Defects In CdZnTe Research Articles

Defect levels characterized by photoconductivity and thermally stimulated current in CdZnTe crystals

Deep-level defects in CdZnTe (CZT) crystals were studied by combining photoconductivity (PC) and thermally stimulated current (TSC) measurements. The stretched-exponential function could well describe the time-dependent photocurrent decay kinetics, and the decay time constant was fitted to be ~54 s for CZT1 and ~98 s for CZT2, respectively. TSC spectra were analyzed through SIMPA fitting, and the total defect density was calculated to be about 1.14 × 1016 cm−3 in CZT1 and 1.80 × 1016 cm−3 in CZT2, respectively. Deep donor (Te antisites) could be considered as dominating the photoconductivity decay process. The electron mobility was fitted in Time-of-Flight (TOF) spectra to be about 783 cm2/Vs in CZT1 and 717 cm2/Vs in CZT2, respectively. Approximately the same electron mobility of the two CZT crystals corresponds to similar concentration of all the defect traps. The mobility-lifetime (μτ) product for electrons could be fitted by Hecht equation to be about 1.61 × 10−3 cm2/V in CZT1 and 3.01 × 10−4 cm2/V in CZT2, respectively. Higher concentration of deep donor (Te antisites) in CZT2 compared to that in CZT1 will lead to lifetime reduction and the resultant lower (μτ)e product.

The mechanism of defects effect on carrier transport by employing multi-wavelength sub-bandgap lights on CdZnTe crystal

The effect of different point defects in CdZnTe (CZT) crystals on carrier transport performance were discussed. The energy level and concentration of the traps were determined by the photo-induced current transient spectroscopy (PICTS). By adding the infrared light illumination into the PICTS experiment, the effect of sub-bandgap light on defects was observed. Meanwhile, several DC photoconductive experiments were done under different wavelength lights were used to explore the effect of the different defects on carrier transport properties of CZT crystals. Furthermore, the energy spectrum test was used to investigate the effect of sub-bandgap light on the energy resolution of CZT detectors. The data results showed that sub-bandgap light could effectively change the ionization state of defects. The doubly ionized Te antisite (TeCd2+) had the greatest effect on mobility-lifetime product(μτ) of electrons owing to its stronger ability of capturing electrons and energy level located around the midgap. While the secondary ionized Cd vacancy (VCd2−), as considered as a hole trap, had the greatest effect on μτ-product of holes. Consequently, choosing suitable infrared light can improve the detection performance of CZT crystal effectively.

Two-Step Annealing to Remove Te Secondary-Phase Defects in CdZnTe While Preserving the High Electrical Resistivity

The presence of Te secondary-phase defects (i.e., Te inclusions and Te precipitates) is a major factor limiting the performance of CdZnTe (CZT) X-ray and gamma-ray radiation detectors. We find that Te secondary-phase defects in CZT crystals can be removed through postgrowth two-step annealing without creating new trapping centers (i.e., prismatic punching defects). Two-step annealing (with the first under a Cd pressure and the second one under a Te pressure) was demonstrated to be effective in removing the Te secondary-phase defects, while preserving the electrical resistivity of the CZT detector. The first step involves annealing of semi-insulating CZT under a Cd overpressure at 700 °C/600 °C (CZT/Cd) for 24 h, which completely eliminated the Te-rich secondary-phase defects (Te inclusions). However, it resulted in a lower resistivity of the samples (down to $2\times 10^{4-6}\,\,\Omega \cdot \text {cm}$ ). A subsequent annealing step involves processing CZT under a Te ambient condition at 540 °C/380 °C (CZT/Te) for 120 h, which restored the crystal’s resistivity to $6.4 \times 10^{10}\,\,\Omega \cdot \text {cm}$ without creating new Te secondary-phase defects. However, Te inclusions reappeared in the case of unnecessarily long Te ambient annealing. Pulse-height spectra taken with the two-step annealed CZT detectors showed an improved detector performance due to a reduced concentration and the size of Te secondary-phase defects.

Distinctive distribution of defects in CdZnTe:In ingots and their effects on the photoelectric properties**Project supported by the National Natural Science Foundations of China (Grant Nos. 51502244, 51702271, U1631116, and 51372205), the National Key Research and Development Program of China (Grant Nos. 2016YFF0101301 and 2016YFE0115200), the Fund of the State Key Laboratory of Solidification Processing in Northwestern Polytechnical University, China (Grant

Photoelectric properties of CdZnTe:In samples with distinctive defect distributions are investigated using various techniques. Samples cut from the head (T04) and tail (W02) regions of a crystal ingot show distinct differences in Te inclusion distribution. Obvious difference is not observed in Fourier transform infrared (FTIR) spectra, UV–Vis–NIR transmittance spectra, and I–V measurements. However, carrier mobility of the tip sample is higher than that of the tail according to the laser beam induced current (LBIC) measurements. Low temperature photoluminescence (PL) measurement presents sharp emission peaks of D0X and A0X, and relatively large peak of D0X (or A0X) / Dcomplex for T04, indicating a better crystalline quality. Thermally stimulated current (TSC) spectrum shows higher density of shallow point defects, i.e., Cd vacancies, , etc., in W02 sample, which could be responsible for the deterioration of electron mobility.

Analysis of Deep and Shallow Traps in Semi-Insulating CdZnTe

Trap levels which are deep or shallow play an important role in the electrical and the optical properties of a semiconductor; thus, a trap level analysis is very important in most semiconductor devices. Deep-level defects in CdZnTe are essential in Fermi level pinning at the middle of the bandgap and are responsible for incomplete charge collection and polarization effects. However, a deep level analysis in semi-insulating CdZnTe (CZT) is very difficult. Theoretical capacitance calculation for a metal/insulator/CZT (MIS) device with deep-level defects exhibits inflection points when the donor/acceptor level crosses the Fermi level in the surface-charge layer (SCL). Three CZT samples with different resistivities, 2 × 104 (n-type), 2 × 106 (p-type), and 2 × 1010 (p-type) Ω·cm, were used in fabricating the MIS devices. These devices showed several peaks in their capacitance measurements due to upward/downward band bending that depend on the surface potential. Theoretical and experimental capacitance measurements were in agreement, except in the fully compensated case.

Tellurium Secondary-Phase Defects in CdZnTe and their Association With the 1.1-eV Deep Trap

Defects located at the E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> - 1.1 (eV) level, which are electro-optically active deep traps, generally have been overlooked since their presence cannot be detected except for those in highly resistive CdTe compounds. The origin of this trap is still debated on whether it is from Te vacancies or from dislocations induced by secondary phase defects in Te. We have grown high-resistivity Te-rich CZT ingots to clarify the origin of the 1.1 eV defects and to analyze the defect levels in the CZT samples by current deep level transient spectroscopy (I-DLTS)and photoluminescence (PL). From the analysis, defect levels such as shallow acceptor/donor, A-centers, Cd vacancies and Te antisite appeared to be in both high and low concentrations of Te inclusions, but the level of 1.1-eV defects exhibited dependence on the density of Te inclusion in the CZT samples. We also evaluated the effect of the 1.1-eV deep-level defects on the detector's performance in point view of carrier trapping and de-trapping.

Carbon Coating and Defects in CdZnTe and CdMnTe Nuclear Detectors

CADMIUM zinc telluride (CdZnTe) and cadmium manganese telluride (CdMnTe) are prime materials for detecting X-rays and gamma-rays at room temperature due to their high average atomic numbers that are essential to having high stopping-power for incident high-energy electromagnetic radiations. A major obstacle in developing CdZnTe and CdMnTe detectors lies in growing crystals free from defects, such as Te inclusions, dislocations, sub-grain boundary networks, and precipitates. We present the results of our study of the relationship between carbon coating of the growth ampoule and dislocations in CdZnTe and sub-grain boundary networks in CdMnTe, grown by Bridgman method. For the CdZnTe crystals, a carbon-coating of $2~\mu\hbox{m}$ on the ampoule generated fewer dislocations than did a thinner $0.2 - \mu\hbox{m}$ carbon-coated one. Furthermore, the ampoule’s design (normal- or tapered-shape) did not affect the densities of etch pits as much as did the thickness of the carbon-coating. For a CdMnTe ingot with a carbon coating of about $2~\mu\hbox{m}$ , created by cracking spectroscopic-grade acetone at $\sim {900}^\circ {\rm C}$ , we observed very few grain boundaries and grain-boundary networks.

Use of high-granularity CdZnTe pixelated detectors to correct response non-uniformities caused by defects in crystals

Following our successful demonstration of the position-sensitive virtual Frisch-grid detectors, we investigated the feasibility of using high-granularity position sensing to correct response non-uniformities caused by the crystal defects in CdZnTe (CZT) pixelated detectors. The development of high-granularity detectors able to correct response non-uniformities on a scale comparable to the size of electron clouds opens the opportunity of using unselected off-the-shelf CZT material, whilst still assuring high spectral resolution for the majority of the detectors fabricated from an ingot. Here, we present the results from testing 3D position-sensitive 15×15×10mm3 pixelated detectors, fabricated with conventional pixel patterns with progressively smaller pixel sizes: 1.4, 0.8, and 0.5mm. We employed the readout system based on the H3D front-end multi-channel ASIC developed by BNL's Instrumentation Division in collaboration with the University of Michigan. We use the sharing of electron clouds among several adjacent pixels to measure locations of interaction points with sub-pixel resolution. By using the detectors with small-pixel sizes and a high probability of the charge-sharing events, we were able to improve their spectral resolutions in comparison to the baseline levels, measured for the 1.4-mm pixel size detectors with small fractions of charge-sharing events. These results demonstrate that further enhancement of the performance of CZT pixelated detectors and reduction of costs are possible by using high spatial-resolution position information of interaction points to correct the small-scale response non-uniformities caused by crystal defects present in most devices.

Effects of Crystal Growth Methods on Deep-Level Defects and Electrical Properties of CdZnTe:In Crystals

Deep-level defects in CdZnTe: In (CZT) crystals grown by the modified vertical Bridgman (MVB) method and the traveling heater method (THM) were comparatively studied by thermally stimulated current (TSC) measurements. The trap density of Cd vacancy-related acceptor defects is found to be lower in THM-grown crystals compared with crystals grown by the MVB method, since the relatively low growth temperature of the THM will greatly reduce the loss of Cd and decrease the generation of Cd vacancies. Tellurium antisite-related deep donors are dominant in MVB-grown CZT, and Te interstitial-related deep acceptors are dominant in THM-grown CZT crystals grown by the Te-rich solution technique. The compensation of donor and acceptor defects leads to a low concentration of net free electrons in n-type MVB-grown CZT and a relatively high concentration of net free holes in p-type THM-grown CZT. The Fermi level in MVB-grown CZT is positioned below the conduction band by the antisite-related deep donor, while that in THM-grown CZT is positioned above the valence band by the interstitial-related deep acceptor.

Axial distribution of deep-level defects in as-grown CdZnTe:In ingots and their effects on the material׳s electrical properties

We investigated the distribution of deep-level defects in CdZnTe:In ingots along the growth direction using thermally stimulated current (TSC) spectra. Higher concentration of Te antisite-related deep donors were found in the tail region due to Te enrichment in the melt as solidification proceeded. The compensation between InCd+ defects and Cd vacancies results in low concentration of net free electrons. High resistivity with n-type conduction was demonstrated from the I–V analysis. Hall mobility is lower in the tail due to higher concentration of Te antisite-related deep donors.

Effects of excess tellurium and growth parameters on the band gap defect levels in CdxZn1−xTe

This research summarizes an effective way to understand compensation for use of CdZnTe as ambient temperature radiation detector. The indium doped CdZnTe passivates certain detrimental intrinsic defects and defect complexes in the band gap. This was achieved by using a combination of excess tellurium in the starting material (0% to 7.5% by weight) and the process variables during growth, including the imposed temperature gradient, growth rate, and cool-down process. These studies have shown that a combination of slight excess tellurium as well as the cool-down scheme could control certain intrinsic defect levels and defect level complexes in the band gap of CdZnTe by causing favorable carrier compensation. At a macroscopic level, these manipulations help to minimize thermal instabilities during growth and determine the final grain structure, integrity, and yield of the ingot. Also, these manipulations help to control the formation of certain intrinsic defect levels and defect level complexes in the band gap, which have a direct bearing on the ability of the CdZnTe crystals to function as room temperature radiation detectors. The band-gap defects in CdZnTe were studied using the thermally stimulated current (TSC) technique. The thermal ionization energy and capture cross-section for 8 prominent defect levels (current peaks in the TSC spectrum) were calculated using the variable heating rate method. These fitted values were compared to transition energy levels of possible defects in the band gap of CdTe and purity data of CdZnTe samples used in this study. The theoretical values of the transition energy levels of defects in the band gap of CdTe were determined by the first principle band gap structure studies as well as our earlier studies on defects and defect levels in the band gap of CdTe. We have tentatively associated some prominent current peaks to certain defect levels and defect level complexes in Cd1−xZnxTe. The improvement in the detector properties was correlated to the reduction of a proposed deep level defect complex (TeCd + VCd) (thermal ionization energy &amp;gt;0.8 eV and capture cross-section of 10−13 to 10−14 cm2), and the reduction of the ionized species corresponding to an acceptor defect level (thermal ionization energy ∼0.2[03] eV), associated with dislocations/dislocation complexes with Te clusters. The best crystals tested had an average μτe (electrons) of 1.8 × 10−3 cm2/V, a peak-to-valley ratio of 2.0 for the 122 keV x-ray peak using a Co-57 source and bulk resistivity the order of 3 × 1010 Ω cm. The best radiation detector crystals corresponded to those grown with an excess tellurium of 0.5% (by weight in the starting CdZnTe charge) grown at a rate of 0.5 to 0.86 mm/h with an imposed temperature gradient of &amp;gt; 50 °C/in. There was a high yield without any post-processing of the ingots.

The growth of silver and copper single crystals on silicon and the selective removal of silicon by electrochemical displacement : W. K. Zwicker and S. K. Kurtz. Acta Electronica16 (4), 331 (1973). (In French)

Following our successful demonstration of the position-sensitive virtual Frisch-grid detectors, we investigated the feasibility of using high-granularity position sensing to correct response non-uniformities caused by the crystal defects in CdZnTe (CZT) pixelated detectors. The development of high-granularity detectors able to correct response non-uniformities on a scale comparable to the size of electron clouds opens the opportunity of using unselected off-the-shelf CZT material, whilst still assuring high spectral resolution for the majority of the detectors fabricated from an ingot. Here, we present the results from testing 3D position-sensitive 15×15×10 mm3 pixelated detectors, fabricated with conventional pixel patterns with progressively smaller pixel sizes: 1.4, 0.8, and 0.5 mm. We employed the readout system based on the H3D front-end multi-channel ASIC developed by BNL's Instrumentation Division in collaboration with the University of Michigan. We use the sharing of electron clouds among several adjacent pixels to measure locations of interaction points with sub-pixel resolution. By using the detectors with small-pixel sizes and a high probability of the charge-sharing events, we were able to improve their spectral resolutions in comparison to the baseline levels, measured for the 1.4-mm pixel size detectors with small fractions of charge-sharing events. These results demonstrate that further enhancement of the performance of CZT pixelated detectors and reduction of costs are possible by using high spatial-resolution position information of interaction points to correct the small-scale response non-uniformities caused by crystal defects present in most devices.