Wm. C. Hughes

Indium‐Hg vacancy interactions in Hg1−xCdxTe measured by perturbed angular correlation

The interaction of mercury vacancies with dopant indium atoms in Hg0.79Cd0.21Te was studied using the perturbed γγ angular correlation (PAC) technique. Two dominant PAC signals, characterized by quadrupole interaction strengths νQ1=83 MHz and νQ2= 91 MHz and asymmetry parameters η1=η2=0.08, were observed and attributed to one or more In‐VHg complexes. The complexes appeared after annealing doped samples at T≥350 °C in vacuum and quenching. The fraction of In atoms associated with vacancies was increased further by annealing at 80 °C for ≳10 h. The In‐vacancy complexes vanished on annealing in a Hg‐saturated atmosphere.

Observation of indium-vacancy and indium-hydrogen interactions in Hg 1− x Cd x Te

We have used a nuclear hyperfine technique, perturbed γγ angular correlation (PAC), to study the interactions between111In and native defects and impurities in Hg1−xCdxTe. The PAC technique uses the quadrupole interaction of111In with local electric field gradients to characterize the local environment of this donor dopant. We observed that when In was diffused into a bulk or thin film sample of Hg1−xCdxTe (x=0.21 and x=0.3) at 350°C and the sample was slow cooled, the In occupied sites with near-cubic symmetry, presumably the substitutional metal site. However, when the sample was quenched, a fraction of the In was incorporated into defects characterized by quadrupole interaction strengthsvQ1 andvQ2 and asymmetries of ν1=ν2=0.08. These defects are attributed to the trapping of a metal vacancy at a next-nearest neighbor site to the In atom. The introduction of hydrogen by boiling the samples in distilled water for >4h eliminated the previously observed PAC signals and created defects characterized byvQ3=35 MHz, ν3 <0.1 andvQ4=MHz, ν4 <0.1. These defects are attributed to the decoration of the In-VHg complex by a hydrogen atom. Hall effect measurements showed that hydrogenation increased the hole concentration in p-type quenched samples and even converted n-type indium-doped samples to p-type. A possible model for hydrogen incorporation which includes self-compensation by vacancy creation is suggested.

Reassessment of the assignment of the InM2+3+VM2+ defect in CdTe and ternary II‐VI compounds

Previous perturbed angular correlation (PAC) spectroscopy measurements on the donor indium in CdTe and its alloys have revealed several defect complexes. One defect characterized by two sets of quadrupole interaction parameters, νQ=83 MHz, η=0.08 and νQ=92 MHz, η=0.08, was observed in Hg0.8Cd0.2Te (x=0.2 MCT) and attributed to the substitutional indium–metal vacancy complex InM2+3+VM2+. A defect characterized by νQ=61±1 MHz and asymmetry parameter η between 0 and 0.19 was seen in CdTe and widely attributed to the same complex. Both of these assignments were based mainly on an observed relationship between complex formation and the loss of metal ions. In this article we present PAC measurements on 111In‐doped x=0.45 MCT (Hg0.55Cd0.45Te). These measurements reveal defects having quadrupole interactions very similar to those seen previously in CdTe and in x=0.2 MCT. Two unique defect fractions f1 and f2, characterized by νQ1=60±3 MHz, η1≊0–0.2, and νQ2=87±4 MHz, η2≊0–0.15, were seen in x=0.45 MCT, in some ca...

The thermodynamics of indium—vacancy pairs in Hg0.79Cd0.21Te

Abstract We have measured the thermodynamic properties of In-V Hg pairs in the infrared material Hg 0.79 Cd 0.21 Te by using the method of perturbed angular correlation (PAC). PAC allows the determination of the absolute fraction of indium atoms in particular defects. Previous measurements have characterized the In-V pairs with hyperfine interaction frequencies of 83 and 92 MHz. These defects were observed in materials which were quenched from > 350dC to retain a large number of metal vacancies. After subsequent low temperature (60 l T A l 110dC) anneals, the fraction of indium atoms that had trapped vacancies increased because of the migration of metal vacancies. By measuring the rate of increase at different temperatures, we determined the vacancy migration energy to be E m = 0.45 p 0.04 eV. Anneals at slightly higher temperatures dissociated the In-V pairs. From the decrease in the In-V pair fraction with temperature, the In-V pair binding energy was found to be E b = 0.44 p 0.15 eV.

Orientation of the electric‐field gradient arising from a vacancy in Hg0.79Cd0.21Te

We have used the perturbed angular correlation technique to measure the orientation of the electric‐field gradients (EFGs) due to vacancy trapping by substitutional indium donors in the II‐VI semiconductor Hg0.79Cd0.2Te. Previously, two hyperfine interaction frequencies were measured and were attributed to the trapping of a metal vacancy at a next nearest‐neighbor site to the indium atom in bulk solid‐state recrystallized materials. In the present experiments, measurements are done on thin‐film samples to find the principal axes of the EFGs. Both EFGs are found to have principal axes parallel to a 〈111〉 crystal axis, despite the fact that a simple point charge model supports a 〈110〉 EFG for this 〈110〉‐oriented In‐VHg complex. A similar situation exists for indium‐vacancy pairing in other II‐VI semiconductors. We propose that the 〈111〉 EFG orientation arises from the electric dipole moments of the highly polarized Te ions in the region of the vacancy.

Perturbed angular correlation observation of vacancy-indium atom defect complexes in (Hg, Cd)Te

Abstract We present the first perturbed angular correlation observations of defect complexes in the technologically important material (Hg,Cd)Te. In particular, we report the observation and identification of In-mercury vacancy complexes in Hg0.79Cd0.21Te. These complexes were observed following a rapid quench of the material after annealing at 350–400 ° C in vacuum. Since the complexes vanished following similar quenching in a Hg atmosphere, we conclude that they involve Hg vacancies. The migration energy of vacancies was calculated to be 0.6 ± 0.2 eV from measurements of the evolution of the In-VHg complexes during annealing between 75 and 100° C.

PAC and XPS studies of II-VI compounds

The perturbed angular correlation (PAC) method has considerable potential for studying near-surface defects of materials on a microscopic scale, especially in combination with standard surface techniques. We have characterized the near-surface and surface regions of II–VI compounds using PAC and X-ray photoelectron spectroscopy (XPS). PAC was used to identify point defect complexes containing the radioactive 111In probe atoms, which were diffused into Cd0.96Zn0.04Te, Cd0.8Mn0.2Te and Hg0.79Cd0.21Te (MCT) samples under vacuum from 200 to 550 °C. After the Cd(Zn)Te samples were annealed at 450 °C for 30 min, 60% of the In atoms occupied unique non-cubic sites, characterized by an interaction frequency νQ = 60 MHz and an asymmetry parameter η=0.2. These sites were attributed to In-(cadmium vacancy) complexes, since this signal was eliminated by subsequent annealing in a cadmium atmosphere. Similar results were obtained for the Cd(Mn)Te samples, but the vacancy complex occured at a lower annealing temperature. The XPS data for Cd(Mn)Te showed that its oxide characteristics differed considerably from those of CdTe. Although oxide formation of CdTe occurs very slowly after sputter cleaning, a significant TeO layer was formed on the Cd(Mn)Te surfaces after only 10 min exposure to air. In addition, deionized water removed the native oxide completely for CdTe but not for Cd(Mn)Te. For MCT, annealing at 350 °C caused the formation of two defect complexes, characterized by frequencies of 83 and 92 MHz, and η ≈ 0.1. These PAC signals vanished after annealing under a mercury atmosphere, indicating that they were also due to In-vacancy complexes.

Perturbed-angular-correlation studies of In-vacancy pairs in Hg1−xCdxTe

The interaction of vacancies with 111In atoms is studied in Hg1−xCdxTe compounds via perturbed-angular correlation (PAC) experiments, for x=0.065, 0.21, 0.44, and 0.95. In the low-x (Hg-rich) compounds, Hg vacancies are created by heating in vacuum. For the x=0.21 alloy, we have previously shown that InC-vacC pairs (A centers consisting of an In atom on a cation site and a vacancy at a neighboring cation site) are abundant after quenching from elevated temperatures. These defects are characterized by two PAC signals with quadrupole interaction frequencies νQ1=83 MHz and νQ2=92 MHz, and asymmetry parameters η1=η2=0.08. For the x=0.065–0.44 alloys, the data presented in this article show that the fractions f1 and f2 of In atoms associated with these two frequencies vary with x according to whether one or two Hg atoms are nearest neighbors to the Te atom that is bound to the In atom and the vacancy. The data are explained by the polarizable point-ion model. For the x=0.95 compound, PAC signals are observed o...

The temperature-dependent structure of point-defect complexes in silver chloride crystals

Evidence of a temperature-induced, cyclable transition in the structure of point-defect complexes in AgCl crystals is reported. The effect is attributed to an entropy-driven change in the equilibrium structure of defect configurations, analogous to a phase transition in bulk solids. Such a transition is likely to occur in a system where the entropy term is a significant part of the defect Gibbs free energy, and where the energies of different defect configurations are very close. A crude thermodynamic analysis provides an estimate of the energy and entropy changes associated with the transition.

PAC study of diffusion in single crystals: the case of silver vacancy diffusion in indium-doped silver chloride

Abstract A previously unreported dependence of the Abragam-Pound damping constant on the orientation of detectors relative to the crystal axes for dynamic PAC spectra has been observed in AgCl single crystals doped with the PAC probe 111 In. The Winkler and Gerdau analysis of PAC spectra in the case of stochastic, rapidly fluctuating fields is adapted to the case of single crystals. The new analysis predicts the orientation dependence of the damping constant and thus allows the extraction of additional information on the mechanism of diffusion from the PAC data. The AgCl:In data, analyzed in terms of this calculation, provide strong evidence that vacancies are hopping among next-nearest silver sites around the indium atoms. This method of analysis may have general application to the microscopic study of atomic diffusion mechanisms.