EFFECTS OF METAL-ION CHEMISORPTION ON GAAS SURFACE RECOMBINATION - PICOSECOND LUMINESCENCE DECAY MEASUREMENTS

n-GaAs/KOH-Se--/2-(aq) contacts have been studied using real time photoluminescence decay techniques. This system is of interest because metal ion chemisorption improves the steady-state current-voltage properties of GaAs/KOH-Se--/2-(aq)/Pt cells, yielding 16% efficiency under simulated 1-sun illumination conditions. In this work, the luminescence decay dynamics of thin epilayer GaAs samples under high level injection conditions were monitored in contact with KOH-Se--/2-(aq) solutions. The photoluminescence signals decayed more rapidly after metal ion chemisorption than after a fresh etch, indicating that the metal ion treatment induced a more active recombination and/or charge-transfer process than the etch. A finite-difference simulation was used to model the decays and to extract a minority carrier surface recombination velocity, S-min, for these systems. For etched GaAs surfaces, S-min = 5 x 10(3) cm s(-1), while GaAs surfaces that had been etched and then exposed to 0.010 M Co(NH3)(6)(3+) (pH = 11) solutions displayed S-min = 2 X 10(5) cm s(-1). Qualitatively similar behavior was observed for Rh-, Ru-, and Os-treated GaAs surfaces as well. These data are fully consistent with prior suggestions that the primary effect of metal ion chemisorption is to increase the rate of hole transfer to the Se--/2-(aq) electrolyte, as opposed to decreasing surface recombination processes at the GaAs/liquid contact.