Cl-doped CdTe Research Articles

Cl-Doped CdTe Crystal Growth for Medical Imaging Applications

CdTe:Cl doped single crystals were grown under conditions of tellurium excess by using an accelerated crucible rotation technique, modified vertical Bridgman (ACRT-MVB) method. Chlorine dopant levels were kept at 4.4 × 1019 at·cm−3, for all growths, while the Te excess level varied from 3.5 to 15% by weight. The relationship between the detector performance, Te inclusions, and resistivity was investigated in detail. Tellurium excess caused additional nucleation which decreased the average single crystal grain size. At the same time, the increasing Te excess level improved the electrical transport properties. In the three Cl-doped, and one In-doped CdTe crystals, detectors from Cl-doped CdTe grown under 15% Te excess showed better response to gammas and alphas, and high µτ for electrons (1.8 × 10−3 cm2/V), as well as for holes (5.1 × 10−4 cm2/V). The full-width half maximum for the Cl-doped CdTe were very large, as the peaks were broadened, especially at high bias. This could be due to hole trapping in a Cl-related A-center (VCd-ClTe)−, and in Cd- vacancies (VCd)−, and electron trapping in Te-antisites (TeCd)+.

Direct Thermal Growth of Large Scale Cl-doped CdTe Film for Low Voltage High Resolution X-ray Image Sensor

Polycrystalline cadmium telluride (CdTe) X-ray photodetector with advanced performance was fabricated in a Schottky diode form by direct thermal deposition (evaporation) on pixelized complementary metal oxide semiconductor (CMOS) readout panel. Our CdTe X-ray detector shows such a variety of benefits as relatively low process temperature, low cost, low operation voltage less than 40 V, and higher sensitivity and spatial resolution than those of commercial a-Se detectors. CdTe has cubic Zinc Blende structure and maintains p-type conduction after growth in general. For low voltage operation, we succeeded in Cl doping at all stage of CdTe film deposition, and as a result, hole concentration of p-type CdTe was reduced to ~1012 cm−3 from ~1015 cm−3, and such concentration reduction could enable our Schottky diode with Ti electrode to operate at a reverse bias of less than 40 V. Our CdTe Schottky diode/CMOS pixel array as a direct conversion type imager demonstrates much higher resolution X-ray imaging in 7 × 9 cm2 large scale than that of CsI/CMOS array, an indirect conversion imager. To our limited knowledge, our results on polycrystalline CdTe Schottky diode/CMOS array would be very novel as a first demonstration of active pixel sensor system equipped with directly deposited large scale X-ray detector.

Microstructural, optical and electrical properties of Cl-doped CdTe single crystals

AbstractMicrostructural, optical and electrical properties of Cl-doped CdTe crystals grown by the low pressure Bridgman (LPB) method were investigated for four different doping concentrations (unintentionally doped, 4.97 × 1019 cm−3, 9.94 × 1019 cm−3 and 1.99 × 1020 cm−3) and three different locations within the ingots (namely, samples from top, middle and bottom positions in the order of the distance from the tip of the ingot). It was shown that Cl dopant suppressed the unwanted secondary (5 1 1) crystalline orientation. Also, the average size and surface coverage of Te inclusions decreased with an increase in Cl doping concentration. Spectroscopic ellipsometry measurements showed that the optical quality of the Cl-doped CdTe single crystals was enhanced. The resistivity of the CdTe sample doped with Cl at the 1.99 × 1020 cm−3 was above 1010 Ω.cm.

Positron annihilation spectroscopy of vacancy-related defects in CdTe:Cl and CdZnTe:Ge at different stoichiometry deviations.

Positron annihilation spectroscopy (PAS) was used to examine the effect of defined Cd-rich and Te-rich annealing on point defects in Cl-doped CdTe and Ge-doped CdZnTe semi-insulating single crystals. The as-grown crystals contain open-volume defects connected with Cd vacancies . It was found that the Cd vacancies agglomerate into clusters coupled with Cl in CdTe:Cl, and in CdZnTe:Ge they are coupled with Ge donors. While annealing in Cd pressure reduces of the density, subsequent annealing in Te pressure restores . The CdTe:Cl contains negatively-charged shallow traps interpreted as Rydberg states of A-centres and representing the major positron trapping sites at low temperature. Positrons confined in the shallow traps exhibit lifetime, which is shorter than the CdTe bulk lifetime. Interpretation of the PAS data was successfully combined with electrical resistivity, Hall effect measurements and chemical analysis, and allowed us to determine the principal point defect densities.

Cl-doping of Te-rich CdTe: Complex formation, self-compensation and self-purification from first principles

The coexistence in Te-rich CdTe of substitutional Cl-dopants, ClTe, which act as donors, and Cd vacancies, VCd−1, which act as electron traps, was studied from first principles utilising the HSE06 hybrid functional. We find ClTe to preferably bind to VCd−1 and to form an acceptor complex, (ClTe–VCd)−1. The complex has a (0,-1) charge transfer level close to the valence band and shows no trap state (deep level) in the band gap. During the complex formation, the defect state of VCd−1 is annihilated and leaves the Cl-doped CdTe bandgap without any trap states (self-purification). We calculate Cl-doped CdTe to be semi-insulating with a Fermi energy close to midgap. We calculate the formation energy of the complex to be sufficiently low to allow for spontanous defect formation upon Cl-doping (self-compensation). In addition, we quantitatively analyse the geometries, DOS, binding energies and formation energies of the (ClTe–VCd) complexes.

Grain-boundary-enhanced carrier collection in CdTe solar cells.

When CdTe solar cells are doped with Cl, the grain boundaries no longer act as recombination centers but actively contribute to carrier collection efficiency. The physical origin of this remarkable effect has been determined through a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles theory. Cl substitutes for a large proportion of the Te atoms within a few unit cells of the grain boundaries. Density functional calculations reveal the mechanism, and further indicate the grain boundaries are inverted to n type, establishing local p-n junctions which assist electron-hole pair separation. The mechanism is electrostatic, and hence independent of the geometry of the boundary, thereby explaining the universally high collection efficiency of Cl-doped CdTe solar cells.

High Energy Resolution CdTe Schottky Diode <formula formulatype="inline"><tex Notation="TeX">$\gamma$</tex></formula>-Ray Detectors

Schottky diode X-/ γ-ray detectors based on semi-insulating Cl-doped CdTe crystals have been developed and investigated. Both the Schottky and Ohmic contacts were formed by vacuum deposition of Ni electrodes on the opposite faces of (111) oriented CdTe crystals pretreated by Ar ion bombardment with different conditions. Record-low leakage current in the fabricated Ni/CdTe/Ni structure at high voltages (~5 nA at 300 K for the area of 10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at bias voltage of 1500 V) was achieved. The charge transport mechanisms in the detectors are interpreted on the basis of known theoretical models. The developed detectors have shown the record-high energy resolution in the measured spectra of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs (0.42% FWHM). From a comparison of the spectra taken with the detector irradiated from the Schottky contact side and from the opposite side with an Ohmic contact, the concentration of uncompensated impurities (defects) in the CdTe crystals has been determined. The obtained value has been found to be close to the optimal one determined from the calculation results.

Development of 4-Sides Buttable CdTe-ASIC Hybrid Module for X-ray Flat Panel Detector

A 4-sides buttable CdTe-ASIC hybrid module suitable for use in an X-ray flat panel detector (FPD) has been developed by applying through silicon via (TSV) technology to the readout ASIC. The ASIC has 128 times 256 channels of charge integration type readout circuitry and an area of 12.9 mm times 25.7 mm. The CdTe sensor of 1 mm thickness, having the same area and pixel of 100 mum pitch, was fabricated from the Cl-doped CdTe single crystal grown by traveling heater method (THM). Then the CdTe pixel sensor was hybridized with the ASIC using the bump-bonding technology. The basic performance of this 4-sides buttable module was evaluated by taking X-ray images, and it was compared with that of a commercially available indirect type CsI(Tl) FPD. A prototype CdTe FPD was made by assembling 9 pieces of the 4-sides buttable modules into 3 times 3 arrays in which the neighboring modules were mounted on the interface board. The FPD covers an active area of 77 mm times 39 mm. The results showed the great potential of this 4-sides buttable module for the new real time X-ray FPD with high spatial resolution.

Effect of chlorine impurities on the long-wavelength absorption edge of CdTe single crystals

The optical absorption spectra of the nominally undoped and Cl-doped CdTe single crystals in the region of the fundamental long-wavelength absorption edge are studied at room temperature. The crystals are grown by the modified physical vapor transport method. It is shown that the exponential dependence of the absorption coefficient of the undoped and lightly doped samples can be described by the Urbach rule. In the heavily doped crystals, the absorption edge is due to optical transitions involving density-of-state tails formed by fluctuations of the dopant concentration. The typical energy range of fluctuations of the extrinsic potential profile and the concentrations of charged centers are calculated for the heavily doped material.

Correlated Evolution of the Structural and the Optical Properties During the Growth of Cl-Doped Polycrystalline CdTe

A study of the structural and the optical properties has been carried out before, upon, and after the coalescence of Cl-doped CdTe islands. Coalescence is found to induce high tensile stresses with the formation of grain boundaries. These stresses are eventually relaxed through the generation of compressive stresses related to the segregation of Cl atoms in the vicinity of grain boundaries. These segregation phenomena also influence the compensation mechanisms: a preferential formation of β acceptor complexes instead of A centers is directly observed by cathodoluminescence measurements. Furthermore, the coalescence and the post-coalescence stresses contribute to an orientation-dependent driving force for the grain growth, which leads to a progressive texturation of the material.

Ion-plasma protective coatings of CdTe surface for X- and gamma-ray detectors

Comparative study of I–V characteristics of “metal-semiconductor-metal” structures without coatings and with protective coatings is presented. Regimes for radio frequency deposition of dielectric coatings on the surface of pixeled Schottky junction based on Cl-doped CdTe single crystals with high resistivity (10 7–10 10) Ω·cm were determined. In particular, the films with composition of (TeO 2) x –(SiO 2) 1 − x have demonstrated high isolation properties. Possible applications of new CdTe pixeled detectors with the studied protective coatings for high resolution spectrometry in X- and gamma-ray sensor devices are discussed.

The effect of chlorine doping concentration on the quality of CdTe single crystals grown by the modified physical vapor transport method

Undoped and Cl-doped CdTe crystals were grown by the modified PVT technique in a vertical configuration. Their structural quality as well as electrical and transport characteristics as a function of a doping concentration was analyzed by means of infrared (IR) spectroscopy, optical microscopy, secondary ion mass spectrometry (SIMS), photoelectrical and Hall effect measurements. In the investigated semi-insulating crystals with resistivity ρ>10 8 Ω×cm doped in the chlorine concentration range N Cl=10 18−10 19 cm −3 the best transport and structural characteristics were observed. The formation of precipitates and inclusions at higher values of N Cl results in worsening of crystalline quality as well as a decrease of resistivity in the doped samples. The IR transmission spectra in heavily doped samples are determined by the light scattering on these structural imperfections, whereas optical transitions between the heavy hole and light hole valence bands dominate spectra in undoped ones. The absorption peak caused by these transitions seems to be observed for the first time in undoped CdTe.

ＣｄＴｅ単結晶表面に対するプラズマエッチングとＣｄ雰囲気中熱処理の効果

The effect of H2 plasma irradiation on a Cl-doped CdTe(111)B crystal was studied by comparing with that of the same irradiation but combined with a post-irradiation thermal annealing in a Cd ambient. Photoluminescence (PL) property of the CdTe crystal was found to be degraded by a two-months storage in a low-pressure desiccator. Electron spectroscopy for chemical analysis revealed that native oxide such as TeO2 was formed on the CdTe surface after the storage. Irradiation by H2 plasma was effective to remove the oxide completely. However, the PL property after the H2 plasma irradiation did not recover the original one due to the formation of Cd vacancy by ion bombardment. The post-irradiation annealing was found to be effective to recover the damage by ion bombardment.

Modeling of Point Defects in Cl-Doped CdTe Crystals Annealed in Cd Vapor

Equations are proposed for quasi-chemical reactions leading to the formation of intrinsic and impurity defects and defect complexes in Cl-doped cadmium telluride crystals during annealing in cadmium vapor at different temperatures. The equilibrium constants for the formation of (Cl Te + V Cd 2− )− defect complexes and Te Cd 2+ antisite defects are calculated. An analytical expression is derived for the cadmium partial pressure corresponding to a type conversion. The conditions are established for producing n- and p-type CdTe〈 Cl〉 crystals with controlled carrier concentration.

Gamma-radiation effect on donor and acceptor states in CdTe and CdTe:Cl

The evolution of photoluminescence (PL) spectra with increasing γ-irradiation dose was monitored for undoped and Cl-doped CdTe crystals. The emission intensities and the Huang-Rhys factors for different PL lines were determined as a function of the exposure dose. In irradiated CdTe:Cl, an irradiation-induced enhancement of the electron-phonon coupling for the A-center PL was attributed to decreasing concentration of the donor–acceptor pairs responsible for this band. Gamma-irradiation of undoped CdTe at a dose larger than 10kGy resulted in quenching of the A-center PL at 1.4eV, the donor–acceptor line in the edge luminescence at 1.55eV, as well as emission of the donor-bound excitons. Observed peculiarities are explained by a decrease of concentration of isolated donors, presumably through the creation of complexes with intrinsic defects generated by γ-rays.

Annealing effects on defect levels of CdTe:Cl materials and the uniformity of the electrical properties

CdTe crystals grown by the traveling heater method (THM) often show a pronounced nonuniformity along the ingots due to the thermal irregularities, the Te-excess growth conditions resulting from the retrograde slope of the solidus line of the phase diagram, and from the introduced impurities. In addition, structural defects can be present that influence the electrical and optical properties. The aim of this work is to study the annealing effects of Cl-doped CdTe on the uniformity of the material, the defects, the resistivity, the /spl mu//spl tau/ product, and on the detection properties. Samples have been annealed under various pressures (vacuum, argon, CdCl/sub 2/) and include different temperature stages between 250/spl deg/C and 850/spl deg/C. An increase of the resistivity was observed after a thermal treatment of these samples under argon pressure. In this case, the highest resistivity was obtained by annealing samples at 460/spl deg/C. The presence of CdCl/sub 2/ during the annealing leads to a better uniformity of the materials. The defects present in the materials have been investigated by photo-induced transient current spectroscopy (PICTS), thermo-electric emission spectroscopy (TEES), and thermally stimulated current (TSC) methods, which allow the calculation of their activation energy, their cross section, and their concentration.

Study of the resistivity mapping in CdTe:Cl-correlation with annealing and Te-precipitates

The presence of defects in Cl-doped CdTe crystals influences the electrical properties and the homogeneity of the material. The aim of this paper is to study the effects of thermal treatment on the precipitates present in the volume of the material and thus on the uniformity of the material in regards to both resistivity and precipitate concentrations. The samples are annealed under argon pressure. The number of precipitates, generally of Te, is measured using infrared microscopy. In addition, a low-cost contactless measurement technique for resistivity mapping has been developed. This technique presents a good spatial resolution of approximately 1 mm and a resistivity range measurement of 10/sup 6/-10/sup 10/ /spl Omega/-cm. The relation between the number of Te-precipitates and the resistivity maps is investigated. A decrease in the number of Te-precipitates after sample annealing is observed. Furthermore, the resistivity decreases with an increase in the number of precipitates in the material. The uniformity of the annealed samples is discussed. The correlation of the resistivity maps and the Te-precipitates is also given, illustrated by comparison topography with resistivity and Te-precipitate concentrations. The contactless resistivity measurement technique is also described.

Photoluminescence spectra of Cl-doped CdTe crystals

A detailed analysis of photoluminescence (PL) spectra has been done to identify the origins of PL emissions in solution-grown Cl-doped CdTe crystals. In the exciton emission region, a sharp peak at 1.590 eV and a broad peak at 1.586 eV are observed in the Cl-doped CdTe crystals and their intensities are enhanced with increasing amount of Cl. On the basis of the temperature-dependence measurements, the two features are attributed to the recombination of excitons bound to two different acceptors. From the variations in the PL spectra of the Cl-doped CdTe crystals before and after Cd annealing, it is demonstrated that the feature at 1.590 eV is associated to exciton bound to complex acceptor formed with a Cd vacancy and two Cl donors (V Cd-2Cl Te), and the feature at 1.586 eV is associated to exciton bound to another Cl-related complex acceptor of (V Cd-Cl Te). The ionization energies for (V Cd-2Cl Te) and (V Cd-Cl Te) complex acceptors are found to be about 45 and 120 meV, respectively.

Temperature-gradient-solution grown CdTe crystals for γ-ray detectors

Pure and Cl-doped CdTe crystals were grown from Te-rich melt, by the TGSG method. The crystalline characteristics of the CdTe crystals grown with various axial temperature gradients have been investigated. An optimum value of 23°C/cm with a translation rate of 18 mm/d produced good quality CdTe crystals with low etch pit density (∼104 cm −2) and high infrared transmittance (>60%). The compensation effects of Cl doping were analyzed by means of van der Pauw measurements, the transient charge technique, infrared spectroscopy and photoluminescence analysis. The room temperature resistivity of the undoped crystals was on the order of 10 3 Ω cm, and >10 9 Ω cm at Cl-doping concentrations ranging from 500 to 5000 ppm. The best values of the mobility-lifetime product 8.3×10 −4 cm 2/V for electrons and 7.5×10 −5 cm 2/V for holes were obtained at Cl concentration levels of 500 ppm. The γ-ray detectors with a size of 5×5×1 mm 3 fabricated from the high-resistivity Cl-doped CdTe crystals showed energy resolutions (FWHM) of 7.0 keV for 122 keV γ-ray from 57Co and 12.9 keV for 356 keV γ-ray from 133Ba; no polarization effect was observed.

CdTe:Cl and CdTeSe:Cl Single Crystals Application for Radiation Detectors

ABSTRACTWe report on electrical and optical properties of vertical Bridgman grown Cl-doped CdTe including the ternary compositions Cd0.9Zn0.1Te and CdTe0 9Se0.1 with respect to application as a radiation detector. Based on Hall effect measurements, photoinduced current spectroscopy (PICTS) and photoluminescence we infer that high resistive material with good performance is controlled by deep level defects. The resistivity is calculated as a function of the shallow acceptor concentration (Cl-A-centers) with the conclusion that a deep donor state at mid gap must be present.