Performance of P-I-N CdZnTe Radiation Detectors and Their Unique Advantages

Abstract

We present the performance characteristics of CdZnTe radiation detectors with a new P-I-N design and their unique advantages over metal-semiconductor-metal (M-S-M) devices. In M-S-M CdZnTe detectors the bulk resistivity of the substrate largely determines the leakage current. High leakage current is a dominant noise factor for CdZnTe detector arrays, coplanar detectors, and detectors used for low X-ray energy applications. P-I-N devices provide low leakage currents. Early CdZnTe detectors exhibited polarization, were limited to small detection volumes, and some required high deposition temperatures. We have developed a new heterojunction design which can be deposited at low temperatures so that even high-pressure Bridgman CdZnTe can be used. Using the P-I-N design, CdZnTe detectors with high detection volumes (>200 mm 3 ) were fabricated and exhibited low leakage current, good energy resolution, and no polarization. These detectors have significant advantages over M-S-M detectors in three specific areas. First, X-ray fluorescence studies require detectors with low leakage currents to provide less spectral broadening due to electronic noise. Second, less expensive vertical Bridgman CdZnTe material can be used for imaging applications since it normally possesses too low of a bulk resistivity to be useful as a M-S-M detector. Third, leakage currents across the anode grid in large volume coplanar detectors can be significantly reduced. INTRODUCTIONCompound semiconductors such as CdTe and HgI 2 have been investigated for room temperature gamma-ray detection since they have a high average atomic number (resulting in large attenuation coefficient) and a wide bandgap (which enables room temperature operation). However, major issues such as poor charge collection efficiency and non-ideal ohmic contacts have limited their potential. In CdTe, deep traps reduce the mobility-lifetime product (jIt') of holes causing long low-energy tails, leading to poor energy resolution. Poor charge collection also limits the detection volume. Increasing bias will improve the energy resolution; however, CdTe's low resistivity (10' ohm-cm) and M-S-M design prevent operation at high bias due to high leakage current [1,2].A significant advancement in radiation detector technology has been the development of CdZnTe alloys by the high pressure Bridgman (HPB) technique [3,4,5]. CdTe with Zn (in the range of 4 to 20% Zn) increases the resistivity to mid 10'0 ohm-cm. This improves the overall performance of the device, but there are still problems with low energy tailing and poor peak-tovalley ratios in large volume detectors. The electron gt product does not improve, and the hole ýiv product decreases when adding Zn; deep traps still limit the energy resolution and detection volume.Several approaches such as thermoelectric cooling, improved pulse processing methods, and new device designs are being researched to improve detector energy resolution [6,7,8].

X-ray fluorescence spectrometry

“…The performance of a PIN-type CdZnTe detector employing less expensive material, which was produced using either the vertical or high-pressure Bridgman process, was reported by Sudharsanan and coworkers. 92,93 Detectors of large volume (w200 mm 3 ) were produced and the new PIN design offered low leakage currents and no charge polarization effects. Ivanov and colleagues, 94,95 reported the use of an M-p-N CdTe detector that exhibited a low leakage current and, when operated at 230 ³C, provided an energy resolution of 420 eV at 5.9 keV.…”

Surface processing of CdTe crystals by an excimer laser and its application in fabricating nuclear radiation detectors

“…This technique has an advantage that extremely high temperature can be achieved at the semiconductor surface without affecting the bulk because of the high absorption coefficient of excimer laser in semiconductor materials [1,2]. This is of particular interests for those semiconductors, like CdTe, which are vulnerable to high temperature processing [3,4]. This useful feature of excimer laser makes surface processing of semiconductors possible in two different ways.…”