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 li quid-helium temperature, of the excitonic e1-hh1 transition in the wel l. The ensemble of the data, taken over a wide range of optical excita tion levels, for various values of the tunneling-barrier thickness, an d before and after passivation of the surface by hydrogen, allows a de scription both of the details of the tunneling mechanism and of the ch aracter 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, antisit e-As donors as dominating surface trap's, provides an appropriate desc ription of the state distribution at the interface between AlGaAs and its oxide; (iii) hole accumulation in surface states, resulting from t he nominally different unipolar tunneling probability for the two carr iers (and increasing with excitation level), generates a dipole electr ic field across the tunneling barrier, extending into the well; (iv) h ydrogenation efficiently passivates electron trapping in surface state s, but not hole tunneling and the consequent generation of a surface f ield by illumination; (v) the experimental findings agree with a model for ambipolar tunneling based on a self-consistent quantum-mechanical approach.