Effect of above band-gap illumination on structure of free exciton reflection spectra of ZnSe

The fine structure of free exciton reflection spectra is often observed in bulk and heteroepitaxial ZnSe epilayers of thickness higher than the strain relaxation thickness. This modification of excitonic lines gives no way of direct correlation between optical spectra and energetic structure of semiconductors. We present results on well-resolved measurements and theoretical treatment of reflectance spectra in the free-exciton region of MOVPE-grown ZnSe/GaAs epilayers under additional above band-gap illumination. The measurements were performed at T = 77 K, in order to eliminate the polariton phenomena from consideration. A radical change was observed, even the total reversion of the spectral line contour after external illumination of the power of P = 1 mW. A multilayer model of the near-surface region was used to analyze the mechanism of lineshape formation, the resonance energy and the decay parameter of free excitons. They were estimated by fitting procedure. It was shown that the spike structure of the ZnSe reflection spectra at high values of damping parameter of excitons is mainly due to the surface electric field and excitonic Stark effect. The characteristic of the electric field distribution in the space-charge region was evaluated for various illumination conditions. The nature of illumination induced surface charged states is discussed. (C) 2000 Elsevier Science S.A. All rights reserved.