TEM and HREM Studies of As-Grown Low-Temperature Molecular Beam Epitaxial In
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TEM AND HREM STUDIES OF AS-GROWN LOW-TEMPERATURE MOLECULAR BEAM EPITAXIAL InO.52Al0.48As/InP HETEROLAYERS A. CLAVERIE*, Z. LILIENTAL-WEBER, P. WERNER, W. SWIDER, P. K. BHATIACHARYA**, S. GUPTA** and G. MOUROU** Materials Science Division, Lawrence Berkeley Laboratory, Berkeley CA 94720 * Centre d'Elaboration des Materiaux et d'Etudes Structurales, CNRS, BP 4347, 29 rue J. Marvig 31055 Toulouse Cedex, France ** Dept. of Electrical Engineering and Computer Science, The University of Michigan, Ann Arbor, MI 48109-2122 ABSTRACT Transmission and High Resolution Electron Microscopy have been used to study the dramatic changes in crystalline quality which occur in As-rich In0.52A10.48As/InP layers 0 when the growth temperature is lowered. We have found that for temperatures as low as 200 C and for a flux ratio of 20, the layers can be of high quality. For the lowest growth temperature of 150 0C, pyramidal defects as well as hexagonal As grains are found which are characteristic of the breakdown of the monocrystalline growth in these layers. A mechanism for the formation of these defects is proposed based on their crystallographic structure. The fast photo response previously observed in these layers must be related to the presence of these defects. INTRODUCTION InAlAs layers grown under low-temperature conditions have been recently used as buffer layers for field-effect transistors, resulting in improved device performance [1]. It has previously been shown [2] that recombination times as low as 400 fs could be obtained for 0 InAlAs grown at a temperature as low as 150 C. This material therefore shows great potential for the realization of subpicosecond optoelectronic switches and other applications. There are striking similarities between this material and the more well studied "low-temperature" GaAs. 0 Both materials are generally grown at low temperature (in the 150-300 C range) under arsenic overpressure and thereafter exhibit semi-insulating properties and fast response [3]. We have shown in a previous paper that the stoichiometry of the layers as measured by particle induced x-ray emission (PIXE) experiments is a strong function of growth temperature [4]. It was found that, as in the case of LT GaAs, the semi-insulating properties of the In0.52A10.48As/InP layers are related to the presence of a large fraction of excess As in these layers, up to several percent. The content of excess As (1-2%) in the layers is increased when lowering the growth temperature ( 200 0 C. At a lower growth temperature (Tg=150 C), pyramidal defects are formed which mostly consist of twins, dislocations and small lamella of hexagonal As. A special feature is the occurance of ordering of group-Ill elements observed at 200'C, which is likely related to the larger changing of the element ratio. However, for the moment we can not clearly state how much additional As is incorporated in the layers in dependence on Tg. The lower the growth temperature, the higher should be the amount of this excess arsenic that can be incorporated. If this amount
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