Ballistic-electron-emission spectroscopy of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ heterostructures: Conduction-band offsets, transport mechanisms, and band-structure effects

We report an extensive investigation of semiconductor band-structure effects in single-barrier ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ 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 $(0\mathrm{}<~x<~1)$. The initial (Γ) BEES thresholds for ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ 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 ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ X valley states. Low-temperature spectra show a marked reduction in this intervalley current component, indicating that intervalley phonon scattering at the GaAs/${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ interface produces a significant fraction of thisX valley current. A comparison of the BEES thresholds with the expected composition dependence of the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ Γ, L, and X points yields good agreement over the entire composition range.