Role of the X minimum in transport through AlAs single-barrier structures

We report an electrical transport and electroluminescence (EL) spectroscopy study of single-barrier GaAs-AlAs-GaAs (Formula presented) tunnel structures with a barrier thickness in the range 4.5-8.0 nm. The results permit us to determine directly the relative roles of nonresonant Γ-Γ tunneling and resonant (Formula presented) intervalley transfer in the transport through the indirect-gap tunnel barriers. The (Formula presented) transport is shown to take place predominantly without conservation of transverse wave vector (Formula presented) with (Formula presented)-conserving scattering via (Formula presented) states only significantly close to the onset of (Formula presented) intervalley transfer. By detecting extremely weak EL arising from excited (Formula presented) states we show that the complete (Formula presented) transport process is very strongly sequential and determine, quantitatively, the comparative time scales for (Formula presented) and inter-X-level scattering. The bias-dependent (Formula presented) and Γ-Γ transport times are determined for AlAs barrier widths in the range 3.0-10 nm. The intervalley (Formula presented) transport model yields results in good agreement with experiment and demonstrates that, providing intervalley (Formula presented) transfer is energetically possible, nonresonant Γ-Γ tunneling only contributes significantly to the transport characteristics for barrier widths of ∼3 nm or less.