Linear x-ray detector array made on bulk CdZnTe for 30~100 keV energy

Journal of Electronic Materials, Jun 1997 by Yoo, S S, Jennings, G, Montano, P A

S.S. YOO,*^ G. JENNINGS,^ and P.A. MONTANO*^

*Microphysics Laboratory, Department of Physics, University of Illinois at Chicago, Chicago, IL 60607-7059

^Material Science Division, Argonne National Laboratory, Argonne, IL 60439

A CdZnTe detector grown by the high pressure Bridgman (HPB) growth technique was tested using high energy x-rays (30~100 keV), and the performance was compared with a commercially available NaI scintillating detector of 5 cm thickness. The charge collection efficiency of a CdZnTe detector is as high as 90% at relatively low electric field, 600 V/cm. At high x-ray photon energies, the detection efficiency is reduced due to the thickness of the CdZnTe. A 32 channel linear array was fabricated on 1.2~1.7 mm thick CdZnTe, of which the detector area was 175 x 800 (mu)m^sup 2^ and the pitch size 250 (mu)m. The measured dark current for the 16 element detector was as low as 0.1 pA at 800 V/cm with an excellent uniformity. Energy spectra were measured using a Co^sup 57^ radiation source. A small pixel effect and charge sharing were observed. The energy resolution was improved and compared with the large area detector. The array detector gave an average 5.8% full-width-half-maximum (FWHM) at 122 keV photopeak. The large area detector of the same material before fabrication exhibited a low energy tail at the photopeak, which limits the photopeak FWHM to 8%.

Key words: CdTe, CdZnTe, gamma-ray detector, high pressure Bridgman, linear array, radiation detector, x-ray detector

INTRODUCTION

The new high photon intensities available in the third generation synchrotron x-ray sources, such as the Advanced Photon Source at Argonne National Laboratory, require new x-ray detectors, particularly in the 30~100 keV x-ray photon energy range. In Compton inelastic or diffraction scattering measurement, the use of high energy x-rays as a probe beam is of a great advantage to improve its resolution. Particularly for successful Compton scattering profile measurement with sub-atomic momentum resolution, one needs a semiconductor detector array, which provides both energy resolution and real time measurement capability.1,2

Currently there are numerous x-ray detectors commercially available for low energy x-ray experiments. Most of semiconductor detectors are based on Si or Ge material, and various high performance detectors, such as drift detectors, photodiodes, or CCDs, are widely used. However, at high x-ray photon energies, the use of Si detectors are greatly limited because of the reduced stopping power. Relatively higher stopping power or denser Ge material seems to be a good candidate for the detection of such high energy photons. However, cooling at cryogenic temperature greatly inhibits the advantage of using Ge materials for high energy photon detectors.

An alternative choice is to use CdTe based materials, which have a wide band gap as well as a high atomic number. The high intrinsic resistivity of CdTe is of a great benefit to operate the detectors at room temperature. Extensive research work was carried out to realize room temperature operation of radiation detectors using bulk grown CdTe.3,4 However, a polarization effect, which causes a progressive decrease in the counting rate with time, is often seen in C1 doped CdTe detectors.5,6 This seems to be a limitation to using the CdTe material for further advanced radiation detector applications. When the CdTe detector size is small within the carrier trapping length, such as the case of 10~15 (mu)m thick molecular beam epitaxially (MBE) grown CdTe layers of high crystalline quality, the polarization effect was not noticeable, and consistent response was obtained at least during the measurement period of a few days.7 In spite of availability of large area high quality materials, the thickness of MBE grown CdTe layers is usually limited to 10~20 ,m because of its slow growth rate. Therefore, the use of the MBE CdTe detectors is limited to the low x-ray energy range of 10~20 keV or applications where high spatial resolution as well as fast temporal response are needed.8

HPB grown CdZnTe has been of great interest due to its ability to produce a large volume of radiation detector quality materials.9,10 Recently, extensive research was carried out to achieve high efficiency detectors using this material.11,12 CdTe alloyed with 10~20% Zn was reported to have an intrinsic resistivity nearly two orders of magnitude higher than that of Cl doped CdTe.13 It was also reported that the polarization effect does not appear for HPB grown CdZnTe detectors at least for a week, and the variation of the counting rates observed during this period was within the noise level of their electronic system.14

FABRICATION AND MEASUREMENT

HPB grown CdZnTe detectors with 10% Zn were commercially purchased. The thickness varied from 1.25 to 1.7 mm with a 1 x 1 cm^sup 2^ area. After the CdZnTe sample was mounted on a carrier using low temperature melting wax, which also protects the underlying Au contact metal from chemical processing, the top Au contact was removed in a light concentration of Brmethanol solution. Photolithography was carried out to define 4 arrays of 32 channel elements, then followed by electroless Au deposition. The sample was coated with photoresist in order to protect the surface during the following sawing process, and cut into four linear arrays using a diamond wire saw at slow speed. Then the whole sample was dipped in 2~3 day old 5% Br in methanol etchant to remove the damaged side wall by the diamond wire saw. Linear arrays were mounted on ceramic substrates using silver paint or epoxy, which allows the access of bias to the bottom contact. For current-voltage measurements, a linear array was mounted on a flat package, and gold ball wire bonding was made on the test 16 elements. When measurements were performed, the linear array was mounted on a printed circuit board, and only one channel was capacitively coupled to a eV-5091 preamplifier using a surface mount ceramic capacitor as illustrated in Fig. 1.