The Initial Stages of MBE Growth of InSb on GaAs(100) - A High Misfit Heterointerface
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THE INITIAL STAGES OF MBE GROWTH OF InSb ON GaAs(100) - A HIGH MISFIT HETEROINTERFACE. C.J.KIELY, A.ROCKETr*, J-I. CHYI* AND H.MORKOC* Department of Materials Science and Engineering, University of Liverpool, England. * Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, IL 61801, USA. ABSTRACT The initial stages of heteroepitaxy of InSb on GaAs(100) grown by MBE have been studied by transmission electron microscopy. Three dimensional InSb island growth occurs in which the majority of the 14.6% misfit strain is accommodated by a square array of a/2 edge-type misfit dislocations. The implications of each island having a well defined defect array before coalescence into a continuous epilayer are discussed. Some 60 0 -type a/2 interfacial defects and associated threading dislocations are also observed in coalesced films and possible reasons for their existence are explained. A strong asymmetrical distribution of planar defects in the InSb islands is observed and the origin of the asymmetry is discussed. Finally some evidence for local intermixing in the vicinity of the interface is presented. INTRODUCTION Recently several groups worldwide have reported successfully growing InSb layers on GaAs (100) by molecular beam epitaxy despite a 14.6% lattice mismatch [1-4]. The potential advantage of growing InSb layers (i.r. detectors) directly on a GaAs substrate is that signal processing could then be integrated with the detector on a single substrate chip, leading to significant gains in device performance. Hall effect measurements have typically been used to assess the electrical quality of the MBE grown InSb layers [1-4]. Reports of room temperature electron mobilities for films between 5 and 10 gtm thick lie in the range 5.7 - 6.7 x 104 cm 24 V s-1, which are still 2 1 somewhat lower than those obtained for bulk InSb (7.6 - 8.4 x10
cm V s- ). Several
detailed microstructural characterisations of the InSb/GaAs interface have also been reported recently [4-6]. A common characteristic of the epilayers produced to date seems to be a rather high density of threading dislocations (106- 107 cm- 2 ). Such defects are undesirable since they act as scattering centres for electrons and hence reduce the electron mobility of the film. High resolution electron microscopy studies of the InSb/GaAs interface [5,6] have revealed that most (although not all) of the misfit strain is accommodated by a square array of pure-edge a/2 Lomer-type interfacial misfit dislocations spaced on average 30A apart. However, some 600 -type a/2 defects are also occasionally noted at the interface, and it is these which often have epi-threading segments associated with them [7]. RHEED studies [1,3] have indicated that the early stages of layer growth involve three dimensional nucleation and formation of a non-pseudomorphic interface. In this paper we present a TEM examination of the initial stages of MBE InSb growth on GaAs(100). We discuss in some detail the formation mechanism of the Lomer-type misfit dislocations and suggest reaso
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