A Study of Dark Currents on Various Modified Surfaces of Mercuric Iodide Single Crystals
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A STUDY OF DARK CURRENTS ON VARIOUS MODIFIED SURFACES OF MERCURIC IODIDE SINGLE CRYSTALS H. N. Jayatirtha, M. Azoulay, M.A. George and A. Burger, NASA Centerfor PhotonicMaterials and Devices, Department of Physics, Fisk University, Nashville, TN 3720& ABSTRACT The surface modifications that occur on a-mercuric iodide surfaces during the fabrication process are crucial for the development of a low noise, high resolution X-ray spectrometer. In this study, we discuss the effects of surface treatments on the dark currents and the results are correlated with surface morphology studies that were carried out by the atomic force microscopy (AFM) technique. I. INTRODUCTION Mercuric iodide is an important material for the detection of X-rays and gammarays at room temperature due to its high atomic numbers (Z = 80, 53) and large band gap energy of 2.29 eV (300 K) [1]. At energies below 100 keV, its energy resolution approaches that of Si and Ge detectors and has a unique advantage of room temperature operation [2]. Crystals of HgI 2 grown so far, have allowed for the fabrication of efficient low-energy X-ray spectrometers with excellent resolution and gamma-ray spectrometers with good resolution and relatively low efficiency [3]. It has been shown that the surface effects are extremely important for the fabrication of HgI2 X-ray spectrometers, since the penetration depth of X-rays in HgI 2 is only several microns [4]. Therefore, the collection of free charge carriers generated by the absorbed X-ray photons is limited both by surface recombination and bulk trapping process. Recent work at our laboratory, on surface morphology of a-mercuric iodide, using atomic force microscopy (AFM), has shown that surface features change as a result of various surface conditions [5]. The surface modifications that occur on a-mercuric iodide during the fabrication process are crucial for the development of low noise, high resolution X-ray spectrometer [6]. Hence, an effort has been made in this study to understand the effects of various surface conditions on the dark leakage current and to correlate the results with the surface morphology studies using the AFM technique. II. EXPERIMENTAL Vapor grown single crystals of a-mercuric iodide of nuclear radiation detector quality were supplied by the EG&G/Energy Measurements, Santa Barbara Operations laboratory. The crystals were cleaved along the crystallographic c-plane to form plates about 1 mm thick, of an area about 40 mm2 .
-e--Pt Lead Humiseal
Aquadag
SHgl 2 Crystal
Encapsulant
Figure 1. Schematic diagram of a mercuric iodide crystal with electric contacts. Mat. Res. Soc. Symp. Proc. Vol. 302. ©1993 Materials Research Society
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Table I: Summary of samples with various surface conditions used in the analysis. Sample No.
Surface Treatment
Symbol
1
As-cleaved
C
2
Cleaved, vacuum exposed
CV
3
Cleaved, etched in KI
CE
4
Cleaved, etched in KI, vacuum exposed
CEV
Crystals with various surface conditions including as-cleaved (C), cleaved-vacuum etched (CV), cleaved-chemically
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