GaAs Substrates for High-Power Diode Lasers

Abstract

GaAs substrate crystals with low dislocation density (Etch-Pit Density (EPD) < 500 cm⁻²) and Si-doping (≈ 10¹⁸ cm⁻³) are required for the epitaxial production of high-power diode-lasers. Large-size wafers (≥ 3 in) are needed for reducing the manufacturing costs. These requirements can be fulfilled by the Vertical Bridgman (VB) and Vertical Gradient Freeze (VGF) techniques. For that purpose we have developed proper VB/VGF furnaces and optimized the thermal as well as the physico-chemical process conditions. This was strongly supported by extensive numerical process simulation. The modeling of the VGF furnaces and processes was made by using a new computer code called CrysVUN++, which was recently developed in the Crystal Growth Laboratory in Erlangen.

GaAs crystals with diameters of 2 and 3 in were grown in pyrolytic Boron Nitride (pBN) crucibles having a small-diameter seed section and a conical part. Boric oxide was used to fully encapsulate the crystal and the melt. An initial silicon content in the GaAs melt of c(Si_melt) = 3 × 10¹⁹ cm⁻³ has to be used in order to achieve a carrier concentration of n = (0.8−2) × 10¹⁸ cm⁻³, which is the substrate specification of the device manufacturer of the diode-laser. The EPD could be reduced to values between 500 cm⁻² and 50 cm⁻² with a Si-doping level of 8 × 10¹⁷ to 1 × 10¹⁸ cm⁻³. Even the 3 in wafers have rather large dislocation-free areas. The lowest EPDs (< 100 cm⁻²) are achieved for long seed wells of the crucible.