Hg1-xCdxTe Films Research Articles

Electrical and microscopic characterization of p+-type layers formed in HgCdTe by arsenic implantation

The Hall-effect (with mobility spectrum analysis) and capacitance measurements were combined with transmission electron microscopy for the study of molecular-beam epitaxy-grown n-type Hg1-xCdxTe (x = 0.22) films subjected to various operations used in the fabrication of p +-n photodiode structures. The operations included arsenic ion implantation, activation annealing and extra annealings aimed at investigation of the details of the p +-n junction formation. A detailed characterization of p+-type layers formed by the implantation was done. A high degree of arsenic activation by activation annealing was demonstrated, along with the annihilation of implantation-induced extended (dislocation loops) and quasi-point defects, and the disappearance of electrically active centres associated with them. The annealing was also shown to cause the activation of residual acceptors, changing the degree of electrical compensation in the n-‘base’ of some of the p +-n structures.

Direct comparison of the results of arsenic ion implantation in n– and p–type Hg0.8Cd0.2Te

Optical reflectance in the visible wavelength range, transmission electron microscopy, and the Hall-effect measurements with mobility spectrum analysis have been used for the direct comparison of the results of arsenic ion implantation in samples with n– and p–type conductivity fabricated on the basis of a Hg1-xCdxTe film with x = 0.22 grown by molecular beam epitaxy on a Si substrate. Optical reflectance studies showed that the effect of ion implantation on the properties of the surface was very similar in n– and p–type material. Transmission electron microscopy showed that defect patterns were also similar in n– and p–type samples in terms of the thicknesses of implantation-damaged layers and types of defects formed. Electrical studies demonstrated that low- and mid-mobility electrons induced by the implantation in n– and p–type material had similar average concentration and mobility. It was concluded that the nature of donor defects that produced these electrons was the same in n– and p–type HgCdTe, and it was suggested that the defects in question were interstitial mercury atoms captured by dislocation loops and by quasi-point defects formed as a result of implantation.

Strong Disorder in HgCdTe Studied with Optical Methods and X-Ray Diffraction

Optical transmission, photoluminescence, and X-ray diffraction have been used for studying structural disorder in Hg1-xCdxTe (x=0.3–0.4) films grown by Molecular-Beam Epitaxy on GaAs and Si substrates. According to all three methods, studied films immediately after the growth showed quite different scale of the disorder. After thermal annealing films showed similar optical properties, yet their structural properties remained different. It appears that the ability of Hg1-xCdxTe to gain good optical quality under annealing for considerably disordered initial material is not directly structure-related.

Maximum entropy mobility spectrum analysis of LPE-grown and anodic oxidated Hg1-xCdxTe(x=0.237)

In this paper, magneto-transport properties of the LPE-grown and anodic oxidated p-type Hg1-xCdxTe(x=0.237) films have been studied by using maximum entropy mobility spectrum analysis (ME-MSA) technique. It can be found that the high-mobility electron (μe∼2 × 104cm2/Vs) has considerable contributions to the conduction of anodic oxidated Hg1-xCdxTe(x=0.237) film, but not in LPE-grown Hg1-xCdxTe(x=0.237) film. The high-mobility electron maintains dominant contributions from 11k to 150k, which can be attributed to two-dimensional electron gas in the inversion layer of anodic oxidated p-type Hg1-xCdxTe(x=0.237) film. In addition, we also observe the nonphysical contributions of low mobility electrons (μe∼0.08 × 104cm2/Vs) in mobility spectrum of both LPE-grown and anodic oxidated p-type HgCdTe films. The low-mobility electrons, so-called mirror peaks, can be interpreted as a consequence of magnetic freeze-out of holes in vacancy-doped HgCdTe, which disappeared at T=150k.

Optical and electrical studies of arsenic–implanted HgCdTe films grown with molecular beam epitaxy on GaAs and Si substrates

A defect study was performed on arsenic-implanted Hg1-xCdxTe (x=0.23–0.30) films with graded-gap surface layers, grown with molecular-beam epitaxy on GaAs and Si substrates and designed for fabrication of ‘p+–n’-type photodiodes. First, formation of n+–p structure was investigated in p-type material, in order to study radiation-induced donor defects. Next, formation of p+–n structure was investigated in the course of implantation in n-type material and arsenic activation annealing. Influence of the graded-gap surface layer was found to be expressed in the degree of saturation of the concentration of radiation-induced defects, with results obtained on arsenic- and boron-implanted material differing due to the difference in the ion masses.

Properties of arsenic–implanted Hg1-xCdxTe MBE films

Defect structure of arsenic-implanted Hg1-x Cdx Te films (x =0.23–0.30) grown with molecular-beam epitaxy on Si substrates was investigated with the use of optical methods and by studying the electrical properties of the films. The structural perfection of the films remained higher after implantation with more energetic arsenic ions (350 keV vs 190 keV). 100%-activation of implanted ions as a result of post-implantation annealing was achieved, as well as the effective removal of radiation-induced donor defects. In some samples, however, activation of acceptor-like defects not related to mercury vacancies as a result of annealing was observed, possibly related to the effect of the substrate.

Hole Transport in Arsenic-Doped Hg1−x Cd x Te with x ≥ 0.5

Hole transport in arsenic-doped p-type Hg1-xCdxTe epitaxial layers with x ≥ 0.5 has been studied employing Hall-effect measurements and theoretical modeling of hole scattering mechanisms. The hole transport parameters extracted from four different Hg1-xCdxTe films with x = 0.50, 0.56–0.58, 0.65, and 0.80, were analyzed using an iterative solution of Boltzmann’s transport equation. Hole mobilities in the samples with x values of 0.5 and 0.56–0.58 were found to be predominantly limited by ionized impurity scattering, and exhibited relatively high impurity compensation ratios ≥2. The sample with x = 0.65 exhibited the highest hole mobility, a low compensation ratio of 1.05, and mobility characteristics were limited predominantly by polar optical phonon scattering at temperatures ≥200 K. Hole mobility in the sample with x = 0.80 was found to be limited by polar optical phonon scattering and ionized impurity scattering (compensation ratio 1.20–1.56). Although the sample temperatures employed were not sufficiently low to unambiguously discriminate the scattering strength of static strain and dislocations, the experimental hole mobility characteristics cannot be adequately modeled if these two mechanisms are neglected. The ionization energy of the arsenic acceptor impurities was found to exhibit a quadratic dependence on the CdTe mole fraction.

Hg1-xCdxTe as the Bottom Cell Material in Tandem II-VI Solar Cells

AbstractWe have measured the electrical, optical and morphological properties of as-grown Hg1-xCdxTe films prepared by r.f. sputtering. The Hg1-xCdxTe films were grown at substrate temperatures ranging from 25°C to 150°C. Films grown at temperatures lower than or equal to 70°C were highly resistive (≥105 Φ-cm) and were not measurable by our Hall apparatus. Optical transmission data show that the band gap of these films ranged from 0.8 eV-1.5 eV, satisfying the optimum band gap criteria. Plan view and cross sectional SEM studies indicate that the films grown at 85°C and 100°C have larger grains with compact grain boundaries and these films typically yield the best cell performance. Spectroscopic ellipsometry studies are being used to estimate the band-gap as a function of substrate temperature.We have also studied the effect of various back contacts and have fabricated complete solar cells. We found that Cu-Au serves as the best ohmic back contact to CdS/HgCdTe solar cell. Our preliminary results of J-V analyses on the complete solar cell show that the efficiency is mainly limited by the short-circuit current. Electrical-bias-dependent QE measurements indicate voltage-dependent current collection mainly in the long wavelength region. Further optimization of growth parameters and CdCl2 treatment needs to be carried out to improve the cell performance.

Relationship between lattice parameters and compositions of molecular beam epitaxial Hg1－xCdxTe films

High_resolution x_ray diffraction technology is applied to the measurement of la ttice parameters and the study of strain for Hg1-xCdxTe fi lms gr own by molecular beam epitaxy. The results show that there exist both perpendicu lar strain and shear strain in Hg1-xCdxTe films. Based on crystal elastic theory, strain states in Hg1-xCdxTe films are analyz ed and calculated, and the lattice parameters of 1-xCdxTe film s at the relaxation state are obtained. It is found that the relationship betwee n lattice parameters and compositions of Hg1-xCdxTe films agrees with Vegard's law, rather than Higgins formula proposed in earlier research. It is also found that the lattice parameters can be derived from the measured data of (224) plane spacing, and that the compositions of Hg1-xCdxTe films can also be evaluated by using Vegard's law with an error about 0 01.

Optimizing boron implantation dose of HgCdTe infrared detectors

Using the material chip technology,large area photodiodes of n-on-p structure with different boron implantation dose are fabricated on the Hg1-xCdxTe film for mid-infrared wavelength region(x=0.291).Current-voltage characteristics of the photodiodes are measured at 77K and zero bias resistance-area products of different photodiodes are fitted from the data in the voltage range of -0.2—0.08V.The study indicated that the R0A products of different elements depended distinctly upon the implanted boron dose.A large R0 value has also obtained in another chip with x=0.2743.All the samples in this study are grown by Riber 32P molecular-beam epitaxy system and all the junctions-forming process is same to the standard planar technology but using a series of metallic masks during the boron ion implantation.

A dual-phase-lag diffusion model for predicting thin film growth

A dual-phase-lag diffusion (DPLD) model, which extends Fick's law by including two lagging times, j for the mass flux vector and for the density gradient, is developed to predict thin film growth. Depending upon the phase lag ratio /j, the DPLD model uniquely characterizes four types of growth kinetics as reported in the literature. The model validation with experimental data of silicon oxidation and Hg1-xCdxTe film deposition demonstrates that the present model captures the anomalous behaviour of thin film growth from the very beginning of the process to relatively long times very well.

CHARACTERIZATION OF Hg1-xCdxTe EPITAXIAL FILMS ON VARIOUS VICINAL PLANES

The Hg1-xCdxTe films were prepared by a dipping liquid phase epitaxy technique on spherical CdTe substrate with several different misoriented facets. They were characterized by means of micrograph,infrared absorption spectra, Raman scattering, and X ray double crystal diffraction. The results show that-near facet (δ<1°) and terrace free (δ≈1.2 °) growth modes are superior in both the planarity and the amount of precipitates.

Influence of the liquid phase epitaxial growth conditions on composition profile for Hg1-xCdxTe film

In this paper, the expressions for the effective segregation coefficient and composition depth profile of Hg1-xCdxTe liquid phase epitaxial (LPE) film are presented and compared with the experimental data. The expressions are derived based on the associated solution model and the phase diagram theory of Hg1-xCdxTe. The results show that supercooling, cooling rate, liquid composition, and nonequilibrium Hg pressure over the solution will result in the changes of average composition and composition-depth profile of the grown film. Adequate Hg loss, slow cooling rate, and proper prescription of liquid composition can improve the uniformity of longitudinal composition of Hg1-xCdxTe LPE film.

Study on the composition profile of a Hg1-xCdxTe epitaxy film by infrared transmission spectroscopy

Based on our empirical rules for the intrinsic absorption coefficient and refractive index of Hg1-xCdxTe in Hougen's model, a useful method for determining the composition profile of an epitaxy layer from room-temperature infrared transmittance spectroscopy is presented. The compositional depth non-uniformity of Hg1-xCdxTe film samples grown by liquid-phase epitaxy, metal-organic chemical vapour deposition and molecular beam epitaxy techniques is determined using this method and compared with that from secondary ion mass spectroscopy and X-ray microprobic measurements.

Determination of strains in Hg1-xCdxTe thin film material

Determination of strains in Hg_1-<i>x</i>Cd_<i>x</i>Te thin film material

Raman studies of composition and structural ordering in Hg1-xCdxTe

Information on both the alloy composition and the lattice properties of Hg1-xCdxTe for x-values between 0.20 and 0.31 have been obtained by resonance Raman spectroscopy with laser photon energies between 2.35 and 2.7 eV. In addition to the HgTe-like TO and LO modes and the CdTe-like LO mode, the authors have studied the resonance behaviour of the mode at 133 cm-1, which has been identified as originating from the preferential clustering of 3Hg and 1Cd about the Te. They find that the intensity of this peak for various bulk and epitaxially grown samples is unusually large only near the E1 resonance. Pulsed laser annealing with an Nd:YAG-pumped dye laser strongly suppresses this mode in all samples, suggesting that extremely rapid epitaxial regrowth may inhibit the 3:1 cluster formation. Detailed comparisons are presented for Hg1-xCdxTe films grown by liquid phase epitaxy (LPE), molecular beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD) procedures.

Molecular beam epitaxy of CdTe and Hg1-xCdxTe ON GaAs (100)

Using the molecular beam epitaxial (MBE) technique, CdTe and Hg1-xCdxTe have been grown on Cr-doped GaAs (100) sub-strates. A single effusion cell charged with polycrystal-line CdTe is used for the growth of CdTe films. The CdTe films grown at 200 °C with a growth rate of ~ 2 μm/hr show both streaked and “Kikuchi” patterns, indicating single crystalline CdTe films are smoothly grown on the GaAs sub-strates. A sharp emission peak is observed at near band-edge (7865 A, 1.577 eV) in the photoluminescence spectrum at 77 K. For the growth of Hg1-xCdxTe films, separate sources of HgTe, Cd and Te are used. Hg0.6Cd0.4Te films are grown at 50 °C with a growth rate of 1.7 μm/hr. The surfaces are mirror-smooth and the interfaces between the films and the substrates are very flat and smooth. As-grown Hg0.6Cd0.4Te films are p-type and converted into n-type by annealing in Hg pressure. Carrier concentration and Hall mobility of an annealed Hg0.6Cd0.4Te film are 1 × 1017 cm−3 and 1000 cm2/V-sec at 77 K, respectively.