Incorporation of Extrinsic Defects in HgI 2 During Detector Fabrication
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INCORPORATION OF EXTRINSIC DEFECTS IN HgI 2 DURING DETECTOR FABRICATION
J.M. Van Scyoc*, T.E. Schlesinger*, R.B. James"*, A.Y. Cheng***, C. Ortale***, And L. van den Berg*** *Department of Electrical and Computer Eng., Carnegie Mellon University, Pittsburgh, PA "**Sandia National Laboratories, Livermore, CA ***EG&G Energy Measurements, Goleta, CA ABSTRACT The incorporation of extrinsic defects into mercuric iodide substrates during detector fabrication can be extremely detrimental to device performance. In particular, extrinsic defects can act as trapping and recombination centers, and they can reduce charge collection efficiencies, decrease gt product, or contribute to polarization effects in nuclear detectors. In this paper we present results of processing, photoluminescence, and electromigration experiments that clearly show that extrinsic defects can be incorporated in mercuric iodide during detector fabrication. By observing the luminescence features characteristic of Cu and Ag in mercuric iodide, we show that both these materials are taken up by mercuric iodide crystals during etching with KI if the etching solution is contaminated with these elements. The migration of material from contacts into the crystal, as shown by resistance measurements, is also presented. We infer from this work that other defects which are detrimental to device performance may also be incorporated in mercuric iodide if insufficient care is taken during device fabrication. Suggestions are therefore made as to some of the precautions that must be taken in order to realize the highest quality detectors. INTRODUCTION Red mercuric iodide (a-HgI2 ) is well suited for use in room temperature radiation detectors because of many of its properties [1-5]. These properties include high atomic numbers (ZHg=80 and Z1=53), low electron-hole creation energy (4.2eV at 300K) [3-5], and high resistivity (1013fl*cm) [5]. However, detector quality is still limited by many material and processing issues, resulting in low manufacturing yields, high production costs [6], and long term instability. Defects introduced from the starting material and during the processing are possibly responsible for these problems. Several steps are required to produce mercuric iodide detectors. After purification of the starting material and crystal growth, slices are cut from the larger crystal and polish etched in KI solution. Then contacts are deposited, and the detectors are mounted on alumina substrates. During this production many individual processes are involved. Chemical etching, material heating, and vacuum exposure art encountered throughout detector formation. It has been shown that these steps can have noticeable effects on the material properties [7]. The contacting process is also critical in detector fabrication. Choice of contact material and method of deposition have a large impact on the quality and stability of the detector. Several materials have been studied for use as contacts, although palladium has been standardized as the current metal of choice. E
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