High-Energy X-ray Diffraction and Topography Investigation of CdZnTe

Journal of Electronic Materials, Jun 2005 by Carini, G A, Camarda, G S, Zhong, Z, Siddons, D P, Et al

High-energy transmission x-ray diffraction techniques have been applied to investigate the crystal quality of CdZnTe (CZT). CdZnTe has shown excellent performance in hard x-ray and gamma detection; unfortunately, bulk nonuniformities still limit spectroscopic properties of CZT detectors. Collimated high-energy x-rays, produced by a superconducting wiggler at the National Synchrotron Light Source's X17B1 beamline, allow for a nondestructive characterization of thick CZT samples (2-3 mm). In order to have complete information about the defect distribution and strains in the crystals, two series of experiments have been performed. First, a monochromatic 67 keV x-ray beam with the size of 300 × 300 μm^sup 2^ was used to measure the rocking curves of CZT crystals supplied by different material growers. A raster scan of a few square centimeter area allowed us to measure the full-width at half-maximum (FWHM) and shift in the peak position across the crystal. The rocking curve peak position and its FWHM can be correlated with local stoichiometry variations and other local defects. Typically, the FWHM values ranging from 8.3 arcsec to 14.7 arcsec were measured with the best crystal used in these measurements. Second, transmission white beam x-ray topography (WBXT) was performed by using a 22 mm × 200 μm beam in the energy range of 50 keV to 200 keV. These types of measurements allowed for large area, high-resolution (50 μm) scans of the samples. Usually, this technique is used to visualize growth and process-induced defects, such as dislocations, twins, domains, inclusions, etc. the difference in contrast shows different parts of the crystal that could not be shown otherwise. In topography, good contrast is indicative of a high quality of the sample, while blurred gray shows the presence of defects. Correlation with other techniques (e.g., infrared (IR) mapping and gamma mapping) was also attempted. Our characterization techniques, which use highly penetrating x-rays, are valid for in-situ measurements, even after electrical contacts have been formed on the crystal in a working device. Thus, these studies may lead to understanding the effects of the defects on the device performance and ultimately to improving the quality of CZT material required for device fabrication. It is important to study crystals from different ingot positions (bottom, center, and top); consequently, more systematic studies involving scans from center to border are planned.

Key words: High-energy x-ray diffraction, rocking curve, topography, CZT, crystal quality, detectors characterization

High-Energy X-ray Diffraction and Topography Investigation of CdZnTe

INTRODUCTION

CZT1,2 (Cd^sub 1-x^Zn^sub x^Te) is an attractive material for the fabrication of high-energy detectors. Its large band gap (E^sub gap^ [congruent with] 1.6 eV) allows it to operate at room temperature, and the high atomic numbers of the elements composing the material (Z^sub Cd^ = 48, Z^sub Zn^ = 30, and Z^sub Te^ = 52) give a high quantum efficiency. In principle, detectors are simple devices that directly convert charge pairs generated by ionizing radiation, such as x-rays, gamma rays, and beta particles, into electrical signals. These advantages combined with very good resolution make CZT detectors suitable for a wide range of applications such as nuclear weapons monitoring, imaging devices for medical, and astronomy applications.

Detector grade material has a composition of x = 0.04-0.2, a resistivity ρ [approximate] 1010 Ωcm, and mobilitylifetime product for electrons μ^sub e^τ^sub e^ [approximate] 10^sup -3^ cm^sup 2^/V and for holes μ^sub h^μτ^sub h [approximate] 10^sup -5^ cm^sup 2^/V.

Historically, the use of the high-pressure Bridgman3 technique for CZT crystal growth led to the production of material for larger area detectors. Unfortunately, structural defects such as twins, dislocations, inclusions, grain, and tilt boundaries result in a low crystalline quality and limit the detector's performances.4,5

High-energy transmission x-ray diffraction techniques6,9 provide a powerful way to study bulk properties of thick crystals (millimeters). As detectors are directly fabricated using a crystal slab, this investigation is suitable to determine the relationship between structural defects and detector response. Highly penetrating x-rays are used for in-situ measurements, even after electrical contacts have been formed on the crystal in a working device. The energy used for these measurements makes reliable the simultaneous measurement of diffraction and detector response. These studies may ultimately lead to the improved quality of CZT material required for device fabrication.10

EXPERIMENT

The X17B1 beamline at BNL's National Synchrotron Light Source provides a collimated highenergy x-ray beam produced by a superconducting wiggler. The experimental hutch was setup for two series of experiments using transmission diffraction techniques. In both experiments, samples, in their holder, were mounted in two translation (x-y) stages located in the center of the triple axis (Fig. 1a and b).