High-Speed Epitaxy Using Supersonic Molecular Jets
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HIGH-SPEED EPITAXY USING SUPERSONIC MOLECULAR JETS DJULA ERES Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN. ABSTRACT This paper discusses the use of supersonic jets of gaseous source molecules in thin film growth. Molecular jets in free form with no skimmers or collimators in the nozzle-substrate path were used in the investigation of basic film growth processes and in practical film growth applications. The Ge growth rates were found to depend linearly on the digermane jet intensity. Furthermore, the film thickness distributions showed excellent agreement with the distribution of digermane molecules in the jet. High epitaxial Ge growth rates were achieved on GaAs (100) substrates by utilizing high-intensity pulsed jets. The practical advantages and limitations of this film growth technique are evaluated, based on the results of microstructural and electrical measurements of heteroepitaxial Ge films on GaAs (100) substrates. INTRODUCTION Over the past decade, new film growth methods have been developed that extend control over a wider range of growth parameters, and consequently provide ways to grow better thin films and more sophisticated artificially structured materials. For example, the kinetic restrictions embodied in atomic (or molecular) layer epitaxy have demonstrated considerable success in controlling the growth of compound semiconductors at the monolayer level [1]. It has been claimed that the application of surface-active species in heteroepitaxial film growth prevents island growth by altering the interface energetics through adsorption of a third species which reduces the surface free energy [2]. Similarly, the driving force behind most other innovations in film growth methods is the search for a higher degree of external control over the process. Despite the fact that gaseous sources have numerous advantages over elemental sources used in molecular beam epitaxy (MBE), they are not widely used in the MBE environment for the growth of group IV heterostructures because of the poor understanding of the underlying growth processes. The main advantage of using gaseous molecules is that they can provide much higher pressures for beam formation than the evaporative oven sources used in MBE, translating into higher beam fluxes and potentially higher film growth rates. A supersonic free jet is a primitive beam source that combines the directional advantages of a beam growth method with the high intensities available from gaseous sources. A supersonic molecular jet is generated by expanding a binary gas mixture through a small nozzle orifice from a high stagnation (reservoir) pressure into a continuously pumped vacuum chamber. The gas mixture consists of a small amount (5%) of heavy molecules "seeded" (mixed) into a lighter inert carrier gas such as helium. During the expansion the random thermal motion of molecules is converted into a directional flow along the expansion axis, resulting in a strongly forward-peaking intensity distribution [3]. We have utilized both continuous and pulsed m
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