| Popis: |
In MOVPE growth of epilayers of the ternary composition Ga0.47In0.53As, nominally lattice matched to InP substrates, a complete change in the occupancy of the group V sublattice is involved. At the commencement of growth, the low concentration of PH3 needed to stabilize the InP surface must be changed to an AsH3 mixture. To achieve the sharp interfaces required in many device applications it is consequently necessary to adopt a low growth rate and fast gas compositional switching. To assess the magnitude of the problem, interfaces obtained in two types of growth equipment have been studied by TEM, both in plan view and cross-section. A very disturbed interface region was found which gave rise to numerous threading defects extending into the ternary layer. Micrographs of the interface show Moire fringes which can be interpreted as lattice spacing variations, corresponding to a lattice mismatch approaching the difference between InP and InAs lattices. The micrographs also show evidence that nucleation of the disturbed region is non-uniform. Modifications to the reactor to reduce the time taken for gas compositional switching, and the use of an InP buffer layer, resulted in a substantial improvement in the interface region, with a consequent reduction in the density of threading defects in the epilayer. |
