Te Layers Research Articles

A Comparison of Atomic Layers Formed by Electrodeposition of Selenium and Tellurium Scanning Tunneling Microscopy Studies on Au(100) and Au(111)

Structures formed by the electrodeposition of atomic layers of chalcogenides Se and Te, on Au(100) and Au(111), are described and compared. Each element, on each surface, forms a low coverage structure, consisting of atoms packed simply in high coordinate sites at distances just above their van der Waals diameter. As coverages are increased above this level, structures composed of chalcogenide atom chains or rings are formed. It is proposed that these chains or rings have significant molecular character, involving orbital overlap of adjacent chalcogenide atoms. Mechanisms are described to account for the formation of these chains and rings. Discussion is also presented concerning the appearance of triangular phase boundaries for both chalcogenides on Au(111). In the case of Se, isolated triangles, about 4–6 nm on a side are distributed across the surface, whereas a network of triangular phase boundaries is observed in the deposition of Te. The triangular phase boundaries in Se appear to result from the nucleation of domains in different threefold sites on Au(111). For Te, however, it is proposed that the triangular domains and phase boundaries are the result of Te atoms being too large to form an extended structure. © 1999 The Electrochemical Society. All rights reserved.

Dielectric and pyroelectric properties of ordered CdZnTe layers grown by MOCVD

Cd 1− x Zn x Te (0.1⩽ x⩽0.86) layers were epitaxially deposited on (1 0 0) CdTe and on Cd 0.96Zn 0.04Te substrates by MOCVD, and some of their electrical and optical properties were investigated. The dielectric constant as a function of temperature was found to have a peak at a Curie temperature. At this same temperature the slope of the resistivity showed a sharp change. It was found that the layers were pyroelectric. The pyroelectric coefficient varied from 2.3×10 −11 to 5.1×10 −10 C K −1 m −2, depending on Zn content ( x) and growth conditions. It was observed that the Curie temperature shifts in the direction of higher temperatures when the Zn content of the layers increases. The observed results are consistent with the results obtained on CdZnTe monocrystals. A correlation was observed between the pyroelectric properties of CdZnTe and valence band splitting effect, measured by the photoluminescence method at 77 K.

Influence of structural defects on lattice parameter and measured composition of Hg 1− xCd xTe epilayers

Structural and compositional parameters of the Hg 1− x Cd x Te layers grown by metal-organic vapor deposition on (2 1 1)-oriented CdTe substrates were investigated by high-resolution X-ray diffraction, high-resolution scanning electron microscopy, energy dispersive spectroscopy in scanning electron microscopy and Fourier transform infra-red spectroscopy in a transmission mode. Divergence in Cd concentrations, obtained by different methods, is explained in terms of extended defects which cause lattice swelling of the Hg 1− x Cd x Te matrix. Strong correlation between defect density and lattice swelling was established by absolute measurements of lattice parameters (Bond method) and by direct defect imaging. It is proposed to use high-resolution Bond technique for nondestructive monitoring of the layer quality.

Investigation of Hg 1-X Cdx te epitaxial vapor phase growth under isothermal conditions

The Hg 1. x Cd x Te layers were grown by vapor phase epitaxy on Cd-terminated {111} CdTe single crystal substrates from a HgTe solid source under isothermal conditions in a semi-closed system with controlled Hg vapor pressure. The growth kinetics were investigated in the temperature region from 420?C to 550?C with different source to substrate spacings, varying from 1 to 11 mm. It was found that the dependence of the growth rate on temperature could be well described by an Arrhenius type equation with an activation energy of 80 kJ/mol in the investigated temperature interval. The activation energies for the crystallization were the same for all the investigated source to substrate spacing.This activation energy value points to the importance of a solid-state diffusion process in the Hg 1-x Cdx Te/CdTe epitaxial couple obtained by isothermal growth under the given experimental conditions.

Solar Cell Contacts Using Nano-Sized Dispersions

ABSTRACTNano-sized dispersions have been employed as precursor inks for the spray deposition of contacts to both Si and CdTe materials. In the case of Si, nano-sized Al particles (nano-Al) were dispersed and spray deposited onto p-type Si. Annealing above the eutectic temperature causes alloy formation yielding a p+ layer with p ∼ 10−4 Ω•cm. For CdTe, nano-sized Te particles (nano-Te) were dispersed and sprayed onto CdTe/CdS/SnO2/glass heterostructures. Contact to the CdTe layer occurred during a 30 min anneal in He (T = 215 to 255 °C). These solar cells were finished by spin-coating the Te layer with Ag paint and subsequently annealing in air (100 °C / 1 h). This approach produces solar cells with open circuit voltages (Voc) from 720 to 800 mV, short circuit current densities (Jsc) from 18 to 20 mA/cm2 and efficiencies up to 10.3%. The performance of these cells was similar to those produced using the standard NREL contact.

Growth and physicochemical characterization of CuAlTe2 films obtained by reaction, induced by annealing, between Cu/Al/Te/Al/Cu... Al/Cu/Al/Te layers sequentially deposited

CuAlTe2 thin films have been synthesized by annealing under an argon flow a multilayer structure of thin Cu, Al and Te layers sequentially deposited by evaporation under vacuum. The films have been characterized by X-ray diffraction, microprobe analysis, photoelectron spectroscopy and Raman scattering. At the end of the process, the XRD spectra demonstrate that textured CuAlTe2 films have been obtained with preferential orientation of the crystallites along the (112) direction. The Raman patterns are in good accordance with the reference. The XPS spectra show that the binding energies of the elements are in good agreement with bonds of CuAlTe2. Even after a decrease of the oxygen contamination by improvement of the depositing process the oxygen present in the films is found to be about 12 at %.

Electrical, Magnetic, and Structural Properties of Sn1-xMnxTe Layers Grown by Molecular Beam Epitaxy

Electrical, Magnetic, and Structural Properties of Sn_1-xMn_xTe Layers Grown by Molecular Beam Epitaxy

Electrical properties of HgCdTe epilayers doped with silver using an AgNO3 solution

We employed AgNO3 solutions for doping Ag in liquid phase epitaxy (LPE) grown Hg0.78Cd0.22Te epilayers and found that the minority carrier lifetimes became longer so that the diode properties improved. After annealing LPE grown Hg(1-x)Cd(x)Te layers (x=0.22) in Hg atmosphere, the epilayers were immersed in an AgNO3 solution at room temperature. The typical carrier concentrations of holes was 3 × 1016 cm−3 at 77K. These values were almost the same as for the nondoped wafers. Also, its acceptor level was 3 to 4 meV. This shows that the Ag was activated. The doped crystals have lifetimes several times longer than those of the nondoped crystals. Numerical fitting showed the lifetime was limited mostly by the Auger 7 process. The Shockley-Read-Hall recombination process was not effective. To examine the Ag-doped wafer, we fabricated photodiodes using standard planar technology. The diodes have an average zero-bias resistance of several MΩ and a shunt resistance of about 1 GΩ for a 10 µm cutoff wavelength at 78K. These values are about four times higher than those of nondoped diodes. The photo current is also two times higher at the same pixel size. This shows that the quantum efficiency is increased. The extension of the lifetime contributes to the high resistance and the high quantum efficiency of the photodiode.

Surface properties of passivated

We have investigated the properties of surfaces after passivation with various treatments, either by electrochemical processes or by immersion in chemical baths. From the C(V) characteristics of metal-insulator-semiconductor structures, we have determined the density of fixed interface charges deduced from the flat-band voltage. The density of slow traps is obtained from the width of the anticlockwise hysteresis and the energy distribution of fast interface states is reported for all passivation processes. The treatment by immersion in bromine-methanol solution including a small amount of S gives the best interface properties. The energy distribution of fast interface states then presents a broad U-shape with a minimum lower than . The surface roughness as measured by atomic force microscopy is notably reduced after this treatment and the x-ray photoelectron spectroscopy of the samples so obtained reveals the presence of a residual Te layer on the surface.

Preparation of Hg 1− xCd xTe (0.1≤ ×≤0.5) epitaxial layers by two-stage evaporation–condensation–diffusion method

The present investigation is devoted to the further study of the two-temperature evaporation–condensation–diffusion (ECD) method. Experiments have been performed to determine the Hg vapour pressure influence on rates of HgTe evaporation and condensation, morphological peculiarities of the initial stages of Hg 1− x Cd x Te layer growth, and interdiffusion in the HgTe–CdTe system. It is shown that the classical ECD method for Hg vapour pressures higher than 4 atm becomes inefficient due to the several reasons discussed in this paper. We suggest a two-stage epitaxy process for Hg 1− x Cd x Te and other solid solutions. In the first stage, a structurally perfect layer with a desired thickness is formed at low Hg pressure. In the second stage, the Hg vapour pressure is fixed. This provides the necessary composition of the layer surface. High-quality Hg 1− x Cd x Te layers with the surface composition x s up to 0.5 are reproducibly obtained by this proposed technique.

Phase-Change Optical Disk Having a Nitride Interface Layer

A thin nitride layer formed at the interface of a Ge–Sb–Te recording layer and a ZnS–SiO2 protective layer successfully suppresses the phenomenon that reflectivity or signal amplitude becomes markedly small due to repeated overwrites. Based on secondary ion mass spectrometry (SIMS) observations, the 5-nm-thick interface layer was found to restrain sulfur atoms in the ZnS–SiO2 layer from diffusing into the Ge–Sb–Te layer and from changing the optical characteristics of the layer. Among several nitride materials, germanium nitride (Ge–N) sputtered film is found to have the most suitable properties as an interface layer: high barrier effect and good adhesiveness with Ge–Sb–Te and ZnS–SiO2 layers. The optical disk having the Ge–N interface layer achieves more than 5×105 cycles of overwrites with almost no changes in signal amplitude, reflectivity and jitter based on DVD-RAM specifications. The disk shows no degradation such as cracking, peeling, and corrosion after exposure to accelerated environmental conditions of 90°C and 80% RH for 200 h.

In Situ Optical Second Harmonic Rotational Anisotropy Measurements of an Au(111) Electrode during Electrochemical Deposition of Tellurium

The electrodeposition of Te on a single-crystalline Au(111) electrode was studied with 1064-nm-excited SH rotational anisotropy measurements. The SH rotational anisotropy was significantly changed with the first underpotential deposition (upd) of Te, and the bulk Te deposition attenuated the anisotropic character of the overall surface symmetry. The change in the SH rotational anisotropy during the first upd of Te was examined using two different models. The first model considered only the contribution of the Au(111) surface to the SHG response, while the second one took into account the contributions of both the Au(111) substrate and the adsorbed Te layer. In the former case, the change in the SH rotational anisotropy can be explained by considering the quenching of the nonlinear susceptibility of the Au(111) surface. The analysis based on the latter model resulted in a rotation angle of 608 for the adsorbed Te layer against the the Au(111) lattice. This value was not consistent with that expected from t...

Transient photo-reflectance of CdMnTe MBE layers: optical path modulation

In photo-reflectance of Cd 1− x Mn x Te layers, a new modulation mechanism of thermo-magnetic origin, optical path modulation, gives additional lines. These lines are used for volume probing of magnetic relaxation. The comparison with the free-exciton line, used as a surface probe, gives an insight into spin relaxation and heat diffusion phenomena. The heat diffusion coefficient in our sample ( x = 0.16) is estimated to be about 1 cm 2/s and the spin-lattice relaxation time in superfluid He about 330 ns.

Iodine and arsenic doping of (1 0 0)HgCdTe/GaAs grown by metalorganic vapor phase epitaxy using isopropyl iodide and tris-dimethylaminoarsenic

We report in situ n- and p-type doping of HgCdTe/CdTe/(1 0 0)GaAs grown by metalorganic vapor phase epitaxy (MOVPE), using the interdiffused multilayer process (IMP). HgCdTe was doped either with iodine from isopropyliodide (IPI) for n-type doping or with arsenic from tris-dimethylaminoarsenic (DMAAs) for p-type doping. Standard bubbler configurations were used for dopant precursors. DiPTe, DmCd and elemental Hg were used as sources. HgCdTe layers of 10 μm were grown on (1 0 0)GaAs at 370°C. Dopant gases were allowed into the reactor only during the CdTe growth cycle of the IMP process. HgCdTe layers were Hg-annealed at 415°C for 15 min and at 220°C for 3 h, consecutively. Iodine-doped HgCdTe layers had 77 K electron concentrations of 6 × 10 15 to 5 × 10 17 cm −3. Hall mobility decreased with increasing doping concentration. Arsenic-doped HgCdTe layers had 77 K hole concentration of 2 × 10 16 to 7 × 10 17 cm −3 with about 100% of arsenic activated. Composition x of Hg 1− x Cd x Te layers was not influenced by the arsenic doping. The above results show that IPI and DMAAs precursors are excellent n- and p-doping sources for HgCdTe.

Achievement of low p-type carrier concentration for MOCVD growth HgCdTe without an annealing process

The electrical characteristics of as-grown epitaxial HgCdTe layers grown by metalorganic chemical vapor deposition were investigated. We propose a new cap layer in order to obtain as-grown HgCdTe layers with low p-type carrier concentration. The new cap layer called the “double-cap layer” consists of a graded Cd content Hg 1− x Cd x Te layer with x = 0.2–0.7 and a CdTe cap layer which was continuously grown at low temperature on the graded HgCdTe layer. The carrier concentration of HgCdTe layers with the new cap layer was about 3 × 10 16 cm −3. The p-type carrier concentration has been reduced further to 2 × 10 15 cm −3 (for x = 0.22) by lowering the growth temperature and increasing the partial pressure of Hg. This carrier concentration is lower than the estimated Hg vacancy concentration from considerations of the thermal equilibrium conditions.

Molecular beam epitaxial growth and performance of integrated multispectral HgCdTe photodiodes for the detection of mid-wave infrared radiation

In situ doped HgCdTe two-color detectors with the n—p—n geometry were grown by molecular beam epitaxy, for the simultaneous detection of two closely spaced bands in the mid-wave infrared spectrum. The average near-surface etch pit densities in these layers were 5 × 10 6 cm −2 which is a factor of 10 higher than that observed for the lattice-matched growth of Hg 1− x Cd x Te ( x = 0.22) layer on Cd 0.96Zn 0.04Te substrates. The 0.04% lattice mismatch between the Hg 1− x Cd x Te ( x = 0.35) epilayer and the Cd 0.96Zn 0.04Te substrate produces plastic deformation of the epilayer which results in an increased dislocation densities in the epilayer. The alloy composition across the device structure along the growth direction was determined by secondary ion mass spectrometric analysis, and deviated by less than 1% from the target. The device structures were processed as diodes with the mesa architecture and tested. The spectral response of the detectors at 77 K was characterized by sharp turn off at 3.7 and 4.4 μm. R o A values in excess of 1 × 10 6 Ω cm 2 and quantum efficiencies greater than 75% were measured for diodes in each band.

Investigation of the Mechanism of the Enhanced Z3DT in PbTe Based Superlattices

AbstractVarious possible mechanisms for the recently discovered enhanced Seebeck coefficient S in PbTe/Te superlattices relative to the corresponding PbTe bulk are investigated. Among the various mechanisms which can account for the enhanced S, the energy dependent τ model (τ ∼ ɛr) seems the most plausible. Here the effective scattering parameter r is preferably increased due to the extra scattering by the periodic Te layers introduced in the superlattice. Other transport properties including the longitudinal magnetoresistance are also discussed.

The role of 2D islands in the epitaxial growth of (001) CdTe

We present an extensive study of the homoepitaxial growth of (001) CdTe with a particular emphasis on the variation of the lattice parameter and on the determination of the surface stoichiometry during the growth as well as the related evolution of the growth rate. An oscillatory behavior of the in plane lattice parameter during the 2D homoepitaxial growth of (001) CdTe is demonstrated. It is associated to a deformation, induced by the surface reconstructions, at the free edges of the small islands formed during the growth by molecular beam epitaxy (MBE) or atomic layer epitaxy. During the MBE growth, the presence on top of the CdTe growing surface of both chemisorbed dimerized tellurium atoms and weakly bonded Te atoms adsorbed on top of this Te layer is evidenced. The influence of the stoichiometry on the evolution of the growth rate is demonstrated. Moreover, an influence of strain on the growth mechanisms of CdTe has been observed.

Measuring and calculating the Hall mobility of semiconductor epitaxial layers by a contactless method

A new probe was made from microwave dielectric waveguides. Using this probe, we can measure the magnetoresistance of Al x Ga 1− x As, Hg 1− x Cd x Te and other epitaxial layers easily. It is a contactless and non-destructive method. During the test, we do not need to specially treat the surface and the shape of the samples. The measuring area is about 4 × 4 mm 2. This article also presents a method of calculating the mobility of these samples from the magnetoresistance data for different semiconductor materials.

The effect of hydrogenation in Hg xCd 1 − xTe thin films grown on p-CdTe (211) B substrates

Fourier transform infrared (FTIR) transmission, Hall effect, and nuclear resonance reaction measurements have been carried out to investigate the effect of hydrogenation on the deep levels and the hydrogen depth profiling in nominally undoped Hg x Cd 1 − x Te layers grown on undoped p-CdTe (211) B-orientation substrates by molecular beam epitaxy. After hydrogenation, the FTIR spectra showed that the transmittance intensity increased in comparison to that of the as-grown Hg x Cd 1 − x Te and that the absorption edge shifted to the short wavelength range. Hall effect measurements showed that the carrier concentration decreased and the mobility increased after hydrogenation. After hydrogenation, p-type Hg x Cd 1 − x Te is converted to n-type Hg x Cd 1 − x Te with high resistivity. Nuclear resonance raaction measurements show that the concentration and the penetration depth of the hydrogen atom in n-Hg 0.77Cd 0.23Te are 3.5% and 640Å, respectively. The areal density of the hydrogen-containing layer at the surface of the hydrogenated n-Hg 0.77Cd 0.23Te film is 4.39 × 10 15atoms/cm 2. These results indicate that hydrogen atoms not only effectively passivate impurities or defects in the Hg x Cd 1 − x Te film but also change the carrier type of p-Hg x Cd 1 − x Te.