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 Cd_0.9Zn_0.1Te (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, μτ_e≈(2–5)×10⁻³ and μτ_h≈(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.     

Precipitation in CdTe crystals studied through mie scattering

Below gap optical losses in as-grown n-type CdTe crystals were analyzed in terms of free carrier absorption and Mie extinction due to Te precipitates. Experimental absorption spectra measured between 2 to 20 μm exhibited the well-known free carrier absorption behavior α_FCA∼λ^x with x=3 due to scattering by polar optical phonons. In shorter wavelength regions below 6 μm, however, additional contributions to the light loss due to absorption and scattering by precipitates were also observed. Assuming a log-normal size distribution, the precipitate extinction spectra were calculated according to Mie theory within the electric and magnetic dipole and electric quadrupole approximation. A comparison with the experimental spectra identifies the precipitates and enables estimation of their sizes and total number density. In this investigation, both undoped and In-doped CdTe crystals grown from stoichiometric melts by vertical asymmetric Bridgman method were used. It was found that In doping, in general, suppresses Te precipitation. At high doping level (melt containing∼10¹⁹ In atoms cm⁻³), the formation of In₂Te₂ is also indicated. It is demonstrated that the Mie extinction analysis offers an, expedient method to rapidly analyze the precipitates in CdTe and in similar other wide gap materials in a nondestructive manner.     

Current response of a TlBr detector to <Superscript>137</Superscript>Cs γ-ray radiation

The current response of a TlBr detector to ¹³⁷Cs γ-ray radiation has been studied in the dose-rate range 0.033–3.84 Gy/min and within the voltage range 1–300 V; the detectors are based on pure and doped TlBr crystals grown from the melt by the Bridgman-Stockbarger method. The mass fraction of Pb or Ca introduced into the TlBr crystals was 1–10 ppm for Pb and 150 ppm for Ca. The current response of nominally undoped TlBr samples was nearly linear over two decades of studied dose rates. Deep hole levels associated with cationic vacancies V _c ⁻ determine the dependence of the current response on the voltage in the high electric fields. The parameters of the carriers’ transport μτ are determined. The TlBr crystals grown in vacuum and in the bromine vapor exhibit a large mobility-lifetime product of 4.3 × 10⁻⁴ and 6.4 × 10⁻⁵ cm²V⁻¹, respectively. The value of μτ is in the range (4–9) × 10⁻⁵ cm²V⁻¹ for crystals doped with a divalent cation.     

Electrical properties of CdTe crystals grown by vuvg from nonstoichiometric charges

High purity CdTe crystals were grown by the vertical unseeded vapor growth technique. The growth charge composition varied between CdTe +0.1 mol% Cd and CdTe + 0.1 mol% Te. At a deviation from stoichiometry of δTe > 0.05 mol% in the charge p-type crystals with an electrical resistivity of 10⁶ Ωcm were obtained, n-type crystals were obtained with charge compositions of δTe < 0.02 mol%, with the resistivity ρ increasing with δTe from 10³ to < 10⁹ Ωcm. Maximum resistivity ∿2.10⁹ Ωcm was found when δTe ∿0.02 mol%. In all cases, the crystal composition shows a higher Cd concentration of ∿0.02 mol% relative to the charge, suggesting noncongruent sublimation of CdTe.     

X-ray Diffraction Imaging of Improved Bulk-Grown CdZnTe(211) and Its Comparison with Epitaxially Grown CdTe Buffer Layers on Si and Ge Substrates

Large-area high-quality Hg_1–x Cd _x Te sensing layers for infrared imaging in the 8 μm to 12 μm spectral region are typically grown on bulk Cd_1–x Zn _x Te substrates. Alternatively, epitaxial CdTe grown on Si or Ge has been used as a buffer layer for high-quality epitaxial HgCdTe growth. In this paper, x-ray topographs and rocking-curve full-width at half-maximum (FWHM) data will be presented for recent high-quality bulk CdZnTe grown by the vertical gradient freeze (VGF) method, previous bulk CdZnTe grown by the vertical Bridgman technique, epitaxial CdTe buffer layers on Si and Ge, and a HgCdTe layer epitaxially grown on bulk VGF CdZnTe.     

Spectroscopic evaluation of n-type CdZnTe gamma-ray spectrometers

This work focuses on the evaluation of the spectroscopic performance of n-type CdZnTe gamma-ray spectrometers, grown by a modified horizontal Bridgman Technique developed at IMARAD Imaging Systems Ltd. Two types of devices are studied: (i) detector arrays grown and produced by IMARAD and employing ohmic indium contacts and (ii) detectors and arrays fabricated at Technion in crystals provided by IMARAD, employing different types of contacts. Alpha particle spectroscopy as well as gamma-ray spectroscopy is used to evaluate and characterize the energy resolution of gamma-ray spectrometers fabricated on n-type CdZnTe grown by a modified horizontal Bridgman and doped with indium. The electron and hole mobility lifetime products of the n-type CdZnTe material grown by IMARAD are estimated by measuring the dependence of charge collection efficiency upon the bias voltage, using a calibrated multichannel analyzer. The measured results indicate that the average electron and hole mobility-lifetime products are, respectively, of the order of μ_nτ_n=(1–2)·10⁻³ cm²/V and μ_pτ_p=6·10⁻⁶ cm²/V. The measured energy resolution of 122 keV photons is −(5–6)% when the source is not collimated and is reduced to −4.5% when the source is collimated. These results are obtained with ohmic cathode as well as with a rectifying cathode. A statistical model for the calculation of the pulse height spectra as a function of photon energy, electron and hole mobility-lifetime products and applied electric field, which has been recently reported in Applied Physical Letters, is used to determine the role of incomplete charge collection in the spectral performance of the n-type CdZnTe spectrometers. The comparison between the measured and modeled results indicates that the dark noise, cross talk and non-uniformity are the main limiting factors of the spectral performance of the n-type spectrometers rather than incomplete charge collection. The good spectroscopic performance of the arrays under study is attributed to an adequate hole mobility lifetime for the geometry of the pixilated arrays. The study indicates that the n-type CdZnTe spectrometers are useful for a wide range of imaging applications.     

Growth and structural properties of low defect,sub-grain free CdTe substrates grown by the horizontal bridgman technique

Large area, low defect CdTe substrates are essential for high quality epitaxy of HgCdTe in infrared detector applications. Vertical Bridgman (VB) CdTe normally exhibits higher than desired dislocation density and sub-grain structure. A seeded Horizontal Bridgman (HB) technique has been used to grow CdTe single crystals which exhibit superior crystalline qualities when compared to standard VB substrates. The HB grown CdTe crystals were not intentionally doped and had resistivities in the 10⁷ ohm-cm range. The etch pit density (EPD) near the seed and the tail end sections is 5 × 10⁴ cm⁻²s, while wafers from the middle section of the ingot have EPDs in the 10⁴ cm⁻² range. Furthermore, HB EPD patterns indicate the absence of sub-grain boundaries. X-ray rocking curves are very sharp and exhibit FWHMs as low as 9 arc-sec. By comparison, the best samples from standard VB CdTe ingots exhibit x-ray rocking curves with FWHMs in the >30 arc-sec range. The IR transmission of HB material is as high as 57% in the 2.5 to 20 μm region. Results of electrical and optical characterization are presented.     

Evaluation of Mn Uniformity in CdMnTe Crystal Grown by the Vertical Bridgman Method

Cd_1−x Mn _x Te is a typical diluted magnetic semiconductor, as well as substrate for the epitaxial growth of Hg_1−x Cd _x Te. In this paper, the homogeneity of a Cd_1−x Mn _x Te (x = 0.2) single-crystal ingot grown by the vertical Bridgman method was studied. The crystal structure and quality of the as-grown ingot were evaluated. Near-infrared (NIR) transmission spectroscopy was adopted to develop a simple optical determination of the Mn concentration in the as-grown ingot. A correlation equation between cut-off wavelength λ _co from NIR transmission spectra and Mn concentration by inductively coupled plasma atomic emission spectrometry (ICP-AES) was established. Using this equation, we investigated the Mn concentration distribution in both the axial and radial directions of the ingot. It was found that the segregation coefficient of Mn in the axial direction of the ingot was 0.95, which is close to unity. The Mn concentration variation in the wafers from the middle part of the ingot was 0.001 mole fraction. All these results proved that homogeneous Cd_0.8Mn_0.2Te crystals can be grown from the vertical Bridgman method.     

Solution Growth and Thermoelectric Properties of Single-Phase MnSi<Subscript>1.75−<Emphasis Type="Italic">x</Emphasis></Subscript>

We have investigated the crystal growth of single-phase MnSi_1.75−x by a temperature gradient solution growth (TGSG) method using Ga and Sn as solvents and MnSi_1.7 alloy as the solute, and measured the thermoelectric properties of the resulting crystals. Single-phase Mn₁₁Si₁₉ and Mn₄Si₇ crystals were grown successfully using Ga and Sn as solvents, respectively. The typical size of a grown ingot of Mn₁₁Si₁₉ was 2 mm to 4 mm in thickness and 12 mm in diameter, whereas Mn₄Si₇ had polyhedral shape with dimensions in the range of several millimeters. The single-phase Mn₁₁Si₁₉ has good electrical conduction (ρ = 0.89 × 10⁻³ Ω cm to 1.09 × 10⁻³ Ω cm) compared with melt-grown multiphase higher-manganese silicide (HMS) crystals. The Seebeck coefficient, power factor, and thermal conductivity were 77 μV K⁻¹ to 85 μV K⁻¹, 6.7 μW cm⁻¹ K⁻² to 7.2 μW cm⁻¹ K⁻², and 0.032 W cm⁻¹ K⁻¹, respectively, at 300 K.     

Optical and Structural Properties of CdTe Grown by Molecular Beam Epitaxy at Low Temperature for Resonant-Cavity-Enhanced HgCdTe Detectors

Investigation into resonant-cavity-enhanced (RCE) HgCdTe detectors has revealed a discrepancy in the refractive index of the CdTe layers grown by molecular beam epitaxy (MBE) for the detectors, compared with the reported value for crystalline CdTe. The refractive index of the CdTe grown for RCE detectors was measured using ellipsometry and matches that of CdTe with an inclusion of approximately 10% voids. X-ray measurements confirm that the sample is crystalline and strained to match the lattice spacing of the underlying Hg_(1−x)Cd_(x)Te, while electron diffraction patterns observed during growth indicate that the CdTe layers exhibit some three-dimensional structure. Secondary ion mass spectroscopy results further indicate that there is enhanced interdiffusion at the interface between Hg_(1−x)Cd_(x)Te and CdTe when the Hg_(1−x)Cd_(x)Te is grown on CdTe, suggesting that the defects are nucleated within the CdTe layers.     

IBIC characterization of charge transport in CdTe:Cl

Studies of charge transport uniformity in bulk CdTe:Cl have been carried out using ion-beam-induced charge (IBIC) imaging. High resolution maps of charge collection efficiency, mobility-lifetime product (μτ), and drift mobility (μ) were measured using a scanning microbeam of 2 MeV protons focused to a beam diameter of ∼3 μm. Excellent charge transport uniformity was observed in single crystal CdTe:Cl, with electron μτ values of up to 5 × 10⁻³ cm²/V s. The presence of extended defects such as tellurium inclusions was also studied using IBIC, and their influence on the charge transport performance of CdTe detector structures is discussed. The text was submitted by the authors in English.     

High-Quality (211)B CdTe on (211)Si Substrates Using Metalorganic Vapor-Phase Epitaxy

High-quality (211)B CdTe buffer layers are required during Hg_1−x Cd _x Te heteroepitaxy on Si substrates. In this study, direct metalorganic vapor-phase epitaxy (MOVPE) of (211)B CdTe on Si, as well as CdTe on Si using intermediate Ge and ZnTe layers, has been achieved. Tertiary butyl arsine was used as a precursor to enable As surfactant action during CdTe MOVPE on Si. The grown CdTe/Si films display a best x-ray diffraction rocking-curve full-width at half-maximum of 64 arc-s and a best Everson etch pit density of 3 × 10⁵ cm⁻². These values are the best reported for MOVPE-grown (211)B CdTe/Si and match state-of-the-art material grown using molecular-beam epitaxy.     

Nature of the donor action of Gd impurity in crystals of lead and tin telluride

Electron spin resonance and the Hall effect are investigated in n-Pb_1−x Sn_xTe:Gd crystals grown from melt. It is found that there is no direct correlation between the free electron density and the density of the Gd³⁺ impurity in these crystals. The conclusion is drawn that the the electron conductivity of Pb_1−x Sn_xTe:Gd crystals is not caused by the Gd impurity but by intrinsic defects of the crystal lattice which have zero activation energy due to the Gd impurities. Fiz. Tekh. Poluprovodn. 32, 1331–1333 (November 1998)     

Effect of the concentration of uncompensated impurities on the properties of CdTe-based X- and γ-ray detectors

Measurements of the ⁵⁵Fe-isotope emission spectra and the photosensitivity of CdTe detectors with a Schottky diode, and also the temperature dependence of the resistivity of a CdTe crystal ((2–3) × 10⁹ Ωcm at 300 K) have been used to determine the concentration of uncompensated donors (1–3) × 10¹² cm⁻³. Similar measurements performed for Cd_0.9Zn_0.1Te crystals with the resistivity (3–5) × 10¹⁰ Ω cm at 300 K have shown that the concentration of uncompensated donors in this case is lower by approximately four orders of magnitude. The results of calculations show that, due to such a significant decrease in the concentration of uncompensated donors, the efficiency of X- and γ-ray radiation detection in the photon energy range 59 to 662 keV can decrease by one-three orders of magnitude (depending on the photon energy and the lifetime of charge carriers in the space-charge region). The results obtained account for the apparent poor detecting properties of the Cd_0.9Zn_0.1Te detectors.     

Development of the high-pressure electro-dynamic gradient crystal-growth technology for semi-insulating CdZnTe growth for radiation detector applications

The high-pressure electro-dynamic gradient (HP-EDG) crystal-growth technology has been recently developed and introduced at eV PRODUCTS to grow large-volume, semi-insulating (SI) CdZnTe single crystals for room-temperature x-ray and gamma-ray detector applications. The new HP growth technology significantly improves the downstream CdZnTe device-fabrication yield compared to earlier versions of the HP crystal-growth technology because of the improved structural and charge-transport properties of the CdZnTe ingots. The new state-of-the-art, HP-EDG crystal-growth systems offer exceptional flexibility and thermal and mechanical stability and allow the growth of high-purity CdZnTe ingots. The flexibility of the multi-zone heater system allows the dynamic control of heat flow to optimize the growth-interface shape during crystallization. This flexibility combined with an advanced control system, improved system diagnostics, and realistic heat-transport modeling provides an excellent platform for continuing process development. Initial results on large-diameter (140 mm), SI Cd_1−xZn_xTe (x=0.1) ingots grown in low temperature gradients with the HP-EDG technique show reduced defect density and complete elimination of ingot cracking. The increased single-crystal yield combined with the improved charge transport allows the fabrication of large-volume, high-sensitivity, high energy-resolution detector devices at increased yield. The CdZnTe ingots grown to date produced large-volume crystals (≥1cm³) with electron mobility-lifetime product (μτ_e) in the (3–7) × 10⁻³ cm²/V range. The lower-than-desired charge-transport uniformity of the HP-EDG CdZnTe ingots is associated with the high density of Te inclusions formed in the ingots during crystallization. The latest process-development efforts show a reduction in the Te-inclusion density, an increase of the charge-transport uniformity, and improved energy resolution of the large-volume detectors fabricated from these crystals.     

Cyclic Annealing During Metalorganic Vapor-Phase Epitaxial Growth of (211)B CdTe on (211) Si Substrates

High-quality (211)B CdTe buffer layers on Si substrates are required to enable Hg_1–x Cd _x Te growth and device fabrication on lattice-mismatched Si substrates. Metalorganic vapor-phase epitaxy (MOVPE) of (211)B CdTe on Si substrates using Ge and ZnTe interlayers has been achieved. Cyclic annealing has been used during growth of thick CdTe layers in order to improve crystal quality. The best (211)B CdTe/Si films grown in this study display a low x-ray diffraction (XRD) rocking-curve full-width at half-maximum (FWHM) of 85 arcsec and etch pit density (EPD) of 2 × 10⁶ cm⁻². These values are the best reported for MOVPE-grown (211) CdTe/Si and are comparable to those for state-of-the-art molecular beam epitaxy (MBE)-grown CdTe/Si.     

Tellurium-rich growth and laser fabrication of lead-tin-telluride (Pb1−xSnxTe: 0.06<x<0.08)

Single crystals of Pb_1−x Sn_x Te (0.06<x<0.08) have been grown by using an ingot-nucleation technique from a Te-rich source. The as-grown crystals have a p-type carrier concentration around 10¹⁹ cm⁻³ and dislocation density as low as 10³ cm⁻². Diode lasers fabricated from these crystals have contact resistances of 2×10⁻⁵ Ω-cm² and a single-mode single-ended output power of 750 μW at heat sink temperatures around 15 K.     

Effect of superstoichiometric components on the spectral and kinetic characteristics of the luminescence of ZnSe crystals with isovalent impurities

The spectral and kinetic parameters of the X-ray luminescence of ZnSe crystals doped with Zn, Se, and Te were investigated during the growth process at temperatures in the range 80–500 K, and also after annealing in Zn vapor. ZnSe crystals grown from a stoichiometric mixture, or mixture containing chalcogenide impurities, typically produce the minimum level of afterglow and a rapid rise of X-ray luminescence, as well as a shift of its peak from the infrared region toward shorter wavelengths after annealing in zinc. ZnSe crystals grown from material with excess of Zn have a relatively low X-ray luminescence yield and a substantial level of afterglow. It is assumed that the growth of Te-activated crystals is accompanied by the development of thermally stable complexes of the form V _ZnTe_Se that act as radiative recombination centers. The introduction of excess Zn into the initial mixture produces a reduction in the concentration of V _Zn and, hence, in the concentration of radiative recombination centers. It is shown that, for free-electron concentrations n<10¹⁸ cm⁻³, the afterglow time constant τ can be described as a function of n by a model of radiative recombination that involves a single impurity level, whereas for n>10¹⁸ cm⁻³, the time constant decreases with increasing n, which cannot be explained in terms of the simple model. It is suggested that radiative recombination centers of a new type are produced as a result of prolonged annealing in Zn vapor. Fiz. Tekh. Poluprovodn. 31, 1211–1215 (October 1997)     

Electrical properties and compositional distributions of CVT and PVT grown Hg1−xCdxTe epilayers

Epitaxial layers of Hg_1−xCd_x Te were grown on CdTe substrates by the chemical vapor transport technique using Hgl₂ as a transport agent. The epilayers were of nearly uniform composition both laterally and to a depth of about one-half of the layer thickness. By comparison, the composition varied continuously throughout the depth of the layer for epilayers grown by the physical vapor transport technique. Layers were grown both p- and n-type with carrier concentrations on the order of 10¹⁷ cm⁻³. Low-temperature annealing was used to convert the p-type layers into n-type. The room-temperature carrier mobilities of as-grown and converted n-type layers ranged from 10³ to 10⁴ cm²/V-s depending on the composition and are comparable to previous literature values for undoped Hg_1−xCd_xTe crystals.     

Effects of excess tellurium on the properties of CdZnTe radiation detectors

Room-temperature radiation detectors have been fabricated on high-resistivity, indium-doped Cd_0.90Zn_0.10Te crystals grown under different amounts of excess Te. The effects of the excess Te on the properties of the detectors are explained by a simple model using only three parameters: the density of Cd vacancies, the density of Te antisites (Te at Cd sites), and the deep level of doubly ionized Te antisites. The best detectors, which can resolve the low-energy Np-L and Te-K peaks as well as Cd and Te escape peaks of ²⁴¹Am, are produced from crystals grown with 1.5% excess Te. The detectors fabricated from crystals grown without excess Te are unable to resolve any characteristic-radiation peaks of ²⁴¹Am and ⁵⁷Co. This result is explained by a model of networked p-type domains in an n-type matrix or vice versa, which is caused by the lack of sufficient deep-level Te antisites. Such conduction-type inhomogeneity causes massive electron and hole trapping. As for the detectors fabricated from Cd_0.90Zn_0.10Te crystals grown with 2% and 3% excess Te, they are able to resolve the ²⁴¹Am 59.5-keV, ⁵⁷Co 122-keV, and ⁵⁷Co 136-keV radiation peaks. However, the full-width at half-maximum (FWHM) values of these peaks are broadened, especially the high-energy ⁵⁷Co peaks. These phenomena are attributed to the hole and, possibly, electron trapping by Cd vacancies and Te antisites, respectively. The result of the analysis indicates that sufficient Te antisites and a low density of carrier traps in Cd_0.90Zn_0.10Te are essential for producing high-quality radiation detectors. In the analysis, it was discovered that most of the excess Te, on the order of 1–2 × 10²⁰ cm⁻³, remain electrically inactive. A possible explanation for this phenomenon is that the excess Te atoms form neutral Te-antisite and Cd-vacancy complexes, such as Te_Cd·(V_Cd)², during the post-growth cooling process.