Hole mobility enhancement in GaAs/p-Al_0.3Ga_0.7As QW-HEMT structures with (4 1 1)A super-flat interfaces grown by MBE

Be-doped GaAs/p-Al_0.3Ga_0.7As QW-HEMT structures with well width of L_w = 2, 7, 10, 20 and 50 nm were simultaneously grown on (4 1 1)A and (1 0 0) GaAs substrates by MBE. Be-doping concentration in the p-Al_0.3Ga_0.7As layer was 1.4 × 10¹⁸cm^-3, and the undoped Al_0.3Ga_0.7As spacer layer was 6 nm thick. Hole concentration was about 6 × 10¹¹ cm^-2 for L_w > 7 nm. Hole mobility at 10K in the conventional (1 0 0) samples remained almost constant (about 20 000 cm²/(Vs) for the current in the [0 1 1] direction) with decreasing L_w down to 7 nm and it rapidly decreased to 2760 cm²/(Vs) at L_w = 2 nm. On the other hand, in the (4 1 1)A samples, hole mobility (10 K) increased from 17 500 cm²/(Vs) (L_w = 50 nm) to 33 900 cm²/(V s) (L_w = 10 nm) and dropped rapidly down to 4090 cm²/(V s) (L_w = 2 nm) for a current direction of [0 1 1]. This significant enhancement of hole mobility in the (4 1 1)A samples is mainly due to (1) the significantly reduced interface roughness scattering of 2DHG when using the (4 1 1)A super-flat interfaces and (2) the reduced effective mass of holes in the narrow (4 1 1)A QWs (L_w = 7-20 nm). Shubnikov-de Haas (SdH) measurements on the (4 1 1)A QW sample (L_w = 7 nm) confirmed the reduced effective mass (0.30m₀) of holes.