CdxHg1−xTe thin film galvanomagnetic devices

The present paper concerns evaporated thin film Hall generators of Cd_xHg_1?xTe. We first give some information about the technology of Cd_xHg_1?xTe thin films and then we consider the construction and characteristics of these galvanomagnetic devices.     

Blue-shift in photoluminescence of ion-milled HgCdTe films and relaxation of defects induced by the milling

Simultaneous measurements of electrical conductivity, the Hall coefficient, and photoluminescence (PL) spectra of ion-milled Hg_1 − xCd_xTe films (x ∼ 0.30 and 0.38) were performed during post-milling ageing of the films at 293 K. In the course of the PL study, a ‘relaxation’ of the blue-shift of the PL band of ion-milled Hg_0.70Cd_0.30Te was observed. The relaxation was caused by the decrease of the electron concentration due to gradual disintegration of defects induced by the milling. It is shown that while ion milling substantially changes the electrical properties of Hg_1 - xCd_xTe, its PL spectrum in the long-term is affected insignificantly.     

The structure of vacuum-evaporated CdxHg1−xTe thin films

The results of investigations on the structure of vacuum-evaporated Cd_xHg_1?xTe thin films 0.5–12 μm thick and with 0 < × < 0.2 are presented.The investigations were carried out using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. An analysis was carried out of the degree of order of the thin film structure, and the phase and chemical composition, the lattice parameters and the crystal grain size were determined.These investigations contribute to an analysis of the effect of carrier scattering in thin Cd_xHg_1?xTe films; furthermore, they form a basis for the introduction of certain changes in the technological process.     

Chemical substitution of Cd ions by Hg in CdSe nanorods and nanodots: Spectroscopic and structural examination

The chemical substitution of cadmium by mercury in colloidal CdSe quantum dots (QDs) and nanorods has been examined by absorption, photoluminescence and Raman spectroscopy. The crystalline structure of original CdSe QDs used for Cd/Hg substitution (zinc blende versus wurtzite) shows a strong impact on the optical and structural properties of resultant Cd_xHg_1−xSe nanocrystals. Substitution of Cd by Hg in isostructural zinc blende CdSe QDs converts them to ternary Cd_xHg_1−xSe zinc blende nanocrystals with significant NIR emission. Whereas, the wurtzite CdSe QDs transformed first to ternary nanocrystals with almost no emission followed by slow structural reorganization to a NIR-emitting zinc blende Cd_xHg_1−xSe QDs. CdSe nanorods with intrinsic wurtzite structure show unexpectedly intense NIR emission even at early Cd/Hg substitution stage with PL active zinc blende Cd_xHg_1−xSe regions.     

Compositional and spin–orbit control on the electronic structure and optical characteristics of ZnHgTe alloys using mBJ–GGA approach

Employing systematic first-principle calculations to the family of tellurium II–VI compounds such as Hg- and Zn-based semiconductors and their related Zn_1?x Hg _x Te ternary alloys, we have simulated the electronic and optical characteristics incorporating the spin–orbit coupling effect. The salient features such as the band gaps and the optical spectra with a satisfactory adequate approach are computed with the so-called modified Becke–Johnson exchange correlation potential as implemented in the full-potential linearized augmented plane-wave scheme. The theoretical finding surmises that a topological insulating state may be caused with 25% of Zn concentration incorporated in HgTe material. Intriguingly, a band gap is conclusively developed near 0.25 eV in Zn_0.25Hg_0.75Te alloy. The relevant components like the band structure, optical response functions such as the real and imaginary parts of dielectric function, spectral dependence of optical conductivity, reflectivity spectrum, refractive index, electron energy loss function, and absorption coefficient are established for the bulk Zn_1?x Hg _x Te ternary alloys, while Zn composition spans in the range 0–1. The overall accordance between our results and other theoretical reports as well as experimental realization is fairly good. We infer that the current work may be beneficial for optical emitters/converters in solar cell devices applications.     

Group V acceptor doping of CdxHg1−xTe layers grown by metal-organic vapour phase epitaxy

Layers of Cd_xHg_1−xTe have been grown by metal-organic vapour phase epitaxy and doped with arsenic and with phosphorus. This has been achieved by establishing metal-rich gas phase conditions during growth and obviates the need for high temperature “activation” type anneals. Data from Hall measurements and secondary ion mass spectrometry on annealed samples are in close agreement.     

Photoluminescence of CdHgTe based nanoheterostructures

We have studied the photoluminescence (PL) spectra of Cd _x Hg_{1 − x} Te/Cd _y Hg_{1 − y} Te nanohetero-structures grown by molecular beam epitaxy on CdTe/ZnTe/GaAs substrates. The width of potential wells in the structures was varied within d = 12−200 nm and the material composition was changed within x ∼ 0.25–0.40 in the well and y ∼ 0.68–0.82 in the barrier layers. The PL spectra of samples with d ≤ 33 nm exhibit transitions between quantum confinement levels. The samples with d > 50 nm display the PL due to excitons localized on composition fluctuations, which is characteristic of Cd _x Hg_{1 − x} Te epilayers with thicknesses above 3 μm. It is established that the exciton PL band in Cd _x Hg_{1 − x} Te exhibit broadening that is determined both by stochastic fluctuations of the composition and by its macroscopic inhomogeneities.     

Electrical properties of CdxHg1−x Te and ZnxCdyHg1−x−y Te modified by low-energy ion bombardment

The phenomenon of conversion of the conductivity type in p-type samples of the Cd_xHg_1−x Te (0.28≤x≤0.55) and Zn_xCd_yHg_1−x−y Te solid solutions bombarded by low-energy argon ions was studied. It is shown that a necessary condition for the conversion effect in Cd_xHg_1−x Te with 0.28≤x≤0.39 is ion neutralization in the bombarding beam. The dependence of the conversion layer thickness in Cd_xHg_1−x Te on the solid solution composition agrees with that predicted by the diffusion model of ion-bombardment-induced conversion of the conductivity type.     

Conductivity compensation in CdZnTe: Cl crystals with variable zinc content

The effect of cadmium vapor pressure during postgrowth annealing on the compensation of conductivity has been studied in semi-insulating Cd_1?x Zn _x Te:Cl crystals with variable zinc content (x = 0.005, 0.01, and 0.05), which are used in nuclear radiation detectors. At a small zinc content (x = 0.005 and 0.01), the main role in the in Cd_1?x Zn _x Te:Cl crystals is played by the cadmium point defects. In crystals with a higher zinc content (x = 0.05), the compensation of charged defects is incompletely controlled by changing the cadmium vapor pressure, which is evidence of a significant influence of the zinc point defects.     

Production of double-layer epitaxial structures based on the solid solution of the Cd-Hg-Te system using a combination of LPE and MOCVD techniques

The technique of growing CdTe and Cd _x Hg_{1 ? x} Te layers utilizing chemical metalloorganic compound vapor deposition (MOCVD) onto a CdZnTe(111) substrate with a preliminarily deposited Cd _y Hg_{1 ? y} Te layer using liquid phase epitaxy (LPE) has been developed. No noticeable changes in the electrophysical parameters and composition of the Cd _y Hg_{1 ? y} Te layer take place under such conditions during MOCVD deposition.     

Growth and properties of CdxHg1−xTe crystals

It has been found that there are two main factors complicating the preparation of Cd_xHg_1–xTe. The marked difference between the liquidus and solidus curves of the CdTe-HgTe pseudo-binary system gives the expected problems of segregation of CdTe with respect to HgTe during growth but there are also problems due to the segregation of any excess Te in the melt. A 2% excess of Te can give rise to pronounced constitutional supercooling effects. To avoid this, careful control of melt stoichiometry is required. This is made difficult by the high vapour pressure of mercury over the melt, the value of which is not known with great accuracy.The conditions of melt stoichiometry required for crystal growth do not necessarily give material of the required type and resistivity and this must be adjusted after growth by annealing at a controlled mercury pressure at a fixed temperature.     

Structure and optical properties of polycrystalline Cd1−xZnxTe thin films prepared by simultaneous evaporation

Cd_1–xZn_xTe films were deposited by simultaneous evaporation of CdTe and ZnTe. These Cd_1–xZn_xTe films were of cubic phase, and strongly (1 1 1) oriented as deposited. Predominant direct optical transitions were observed and the band gap varied with zinc content in a non-linear way. A structure development of CdS/CdTe/ZnTe : Cu solar cells with a Cd_1–xZn_xTe buffer layer was proposed.     

Hall probe Cdx Hg1-x Te for use in magnetic investigations of Nb3Sn superconducting layers

The critical temperature of a superconducting Nb₃Sn layer has been measured by repulsion of magnetic flux as a function of temperature, using a Hall probe. Hall probes of an active semiconductor film, Cd_xHg_1-xTe (x = 0.175), have been made by thermal evaporation in a vacuum. The chemical composition of the Cd_xHg_1-xTe thin films have been determined by spectrophotometric analysis. The Hall probes have been characterized by electric measurements over a temperature range of 4.2–18.5 K in a magnetic field. The probes are particulary suitable for magnetic measurements of superconducting Nb₃Sn layers at low temperatures.     

Studies on growth mechanism of HgCdTe epilayer on Si grown by HWE

In this paper, the mechanism of Hg_1−xCd_xTe/Si heterojunction grown by HWE (Hot Well Epitaxy) was studied. Opitical characterizations were shown with FTIR, the composition x = 0.39 was deduced by using MIR transmittance, the absorbing peak at 319.4 cm⁻¹ was measured by FIR transmittance, 319.4 cm⁻¹ confirmed the existence of Si–Te bond of at Si/HgCdTe interfacial layer. The I-V characteristics at both room temperature and 77 K of HgCdTe (n-type)/Si (p-type) heterojunction show that the good p-n heterojunction properties was obtained by using HWE. XRD study confirmed the formation of (111) oriented HgCdTe on (211) Si. Morphology of a cross section observed using EPMA indicates the columnar growth of HgCdTe. An analysis of interfcial layer by EPMA indicated presence of three layers composed of Si + Te, Si + Te + Hg and Si + Te + Cd + Hg. Among them, the most important one is the first layer. The problem of lattice mismatch and the difference of thermal expansion coefficient between Si and CdTe or HgTe may be improved by formation of Si–Te stable chemical bond through bybridization orbital bonding between Si and Te. The second and third layers are formed by evaporation-interdiffusion. Formation of the whole interfacial layer provides the appetency for the growth of (111) Hg_1−xCd_xTe epilayer on (211) Si substrate.     

Investigation of the spectra of light reflected off CdxHg1 − xTe thin films

The spectra of the fundamental reflectivity of Cd_xHg_{1 ? x}Te solid solution thin films thermally evaporated from raw polycrystalline material onto mica substrates have been investigated in the 1.7?5.5 eV energy range. Measurements of the positions of the reflection maxima as a function of the chemical composition determined by the spectrophotometric method for compositions from x = 0.00 up to x = 0.25 have been carried out.From these investigations a linear dependence of the positions E₁ of the reflection maxima on the composition x has been obtained. The results thus obtained are compared with results published in the literature for Cd_xHg_{1 ? x}Te single-crystal solid solutions as well as for amorphous and polycrystalline thin films of some other materials. Finally, the possibility of obtaining a criterion for the degree of crystalline order in a thin film is considered.     

Anodic polarization of HgTe,CdTe and Hg0.8Cd0.2Te — Oxide formation kinetics and composition

A comparative study of the anodic polarization behaviour of HgTe, CdTe and Hg_0.8Cd_0.2Te at constant current density was undertaken. It is argued that metal dissolution starts first, which renders the semiconductor surface negatively charged and tellurium rich. The process continues until the overpotential across the interface rises sufficiently to dissociate the tellurium at which point tellurium dissolution and oxidation begins. The continuation of this process evidently requires that the metal dissolution follows. This dissolution-precipitation mechanism for oxide nucleation is supported here. It is predicted that the molar ratio of HgTeO₃ to CdTeO₃ in the anodic oxide on Hg_0.8Cd_0.2Te should be 1.1. This is in good agreement with the experiments. The overvoltages required to initiate oxidation are shown to decrease when the current density is increased.     

Molecular beam epitaxy of CdTe and CdxHg1−xTe

Epitaxial layers of Cd_xHg_1?xTe were grown by molecular beam epitaxy on the Te(111) face of CdTe substrates at up to 120 °C. These layers show good crystallinity and a low carrier concentration at 77 K. Their low Hall mobility which seems to be due to twins can be increased by a post-growth annealing.     

A simple reactor for growth of HgCdTe films using LPE technique

A reactor for growth of Hg_1−x Cd _x Te epilayers by liquid phase epitaxy has been designed and developed. The layers have been successfully grown using this liquid phase epitaxy (LPE) reactor.     

Zinc incorporation into CdTe quantum dots in glass

We report zinc incorporation into CdTe nanoparticles grown by two-step solid phase precipitation in commercial borosilicate glass quenched from the melt, based on a co-evaluation of the results of resonant Raman and optical absorption measurements. Resonant Raman spectra display a two-peak structure at wave-number positions corresponding to ternary compound Zn_xCd_1?xTe. We attribute the higher intensity peak between 190 and 195 cm^?1 to the first harmonic of the zone-center longitudinal optical mode (LO₁) and, the lower intensity peak between 157 and 160 cm^?1 to the second harmonic (LO₂) of Zn_xCd_1?xTe crystal. The wave-number of vibrational Raman modes indicates that zinc content varies between 39 and 50% during the growth of quantum dots. The asymptotic absorption edge against heat-treatment time plot extrapolates to a bulk band gap of 1.714 eV which sets a lower limit of 31% for zinc incorporated into quantum dots which is consistent with the results of resonant Raman measurements. The energetic position of asymptotic absorption edge of 1.592 eV and an additional unresolved weak structure in Raman spectrum between 166 and 182 cm^?1 observed for as-received glass might serve as a evidence for the occurrence of a different nanocrystalline phase with 13% zinc content.     

Influence of SiO2 surface passivation on carrier lifetimes in n-type Hg0.7Cd0.3Te

The influence of an SiO₂ surface passivation layer on the electronic transport and carrier recombination properties of n-type Hg_0.7Cd_0.3Te crystals has been investigated. An increase of the excess carrier lifetimes was observed, which was due to the formation of a built-in potential at the HgCdTe/SiO₂ interface. As the recombination performance of the investigated samples is comparable to that of standard Hg_0.7Cd_0.3Te material, SiO₂ passivation of Hg_1t-xCd_xTE is potentially able to replace the commonly used anodic oxide passivation.