Ballistic-electron-emission spectroscopy of Al_xGa_1-xAs/GaAs heterostructures: conduction-band offsets, transport mechanisms, and band-structure effects

Abstract

We report an extensive investigation of semiconductor band-structure effects in single-barrier Al_xGa_1-xAs/GaAs heterostructures using ballistic-electron-emission spectroscopy (BEES). The transport mechanisms in these single-barrier structures were studied systematically as a function of temperature and Al composition over the full compositional range (0≤x≤1). The initial (Γ) BEES thresholds for Al_xGa_1-xAs single barriers with 0≤x≤0.42 were extracted using a model which includes the complete transmission probability of the metal-semiconductor interface and the semiconductor heterostructure. Band offsets measured by BEES are in good agreement with previous measurements by other techniques which demonstrates the accuracy of this technique. BEES measurements at 77 K give the same band-offset values as at room temperature. When a reverse bias is applied to the heterostructures, the BEES thresholds shift to lower voltages in good agreement with the expected bias-induced band-bending. In the indirect band-gap regime (x>0.45), spectra show a weak ballistic-electron-emission microscopy current contribution due to intervalley scattering through Al_xGa_1-xAs X valley states. Low-temperature spectra show a marked reduction in this intervalley current component, indicating that intervalley phonon scattering at the GaAs/Al_xGa_1-xAs interface produces a significant fraction of this X valley current. A comparison of the BEES thresholds with the expected composition dependence of the Al_xGa_1-xAs Γ, L, and X points yields good agreement over the entire composition range.