CdZnTe Crystals Research Articles

Performance updating of CdZnTe strip-drift detectors

Efficiency of strip-drift structure of contacts for improvement of charge collection conditions in CdZnTe detectors was shown some years ago. For such detector with area 10×10 mm 2 and thickness 3 mm of crystal, produced by eV products we obtained energy resolution 1.9 and 12.0 keV on energies 59.6 and 662 keV correspondingly. Recently, significant progress was done in CdZnTe crystals growth technology. In the present paper we present preliminary result of performance updating of CdZnTe strip-drift detectors based on crystal of 10×10×6 mm 3 produced by Yinnel Tech company. Results of crystal input characterization, the value of leakage current and inter-strip resistivity for all strips and energy resolution for collecting strips of detector are presented. The energy resolutions 1.8 and 11.0 keV on energies 59.6 and 662 keV correspondingly were obtained although the thickness of detector was two times more than earlier.

X-ray Topography to Characterize Surface Damage on CdZnTe Crystals

AbstractSynthetic CdZnTe or “CZT” crystals can be used for room temperature detection of α- and γ-radiation. Structural/morphological heterogeneities within CZT, such as twinning, secondary phases (often referred to as inclusions or precipitates), and polycrystallinity can affect detector performance. As part of a broader study using synchrotron radiation techniques to correlate detector performance to microstructure, x-ray topography (XRT) has been used to characterize CZT crystals. We have found that CZT crystals almost always have a variety of residual surface damage, which interferes with our ability to observe the underlying microstructure −for purposes of crystal quality evaluation. Specific structures are identifiable as resulting from fabrication processes and from handling and shipping of sample crystals. Etching was found to remove this damage; however, our studies have shown that the radiation detector performance of the etched surfaces was inferior to the as-polished surface due to higher surface currents which result in more peak tailing and less energy resolution. We have not fully investigated the effects of the various types of inducible damage on radiation detector performance.

Crystal Growth and Characterization of CdTe and Cd0.9Zn0.1Te for Nuclear Radiation Detectors

AbstractCdTe and Cd0.9Zn0.1Te (CZT) crystals have been studied extensively at EIC Laboratories, Inc. for various applications including x- and γ-ray imaging and high energy radiation detectors. The crystals were grown from in-house zone refined ultra pure precursor materials using a vertical Bridgman furnace. The growth process has been monitored, controlled and optimized by a computer simulation and modeling program (MASTRAPP). The grown crystals were thoroughly characterized after sequential surface passivations and post-growth annealing treatments with and without component overpressures. The infrared (IR) transmission images of the post-treated CdTe and CZT crystals showed average Te inclusion size of ∼10 μm for CdTe crystal and ∼8 μm for CZT crystal. The etch pit density was ≤ 5×104 cm−2 for CdTe and ≤ 3×104 cm−2 for CZT. Various planar and Frisch collar detectors were fabricated and evaluated. From the current-voltage measurements, the electrical resistivity was estimated to be ∼1.5×1010 Ω·cm for CdTe and 2-5×1011 Ω·cm for CZT. The Hecht analysis of electron and hole mobility-lifetime products (μτe and μτh) showed μτe=2×10−3 cm2/V (μτh=8×10−5 cm2/V) and μτ3-6×10−3 cm2/V (μτh=4-6×10−5 cm2/V) for CdTe and CZT, respectively. Final assessments of the detector performance have been carried out using 241Am (60 keV) and 137Cs (662 keV) energy sources and the results are presented in this paper.

Optical and electrical properties of indium-doped Cd 0.9Zn 0.1Te crystal

The indium-doped CdZnTe crystal was characterized by PL spectra at 10 K and IR transmission spectra, as well as its current–voltage behavior at room temperature. The results showed that indium atoms substituted for the Cd vacancy and produced ionized donors [In Cd] +, i.e. indium doping element recombined with [V cd] 2− and formed the singly negative defect complex A-center [ In Cd + V Cd 2 - ] - and the neutral ones [ 2 In Cd + V Cd 2 - ] 0 and [ In cd + ( In Cd + V Cd 2 - ) - ] 0 . Indium doping also increased the IR absorption of the CdZnTe crystal due to the lattice absorption and free-carrier absorption. Meanwhile, indium doping increased the electrical resistivity and decreased the leakage current of CdZnTe crystals remarkably.

Thermal treatment of detector-grade CdZnTe

To improve the quality of CdZnTe (CZT) crystal, a thermal treatment was conducted by using Cd 1− y Zn y alloy as an annealing source. Guided by our calculations of the correlations of the partial pressures between CZT crystal and Cd 1− y Zn y alloy, optimized conditions for the gas–solid equilibrium annealing of CZT were experimentally studied. Characterizations revealed that the concentration distribution was uniform, and the defect density was decreased by more than one order of magnitude. The structural and electrical properties of the CZT crystal were evidently improved by this thermal annealing.

Performance Comparison of a Large Volume CZT Semiconductor Detector and a LaBr$_3$(Ce) Scintillator Detector

The development of portable nuclear instrumentation demands compact high sensitivity detectors operated at room temperature. The sensitivity of these detectors mainly depends on two parameters: absolute efficiency and energy resolution. In order to provide high efficiency, large volumes are needed. For semiconductor detectors able to operate at room temperature, the largest effective volumes with acceptable resolution are achieved with CdZnTe coplanar-grid and pixel detectors. On the other hand, new scintillation materials were recently developed with spectrometric capabilities only reachable some years ago with semiconductors. In this work we compare the performance of two state of the art detectors of different technologies with a relative large volume: a coplanar-grid CdZnTe detector with dimensions 15 mmtimes15 mmtimes10 mm and a LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Ce) crystal with volume 18 mmtimes18 mmtimes30 mm. The CdZnTe crystal was made by Yinnel Tech (USA) and the detector was manufactured by BSI (Latvia). The LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Ce) scintillator was grown and encapsulated by Saint-Gobain (France). The energy resolution for the CZT detector is 2.05% FWHM at 662 keV. The resolution for the scintillator at this energy was near 3%. The total efficiency was studied in a setup with calibrated point sources. The experimental spectra were compared with Monte-Carlo simulations performed with Geant4. The implications of the results of this comparison are discussed in the context of the practical use of these units

Effect of surface treatments on the electrical and optical properties of CdZnTe single crystal

The effects of surface treatments on the electrical and optical properties of CdZnTe (CZT) crystals were studied experimentally. The IR transmittance of CZT after cutting is only 15%, after chemically polishing with Br-MeOH it increased to 63%. The reflection of surface roughness and absorption of surface-damaged layer should be responsible for the variation. After aging and passivation,CZT possesses low IR transmittance. After fine polishing, the surface oxide layer was removed and the IR transmittance increased. CZT wafers with high roughness possess low surface sheet resistance, and the Te enrichment introduced by etching with Br-MeOH results in high surface leakage current. The effects of etching and passivation on surface trap state have been analyzed with photoluminescence spectra. Etching can deprive effectively the surface damage layer, and passivation can minimize the surface trap state density.

Vapour growth of Cd(Zn)Te columnar nanopixels into porous alumina

The vapour phase growth (VPG) of CdTe and Cd 1− x Zn x Te was performed in order to investigate the formation of Cd(Zn)Te columnar nanostructures, which could serve as a basis for micropixels usable for further development of X- and gamma-ray high-resolution imaging devices. The possibility to form the “Cd(Zn)Te-in-porous alumina” nanostructures by VPG has been demonstrated. The Cd(Zn)Te crystals integrated into nanoporous alumina have shown to have photoluminescence properties compatible with those of the bulk crystals and planar epitaxial layers. Further investigations are going on to improve the structural quality of Cd(Zn)Te nanocrystals.

Photoluminescence analysis on the indium doped Cd0.9Zn0.1Te crystal

Photoluminescence spectra are used to characterize high resistivity of In-doped CdZnTe crystal. Shallow level donor-acceptor pair (DAPs) peak at 1.6014eV is found due to shallow donor and acceptor compensation related defects, which forms the [VCd–InCd]− singly negative complex and the neutral complex [VCd–2InCd]. Cd vacancy acceptors are compensated dominantly due to indium doped [InCd]+ donor and Te antisite [TeCd]4+ donor, which improves the resistivity of CdZnTe:In crystal. Energy level model diagram of CdZnTe:In crystal is discussed. Current-voltage measurement results confirm that Cd vacancy compensation by doped indium improves the resisitivity of CdZnTe crystal.

The multiscale spectral and spatial character of photoresponse in CdZnTe crystals

Experiments give evidence of the spectral and spatial instabilities of the photoresponse in CdZnTe crystals, which are caused by the transformation of fields created by ensembles of structural defects of various types and scales. Wavelet analysis of the photocurrent as a function of the light wavelength (I = f(λ)) and the spatial coordinate (I = f(x)) reveal a correlation between the spectral instability and the spatial arrangement of crystal regions where this instability arises.

Simulation, modeling, and crystal growth of Cd0.9Zn0.1Te for nuclear spectrometers

High-quality, large (10 cm long and 2.5 cm diameter), nuclear spectrometer grade Cd0.9Zn0.1Te (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 (ρ≈2.8 × 1011 Θ cm), good charge-transport properties [electron and hole mobility-life-time product, μτe≈(2–5)×10−3 and μτh≈(3–5)×10−5 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 ∼3-nm surface roughness layer. Nuclear detection measurements on the single-element CZT detectors with 241Am and 137Cs 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.

Study on the effect of Cd-diffusion annealing on the electrical properties of CdZnTe

In order to meet the requirements for the device design of radiation detectors, CdZnTe (or Cd 1− x Zn x Te) crystals grown by Vertical Bridgman Method often need subsequent annealing to increase their resistivity. The nature of this treatment is a diffusion process. Thus, it is meaningful to relate the change of resistivity to the diffusion parameters. A model correlating resistivity and conduction type of CdZnTe with the main diffusion parameter—diffusion coefficient—is put forward in this paper. Combining the model with the analysis of our experimental data, D Cd=1.464×10 −10, 1.085×10 −11 and 4.167×10 −13 cm 2/s are the values of Cd self-diffusion coefficient in Cd 0.9Zn 0.1Te at 1073, 973 and 873 K, respectively. The data coincide closely with the Cd self-diffusion coefficient in CdTe provided by different authors [E.D. Jones, N.M. Stewart, Self-diffusion of cadmium in cadmium telluride, J. Crystal Growth 84 (1987) 289–294; P.M. Borsenberger, D.A. Stevenson, J. Phys. Chem. Solids 29 (1968) 1277; R.C. Whelan, D. Shaw, in: D.G. Thomas (Ed.), II –VI Semiconductor Compounds, Benjamin, New York, 1967, p. 451]. With the data, the effects of annealing time on the change of resistivity and conduction type for Cd 0.9Zn 0.1Te wafers, which are annealed in saturated Cd vapor at 1073, 973 and 873 K, were simulated, and good consistency was found. This work suggests an alternative way to obtain the diffusion coefficient in semiconductor materials and also enables ones to analyze the diffusion process quantitatively and predict the annealing results.

TCT characterization of different semiconductor materials for particle detection

The development of digital semiconductor based X-ray detectors necessitates a detailed understanding of the applied sensor material. Under this premise a broad-band transient current technique (TCT) setup has been developed and used to characterize different semiconductors. The measurements are based on the generation of electrical charges within the sensor material and the subsequent time-resolved analysis of the charge carrier movement. From the recorded current pulses the charge collection efficiency, the charge carrier mobility and the electric field profile have been extracted. The examined materials are silicon p in n diodes, ohmic and Schottky contacted CdTe detectors, CdZnTe (CZT) crystals with Schottky contacts as well as two single-crystal CVD-diamonds.

Research of CdZnTe detector based portable energy dispersive spectrometer

A kind of excellent CdZnTe crystal has been grown in Yinnel Tech, Inc. in recent years. Based on these CdZnTe crystals and some new techniques, a portable energy-dispersive spectrometer has been constructed which has yielded good results. CdZnTe detector has a 3% relative resolution in high-energy field and can detect gamma rays at room temperature. An integrated circuit based on preamplifier and shaping amplifier chips is connected to the detector. Voltage pulses are transformed into digital signals in MCA (multichannel analyzer) and are then transmitted to computer via USB bus. Data process algorithms are improved in this spectrometer. Fast Fourier transform (FFT) and numerical differentiation (ND) are used in energy peak's searching program. Sampling-based correction technique is used in X-ray energy calibration. Modified Gaussian-Newton algorithm is a classical method to solve nonlinear curve fitting problems, and it is used to compute absolute intensity of each detected characteristic line.

The temporal and spatial instability of photoelectric response of the CdZnTe crystals

The spectral, temporal, and spatial instabilities of the photoelectric response I of CdZnTe crystals with a variety of structural inhomogeneities are revealed. It is shown that the character and interrelation of these instabilities are most pronounced in the diagrams dI(t)/dt = f(I(t)) and dI(v)/dv = f(I(v)), where v is the frequency and t is time in which the processes accompanying the stability loss are clearly seen.

Performance characteristics of Frisch-ring CdZnTe detectors

The performance characteristics of Frisch-ring CdZnTe (CZT) detectors are described and compared with other types of CZT devices. The Frisch-ring detector is a bar-shaped CZT crystal with a geometrical aspect ratio of /spl sim/1:2. The side surfaces of the detector are coated with an insulating layer followed by a metal layer deposited directly upon the insulator. The simple design operates as a single-carrier device. Despite the simplicity of this approach, its performance depends on many factors that are still not fully understood. We describe results of testing several detectors fabricated from CZT material produced by different vendors and compare the results with numerical simulations of these devices.

Metal–CdZnTe contact and its annealing behaviors

The electrical properties of different metal–CdZnTe contacts by sputtering deposition method are investigated by current–voltage. The results show that Au is the most suitable electrical contact materials, which forms the nearly ideal Ohmic contact with high resistivity p-CdZnTe crystals. Ohmicity coefficient b is the closest to 1 after 10 min annealing at 333 K, which is analyzed by current–voltage characteristics. XPS analyses show that Au atoms diffuse into CdZnTe during annealing process and Cd and Te atoms diffuse into Au contact. Diffused Au atoms do not form any compound with any element in CdZnTe crystal. PL spectra results of Au deposition on CdZnTe crystals at 10 K show that the inter-diffused donors [Au] 3+ recombine with acceptors [ V Cd] 2− during sputtering process. Meanwhile, the intensity of ( D complex) peak of with Au contact increases sharply in comparison with un-deposited CdZnTe crystal and donor [Au] 3+ and [ A u 3 + ⋅ V Cd 2 − ] + can compensate Cd vacancy [ V Cd] 2− wholly.

Upgrading of CdZnTe by annealing with pure Cd and Zn metals

A method for the annealing of a CdZnTe crystal is described in this paper. Pure Cd and Zn metals are used as annealing sources, which simultaneously provide exact Cd and Zn equilibrium partial pressures for CdZnTe at a certain temperature. Characterizations reveal that homogeneity is highly improved and the defect densities are decreased by more than one order, and hereby the structural, optical and electrical properties of the CdZnTe crystal are evidently improved by this annealing. Investigation of the temperature dependence of the CdZnTe quality after annealing shows 1073 K to be the preferable annealing temperature for CdZnTe. This annealing process has already been demonstrated to be superior to approximate equilibrium partial pressure annealing by using Cd1−yZny alloy as the annealing source.

Optical monitoring of partial vapor pressures in CdTe and CdZnTe systems: a new tool for material technology development

A technique for the determination of the partial pressures of Cd and Te2 vapors in equilibrium with cadmium zinc telluride (CdZnTe) crystals has been developed. The technique has shown a large range of applicability. First of all, the technique can be used for the in-situ monitoring of the partial vapor pressures in the case of technological relevant processes, such as material purification and crystal growth. Moreover, the technique is applied to the characterization of the composition of CdZnTe crystals with a sensitivity of at least two orders of magnitude more than previously reported techniques

Effect of hydrogenation on the electrical and optical properties of CdZnTe substrates and HgCdTe epitaxial layers

In this paper, we report the experimental observations on the effect of plasma hydrogenation in passivating intrinsic point defects, shallow/deep levels and extended defects in low-resistivity undoped CdZnTe crystals. The optical absorption studies show transmittance improvement in the below gap absorption spectrum. Using variable temperature Hall measurement technique, the shallow defect level on which the penetrating hydrogen makes complex, has been identified. In ‘compensated’ n-type HgCdTe epitaxial layers, hydrogenation can improve the resistivity by two orders of magnitude.