Rheed Measurement and Chemical Kinetics of Chemical Beam Epitaxial growth of GaAs
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RHEED MEASUREMENT AND CHEMICAL KINETICS OF CHEMICAL BEAM EPITAXIAL GROWTH OF GaAs T.H. Chiu AT&T Bell Laboratories, Holmdel, New Jersey 07733 ABSTRACT Recent efforts employing reflection high energy electron diffiaction measurements to study the chemical beam epitaxial growth of GaAs is reviewed. A reaction model which assumes the dominance of Ga alkyls and their derivatives adsorbed on the growing surface can explain most of the growth results in a consistent way. Dynamic evolution of the reconstruction pattern of the adsorbed triethylgallium or trimethylgallium overlayer illustrates how the alkyl-Ga bonds are cleaved sequentially. The growth rate dependence on temperature and incident flux can be fitted quite well in this reaction model. In the absence of As flux, the existence of a metastable Ga alkyl overlayer makes possible the atomic layer epitaxy of GaAs. Introduction Since the first successful preparation of GaAs film using metal alkyls [1], metalorganic vapor phase epitaxy (MOVPE) of rn-V compound semiconductors has been developed rapidly in the last two decades. Although the reaction steps in MOVPE have been studied quite extensively, the growth chemistry remains inadequately understood [2]. A combination of complicated gas phase and surface reactions in addition to limitations on the available in situ monitoring techniques has made it difficult to characterize the process. Recently, growth of high quality rn1-V compounds under high vacuum conditions by chemical beam epitaxy (CBE) or metalorganic molecular beam epitaxy (MOMBE) have been demonstrated [3]. In high vacuum the mass transport takes the form of molecular beams, and the growth is expected to be limited by surface chemical kinetics only. The high vacuum also made available the use of reflection high energy electron diffraction technique (RHEED) to monitor the growing surface. As in solid source molecular beam epitaxy (MBE), the RHEED intensity oscillation frequency for the growth of (001) GaAs corresponds to monolayer formation of GaAs, and is most useful in growth rates determination. In CBE the growth rates is related to the group III alkyls pyrolysis. The dependence of growth rates on substrate temperature and incident flux provides direct test of the chemical kinetics model which assumes sequential alkyl-Ga bond cleavage [4]. Furthermore, the reconstruction pattern during the deposition of Ga alkyl in the absence of As flux provides useful information of how the triethylgallium (TEG) or trimethygallium (TMG) molecules adsorb on the surface and decompose subsequently. Depending on the substrate temperature, the adsorption of Ga alkyl may proceed in a self-limited way. This makes possible the growth of GaAs by atomic layer epitaxy (ALE) when the substrate is exposed to TMG and As flux alternately [5]. Experimental The experiments were performed in a Riber MBE-2300 system reconfigured for CBE process. TEG mixed with hydrogen in a ratio of 1:8 and pure TMG have been used for the growth of GaAs. Gas lines and quartz delivery cell were kept at 50-1
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