Low-temperature instabilities of the electrical properties of Cd<Subscript>0.96</Subscript>Zn<Subscript>0.04</Subscript>Te:Cl semi-insulating crystals

The electrical properties in the temperature range 295–430 K and low-temperature (4.2 K) photoluminescence of Cd_1?xZn_xTe:Cl semi-insulating crystals grown from melts with a variable impurity content (C _Cl ⁰ = 5 × 10¹⁷–1 × 10¹⁹ cm^?3) are investigated. Nonequilibrium processes leading to a decrease in carrier concentration are observed in all the samples at low temperatures (T = 330–385 K). These changes are reversible. The activation energy of these processes E _a is found to be 0.88 eV. As with semi-insulating CdTe:Cl, the observed phenomena can be explained by a change in the charge state of background copper atoms: Cu_Cd ? Cu_i. The introduction of Zn changes the ratio of the concentrations of shallow-level donors Cu_i and Cl_Te from their levels in the initial material.     

Exfoliation and blistering of Cd<Subscript>0.96</Subscript>Zn<Subscript>0.04</Subscript>Te substrates by ion implantation

As part of a series of wafer bonding experiments, the exfoliation/blistering of ion-implanted Cd_0.96Zn_0.04Te substrates was investigated as a function of postimplantation annealing conditions. (211) Cd_0.96Zn_0.04Te samples were implanted either with hydrogen (5×10¹⁶ cm⁻²; 40–200 keV) or co-implanted with boron (1×10¹⁵ cm⁻²; 147 keV) and hydrogen (1–5×10¹⁶ cm⁻²; 40 keV) at intended implant temperatures of 253 K or 77 K. Silicon reference samples were simultaneously co-implanted. The change in the implant profile after annealing at low temperatures (<300°C) was monitored using high-resolution x-ray diffraction, atomic force microscopy (AFM), and optical microscopy. The samples implanted at the higher temperature did not show any evidence of blistering after annealing, although there was evidence of sample heating above 253 K during the implant. The samples implanted at 77 K blistered at temperatures ranging from 150°C to 300°C, depending on the hydrogen implant dose and the presence of the boron co-implant. The production of blisters under different implant and annealing conditions is consistent with nucleation of subsurface defects at lower temperature, followed by blistering/exfoliation at higher temperature. The surface roughness remained comparable to that of the as-implanted sample after the lower temperature anneal sequence, so this defect nucleation step is consistent with a wafer bond annealing step prior to exfoliation. Higher temperature anneals lead to exfoliation of all samples implanted at 77 K, although the blistering temperature (150–300°C) was a strong function of the implant conditions. The exfoliated layer thickness was 330 nm, in good agreement with the projected range. The “optimum” conditions based on our experimental data showed that implanting CdZnTe with H⁺ at 77 K and a dose of 5×10¹⁶/cm² is compatible with developing high interfacial energy at the bonded interface during a low-temperature (150°C) anneal followed by layer exfoliation at higher (300°C) temperature.     

Bias-Dependent Ultraviolet Photodetection by Au-Mg<Subscript>0.1</Subscript>Zn<Subscript>0.9</Subscript>O/ZnO-Ag Structure

We report on the bias-dependent photodetection of two different wavelengths of photons in the ultraviolet (UV) region by Au-Mg_0.1Zn_0.9O/ZnO-Ag structure deposited on a glass substrate by the sol-gel technique. Without the cap layer of Mg_0.1Zn_0.9O, the current-voltage (I-V) characteristic is symmetric and linear for positive and negative bias conditions. The presence of a cap layer on top of ZnO resulted in an asymmetric I-V curve and also decreased the dark current for both bias voltages applied to the Au contact. The structure shows maximum photoresponse at 370 nm and 320 nm when the Au contact is positively and negatively biased, respectively. The photo-to-dark current density ratio is about 48 at +20 V.     

Uniformity and reproducibility studies of low-pressure-grown Cd<Subscript>0.90</Subscript>Zn<Subscript>0.10</Subscript>Te crystalline materials for radiation detectors

A large number of room-temperature detectors have been produced from CdZnTe crystals grown with 10% Zn and 1.5% excess tellurium by the low-pressure, vertical-Bridgman technique. Radiation spectra obtained by these crystals using a ²⁴¹Am source reveal the characteristic 59.5-keV line as well as the six low-energy peaks, which include the Cd and Te escape peaks. Similarly, ⁵⁷Co spectra obtained also show a very well-defined 122-keV peak with a 3:1 peak-to-valley ratio. Seven CdZnTe crystals have been grown for reproducibility studies. Four of these crystals have resistivities over 1E9 Ω-cm. Considering that the indiumdoping level is on the order of 2E15 cm⁻³, the reproducibility is excellent. The theoretical basis of the high-resistivity phenomenon in CdZnTe is discussed in reference to a previous paper. The uniformity of these 6-in.-long CdZnTe crystals is studied, and various measurements are carried out, both laterally and vertically, along the boule. It is determined that, in general, roughly a 3.5-in. section near the middle of the 6-in. boule has sufficient resistivity for producing radiation detectors. This nonuniformity along the vertical direction is caused mostly by the composition change of Cd, Zn, Te, and In-doping level in the growth melt caused by differences in the segregation coefficients of these elements. Although, variations in resistivity are seen across some of the wafer slices, most show very good uniformity with high breakdown voltage. Some of the variations are attributed to the different grains within the boule. Similar results are seen in the measured radiation spectra obtain on 4 mm × 4 mm × 2 mm samples from different locations across the wafer, where some samples show well-resolved secondary peaks, while others display only the primary spectral lines.     

Exact self-compensation of conduction in Cd<Subscript>0.95</Subscript>Zn<Subscript>0.05</Subscript>Te:Cl crystals in a wide range of Cd vapor pressures

The process of self-compensation in Cd_0.95Zn_0.05Te:Cl solid-solution crystals has been studied by annealing single crystals under a controlled Cd vapor pressure, with subsequent measurements of the Hall effect, photoluminescence, carrier lifetime and mobility, and photocurrent memory in the annealed crystals. By means of this annealing, conditions of thermal treatment that make it possible to fabricate low-conductivity samples with a low carrier density, 10⁷–10¹¹ cm^?3, are defined. In these samples, a p → n conduction inversion is observed at a higher free-carrier density (n, p ≈ 10⁹ cm^?3) and the dependence of the electron density on the Cd vapor pressure exhibits a more gentle slope than in the case of CdTe:Cl crystals. The obtained data are discussed in terms of a self-compensation model in which intrinsic point defects act as acceptors with deep levels. This level is attributed to a Zn vacancy, which remains active at high Cd pressure.     

Wafer bonding of (211) Cd<Subscript>0.96</Subscript>Zn<Subscript>0.04</Subscript>Te on (001) silicon

We have successfully bonded (211) cadmium zinc telluride (CZT) substrates onto (001) Si substrates for subsequent epitaxial-layer deposition of mercury cadmium telluride device layers. Silicon-nitride intermediate layers were employed as they provide both low surface roughness, which is necessary for bonding, and low absorption in the 1–10-μm range. Prior to bonding, the SiN layers were activated using oxygen plasma. Transmission infrared (IR) imaging showed >70% bonded area of a 10 mm×10 mm CdZnTe substrate onto a Si substrate. After the initial bond, the structure was exposed to a low-temperature anneal (150°C) for extended periods of time (22 h) to increase the bond strength. This process was sufficient to produce a CdZnTe on silicon structure that was able to withstand subsequent chemical-mechanical polishing (CMP) of the CZT substrate. We also investigated CMP of the transferred CdZnTe to improve the surface for subsequent epitaxial deposition. A Br/ethylene glycol/methanol solution produced the lowest damage levels, as determined by triple axis x-ray diffraction (TAD) while a standard silica/NaOH treatment produced a surface with <0.5-nm root mean square (RMS) roughness.     

Infrared transmission spectra of Cd<Subscript>1−x</Subscript>Zn<Subscript>x</Subscript>Te (x = 0.04) crystals

Three indicators (T₁₀₀₀, T₅₀₀₀, and T₁₀₀₀/T₅₀₀₀) are used to appraise the infrared (IR) transmission spectra for Cd_1−xZn_xTe (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.     

A modified vertical Bridgman method for growth of high-quality Cd<Subscript>1−x</Subscript>Zn<Subscript>x</Subscript>Te 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.     

Mn<Subscript>0.1</Subscript>Ag<Subscript>0.9</Subscript>In<Subscript>4.7</Subscript>S<Subscript>7.6</Subscript> single crystals: Crystal structure,band gap,and thermal expansion

Mn_0.1Ag_0.9In_4.7S_7.6 single crystals are for the first time grown by the Bridgman method (vertical variant). The single crystals crystallize in the cubic spinel structure. The band gap of the single crystals is determined from the transmittance spectra in the region of the fundamental absorption edge at temperatures of T = 295 and 80 K. Thermal expansion is studied by the dilatometric method in the temperature range 80–500 K. The coefficients of thermal expansion, the Debye temperatures, and the rms (root mean square) dynamic displacements of atoms are calculated.     

Positron-defect profiling in Cd<Subscript>1−x</Subscript>Zn<Subscript>x</Subscript>Te wafers after saw cutting

Near-surface damage induced by saw cutting of ingots of Cd_1−xZn_xTe was investigated by positron-defect depth profiling. The damage extends to several micrometers depth and depends on the cutting apparatus. The samples were polished and etched repeatedly, followed each time by positron-depth profiling. New subsurface damage created during the polishing process is observed. No new damage is observed after etching. Positron-depth profiling is suggested as a diagnostic tool to monitor the quality of sample surfaces.     

Ultraviolet Photodetection Properties of a Pt Contact on a Mg<Subscript>0.1</Subscript>Zn<Subscript>0.9</Subscript>O/ZnO Composite Film

We report on the ultraviolet (UV) photodetection properties of a Pt contact on a sol-gel Mg_0.1Zn_0.9O/ZnO composite structure on a glass substrate. In the dark, the current–voltage (I–V) characteristics between the Pt and Ag contacts on the top of the ZnO film were linear while that on the Mg_0.1Zn_0.9O/ZnO composite film were rectifying, suggesting the formation of a Schottky diode on the latter. The ideality factor, n, and the reverse leakage current density, J _R , of the Schottky diode were greater than 2 and 2.36 × 10⁻² A cm⁻² at −5 V, respectively. Under ultraviolet light, the I–V characteristics become linear. The maximum photo-to-dark current ratio observed was about 63. The composite film showed good sensitivity to UV light with wavelengths of less than 400 nm, though the photoresponse process was found to be slow.     

Anti-stokes luminescence of Zn<Subscript>0.75</Subscript>Cd<Subscript>0.25</Subscript>S microcrystals annealed in the presence of oxygen

The nature of anti-Stokes luminescence centers excited in the temperature range 77–300 K by light with wavelengths from 620 to 710 nm and intensities in the range 10¹⁵–10¹⁶ photons/cm² in Zn_0.75Cd_0.25S microcrystals annealed in the presence of oxygen has been investigated. It is shown that the centers of two-photon excitation of this luminescence are clusters of native metal oxides (ZnO) _n and (CdO) _n , whose energy levels in the Zn_0.75Cd_0.25S band gap are 1.70–1.95 eV below the bottom of the conduction band.     

Potential of using the Cd<Subscript>0.8</Subscript>Hg<Subscript>0.2</Subscript>Te alloy in solar cells

Surface-barrier diodes based on the Cd_xHg_1?x Te alloy (x ～ 0.8) sensitive in the wavelength range 0.3–1.1 μm, which were obtained by etching (bombardment) of the surface of the p-type crystal with argon ions, are studied. Using the measured spectral absorption and reflection curves, as well as the parameters of the diode structure, which were found from electrical characteristics, the spectra of photoelectric quantum efficiency of diodes are calculated. The results of the calculation of photoelectric parameters of the Gd_0.8Hg_0.2Te-based diodes are given in comparison with the CdTe-based and Si-based solar cells. For the AM1.5 solar irradiation conditions, the open-circuit voltage and short-circuit current, as well as values of limiting efficiency, are determined.     

Impulse Response Time Measurements in Hg<Subscript>0.7</Subscript>Cd<Subscript>0.3</Subscript>Te MWIR Avalanche Photodiodes

The response time of front-sided illuminated n-on-p Hg_0.7Cd_0.3Te electron avalanche photodiodes (e-APDs) at T = 77 K was studied using impulse response measurements at λ = 1.55 μm. We measured typical rise and fall times of 50 ps and 800 ps, respectively, at gains of M ≈ 100, and a record gain-bandwidth (GBW) product of GBW = 1.1 THz at M = 2800. Experiments as a function of the collection width have shown that the fall time is strongly limited by diffusion. Variable-gain measurements showed that the impulse response is first-order sensitive to the level of the output amplitude. Only a slight increase in the rise time and the fall time was observed with the gain at constant output amplitude, which is consistent with a strongly dominant electron multiplication. Comparisons of the experimental results with Silvaco finite element simulations confirmed the diffusion limitation of the response time and allowed the illustration of the transit time and RC effects.     

Pressure dependence of intersubband transitions in HgTe/Hg<Subscript>0.3</Subscript>Cd<Subscript>0.7</Subscript>Te superlattices

The optical absorption coefficient of HgTe/Hg_0.3Cd_0.7Te superlattices (SLs) and its pressure dependence has been investigated at hydrostatic pressures up to 30 kbar at room temperature. The corresponding intersubband transition energies result from a comparison of experimental and theoretical absorption coefficients. The latter is based on the band structure, which is calculated using Kane’s four-band (8×8 k · p) model together with the envelope function approximation. The experimental linear pressure coefficients of the H1 — E1 and H1 — L1 intersubband transitions are in good agreement with the theoretical values, e.g., 7.15±0.3 meV/kbar and 6.2±0.3 meV/kbar compared to 7.4 and 6.4 meV/kbar, respectively. this is in stark contrast to the pressure dependence of ≤1 meV/kbar of the photoluminescence (PL) peaks of a similar SL reported in the literature. Consequently, we conclude that the reported PL peaks are not due to intersubband transitions and that the k · p model correctly reproduces the electronic band structure and its pressure dependence of HgTe/Hg_1−xCd_xTe SLs.     

Experimental study of non-stoichiometry in Cd<Subscript>1−x</Subscript>Zn<Subscript>x</Subscript>Te<Subscript>1±δ</Subscript>

In this communication, the latest experimental results are presented on a composition of Cd_1−xZn_xTe_1±δ, both x and δ, determined by the high-precision, vapor-pressure scanning method. The space arrangement of the single-phase solidus volume of Cd_1−xZn_xTe_1±δ in the pressure-temperature-composition (P-T-X) phase space has been derived from direct vapor-pressure measurements in the temperature range up to 1,375 K. From the experimental data for x=0.0, 0.05, 0.1, 0.15, 0.25, 0.5, 0.75, 0.8, 0.9, and 1, the complete phase diagram of Cd_1−xZn_xTe_1±δ was constructed. Transformation of the diagram was traced for the whole range of existence of the Cd_1−xZn_xTe_1±δ solid solution as a function of ZnTe concentration. Increase of ZnTe in the solid solution results in the extension of the homogeneity range and a shift of the solidus toward Te, so that already for x≥0.15 the solidus does not contain the stoichiometric, (n_Cd+n_Zn):n_Te=50 mol.% plane. From detailed experimental studies, maximum non-stoichiometry as a function of the temperature was determined in a wide temperature range for Te and metal solubility. Temperature dependencies of the partial pressures of vapor-phase species for different crystal compositions, X_S=const, were deduced from the vapor-pressure experiment for the x=0.05 section of the diagram. From these results, relationships are derived between crystal composition, vapor pressure, and composition of the conjugated vapor.     

A Scanning Probe Microscopy Study of Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te

The effects of several ex vacuo methods used in the surface preparation of Cd_1−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.     

Low-Temperature Thermoelectric Properties of Co<Subscript>0.9</Subscript>Fe<Subscript>0.1</Subscript>Sb<Subscript>3</Subscript>-Based Skutterudite Nanocomposites with FeSb<Subscript>2</Subscript> Nanoinclusions

Bulk thermoelectric nanocomposite materials have great potential to exhibit higher ZT due to effects arising from their nanostructure. Herein, we report low-temperature thermoelectric properties of Co_0.9Fe_0.1Sb₃-based skutterudite nanocomposites containing FeSb₂ nanoinclusions. These nanocomposites can be easily synthesized by melting and rapid water quenching. The nanoscale FeSb₂ precipitates are well dispersed in the skutterudite matrix and reduce the lattice thermal conductivity due to additional phonon scattering from nanoscopic interfaces. Moreover, the nanocomposite samples also exhibit enhanced Seebeck coefficients relative to regular iron-substituted skutterudite samples. As a result, our best nanocomposite sample boasts a ZT = 0.041 at 300 K, which is nearly three times as large as that for Co_0.9Fe_0.1Sb₃ previously reported.     

Comparative studies of p-type InP layers formed by Zn<Subscript>3</Subscript>As<Subscript>2</Subscript> and Zn<Subscript>3</Subscript>P<Subscript>2</Subscript> diffusion

The Zn₃As₂ and Zn₃P₂ were used as Zn-diffusion sources to form a p-region in undoped-InP wafers. The p-type InP formed by Zn diffusion from a Zn₃P₂ source has higher transmittance over the testing-spectrum range 1,000–1,700 nm versus Zn diffusion from a Zn₃As₂ source. In the case of a p-type region formed from a Zn₃As₂ source, x-ray photoelectron spectroscopy (XPS) showed As atoms were reduced from the oxide state and formed an InAs composition, which introduces more absorption loss.     

Polymorphic transformations and thermal expansion in AgCuSe<Subscript>0.5</Subscript>(S,Te)<Subscript>0.5</Subscript> crystals

The high-temperature X-ray diffraction technique is used to study AgCuSe_0.5(S,Te)_0.5 crystals. It is shown that, at room temperature, the AgCuSe_0.5S_0.5 crystal is composed of Cu_1.96S and AgCuSe phases. At a temperature of 695 K, these phases transform into a single face-centered cubic (fcc) phase. The transformation is reversible. The AgCuSe_0.5Te_0.5 composition consists of three phases, specifically, Cu₂Te, AgCuSe, and a cubic phase. At 444 K, both orthorhombic phases simultaneously transform into a diamond-like cubic phase. In this transformation, the cubic phase plays the role of a seed. From the temperature dependence of the lattice parameters, the thermal-expansion coefficients of the phases involved in both compositions are calculated for the main crystallographic directions.