Photoluminescence Studies of Impurities and Defects in Mercuric Iodide
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PHOTOLUMINESCENCE STUDIES OF IMPURITIES AND DEFECTS IN MERCURIC IODIDE X.J. BAO*,T.E. SCHLESINGER*,R.B. JAMES**,A.Y. CHENG*** AND C. ORTALE*** *Carnegie Mellon University, Department of Electrical and Computer Engineering, Pittsburgh, PA 15213 "*SandiaNational Laboratories, Advanced Materials Division, Livermore, CA 94450 "s**EG&G Energy Measurements, Inc., Goleta, CA 93116 ABSTRACT We have studied the effects of chemical etching in potassium iodide(KI) aqueous solution, vacuum exposure and bulk heating on the photoluminescence(PL) spectra of mercuric iodide(HgI 2). Different contact materials deposited onto HgI 2 were also investigated, such as Pd, Cu, Al, Ni, Sn, In, Ag and Ta. These processing steps and the choice of a suitable electrode material are very important in the manufacturing of high-quality mercuric iodide nuclear detectors. Comparisons are made between the front surface photoluminescence and transmission photoluminescence spectra.
INTRODUCTION Mercuric iodide has gained increasing attention because it can be used to fabricate high efficiency nuclear detectors operated at ambient temperature, as compared with conventional Si(Li) and Ge(Li) nuclear detectors that have to be kept cool at all times[l]. However, defects that are introduced during detector fabrication as well as due to the interaction of the contact material with the HgI 2 severely limit the manufacturing yield of high quality HgI 2 nuclear detectors[2]. Characterization of the processing steps and various contact materials can provide guidance in the optimization of fabrication processes and the proper choice of a contact material.
RESULTS AND DISCUSSIONS HgI 2 has a bandgap of 2.4eV at 4.2K and produces strong PL at low temperatures. Many workers[3-6] have studied HgI 2 by PL measurement. In this work, an Argon laser tuned to 4880Awas used for excitation source in the PL measurement. Several processing steps were separately studied by PL. These steps include KI etch, vacuum exposure and bulk heating. The effect of KI etching is shown in Fig.1. It can be seen that KI etch effectively exposes a fresh surface that is quite different from the "degraded" surface before the etch. This "degraded" surface can be formed within a day for samples stored in a dessicator. Since the PL spectra taken from a degraded surface resemble those taken from freshly etched samples that are relatively iodine poor (see Fig.2), the degradation was attributed to iodine loss due to sublimation and/or interaction of the HgI 2 with air. The effect of vacuum exposure was similar to KI etch in that it removes a layer of material from the surface. Instead of inducing a preferential loss of iodine or mercury, the vacuum does not produce a significant deviation of stoichiometry as measured with a PL technique. Bulk heating to 100'C in a helium ambient for 10 to 60 minutes was found to decrease P2 to P3 and band 2 to P3 ratios. These changes were attributed to the creation of native defects at elevated temperatures. In Fig.2, P2 to P3 and band 2 to P2 ratios for nine diff
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