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The maximum optical-absorption cross section of Cr2+ ions was evaluated from near-infrared (NIR) absorption spectroscopy and direct measurements of the chromium concentration in Cr2+:CdSe crystals. The emission lifetime of the excited state, 5E, of Cr2+ was measured as a function of Cr2+ concentration in the 2 × 1017-2 × 1018 ions/cm3 range and as a function of temperature from 77-300 K. Lifetime values were as high as ∼6 µs in the 77-250 K range and decreased to ∼4 µs at 300 K because of nonradiative decays. Assuming that most of the Cr dopant is in the Cr2+ state, an optical-absorption cross section σa of (1.94 ± 0.56) × 10-18 cm2 was calculated. Implications for laser performance are discussed.

Transition metals in the group II-VI semiconductors have attracted considerable attention for their application as tunable mid-infrared (IR) lasers at room temperature. Very recently, Co-doped II-VI hosts have been considered as saturable absorber materials. In the present study, we have grown both Cr-doped and Co-doped CdSe by a high-temperature solution growth technique using Se as solvent. The Cr-doped CdSe was grown in 4,000-ppm concentration. The Co-doped CdSe crystals were grown in concentrations of 2,000 ppm and 4,000 ppm. The crystals were characterized by optical-absorption spectroscopy. The absorption-peak position for Cr was 1.92 µm, and for Co, the peak position was 1.8 µm. The doping concentrations for Cr-doped samples were estimated from optical-absorption spectra using the Beer-Lambert law. Good-quality, crack-free single crystals were grown; however, our near-IR transmission microscopy showed Se inclusions, which were also confirmed by differential scanning calorimetry (DSC). The electrical-resistivity dependence on doping concentrations is presented and discussed. Glow discharge mass spectrometric (GDMS) measurements for the Co-doped sample are reported. The absorption cross section for Co was calculated for the first time using the Beer-Lambert law.

Cr 2+ :ZnSe is now known as a room-temperature widely tunable solid-state mid-infrared laser material. Optimization of laser performance requires first and foremost optimization of the Cr 2+ -doping level in the crystals. A simple theoretical model has been developed for evaluation of the thermal diffusivity of Cr 2+ ions in bulk ZnSe single crystals and polycrystalline window materials, from optical absorption measurements in the 1200–2800 nm spectral region. Sputtered metallic chromium and powders have been used as dopant sources. Diffusion appears to be faster in polycrystalline materials than in single crystals for annealing temperatures below 910°C. The activation energy, which is lower in polycrystals, depends on the nature of the dopant source. For mid-IR laser application, the model predicts for example, non-negligible optical losses due to passive self-absorption in the 2000–3000 nm spectral region for chromium-doped polycrystalline materials after annealing at temperatures above 900°C for 1.75 days.

Characterization studies of Cd1−xZnxTe (0

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 .

The preparation and thermophysical properties of AgGaTe 2 crystals are reported. The synthesis from pure elements was performed in a rocking oven and scanning electron microscopy and energy dispersive X-ray elemental analysis show the presence of additional phases. From simultaneous differential calorimetry and thermogravimetric analysis (SDT) measurements we have found that AgGaTe 2 melts at 725.7°C, with a heat of fusion of 104.8 J/g −1 . High temperature powder X-ray diffraction data confirmed the chalcopyrite structure and yielded the thermal expansion coefficients along the a - and c- axes to 500°C. Heating above that temperature, under inert gas flow, causes the material to sublime incongruently and produce a film of gallium rich surface. Experimental values of thermal diffusivity and estimates of the thermal conductivity values are also presented.

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.

The spectral resolution of cadmium zinc telluride (CZT) room temperature nuclear radiation detectors is often limited by the presence of conducting surface species that increase the surface leakage current. Surface passivation plays an important role in reducing this surface leakage current and thereby decreasing the noise of the detectors and improving the spectral energy resolution. Chemical etching with a Br-MeOH solution leaves CZT surfaces rich in Te and is considered as one of the primary causes of the increased surface leakage current. Previous studies have shown that hydrogen peroxide (H2O2) forms oxides of tellurium on the CZT surface and thus acts as a good passivating agent. In this study we will present results on the use of potassium hydroxide (KOH) as an alternative passivating agent. The KOH aqueous solution leaves a more stoichiometric (evaluated from the trends in the surface Cd:Te ratio) and smoother CZT surface. The passivation effects of KOH solution on the surface of the CZT have been characterized by current-voltage measurements for different KOH concentrations and etching times for both parallel strip electrodes as well as a metal-semiconductor-metal configuration. The surface chemical composition and its morphology were studied by scanning x-ray photoelectron spectroscopy and atomic force microscopy. The comparison and demonstration of improvements in the spectral resolution of the CZT detectors (based on 241Am spectra) with and without the KOH treatment are presented.

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$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.

A large photorefractive effect was measured in CdS0.8Se0.2:V and Cd0.55Mn0.45Te:V ternary crystals which shows promise for many device applications such as optical signal processing and optical limiting. In this study, we compared the results obtained from two-wave mixing experiments with Cd0.55Mn0.45Te:V and CdS0.8Se0.2:V crystals at the 633 nm wavelength. As the signal to pump beam ratio was varied from 10−1 to 10−4, Cd0.55MN0.45Te:V and CdS0.8Se0.2:V showed a maximum photorefractive gain of 0.17 and 0.20 cm−1, respectively. The grating formation time of both the crystals were measured to be in the milli-second range at an incident intensity of 200 mW/cm2.

ZnSe:Cr2+ is an attractive candidate as a room-temperature tunable solid-state laser with output in the 2–3 µm range. Passive absorption losses in this emission range currently limit laser performance. In this study, we use absorption and photoluminescence spectroscopies at 5 and 296K to address the origin of these optical losses. A series of diffusion-doped ZnSe:Cr single-crystal samples with Cr2+ concentrations in the range from 2×1017 cm−3 to 9×1019 cm−3 were obtained using CrSe powder as the dopant source. We find that trace amounts of Fe2+ produce absorption in the 2–3 µm range. Also, we have obtained data on a 680 nm absorption band observed in ZnSe:Cr which has been assigned to an internal transition of Cr2+. In our series of samples, the relative intensities of the 680 nm absorption band do not track the relative intensities of the 1.8 µm band (known to be due to Cr2+), although excitation near 680 nm does produce weak Cr2+ luminescence. Our absorption data do not support the current assignment of the 680 nm absorption as being an internal transition of the Cr2+ ion.

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 1.0–10.2×1019 cm−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.

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.

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.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.

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.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.