Heterojunction-induced phenomena in Hall effect and photoconductivity measurements of epitaxial AlxGa1−xAs

Abstract

Temperature-dependent Hall effect measurements are reported for n-type, Si-doped Al0.32Ga0.68As films grown on semi-insulating GaAs substrates. Results are presented for films grown with and without undoped Al0.32Ga0.68As buffer layers. For T≥200 K, the free electron concentration in these Si-doped films has an exponential dependence on temperature with an activation energy of ∼93 meV. The temperature-independent free-electron concentration observed for T≤100 K is due to conduction in the GaAs substrate near the substrate-epilayer interface (i.e., a two-dimensional electron gas). Hall mobilities measured in thin (≤5 μm) AlxGa1−xAs films, particularly at low temperatures, can be dominated by this conduction mechanism and are thus invalid as a measure of the transport properties of the AlxGa1−xAs films. In addition, photoconductivity data are presented which show that the persistent photoconductivity effect observed in these films is due primarily to charge separation at the AlxGa1−xAs/GaAs heterojunction.