Band-tail shape and transport near the metal-insulator transition in Si-doped Al(0.3)Ga(0.7)As

In the present work, an infrared LED is used to photodope MBE-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 (1x10^19 cm^-3 for sample 1 (S1) and 9x10^17 cm^-3 for sample 2 (S2)) and vary the effective electron density between 10^14 and 10^18 cm^-3. The metal-insulator transition (MIT) for S1 is found to occur at a critical carrier concentration of 5.7x10^16 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 (DOS) in Al(0.3)Ga(0.7)As is found electrically through transport measurements. It is determined to be exponential in character, with an exponent of -1.25 for S1 and -1.38 for S2.