Development of Large-Diameter and Very High Purity Ge Crystal Growth Technology for Devices

Abstract

High-purity germanium (HPGe) single crystals find their applications as radiation detectors especially for spectroscopy of high-energy photons and particles in nuclear physics. Growing “detector-grade” HPGe single crystals, with tailored structural defects and controlled impurity content, uniform throughout the crystal is a challenging task in this type of crystalline semiconductor material. All different process steps for HPGe, namely, i) reducing germanium dioxide powder into Ge metal; ii) zone refining of intrinsic source material (6 N) up to an ultrahigh purity (13 N); iii) Czochralski growth of high-purity single crystals, are developed in house. A specific preparatory coating process for zone refining is entrenched as part of this work. Gallium, a dominant impurity usually found in HPGe, can be avoided. Using these advanced process know-hows, a technology to grow volume Ge single crystals of large diameter (3 in.) with very high purity (net carrier concentration ≈ 5 × 1010 cm−3 or equivalent to 1 part per trillion level) is successfully developed. The zone-refined Ge bars and the crystals show p-type conductivity. Some crystals exhibit n-type conductivity, and n–p or p–n transition is encountered in a single growth experiment. Besides the first results of encompassing very high-purity crystals with uniform diameter, a controlled dislocation density (≈104 cm−2) can be realized.