A. Burger

Photoluminescence study of deep levels in Cr-doped ZnSe

Single crystals of intrinsic ZnSe were grown by the seeded physical vapor transport method and the diffusion doping was utilized to incorporate Cr in these crystals. The radiative recombinations in these samples with Cr concentration in the range 1.0–10.2×1019u2009cm−3 were studied by the steady state photoluminescence technique. It was found that the Cr deep centers inhibit the band-to-band emission in Cr-doped ZnSe. Except in undoped single crystals, no emission corresponding to the band-to-band transition was observed from any of the doped samples. Instead, the higher wavelength emissions associated with Cr deep levels were obtained. This capture of photoexcited carriers by deep centers was verified using different excitation wavelengths. The role of chromium impurities in nonradiative recombination processes was also evidenced from the sharp decreases in the deep level emission intensity with increasing Cr concentration.

Characterization of metal contacts on and surfaces of cadmium zinc telluride

Abstract In the past several years significant progress has been made in building a database of physical properties for detector quality Cd x Zn 1− x Te (CZT) ( x =0.1–0.2) crystal material. CZTs high efficiency combined with its room temperature operation make the material an excellent choice for imaging and spectroscopy in the 10–200xa0keV energy range. For detector grade material, superior crystallinity and high bulk resistivity are required. The surface preparation during the detector fabrication plays a vital role in determining the contact characteristics and the surface leakage current, which are often the dominant factors influencing its performance. This paper presents a surface and contact characterization study aimed at establishing the effects of the surface preparation steps prior to contacting (polishing and chemical etching), the choice of the metal and contact deposition technique, and the surface oxidation process. A photoconductivity mapping technique is used for studying the effects of different surface treatments on the surface recombination.

Controlled micro oxidation of CdTe surface by laser irradiation: A micro‐spectroscopic study

A small area of CdTe single crystal within a spot of ≤2 μm diameter has been oxygenated under ambient conditions using 514.5 nm radiation from Ar+ laser. The oxygenation could be monitored using laser Raman spectroscopy, and it has been established that the asymmetric TeO2−3 ion is formed on the surface. The significance of the process for basic material science and technological applications in electronic and optoelectronic devices has been emphasized.

Study of surface recombination velocity of Cd1−xZnxTe radiation detectors by direct current photoconductivity

The surface recombination velocity of a Cd1−xZnxTe (CZT) radiation detector treated by mechanical polishing and by a standard 5% bromine in methanol chemical etch is reported. The light power dependence of the surface recombination velocity was measured using dc photoconductivity. The results reveal that the surface recombination velocity is a function of the electron generation rate, which can be described by a Shockley–Read one-center model. It was observed that the surface recombination velocity of the CZT detector treated by polishing only is much larger than that treated with polishing followed by chemical etching. The correlation of dc photoconductivity and low-temperature photoluminescence measurements of the CZT detector is also discussed.

Comparative gamma spectroscopy with SrI 2 (Eu), GYGAG(Ce) and Bi-loaded plastic scintillators

We are developing new scintillator materials that offer potential for high resolution gamma ray spectroscopy at low cost. Single crystal SrI<inf>2</inf>(Eu) offers ∼3% resolution at 662 keV, in sizes of ∼1 in³. We have developed ceramics processing technology allowing us to achieve cubic inch scale transparent ceramic scintillators offering gamma spectroscopy performance superior to NaI(Tl). Our bismuth-loaded plastic scintillator demonstrates energy resolution of ∼8% at 662 keV, for samples of ∼0.5 cm³.

The effects of chemical etching on the charge collection efficiency of {111} oriented Cd/sub 0.9/Zn/sub 0.1/Te nuclear radiation detectors

We report the effects of different etchants on the mobility-lifetime product (/spl mu//spl tau/) of cadmium zinc telluride (CZT). The /spl mu//spl tau/ values were obtained via the Hecht relation. The effects of surface recombination were also investigated using photoconductivity measurements. Results were obtained for a {111} oriented CZT single crystal. We find that the surfaces behave differently after etching. Values for surface recombination rates for [111]A and [111]B surfaces can vary by up to two orders of magnitudes for the same etchant. This large variation in surface recombination rates affects the charge collection efficiency and detector performance. We report surface treatments that enhance the charge collection efficiency for [111]A and [111]B surfaces. Data describing the detector response for Am/sup 241/ spectra current-voltage relationship, and bulk /spl mu//spl tau/ values and surface recombination rates are shown for different etchants.

Luminescence and spectroscopic behavior of Eu3+-doped Y2O3 and Lu2O3 epitaxial films grown by pulsed-laser deposition

We report on the fabrication of highly epitaxial Y2O3:Eu3+ and Lu2O3:Eu3+ films on sapphire and Si single crystalline, and quartz substrates under different growth and optimization conditions using pulsed-laser deposition technique. The films were characterized using x rays, microstructure, and spectroscopic measurements. Our results demonstrate superior spectroscopic performance on the films grown on sapphire substrates due to the high quality and epitaxial nature of the film. The surface morphology of the epitaxial films controls the photoluminescence property of these phosphors film.

Effects of surface Processing on the performance of Cd/sub 1-x/Zn/sub x/Te radiation detectors

The effects of polishing on the electron mobility-lifetime product of a detector-grade Cd/sub 1-x/Zn/sub x/Te (CZT) crystal is reported. The two surfaces of the crystal were deposited with Au layers and illuminated with subband light. A direct current (dc) photocurrent technique was used to measure the electron mobility-lifetime product as a function of the illuminating power. The measured dependence of the electron mobility-lifetime product on the photogenerated electron concentration was highly affected by the condition of the irradiated surface. This discovery has important implications for processing of CZT surfaces for radiation detector applications.

RAMAN AND PHOTOLUMINESCENCE STUDIES ON INTRINSIC AND CR-DOPED ZNSE SINGLE CRYSTALS

Intrinsic and Cr-doped single crystals of ZnSe, grown by the seeded physical vapor transport technique, are studied using Raman and photoluminescence spectroscopic techniques. The coupling between the longitudinal–optical (LO) phonon and the hole plasmons, in Cr-doped ZnSe, results in a downward shift of the LO peak with increasing dopant concentration. The ratio of integrated intensities of LO and transverse–optical (TO) modes in ZnSe:Cr shows a systematic increase, with increasing temperature and decreasing excitation wavelength. This occurs due to the interaction of the LO phonon field with the surface electric field in the depletion layer as an enhancement of LO mode intensity. The localized Raman modes of CrZn–Se are seen in Cr-doped samples, which corroborate the photoluminescence findings of the presence of Cr2+ and Cr1+ deep levels. The LO and TO modes of intrinsic ZnSe show upward renormalization in their peak positions with decreasing excitation wavelength. The interaction of discrete phonons wi...

SrI2, a Novel Scintillator Crystal for Nuclear Isotope Identifiers

The growth and scintillating properties of undoped and Eu2+ doped Strontium Iodide indicate excellent potential for gamma ray spectroscopy. Energy resolution at 662 keV was found to be as good as 2.7% at 662 keV. The effect of purification by zone refining was also studied and crystal growth of SrI2 by the Bridgman technique was found to be less subject to cracking compared to the growth of lanthanum halide scintillators.