Study of Surface Passivation and Contact Deposition Techniques in CdZnTe X-Ray and Gamma-Ray Detectors Dominique E. Jones, Stephen U. Egarievwe, Member, IEEE, Anwar Hossain, Ifechukwude O. Okwechime, Mebougna L. Drabo, JaQueeta Hall, Aaron L. Adams, Stephen O. Babalola, Giuseppe S. Camarda, Aleksey E. Bolotnikov, Member, IEEE, Wing Chan, Ralph B. James, Fellow, IEEE Abstract–Passivation and improved contact deposition techniques are known to reduce the surface leakage current and decrease noise levels of CdZnTe X-Ray and gamma-ray detectors by improving their spectral energy resolutions. This paper presents a comparative study of surface passivation process in CdZnTe X-Ray and gamma-ray detectors. The experimental study compares three surface passivation processes: mechanically polished with a 0.9μm Alumina Powder (Al 2 O 3 ) finishing; mechanically polished with a 0.9μm Alumina Powder (Al 2 O 3 ) and etched with HBr + H 2 O 2 solution for 2 minutes; mechanically polished with a 0.9μm Alumina Powder (Al 2 O 3 ) and chemo- mechanically polished with bromine-methanol-ethylene glycol solution. The results show that chemo-mechanical polishing with bromine-methanol-ethylene glycol solution proved to be the best method out of the three for reducing surface leakage current. The preliminary results of aging studies on H 2 O 2 (Hydrogen Peroxide), NH4F (Ammonium Fluoride) + H 2 O 2 + H 2 O and (NH 4 ) 2 S (Ammonium Sulfide) passivation agents are also presented. I. INTRODUCTION ADMIUM Zinc Telluride (CdZnTe) has proven to be a good candidate for room-temperature X-ray and gamma- ray detectors used throughout many applications such as nuclear nonproliferation, national security and medical imaging systems [1]–[8]. However, their performances are still limited by material defects from surface fabrication processes. Such defects can enhance the surface leakage, which limits the maximum bias application due to the high conductivity of the surface. This effect distorts the signal, making the detectors less usable for gamma-ray spectroscopy. Surface properties can influence the electric field inside the detectors, and affect charge transport and signal formation [1], [2]. Manuscript received November 16, 2012. This work was supported in part by the U.S. Department of Homeland Security (DHS) under Grant No. 2012- DN-077-ARI065-02; by the National Science Foundation (NSF) under Grant No. 1140059; by the U.S. Nuclear Regulatory Commission (NRC) under Grant No. NRC-27-10-514; and by the U.S. Department of Energy (DOE) through Brookhaven National Laboratory (BNL). D. E. Jones, S. U. Egarievwe, I. O. Okwechime, M. L. Drabo, J. Hall, A. L. Adams, and W. Chan are with the Engineering, Construction Management and Industrial Technology Department, Alabama A&M University, Huntsville, AL 35810-1015 USA (e-mail: stephen.egarievwe@aamu.edu). S. O. Babalola is with the College of Engineering, Technology and Physical Sciences, Alabama A&M University, Huntsville, AL 35810-1015 USA. A. Hossain, G. S. Camarda, A. E. Bolotnikov, and R. B. James are with the Department of Nonproliferation and National Security, Brookhaven National Laboratory, Upton, NY 11973-5000 USA (e-mail: rjames@bnl.gov). Most damage to the surface is induced during the cutting phase. Therefore, mechanical polishing and chemical etching are applied to smooth the rough surfaces, which tend to contain trapping centers, enhancing the leakage current [3]. Chemical etching is known to be vital during the fabrication process because it produces a cleaner defect-free surface for electrode deposition. Chemical etchants react with the surfaces by removing material from the top layers and leaving behind a smoother surface [1]. An etchant with bromine methanol produces lower surface roughness compared to other etchants by removing the native oxide layer on the surface. The interface between metals and semiconductors plays an important role in determining the contact characteristics. Sang et al [7] pointed out that there always exists thermal stress on the contact interface caused by the mismatch of the thermal expansion coefficient between the electrode material and the CdZnTe crystal. This is due to a drastic change of temperature during the electrode deposition procedures, or the surrounding temperature of the detectors. The electroless contact deposition technique is widely used not only because of its simplicity, but it also creates a stronger chemical bond between metal and semiconductor compared to the sputtering and evaporation techniques [4]. Sputtering and evaporation deposition methods can create defects in the interface region, which leads to trapping effects. Nemirovsky et al [5] also found that gold (Au) is the contact of choice because it is chemically and mechanically stable enough to allow bonding, prolonged operation and reduction in the leakage current. Some concluded that compared to other metal contacts and techniques, the electorless Au deposition method creates more inter-diffusion between the contact and the CdZnTe material, resulting in better gamma response [6]. Sang’s team also concluded that compared to Al, In, and Pt contacts and depositions, Au electrodes formed quasi-ohmic contact on high resistivity CdZnTe [7]. Therefore, the electroless Au contact technique was used in this study. It is known that the inter-strip resistance of CdZnTe detectors has to be high enough to reduce surface leakage current. Passivation is known to reduce the surface conductivity, decrease the noise and improves the spectral energy resolution of the detectors [8]. Although the chemical etching can produce a smooth surface, the surface becomes nonstoichiometric and has been observed to be tellurium- enriched after the chemical etching [9], [10]. Tellurium- enriched surfaces act as trapping centers and reduce the C 2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS/MIC) R04-26 978-1-4673-2030-6/12/$31.00 ©2012 IEEE 4124