Kuo-Tong Chen

Cr/sup 2+/-doped zinc chalcogenides as efficient, widely tunable mid-infrared lasers

Transition-metal-doped zinc chalcogenide crystals have recently been investigated as potential mid-infrared lasers. Tetrahedrally coordinated Cr/sup 2+/ ions are especially attractive as lasants on account of high luminescence quantum yields for emission in the 2000-3000-nm range. Radiative lifetimes and emission cross sections of the upper /sup 5/E state are respectively /spl sim/10 /spl mu/s and /spl sim/10/sup -18/ cm/sup 2/. The associated absorption band peaked at /spl sim/1800 mm enables laser-diode pumping of the Cr/sup 2+/ systems. Laser demonstrations with ZnS:Cr and ZnSe:Cr (using a MgF/sub 2/:Co/sup 2+/ laser pump source) gave slope efficiencies up to 30%. Excited-state-absorption losses appear small, and passive losses dominate at present. Tuning experiments with a diffraction grating produce a tuning range covering at least 2150-2800 nm. Laser crystals can be produced by Bridgman growth, seeded physical vapor transport, or diffusion doping. Zinc chalcogenide thermomechanical properties of interest for medium-to-high-power operation compare favorably with those of other host materials, except for the larger refractive-index derivative dn/dT.

Low‐temperature photoluminescence of detector grade Cd1−xZnxTe crystal treated by different chemical etchants

Low‐temperature photoluminescence (PL) spectra of detector grade Cd1−xZnxTe (x=0.1) have been measured to obtain information about shallow level defect concentration introduced during mechanical polishing and chemical etching processes. We present here a comparative PL study of Cd0.9Zn0.1Te crystals treated by different chemical solutions used for nuclear detector surface treatment. The results show that the 5% Br–MeOH+2%Br–20% lactic acid in ethylene glycol treatment combines the advantages of bromine and lactic acid for chemical etching and results in the best surface condition, as evidenced by the largest I(D0,X)/Idef intensity ratio and the narrowest full width at half‐maximum of the main peak (D0,X). Changes in the surface morphology were also analyzed by atomic force microscopy and correlated with the PL results. Current–voltage (I–V) curves and the room‐temperature 55Fe spectral response of the sample etched by the best treatment are also presented and discussed.

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.

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...

Chemical analysis of metal impurity distribution of zone-refined mercuric iodide by ICP-AES and DSC

A mercuric iodide single crystal is being developed for X-ray and gamma-ray detector applications where high-purity starting material is required. Zone-refining processing has been proven to be an effective step in the purification of large amounts of mercuric iodide for crystal growth. In this study we used the Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) to identify and determine the distribution of impurity concentrations along the ampoule after zone-refining mercuric iodide. The results show that for Ag, Cu, Fe, Mg, Ca, Zn, Cr and Al, the zone-refining process does sweep the impurities to the last-to-freeze zone, due to an effective distribution coefficient, keff < 1. For Na, Ni, Cd, Mn and Pb the concentration gradient seems to be fairly independent of the position along the ingot. Differential Scanning Calorimetry was also employed to investigate the deviation from stoichiometry caused by the zone-refining process, and the results indicated that the first-to-freeze section is Hg-rich, and the middle section tends to become slightly Hg-rich, while the last-to-freeze section becomes I-rich.

Seeded physical vapor transport of cadmium-zinc telluride crystals: growth and characterization

Crystals ofCdl -xZnxTe with x = 0.2 and 40 g in weight were grown on monocrystalline cadmium-zinc telluride seeds by closed-ampoule physical vapor transport with or without excess (Cd + Zn) in the vapor phase. Two post-growth cool-down rates were used. The crystals were characterized using low temperature photoluminescence, atomic force microscopy, chemical etching, X-ray diffraction and electrical measurements. No formation of a second, ZnTe-rich phase was observed.

Recent CdZnTe detector fabrication developments

High resistivity CdZnTe has attracted extensive interests for its use as x-ray and gamma-ray detector operated at room and elevated temperature due to its superior electro-optical and structural properties compared to other materials. The post- growth surface annealing under constituent over pressure, passivation, oxidation and chemical etching in terms of restoring stoichiometry, reducing surface damage and charge carrier trapping centers have been investigated by low temperature photoluminescence, x-ray photoelectron spectroscopy and atomic force microcopy. The choice of contacting metal and deposition method also plays an important role in lowering surface leakage current and increasing the ohmic behavior between contact and detector, and subsequently directly affects the detector performance. The data revealed by spectroscopic and microscopic measurements were correlated with current-voltage and x-ray spectrum results to give an improved detector fabrication process based on CdZnTe detector. These results are presented and discussed.

Raman Scattering Study of H 2 O 2 -Etched Zn 0.1 Cd 0.9 Te Surfaces

Raman study of as-grown and H 2 O 2 -etched surfaces of Zn 0.1 Cd 0.9 Te single crystal has been performed. A distribution of Te precipitates on the surface of as-grown ZCT, which increases after etching, has been encountered. With high irradiation powers, due to the oxidation of the surfaces, new bands of TeO 3 2− are evolved. A downward shift in the peak position, as well as a halfwidth broadening, of all Raman modes has been observed with increasing laser power. The phenomenon, due to the formation of insulating oxide of tellurium in a dispersion of Te precipitates and vice-versa, has been attributed to the quantum confinement of phonons.

Performances of CdTe and Cd1-xZnxTe gamma-ray detectors at elevated temperatures

This comparative study of the performances of CdTe and Cd1--x)ZnxTe detectors at temperatures in the 25 - 70 degree(s)C range shows that Cd0.8Zn0.2Te has a better performance as a gamma-ray detector at elevated temperatures. Peak position shifts as function of temperature are less pronounced in spectra obtained with Cd0.8Zn0.2Te detectors than in spectra from CdTe. The Cd0.8Zn0.2Te detectors exhibited a FWHM of 24% (at 60 degree(s)C) at 81 keV without significant deterioration due to the heating cycles.

Metal contact formation on zinc cadmium telluride detector material

The understanding of the electric contact formation process is critical in exploiting the full potential of Cd{sub 1{minus}x}Zn{sub x}Te (CZT) material for room temperature detection applications. The metal-semiconductor electrical characteristics were shown in this study to be strongly affected by the surface preparation steps prior to metallization (polishing, and chemical etching), the choice of the metal and contact deposition technique, and by the subsequent surface passivation of CdZnTe. In this paper, the authors also present the implementation new detector fabrication processing steps resulting in a significant lowering of the dark leakage current and an improvement in the detector performance.