Article
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Group II-VI narrow band gap compounds CdTe, ZnCdTe and CdSeTe are known as the most suitable semiconductor materials for the room temperature gamma- and X-ray detectors. In this work, electronic properties of a quadrupole compound ZnCdSeTe were investigated. Chlorine, copper and oxygen doped host material was synthesized from the grinded mixture of 2 mol% ZnTe, 36 mol% CdTe, and 62 mol% CdSe, to keep a hexagonal structure of crystals. Precautions were applied to achieve an uniform doping, high quality of the crystal surfaces, and to remove residue phases after the thermal treatments. Fabricated polycrystalline samples showed a high performance from NIR via VIS and UV to X-ray band, with sharp donor-acceptor pair peak at 922 nm, and dynamic range above 104. High stability, good linearity and performance of samples was measured using X-ray source with Cu-anode, at 40 kV.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

ResearchGate has not been able to resolve any citations for this publication.
Article
Full-text available
Direct growth of high-quality, thick CdTe (211) epilayers, with thickness up to 100 µm, on Si (211) substrates in a vertical metalorganic vapor phase epitaxy system is reported. In order to obtain homo-orientation growth on Si substrates, pretreatment of the substrates was carried out in a separate chamber by annealing them together with pieces of GaAs at 800–900°C in a hydrogen environment. Grown epilayers had very good substrate adhesion. The full-width at half-maximum (FWHM) value of the x-ray double-crystal rocking curve from the CdTe (422) reflection decreased rapidly with increasing layer thickness and remained between 140–200 arcsec for layers >18 µm. Photoluminescence measurement at 4.2 K showed high-intensity, bound excitonic emission and very small defect-related deep emissions, indicating the high crystalline quality of the grown layers. Furthermore, a CdTe/n+-Si heterojunction diode was fabricated that exhibited clear rectifying behavior.
Article
Full-text available
ZnTe:O powder phosphors were successfully prepared by a dry synthesis process using gaseous doping and etching media. It was found that dry doping by O2 through ball-milling was an effective way to synthesize ZnTe:O powder phosphors and produced a red emission centered at 680 nm with a decay time of 1.1 µs. The emission intensity of dry O2-doped samples was three times more intense than from ZnO-doped samples, possibly due to a more uniform distribution of oxygen substitution on tellurium sites. The samples annealed in a 95% N2/5% H2 forming gas atmosphere exhibited a x-ray luminescent efficiency five times higher than did powders annealed in vacuum or N2 atmosphere. This enhancement was attributed to the removal of surface tellurium oxides. ZnTe:O phosphor screens were prepared with x-ray luminescence efficiencies equivalent to 56% of ZnSe:Cu,Ce,Cl and 76% of Gd2O2S:Tb screens under 17-keV radiation. An x-ray imaging resolution of 2.5 lines/mm was resolved, the same as that measured for commercial ZnSe:Cu,Ce,Cl and Gd2O2S:Tb screens. These results indicate that ZnTe:O is a promising phosphor candidate for synchrotron x-ray imaging applications.
Article
CdTe is one of the most encouraging semiconductor materials in the field of room temperature γ- and X-ray spectroscopy. To improve the detector properties, the ternary systems (Cd,Zn)Te and Cd(Te,Se), and CdTe were grown by vertical Bridgman technique. To achieve low noise detectors, the resistivity of all materials was increased by chlorine doping. The crystals were characterized by electrical (Hall measurements, photoinduced current transient spectroscopy) methods. The numbers of deep levels influencing the resistivity were reduced by introducing Se into the CdTe system. A common deep level close to the middle of the bandgap has been identified, responsible for the compensation effect in all three systems. In addition high resistivity and n-type conductivity were achieved in CdTeSe materials for the first time. Detectors were tested by irradiation with α particles and low γ-rays. Best charge collection efficiency (CCE) of 91% was recorded for CdTe 0.9Se 0.1 for both radiation types and smallest FWHM of about 10% at 60 keV.
Article
High-quality, large (10 cm long and 2.5 cm diameter), nuclear spectrometer grade Cd{sub 0.9}Zn{sub 0.1}Te (CZT) single crystals have been grown by a controlled vertical Bridgman technique using in-house zone refined precursor materials (Cd, Zn, and Te). A state-of-the-art computer model, multizone adaptive scheme for transport and phase-change processes (MASTRAP), is used to model heat and mass transfer in the Bridgman growth system and to predict the stress distribution in the as-grown CZT crystal and optimize the thermal profile. The model accounts for heat transfer in the multiphase system, convection in the melt, and interface dynamics. The grown semi-insulating (SI) CZT crystals have demonstrated promising results for high-resolution room-temperature radiation detectors due to their high dark resistivity ({rho} {approx} 2.8 x 10{sup 11} {Theta} cm), good charge-transport properties, electron and hole mobility-life-time product, {mu}{tau}{sub e} {approx} (2-5) x 10{sup -3} and {mu}{tau}{sub h} {approx} (3-5) x 10{sup -5} respectively, and low cost of production. Spectroscopic ellipsometry and optical transmission measurements were carried out on the grown CZT crystals using two-modulator generalized ellipsometry (2-MGE). The refractive index n and extinction coefficient k were determined by mathematically eliminating the {approx}3-nm surface roughness layer. Nuclear detection measurements on the single-element CZT detectors with {sup 241}Am and {sup 137}Cs clearly detected 59.6 and 662 keV energies with energy resolution (FWHM) of 2.4 keV (4.0%) and 9.2 keV (1.4%), respectively.
Article
We report on the design, fabrication, and performance of CdZnTe gamma-ray detectors with a new P-I-N structure for spectroscopic applications. Highpressure and conventional vertical-Bridgman CdZnTe crystals were used for detector fabrication. P and n layers were deposited by thermal evaporation, and by optimizing the deposition conditions we achieved low leakage current (approximately 15 nA at 1000 V) and good performance. Spectral response data at high bias voltages showed improved energy resolution and peak-to-valley ratios for 241Am and 57Co compared to metal-semiconductor-metal detectors.
Article
The influence of oxygen codopant upon the optoelectronic properties of chlorine-doped CdTe films is being investigated. It is shown that a small quantity of oxygen decreases the resistivity of films, whereas at higher concentrations oxygen codopant increases the resistivity of films up to 6 orders of magnitude. A subsequent annealing in tellurium vapor pressure decreases the resistivity of films. It is supposed that an anomalous resistivity drop around 0.22kPa is caused by shallow acceptor complexes that oxygen forms with group I impurities like copper and silver. At higher concentrations oxygen forms isoelectronic complexes with cadmium vacancies, which cause a high resistivity of films. Te annealing extracts oxygen from the films as Te forms with dissolved oxygen tellurium oxide TeO2 which easily sublimates. Photoconductivity of the oxygen and chlorine-doped CdTe films is poor, or is not detected.
Article
The optoelectronic properties of oxygen, copper and chlorine doped CdTe thin films are being investigated. It is found that Cu co-dopant increases the p-type resistivity of CdTe:Cl films up to some orders of magnitude whereas, oxygen co-dopant increases the resitivity of CdTe:Cl:Cu films up to some orders of magnitude, and increases the quantity of liquid phase at thermal treatments. It is thought that Cu complexes, with oxygen and the isoelectronic complex defects VCl2ClTe, can limit the efficiency of CdS/CdTe solar cells.
Article
The defect structure and conductivity type in CdTe layers formed with CdCl2 flux are investigated. It is stated that chlorine outdiffusion combined with the slow cooling thermal treatment under tellurium pressure enables one to get high stable p-conductivity in CdTe layers recrystallized with CdCl2. p-type resistivity under 100 Ω · cm and a dark-to-light resistance ratio under 1.02 have been achieved. It is supposed that high p-type conductivity is originated from a shallow acceptor complex (Te2−iCl+Te)− or/and (V2−CdCl+Te)−. Interstitial tellurium is proposed as the 1.4 eV photoluminescence band killer in CdTe.
Article
The photoluminescence spectra and formation of the complex defects in the Cl doped CdTe p-type layers are investigated. It is supposed that at low Cl concentration complex defects TeiClTe and VCdClTe are responsible for the low p-type resistivity in CdTe. At higher Cl concentration formed neutral complexes Tei2ClTe and VCd2ClTe increased the p-type resistivity. At high Cl concentration excess ClTe caused a high resistivity and photoconductivity of CdTe. It is proposed that Tei and complex defects with Tei caused the 1.4eV PL band quenching and the 1.0eV PL band formation. The unstable 1.25eV PL band is supposed to be due to VCd-VTe pair. Acceptor impurities in initial material as Cu and its complexes can increase the p-type resistivity value in CdTe: Cl.
Article
We have studied the defect levels in as grown and post growth processed cadmium telluride (CdTe) using thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) techniques. We have extracted the thermal energy (Eth) and trapping cross section (σth) for the defect levels using the initial rise and variable heating rate methods. We have identified 10 different defect levels in the crystals. Thermal ionization energy values obtained experimentally were compared to theoretical values of the transition-energy levels of intrinsic and extrinsic defects and defect complexes in CdTe determined by first-principles band-structure calculations. On the basis of this comparison, we have associated the observed ionization levels with various native defects and impurity complexes.
Article
Chemical polishing is a process of crucial importance in the manufacture of epiready substrates for molecular beam epitaxy (MBE) of high-quality HgCdTe layers. With the aim of fabrication of (211) CdZnTe substrates, we focused on the fundamental research of polishing processes with respect to reducing subsurface damage. Wafers of the orientation (211) were prepared from the as-grown crystals by a process flow including oriented slicing, several steps of mechanical polishing, and finally chemical polishing. In the prechemical polishing process, several free and bound abrasives were applied to reach the surface roughness close to 1 nm. The surface polishing treatment included testing of the surface quality after each polishing step. We used an interferometer profiler, which yields detailed surface maps. Within chemical polishing processing, we have looked for an optimum composition of etchant based on the bromine-methanol/ethylene glycol solution and adequate polishing pad. We studied the substrate surface quality dependence on the rotation speed of the plate, sample loading weights, and duration of polishing. Correlation between the final surface roughness and layer thickness removed was established. The chemical polishing with a very low concentration of Br-methanol/ethylene glycol solution was found to yield very good CdZnTe surfaces with a perfect flatness.
Article
A CdZnTe detector grown by the high pressure Bridgman (HPB) growth technique was tested using high energy x-rays (30∼100 keV), and the performance was compared with a commercially available Nal scintillating detector of 5 cm thickness. The charge collection efficiency of a CdZnTe detector is as high as 90% at relatively low electric field, 600 V/cm. At high x-ray photon energies, the detection efficiency is reduced due to the thickness of the CdZnTe. A 32 channel linear array was fabricated on 1.2∼1.7 mm thick CdZnTe, of which the detector area was 175 × 800 µm2 and the pitch size 250 µm. The measured dark current for the 16 element detector was as low as 0.1 pA at 800 V/cm with an excellent uniformity. Energy spectra were measured using a Co57 radiation source. A small pixel effect and charge sharing were observed. The energy resolution was improved and compared with the large area detector. The array detector gave an average 5.8% full-width-half-maximum (FWHM) at 122 keV photopeak. The large area detector of the same material before fabrication exhibited a low energy tail at the photopeak, which limits the photopeak FWHM to 8%.
Article
The timing characteristics of a planar Cd1-xZn<sub>x </sub>Te sample at each frequency of a scanning square-wave test pattern, has been measured. This study is aimed at evaluating the speed characteristics of a Cd1-xZnxTe detector for X-ray imaging and computed tomographic (CT) applications. The experimental results of this study indicate that the temporal response of a Cd1-xZnxTe detector based X-ray system, improves significantly by optimizing the X-ray tube and detector parameters