Temperature gradients, dopants, and dislocation formation during low-pressure LEC growth of GaAs

Abstract

It was widely accepted that in the presence of high thermal gradients thermoelastic stress is responsible for the dislocation structure observed in GaAs crystals grown from the melt. Recent studies suggest that another mechanism must be operative when growth takes place in the presence of sufficiently low thermal gradients. In this work we will review the current status of low pressure, low thermal gradient GaAs crystal growth. Our previous studies have shown that undoped GaAs crystals grown under low pressure by the liquid encapsulated Czochralski technique from melts with axial temperature gradients of 6°C/cm at the solid/liquid interface have dislocation densities of ∼ 3x10 3 cm -2 throughout the boule. Our recent work shows that growth at an order of magnitude slower pull rate has no effect on this dislocation density, but does result in more extensive polyganization. Increased pull rates, however, increase the dislocation density. Growth of crystals on the 〈111〉 axis results in dislocation densities 2.5 times greater than observed in crystals grown on the 〈100〉 axis. For GaAs, LEC growth using Dash-type necking is ineffectual for reducing dislocation densities. Doping with indium, silicon or tellurium at concentrations above “critical” values significantly reduces the dislocation density. Doping with tin, phosphorus, zinc or germanium has no observable effect. Also, there is no effect of dopant segregation on dislocation density. These observations together with others reported in the literature strongly support a mechanism wherein dislocations are formed as a result of microscopic stress fields (exceeding the critical resolved shear stress) that arise from inhomogeneous vacancy incorporation at the solid/liquid interface. The dislocation density is determined, then, by the Ga and As vacancy concentrations, and therefore can be affected by either stoichiometry or dopant additions.