Transformation steps of microstructures in photodeposited films of a-Se
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IBM Research, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 (Received 17 March 1988; accepted 25 August 1988) Amorphous selenium (a-Se) colloid particles have been directly photodeposited onto insulating substrates. Scanning Electron Microscopy (SEM) supported by kinetic adsorption measurements reveals the origin of the photodeposition process. The studies show that film formation occurs through a sequence of particle precipitation, growth, and photoadsorption. A plastic flow and coalescence process creates a continuous monolayer before a fast deposition rate of Se on the deposited materials commences. Surface forces seem to have an important role in the creation of the morphological structures of the adsorbed particles.
I. INTRODUCTION In recent years, many works have appeared in the literature describing laser photodeposition processes.1 The photodeposition process (PD) of a-Se has been investigated since 1975 by various methods.2"4 The basic phenomenon involves photo-enhanced adsorption of nearly monodisperse spherical particles of submicron diameter onto insulating substrates. Previous papers have dealt with the active growth period2 when the photoadsorption occurs on a continuous monolayer of already deposited a-Se material. This paper presents a better view obtained from observations made on the surface morphology during the initial adsorption stage, so-called "incubation period."2 We shall also discuss the influence of photodeposition controlling parameters on the film structure development before the creation of the first continuous monolayer. A simple model will be used to estimate from the nonelastic/elastic deformation ratio the different structures obtained by photodeposition. II. SEM INVESTIGATION OF FILM STRUCTURE Figure 1 shows several SEM micrographs which allow us to divide the first covering steps as follows: (a) Adsorption of spherical particles dispersed quite randomly on the substrate surface. (b) Growth of the adsorbed particles occurs until near neighbors come into contact. (c) A bridging effect initiates an apparent coalescence and plastic flow redistribution between particles. (d) A final merging process creates a continuous film structure and the discrete particle morphology disappears. During the initial particle adsorption, we found that the type of substrate and its surface pretreatment were the main factors which determine the rate of appearance of newly adsorbed colloid particles on the surface. An interesting feature was the relatively narrow size distribution of "On leave from the Center for Technological Education, aff. Tel-Aviv University, Holon 58368, Israel. J. Mater. Res., Vol. 4, No. 1, Jan/Feb 1989
the spherical particles appearing on the substrate during stage (a), as shown by the histograms in Fig. 2. The following conclusions are also drawn from the histograms: (1) For different aging conditions, the particle diameter distribution is within a restricted range 50-700 nm. (2) The modal value of the particle diameter rises with aging temperature T
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