Homogenization of EL2 defect concentration of dislocation-free liquid-encapsulated-Czochralski GaAs:In in its growth direction

Abstract

Homogenization of EL2 point defect concentration is attempted for dislocation-free In-doped liquid-encapsulated-Czochralski (LEC) GaAs in its growth axial direction. The investigation reveals first the existence of characteristic axial distributions of [EL2] and resistivity which reflect a thermal history of dislocation-free as-grown crystal ingot. The variations of these quantities are small but cannot be homogenized even after a postgrowth heat treatment at 950 °C. The origin of the axial inhomogeneity is shown to be caused by inhomogeneous distribution of As precipitates in the growth direction. The arsenic (As) precipitates in the bulk crystal, which were formed during the LEC growth, are found to be dissolved by a heat treatment at 1200 °C followed by a fast cooling. After the 1200 °C heat treatment and the succeeding heat treatment at 950 °C to stabilize the EL2 defect concentration, a highly improved axial homogeneity is achieved in the distributions of both [EL2] and resistivity. From the amount of [EL2] change induced by the dissolution of the As precipitates, the amount of precipitated As in an as-grown crystal is estimated to be equivalent to an EL2 concentration on the order of 1015 cm−3 at the seed end and nearly zero at the tail end of the crystal ingot. Also discussed is a mechanism to produce the characteristic axial distribution of the As precipitates in an as-grown crystal ingot.