Development of X-ray, Gamma Ray Spectroscopic Detector Using Epitaxially Grown Single Crystal Thick CdTe Films
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1164-L05-01
Development of X-ray, Gamma Ray Spectroscopic Detector Using Epitaxially Grown Single Crystal Thick CdTe Films M. Niraula, K. Yasuda, H. Ichihashi, Y. Kai, A. Watanabe, W. Yamada, H. Oka, T. Yoneyama, K. Matsumoto, T. Nakanishi, D. Katoh, H. Nakashima, and Y. Agata Nagoya Institute of Technology, Graduate School of Engineering Gokiso, Showa, Nagoya 466-8555, Japan ABSTRACT In this paper, we review our efforts in the spectroscopic detector development using epitaxially grown thick single crystal films of CdTe. The films were grown on GaAs and Si substrates using metalorganic vapor phase epitaxy growth technique. High crystalline quality thick single crystal CdTe films (>260 µm) were obtained where the growth rates could be varied from 10 to 70 µm/h by adjusting the precursor’s flow rates, ratios and the substrate temperatures. Spectroscopic detectors were fabricated in a p-CdTe/n-CdTe/n+-GaAs or p-CdTe/n-CdTe/n+-Si heterojunction diode structure. Both types of detector were capable of detecting and resolving energy peaks from a gamma ray source. However, the spectroscopic performance of p-CdTe/nCdTe/n+-Si detectors was better than that of the p-CdTe/n-CdTe/n+-GaAs detectors. Details on the growth characteristics, detector fabrication and the detector performance are reported. Furthermore, current challenges in this detector fabrication technique are discussed. INTRODUCTION CdTe and CdZnTe are the most widely explored compound semiconductors for x-ray, gamma ray detector applications. Their unique physical properties such as high average atomic number and density give high detection efficiency for the impinging photons, while the wide bandgap values make them suitable for room temperature operation. Due to intensive research carried out in recent years, material properties, especially the charge transport properties of electron, have significantly improved [1-4]. Small spectroscopic detectors and small-area imaging array with high-energy resolutions are already developed and some of them are now commercially available. Today, CdTe, CdZnTe detector crystals are grown by melt-growth techniques, such as the high-pressure or vertical Bridgman or the traveling heater method [1-5]. High-resistivity crystal required for detector fabrication is obtained by compensating background impurities and native defects with external dopants [1-3]. Unfortunately, melt-grown bulk crystals presently contain numerous crystal defects, such as grain boundaries, twins, Te-inclusions, cracks and pipes, and in some cases exhibit polycrystallinity that lead to poor electrical transport properties and inhomogeneity, which limit detector size to small dimensions. The detector grade crystal wafers are often obtained by screening and separating portions of single crystalline defect-free crystals from the large volume crystal ingots. This obviously decreases the crystal yields, thus increasing the overall detector cost, and also limits the size of the available wafers. This hampers the development of large-area and high-sensitivity, high
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