Development of Large‐Diameter and Very High Purity Ge Crystal Growth Technology for Devices
Rajappan Radhakrishnan Sumathi, Alexander Gybin, Kevin‐P. Gradwohl, Pradeep Chandra Palleti, Mike Pietsch, K. Irmscher, Natasha Dropka, Uta Juda
Abstract
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 × 10 10 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 (≈10 4 cm −2 ) can be realized.