700 Mescher, Schlesinger, Toney, Brunett, and James 700 Special Issue Paper Journal of Electronic Materials, Vol. 28, No. 6, 1999 (Received November 5, 1998; accepted February 5, 1999) Development of Dry Processing Techniques for CdZnTe Surface Passivation M.J. MESCHER, 1 T.E. SCHLESINGER, 1 J.E. TONEY, 2 B.A. BRUNETT, 3 and R.B. JAMES 3 1.--Electrical and Computer Engineering Department, Carnegie Mellon University, Pittsburgh, PA. 2.--Spire Corporation, Boston, MA. 3.--Sandia National Laboratories, Livermore, CA A method for passivating the surface of Cd 1–x Zn x Te (CZT) x-ray and gamma ray detectors using relatively simple dry processing techniques has been developed. Leakage currents were significantly reduced for several processing methods. CZT samples were exposed to an oxygen plasma and/or coated with a reactively sputtered silicon nitride layer. Several parameters of the oxygen plasma step were found to be important for achieving enhanced surface resistivity. SiN X has been previously characterized and was used because of its high dielectric quality and low deposition temperature. Reduction in leakage current after passivation by a factor of as much as twenty is demonstrated. Results are also presented which give a measure of the long-term stability of the passivating layers. Key words: Cadmium zinc telluride, oxygen plasma, reactive sputtering, silicon nitride, surface passivation INTRODUCTION Cadmium zinc telluride (CZT) has shown great promise as a material to be used for the production of large-volume x- and gamma-ray spectrometers operating at room temperature. However, the performance of spectrometers fabricated from CZT crystals are often limited by leakage current in the devices. The leakage current acts as a source of noise which reduces the ability of the detectors to spectrally resolve the unique radiological emissions of a wide variety of isotopes. Current crystal preparation methods typically include a mechanical polish and a wet etch containing Br in methanol 3–9 or Br in ethylene glycol. 10 Although these etches can leave a smooth surface, this surface is not stoichiometric and has been shown to be Te-rich for CdTe 8 and CdZnTe. 3 As a result, the surface possesses a resistivity which is lower than the bulk resistivity of approximately 10 11 Ω-cm. The current which passes through this low- resistivity region on the surface can be a large fraction of the total leakage current in the device. When a CZT crystal is fabricated into a device which functions in an enhanced mode called “electron-only,” 11–13 the design typically requires there to be closely spaced electrodes across which significant potentials are applied. Under such circumstances, the need to reduce surface leakage currents is increased. Specifically, a method is required to passivate, or reduce the conductivity of, the surface layer (Fig. 1). It is important that this passivation also be stable over time. Previous methods of passivation include variations in the wet etch typically used after mechanical polishing, wet chemistry surface oxidation using peroxides, 1–3 and atomic oxygen bombardment. 14 In these studies, the increase in surface resistivity was attributed to the formation of thin layers (approximately 20Å) of Te oxides, primarily TeO 2 . These methods typically result in surface leakage reductions of between 20 and 500%. One group reports reductions of factors greater than ten; 10 however, the passivation methods used in these studies have not as yet been discussed in detail. The methods listed above suffer from various drawbacks. Wet chemistry oxidation must typically be done before electrode deposition. As a result, these contacts are typically blocking because of the underlying oxide layer. The apparatus required for atomic oxygen bombardment is costly and the times required to obtain significant reduction in leakage currents are substantial. 14 Also, the long-term stability of the thin passivation layers