Distribution Of Te Inclusions Research Articles

Modified Bridgman assisted ZnTe crystal growth for THz device applications

Zinc Telluride (ZnTe) single crystals have enormous potential for generating and detecting broadband terahertz (THz) radiation, making them suitable for a wide range of applications in communication, defence security and biomedical applications. The quality and homogeneities of the crystal plays an important role in obtaining THz response. This paper elaborates on the steps involved in the crystal growth of ZnTe and the characterization technique utilized for the determination of quality of the grown crystal, identifications of defects and qualifications of THz signals. Single crystal was grown by the Bridgman technique in vacuum sealed carbon-coated quartz ampoule with excess tellurium (Zn:Te/3:7). To obtain maximum THz response, crystals were oriented along (110) direction using a Laue diffraction pattern and sliced accordingly. Three different crystals along the length (bottom-middle-top) of size ∼ 10 mm × 10 mm are taken for analysis. The crystallinity of ZnTe was confirmed with X-ray diffraction pattern and lattice parameters were obtained using Rietveld refinement. Raman analysis was carried out at three different regions of the grown crystal which revealed the presence of same high-intensity mode at 413 cm−1 in all the regions confirming the quality of the crystal. The EDS compositional analysis confirms the stoichiometry along the grown crystal length. Optical bandgap of 2.216 eV was measured by UV-Vis-NIR spectra and >55 % transmission was obtained across the grown crystal length. The size and distribution of Te inclusions were measured using IR microscopy. The crystals were subjected to terahertz investigation in the range of 0.1–4 THz using a time domain spectrometer. All the crystals exhibited negligible absorption coefficients in the 0.1–3.75 THz range, confirming the maximum transmission in the THz domain. The crystals also exhibit excellent electro-optic detection which is confirmed by signal-to-noise ratio in the measured range.

Improvement of charge collection efficiency of the CdZnTe detectors by decreasing the Te inclusions

CdZnTe is a promising material for room-temperature radiation semiconductor detectors. However, their radiation response performance might be deteriorated owing to defects within the material such as Te inclusions. This study employs the Cd thermal annealing process to reduce the size of these Te inclusions. An ampoule containing Cd powder and CdZnTe was sealed under the vacuum conditions of 2.5 × 10−5 Torr for Cd-vapor annealing. Subsequently, the ampoule was annealed at 783 K for 64 h. An IR camera was used to observe the distribution of Te inclusions in the CdZnTe before and after annealing. As a second experiment, NH4F + H2O2 passivation was performed to suppress the increased leakage current due to Cd-vapor annealing. Then an 241Am radioisotope source was used to obtain a radiation response spectrum. The improved charge collection efficiency was about 35%.

Correlation between Te inclusions and the opto-electrical properties of CdMnTe and CdMgTe single crystals

In this paper, the effects of Te inclusions on the optical and electrical properties of CdMnTe and CdMgTe crystals grown by the modified vertical Bridgman method were investigated. For CdMnTe ingot, the average size and density distribution of Te inclusions along axial direction were in the range of 12 ~ 29 μm and 1.53 × 104 cm−2 ~ 2.81 × 105 cm−2, respectively. For CdMgTe ingot, the average size and density distribution of Te inclusions were in the range of 6 ~ 20 μm and 8.17 × 103 cm−2 ~ 2.42 × 104 cm−2, respectively. The density and size of Te inclusions increased gradually from the first-to-freeze part to the last-to-freeze part for CdMnTe ingot, while that of CdMgTe ingot first decreased and then increased. For both CdMnTe and CdMgTe ingots, the resistivity, IR transmittance and FWHM of (D0, X) peak first increased and then decreased from the first-to-freeze part to the last-to-freeze part. The middle-to-freeze parts of both ingots had the best crystal quality. For both CdMnTe and CdMgTe detectors, the energy resolution and (μτ)e value were first increased and then decreased from the first-to-freeze part to the last-to-freeze part. The detectors fabricated by the middle-to-freeze parts of both crystals had the best performance and can could meet the requirements of room temperature radiation detection.PACS: 78.30.Fs; 78.55.Et; 81.40.Ef.

Anomalous Te Inclusion Size and Distribution in CdZnTeSe

The effect of selenium in CdZnTe was investigated by characterizing the Cd0.9Zn0.1Te0.98Se0.02 crystal. The bandgap of Cd0.9Zn0.1Te0.98Se0.02 is smaller than Cd0.9Zn0.1Te. In addition, its value in ingot follows the zinc-segregation tendency than that of selenium. There was a recognizable difference in the amounts of compensating dopants, the distribution of Te inclusions, and the performance of the detector. The amounts of indium doping for the compensation were reduced by the addition of selenium, which provided the highest partial pressure in the melts. Thus, the generation of Cd vacancies, which is a native defect in CdZnTe, was prevented. In addition, the size of Te inclusions varied from the tip, middle, and heel of the CdZnTeSe ingot. The addition of selenium may change the thermal conductivity of CdZnTeSe melts, which is responsible for reducing the bulging of the retrograde solidus line near stoichiometry. The planar CdZnTeSe detector showed a 59.5-keV gamma peak for Am-241 with an 11% of energy resolution.

Revealing Te inclusions on VGF-grown CdZnTe crystals by using low energy argon ion irradiation: Influence of the substrate temperature

Cd1−xZnxTe (CZT) crystals find interesting uses in a variety of detector-type applications. However, it is well known that the concentration and distribution of Te inclusions within a device are one of the major contributions to the degradation of CZT detectors. In addition, Te precipitates on the surface can also increase the leakage current, thereby deteriorating the device performance.Our previous studies clearly revealed a pronounced effect when CZT surfaces were irradiated with Ar ions at room temperature. Even for small fluences, a removal of polishing scratches on the sample surfaces was observed. Correlated to this effect, an important enhancement in the luminescence intensity of the irradiated samples was observed. Te inclusions also tend to aggregate after the irradiation process and are also eliminated from the surfaces for the highest ion fluences.In this work we continue our previous work by studying the effect of the temperature of the sample in the range 300–600 K during Ion Beam Sputtering of CZT crystals surfaces at different fluences, by means of Scanning Electron Microscopy, Atomic Force Microscopy, and μ-Photoluminescence. It is expected that increasing the temperature of CZT samples during the irradiation process will enhance the beneficial effects this technique has shown when trying to reduce surface Te inclusions and to increase the luminescence properties of CZT crystal surfaces.

Distribution of Te Inclusions in CdMnTe Crystal Grown by Traveling Heater Method

This study investigated Te inclusions in CdMnTe crystals grown by the traveling heater method (THM). Three CdMnTe ingots were grown under different growth temperatures of 850°C, 900°C, and 950°C by the THM method. In the CdMnTe ingots, the concentration of Te inclusion was lower in the middle-to-deposit part, and the size of Te inclusions kept growing till the last-to-deposit part. The concentrations of Te inclusions along the CdMnTe ingots were in the range of 104–106 cm−3, with the size in the range of 3–16 μm. CdMnTe crystals grown under 900°C had the lowest density and size of Te inclusion, due to the smooth and near-planar growth interface as revealed by infrared (IR) transmission microscopy. The relationship between the origin of Te inclusions and growth interface was analyzed during the THM growth of CdMnTe. IR transmission spectrometry was used to characterize Te inclusions in CdMnTe crystals. Current–voltage (I–V) and energy response measurements were carried out to reveal the resistivity (ρ) and energy resolution of CdMnTe detectors. The effects of Te inclusions on the optical and electrical properties of CdMnTe crystals for nuclear detector applications are discussed.

Characterization of Cr-doped ZnTe Crystals Grown by Temperature Gradient Solution Growth (TGSG)

Cr-doped ZnTe ingot was grown by temperature gradient solution growth (TGSG), using CrTe and Te as dopant source and solvent, respectively. Single crystals in the head of the ingot have larger size of more than 10 mm× 10 mm and less Te inclusions. The size, shape and distribution of Te inclusions well reflect the thermal field distribu- tion of the ingot. Radial asymmetric thermal field of the ingot leads to radial asymmetric distribution of Te inclusions. The thermal migration of Te inclusions under thermal gradient results in their merging, enlargement and enlongation. Defects such as cracks and voids can be introduced into ZnTe matrix by Te inclusions. Some excess amount of CrTe is rejected by the advancing interface, which indicates that the present growth method has self purification effect. The re- sistivity of Cr-doped ZnTe ( about 1000 Ω·cm ) is slightly higher than that of ZnTe (about 300 Ω·cm). Cr-doped ZnTe exhibits an absorption centered at about 1750 nm, which means Cr 2+ ions are successfully incorporated into ZnTe. IR

Narrow shape distribution of Te inclusions in ZnTe single crystals grown from Te solution

The shape and distribution of Te inclusions in ZnTe crystals grown from Te solution were studied. Polyhedron type Te inclusions were mainly observed. The relative size of the facets bounding the polyhedron Te inclusions is found very sensitive to the local crystalline quality of the matrix, which in turn determines the shape and distribution of Te inclusions. However, on the macroscopic perspective, the overall three-dimensional (3D) shape of polyhedron Te inclusions “focuses on” a truncated octahedron (tetradecahedron) bounded by {100}, and {111}Te&Zn planes, which is different from the reported results, when only tetrahedron Te inclusions with {111}Te/Zn facets were found. Te inclusions in ZnTe matrix exhibit a much narrower shape distribution than do in CdZnTe, where both truncated octahedron and tetrahedron shape Te inclusions coexist. This shape distribution phenomenon was discussed for the first time based on the growth rate dispersion (GRD) theory, which leads us to believe that, the defect density and stress in the matrix may be important factors affecting the shape distribution.

Te Inclusions in CdMnTe Crystal Grown by the Traveling Heater Method and their Effects on the Electrical Properties

The concentration and size distribution of Te inclusions in CdMnTe (CMT) are key factors in nuclear devices performance. High-concentration and large-size Te inclusions degrade the performance drastically, especially for electrical properties. In this paper, the Te inclusions along the axial and radial directions in CMT crystal grown by the Traveling Heater Method (THM) were revealed by Infrared Microscopic System. The size of Te inclusions is 5-17 μm and the concentration is (2.5-4.6) ×105cm-3. The Current-Voltage measurements were performed and the resistivity of the CdMnTe crystal with excess Te was in the range of 108-109Ωcm, and the correlation between the Te inclusions and resistivity was investigated. The large size (>17 μm) and high concentration (4.5 of ×105cm-3) Te inclusion decrease the resistivity of the CMT crystal seriously.

‘Star‐like’ defects in Cd‐annealed CdZnTe crystals—an experimental study of their origin and formation mechanism

AbstractWe conducted low‐temperature annealing experiments at temperatures slightly above and below the melting point of Te to clarify the effects of the state of Te inclusions (solid or liquid) upon the formation of ‘star‐like’ defects in Cd‐annealed CdZnTe (CZT). We also carried out post‐growth annealing experiments with and without using Cd vapor to clarify the mechanism of formation of such defects. We demonstrated that these ‘star‐like’ defects are due to the reaction between in‐diffused Cd atoms and the molten Te inclusions, but we found no observable ‘one‐to‐one’ correlation between ‘star‐like’ defects and Te inclusions. The non‐uniform distribution of Te inclusions in the CZT matrix could account for this phenomenon since the punching distance of the dislocations depends on the volume fraction of inclusions within the matrix.

Fabrication and Characterization of ${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te}$ Schottky Diodes for High Resolution Nuclear Radiation Detectors

Cadmium zinc telluride (CZT) Schottky diodes with very low reverse leakage current have been fabricated and characterized for high resolution gamma ray detectors. The diodes were made using <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te}$</tex></formula> detector grade crystals grown from zone refined Cd, Zn, and Te (7N) precursor materials using low temperature tellurium solvent method. Various crystallographic defects including Te-inclusions/precipitates were identified and characterized using electron beam induced current (EBIC) measurement and thermally stimulated current (TSC) spectroscopy. The EBIC images were correlated with transmission infrared (TIR) images of CZT crystals and the EBIC contrast was attributed to the nonuniformities in spatial distribution of Te inclusions. Characterization by TSC revealed shallow and deep level centers with activation energies 0.25–0.4 eV and 0.65–0.8 eV respectively, which were attributed to intrinsic defects associated with Te inclusions. Pulse height measurements were carried out using <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$^{137}{\rm Cs}$</tex></formula> (662 keV) radiation source and energy resolution of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim 1.51}\%$</tex></formula> full width at half maximum (FWHM) was obtained from the as-grown boule.

Characterization of CdZnTe after argon ion beam bombardment

The objective of this work is to analyze the effects of argon ion irradiation process on the structure and distribution of Te inclusions in Cd1-xZnxTe crystals. The samples were treated with different ion fluences ranging from 2 to 8×1017cm−2. The state of the samples before and after irradiation were studied by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Cathodoluminescence, Photoluminescence, and micro-Raman spectroscopy. The effect of the irradiation on the surface of the samples was clearly observed by SEM or AFM. Even for small fluences a removal of polishing scratches on the sample surfaces was observed. Likely correlated to this effect, an important enhancement in the luminescence intensity of the irradiated samples was observed. An aggregation effect of the Te inclusions seems to occur due to the Ar bombardment, which are also eliminated from the surfaces for the highest ion fluences used.

Size and distribution of Te inclusions in THM as-grown CZT wafers: The effect of the rate of crystal cooling

The concentration and size distribution of Te inclusions/precipitates in CZT are key factors in a device's performance. High concentrations can degrade the performance drastically, especially for long drift-length devices (more than about 10-mm thick). Here, we extend our previous findings on the concentration and size distribution of Te inclusions/precipitates in CZT wafers grown by the THM technique, by considering the rate of cooling of the ingots. We measured their distribution along the diameter of the wafers in a conventional slow-cooled and a fast-cooled ingot. The overall average concentration of Te inclusions/precipitates for the slow-cooled sample was less than 1×10 5 cm −3, attesting to their suitability for fabricating thick radiation-detection devices.

Distribution of Te inclusions in a CdZnTe wafer and their effects on the electrical properties of fabricated devices

We quantified the size and concentration of Te inclusions along the lateral- and the growth-directions of a ∼6 mm-thick wafer cut axially along the center of a CdZnTe ingot . We fabricated devices, selecting samples from the center slice outward in both directions, and then tested their response to incident X-rays. We employed, in concert, an automated IR transmission microscopic system and a highly collimated synchrotron X-ray source that allowed us to acquire and correlate comprehensive information on Te inclusions and other defects to assess the material factors limiting the performance of CdZnTe detectors.

Size and distribution of Te inclusions in detector-grade CdZnTe ingots

AbstractTo observe the Te inclusions distribution along the axial direction of CdZnTe ingots, batches of as-grown detector-grade CdZnTe crystals grown by vertical Bridgman method, were investigated using IR transmission imaging. However, there is not a rigorous regularity for the micron-scale Te inclusions distribution along the axial direction of CdZnTe ingots according to the measurements of the present investigation. The size and distribution variations of Te inclusions are attributed to the growth velocity, furnace temperature situation and morphological changes at the liquid-solid interface during crystal growth. Electron mobility lifetime products were discussed in conjunction with Te inclusions. However, no strict correlations are identified between the sizes and concentrations of Te inclusions and the γ-ray spectroscopy response. Extended defects in CdZnTe crystals, which are not visible via IR transmission imaging, potentially attribute to CdZnTe detectors performance degrades.

Fluctuations in induced charge introduced by Te inclusions within CdZnTe radiation detectors

Recently, homogenization theory based on a multiple-scale perturbation of the electron transport equation has been used to derive a mathematical framework for modeling the excess charge lost to Te inclusions within radiation detectors based on semi-insulating cadmium zinc telluride (CdZnTe). In that theory, the heterogeneous material is mathematically replaced by a homogenized CdZnTe crystal whose effective electron attenuation length incorporates the additional uniform electron trapping caused by the inclusions. In this paper, the homogenization theory is extended to incorporate fluctuations in the induced charge (i.e., charge collection nonuniformities) introduced by the random position and size distributions of a noncorrelated population of small (i.e, &amp;lt;20 μm) Te inclusions. Analysis of the effective parameters derived within the homogenized framework is used to develop a probability distribution of effective electron attenuation lengths, and therefore effective mobility-lifetime products, as a function of both the position and size distribution of Te inclusions. Example distributions are detailed for the case of an exponential size distribution at various number densities. Further, it is demonstrated that the inclusion-induced material nonuniformities derived in this paper can be numerically sampled efficiently, making them applicable to Monte Carlo device simulation of realistic CdZnTe detectors. Simulated charge induction maps and pulse-height spectra are presented and compared to recently published measurements.

THM Growth and Characterization of 100 mm Diameter CdTe Single Crystals

The THM growth technology for 100 mm diameter CdTe single crystals has been intensively developed. In consequence of the optimization of growth conditions, we have succeeded in controlling growth interface shape and growing 100 mm diameter CdTe single crystals with 300 mm in length. Concerning the behavior of Te inclusions in the grown crystal, their size and density were investigated by IR transmission microscopy. The size distribution of Te inclusions was divided into two groups, and the density of them was less than 1 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Charge transport properties of the grown crystal were investigated by using the ldquomutau-modelrdquo spectral fitting method, and were found to be quite uniform all over the wafer. To investigate the homogeneity of radiation detector performances, Ohmic type and Schottky type detectors with 4 mm times 4 mm times 1 mm were fabricated from the left half and the right half of the 100 mm diameter wafer, respectively. Standard deviations of their energy resolutions for the 122 keV line from <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> Co were less than 6%. This excellent uniformity is essential for the room temperature semiconductor detectors in the major application areas, such as medical imaging, non-destructive inspection and homeland security.

Study of Te inclusions in CdMnTe crystals for nuclear detector applications

The concentration, size and spatial distribution of Te inclusions in the bulk of CdMnTe crystals mined from two batches of ingots were studied. An isolated planar layer decorated with Te inclusions was identified in CdMnTe crystals from the second ingot. The internal electric field of a CMT crystal was probed by infrared (IR) imaging employing Pockels electro-optic effect. The effect of an isolated plane of Te inclusions on the internal electric-field distribution within the CdMnTe crystal was studied. Space charge accumulation around the plane of Te inclusions was observed, which was found to be higher when the detector was reverse-biased. The effects of the plane of Te inclusions on the electric-field distribution within the CdMnTe crystal, and the quality of CdMnTe crystals for nuclear detector applications are discussed.

Characterization of the charge transport uniformity of CdZnTe crystals for large-volume nuclear detector applications

Mounting evidence indicates that charge transport nonuniformity is one of the most important obstacles limiting the energy resolution of large-volume (/spl ges/1 cm/sup 3/) CdZnTe nuclear detectors. Better understanding of the defects responsible for charge transport nonuniformity is needed to develop advanced crystal growth processes producing CdZnTe crystals with more uniform defect distribution and, hence, more uniform charge transport. Better material screening techniques are also required to determine the charge transport uniformity of the CdZnTe crystals at the ingot or slice level in order to maximize the detector fabrication yields in manufacturing large-volume CdZnTe nuclear detectors. Here, we present results from a program aimed at implementing a defect and charge transport uniformity characterization process to provide feedback to crystal growth process development efforts and material selection for large-volume CdZnTe detector manufacturing. We first focused on recent results suggesting that structural nonuniformity of CdZnTe crystals around Te inclusions is responsible for electron transport nonuniformity and diminished energy resolution of large-volume CdZnTe coplanar-grid detectors. The spatial size and density distribution of Te inclusions was measured with infra-red microscopy. A charge transport mapping station employing a collimated alpha source was used to study the uniformity of carrier transport in CdZnTe crystals.