Interaction mechanisms of near-surface quantum wells with oxidized and H-passivated AlGaAs surfaces

The tunneling mechanism of electrons and holes to surface states from near-surface Al0.3Ga0.7As/GaAs quantum wells has been investigated by steady-state and time-resolved photoluminescence spectroscopy, near liquid-helium temperature, of the excitonic e1-hh1 transition in the well. The ensemble of the data, taken over a wide range of optical excitation levels, for various values of the tunneling-barrier thickness, and before and after passivation of the surface by hydrogen, allows a description both of the details of the tunneling mechanism and of the character and behavior of relevant surface states. The main results are summarized as follows: (i) steady-state tunneling is ambipolar, namely, separate for electrons and holes, rather than excitonic; (ii) Spicer's advanced unified defect model for an oxidized GaAs surface, antisite-As donors as dominating surface trap's, provides an appropriate description of the state distribution at the interface between AlGaAs and its oxide; (iii) hole accumulation in surface states, resulting from the nominally different unipolar tunneling probability for the two carriers (and increasing with excitation level), generates a dipole electric field across the tunneling barrier, extending into the well; (iv) hydrogenation efficiently passivates electron trapping in surface states, but not hole tunneling and the consequent generation of a surface field by illumination; (v) the experimental findings agree with a model for ambipolar tunneling based on a self-consistent quantum-mechanical approach.