Cd Overpressure Research Articles

Partial pressure of Cd in equilibrium with stochiometric melt and solid of (Cd0.8Zn0.2)Te between 300 °C and 1250 °C for melt growth

The partial pressures over Cd0.8Zn0.2Te solid and melt were measured by the optical absorption technique on five optical cells loaded with four homogenized samples and one with as-grown crystal. From the measured partials pressures, the three-phase loops with five solidus and four liquidus points were established together with the Cd partial pressure over the melts. A series of crystal growth experiments by directional solidification of Cd0.8Zn0.2Te with controlled Cd overpressure, provided by Cd reservoir temperatures from 750 to 935 °C, were conducted. From the Te precipitates content observed by the IR micrographs, the correlation between the Cd reservoir temperature and the Cd0.8Zn0.2Te melt temperature, which resulted in the lowest precipitate density, was established. From the calculated masses of the elements in the vapor phase, the compositions of the condensed phases for each set of partial pressure data were derived. The correlation between the temperature of the Cd reservoir and the temperature of a stoichiometric Cd0.8Zn0.2Te sample, such that the equilibrium Cd overpressure is coexisted, have been established for the Cd0.8Zn0.2Te melt and the solid between 300 and 1250 °C. Using this information, the temperatures of the Cd reservoir and the Cd0.8Zn0.2Te sample can be programed during the melt growth to process intrinsic/stochiometric crystals. Additionally, the correlation on the equilibrium Cd overpressure environment for the Cd0.8Zn0.2Te solid can also be employed for the heat treatment of post-growth annealing to adjust the composition of the grown crystals toward stoichiometry.

Growth Process of Cd0.8Zn0.2Te Crystal to Enhance its Performance as Detector for High-energy Radiation

Introduction: In the applications of room temperature detector for high-energy radiation, there are two critical requirements for the semiconducting material cadmium zinc telluride (CdZnTe): (1) high electrical resistivity to reduce the bulk leakage current and (2) low levels of structural defects which hinder the detectivity as trapping and recombination centers for the carriers. To enhance the performance of the detectors, an optimal single process has been developed in the melt growth of Cd0.8Zn0.2Te by directional solidification with controlled Cd overpressure to maximize the electrical resistivity as well as minimize the structural defects, including Te precipitates/inclusions of the grown CdZnTe crystals. Method: Using the phase diagram data of pressure-temperature-composition (P-T-X), melt growth of Cd0.8Zn0.2Te crystals by directional solidification from a starting melt at 1145oC has been performed with various Cd overpressures controlled by the temperature of a Cd reservoir. The grown crystals were sliced and were characterized by electrical resistivity measurements and chemical analysis of Glow Discharge Mass Spectroscopy (GDMS). The structural defects were studied by the infrared (IR) transmission images taken by an IR microscope. Result: By doping of In (4 – 6 ppm, atomic) and growing with a Cd reservoir in the range of 785 to 825oC, the electrical conductivity were consistently higher than 109cm and up to 2x1011cm. From the trend of the Te precipitates density observed by the IR micrographs, it was concluded that a Cd reservoir temperature of 820+10oC resulted in the lowest precipitate density. Conclusion: The employment of a Cd reservoir temperature of 820+5 oC during the growth process will provide the optimal Cd pressure over the melt at 1145oC to maximize the electrical resistivity as well as minimize the structural defects, including Te precipitates/inclusions of the grown Cd0.8Zn0.2Te crystals. Discussion: Since the solids of different compositions, x in the Cd1-xZnxTe system, have different liquidus/solidus temperatures as well as different homogeneity ranges, the procedure presented here for the Cd0.8Zn0.2Te solid may not applicable to other compositions.

Enhanced p-Type Doping in Polycrystalline CdTe Films: Deposition and Activation

An in situ nonequilibrium method to increase hole density in polycrystalline CdTe thin films to 1016 cm−3 using group V substitution on Te is presented. Single-phase CdTe films doped with P, As, and Sb were deposited at 550 °C at 100–200 nm/s onto moving cadmium sulfide/high resistance transparent buffer layer/transparent conductive oxide /glass superstrates by vapor transport deposition in Cd overpressure from high purity compound sources. Doping levels before and after activation were determined by capacitance-voltage analysis of diagnostic devices. Secondary ion mass spectrometry depth profiling confirmed dopant incorporation levels of 1017–1018 atoms/cm3 in as-deposited films. Electronic activation was carried out by post-deposition annealing in Cd or CdCl2 vapor with fast cooling, increasing acceptor concentrations to >1015 cm−3 for P and >1016 cm−3 for As and Sb, compared with mid −1014 cm−3 acceptor levels for undoped CdTe films. The activation methods are compatible with post-deposition processing presently used for high-efficiency CdTe solar cells. For the dopants As and Sb, the acceptor concentration increased by substitutional AsTe and SbTe formation, respectively, which was validated by cathodoluminescence spectroscopy.

Study of thin film poly-crystalline CdTe solar cells presenting high acceptor concentrations achieved by in-situ arsenic doping

Doping of CdTe using Group-V elements (As, P, and Sb) has gained interest in pursuit of increasing the cell voltage of CdTe thin film solar devices. Studies on bulk CdTe crystals have shown that much higher acceptor concentration than the traditional copper treatment is possible with As, P or Sb, enabled by high process temperature and/or rapid thermal quenching under Cd overpressure. We report a comprehensive study on in-situ As doping of poly-crystalline CdTe solar cells by MOCVD, whereby high acceptor densities, approaching 3 × 1016 cm−3 were achieved at low growth temperature of 390 °C. No As segregation could be detected at grain boundaries, even for 1019 As cm−3. A shallow acceptor level (+0.1 eV) due to AsTe substitutional doping and deep-level defects were observed at elevated As concentrations. Devices with variable As doping were analysed. Narrowing of the depletion layer, enhancement of bulk recombination, and reduction in device current and red response, albeit a small near infrared gain due to optical gap reduction, were observed at high concentrations. Device modelling indicated that the properties of the n-type window layer and associated interfacial recombination velocity are highly critical when the absorber doping is relatively high, demonstrating a route for obtaining high cell voltage.

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.

Experimental and theoretical comparison of Sb, As, and P diffusion mechanisms and doping in CdTe

Fundamental material doping challenges have limited CdTe electro-optical applications. In this work, the As atomistic diffusion mechanisms in CdTe are examined by spatially resolving dopant incorporation in both single-crystalline and polycrystalline CdTe over a range of experimental conditions. Density-functional theory calculations predict experimental activation energies and indicate that As diffuses slowly through the Te sublattice and quickly along GBs similar to Sb. Because of its atomic size and associated defect chemistry, As does not have a fast interstitial diffusion component similar to P. Experiments to incorporate and activate P, As, and Sb in polycrystalline CdTe are conducted to examine if ex situ Group V doping can overcome historic polycrystalline doping limits. The distinct P, As, and Sb diffusion characteristics create different strategies for increasing hole density. Because fast interstitial diffusion is prominent for P, less aggressive diffusion conditions followed by Cd overpressure to relocate the Group V element to the Te lattice site is effective. For larger atoms, slower diffusion through the Te sublattice requires more aggressive diffusion, however further activation is not always necessary. Based on the new physical understanding, we have obtained greater than 1016 cm−3 hole density in polycrystalline CdTe films by As and P diffusion.

In Situ Arsenic Doping of CdTe/Si by Molecular Beam Epitaxy

p-Type doping of the absorbed layer has been a significant challenge for CdTe solar cells. In this work, we report on in situ arsenic doping of molecular beam epitaxy (MBE) CdTe grown on Si(211) and the use of a cadmium overpressure to enhance incorporation. When growing CdTe:As without a Cd overpressure, extremely high As fluxes are required to achieve noticeable amounts of arsenic incorporation. By supplying a Cd flux during growth, the As incorporation increases by an order of magnitude. By including a Cd overpressure during growth, we have obtained single-crystal CdTe:As films with As incorporation concentration of \(1\times 10^{17} \, \mathrm {\rm cm}^{-3}\). An activation anneal was performed on these films in a rapid thermal annealing furnace, resulting in p-type layers with net carrier concentration of ∼5 × 1016 cm−3.

Radiative recombination mechanisms in CdTe thin films deposited by elemental vapor transport

Abstract A photoluminesence (PL) study of the radiative recombination mechanisms for CdTe films deposited under different Cd and Te overpressure by elemental vapor transport is presented. The experiment and analysis have been carried out in the temperature range of 12–130 K. The intensity of the PL laser excitation beam was varied by two orders of magnitude. It has been established that the bands in the 1.47–1.50 eV are determined by transitions involving shallow D and A states and the 1.36x–1.37x eV band is due to band to level transitions. Deep transitions at 1.042 eV and 1.129 eV are due to radiative transitions to levels determined by CdTe native defects.

Prismatic punching defects in CdTe compounds

The origin and nature of star-shaped prismatic punching defects (PPDs) were analyzed by white-beam X-ray diffraction topography (WBXDT) and photoluminescence (PL) measurements. PPDs were observed in CdTe-based compounds that had Te-rich secondary phase defects and were annealed only under Cd overpressure. The evolutional features of the PPDs during annealing were similar to those of a diffusion process, that is, the in-diffusion of Cd vapor via Cd vacancies. PL mapping around the PPDs reveals that the stoichiometry of PPDs is the same as that of a normal matrix, and that PPDs are just structural defects with non-radiative emission induced by the exothermic reaction between Cd and Te. We find that Te-rich secondary phase defects can be removed without the generation of star-shaped PPDs, if the exothermic reaction rate can be slowed down to reduce the thermal gradient between the Te-rich region and the surrounding matrix.

Mercury cadmium selenide for infrared detection

Samples of HgCdSe alloys were grown via molecular beam epitaxy on thick ZnTe buffer layers on Si substrates. Two Se sources were used: an effusion cell loaded with 5N source material that produced a predominantly Se6 beam and a cracker loaded with 6N material that could produce a predominantly Se2 beam. The background electron concentration in as-grown samples was significantly reduced by switching to the Se cracker source, going from 1017–1018 cm−3 to 3–5 × 1016 cm−3 at 12 K. The concentration remained low even when the cracking zone temperature was lowered to produce a predominantly Se6 beam, which strongly suggests that a major source of donor defects is impurities from the Se source material rather than Se species. Secondary ion mass spectroscopy was performed. Likely donors such as F, Br, and Cl were detected at the ZnTe interface while C, O, and Si were found at the interface and in the top 1.5 μm from the surface in all samples measured. The electron concentration for all samples increased when annealed in a Cd or Hg overpressure and decreased when annealed under Se. This suggests the presence of native defects such as vacancies and interstitials in addition to impurities. Overall, by switching to higher purity Se material and then annealing under Se overpressures, the background electron concentration was reduced by an order of magnitude, with the lowest value achieved being 9.4 × 1015 cm−3 at 12 K.

Semi insulating CdTe:Cl after elimination of inclusions and precipitates by post grown annealing

We present in this contribution results of two-step annealing, when the CdTe:Cl doped samples are at first annealed under Cd overpressure to remove inclusions and the re-annealed under Te overpressure to restore the high resistivity state. Investigation of samples after Cd rich annealing by infrared microscope has proven, that all inclusions are removed. Also Te nano precipitates were strongly influenced by the annealing process. The resistivity of the samples after Te-rich annealing was restored to values ( ∼ 108-109Ωcm). We observed, however, decrease of mobility-lifetime product of electrons from 10−3cm2/Vs to 10−4cm2/Vs. In order to understand the reason of this decrease we performed a study of point defects before and after annealing by thermoelectric effect spectroscopy. It shows a decrease of concentrations of most deep levels after two-step annealing. This behavior is completely different compared to past annealing studies, where concentration of deep levels strongly increased after annealing. The only level with an increased concentration in the current study is the midgap level (E ∼ 0.8 eV). At the same time we observed increase of micro-twins in the samples investigated by transmission electron microscopy. The decrease of charge collection efficiency after two-step annealing may be therefore connected with re-arrangement of near midgap levels due to increase of concentrations of structure defects (micro twins, dislocations) that accumulate in their surroundings point defects with energy ∼ 0.75 eV.

Temperature-gradient annealing of CdZnTe under Te overpressure

The performance of CdZnTe (CZT) detectors is limited not only by conventional carrier-trapping to point defects but also by trapping at macroscopic Te secondary-phase defects, such as Te inclusions and Te precipitates. The aim of this research is to remove these secondary-phase defects via thermomigration, and to obtain high resistivity of the material by creating Te antisites through annealing in a high Te overpressure. We annealed Te-rich CZT samples in the temperature range between 500 and 700°C under Te overpressure with a temperature gradient of 50–60°C/cm. We investigated the effects of annealing under these conditions by IR transmission microscopy, current–voltage measurements, photoluminescence, and white-beam X-ray diffraction topography (WBXDT) measurements comparing the findings with those from isothermally annealed CZT samples under Cd overpressure. We proved experimentally that Te inclusions present in Te-rich CZT melts contain void fractions. We attributed the complex defect of Te antisites with Cd vacancies in our annealing experiment as the deep level defect that pins the Fermi near the middle of the bandgap, rather than the Te antisite.

Effect of ${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te\!:\!In}$ Crystals Annealing on Their High-Temperature Electrical Properties

We studied the electrical properties of Cd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Te:In (CZT) single crystals with [In]=3*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> at/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> at its high-temperature point-defect (PD) equilibrium state under a Cd overpressure (P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cd</sub> ). We detailed the influence of thermal treatment and the deviation of stoichiometry on the electron concentration, observing unexpectedly high conductivity and an increase in free-electron density ( ~ 1.5-2 orders of magnitude) when annealing the sample at 770 K under a Cd vapor pressure (0.01 atm.). Prolonged exposure of the samples under these conditions lowered the electron density by two approximately orders-of-magnitude until it approached the intrinsic value. The electron mobility after such treatment increased to CZT's maximal values at ~ 460 K (650-700 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /(V*s)). Therefore, such annealing can be effective in assuring high-resistive CZT detectors after crystal growth, or by special treatment, thereby eliminating the inclusions. We analyzed these data in the framework of Kröger's theory of quasi-chemical reactions, and compared the findings to those obtained for undoped CdTe.

Inclusions Elimination and Resistivity Restoration of CdTe:Cl Crystals by Two-Step Annealing

The aim of this work was to find an effective annealing treatment leading to a complete elimination of Te inclusions and simultaneously to conservation of the resistivity of CdTe:Cl crystals with varying chlorine doping level. This goal was reached by an application of two post-growth annealing steps. The first annealing step performed under Cd overpressure led to a significant reduction of Te inclusions. However, Cd-rich thermal treatment converted crystals into high-conductive n-types and re-annealing of the material in a Te atmosphere was necessary to restore the material resistivity. After application of the second annealing step under Te overpressure, resistivity higher than 108 Ωcm was observed only in samples with high chlorine doping levels of nCl ≈ 2 × 1017 cm-3. In addition, a new emission at 1.26 eV was observed in the photoluminescence spectra after two-step annealing. The size of Te inclusions, resistivity, photoluminescence spectra and detection properties of CdTe:Cl crystals before and after the two-step thermal treatment are discussed.

Melt growth of high-resistivity CdZnTe crystals by controlling Cd over-pressures

In this study, Cd 1− x Zn x Te crystals, with x=0.20, were grown unseeded by closed-ampoule vertical directional solidification (Bridgman) technique under controlled Cd over-pressures. CdZnTe crystals of high electrical resistivity have been reproducibly obtained with the In dopant concentration of 4–6 ppm, atomic and a Cd reservoir temperature between 785 and 825 °C. A combination of three process modifications have contributed to the accomplishment: (1) homogenizing the ampoules of starting materials under the vacuum environment to minimize contamination, (2) performing growth under controlled Cd over-pressures with the optimal In dopant, and (3) controlling post-growth cooling procedures to maintain uniformity of electrical properties over the crystal.

Defect levels and thermomigration of Te precipitates in CdZnTe:Pb

Semi-insulating Cd 0.9Zn 0.1Te:Pb crystals were grown by the vertical Bridgman method. Measurements of the current deep level transient spectroscopy (I-DLTS) revealed three trap levels in this material. Unlike other compensating dopants, CdZnTe:Pb samples do not show any Cd-vacancies defects and A-center levels. We subjected them to temperature-gradient annealing in Cd overpressure at 490–717 °C, and recorded an exponential relationship between the annihilation time of Te precipitates and the annealing temperature. The energy resolution of an annealed CdZnTe:Pb detector, tested using a 137Cs radioactive source, gave an energy resolution of 2.5%.

Preparation of Inclusion and Precipitate Free Semi-Insulating CdTe

Annealing conditions used for the preparation of semi-insulating CdTe after elimination of inclusions and precipitates are discussed. Second phase defects are eliminated by post-growth stoichiometric annealing and the semi-insulating CdTe is prepared by re-annealing under Te or Cd overpressure. In case of In-doped samples with high In concentration, semi-insulating CdTe with resistivity approximately 109 Omegacm was prepared by annealing under Te overpressure. In-situ high temperature measurement of the electrical conductivity and Hall coefficient is used to find annealing conditions for undoped and slightly In-doped CdTe. The dominant extrinsic acceptor level with a concentration of NA = 4 times 1015 cm-3 was determined and Cd-rich annealing was used for the preparation of semi-insulating CdTe.

Component Overpressure Growth and Characterization of High-Resistivity CdTe Crystals for Radiation Detectors

Spectrometer-grade CdTe single crystals with resistivities higher than 109 Ω cm have been grown by the modified Bridgman method using zone-refined precursor materials (Cd and Te) under a Cd overpressure. The grown CdTe crystals had good charge-transport properties (μτe = 2 × 10−3 cm2 V−1, μτh = 8 × 10−5 cm2 V−1) and significantly reduced Te precipitates compared with crystals grown without Cd overpressure. The crystal growth conditions for the Bridgman system were optimized by computer modeling and simulation, using modified MASTRAPP program, and applied to crystal diameters of 14 mm (0.55′′), 38 mm (1.5′′), and 76 mm (3′′). Details of the CdTe crystal growth operation, structural, electrical, and optical characterization measurements, detector fabrication, and testing using 241Am (60 keV) and 137Cs (662 keV) sources are presented.

Numerical simulation of temperature and pressure fields in CdTe growth experiment in the Material Science Laboratory (MSL) onboard the International Space Station in relation to dewetting

A global thermal modelling of a cadmium telluride (CdTe) space experiment has been performed to determine the temperature field within the sample cartridge assembly of the Material Science Laboratory–low gradient furnace (MSL-LGF) apparatus. Heat transfer and phase change have been treated with a commercial CFD software based on a control volume technique. This work underlines the difficult compromise between enhancing the crystal quality and the occurrence of the dewetting phenomenon when using a Cd overpressure or inert gas in the ampoule.

Development of inclusion-free CdZnTe substrates from crystals grown by the vertical-gradient freeze method

With the aim of fabrication of (111) and (211) CdZnTe inclusion-free substrates for molecular-beam epitaxy (MBE) and liquid-phase epitaxy (LPE) growth of mercury cadmium telluride (MCT), we focused on fundamental research of the process of crystallization and cooling to room temperature. Based on the study of inclusion formation in dependence of Cd overpressure above the melt, an optimized process of crystal solidification was established. A key result of this study is the position of the Cd pressure, where inclusion-free crystals were fabricated without post-growth annealing. The crystals with a diameter of 100 mm and a height 40–50 mm were fabricated by the vertical-gradient freeze method (VGFM). The resulting ingots exhibit very good crystallographic quality, with a single-crystalline part filling 60–80% of the crystal volume. Substrates with orientation (111) and (211) and dimensions up to 4 × 4 cm were fabricated.