Dislocation reduction in large-diameter LEC GaAs growth: I. Low gradient growth and indium doping

Abstract

The effects of passive heat shielding, ambient pressure control, and indium doping on the properties of large-diameter, semi-insulating GaAs crystals grown by the liquid encapsulated Czochralski (LEC) technique have been investigated. Dislocation densities in the “ring” region of undoped crystals as low as 3700 cm -2 were achieved by low gradient growth compared to levels of over 20,000 in standard LEC material. A reduction in operating pressure from 300 to 150 psi led to further reductions to levels as low as 2800 cm -2. Effectively dislocation-free material with dislocation densities ranging from 10 to 2000 cm -2 was grown in the low gradient configuration by isovalent doping with indium at concentration levels of (6–9) × 10 19 cm -3. A new phenomenon of critical isovalent doping was identified, which establishes a critical indium concentration for minimum dislocation density. Critical doping is explained in terms of competing effects of lattice “hardening” and reduced thermal transport due to indium. It was found that background impurity and point defect concentrations remain substantially unaffected by passive shielding, although some differences between the properties of undoped and indium doped crystals were identified. Finally, from the standpoint of crystal manufacturing, it was demonstrated that wafer yield per crystal of an isovalent hardening growth process would be at least 30% lower than a comparable undoped growth process due to the limitation of constitutional supercooling.