DC photoconductivity study of semi-insulating Cs(1-X)Zn(X)Te crystals

Journal of Electronic Materials, Jun 2001 by Cui, Y, Wright, G W, Ma, X, Chattopadhyay, K, Et al

Room temperature direct current (DC) photocurrents were measured for detector grade Cd^sub 1-x^Zn^sub x^Te (CZT) crystals in the spectral range of 400-1000 nm as a function of light intensity and voltage. The photocurrent data were analyzed and fit to a theoretical model to extract the electrical transport properties for high-- resistivity detector grade CZT material. Using the DC photocurrent measurements, the mobility-lifetime ((mu)(tau)) products and the surface recombination velocities for both electrons and holes were measured. For this study the CZT detectors had Au contacts, and the surfaces were treated in a standard 5% bromine in methanol etching solution. The correlation of the DC photocurrent measurements and detector performance is also reported.

Key words: DC photocurrent, surface recombination, CZT, detector performance, gamma-ray detectors

INTRODUCTION

Cadmium zinc telluride (CZT) is an important semiconductor material for room temperature gamma-ray and x-ray detectors because of its wide band gap, high atomic number of the constituents, and high resistivity. It also has a reputation of good electron transport properties, which are usually characterized by a relatively high value of product of electron mobility-- lifetime ((mu)(tau)). The (mu)(tau) product is a critical performance parameter of semiconductor detectors, because it determines the charge collection efficiency (CCE). The most common procedure to measure ((mu)(tau)) is by using strongly absorbed ionizing radiation (x-ray, alpha particles, or high energy electrons), record the collected charge as a function of the applied voltage bias and fit the results to the Hecht equation.1 The Hecht equation is oversimplified, because it assumes a uniform internal electric field and no surface recombination effects. For HgI^sub 2^ crystals, it was realized that surface recombination effects might play an important role in the carrier transport.2-4 Moreover, in CZT material, it was reported that surface effects affected charge collection efficiency.5 Several surface treatments and contact studies on CZT have been performed.6-15 Although the effects of those procedures were correlated with detector performance, the effects have not been directly related to the surface recombination of charge carriers.

A direct current (DC) photoconductivity study of the detector grade CZT crystals is helpful to characterize and improve the transport properties of CZT. By applying different voltage polarities to the illuminated electrode, electron and hole photocurrents can be measured separately, and the corresponding ((mu)(tau)) can be obtained respectively. As a technique, DC photoconductivity measurement of the semiconductor crystals has been well established for decades. The main purpose of this paper is to correlate the detector performance with DC photocurrent measurements, and to distinguish surface effects from bulk effects. We measured the photocurrent of detector grade CZT crystals in the spectral range of 400-1000 nm as a function of light intensity and voltage. The theoretical model developed by Many16 can determine both the ((mu)(tau)) products and surface recombination velocities. This model has been successfully employed to study the surface generation and recombination effects in the photoconductivity of HgI^sub 2^ single crystals.2 In this paper, we show that the recombination velocities of two surfaces can differ dramatically even though both surfaces are treated in the same etching solution. It should be noted that the theoretical analysis is valid for a contactless experiment without injecting electrons and holes from electrodes, and for a uniform internal electric field. To make a good detector, it is necessary to have good electrical contacts, preferably ohmic contacts. The effects of contacts make the theoretical analyses complicated for some detectors.17 However, it is found that even if the contacts are ohmic or injection types, Many's theoretical model is still applicable as an approximation.

EXPERIMENTAL

Two CZT samples with dimensions of 10x10x2 mm^sup 3^ and 10x10x10 mm^sup 3^ were polished on a mechanical polisher with a 0.05 (mu)m particle size alumina suspension and rinsed in methanol. The samples were then treated in a standard 5% bromine-in-methanol etching solution for two minutes. Au contacts were then deposited by sputtering, and Pd leads were attached to contacts using a colloidal graphite suspension in water. Finally, the devices were covered with a protective coating. The experimental setup for detector performance measurement using an241Am point source has been reported earlier.11 DC photocurrent measurements were carried out with a custom-made computer-interfaced photocurrent measurement system, including a 100 W Quartz Tungsten Halogen series Q source (Oriel Instruments, Model 60066) and a motorized 1/8m Monochromator (Oriel Instruments, Cornerstone 130(TM), Model 74000). The spectral response was measured using an optical power meter (Oriel Instruments, Model 70310). A bias was applied by a high voltage supply (Tennelec, TC952). The voltage dependence of the photocurrents under a specific wavelength illumination was measured with a high voltage source measurement unit (Keithley, Model 237). The light intensities were measured using an optical power meter (Newport, model 1815-C). Optical absorption measurements were carried out with a Cary 500 scan UV-Vis-NIR spectrophotometer. All measurements reported in this paper were made at room temperature.