Fabrication Techniques for Reverse Electrode Coaxial Germanium Nuclear Radiation Detectors

Germanium detectors with reverse polarity coaxial electrodes have been shown to exhibit improved resistance to radiation damage as compared with conventional electrode devices. However, the production of reverse electrode devices involves the development of new handling and fabrication techniques which has limited their wider application. We have developed novel techniques which lead to a device which is simple to fabricate, environmentally passivated and surface state adjusted. 1) n+ Contact. The n+ contact in the central hole in the crystal is formed by lithium, electroplated from a LiNO3-KNO3 eutectic molten salt bath held at the diffusion temperature. The outside surface of the crystal is protected during electrolysis by teflon pipe tape. Electrolysis has proven to be fast and convenient and it provides a rugged surface for handling during further processing. 2) p+ Contact. The p+ contact on the outer surface of the crystal is formed by boron ion implantation. The crystal is held by a shaft through the central hole and implanted with 1015cm-2, 25 KeV boron ions. No post-implant anneal is needed. 3) Passivation and Surface State Adjustment. The open ends of the coaxial detector have their surfaces passivated and the surface states adjusted to flatband condition by sputtering germanium in an argonhydrogen mixture. A special sputtering fixture is described for conveniently mounting the detector during sputtering. Due to the combination of thin external contacts and interelectrode surface state adjustment, the detector has complete charge collection over nearly 100% of the original crystal volume.