Compounds CdTe Research Articles

Volumetric effects of ZnTe, CdTe and HgTe compounds at melting and subsequent heating

The temperature dependence of the density of CdTe, ZnTe and HgTe compounds was studied by means of the γ– ray attenuation method both in solid and liquid states. A decrease in the density in the course of the crystal → melt phase transition and subsequent heating for ZnTe and CdTe is obtained. The volume effect during the HgTe melting is negative. According to our evaluations, the increase of the external pressure should result in an increase of the fusion temperature for ZnTe and CdTe. The dT/dP coefficient of the HgTe at the melting point is negative. The density of the molten HgTe compound changes along a curve with a maximum. This anomaly is explained by the thermal dissociation of the HgTe with the formation of 2 complex associates in the liquid phase. The thermal expansion of the Zn and Cd tellurides is considered on the basis of the characteristics of the interatomic bonds strength.

Shallow donor muonium states in II-VI semiconductor compounds

Experimental data on shallow donor muonium states in the $\mathrm{II}\mathrm{\ensuremath{-}}\mathrm{VI}$ semiconductor compounds CdS, CdSe, CdTe, and ZnO are presented. These are characterized by very weak hyperfine interactions amounting to approximately ${10}^{\ensuremath{-}4}$ of the free-atom value, and by donor levels with binding energies comparable to those calculated on the hydrogenic model of the shallow centers. The data are discussed in terms of a model generalizing the knowledge of muonium and hydrogen states in tetrahedrally bonded semiconductors. Within this model the shallow muonium state is attributed to muonium bound to the anion of the $\mathrm{II}\mathrm{\ensuremath{-}}\mathrm{VI}$ compound and located at the antibonding site or close to it in the bond direction.

Phonons on II-VI (110) semiconductor surfaces

We have studied the phonon dispersion and density-of-states on the (110) surface of II-VI compounds CdTe, ZnTe, and ZnS in their zinc-blende phase by applying the adiabatic bond charge model. The relaxed surface atomic geometry and the corresponding bond charge information is obtained from the application of an ab initio pseudopotential calculation. The origins of various surface phonon modes are discussed and their variation for different compounds are analyzed in terms of the reduced mass and total mass differences. Furthermore, we have related some differences between surface modes on II-VI (110) and III-V (110) in terms of the ionicity factor.

Bulk growth of quasi-binary quaternary alloys

Crystal growth experiments were conducted by: (a) melting together two III–V compounds (InAs, GaSb) and (b) melting together one III–V compound (InSb or GaSb) and one II–VI compound (CdTe or HgTe). These melts contain four different atoms, but in comparison to the conventional quaternary alloys A x C 1− x B y D 1− y , have a constrain in the composition given by x= y, and therefore can be designated as “quasi-binary”. The composition of these melts is (AB) 1− x (CD) x . Bulk crystals were produced from these melts by directional solidification and Czochralski pulling. Our goal was to determine whether the grown crystals will remain “quasi-binary” or, due to segregation will turn conventional quaternary x≠ y. The grown crystals were free of cracks, which can be explained by low lattice mismatch. Using the electron probe micro-analysis (EPMA), it was determined that quasi-binary (InSb) 1− x (CdTe) x crystals were obtained. The other systems yielded conventional quaternary specimens with x≠ y.

Measurement and Assessment of the Thermodynamic Properties and the Phase Diagram of the Cd–Te System

The enthalpy of formation of the CdTe compound was determined from its heat of solution at 773 K using a twin solution calorimeter. The heat contents of binary Cd-Te compositions were measured by a drop calorimeter in the concentration range from X Te = 0.50 to 0.95 and in the temperature range from 800 to 1500 K. A method of thermodynamic analysis was applied to the obtained data to derive the other thermodynamic properties. For the first lime enthalpies of mixing of the liquid Cd-Te system are derived from the experimental data. The enthalpy and entropy of mixing of the liquid Cd-Te phase show large negative values in the neighborhood of the compound composition. These strongly temperature dependent anomalies are explained by a chemical short range order in liquid alloys. The obtained thermodynamic and phase diagram data for the Cd-Te system Were used to reassess the system following CALPHAD standard. An associated solution model for the liquid phase and standard Gibbs energy functions for the three solid phases (Cd, CdTe and Te) were used. The phase diagram and thermodynamic properties calculated from the reassessed system agree well with the experimental data.

Electrical and optical investigation on doping of II-VI compounds using radioactive isotopes

Using radioactive isotopes of shallow dopants (Ag, As, Rb) as well as of native or isoelectronic elements (Se, Te, Cd, Sr) which were incorporated as host atoms and then transmuted into relevant dopants (transmutation doping) we investigated doping phenomena occurring in the wide band gap II–VI compounds CdTe, ZnTe, ZnSe and SrS by the classical methods of semiconductor physics: Hall effect, C–V and photoluminescence measurements. Thus, we could assign unambiguously defect features in electrical and photoluminescence measurements to extrinsic dopants by means of the half lives of radioactive decay. In As doped ZnSe samples we observed two states: a metastable effective mass like state and a deep state. The occurrence of the latter state is always linked with the high resistivity of As doped ZnSe crystals. The transmutation doping experiments reveal that the so-called self-compensation typical for wide band gab II–VI compounds can be overcome when the thermal treatment for dopant incorporation is time separated from its electrical activation, achieved using transmutation at room temperature. Under these conditions we found an almost one-to-one doping efficiency relative to the implanted dose. Thus, these investigations are a contribution to understanding compensation phenomena occurring due to interactions between dopants and native defects during conventional doping treatments.

Shallow doping of wide band-gap II–VI compounds

Using radioactive isotopes of shallow dopants (Ag, As, Rb) as well as of native or isoelectronic elements (Se, Te, Cd) which were incorporated as host atoms and then transmute into relevant dopants (transmutation doping), we investigated the doping phenomena occurring in wide band-gap II–VI compounds CdTe, ZnTe and ZnSe by the classical methods of semiconductor physics Hall effect, C–V and photoluminescence measurements. Thus, we could assign unambiguously defect features in electrical and photoluminescence measurements to extrinsic dopants by means of the half lives of radioactive decay. In As-doped ZnSe samples we observed two states: a metastable effective mass-like state and a deep state. The occurrence of the latter state is always linked with the high resistivity of As-doped ZnSe crystals. The transmutation doping experiments reveal, that the so-called self-compensation being typical for wide band-gap II–VI compounds can be overcome if the thermal treatment for dopant incorporation is time separated from its electrical activation, here due to transmutation at room temperature.

Surface processing of CdTe compound semiconductor by excimer laser doping

Laser processing technique has been developed to achieve heavy impurity doping in II–VI compound semiconductor such as CdTe which is excellent material for application in high energy flux detector because of its high absorption coefficient and energy resolution due to large atomic number and high carrier mobility. At first, transient temperature increment in CdTe due to excimer laser radiation will be discussed using computer simulation data. Using KrF excimer laser with a pulse width of 20 ns and a wavelength of 248 nm, one shot of laser of energy 80 mJ/cm 2 increases the surface temperature of CdTe more than 1000°C, the melting point of CdTe. As the laser pulse is very short and the penetration depth of UV laser light is very short, the depth of processed and modified layer is limited to few hundred angstroms. Secondly, we have experimentally studied the laser doping process for the application to detectors of high energy flux. Resistivities of CdTe surface processed by laser doping drastically decreased from 10 5 to 10 −1 Ω cm. p–i–n diode was thus fabricated on a high resistivity single crystal CdTe using excimer laser doping on one side for p-layer and n-type CdTe grown epitaxially on the other side. The p–i–n diodes resulted showed promising characteristics and high energy sensitivity. This technique is also promising to form ohmic contacts in other II–VI semiconductor devices. Because we can adapt this process at the final stages during device fabrication and also the laser effect is limited to a very thin layer, hence there will be no influence in the bulk characteristics of the device.

Growth of CdZnTe single crystals for radiation detectors

A new growth technique of CdZnTe single crystals for room-temperature radiation detectors is developed. It is based on a free growth from the vapour phase on a oriented single crystal seed. Binary polycrystalline CdTe and ZnTe compounds are placed and heated separately to produce divided flows which were introduced into the crystal growth zone where they mixed before deposition on the seed. Monocrystalline and homogeneous materials with high electrical resistivity, low defect density, and crystal volume up to ∼50 cm 3 are prepared by this technique.

Electrodeposition of CdTe Thin Films on Au(111)/Polyethylene.

Electrolyte/metal interface is interesting as the place of producing material because we can control the process dynamics by means of applied potential to metal. In this paper, we electrodeposited CdTe thin films on Au(111)/polyethylene in 50mM H2SO4+1mM CdSO4+0.1mM TeO2 aqueous solution. The electrodeposited thin films were investigated by X-ray Photoelectron Spectroscopy (XPS), Electro-Chemical Atomic Force Microscopy (EC-AFM) and electrochemical measurement. The film electrodeposited at-1000mV (vs. Hg/Hg2SO4) consisted mainly of CdTe compound and was much less oxidizable than Cd film and Te film. Based on the result of in-situ EC-AFM measurement, we ascertained the CdTe deposition mechanism under this condition.

Nitrogen doping in molecular-beam epitaxy growth of II–VI semiconductors

We report on the p-doping properties of the II–VI related compounds ZnTe, CdTe, BeTe and MgTe. Following a comparative study of nitrogen doping in these materials, we perceive the formation of nitride compounds as responsible for the trend observed in N-doped telluride materials. We have also investigated a possible enhanced intermixing process in CdZnMgTe/CdZnTe and ZnSe/BeTe superlattices due to nitrogen incorporation.

Pd-defect-complexes in ZnTe and CdTe and interaction with group-V-elements

In the II–VI compounds ZnTe and CdTe, tracer diffusion and perturbed angular correlation experiments revealed a hindered diffusion of Pd at elevated temperature in tellurium-rich material due to the formation of Pd-metal-vacancy complexes. In metal-rich n-type material, Pd distributes by diffusion over the entire sample volume. Additional group-V element implantation has different effects in ZnTe and CdTe: in metal-rich ZnTe, the Pd diffusion is hindered and complex formation at Pd is found; in metal-rich CdTe, the Pd diffusion seems to be unchanged and, in the case of N, a complex is formed at elevated temperatures interpreted as consisting of interstitial Pd and interstitial N.

Nitrogen doping of Te-based II–VI compounds during growth by molecular beam epitaxy

We present a detailed study of p-type doping of Te-based compounds (CdTe, ZnTe) and alloys (CdMgTe, ZnMgTe, and CdZnTe) during molecular beam epitaxy, using nitrogen atoms produced by a direct current glow plasma source. Characterization includes capacitance–voltage, Hall effect, low temperature reflectivity and luminescence, double crystal x-ray diffraction (XRD), nuclear reaction analysis and secondary ion mass spectroscopy. Doping introduces shallow hydrogenic acceptors NTe, whose ionization energy was determined. For ZnTe, doping as high as p≈1020 cm−3 can be obtained when ≈1.5×1020 cm−3 nitrogen atoms are incorporated in the layer. This doping level decreases considerably for CdZnTe and ZnMgTe alloys as the Zn content decreases. The highest concentration obtained for CdTe is p≈1018 cm−3. For a CdTe layer with a doping level p≈1017 cm−3 the hole mobility is μp=235 cm2/V s at 65 K. In ZnTe, incorporation of nitrogen atoms in NTe configurations induces a noticeable change of the lattice parameter, a change which was not observed in CdTe:N layers. The XRD pattern of ZnTe:N/ZnTe pseudosuperlattices allows measurement of the Zn–N bond distance dZn–N=2.16±0.05 Å. Study of the doping efficiency as a function of the growth conditions indicates that the compensation mechanism is related to the formation of nitrogen interstitial defects or complex defects involving metal vacancies, but no deep center has been detected in luminescence. A comparative study of various doped telluride materials indicates that presence of Zn atoms strongly enhances the solubility limit of NTe, hence the key role of Zn in the doping efficiency. Results are discussed on the basis of geometric and thermodynamic considerations. The first argument is that the Zn–Te distance is the closest fit to the metal–N bond distance, and this minimizes the elastic contribution to the energy of formation of the NTe acceptor. As a corollary, the doping efficiency decreases when the alloy lattice parameter increases. The second argument compares the formation enthalpies ΔHF of II3N2 nitride (II being Zn, Cd, or Mg) to that of the IITe compound.

Characterization of Zn-alloyed CdTe compound semiconductors processed in microgravity on USML-1 and USML-2

Three of the four CdZnTe crystals grown on USML-1 and 2 have been analyzed with respect to hydrostatic and buoyant gravitational influences. Characterization was carried out using Synchrotron White Beam X-ray Topography, Fourier Transfer Infrared (FTIR) Spectroscopy and chemical etching. It was found that in the absence of hydrostatic pressure the liquid separated from the ampoule walls, depending on influences such as: volumetric fill-factor, level of constraint, residual g-vector, ampoule geometry and growth conditions. Regions solidified without wall contact were found to be virtually free from twinning, suggesting that these pervasive defects are surface nucleated. Further, these regions showed dramatic reductions in the density of dislocations, from 800,000 (l-g) to 800 (μ-g). This was attributed to thermo-mechanical stress reduction within the flight samples. Regions of partial wall contact showed defect gradients, high on the wall side and low on the free surface side. These results are consistent with our thermo and thermo-mechanical stress model.

Comparative Pulmonary Absorption, Distribution, and Toxicity of Copper Gallium Diselenide, Copper Indium Diselenide, and Cadmium Telluride in Sprague–Dawley Rats

Copper gallium diselenide (CGS), copper indium diselenide (CIS), and cadmium telluride (CdTe) are novel compounds used in the photovoltaic and semiconductor industries. This study was conducted to characterize the relative toxicities of these compounds and to evaluate the pulmonary absorption and distribution after intratracheal instillation. Female Sprague–Dawley rats were administered a single equimolar dose (70 m m) of CGS (21 mg/kg), CIS (24 mg/kg), CdTe (17 mg/kg), or saline by intratracheal instillation. Bronchoalveolar lavage fluid (BALF) protein, fibronectin, inflammatory cells, lung hydroxyproline, and tissue distribution were measured 1, 3, 7, 14, and 28 days after instillation. Relative lung weights were significantly increased in CIS- and CdTe-treated rats at most time points. Inflammatory lesions in the lungs consisting of an influx of macrophages, lymphocytes, and PMNs were most severe in CdTe-treated rats, intermediate in CIS-treated rats, and minimal in rats receiving CGS. Hyperplasia of alveolar type 2 cells was present in CIS- and CdTe-treated rats and was greatest in CdTe-treated rats. Pulmonary interstitial fibrosis was observed in CdTe-treated rats at all time points. All three compounds caused marked increases in total BALF cell numbers, with the greatest increase observed in CIS-treated rats. BALF protein, fibronectin, and lung hydroxyproline were significantly increased in all treated animals and were highest in CdTe-treated animals. There was no apparent pulmonary absorption or tissue distribution of CGS. Indium levels increased in extrapulmonary tissues of CIS-treated rats, although Cu and Se levels remained unchanged. CdTe was absorbed from the lung to a greater extent than CGS and CIS. Cd and Te levels decreased in the lung and increased in extrapulmonary tissues. Of these compounds CdTe presents the greatest potential health risk because it causes severe pulmonary inflammation and fibrosis and because it is readily absorbed from the lung may potentially cause extrapulmonary toxicity.

Thermoreflectance spectroscopy of Te-based II–VI compounds ZnTe and CdTe

Thermoreflectance (TR) spectra of Te-based II–VI compounds ZnTe and CdTe are presented. Measurements are carried out on the bulk single crystals in the 2.0–5.6 eV photon-energy range at room temperature. The TR spectra obtained reveal distinct structures at energies of the E0, E0+Δ0, E1, E1+Δ1, E0′, and E2 critical points (CPs). Detailed analyses are made using a simplified model of the interband transitions. Results suggest that the E0 and E0+Δ0 structures can be characterized by a three-dimensional M0 CP, the E1 and E1+Δ1 structures by a two-dimensional (2D) M0 CP, and the E0′ and E2 structures by a damped harmonic oscillator (a broadened 2D M1 CP). It is also concluded that the broadening terms cannot be neglected in the analysis of TR spectra.

Synchrotron characterization of Zn-alloyed CdTe compound semiconductors processed in microgravity on STS 50 and 73

Synchrotron characterization of Zn-alloyed CdTe compound semiconductors processed in microgravity on STS 50 and 73

Dimensional reduction in II‐VI materials: A2Cd3Q4 (A = K, Q = S, Se, Te; A = Rb, Q = S, Se), novel ternary low‐dimensional cadmium chalcogenides produced by incorporation of A2Q in CdQ

AbstractThe synthesis of the isomorphous, layered chalcogenides K2Cd3S4 (I), Rb2Cd3S4 (II), K2Cd3Se4 (III), Rb2Cd3Se4 (IV), and K2Cd3Te4 (V) in molten A2Qx fluxes in reported (A = K, Rb; Q = S, Se, Te; x = 2 to 3). The compounds form as (Cd3Q4) layers interspersed with A + cations; the layers are composed of Cd3Q2−4 units shaped as truncated cubes. The compounds have room‐temperature band gaps of 2.75, 2.92, 2.36, 2.37, and 2.26 eV for I, II, III, IV, and V, respectively, and also display strong photoluminescence. The thermal analysis data for all compounds are reported. The properties of these compounds are compared with those of the three‐dimensional compounds CdS, CdSe, and CdTe, as well as those of the nanometer‐sized CdQ clusters. A conceptual context is presented to connect all these different types of compounds.

Transition-metal impurities in II–VI semiconductors: characterization and switching of charge states

Recent progress is reviewed regarding 3d transition-metal impurities (Sc … Fe) in those II–VI compounds CdTe, ZnTe, and ZnSe which are nowadays most often studied. Based on extensive studies of their electronic properties by means of optical and EPR methods, as well as combinations between them, conclusions can be drawn on their incorporation into the lattice, their energy level structure, and relationships between these characteristics.

Plasma nitrogen doping efficiency in molecular beam epitaxy of tellurium-based II–VI compounds

We report on the p-type doping of tellurium-based compounds (CdTe, ZnTe) and alloys (CdMgTe, ZnMgTe, ZnCdTe) using nitrogen atoms from a plasma source in molecular beam epitaxy. A systematic decrease of the doping efficiency is observed as the Zn content decreases. We discuss the key role of the Zn content for the nitrogen doping efficiency, in the framework of two hypotheses: local strain induced in the lattice by the presence of nitrogen and formation of compounds involving nitrogen atoms.