xCdxTe Crystals Research Articles

Reaction constants for main cationic native defects in narrow-gap Hg1−xCdxTe crystals

The equilibrium defect structure of Hg1−xCdxTe crystals and the diffusion of mercury in this material have been investigated. The results are presented. The results were obtained by using an optical method for free carrier concentration measurements at T≈300K. A new model for mercury diffusion was used, taking into account the effect of charged defect electric drift. As result, a consistent system of reaction constants for mercury vacancies and the mercury interstitials in Hg1−xCdxTe crystals has been obtained.

A two-stage technique for single crystal growth of HgCdTe using a pressurized Bridgman method

A two-stage technique has been used to grow large diameter Hg1−xCdxTe crystals (x=0.214) using a pressurized Bridgman (P-Bridgman) method, with a starting charge of x=0.06. The two-stage technique has enabled the growth of large crystalline ingots of HgCdTe 40mm in diameter at the very low growth temperature of 680–720°C. It was shown that the HgCdTe crystal had a homogeneous composition of x=0.214 along the growth direction.

Study of ZnCdTe crystals as terahertz wave emitters and detectors

We report a systematic study of ternary ZnCdTe crystals as terahertz wave emitters and sensors with a femtosecond Ti:sapphire laser. Experimental measurements of the generation and detection of terahertz waves from 〈110〉-oriented Zn1−xCdxTe crystals indicate the optimum composition x=0.05. We also report generation and detection of terahertz wave from doped crystals. For terahertz wave applications, the resistivity of these crystals should be greater than 100 Ω cm.

Activation-controlled conduction and metal-insulator transition in the impurity band of narrow-gap p-Hg1−x CdxTe doped crystals

Metal-insulator transition and hopping conduction are studied in narrow-gap copper-doped p-Hg1−xCdxTe crystals in a wide range of temperatures, compositions x, and impurity concentrations. It is shown that, as distinct from wide-gap semiconductors, a characteristic size of shallow acceptor wave function responsible for the hopping mechanism of conduction is determined by the effective mass of a heavy, rather than light, hole. The presence of low-temperature ɛ2-conduction over the delocalized states of positively charged acceptors near the metal-insulator transition is established.

Mechanism for Conversion of the Type of Conductivity in p-Hg1–xCdxTe Crystals upon Bombardment by Low-Energy Ions

The main peculiarities of the p–n conversion of the type of conductivity in narrow-band p-Hg1–xCdxTe solid solutions containing vacancies of mercury upon bombardment by low-energy ions are explained based on the traditional notions about the chemical diffusion of mercury. These peculiarities are related, on the one hand, to the features of the defect formation in Hg0.8Cd0.2Te (containing a small amount of high-mobility interstitial mercury atoms with a great amount of low-mobility vacancies) and, on the other hand, to the high concentrations of intrinsic electrons and holes efficiently screening the electric field of the defect layer. The high conversion rate realized upon ion bombardment, as compared to the conversion rate taking place upon annealing in mercury vapors, is due to the fact that nonequilibrium interstitial mercury atoms are produced in abundance near the surface of the crystal subject to bombardment. This effect depends substantially on the electric field appearing near the outer boundary of the converted layer; therefore, as the Hg content, and, hence, the width of the forbidden band, is increased, one should expect a noticeable decrease in p-n conversion rate.

Is Hg1 − xCdxTe grown epitaxially by the travelling heater method? Does it matter?

Crystal growth by the travelling heater method (THM) is considered to be a large scale, continuous, liquid phase epitaxial (LPE) process [e.g. R. Triboulet et al. J. Crystal Growth 79 (1986) 695]. We studied the structural perfection of Hg1 − xCdxTe crystals grown by THM using preferential chemical etching and X-ray double crystal rocking curves. In principle, THM is similar to LPE; in practice, the Hg1 − xCdxTe crystals are not epitaxially grown. The high temperature gradients involved in the THM growth process cause a stress field. This stress creates dislocations, which pile up by polygonization to form a veining pattern of low angle sub-grain boundaries (LASGBs) in the crystal. Such crystals, consisting of a mosaic of small, almost perfectly aligned single crystals, may only be considered “oriented grown”. The LASGBs have a major detrimental effect on the performance of photodiode arrays for the detection of infrared radiation. Diode leakage depends on the etch-pit density along the LASGB, the proximity of the diode and the extent of intersection of the diode by the LASGBs.

Hole scattering mechanisms in Hg1−xCdxTe

In this paper, we analyze and discuss the roles of nine different scattering mechanisms—ionized impurity, polar and nonpolar optical, acoustic, dislocation, strain field, alloy disorder, neutral impurity, and piezoelectric—in limiting the hole mobilities in p-type Hg1−xCdxTe crystals. The analysis is based on obtaining a good fit between theory and experiment for the light and heavy hole drift mobilities by optimizing certain unknown (or at the most vaguely known) material parameters such as the heavy hole mobility effective mass, degree of compensation, and the dislocation and strain field scattering strengths. For theoretical calculations, we have adopted the relaxation time approach, keeping in view its inadequacy for the polar scattering. The energy dispersive hole relaxation times have been drawn from the published literature that take into account the p-symmetry of valence band wave functions. The temperature dependencies of multiple charge states of impurities and of Debye screening length have been taken into account through a numerical calculation for the Fermi energy. Mobility data for the present analysis have been selected from the HgCdTe literature to represent a wide range of material characteristics (x=0.2–0.4, p=3×1015–1×1017 cm−3 at 77K, μpeak≅200-1000cm2V−1s−1). While analyzing the light hole mobility, the acoustic deformation and neutral impurity potentials were also treated as adjustable. We conclude that • the heavy hole mobility is largely governed by the ionized impurity scattering, unless the strain field or dislocation scattering below 50K, or the polar scattering above 200K, become dominant; • the light hole mobility is mainly governed by the acoustic phonon scattering, except at temperatures below 30K where the neutral impurity, strain field and dislocation scattering also become significant; • the intervalence scattering transitions make negligible impact on the heavy hole mobility, but virtually limit the light hole mobility; • the alloy disorder scattering does not dominate in any temperature region, although it exercises some influence at intermediate temperatures; • the heavy hole mobility effective mass ratio mhh/mo∼-0.28–0.33 for crystals with x<0.4; and • the light hole band deformation potential constant is ∼12 eV.

X-ray asymptotic Bragg diffraction: study of interfaces between Hg 1− xCd xTe crystals and films

Interfaces between Hg 1− x Cd x Te crystals and oxide films have been investigated by X-ray asymptotic Bragg diffraction. The parameters which characterize this interface on the crystal polar faces (1 1 1) have been found. The effect of the crystal electric field on the ion transport in the subsurface region is discussed.

Electrical properties and compositional distributions of CVT and PVT grown Hg1−xCdxTe epilayers

Epitaxial layers of Hg1−xCdx Te were grown on CdTe substrates by the chemical vapor transport technique using Hgl2 as a transport agent. The epilayers were of nearly uniform composition both laterally and to a depth of about one-half of the layer thickness. By comparison, the composition varied continuously throughout the depth of the layer for epilayers grown by the physical vapor transport technique. Layers were grown both p- and n-type with carrier concentrations on the order of 1017 cm−3. Low-temperature annealing was used to convert the p-type layers into n-type. The room-temperature carrier mobilities of as-grown and converted n-type layers ranged from 103 to 104 cm2/V-s depending on the composition and are comparable to previous literature values for undoped Hg1−xCdxTe crystals.

Twinning Effects in Hg1−xCdxTe(x˜0.2) Grown by THM in the 〈111〉 Direction

AbstractHg1−xCdxTe crystals with x˜0.2 were grown by the traveling heater method (THM), in either the [111]A or [111]B directions, using oriented CdTe seeds. Twins are sometimes formed during the growth of these crystals. In crystals grown in the [111]A direction the twins, of orientation [511]B, are constantly growing at the expense of the original [111]A oriented grain. Growth in the [111]B direction, on the other hand, suppresses the growth of the twin domain. Photodiodes and capacitors realized on the (111)A plane are markedly superior to those on the twin plane, [511]B. The difference is due to higher fixed charge and larger fast surface state densities in the case of the [511]B plane. These effects are explained by the lattice structures in the {111} and {511} planes and their possible influence on surface reactivity.

Thermodynamic study of solid-vapour equilibrium in the (Hg 1−xCd x) 1−yTe y system

A quantitative relationship between composition, temperature and partial pressures for HgCdTe materials under solid-vapour equilibrium state has been calculated for the first time. The calculation results have been used to explain some experimental phenomena reasonably well. These results are an important complement to the phase diagrams of the HgCdTe ternary system, which are very useful for VPE and annealing technologies of Hg 1−xCd xTe crystals.

Twinning Effects in Hg1−xCdxTe (x∼0.2) Grown by Thm in The &lt;111&gt; Direction

ABSTRACTHg1−xCdxTe crystals with x∼0.2 were grown by the traveling heater method (THM), in either the [111]A or [111]B directions, using oriented CdTe seeds. Twins are sometimes formed during the growth of these crystals. In crystals grown in the [11]A direction the twins, of orientation [511]B, are constantly growing at the expense of the original [111]A oriented grain. Growth in the [111]B direction, on the other hand, suppresses the growth of the twin domain. Photodiodes and capacitors realized on the (111)A plane are markedly superior to those on the twin plane, (511)B. The difference is due to higher fixed charge and larger fast surface state densities in the case of the (511)B plane. These effects are explained by the lattice structures in the {111} and {511} planes and their possible influence on surface reactivity.

Depth distribution of radiation defects in implanted Hg1−xCdxTe crystals

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Surface properties of p-Hg1−xCdxTe

Electrical conductivity σ and Hall coefficient RH have been measured in p-Hg1−xCdxTe crystals with x≊0.2 in the temperature region 4.2–300 K. The influence of a crystal surface on galvanomagnetic properties is observed only in the temperature region T&amp;lt;20 K. Both positive and negative charges are formed on the surface under the influence of a natural air atmosphere after etching in BrMeOH. The formation of an accumulation layer on p-HgCdTe has been observed for the first time. The growth of an inversion layer was studied over one year after etching. The concentration of surface electrons reached the value 3.5 × 1012 cm−2. In the accumulation layer, the hole surface concentration rapidly increased within two months from 1013 cm−2 to 1014 cm−2. The bending of the bands has been calculated, first, by a classical method of solving the Poisson equation and second, by a quantum method using the Zawadski model of a triangular asymmetrical potential well. Preliminary study shows that the type of layer created, inversion or accumulation, can be connected with the orientation of the surface.

Topographic methods of studying defects in Hg 1−xCd xTe crystals

In this paper, three topographic methods of studying defects in Hg 1− x Cd x Te crystals are described: the back-reflection method with a “whiter” beam, the scanning-reflection method with monochromatic radiation, and the glancing-incidence method with a “white” beam. Unique results obtained by these methods are shown: topographs as a referable criterion of various defects are laid down and explanation of the topographs is carried out theoretically. A set of topographs with reversed contrasts is made and the microorientation differences among the mosaic blocks are determined. Topographs of the intensive strain field in the core region of ingots prepared by the quench-recrystallization method are made and the mechanism of crystal growth is interpreted according to these topographs. In addition, typical topographs made by the three methods respectively, are shown.

Radiation-induced defects in implanted Hg1–xCdxTe crystals

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Ion beam milling effect on electrical properties of Hg1−xCdxTe

Changes induced near the surface of Hg1−xCdxTe crystals due to irradiation caused by low-energy ion beam milling have been investigated. The milling induces a thick n-type layer near the surface, more than 10 μm thick, within 2–3 min. Most of this layer is well-behaved n type with carrier concentration on the order of 1016 cm−3, electron mobility higher than 105 cm2/V s at 77 K, and relatively large lifetime. The temperature dependence is characteristic of uncompensated material, and compensation in highly compensated materials is reduced considerably. The sidewall effect is small; the direction of propagation obeys probably a cosine law. A three-step mechanism is proposed to account for these phenomena, which consists of creation of extended defects, fast propagation of metal ions along them, accompanied by annihilation of metal vacancies, and recombination of defects.

Radiation defects in Ar+-ion-implanted Hg1?xCdxTe crystals

A study has been made of defect formation in Hg1−xCdxTe crystals after implantation with argon ions possessing energies of 50 and 150 keV over a wide range of radiation doses. The spatial distributions of electrically active radiation defects, measured by electrophysical methods, are compared with extended lattice imperfections, determined from Rutherford backscattering spectra. The distribution of radiation donors has been found to be shifted deeper into the semiconductor beyond the region of the interstitial impurity distribution and radiation donors have been found to be neutralized by extended lattice imperfections. The paper discusses the principal factors in the formation of the profiles of the carrier concentration distribution in the implanted cadmium telluride-mercury layer.

Investigation of radiation defects in electron irradiated Hg1−xCdxTe crystals using positron annihilation

AbstractThe electron characteristics of defects in the initial and electron irradiated Hg1−xCdxTe (2–3 MeV, 1018 cm−2, 300 K) crystals using the positron annihilation method have been investigated. The data of electric measurements are confirmed on connection of p‐type conductivity with vacancy defects of metal sublattice initial crystals Hg1−xCdxTe. An analysis of correlation curves of irradiated crystals has shown a possibility of formation of associations of initial defects and radiation damages of vacancy type during radiation process. The presence of narrow component on correlation curves in the region of small angles is associated with formation of positronium states localized in the region of radiation defect complex of vacancy type. Identification of positron‐sensitive defects with electrically active radiation induced ones has been carried out according to the results of isochronal annealing of irradiated crystals.

Atomic migration and surface evaporation of Hg in Hg1−x CdxTe crystals observed by 40-MeV O5+ ion backscattering method

Dissociation and evaporation of Hg from the near-surface region of Hg1−xCdxTe crystals was investigated, up to several microns, by means of 40-MeV O5+ backscattering. Samples with x=0.18 and 0.32 were heat treated in a vacuum for 60 min at various temperatures up to 320 and 340 °C, respectively. The change of the backscattering spectra with the temperature of heat treatment indicated that the Hg atoms outdiffused partly from the near-surface region of the crystals at higher temperatures than 280 and 320 °C for x=0.18 and 0.32, respectively. This behavior of outdiffusion is in contrast to the case of HgTe crystal.