Photo-Induced Reversible Modification of II-VI Semiconductor Surface Composition and Structure
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PHOTO-INDUCED REVERSIBLE MODIFICATION OF 1I-VI SEMICONDUCTOR SURFACE COMPOSITION AND STRUCTURE P.D. BREWER, J.J. ZINCK, and G.L. OLSON Hughes Research Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265
ABSTRACT It is shown that the composition and structure of CdTe and CdS surfaces can be reversibly controlled by excimer laser irradiation at fluences below the melting threshold. The removal rate is observed to depend exponentially on laser fluence up to the melting threshold. The translational energies of products desorbed from laser-irradiated CdTe surfaces were determined using time-of-flight spectrometry and are well-described by a Maxwellian velocity distribution. The dynamics of the photo-stimulated desorption process are correlated with the laser-induced changes in composition, and it is shown that the data are consistent with a thermal mechanism for desorption. A model is introduced which describes the reversible, fluence-dependent changes in composition and structure in terms of the kinetic competition between formation and desorption processes at the semiconductor surface.
INTRODUCTION A comprehensive understanding of laser-induced desorption of compound semiconductor surfaces requires information on the identity and dynamics of the photo-desorbed products and the effect of laser irradiation on surface structure and composition. Extensive studies of the translational and angular distributions of desorbed species from a variety of compound semiconductor surfaces have been conducted [ 1-61. However, many of the observations in those studies, such as athermal velocity distributions and non-cosine angular distributions, which helped craft the initial thinking about the mechanism of the laser-surface interaction, were due to multiple collisions in the early stages of the desorption [71 and not a result of an electronically excited surface process as originally proposed 181. This has forced a reevaluation of the mechanisms responsible in the laser-surface interaction and emphasize the need to correlate surface structure with desorption dynamics in understanding these complex phenomena. Namiki er al. [1,61 showed that species desorbed from compound semiconductor surfaces have average translational energies in the thermal range (-0.1 eV). They suggested that the dimerization of the non-metallic element on the surface plays an important role in the desorption process, and that there exists a threshold fluence for desorption which depends inversely on the bond ionicity of the material. However, there are some exceptions to these general findings. For example in GaN an anomalous average translational energy is observed for Ga atoms (-3 eV) as compared to N2 molecules (< 0.1 eV) desorbed under nitrogen laser irradiation [11. Since the laser-induced changes in surface composition or structure during the laser desorption experiments have not been well characterized, the origins of these anomalies may arise from surface defects or disorder caused by the laser radiation. In this paper we examine the fluence-depende
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