Identification of individual and aligned microdefects in bulk vertical Bridgman- and liquid encapsulated Czochralski-grown GaAs

Abstract

A systematic study of microdefects (precipitates) in undoped and Si-doped vertical Bridgman (VB)-grown and updoped liquid encapsulated Czochralski (LEC)-grown GaAs crystals was performed using infra-red (IR) and differential interference contrast (DIC) microscopy, diluted sirtl-like etchants used with light (DSL) etching, transmission electron microscopy (TEM) and X-ray topography. Apart from hexagonal As precipitates, which were reported in many publications, inclusions of crystalline As 2O 3 and As 2O 5 have been identified for the first time by selected-area diffraction (SAD) and from lattice spacing of Moiré patterns in undoped GaAs. The crystalline precipitates are often associated with voids. In Si-doped annealed GaAs, however, only voids have been found. The average size of precipitates in LEC material is about 100 nm, while in VB they can reach 600 nm. Most of the precipitates decorate dislocations (DPs), but some were also found in the dislocation-free matrix as single inclusions or linear arrays of precipitates (LAPs). The latter defect pattern has not yet been reported. We shall present a tentative formation mechanism of LAPs. From the TEM analysis it follows that the nature of these precipitates is the same as that of the DPs. It has been established that microdefects are surrounded by large (compared to the size of precipitates themselves) zones which have recombinative properties for photo-generated carriers, leading to the formation of hillocks during photo-etching before the defects are dissolved to form S-pits. The S-pits formed during DSL etching and jet etching in H 2SO 4:H 2O 2:H 2O solution used to prepare TEM specimens are elongated and run along the [1 1 ̄ 0] direction. From TEM it follows that the large microdefects in VB material have the form of a regular tetrahedron with the side walls bounded by {111} planes and the edges along the 〈110〉 directions.