Reduction of Defects in Highly Lattice Mismatched InGaAs Grown on GaAs by MOCVD
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INTRODUCTION Epitaxial growth of heavily lattice mismatched InGaAs layer on GaAs substrates has been of great interests since a wide selection of opto-electronic devices can be obtainable with various energy bandgap materials. InGaAs/GaAs heterostructure has a lattice mismatch with a maximun value of 7.2%, which can introduce misfit strain and create a high density of defects in the growing layer. This result introduces a non-uniform surface morphology, namely cross-hatch pattern (CHP) which was first reported by Burmeister [1] in GaAsP/GaAs heterostructure. The films should have low density of dislocations at the interface and mirror-like surface morphology by elimination of CHP for a good device performance. The strain relief mechanism in InGaAs/GaAs heterostructure has been intensely investigated. The initial growth stage of highly strainded InGaAs layer on GaAs shows 3dimensional (D) islands growth which is quite different behavior from that of homo-epitaxy [2]. Misfit dislocations are generated eventually in order to make more efficient relaxation after the size of islands becomes large enough. The type of misfit dislocations is closely related to the initial growth mode [3]. It is suggested that the non-uniform lateral growth initiates the formation of CHP [4,5]. The uniform lateral growth at the initial stage is essential to preserve the interface quality and to maintain 2-D morphology. In this work, a new type of InGaAs/GaAs multi-layer structures was grown by MOCVD technique to maintain a uniform lateral growth in 2-D fashion. A thin In or Ga metal film was embedded between the heterostructure of InGaAs/GaAs by alternate supply of the group III and V gaseous sources. A role of the metallic layer at the interface is to improve two-dimensional growth mode at the initial stage and to reduce the misfit dislocation density by effective relaxation of the interfacial strain. 649 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
EXPERIMENTAL Four different InGaAs multilayer structures were grown on GaAs (100) oriented substrates by low pressure MOCVD technique. The samples were grown at the substrate temperature of 435°C and the chamber pressure of 20 torr. The sources of In, Ga and As were trimethylindium (TMI), triethygallium (TEG), and 100 % AsH 3. The sequence of gas switching for growth of the samples is illustrated in Fig. 1. Sample A is a typical InGaAs layer grown directly on GaAs buffer layer. The thickness of the epitaxial layer is about 1.6 gtm. Sample B has an additional buffer layer of linearly graded compositional In.Gal.,As with a indium mole fraction upto 0.85. The thickness of the graded layer is obtained about 340 nm. In the growth of Sample C, alternative gaseous sources of TMI and arsine were supplied to the buffer layer with a few second interruption. After growth of the GaAs buffer layer, the gas supply of both TMG and AsH 3 was stopped for a few second to clear the residual gas in growth chamber. Then the In metal prelayer was deposited on top of the GaAs buffer layer fo
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