Disordering of a Short Period GaAs-AlGaAs Superlattice by C Diffusion
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DISORDERING OF A SHORT PERIOD GaAs-AIGaAs SUPERLATTICE BY C DIFFUSION Z. JAMAL AND P. J. GOODHEW Department of Materials Science and Engineering, University of Liverpool, Liverpool, L69 3BX, United Kingdom ABSTRACT Carbon doping of GaAs and AlGaAs is easily carried out during chemical beam epitaxy by controlling the proportions of the precursors2 0 trimethylgallium and triethylgallium. The diffusion of C introduced at levels of up to 10 cm-3, has been studied by SIMS and by TEM. In the latter case the destruction of short period GaAs-Al.3Ga.7As superlattices was monitored in order to assess the extent of diffusion after annealing at 8000C. The results confirm that C is quite a stable dopant. In addition, the destruction of the superlattices indicates that the diffusion mechanism of C involves group mIsublattice sites. INTRODUCTION Lately, there has been much interest in using C as an alternative p-type dopant in GaAs. Successful C doping has been achieved by using either intrinsic or extrinsic [1,2,3] C sources. Compared to Zn and Be, the two commonly used p-type sources, C is very stable to high temperature annealing and has a much lower diffusion coefficient [1,3]. Further, C2 is also a very efficient high doping source, capable of achieving hole concentrations up to 10 1cm- 3 [4]. All these properties are important for device application, especially in heterojunction bipolar transistors (HBT) where high doping and abrupt interfaces are required. However, during high temperature annealing of highly C-doped GaAs and AIGaAs, the hole concentration and the mobility has been observed to reduce, although no change was detected in the total concentration of C [1,5]. The changes in the electrical properties are attributed either to C compensating by switching sites from CAs to CGa or CAs moving to neutral interstitial sites. It has also been suggested that misfit dislocations are formed at high level of C doping and these dislocations may act as carrier traps and consequently degrade the electrical properties [6]. So far experimental evidence has not conclusively favoured any of the possible mechanisms. In this paper, we present some results on the diffusion of C through a short period GaAsAlGaAs superlattice which has been annealed at 800°C for several long periods of annealing time. Secondary ion mass spectrometry (SIMS) measurements confirmed that C is indeed very stable to thermal treatment, having only a small value of diffusion coefficient at that particular annealing temperature. In addition, our cross-sectional transmission electron microscopy (TEM) results also show that at the same annealing conditions, C can disorder the superlattice. Clearly the diffusion of C and the enhanced interdiffusion rate of Ga and Al are related. This suggests that the diffusion of C also involves group Ill sublattice sites. EXPERIMENTAL PROCEDURE The aim of this study is to investigate whether the diffusion of C through a GaAs-AlGaAs superlattice would result in the disordering of the superlattice. For this purpose, an epitaxial st
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