High Pressure Bridgman Research Articles

CdTe synthesis and crystal growth using the high-pressure Bridgman technique

Abstract Efficient, safe and cost-effective synthesis of CdTe from elements is rather challenging in silica sealed ampoules due to the high vapor pressure of Cd. In this article, we report on the integrated synthesis and crystal growth of high-purity CdTe using the high pressure Bridgman (HPB) technique that is scalable to large volumes. The process lends itself for cost competitive industrial production of polycrystalline feedstock material for photovoltaics, sensors and electro-optic applications. Cadmium telluride (CdTe) crystals exceeding 1 kg in size were synthesized from elemental Cd and Te sources with purity comparable to state-of-the-art gamma ray detector crystals. In addition, synthesis of highly-doped CdTe feedstock for thin film photovoltaics applications demonstrating effective incorporation of group V (As, Sb) dopants was achieved at growth speeds of ~500 mm/hr. The technique may be applicable to produce other II-VI compounds with volatile components.

Detailed studies of pixelated CZT detectors grown with the modified horizontal Bridgman method

The detector material Cadmium Zinc Telluride (CZT) achieves excellent spatial resolution and good energy resolution over a broad energy range, several keV up to some MeV. Presently, there are two main methods to grow CZT crystals, the Modified High-Pressure Bridgman (MHB) and the High-Pressure Bridgman (HPB) process. The study presented in this paper is based on MHB CZT substrates from the company Orbotech Medical Solutions Ltd. [Orbotech Medical Solutions Ltd., 10 Plaut St., Park Rabin, P.O. Box 2489, Rehovot, Israel, 76124]. Former studies have shown that high-work-function materials on the cathode side reduce the leakage current and, therefore, improve the energy resolution at lower energies. None of the studies have emphasized on the anode contact material. Therefore, we present in this paper the result of a detailed study in which for the first time the cathode material was kept constant and the anode material was varied. We used four different anode materials: Indium, Titanium, Chromium and Gold, metals with work-functions between 4.1eV and 5.1eV. The detector size was 2.0×2.0×0.5cm3 with 8×8 pixels and a pitch of 2.46mm. The best performance was achieved with the low-work-function materials Indium and Titanium with energy resolutions of 2.0keV (at 59keV) and 1.9keV (at 122keV) for Titanium and 2.1keV (at 59keV) and 2.9keV (at 122keV) for Indium. Taking into account the large pixel pitch of 2.46mm, these resolutions are very competitive in comparison to those achieved with detectors made of material produced with the more expensive conventional HPB method. We present a detailed comparison of our detector response with 3D simulations. The latter comparisons allow us to determine the mobility-lifetime-products (μτ-products) for electrons and holes. Finally, we evaluated the temperature dependency of the detector performance and μτ-products. For many applications temperature dependence is important, therefore, we extended the scope of our study to temperatures as low as −30°C. There are two important results. The breakdown voltage increases with decreasing temperature, and electron mobility-lifetime-product decreases by about 30% over a range from 20°C to −30°C. The latter effect causes the energy resolution to deteriorate, but the concomitantly increasing breakdown voltage makes it possible to increase the applied bias voltage and restore the full performance.

Carrier transport properties of HPB CdZnTe and THM CdTe:Cl

Carrier drift mobilities of CdZnTe (CZT) grown by the high-pressure Bridgman (HPB) method and chlorine-doped CdTe grown by the traveling heater method (THM) were measured by a time-of-flight (TOF) technique. A spectrometer-grade crystal of HPB CZT shows room-temperature electron mobility of 960 cm/sup 2//Vs and hole mobility of 56 cm/sup 2//Vs. Both electron and hole mobility of THM CdTe:Cl crystals are higher (1100 cm/sup 2//Vs for electrons and 88 cm/sup 2//Vs for holes) than that of the HPB CZT crystal. Both materials show a saturation of the electron mobility at low temperature around 100 K and a strong decrease of the hole mobility with a lowering of the temperature. Theoretical mobility has been calculated by solving a Boltzmann transport equation assuming several scattering mechanisms such as polar optical phonon, ionized impurity, and alloy scattering. It is concluded from the comparison of the experimental and theoretical temperature dependence that shallow trap-controlled mobilities are observed in both spectrometer-grade CdZnTe and CdTe crystals.

Study of impurity segregation, crystallinity, and detector performance of melt-grown cadmium zinc telluride crystals

A review of growth methods used to produce Cd 1− x Zn x Te (CZT) (0.0< x<0.20) crystals for radiation detector applications is presented. Most of the results emphasize the high-pressure Bridgman (HPB) method. For selected melt-grown HPB ingots, the liquid/solid segregation coefficients of some impurities were measured. The correlation of the impurity content and nuclear detector performance will be discussed. Extended defects and surface and bulk crystallinity were measured using triple and double axis X-ray diffraction techniques (TAD and DAD XRD), X-ray topography, and infrared microscopy. X-ray diffraction maps and IR images were generated and compared to gamma-ray detector tests to correlate macroscopic defects with the nuclear detector responses. Defects states of CZT were also investigated using low-temperature photoluminescence spectroscopy. Comparisons between the material and detector properties for different CZT growth methods will be discussed.

Photoluminescence measurements on undoped CdZnTe grown by the high-pressure bridgman method

The low temperature photoluminescence of Cd0.91Zn0.09Te grown by the high-pressure Bridgman (HPB) method exhibits a neutral donor bound exciton emission (D0X) at 1.65603 eV with its excited state (D0X*) at 1.65798 eV and neutral acceptor bound exciton emissions (A0X) at 1.64566 eV and 1.65201 eV. Assuming a direct generation and subsequent relaxation of excitons at the D0X* state, we demonstrate that the temporal evolution of the above emission bands is well reproduced by a set of rate equations. The resultant radiative-lifetime of 1.4 ns for the D0X and 1.5 and 2.0 ns for the A0Xs are compared with various CdZnTe's (CZTs) grown by the other methods to demonstrate the particular nature of the HPB CZT.

High-pressure bridgman grown CdZnTe for electro-optic applications

The linear electro-optic response of high pressure Bridgman growth semi-insulating Cd0.9Zn0.1Te bulk samples has been characterized. Measurements have been performed in the third optical window for telecommunications around = 1.5 m. The dependence of the Pockels figure of merit on the modulation frequency and on the intensity of the optical probe beam is presented and discussed. Despite the residual optical absorption, Cd0.9Zn0.1Te is not affected by photo-generated auto-inhibition of the Pockels effect. This can be attributed to the action of an efficient intragap recombination center. It is, therefore, a suitable basic material for electro-optic switching of near-infrared beams at low frequency and quasi-continuous wave (cw) regimes. The implementation of a Cd0.9Zn0.1Te-based cross-bar switch for optical communication applications is also presented, which reaches −30 dB of extinction ratio and sub millisecond response time.

The performance effects of crystal boundaries in cadmium zinc telluride radiation spectrometers

The specific performance effects of extended crystal defects in cadmium zinc telluride (CZT) radiation spectrometers have been investigated. While it is generally accepted that defects such as grain boundaries and decorated twins can have a significant deleterious impact on the performance of semiconductor radiation spectrometers, the detailed mechanisms of degradation have been previously unexplored. Because of the difficulties of the high pressure Bridgman (HPB) growth process, CZT materials generally contain a large number of such defects, and the yield of defect-free crystals is comparatively low. High-resolution gamma-ray response mapping has been combined with analytical spectral simulations to relate the anomalies in the measured detector response to the likely originating defect. Examples of several different types of boundaries have been examined in a range of commercial CZT detectors, with the relevant parameters of the boundary being its trapping or channeling nature and the properties of the material on each side of the boundary.

Transport properties of undoped Cd0.9Zn0.1Te grown by high pressure bridgman technique

The electron drift mobility of undoped Cd0.9Zn0.1Te grown by high-pressure Bridgman method is measured by a time-of-flight technique. The sample shows a room temperature mobility and mobility lifetime product of 950 cm2/Vs and 1.6 × 10−4cm2/V, respectively. The mobility increases monotonically with decreasing temperature to 3000 cm2/Vs at 100 K. The dominant scattering mechanism for the electron transport is discussed by comparing with the theoretical mobility obtained by iterative solution of the Boltzmann equation.

Analysis of CZT crystals and detectors grown in russia and the ukraine by high-pressure bridgman methods

Sandia National Laboratories (SNL) is leading an effort to evaluate vertical high pressure Bridgman (VHPB) Cd1−xZnxTe (CZT) crystals grown in the former Soviet Union (FSU) (Ukraine and Russia), in order to study the parameters limiting the crystal quality and the radiation detector performance. The stoichiometry of the CZT crystals, with 0.04<x<0.25, has been determined by methods such as proton-induced x-ray emission (PIXE), x-ray diffraction (XRD), microprobe analysis and laser ablation ICP mass spectroscopy (LA-ICP/MS). Other methods such as triaxial double crystal x-ray diffraction (TADXRD), infrared transmission spectroscopy (IR), atomic force microscopy (AFM), thermoelectric emission spectroscopy (TEES) and laser induced transient charge technique (TCT) were also used to evaluate the material properties. We have measured the zinc distribution in a CZT ingot along the axial direction and also its homogeneity. The (Cd+Zn)/Te average ratio measured on the Ukraine crystals was 1.2, compared to the ratio of 0.9–1.06 on the Russian ingots. The IR transmission showed highly decorated grain boundaries with precipitates and hollow bubbles. Microprobe elemental analysis and LA-ICP/MS showed carbon precipitates in the CZT bulk and carbon deposits along grain boundaries. The higher concentration of impurities and the imperfect crystallinity lead to shorter electron and hole lifetimes in the range of 0.5–2 and 0.1 µs, respectively, compared to 3–20 and 1–7 µs measured on U.S. spectrometer grade CZT detectors. These results are consistent with the lower resistivity and worse crystalline perfection of these crystals, compared to U.S.-grown CZT. However, recently grown CZT from FSU exhibited better detector performance and good response to alpha particles.

CdTe and CdZnTe materials for room-temperature X-ray and gamma ray detectors

Among the semiconductor materials of a wide band gap, CdTe and CdZnTe have attracted most attention as room-temperature X-ray and gamma-ray detectors. Suitable CdTe materials for nuclear detectors and, in particular, for spectrometers, have been developed over the past few decades and are mainly grown via the traveling heater method (THM). However, the manufacture of large homogeneous ingots at relatively low cost has not reached yet a proven stage. Cd 1– x Zn x Te (CZT) materials, mainly grown via the high-pressure Bridgman (HPB) technique, possess several advantages over CdTe and appear to better approach the practicality of providing large volume X-ray and gamma-ray detectors at moderate costs. Continuing effort is still underway to improve the characteristics of both CdTe and CZT materials in order to achieve reproducible detectors for either low- and high-energy gamma rays. This review paper is divided into three parts: The first part describes different structural designs of detectors to improve their spectroscopic characteristics. These include hemispherical detectors, coplanar strip-electrode detectors and monolithic, two-dimensional segmented electrode arrays with pad sizes smaller than their thickness. This part will also describe various electronic methods to compensate for the poor charge collection of holes. The second part compares the characteristics of planar CdTe and CZT nuclear detectors containing metal contacts. Characteristics include: charge collection efficiencies for both electrons and holes indicated by the mobility-lifetime product, energy resolutions, leakage currents and robustness in field use. The third part is devoted to field uses of these detectors. Those include: X-ray fluorescent spectrometers, large volume spectrometers and a new generation nuclear gamma camera for medical diagnostics based on room-temperature solid-state spectrometers.

Methodology for evaluation of mobility-lifetime product by spectroscopy measurements in CdZnTe spectrometers

A novel methodology is presented for extraction of the semiconductor electron and hole mobility-lifetime products by x- and γ-ray spectroscopy analysis. The methodology is based on the analysis of spectroscopy results, namely the maximal and the average charge collection efficiencies as discussed below, for different photon energies at various bias voltages. The methodology enables the evaluation of both electron and hole mobility-lifetime products without the need to use α particle measurements. The evaluation is carried out by a single parameter fitting of the models analyzed in this study. Mobility-lifetime products for CdZnTe substrates grown by Bridgman and high pressure Bridgman (HPB) methods are reported and compared. Typical values of μe⋅τe=2⋅10−3 and μh⋅τh=1⋅10−5 cm2/V are extracted for HPB grown CdZnTe with 10% Zn concentration. Values of μe⋅τe=9⋅10−4 and μh⋅τh=1⋅10−7 cm2/V are obtained for CdZnTe with 10% Zn concentration grown by the Bridgman method.

Linear x-ray detector array made on bulk CdZnTe for 30∼100 keV energy

A CdZnTe detector grown by the high pressure Bridgman (HPB) growth technique was tested using high energy x-rays (30∼100 keV), and the performance was compared with a commercially available Nal scintillating detector of 5 cm thickness. The charge collection efficiency of a CdZnTe detector is as high as 90% at relatively low electric field, 600 V/cm. At high x-ray photon energies, the detection efficiency is reduced due to the thickness of the CdZnTe. A 32 channel linear array was fabricated on 1.2∼1.7 mm thick CdZnTe, of which the detector area was 175 × 800 µm2 and the pitch size 250 µm. The measured dark current for the 16 element detector was as low as 0.1 pA at 800 V/cm with an excellent uniformity. Energy spectra were measured using a Co57 radiation source. A small pixel effect and charge sharing were observed. The energy resolution was improved and compared with the large area detector. The array detector gave an average 5.8% full-width-half-maximum (FWHM) at 122 keV photopeak. The large area detector of the same material before fabrication exhibited a low energy tail at the photopeak, which limits the photopeak FWHM to 8%.

Trapping properties of cadmium vacancies inCd1−xZnxTe

The trapping and thermal emission of holes were studied from a deep acceptor level created during thermal annealing of a ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Zn}}_{\mathrm{x}}$Te (x=0.12) crystal grown by the high-pressure Bridgman (HPB) technique using thermoelectric-effect spectroscopy and thermally stimulated current experiments. The deep level, which is usually absent in as-grown HPB ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Zn}}_{\mathrm{x}}$Te crystals, is assigned to the 2-/- acceptor level of Cd (Zn) vacancies. The thermal ionization energy of the level is ${\mathrm{E}}_{\mathrm{th}}$ =(0.43\ifmmode\pm\else\textpm\fi{}0.01) eV, and the trapping cross section of holes was found to be \ensuremath{\sigma}=(2.0\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$.

Recent progress in Cd 1− xZn xTe radiation detectors

Recent progress in the growth of Cd 1− x Zn x Te (CZT) using the High Pressure Bridgman (HPB) method has demonstrated the potential for greater use of this material in a wide range of applications. Improvements have been made in the preparation of detector blanks, electrical contacts and mounting techniques that have led to improvements in energy resolution, especially at higher photon energies. Most recent studies have indicated that the performance of the material at low (< 150 keV) energies can be significantly improved by cooling the detector to below −30°C with a Thermo-Electric (TE) cooling element. Large volume/large area detectors have been produced, making use of both the latest developments and existing techniques of single polarity charge carrier sensing. Finally, examples of progress in the area of “pixelated” detectors will be presented.

Properties of melt-grown ZnSe solid-state radiation detectors

Zinc Selenide (ZnSe) crystals grown using the High Pressure Bridgman (HPB) technique were used to fabricate solid-state radiation detectors measuring 10/spl times/10/spl times/2 mm/sup 3/. Sputtered platinum and gold contacts were applied to polished detector blanks. Voltage versus current characteristics were determined for the devices at 25/spl deg/C. Pulse height spectra were obtained using /sup 241/Am and /sup 109/Cd at both 25/spl deg/C and 150/spl deg/C with applied bias of 9000 V/cm. Current versus temperature was measured over the temperature range of 30/spl deg/C to 150/spl deg/C. Performance was measured at energies of 22.1 and 59.5 keV over a temperature range of -70/spl deg/C to 170/spl deg/C. Current versus dose rate was measured with 662 keV gamma irradiation. A value of the Mobility-Lifetime product (/spl mu//spl tau/) for electrons was estimated. Time and temperature dependence of photo-peak position using Pulse Height Analysis (PHA) was studied. >

Gamma- and X-ray detectors manufactured from [formula omitted] Te grown by a high pressure bridgman method

Results of recent efforts in the growth of Cd 1−x Zn x Te crystals by a high pressure Bridgman (HPB) method and their use in gamma- and X-ray detector applications are presented. Evidence for crystals of relatively low defect content include etch pit densities of ≤ 10 4 cm −2 , double crystal rocking curve linewidths of 10–15″ and sharp, bright emission lines with excitonic features in low temperature photoluminescence measurements. Resistivities in excess of 10 11 ohm cm are achieved without impurity doping. The resulting low leakage currents lead to good energy resolution, <6% at 59.5 keV for example. The dependence of leakage current on temperature from 233 K to 373 K implies a Fermi level at mid-gap for x=0.2. The results of flash X-ray experiments indicate that the high current sensitivity, low leakage current and good temporal response of Cd 1−x Zn x Te detectors make them attractive candidates for applications involving short pulses at high dose rates.

Properties of CdZnTe crystals grown by a high pressure Bridgman method

CdZnTe crystals have been grown over the full range of alloy composition by a high-pressure Bridgman (HPB) method. Use of the inert gas over pressure reduces loss of material from the melt, and permits growth without the use of sealed ampoules, extending the range of possible alloys, and allowing a choice of crucible materials. The HPB method yields high-quality CdZnTe and enables room-temperature gamma radiation detectors to be made over large area wafers of undoped material for the first time.