Surface recombination in doped semiconductors: Effect of light excitation power and of surface passivation

For n- and p-type semiconductors doped above the 10¹⁶ cm ^-3 range, simple analytical expressions for the surface recombination velocity S have been obtained as a function of excitation power P and surface state density N_T. These predictions are in excellent agreement with measurements on p-type GaAs films, using a novel polarized microluminescence technique. The effect on S of surface passivation is a combination of the changes of three factors, each of which depends on N_T: (i) a power-independent factor which is inversely proportional to N_T and (ii) two factors which reveal the effect of photovoltage and the shift of the electron surface quasi Fermi level, respectively. In the whole range of accessible excitation powers, these two factors play a significant role so that S always depends on power. Three physical regimes are outlined. In the first regime, illustrated experimentally by the oxidized GaAs surface, S depends on P as a power law of exponent determined by N_T. A decrease of S such as the one induced by sulfide passivation is caused by a marginal decrease of N_T. In a second regime, as illustrated by GaInP-encapsulated GaAs, because of the reduced value of S, the photoelectron concentration in the subsurface depletion layer can no longer be neglected. Thus, S^-1 depends logarithmically on P and very weakly on surface state density. In a third regime, expected at extremely small values of P, the photovoltage is comparable to the thermal energy, and S increases with P and decreases with increasing N_T.