].
585 Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
This work investigates experimental evidence for the presence of microscopic potential fluctuations in modulation doped GaN/AlGaN heterostructures. We report on transport results for A10.28Ga 0 72N/GaN MDHs and compare these with predictions from weak localization (WL) theories for a 2DEG. Photoconductivity together with photoluminescence provide additional evidence for the influence of PFs on the recombination processes. EXPERIMENT The investigated Al0.28Ga 0.72N/GaN/A1N/sapphire MDHs were grown by Metal Organic Chemical Vapor Deposition (MOCVD) [7]. The GaN layer was nominally undoped and 3.85 gim thick. The top AlGaN layer included a 5 nm thick undoped spacer layer close to the GaN interface, and a 90 nm thick layer which was Si-doped to a concentration of 3"1018 cm 3 . The transport studies were performed within the temperature range 1.5 - 250 K in a superconducting solenoid magnet Oxford SM 2000. Hall effect measurements were done under an applied magnetic field (B) of 0.3 Tesla. All transport data presented in this paper were taken by a low-amplitude dc-current (1 - 5 ptA) technique. The magnetoresistance was measured in a magnetic field up to 2 T in a direction normal to the plane of the MDHs. Measurements were made on lithographically defined Hall bars with six Al/Ti Ohmic contacts. Stationary and persistent photoconductivity measurements have been performed in a variable temperature cryostat (2 - 300 K) in a standard way [8]. For photoluminescence (PL) measurements, the sample was excited with a Ti: Sapphire solid state picosecond pulsed tunable laser, upconverted with a BBO crystal, to obtain excitation around the GaN bandgap. The detection of the PL spectra was performed with a cooled CCD camera. Photoluminescence excitation (PLE) measurements were performed in the same set-up, where the natural spectral width of the pulsed laser limited the spectral resolution to about 2 meV. Time resolved measurements were performed with a Hamamatsu Syncroscan streak camera system. RESULTS Electrical transport in GaN/A1GaN modulation-doped heterostructures. The variation of the total resistivity Ptot with temperature is shown in Fig. 1. There are three different temperature intervals where ptot shows a characteristic dependence versus temperature. In the initial low-temperature range the value Ptot decreases slightly with T, demonstrating, as we show later, the weakly localized transport. The conductivity transition from this one to free carrier transport is observed at T = 10 - 25 K. The "high" temperature interval T > 100 K shows the well-known phonon assisted increase of Ptot with T. Fig. 2 shows the temperature dependence of mobility and sheet electron concentration in the temperature interval for free carrier transport. The total MDH electron mobility increases with decreasing temperature and saturated at a value of g = 1470 cm 2/Vs at 80 K. It also remained fairly constant for temperatures between 80 and 10 K. The total sheet concent
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