Electron-beam-stimulated processes at CdS surfaces observed by real-time atomic-resolution electron microscopy
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Daniel J. Ehrlich Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02173
(Received 14 April 1986; accepted 2 June 1986) Electron-beam-induced changes in the structure of partially amorphous CdS surfaces have been observed directly by atomic-resolution electron microscopy. A sequence of atomic rearrangements leading to nucleation and growth of cubic CdS and hexagonal Cd has been documented. Inelastic electron collisions lead to crystallization of overlying amorphous CdS material whereas electron-stimulated desorption of S from the underlying CdS crystal results in precipitation of Cd crystallites at the crystalline/amorphous interface. From 100to 500 keV the events are almost energy-independent.
I. INTRODUCTION Recent studies by electron microscopy have provided important insights into the mechanisms of electron-irradiation-induced transformations.’ In this paper we report a study at atomic resolution in which it has been found that moderate energy, 100-500 keV, electrons can induce crystallization and phase separation on CdS surfaces by nonthermal interactions. It is found that amorphous and crystalline regions of these surfaces respond by a very different sequence of events: the former crystallize primarily to cubic CdS while, for the same irradiation, the latter decompose by selective nonthermal electron-stimulated desorption of S, precipitating crystalline Cd. Cadmium sulfide is used in a number of electronic applications. It is sensitive to incident photons and electrons, and the latter are known to cause both microscopic and macroscopic surface modifications even for energies as low as 2 keV.’ Especially given the longstanding interest in CdS as a candidate for electron-beampumped semiconductor laser^,^*^ it is highly relevant to characterize the various electron-stimulated processes as fully as possible. In this paper, we describe real-time in situ electron microscope observations which have provided considerable insight into the mechanics of these events. We have recently reported electron-beam crystallization by the same method.’
II. EXPERIMENTAL Single-crystal, vapor-phase-grown CdS samples in (1010) and (1 120) orientations were chemically polished on one side using the procedures which have been specially developed for causing minimal damage during the preparation of surface-pumped lasers in this material.6.7Despite the optimization of these methods, our observations show that approximately 10-20 atomic lay560
J. Mater. Res. 1 (4), Jul/Aug 1986
http://journals.cambridge.org
ers at the crystal surface are typically amorphized. Polished samples were thinned by reverse-surface jet etching in dilute HC1 or by argon-ion milling. Samples prepared by both thinning methods appeared very similar with no significant density of extended defects; this result is in contrast to previous observations of CdTe.’ Observations were made with the Cambridge University high-resolution electron microscope (HREM),’ operated at 500 kV and with the JEOL 4000EX at Arizona State University,
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