Bulk crystal growth process for compositionally homogeneous gallium indium antimonide substrates

Abstract

The roles of forced and natural convections III the liquid on the radial alloy distribution in vertical Bridgman growth Ga₁₋ₓInₓSb bulk polycrystals (20-50 mm diameter) have been investigated. The shape of the solid-liquid interface has been found to be highly curved under growth conditions necessary for preventing constitutional supercooling such as high axial temperature gradients and low growth rates. Lowering the axial temperature gradient does flatten the interface shape, but extremely slow growth rate (below 0.1 mm/hr) is necessary for avoiding constitutional supercooling. Forced convection induced by melt mixing strategies developed in this work efficiently accelerates the dispersion of the excess solute from the growth interface into the entire melt volume thus leading to a 4-5 fold increase in the growth rate without constitutional supercooling. This also helps to eliminate micro-cracks in the crystals. A comparative study of various melt stirring schemes with different axial temperature gradients and the resulting alloy distribution in the crystals has been presented in this thesis. A combination of axial thermal gradient of approximately 15 °C/cm, accelerated crucible rotation (ACRT) with maximum crucible acceleration up to 100 rpm in 60 seconds and growth rate in the range of 0.2-0.5 mmIhr has been found to be most suitable for radially homogeneous Ga₁₋ₓInₓSb crystals. Bulk polycrystals of 50 mm diameter Ga₁₋ₓInₓSb across the entire composition range (x = 0 - 1) has been successfully grown under these conditions.

In summary, the two primary research contributions pertaining to this thesis are:(a) demonstration of compositionally homogeneous Ga₁₋ₓInₓSb substrates with x = 0 - 1 for the first time and (b) first report of room temperature experimental data for electrical and below bandgap infrared properties of the Ga₁₋ₓInₓSb system.

The below bandgap infrared transmission (up to 25 μm) in undoped Ga₁₋ₓInₓSb bulk crystals has been studied for the first time and found to be limited by native defects such as anti-sites and vacancies found in antimonide based III-V compounds. For the gallium rich alloy compositions (x > 0.5 in Ga₁₋ₓInₓSb),the crystals exhibit p-type conductive behavior with increase in net acceptor concentration with increase in gallium content in the crystals. For x < 0.5 (the indium rich alloy compositions), the crystals exhibit n-type conductivity with increase in net donor concentration with increase in indium content in the crystals. A correlation between the optical transmission and the residual carrier concentration arising from the native acceptors and donors has been observed. Due to donor-acceptor compensation, crystals with alloy compositions in the range of x = 0.5 to 0.7 exhibit high optical transmission for a wide wavelength range (up to 22 μm). The light hole to heavy hole inter-band transition in the valence band and the free electron absorption in the conduction band have been found to be the two dominant absorption processes in undoped Ga₁₋ₓInₓSb system.

The mode of redistributing the excess solute species at the melt-solid interface accumulated during the growth of bulk ternary semiconductor crystals is a key factor determining the spatial alloy composition and crystalline quality. Since the solidification temperatures vary with alloy composition, a control over both heat and mass transport during crystal growth is necessary for achieving spatial compositional homogeneity in the grown crystals. The goal of this work has to been to identify growth conditions necessary for compositionally homogeneous (within I mol% in the radial direction) substrates.