Jean-Olivier Ndap

Preparation conditions of chromium doped ZnSe and their infrared luminescence properties

We report the investigation by photoluminescence lifetime measurements of the near-IR emissions from a series of chromium-doped ZnSe samples, correlated to their preparation conditions. The samples were polycrystalline or single crystals prepared by post growth diffusion doping or single crystals doped during growth by the physical vapor transport method. Room temperature lifetime values between 6 and 8 μs were measured for samples with Cr 2+ concentrations from low 10 17 to high 10 18 cm -3 range. Lifetime data taken down to 78 K was found to be rather temperature independent, reconfirming previous reports indicating a quantum yield of the corresponding emission of close to 100% at room temperature. A strong decrease in the room temperature lifetime was found for chromium concentrations higher than 10 19 cm -3 .

Comparison of cadmium zinc telluride crystals grown by horizontal and vertical Bridgman and from the vapor phase

Characterization studies of Cd1−xZnxTe (0<x<0.24) crystals, CZT, grown by high pressure vertical Bridgman (HPVB), low pressure (LPB) vertical modified Bridgman (VB), horizontal modified Bridgman (HB), and physical vapor deposition (PVD) methods were performed. For selected melt-grown ingots, the liquid/solid segregation coefficients of some of the impurities were established. For most of the crystals, the surface and the bulk crystallinity were determined using triple and double axis X-ray diffraction techniques (TAD and DAD XRD). X-ray topography maps were also used to study macroscopic defects. The difference in properties of CZT grown by these methods are discussed.

Thermal treatments of CdTe and CdZnTe detectors

An irreversible deterioration of CdTe and CdZnTe detectors after heat treatments in the temperature range of 150 - 200 degrees Celsius was reported by several authors; however, the nature of the processes responsible for the detector degradation and increased dark currents is not fully understood. In this study we have prepared CdTe and CdZnTe detectors equipped with Au contacts. The detectors were tested before and after thermal annealing under vacuum. Using combined measurements of current voltage characteristics, low temperature photoluminescence and nuclear spectroscopic measurements, we have attempted to differentiate between the various possible contributions to the detector degradation and elucidate the defect formation process involved.

Defects in CZT crystals and their relationship to gamma-ray detector performance

Abstract This paper reviews some of the progress obtained in the understanding of defects in detector grade cadmium zinc telluride material (CZT). Several techniques have been utilized to elucidate some of the issues related to compositional uniformity, effects of precipitates, grain boundaries, and surface defects related to mechanical and chemical treatments. In few cases, special mapping capabilities had to be developed to allow correlations with detector performance.

Cadmium zinc telluride high-resolution detector technology

Electrode contacting on semiconductor radiation detectors has been a topic of active interest in many recent investigations. Research activities have focused on the morphology and chemistry of modified surfaces using sophisticated preparation techniques and employing characterization methods that are able to discriminate between surface and bulk effects. From an applied point of view, the detector fabrication technology involves a series of fabrication steps which can be optimized. Results of an ongoing effort to improve the performance of high resolution CdxZn1-xTe spectrometers by addressing wafer surface preparation, electrode deposition and contact passivation are described.

High-temperature solution growth of Cr2+ : CdSe for tunable mid-IR laser application

Solid-state tunable laser sources in the mid-IR region have been recently investigated for their potential in remote sensing applications. Room temperature lasers based on single crystals of chromium-doped CdSe are particularly interesting due to their broad band tunability beyond 2 lm spectral region. In this study, we have grown doped single crystals by the high-temperature gradient freezing solution technique, using selenium as the solvent. Crystals of 1.2 cm in diameter and up to 7 cm long, cracks and precipitates free could be obtained. The e!ective segregation coe

High-Efficiency, Broadly Tunable Continuous-Wave Cr2+:ZnSe laser

cient of Cr2‘ ions along the ingot was evaluated. Room temperature emission lifetime values of 3.0}4.5 ls were measured. ( 2000 Published by Elsevier Science B.V. All rights reserved.

Physical properties and evaluation of spectrometer-grade CdSe single crystals

We report room-temperature operation of an all solid-state, continuous-wave Cr2+:ZnSe laser. The laser was pumped by a Tm:YALO laser operating at 1940 nm. Demonstrated Cr:ZnSe laser results include: output power of 380 mW, an absorbed power slope efficiency of 63%, an absorbed power threshold as low as 98 mW, and continuous tunability from 2138 to 2760 nm. All Cr:ZnSe laser crystals were prepared by the diffusion doping technique.

Au/CdZnTe/CdS m-i-n detectors fabricated by sputtering technique

Undoped CdSe single crystals were investigated by current- voltage measurement and low temperature photoluminescence. Crystal quality and surface morphology were also studied by optical microscopy and scanning electron microscopy. Together with gamma spectroscopy, the results show that the CdSe crystals have high resistivities in the 1010 (Omega) (DOT)cm range. This indicates their usefulness for room temperature nuclear radiation detection. Small active volume CdSe detector were fabricated and their performance is presented. A comparison of fundamental material properties between CdSe and CdZnTe detectors is also given.

Thermal diffusion of Cr2+ in bulk ZnSe

The electrical and detection properties of Au/CdZnTe/CdS (m- i-n) gamma-ray and x-ray detectors, fabricated by sputtering technique, have been studied. The CdZnTe crystal was grown by the High Pressure Bridgman method. Current-voltage measurements show that the leakage current is reduced by an order of magnitude when the m-i-n detector is reversed biased. This is due to increased contact barriers, which produced effective blocking contacts. An energy resolution of 3.6 percent for the 59.6 keV line of 241Am was obtained in the reverse bias, with almost no energy resolution for the forward bias due to excessive leakage current.