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.     

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.     

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.     

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.     

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.     

Band gap of CdTe and Cd<Subscript>0.9</Subscript>Zn<Subscript>0.1</Subscript>Te crystals

The band gap E _g of the CdTe and Cd_0.9Zn_0.1Te crystals and its temperature dependence are determined by optical methods. This is motivated by considerable contradictoriness of the published data, which hampers the interpretation and calculation of characteristics of detectors of X-ray and γ radiation based on these materials (E _g = 1.39–1.54 and 1.51–1.6 eV for CdTe and Cd_0.9Zn_0.1Te, respectively). The used procedure of determination of E _g is analyzed from the viewpoint of the influence of the factors leading to inaccuracies in determination of its value. The measurements are performed for well-purified high-quality samples. The acquired data for CdTe (E _g = 1.47–1.48 eV) and Cd_0.9Zn_0.1Te (E _g = 1.52–1.53 eV) at room temperature substantially narrow the range of accurate determination of E _g.     

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.     

Lateral schottky GaN rectifiers formed by Si<Superscript>+</Superscript> ion implantation

Type conversion of p-GaN by direct Si⁺ ion implantation and subsequent annealing was demonstrated by the fabrication of lateral Schottky diodes. The Si⁺ activation percentage was measured as a function of annealing time (30–300 sec) and temperature (1,000–1,200°C), reaching a maximum of ∼30% for 1,200°C, 2-min anneals. The resulting n-type carrier concentration was 1.1×10¹⁸ cm⁻³ for a moderate Si⁺ ion dose of ∼2×10¹⁴ cm⁻². The lateral Schottky diodes displayed a negative temperature coefficient of −0.15 V·K for reverse breakdown voltage.     

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.     

Effect of annealing on the microwave magnetoresistance of thin Ge<Subscript>0.96</Subscript>Mn<Subscript>0.04</Subscript> films

It is established that annealing of ion-implanted thin Ge_0.96Mn_0.04 films with a thickness of 120 nm induces an increase in the microwave resistivity and a change in the mechanism of dephasing of charge carriers. The effect of annealing on the microwave transport properties of the thin films is due to the diffusion-controlled aggregation of dispersed Mn²⁺ impurity ions to form Ge₃Mn₅ clusters.     

Room-temperature photoluminescence of Ga<Subscript>0.96</Subscript>In<Subscript>0.04</Subscript>As<Subscript>0.11</Subscript>Sb<Subscript>0.89</Subscript> lattice matched to InAs

Photoluminescence (PL) has been observed at room temperature from a Ga_0.96In_0.04As_0.11Sb_0.89 quaternary solid solution for the first time. High-quality epitaxial layers of n-type (Te-doped) Ga_0.96In_0.04As_0.11Sb_0.89 with low In content were grown by liquid phase epitaxy (LPE) lattice-matched to InAs(100) substrates from a Ga-rich melt. The PL properties of the material were investigated over a wide temperature range, and the principal radiative transitions were identified. In the temperature range <150 K, donor-acceptor recombination involving the first and second ionization state of native antisite defects was the dominant radiative-recombination process, whereas interband recombination was found to dominate at room temperature.     

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.     

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.     

An improved method for Hg<Subscript>1−x</Subscript>Cd<Subscript>x</Subscript>Te surface chemistry characterization

Hg_1−xCd_xTe surface chemistry has been studied extensively with well-known tools such as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES) in order to advance detector array operability, performance, and yield. Raytheon Vision Systems has pioneered the first application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) as a Hg_1−xCd_xTe surface diagnostic tool to provide unprecedented analysis capability, including analyzing a 0.5-μm-diameter spot, high mass resolution, elemental and molecular composition scrutiny, applicability to insulators, and surface film sensitivity in the part per million range. In this study, data are presented illustrating surface chemistry geometry effects and photoresist redeposition due to common Hg_1−xCd_xTe processing steps including photolithography, bromine etching, and photoresist stripping.     

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.     

Molecular beam epitaxial growth of Cd<Subscript>1−y</Subscript>Zn<Subscript>y</Subscript>Se<Subscript>x</Subscript>Te<Subscript>1−x</Subscript> on Si(211)

We report on the first successful growth of the quaternary alloy Cd_1−yZn_ySe_xTe_1−x(211) on 3-in. Si(211) substrates using molecular beam epitaxy (MBE). The growth of CdZnSeTe was performed using a compound CdTe effusion source, a compound ZnTe source, and an elemental Se effusion source. The alloy compositions (x and y) of the Cd_1−yZn_ySe_xTe_1−x quaternary compound were controlled through the Se/CdTe and ZnTe/CdTe flux ratios, respectively. Our results indicated that the surface morphology of CdZnSeTe improves as the Zn concentration decreases, which fits well with our previous observation that the surface morphology of CdZnTe/Si is poorer than that of CdSeTe/Si. Although the x-ray full-width at half-maximums (FWHMs) of CdZnSeTe/Si with 4% of Zn + Se remain relatively constant regardless of the individual Zn and Se concentrations, etched-pit density (EPD) measurements exhibit a higher dislocation count on CdZnSeTe/Si layers with about 2% Zn and Se incorporated. The enhancement of threading dislocations in these alloys might be due to an alloy disorder effect between ZnSe and CdTe phases. Our results indicate that the CdZnSeTe/Si quaternary material with low Zn or low Se concentration (less than 1.5%) while maintaining 4% total Zn + Se concentration can be used as lattice-matching composite substrates for long-wavelength infrared (LWIR) HgCdTe as an alternative for CdZnTe/Si or CdSeTe/Si.     

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.     

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.     

Accurate Determination of the Matrix Composition Profile of Hg<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te by Secondary Ion Mass Spectrometry

In this report, we present a new secondary ion mass spectrometry (SIMS) analysis technique to provide accurate Cd composition profiles based on the measurement of HgCs⁺ and CdCs⁺ cluster ions. Study of Hg_1–x Cd _x Te samples with different x values shows that x/(1 − x) is linearly proportional to HgCs⁺/CdCs⁺ over the range of x = 0.2 to x = 0.9. This technique allows us to obtain an accurate Cd profile for a multilayer HgCdTe sample with different x values for each layer using a single standard with known x value. (Received 10/15/06; accepted 2/14/07)