ROLE OF THE X MINIMUM IN TRANSPORT THROUGH ALAS SINGLE-BARRIER STRUCTURES

We report an electrical transport and electroluminescence (EL) spectro scopy study of single-barrier GaAs-AlAs-GaAs p-i-n tunnel structures w ith a barrier thickness in the range 4.5-8.0 nm. The results permit us to determine directly the relative roles of nonresonant Gamma-Gamma t unneling and resonant Gamma-X-Gamma intervalley transfer in the transp ort through the indirect-gap tunnel barriers. The Gamma-X-Gamma transp ort is shown to take place predominantly without conservation of trans verse wave vector (k(parallel to)), with k(parallel to)-conserving sca ttering via X-z states only significantly close to the onset of Gamma- X intervalley transfer. By detecting extremely weak EL arising from ex cited X-z states we show that the complete Gamma-X-Gamma transport pro cess is very strongly sequential and determine, quantitatively, the co mparative time scales for X-Gamma and inter-X-level scattering. The bi as-dependent Gamma-X-Gamma and Gamma-Gamma transport times are determi ned for AlAs barrier widths in the range 3.0-10 nm. The intervalley Ga mma-X-Gamma transport model yields results in good agreement with expe riment and demonstrates that, providing intervalley Gamma-X transfer i s energetically possible, nonresonant Gamma-Gamma tunneling only contr ibutes significantly to the transport characteristics for barrier widt hs of similar to 3 nm or less. [S0163-1829(98)04540-8].