Process and yield enhancements for epitaxially grown mercuric iodide crystals

Mercuric iodide single crystals have been used successfully as gamma radiation detectors for many years. Recently a different growth method called `seeded homoepitaxy´ has been studied. This method has the advantage of growing detector-sized volumes with a thickness of about 10mm in a highly accelerated timeframe. The electronic transport properties of a mercuric iodide detector have been shown to depend primarily upon two factors. The first is material quality, which is predominantly determined by the levels of organic and inorganic impurities and the compound stoichiometry. The second is the integrity of the crystalline structure (type and density of defects), which depends mostly upon the thermal growth profile, conditions inside the growth chamber, and seed preparation. Recent improvements made to the epitaxial process have resulted in a detector performance for both planar and pixelated devices that rivals that of traditional ampoule-grown crystals. This increases the usefulness of devices in field applications. Changes in the seed attachment and preparation methods have been successful at reducing crystalline defects by about 90%. This has been substantiated using a method of measuring and recording defect density that combines laser backscatter with digital photography. Improvements in the mercuric iodide growth process, bulk material impurity reduction, and the laser backscatter digital photography will be described and the results discussed.