Cluster Model Study of the Incorporation Process of Excess Arsenic into Interstitial Positions of the GaAs Lattice
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ABSTRACT We study an As 2 molecule approaching a planar, non-reconstructed, As terminated GaAs(001) surface by using a suitable cluster and quantum mechanical ab-initio calculation methods. During our calculations the As2 molecule is always oriented perpendicularly to the surface and its bonding length may vary. We find a metastable position below the growing surface which facilitates the incorporation of the leading As atom into an interstitial position of the crystal during growth. We give a first model for this incorporation process. INTRODUCTION GaAs layers are usually grown by molecular beam epitaxy (MBE). When the substrate temperature is about 600'C these layers are stoichiometric even under the generally used As rich conditions. If the temperature is choosen much lower, say less than about 300 0C, the layers become non-stoichiometric e.g. [1]. A significant amount (up to 1.5%) of excess As is incorporated into the GaAs-lattice which results point defects, such as arsenic antisites, gallium vacancies and especially arsenic interstitials [1-3]. The
incorporation of this excess As during the growth at the low substrate temperatures (LTGaAs) plays the key role in understanding the unique electrical and optical properties of this material. LT-GaAs was brought into the focus of technological interest since it can be used as buffer layer in semiconductor devices or as an active layer for ultrafast optoelectronic devices [1-3]. In a recent theoretical paper [3] we study As2 molecules approaching a planar, non-reconstructed GaAs(001) surface by using suitable clusters and quantum mechanical ab-initio calculation methods. The surface is either As or Ga terminated. During our calculations the As2 molecule is always oriented perpendicularly to the (001) surface. It is only for the case of the As terminated surface that we obtained for the lower As atom of the As2 molecule a metastable position below the growing surface. This paper starts from this metastable position below the growing surface and develops a first theoretical model for the incorporation process of As atoms into interstitial positions during the growth process. We use again quantum mechanical abinitio calculation methods to study the approach process of an As2 molecule to the Asterminated GaAs(001) surfaces. In a second step we study then the incorporation process of the lower As atom (As 2 molecule) into an interstitial position. 233 Mat. Res. Soc. Symp. Proc. Vol. 584 ©2000 Materials Research Society
CLUSTER MODEL AND CALCULATION PROCEDURE We represent the GaAs crystal and its surface by a Ga 2 As5H8 cluster, see Fig. 1. This cluster has the C2 symmetry axis and represents a lx1 unit-cell of the As terminated (001)-surface. The bonds on this surface are left dangling. The other bonds are saturated with hydrogen atoms (Fig. 1) in correspondence to the arguments in [4-8]. The interatomic distances between the Ga and As atoms are chosen according to the lattice constant of 5.654 A, [9]. The Ga-H and As-H bond lengths are fixed to the values of 1.663 A
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