Hydrothermal Production of Wafer Scale Zinc Selenide (ZnSe) Single Crystals

This program will focus on the growth of high-quality, single domain crystals of zinc selenide. The material has significant potential for mid- and far-IR applications, including for the development of infrared countermeasures (IRCM) in response to the Global Strike Operational Imperative. Other potential uses include IR communication, sensing, terahertz imaging, ultrafast spectroscopy, atmospheric monitoring, and various biomedical applications. Zinc selenide is an ideal material for these applications as it has a wider window (0.45 – 22 µm) than any materials currently under development. It also has a good nonlinear optical figure of merit, can be pumped with standard 1 µm lasers, has a high laser damage threshold, low absorption loss and a very low dispersion coefficient. These properties make it almost ideal for nonlinear optical applications in the mid- and far-IR.   The primary limitation to full realization of the potential of ZnSe is the inability to grow high quality single crystals of the material. All existing techniques invariably generate materials that are badly twinned, multidomain polycrystals. This program provides a unique solution to the problem by using a hydrothermal crystal growth technique. It is known that in superheated water (300?C) ZnSe will be transported and grow single crystals. Typically in a hydrothermal fluid, a mineralizer such as hydroxide or chloride will solubilize simple ZnSe powder feedstock and transport it to a cooler zone in a hydrothermal growth chamber and deposit it as a single crystal. Generally the process is a continual one as long as feedstock remains in the chamber, and this can lead to growth of very large crystals. Because the superheated fluid has a very low viscosity and the growing crystal is immersed in an isothermal bath, the quality of the crystals tends to be very high.  Generally single domain crystals with low thermal strain and precise stoichiometry can be grown. This program will exploit this technology to transport ZnSe feedstock to commercially available surrogate single crystal seeds of gallium arsenide. Single crystal wafers of 10 x 10 x 1 mm ZnSe will be grown and provided to Air Force Research Labs for evaluation and subsequent testing. Growth conditions will be optimized to generate a growth protocol that provides maximum quality and suitable growth rates. This protocol will serve as the groundwork to enable future growth of larger (25 mm) wafers and ultimately lead to orientation patterned wafers for quasi-phase matching for lasing throughout the IR region. The development of this wafer growth protocol and the generation of 10 mm single crystal wafers by this protocol are the principal tasks of this program.