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AND EXPERIMENTAL TECHNIQUES

Metal / semiconductor epitaxy forms the basis for several technologies including the formation of ohmic contacts to compound semiconductors. However, most studies have focussed on the very early stages of microstructural evolution for these systems. In this case, the films are composed of isolated nuclei. Later stages of the growth process, including impingement of these independent nuclei followed by their coalescence into larger nuclei and eventually continuous or quasi-continuous film formation remain largely unstudied. We have performed a nm resolution study of Ag / GaAs (110) epitaxy which has followed the growth process from nucleation all the way through quasi-continuous film formation in order to investigate these processes. Other studies of this material system have focused on the very early stages of growth on room temperature (RT) substrates 1. Here, we investigate growth onto both room temperature and 250' C substrates. The experiments were performed in an ultra-high vacuum scanning transmission electron microscope equipped with an extensive specimen preparation chamber 2. Samples were prepared via in vacuo cleavage followed by Ag deposition and imaged all in a vacuum of better than 10-10 mbar. 2.5mm wide strips were cleaved from oriented GaAs wafers so that the long axis of these strips was parallel to the direction. 3 mm diameter discs ultrasonically cut from these strips therefore automatically had a flat with a surface normal. Two 1.0 mm deep slits were cut at ± 0.5 mm from the center of this flat normal to its surface using a 0.15 mm thick diamond wafering saw. This resulted in a 1.0 mm long, 1.0 mm wide 'diving board' which could then be poked in UHV to cleave fresh surfaces for the growth studies. Prior to this UHV cleavage, the sample and its refractory metal holder were degassed at T > Tdeposition so that during growth, the chamber pressure stayed below 3 x 10-10 mbar. Deposition rates from a water-cooled Knudsen cell were 0.5 ML / min with 1 ML being referred to the surface atom density for the bulk terminated GaAs (110) surface. For both the RT and 250' C experiments, samples were observed subsequent to the deposition of 1/2, 1, 2, 4, 8, 16, 32, 64, 128 and 256 ML of Ag. In either case, the same sample was sequentially deposited on and transported to the microscope for imaging. Results were observed to be reproducible for a range 95 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society

of depositions onto both room temperature and 350°C substrates. nm resolution, digitally acquired secondary electron (SE) images were computer analysed to produce the island size distributions presented below. All samples were imaged with the primary beam incident at between 30*and 60" to the normal. For all displayed size distributions, individual size distributions obtained from several micrographs acquired from different areas were averaged to ensure good statistics. RESULTS

Figures 1 display micrographs from a deposition sequence for RT deposition onto GaAs (110