Investigation of Deep Level Defects in Mercuric Iodide by Thermally Stimulated Current Spectroscopy
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INVESTIGATION OF DEEP LEVEL DEFECTS IN MERCURIC IODIDE BY THERMALLY STIMULATED CURRENT SPECTROSCOPY X.J. BAO*, T.E. SCHLESINGER-, R.B. JAMES--, A.Y. CHENG--, C. OlrTALE*'" AND L. VAN DEN BERG" *Carnegie Mellon University, Department of Electrical and Computer Engineering, Pittsburgh, PA 15213 "*Sandia National Laboratories, Advanced Materials Division, Livermore, CA 94450 ""EG&-G Energy Measurcments, Inc., Goleta, CA 93116
ABSTRACT Mercuric iodide (HgI 2 ) single crystals deposited with semitransparent Pd, Al and Ag contacts were studied by thermally stimulated current spectroscopy (TSC). Distinct differences were found among spectra obtained friom samples with different metal contacts, indicating that interactions between the metal contacts and mercuric iodide substrates have strong effects on the deep defect levels in mercuric iodide. The activation energies of some of these defect levels were estimated by taking TSC spectra with different heating rates. In addition, a pyroelectric effect was observed in Ag-contacted samples by thermally stimulated depolarization current technique (TSDC). The implications of these results in device applications of mercuric iodide are discussed.
INTRODUCTION Mercuric iodide is an important semiconducting material for the fabrication of room temperature X-ray and yt-ray detectors[1-5]. Compared with conventional semiconductor nu-
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Al 140
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Temperature (K) Figure 1: TSC spectra taken from samples contacted with semitransparent Ag, Pd, and Al metal layers. The spectra were scaled and displaced along the y-axis for comparison. TSC peaks are marked by the vertical bars. clear detectors such as Si(Li), Ge(Li) and high purity Ge detectors, HgI 2 has the advantages of a larger bandgap (2.14eV at room temperature) and higher atomic numbers (80 and 53 for Hg and I, respectively). The wide ba.ndgap results in a. small dark current, which allows the device to be operated at room temperature. With the efficiencv of the photoelectric Mat. Res. Soc. Symp. Proc. Vol. 209. Q1991 Materials Research Society
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effect proportional to the fifth power of the atomic number, HgI2 also has a higher stopping power for energetic photons and thus a higher detection efficiency. The ability to operate nuclear detectors at ambient temperature without the clumsiness of bulky cooling systems can greatly enhance the development of miniature nuclear detectors[2], X-ray imaging systems[3], and nuclear detection in space[4]. At present, HgI 2 nuclear spectrometers for X-ray detection have resolutions comparable to those of Si or Ge devices[5]. However, the manufacturing yield of high-performance HgI 2 detectors remains relatively low (about 20%)[61. Defects introduced during crystal growth, device fabrication, and aging can all contribute to this low yield. The choice of conducting electrodes is crucial in determining the quality of detectors. Pd and colloidal carbon have been empirically found to yield the best results, although reproducibility is still poor. Other metals suc
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