Optimization of high mobility two-dimensional hole gases

Abstract

Modulation-doped (Al,Ga)As two-dimensional hole gas (2DHG) structures have been grown by molecular-beam epitaxy (MBE) on the (311)A surface of GaAs using silicon as a p-type dopant. Systematic variations of carrier density ps and mobility μ with undoped spacer thickness were observed at 1.7 K, with a peak mobility of 4.95×105 cm2 V−1 s−1 at a sheet carrier density of 2.2×1011 cm−2 occurring at a spacer thickness of 200 Å. The mobility and carrier density of all samples were also measured over the temperature range 300 mK to 4.2 K. These results lead to the conclusion that for samples with spacer thicknesses greater than 400 Å, acoustic phonon scattering limits the mobility at low temperatures, whereas for samples with spacer thicknesses less than this value, ionized impurity scattering was observed to be the dominant scattering mechanism. As observed with electron gases, an increase in mobility was achieved by using a thicker region of lightly doped (Al,Ga)As, giving a sample with a mobility of 7.2×105 cm2 V−1 s−1, at a carrier density of 8.7×1010 cm−2 measured at 300 mK. This value constitutes the highest mobility so far reported, at carrier densities below 1×1011 cm−2.