Production of Electro-Optical Infrared Grade Large Diameter Binary and Ternary III-V Semiconductor Bulk Crystals

Optically transparent bulk semiconductor materials are used as refractive elements in electro-optical infrared (EO/IR) systems operating at wavelengths greater than 2 Ám for selected DoD applications. For this purpose, large clear apertures (greater than 75 mm in diameter) with thicknesses greater than 1 mm are desirable to enable mechanical robustness such that the final elements can be diced, shaped, polished to optical quality specifications. Most of the current EO applications are met by readily available binary III-V compound semiconductor substrates. In the past, crystal growth technology and process parameters for binary compounds could not yield application worth ternary crystals. Due to the intrinsic separation between liquidus and solidus in the pseudo-binary phase diagrams used for growing ternary crystals, alloy segregation is a major issue. This in turn leads to low yield of wafers with same composition andácracking of crystals. In the recent years, United Semiconductors LLC (USLLC) has developed anáinnovative proprietary crystal growth technology based on vertical gradient freeze (VGF) in conjunction with solute feedingáwhich led to the successful demonstration of 50 ľ 150 mm diameter homogeneous bulk ternary crystals with high optical quality and 50-100 mm diameter optical defect free (such as snail-trail free) binary crystals. The issue is cost. To meet optical grade window requirements, multiple crystals are grown and mined for optically compliant wafers. While the yield of optical grade ternary crystals (approximately 10%) may be acceptable for high value optical systems used by the Air Force, the current cost is too high for use in high volume Army sensors and commercial applications. A primary objective here is to increase yield and lower cost, thus providing a path to commercialization for crystal diameters in the 50-150 mm range. Further optimization of the process is necessary to yield parts with consistent optical performance across the clear aperture demonstrated through minimization of scatter from point and macroscopic defects and inhomogeneity arising from microscopic fluctuations. To accelerate the optimization of the crystal growth process, multiple growth chambers shall be used. Another critical need is the metrology development for evaluating the bulk semiconductor material immediately post growth prior to polishing. Selected analytical tools for measuring optical bandgap (using FTIR coupled to an integrating sphere with cryogenic cooling option) and elemental composition (using non-destructive X-ray fluorescence) on as-cut wafers will be established in-house, thus eliminating the time-consuming and expensive need for wafer polishing and post polishing analysis. The rapid feedback will further optimize the growth parameters, thereby enhancing wafer yield and reducing the growth cycle time. Such processes have already been demonstrated successfully.