Cd1−xZnxTe Research Articles

A Scanning Probe Microscopy Study of Cd1−x Zn x Te

The effects of several ex vacuo methods used in the surface preparation of Cd1−x Zn x Te (CZT) have been studied using noncontact atomic force microscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy. Preparation techniques include mechanical lapping, hydroplane bromine-methanol polishing, and in vacuo annealing. The morphology, electrical homogeneity, and local density of states (LDOS) have been studied for each preparation method. Impurities and oxides quickly form on the surface after each preparation method. Annealing in ultrahigh vacuum causes the surface electronic structure to become inhomogeneous whilst the LDOS suggests a compositional change from an oxide surface to p-type CZT.

Study on Removal and Embedding Mechanism of CdZnTe Using Loose Abrasive

Cd1−xZnxTe (CZT) is an excellent ternary compound semiconductor. CZT is the most suitable substrate material for Hg1−yCdyTe epitaxial growth and can make the detector itself. The researchers have done a lot of works on the hard and brittle material removal mechanism in lapping process. however, no published articles are available regarding the removal mechanism and the abrasives embedding mechanism of the soft and brittle material in lapping progress .and there is its own characteristic if the hard abrasives machining the soft and brittle material in lapping progress, the objective of this paper is to build the physical model ,and divide the abrasive into four kinds of abrasives , they are two-body abrasive ,three-body abrasive ,embedded abrasive and small abrasive, and the authors analyze the effects on the material surface of the above abrasives. At last, design series of experiments, through analyzing the results and observe the surface morphology, the authors prove that the physical model is correct.

Characterization of Cd1−xZnxTe crystals grown from a modified vertical bridgman technique

Cd1−xZnxTe (CZT) crystals grown from a modified vertical Bridgman technique were characterized by means of an optical polarized transmission technique using the Pockels effect, low-temperature direct current (DC) photo-conductivity technique, low-temperature photoluminescence (PL) spectroscopy, room-temperature PL mapping technique, and detector performance measurements. Electric field mapping indicates that an approximation of a uniform electric field distribution approximation is generally satisfied for CZT detectors operated at room temperature under typical working conditions. A nonuniform electric field distribution is observed under intense infrared (IR) light illumination, and a model is proposed based on charge generation of defects, trapping, and space-charge effects. The largest hole mobility-lifetime product (μτ)h of the CZT detector measured by DC photoconductivity is 7.0 × 10−4 cm2/V. The detector treated with 2% bromine in methanol chemical etch has a relatively small surface recombination velocity at room temperature, which was obtained from DC photocurrent and detector performance tests, as measured by irradiation of 5.5-MeV α particles and 59.6-keV γ-rays, respectively. We have clearly shown the equivalence of charge collection efficiency results measured by both DC photocurrent and α particle response. Low-temperature DC photocurrent measurements show that surface recombination velocity increases significantly with decreasing temperature from 300 K to 250 K. The effective electron mobility-lifetime product—combination effects of bulk and surface of CZT crystal—increases with increment of temperature. Room-temperature PL mapping measurements indicate uniformity of zinc concentration within CZT crystals. Low-temperature PL spectroscopy shows that the dominant emission peaks are excitons, which are bound to either shallow neutral donors (D0, X) or neutral acceptors (A0, X), depending on the temperature, concentration of donors and acceptors, and the incident light intensity. It was found that the luminescence of (D0, X) depends linearly on the incident laser intensity, while (A0, X) has a nonlinear dependence.

Material quality characterization of CdZnTe substrates for HgCdTe epitaxy

Cd1−xZnxTe (CZT) substrates were studied to investigate their bulk and surface properties. Imperfections in CZT substrates affect the quality of Hg1−xCdxTe (MCT) epilayers deposited on them and play a role in limiting the performance of infrared (IR) focal plane arrays. CZT wafers were studied to investigate their bulk and surface properties. Transmission and surface x-ray diffraction techniques, utilizing both a conventional closed-tube x-ray source as well as a synchrotron radiation source, and IR transmission micro-spectroscopy, were used for bulk and surface investigation. Synchrotron radiation offers the capability to combine good spatial resolution and shorter exposure times than conventional x-ray sources, which allows for high-resolution mapping of relatively large areas in an acceptable amount of time. Information on the location of grain boundaries and precipitates was also obtained. The ultimate goal of this work is to understand the defects in CZT substrates and their effects on the performance and uniformity of MCT epilayers and then to apply this understanding to produce better infrared detectors.

Conductivity switching effect in Cd 1–x Zn x Te films

We report an experimental study of the conductivity switching effect in mono and polycrystalline layers of Cd1–xZnxTe grown by MBE on the Si(001)/Au and n+- GaAs(001) substrates. The current-voltage (I-V) characteristics of metal-semiconductor junctions show that the conductivity of the Cd1–xZnxTe structures exhibit two states: high- and low-resistive. Electrical pulses applied to the layers trigger the switching effect between these two states. The coefficient of resistivity switching is of the order of 104 while its reproducibility is repetitive about 100 switching cycles. The bistablity of the conductance in Cd1–xZnxTe layers supports the hypothesis of the ferroelectricity in this material. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A modified vertical Bridgman method for growth of high-quality Cd1−xZnxTe crystals

In order to deal with the phenomena of Cd evaporation from the raw materials and the heterogeneity caused by the larger-than-unity segregation coefficient of Zn in CdTe during the conventional vertical Bridgman method (VBM) growth of Cd1-xZnxTe (CZT), two modifications—Cd compensation and accelerated crucible rotation technique (ACRT)—are simultaneously adopted to the VBM. By a combination of VBM with the two modifications, several CZT ingots with the dimensions of ∼60×150 mm2 are grown. Structural, optical, and electrical characterization of the as-grown CZT crystals reveals that the application of Cd compensation and ACRT is of obvious efficiency in improving concentration homogeneity, reducing defect density, raising crystal quality and, therefore, upgrading the optoelectronic properties of CZT crystals. Nuclear spectra measurements of detectors fabricated from the as-grown crystals also indicate that both modifications can upgrade the detecting performance of CZT.

Cd desorption induced by Zn exposure during atomic layer epitaxy of CdxZn1–xTe

AbstractThe substitution of surface Cd atoms by Zn atoms during the growth of CdxZn1–xTe by atomic layer epitaxy has been studied as a function of substrate temperature and Zn exposure time by means of the analysis of the photoluminescence spectra of CdxZn1–xTe/ZnTe quantum wells. For substitution and desorption of Cd induced by Zn exposure an activation energy of 1.5 eV was found. The complexity of this desorption process was denoted by the fourth order of the kinetic reaction in the 250–280 °C temperature range. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cd1–xZnxTe high-resistive single crystals growth from a vapor phase

Cd1–xZnxTe high-resistive single crystals growth from a vapor phase

Infrared transmission spectra of Cd1−xZnxTe (x = 0.04) crystals

Three indicators (T1000, T5000, and T1000/T5000) are used to appraise the infrared (IR) transmission spectra for Cd1−xZnxTe (CZT) slices. By comparing the values of these three indicators, four typical types of IR spectra are characterized for CZT crystals. The CZT crystals possessing the four types of IR spectra are different in microstructures, especially the densities and sizes of Te precipitates, the free carrier concentrations, and the resistivities. Mechanisms for the elimination of tiny and dense Te precipitates are given by analyzing the variation of the IR transmittance in the range of 500–5000 cm−1 during the annealing process.

P–T–x–y phase diagram of the Cd–Zn–Te system

The p–T and T–x–y projections of the p–T–x–y phase diagram and its T–x–y isobars and x–y isobar–isothermal sections for the Cd–Zn–Te system were constructed. The character of the sublimation of the Cd1−xZnxTe (CZT) solid solution was discussed. It was shown that quasi–binary approximation is insufficient for CZT congruent transformations.

Study of surface recombination velocity of Cd1−xZnxTe radiation detectors by direct current photoconductivity

The surface recombination velocity of a Cd1−xZnxTe (CZT) radiation detector treated by mechanical polishing and by a standard 5% bromine in methanol chemical etch is reported. The light power dependence of the surface recombination velocity was measured using dc photoconductivity. The results reveal that the surface recombination velocity is a function of the electron generation rate, which can be described by a Shockley–Read one-center model. It was observed that the surface recombination velocity of the CZT detector treated by polishing only is much larger than that treated with polishing followed by chemical etching. The correlation of dc photoconductivity and low-temperature photoluminescence measurements of the CZT detector is also discussed.

First-principles calculations of the effective mass parameters of Al xGa1-xN and Zn xCd1-xTe alloys

First-principles calculations of electronic band structures of the ordered cubic alloys Al xGa1-xN and Cd xZn1-xTe are carried out. The band structures are used to provide e ective masses and Luttinger parameters which are useful in the parametrization of theories based on e ective hamiltonians.

X-ray-Induced Irreversible Changes in the Structure and Electrical Properties of Cd1 – xZnxTe

Changes in the structure and electrical properties of Cd1–xZnxTe real crystals due to x-irradiation with powers commonly used in x-ray crystallography are studied experimentally. The structure is examined by high-resolution x-ray diffraction, and the electrical properties are measured by the method of thermally induced capacitor discharge. It is established that irreversible changes occur in both the structure and the deep-level configuration of the crystals if the x-ray power is above 1 kW. The mechanism of the transformation and a possible way to modify electrical properties by x-irradiation are discussed.

CdZnTe:Cl Crystals for X-Ray Computer Tomography Detectors

Processes of growth of semi-insulating Cd1—xZnxTe:Cl crystals (x = 0.0002 and 0.1) of n-type conductivity are investigated. From the grown crystals detectors for X-ray computer tomography with small value of photocurrent memory (afterglow) (0.1–0.3%) are obtained.

ATOMIC LAYER EPITAXY BY ISOTHERMAL CLOSED SPACE SUBLIMATION

Atomic Layer Epitaxy growth of optoelectronic semiconductors ZnTe, CdTe and CdxZn1-x Te and CdSe films is achieved by using a novel isothermal closed space sublimation system. The quality of the films and the Atomic Layer Epitaxy regime was confirmed by X-Rays and electron diffraction, Transmission Electron Microscope observations and composition measurement of the ternary CdxZn1-x Te alloys. The difference in vapor pressures between the elemental source and the growing surface is the driven force for the growth; this difference being zeroed once the surface is completely. ZnTe growth is regulated at 1 ML/cycle while CdTe is regulated at 0.5 ML/cycle.

DC photoconductivity study of semi-insulating Cd1−xZnxTe crystals

Room temperature direct current (DC) photocurrents were measured for detector grade Cd1−xZnxTe (CZT) crystals in the spectral range of 400–1000 nm as a function of light intensity and voltage. The photocurrent data were analyzed and fit to a theoretical model to extract the electrical transport properties for high-resistivity detector grade CZT material. Using the DC photocurrent measurements, the mobility-lifetime () products and the surface recombination velocities for both electrons and holes were measured. For this study the CZT detectors had Au contacts, and the surfaces were treated in a standard 5% bromine in methanol etching solution. The correlation of the DC photocurrent measurements and detector performance is also reported.

EFFECTS OF ANNEALING CONDITIONS ON THE CRYSTALLIZATION QUALITIES OF Cd1-xZnxTe SLICES

Defects such as point defects, dislocations, impurities and Te precipitates always exist in Cd 1-xZnxTe ( x =0.04) crystals obtained by Bridgman and accelerated crucible rotation technique-Bridgman bulk crystal growth methods. It is important to anneal CdZnTe slices in Cd vapor to eliminate these defects. In this paper, the solid-vapor equilibrium for CdZnTe crystals is first discussed extensively based on the phase diagrams of Cd-Te and the P-T plot of Cd0.96Zn0.04Te. Through the annalysis, the annealing conditions for Cd1-xZnxTe crystals are optimized. Experimental results show that the Zn partial vapor pressure, which is higher than the equilibrium pressure within a certain range, can enhance the diffusion process of the excess Te towards the surface of the slices and thus eliminate Te precipitates. However, saturated Zn pressure causes Zn and Te concentration in surface layers of the slices far away from the stoichiometry. Intensive exchange of atoms between the slice surface and the vapor under this condition would lead to severe damage to the crystallization quality on the surface. The inter diffusion of Zn also inhibits the outer-diffusion of impurities during annealing. Thus a lower Zn partial pressure, but still lies in the solid vapor equilibrium area of the P-T plot, can help to drive out the impurities. According to the crystallization quality and the concentration distribution of slices annealed under different conditions, the annealing of CdZnTe wafers is classified into three kinds, namely de-precipitate annealing, de-impurity annealing and homogenizing annealing. The favorable annealing process should be a multi-step and multi period procedure.

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.

Development of dry processing techniques for CdZnTe surface passivation

A method for passivating the surface of Cd1−xZnxTe (CZT) x-ray and gamma ray detectors using relatively simple dry processing techniques has been developed. Leakage currents were significantly reduced for several processing methods. CZT samples were exposed to an oxygen plasma and/or coated with a reactively sputtered silicon nitride layer. Several parameters of the oxygen plasma step were found to be important for achieving enhanced surface resistivity. SiNX has been previously characterized and was used because of its high dielectric quality and low deposition temperature. Reduction in leakage current after passivation by a factor of as much as twenty is demonstrated. Results are also presented which give a measure of the long-term stability of the passivating layers.

Passivation of CdZnTe surfaces by oxidation in low energy atomic oxygen

A method of surface passivation of Cd1−xZnxTe (CZT) x-ray and gamma ray detectors has been established by using microwave-assisted atomic oxygen bombardment. Detector performance is significantly enhanced due to the reduction of surface leakage current. CZT samples were exposed to an atomic oxygen environment at the University of Alabama in Huntsville’s Thermal Atomic Oxygen Facility. This system generates neutral atomic oxygen species with kinetic energies of 0.1–0.2 eV. The surface chemical composition and its morphology modification due to atomic oxygen exposure were studied by x-ray photoelectron spectroscopy and atomic force microscopy and the results were correlated with current-voltage measurements and with room temperature spectral responses to Ba133 and Am241 radiation. A reduction of leakage current by about a factor of 2 is reported, together with significant improvement in the gamma-ray line resolution.