Band-tail shape and transport near the metal-insulator transition in Si-doped Al_0.3Ga_0.7As

In the present work, an infrared light-emitting diode is used to photodope molecular-beam-epitaxy-grown Si: Al_0.3 Ga_0.7 As, a well-known persistent photoconductor, to vary the effective electron concentration of samples in situ. Using this technique, we examine the transport properties of two samples containing different nominal doping concentrations of Si [1× 10¹⁹ cm^-3 for sample 1 (S1) and 9× 10¹⁷ cm^-3 for sample 2 (S2)] and vary the effective electron density between 1014 and 1018 cm^-3. The metal-insulator transition for S1 is found to occur at a critical carrier concentration of 5.7× 10¹⁶ cm^-3 at 350 mK. The mobilities in both samples are found to be limited by ionized impurity scattering in the temperature range probed, and are adequately described by the Brooks-Herring screening theory for higher carrier densities. The shape of the band tail of the density of states in Al_0.3Ga_0.7 As is found electrically through transport measurements. It is determined to have a power-law dependence, with an exponent of -1.25 for S1 and -1.38 for S2.