Photon-Controlled Growth of Multilayered Structures
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PHOTON-CONTROLLED GROWTH OF MULTILAYERED STRUCTURES DOUGLAS H. LOWNDES, D. B. GEOHEGAN, D. ERES, D. N. MASHBURN and S. J. PENNYCOOK; Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6056 ABSTRACT Pulsed ArF excimer laser (193 nm) photolysis has been used to deposit entirely amorphous and mixed amorphous/polycrystalline superlattice structures containing Si, Ge and Si 3 N4 . High resolution in situ optical reflectivity measurements were used to monitor and/or control deposition. Transmission electron microscope cross-section views demonstrate that amorphous superlattice structures having highly reproducible layer thicknesses (from about 50 to several hundred A), and sharp interlayer boundaries, can be deposited at low substrate temperatures under laser photolytic control. INTRODUCTION The growing number of applications for structures composed of alternating thin layers of crystalline or amorphous materials has given new impetus to the search for alternative low temperature thin film deposition techniques. For semiconductors, low temperatures are needed when multilayered structures contain highly doped adjacent layers, in order to avoid dopant interdiffusion during growth. For semiconductors and other materials, low deposition temperatures minimize in-diffusion of unwanted impurities from the surroundings, prevent interdiffusion with the substrate or other adjacent materials, and provide unique access to any low temperature phases that may exist. However, conventional thermally driven (pyrolytic) chemical vapor deposition (CVD) film growth reactions usually are limited to very low, or even negligible, film growth rates at low temperatures.
In this paper we describe an alternative to pyrolytic growth, namely, direct photolysis of precursor gas molecules, using pulsed ArF (193 nm) excimer laser radiation. The principal advantage of pulsed-laser photolysis for fabrication of amorphous multilayered structures is that high resolution can be obtained in layer thickness, because deposition is inherently "digital": Each laser pulse produces much less than a monolayer of film, on average. Nevertheless, the high pulse repetition rate of excimer lasers permits high deposition rates to be achieved at low temperatures. We also describe an optical reflectivity technique that provides sub-monolayer resolution film thickness monitoring. The combination of high resolution optical monitoring with pulsed laser photolysis can, in principle, provide monolayer accuracy in controlling the average thickness of deposited layers. We report results of experiments in which we have explored deposition of superlattice structures containing amorphous semiconductor and dielectric thin film layers, using these techniques. EXPERIMENTAL CONDITIONS Multilayer growth experiments were carried out in a deposition chamber [1] based on a six-way stainless steel cross and equipped with 2-in.-diam Suprasil windows. All depositions were made onto 2.5-cm square (100) crystalline (c) Si substrates; the substrate temperature was measured
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