Cl Impurities Research Articles

Ab Initio Study of Cl Impurity at GaAs Surfaces

Ab Initio Study of Cl Impurity at GaAs Surfaces

Electronic structure calculation of Mg, Si, P, S, and Cl impurities in aluminium

A fully self-consistent calculation using the multiple scattering X, method is used to study the electronic structure, the relation between charge transfer and electronegativity, the bonding characteristics, and the alloying of Mg with Al. The present investigation indicates that the charge trabsfer from the host Al atoms to the central impurity atom increases with the increase of electronegativity of impurity atoms. It is found that the bonding is metallic for Al-Mg and for the other impurities considered it gradually becomes ionic. The alloying of Mg with Al is more favourable than with other impurities. The results are compared with the available experimental and theoretical data

Evaluation of CdTe(Cl) crystal growth with THM and application to a multichannel detector

Abstract The relationship between the properties of a CdTe radiation detector and its crystal growth condition has been studied. The crystals were grown from an excess Te solution doped with chlorine using radio frequency (RF-THM) and ordinary travelling heater methods (OR-THM). The concentration of chlorine atoms in the CdTe crystal grown by the RF-THM method is estimated to be about 1017 /cm3 and is the same as the concentration in the crystal grown by the OR-THM method. However, the CdTe detector characteristics show excellent properties in the detector fabricated by the RF-THM technique. The depth profile of Cl dopants has been measured by using the SIMS technique and homogeneity of the Cl distribution in the crystal grown by the RF-THM is recognized. It is suggested that the characteristics of the detector depend strongly on the chlorine distribution. In order to investigate the Cl-related emission bands, edge and bound-exciton emission spectra were measured at 4.2 K by photoluminescence. It is suggested that the 1.554 eV emission band is related to the Cl impurity level and that the characteristics of the radiation detector are affected strongly by the appearance of this level. A 46 channel array detector can be produced using high-resistivity CdTe:Cl crystals grown by the RF-THM technique.

Photoluminescence of Chemical Bath Deposited CdSe Films

A photoluminescence study of films grown by the chemical bath deposition method is reported in this work. The films analyzed were deposited on glass substrates having different stoichiometry and thermal treatments. These parameters have a strong influence on the photoluminescence spectra of the films. Lines with higher energetic positions are observed in nearly stoichiometric films. The lines on the emission spectra of thermally treated films are grouped in an interval of 1.634–1.778 eV, while the untreated samples show no emission in this region. Some of the treated films show emission lines attributed to Se vacancies and Cl impurities on the films.

Characterization of anatase type (Sn xTi 1− x)O 2 solid solutions grown by CVD

Anatase type (Sn x Ti 1− x )O 2 solid solutions, which have compositions ranging from x=0 to x=0.02, were grown by chemical vapor deposition (CVD) from TiCl 4, SnCl 4, and H 2O. The solid solutions were studied by inductively coupled plasma (ICP) spectroscopic analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopic analysis, high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). A linear relation between lattice parameters and SnO 2 contents of the solid solutions was found. The c / a ratio of the anatase structure increased as the SnO 2 content increased. Point defects of oxygen non-stoichiometry or Cl impurity were not likely to influence the lattice parameters drastically. Neither amorphous phases nor ordered structures were found in the anatase type (Sn 0.2Ti 0.8)O 2 solid solution crystals by HRTEM or SAED.

Energy shifts and broadening of atomic electron levels near impurity-covered metal surfaces.

The energy shifts and broadening of atomic levels near chemisorbed impurities on a metal surface are calculated using the complex scaling technique. It is shown that the presence of impurities on metal surfaces can both shift the positions and change the widths of electronic levels in hydrogen atoms near the impurity. Results are presented for K, S and Cl impurities

Quantitative analysis of chemical vapor deposition refractory metal silicides

Chemical vapor deposition films of titanium and tungsten silicides have been characterized using secondary ion mass spectrometry (SIMS) and sputtered neutral analysis (SNA). SIMS data for bulk impurities and interfacial contaminants was quantified by reference to ion implant standards generated in situ in the ion microanalyzer. SNA analyses for film stoichiometry were also performed in the same instrument, ionization of the sputtered neutral flux being achieved by gas phase impact of primary ions. The films were found to contain significant levels of H, O, F, and Cl impurities, which largely migrated on annealing either out of the film or to a subsurface interface.

The Distribution of Cl Impurities in SiO2 Films Produced by Thermal Oxidation of Si in Cl‐Containing Ambients

Cl was introduced into films by thermally oxidizing (100) Si in either or gas mixtures at 1100°C. Cl concentration vs. depth profiles in these oxide films were determined by SIMS (secondary ion mass spectrometry). Analysis of the Cl distributions shows that, after incorporation into the network at the interface, in the bulk of the growing oxide the bonded Cl is removed from the network. This process is analyzed quantitatively as a chemical reaction in which reacts to replace Cl with OH. The kinetics of this replacement reaction and of the oxide film growth determine the Cl concentration vs. depth profiles in the grown films. The replacement reaction kinetics are faster when is present than when only is present. This accounts for the significantly different Cl profiles in oxides and oxides. These observations are consistent with previous studies of the redistribution of implanted Cl in oxide films during a second oxidation step.

On the grain boundary segregation of sp‐valence impurities in B.C.C. transition metal

AbstractGrain boundary segregation of sp‐valence impurities in b.c.c. transition metals (α‐Fe) is investigated using a tight‐binding (TB) type electronic theory of s, p, and d‐basis orbitals: Particular attention is paid for Mg, Al, Si, P, S, and Cl impurities at a ∑ = 5 tilt grain boundary in α;‐Fe. Band parameters for host and impurity atoms are determined within Harrison's universal TB scheme and relative electronic energy levels are taken from atomic structure calculations by Herman and Skillman. It is shown that the segregation energy ΔEsegr for sp‐valence impurities is negative (attractive interaction) and exhibits parabolic trend as a function of the number of electrons Nimp (= Nimps+ +Nimpp) in the sp‐band.

Luminescence characterization of residual impurities in CdTe grown by molecular beam epitaxy

The optical properties of CdTe grown by molecular beam epitaxy (MBE) on 〈100〉 InSb substrates have been studied by high resolution photoluminescence and resonant excitation spectroscopy. The layer thickness varied between 2 and 8 μm and the substrate temperature during deposition was in the range of 150 to 300°C. Sharp and strong lines associated with impurity bound exciton recombination are observed and indicate a good crystalline quality as well as a low level of impurity contamination. From a close comparison with the luminescence spectra obtained on bulk material, the acceptor impurities Cu, Li (or Na) and Ag have been unambiguously identified in MBE grown CdTe. Two-electron transitions have also been detected by resonant excitation of the donor bound exciton lines and associated with In and Cl impurities. The appearance of a double band near 1.55 eV is attributed to (D,A) recombination with pairing effects. The relatively high concentration of indium results probably in this case from a contamination by the substrate.

Transport properties of highly doped polycrystalline and amorphous SnO2 films.

The Hall effect in the temperature range 80--500 K has been measured in sprayed ${\mathrm{SnO}}_{2}$ films deposited at low substrate temperatures, 220\ensuremath{\le}${T}_{s}$\ensuremath{\le}440 \ifmmode^\circ\else\textdegree\fi{}C. The data indicate the classical behavior of degenerate semiconductors for polycrystalline films prepared at ${T}_{s}$\ensuremath{\ge}350 \ifmmode^\circ\else\textdegree\fi{}C. With the lowering of ${T}_{s}$, the conduction ceases to be metallic either due to the incorporation of excess Cl donor impurities or the appearance of the amorphous state when ${T}_{s}$300 \ifmmode^\circ\else\textdegree\fi{}C. We interpret the Hall-effect data by a generalized two-band model where a fraction of the electronic states remains strongly localized. Quantitative estimates are given for two types of disorder: a distribution of Cl impurity atoms in a polycrystalline matrix, and an amorphous host-lattice structure. The mobility of amorphous ${\mathrm{SnO}}_{2}$ was measured down to 80 K and the values between 0.4 (80 K) and 1.5 ${\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ (500 K) are compatible with electron transport via extended states in both the conduction-band and impurity-band regimes.

Comparison of the techniques of dispersive and power Fourier transform spectroscopy for investigating the fir properties of imperfect crystals

DFTS has a number of advantages over power FTS, one of which is its great sensitivity, under suitable conditions, to weak spectral features. This has previously been demonstrated with measurements on a wide range of single crystals of high purity. In this paper we demonstrate this advantage for an imperfect crystal, in this case KBr containing Cl impurity. FIR reflection DFTS performed on a sample at room temperature reveals a gap mode due to the Cl impurity at 95 cm −1, together with absorption in the range 50–200 cm −1 arising from lattice strain, with peaks which correspond to features in the one-phonon density of states of KBr. With the sample at room temperature these features are not observable using either reflection or transmission FTS with the same signal-to-noise ratio, although similar features have been observed by transmission FTS in samples rendered transparent by cooling to liquid N 2 temperature.

Magnetic behavior of pressure-quenched CdS containing Cl

Pressure-quenched CdS containing Cl has been shown previously to exhibit both very large diamagnetism and paramagnetism. The diamagnetism observed at 77 K approached Meissner proportion and suggested superconductinglike behavior in this material. The effect is known to depend sensitively on the Cl content of the starting material. This paper describes the results of a survey of the magnetic behavior of pressure-quenched samples prepared from CdS systematically doped with increasing amounts of Cl. The Cl-doped material was prepared in a variety of ways: from mixtures of CdS and Cd${\mathrm{Cl}}_{2}$, by precipitation from aqueous solution, and by acid doping. The differently doped starting materials were analyzed for Cl and other impurities before being pressure quenched. The magnetic susceptibilities were subsequently measured. The results of the survey indicate that the concentration of Cl required to produce specimens with anomalously large magnetism is 0.75\ifmmode\pm\else\textpm\fi{}0.10 wt.%. The technique for the preparation of such material is described.

Emission properties in electrolytically prepared CdTe p-n junctions

In this work p-n CdTe junctions have been prepared by the outdiffusion of Cd assisted by light using nCdTe electrodes in an electrolytic cell configuration. After removing the semiconductor from the electrolyte, electroluminescence experiments were performed in order to study the radiative processes that take place in these photoelectrolytically prepared devices. The observed electroluminescent lines could be explained by supposing two radiative transitions: from Cl impurity level (which is the dopant of the nCdTe) into a VCd-Sn complex, and from the conduction band into a level corresponding to the Cd vacancy. Their TO phonon replicas were also observed.

Electron Spin Resonance Study of Defects Formed in Electron-Irradiated ZnS by Room-Temperature Annealing

Electron spin resonance signals due to isolated Zn vacancies decreased in intensity and new signals ( A 1 and A 2 ) increased as electron-irradiated ZnS containing Cl impurity was kept at room temperature. The features of the new signals are as follows: For the A 1 signal g // =2.0545 and g ⊥ =2.0264, where g // is taken in parallel with the [111] w , and for the A 2 signal g 1 =2.0285, g 2 =2.0238, g 3 =2.0545, and τ=4°, where g 1 and g 2 are taken in the (1\bar10) plane and g 3 in parallel with another axis. The new signals have two excitation bands, two quenching bands, and two enhancing bands around 3.4 and 2.9 eV, 2.9 and 1.9 eV, and 1.3 and 1.0 eV, respectively, at liquid nitrogen temperature. The new signals are ascribed to A centers. A band model is also given to explain the photosensitivity.

Thermal and structural analysis of the effects of chlorine impurities on pressurized CdS

Anomalous large diamagnetism (χv∼−0.25/4π cgs units) and large positive magnetism (χv∼+10−4 cgs units) have recently been reported in pressure-quenched CdS polycrystalline samples by Homan and MacCrone. These effects were shown to be sample material source dependent, and as a first step toward characterizing these samples, we compared the thermal and structural properties of various CdS samples before and after pressurization. Samples containing up to 6-wt. % Cl were prepared by precipitaion from a chloride containing aqueous bath. We find that (i) Cl impurities are probably present in the form of a complex which can decompose on heating to CdS+CdCl2, (ii) melting of a portion of the samples containing high levels of Cl occurs at 535 °C and is attributed to the eutectic temperature of the CdS+CdCl2 mixture, and (iii) heating beyond 535 °C produces complete volatization of the CdCl2 component. The results on samples prepared by mixing of CdS and CdCl2⋅H20 were examined and found to have similar thermal behavior. The cadmium sulfide used in the previous magnetization study was examined and, based on the similarities in the thermal and structural properties, we conclude that the samples which exhibit the anomalous magnetic effects were also prepared by precipitation. In agreement with earlier studies, metastable phases are observed after various pressurizations to above 4.0 GPa.

Concept of Force Variation Due to Charged Defects in Elemental and Compound Semiconductors

AbstractGreen's function calculations of force variation due to charged impurities in elemental and compound semiconductors are analysed on the basis of simple physical arguments. It is strongly suggested that the trends of force variation may not be related with the long range Coulomb interactions, however, the important criteria should be the effect of electric field of charged impurity on its nearest neighbour bonds that mainly affects the short range couplings. Some new experimental results by i.r. method due to Si and Cl impurities in CdTe are also included that lend additional justification to the arguments.

Photoluminescent Properties of CdSe Single Crystals Doped with Donor Impurities

The photoluminescent properties of CdSe single crystals have been investigated in the vicinity of the fundamental absorption edge, varying the concentration of In or Cl impurity over a wide range. Band-to-band emission is observed in crystals with carrier concentrations above 1×1018 cm-3. Both the peaks of the band-to-band emission and the edge emission shift toward shorter wavelengths with increasing carrier concentration. These peak shifts are discussed by taking into consideration the movement of the Fermi level, the shrinkage of the band gap, the screening effect of free carriers, and so on. The emission intensity in the high energy region has the maximum value when the carrier concentration is around 1×1018 cm-3. The emission intensity increases noticeably on annealing the crystal in molten Cd.

Measurements of Impurity Migration in Graphite at High Temperatures Using a Proton Microprobe

The migration of fission products and other impurities through the graphite core of a High Temperature Gas Cooled Reactor is of prime importance in studies of reactor safety. Work in this area is being carried out in which graphite specimens are heated to temperatures up to ~ 3800°C to induce migration of trace elements whose local concentrations are then measured with a proton microprobe. This instrument is a powerful device for such work because of its ability to determine concentrations at a part per million (ppm) level in a circular area as small as 10 ?m while operating in an air environment. Studies show that Si, Ca, Cl, and Fe impurities in graphite migrate from hotter to cooler regions. Also Si, S, Cl, Ca, Fe, Mn, and Cr are observed to escape from the graphite and be deposited on cooler surfaces.

The far-infrared absorption spectra of some alkali halide alloys

A study has been made of the far-infrared transmission spectra of some K x Rb 1− x − Cl yBr 1− y alloys with ( x, y) ≈ 1 mole%. and the results are reported in this paper. An absorption band at 94.5cm −1 in KBr:Cl has been observed, in agreement with the results of other workers. New bands at 99 cm −1 in KBr:Rb. 99 cm −1 in RbCl:K and 103 cm −1 in RbCl:Br have also been found. These bands are attributed to impurity-induced modes in the gaps in the frequency distributions of KBr and RbCl. Comparison of these results with the theoretical results of Mitani and Takeno indicates that the ‘force constants’ associated with Rb and Br impurities are enhanced relative to those associated with K and Cl impurities. The relationship between the results presented here and some features in the infrared reflection spectra of non-dilute alloys is discussed.