Cd Overpressure Research Articles

Electrical properties of CdTe near the melting point

A new experimental setup for the investigation of electrical conductivity (σ) in liquid and solid CdTe was built for a better understanding of the properties near the melting point (MP). The temperature dependence of σ was studied, within the interval 1,050–1,130°C, at defined Cd-partial pressures 1.3–1.6 atm, with special attention to the liquid-solid phase transition. We found that the degree of supercooling decreases with increasing Cd overpressure and reaches the lowest value at 1.6 atm without change of the melting temperature during heating.

Infrared absorption behavior in CdZnTe substrates

Infrared (IR) optical transmission measurements of polished CdZnTe wafers can provide useful information about excess impurities, stoichiometry, and inhomogeneities (precipitates and inclusions). We have investigated the IR transmission behavior of Cd0.96Zn0.04Te between 8 m and 20 m at room temperature. The measurements were made before and after thermal treatments involving control of the Cd and Zn overpressures, which served to minimize the Cd (cation) vacancy population. Our results support the polar optical phonon scattering theory of Jensen, according to which the absorption in donor dominated CdZnTe varies asm with m=3. For material dominated by acceptors, we show that the theoretical absorption by inter-valence band transitions can be approximated by a similar power law with exponent m=1, and that Cd-vacancy dominated wafers are in reasonable agreement with this. We find some wafers in which the asgrown condition exhibits partial compensation of impurity donors by Cd vacancy acceptors, and demonstrate removal of the compensation by annealing to fill the vacancies. In a separate group of wafers, we find that an observed increase in absorption occurring during growth of a HgCdTe layer by liquid phase epitaxy can be explained in terms of an increase in Cd vacancies caused by diffusion of Cd to Te precipitates. This effect can be reversed by annealing in Cd−Zn vapor, which fills vacancies and eliminates some precipitates. Impurity concentrations were measured by glow discharge mass spectrometry (GDMS).

Defect structure of CdZnTe

(Cd,Zn)Te single crystals fabricated in our laboratory by Vertical Gradient Freeze Method were subjected to two types of experiments. In the first experiment, samples were annealed at temperatures 600–900°C under different Cd overpressures. They were quenched after annealing to room temperature and Hall effect and conductivity measurements were performed. In the second experiment, in situ Hall effect and conductivity measurements on neighbor samples were done also at temperatures 700–900°C. We determined equilibrium concentrations of defects at temperatures 700–900°C and the stoichiometry intrinsic line in the p–T diagram comparing the experiments and the theoretical model presented by Berding. Results of the presented analysis can be used to find optimal growth and annealing conditions to reduce precipitation of Cd/Te or to produce intrinsic material suitable for fabrication of (CdZn)Te gamma-ray detectors.

High temperature mobility of CdTe

The Hall mobility of electrons μH is measured in CdTe in the temperature interval 450–1050 °C and defined Cd overpressure in near-intrinsic conditions. The strong decrease of μH above 600 °C is reported. The effect is explained within a model of multivalley conduction where both electrons in Γ1c minimum and in L1c minima participate. The theoretical description is based on the solution of the Boltzmann transport equation within the relaxation time approximation including the polar and acoustic phonon intravalley and intervalley scatterings. The Γ1c to L1c separation ΔE=0.29−10−4T (eV) for the effective mass in the L valley mL=0.35m0 is found to best fit the experimental data. Such ΔE is about four times smaller than it is predicted by first-principle calculations.

CdZnTe substrate producibility and its impact on IRFPA yield

The need for cost effective production of HgCdTe infrared detectors and focal plane assemblies has led to increased attention to the availability of high quality large-area CdZnTe substrates. Reasonable yield of large-area substrates (≥4 cm×6 cm format) is necessary for fabrication of focal plane assemblies (FPAs) now in production, and for future infrared (IR) detectors which are growing in size and complexity. Raytheon’s infrared materials producibility (IRMP) program has addressed this issue, after identifying critical drivers of FPA yield coming from substrates, and targeted certain improvements in substrate process steps for highest impact on large-area substrate yield. Three specific areas of improvements in the substrate process were addressed: (1) compounding of a large 6 kg charge of CdTe; (2) vertical Bridgman growth of 92 mm diameter CdZnTe boules in both quartz and pyrolytic boron nitride (PBN) crucibles; and (3) optimized Cd overpressure control during growth and cool-down of the boule. It was shown that the Cd overpressure and the cooling schedule had the strongest effects on defect populations. The resulting improvements include a 33% increase in wafer yield per unit starting weight, an estimated 50% reduction in substrate cost per cm2, better morphology of epitaxial HgCdTe layers, and improved yield of satisfactory IR detectors. The criteria for selecting substrates have also improved as a result of this work. In addition, photovoltaic detectors were fabricated on wafers from a variety of sources, and tested. Results compare favorably with those on baseline (earlier process) substrates.

Effect of growth conditions on the flatness of micro-crystal terraces of CdTe thin film

An atomic force microscopy study of the surface roughness of CdTe thin films growth by the close-space vapor transport technique on Corning glass substrate under different growth conditions (substrate temperature, Cd overpressure, and annealing) is presented. The roughness measurements show that under certain growth conditions the surface of the micro-crystals is flatter—on the micrometer scale—than in the case of CdTe layers grown by molecular beam epitaxy (MBE) on a CdTe substrate. ©1999 Kluwer Academic Publishers

Gamma Ray Spectrometers Fabricated from Modified Bridgman/Annealed CZT Crystal Material

AbstractThe CZT boule was grown by a modified vertical Bridgman process using in-situ compounding, Cd over-pressure and a pyrolytic boron nitride crucible within a fused ampoule. During growth, the Cd vapor pressure was near 1 atmosphere. These growth conditions tend to give high purity, good stoichiometry, few precipitates, and dislocation densities in the low to mid-104 cm−2 range. The crystals, after polishing, were annealed in a nearly saturated Cd, Zn atmosphere to fill residual Cd-site vacancies and achieve high resistivity in the 1010 γ-cm range. Low temperature photoluminescence study shows very good crystalline quality and a very low concentration of deep level recombinations. Single crystal samples were diced into 1 cm squares for evaluation as gamma ray detectors. The best detector results (4.5% resolution at 60 keV) were achieved for a 2 hour anneal at 850°C.

Cation self-diffusion in

A radiotracer method was used to determine the self-diffusivities of Cd, D(Cd), and Mn, D(Mn), in bulk between 450 and at saturated Cd and Te overpressures. The self-diffusivities were also measured in at . At saturated Cd, D(Cd) and D(Mn) had the same activation energy of 2.12 eV and pre-exponential factors of 1.10 and 1.38 respectively: at Te saturation the activation energies for D(Cd) and D(Mn) were again the same and equal to 2.16 eV with pre-exponential factors of 1.36 and 1.62 respectively. Possible diffusion mechanisms are discussed. The relation between the compositional interdiffusivity and the self-diffusivities is examined, together with the variation of self-diffusivity with composition.

MOCVD grown CdTe investigated by photoluminescence and PAC

Comparative investigations, using perturbed γγ-angular correlation and photoluminescence spectroscopy, were performed on CdTe films grown on GaAs substrates by MOCVD. The incorporation of the radioactive probe atoms 111In into the CdTe films was done via diffusion, implantation, and, for the first time, by in situ doping during MOCVD growth. The latter process was realised by using a modified MOCVD reactor with the precursor trimethylindium (TMIn) labelled with the radioactive isotope 111In. The influence of the doping procedure and the thermal treatment, including annealing under Cd or Te overpressure, was investigated. For two donor-acceptor pairs, involving the donor In and the acceptors As or the Cd vacancy, the results of both techniques were correlated.

Molecular beam epitaxy grown CdTe/α-Sn/CdTe single quantum well structures

CdTe/α-Sn/CdTe single quantum wells (SQW) were successfully prepared by depositing metastable α-Sn layers at substrate temperatures between 120 and 150°C. It was observed that α-Sn has to be grown on a Cd-stabilized CdTe surface and that Cd over-pressure is necessary in growing the CdTe cap layer. The results are confirmed by the observation of a clear lattice image of α-Sn by transmission electron microscopy. Reflectivity measurements indicate a feature corresponding to an absorption edge. Bulk α-Sn has a symmetry induced zero gap. The band edge is believed to be due to the quantum confinement of carriers in the α-Sn layer of the SQW and conforms with calculations using an envelope function approximation approach appropriate for SQWs formed by α-Sn and CdTe. The calculations include the non-parabolicity effect.

Interdiffusion measurements in [formula omitted] superlattices

We have measured the interdiffusion coefficient at CdTe Cd 1 − x Zn x Te interfaces by following the evolution of the X-ray diffraction patterns of superlattices annealed between 360 and 420°C under a Cd overpressure. A linear diffusion model is used to describe the experimental results. The interdiffusion coefficient is given by D Cd (cm 2/s) = 100 exp[−2.5 eV/ kT]. The data agree well with the Cd self-diffusion coefficient measured by other techniques between 500 and 900°C. This shows that only one defect, probably the Cd vacancy, is involved in the diffusion process in bulk material and at CdTe CdZnTe interfaces. At growth temperature generally used for the molecular beam epitaxy of CdTe based superlattices ( T g ⩽ 280° C), the contribution of interdiffusion to the interface morphology can be neglected.

Distribution and genesis of inclusions in CdTe and (Cd,Zn)Te single crystals grown by the Bridgman method and by the travelling heater method

The density and distribution of inclusions of the excess component in CdTe and (Cd,Zn)Te crystals grown by the Bridgman method and by the travelling heater method (THM) have been investigated by transmission infrared (IR) microscopy. A correlation between composition of the metl (melt-solution) and inclusion density has been found. An inclusion-free CdTe Bridgman crystal was grown using a Cd overpressure, a source temperature of 850°C and a melt temperature of 1118°C. The highest inclusion density was detected in a (Cd,Zn)Te THM crystal grown from a Te-rich zone under microgravity conditions. For the first time the kinetics of the inclusion genesis are discussed in detail. The inclusion morphology is very sensitive to the temperature field. In contrast to as-grown crystals, cooling down solution-grown crystals in a very small temperature gradient causes a symmetrical inwards crystallization of the solution as entrapped droplets. In a large temperature gradient, temperature gradient zone melting (TGZM) of the included droplets occurs. Their migration results in {111} facet development at the dissolving interface, although the growing interface is rounded.

Indium Doping of CdTe Grown by Molecular Beam Epitaxy

We report on n-type indium doping of CdTe films grown by molecular beam epitaxy on (001) GaAs substrates. By adjusting the flux of In atoms we can precisely control the carrier concentration over three orders of magnitude — from 8 x 1014 up to 1.3 x 10 18 cm-3 . In agreement with earlier reports we confirmed that Cd overpressure plays an important role in the doping process. The doping appears to be most effective for Cd/Te pressure ratio of 1.5. For this value of Cd/Te pressure ratio essentially 100% efficiency of doping is achieved at low In concentrations (< 10 18 cm -3 ). At higher In concentrations acceptor impurities compensate shallow donors limiting the concentration of free carriers. PACS numbers: 68.55.Ln, 73.61.Ga

Photoluminescence and p-type conductivity in CdTe:N grown by molecular beam epitaxy

An rf nitrogen (N) plasma source has been used to achieve p-type conductivity in molecular beam epitaxy CdTe layers grown with a Cd overpressure. Photoluminescence and secondary ion mass spectrometry measurements have confirmed the incorporation of the N species, and evidence for the resulting p-type conductivity has been obtained using capacitance-voltage and current-voltage techniques. Net hole concentrations as high as 2×1017 cm−3 have so far been achieved, which contrasts with the normally n-type nature of our undoped CdTe layers.

Measurement and control of ion-doping-induced defects in cadmium telluride films

Homoepitaxial p-type CdTe films were grown by coevaporation of CdTe and phosphorus in vacuum, where the phosphorus vapor was ionized and accelerated toward the substrate. Hole densities up to 2×1017 cm−3 were obtained using an ion energy of 60 eV. Effects of residual ion damage were observed using cross-sectional transmission electron microscopy, etch-pit density, and minority-carrier diffusion length measurements. This ion damage is dependent on both the ion dose and the ion energy. Reducing the ion energy below 60 eV results in lower doping densities, but using electron irradiation and Cd overpressure during deposition makes it possible to achieve equivalent doping levels for 20 eV ions while reducing the ion damage. At an ion energy of 20 eV, using electron irradiation of the growing film, and a 0.2% overpressure of Cd, films with hole density of 1×1017 cm−3 and diffusion length of 0.35 μm were obtained. Photovoltaic behavior of the films deposited in different conditions was tested by fabricating n-CdS/p-CdTe heterojunctions.

Donor doping of (211) CdTe epilayers and CdTe/CdZnTe piezoelectric heterostructures by molecular beam epitaxy

Shallow indium donors have been incorporated in (211) CdTe and CdZnTe epilayers by molecular beam epitaxy. For this orientation, no Cd overpressure is necessary to incorporate the indium donors. Essentially 100% activation of donors was obtained in the 1016–7×1017 cm−3 concentration range. Electron mobility was 7300 cm2 V−1 s−1 at 40 K for 3×1015 donors cm−3. Transmission electron microscopy shows that the doped (211) CdTe layers have high structural quality. Optical spectra of modulation-doped (211) CdTe/CdZnTe quantum wells show that an electron gas present in a piezoelectric CdTe well produces strong screening of the internal electric field.

State and distribution of point defects in doped and undoped bridgman-grown CdTe single crystals

Using spark-source mass spectrometry, atomic absorption spectrometry and various optical techniques, a study is made of the concentrations of impurities (total and point-defect concentrations), charge carriers and precipitates as well as of their axial distributions in Bridgman crystals grown with and without an extra Cd source. It is found that the concentrations of substitutional impurities are controlled by Cd and Te vacancies which form during the growth process. When the crystals are cooled after growth, part of the Cd vacancies are occupied by impurity atoms, while part of them cluster to form Te precipitates. The vacancy concentrations are modified, and the latter reaction is suppressed, in the case of growth under Cd overpressure. Intentional doping with halogens leads to formation of persistent Te-site donor/Cd-vacancy complexes.

Indium doping of (001), (111) and (211) CdTe layers grown by molecular beam epitaxy

Indium donor doping of (001), (111) and (211) oriented CdTe layers is performed by MBE (with a Cd overpressure in the case of (001) orientation). For (001) growth the role of the Cd overpressure on the growth characteristics and the doping properties is discussed. For tilted (111) orientation (misoriented a few degrees around the [110] axis) as well as for the (211) orientation, growth temperature as low as 240°C yields untwinned doped layers. Uniform dopings, in the 10 16–10 18 cm -3 range, are reproducible with ≈ 100% doping efficiency and the doped layers show PL spectra dominated by the D 0X and DAP emissions.

Donor activation efficiency and doping profile quality in In-doped CdTe and CdZnTe quantum structures

In-doping of CdTe and Cd 1−x Zn x Te ( x < 20%) layers and quantum structures is performed during molecular beam epitaxy with a Cd overpressure. The In flux is determined as a function of the cell temperature. The role of the Cd overpressure in the activation efficiency is studied. Carrier mobilities of up to 5300 cm 2 V −1 s −1 are observed at light doping levels. In migration and formation of In-related compounds are studied by X-ray photoelectron spectroscopy. The doping profile quality is studied by secondary ion mass spectroscopy: efficient, well localized doping is achieved for low growth temperatures (200–220°C) and low sheet densities (about 10 11 cm −2); otherwise marked migration of the dopant is observed.

Indium doping of CdTe and Cd1−xZnxTe by molecular-beam epitaxy: Uniformly and planar-doped layers, quantum wells, and superlattices

CdTe and Cd1−xZnxTe layers and microstructures were doped with indium donors during their growth at low temperatures (200–220 °C) by molecular-beam epitaxy under Cd overpressure. Uniform and planar doping of layers and local doping of quantum wells and superlattices are presented. Characterization techniques include secondary-ion mass spectroscopy (SIMS), capacitance-voltage and Hall-effect measurements, optical spectroscopy, x-ray double diffraction, and x-ray photoelectron spectroscopy. In the range of indium concentrations 2×1016–1×1018 cm−3, the donor activation efficiency is 100% for uniform doping. A low-temperature carrier mobility of up to 5300 cm2/V s is obtained. The highest measured carrier concentration is 1.3×1018 cm−3; at a higher doping level, strong compensation occurs, related to dopant migration and cadmium vacancy formation. Planar doping also yields ≊100% activation efficiency for moderate values of sheet density (≊1011 cm−2) but has the same limit of about 1018 cm−3 for total carrier concentration. High-structural-quality planar-doped quantum wells and superlattices are obtained. Good localization of dopant is demonstrated by SIMS at low sheet density but at high concentration substantial migration of indium occurs.