Showing papers in "Materials Science and Engineering B-advanced Functional Solid-state Materials in 1994"

Raman and infrared spectroscopic studies of GeGaAg sulphide glasses

Abstract: Gallium sulphide forms stable glasses with GeS2 over a wide range of composition, and in the same system complex materials are obtained by dissolving silver sulphide. In such glasses Ga2S3 acts as a co-former with GeS2 whereas Ag2S plays the role of modifier. Raman scattering and infrared absorption measurements have been carried out on the Ge2S4Ga2S3Ag2S glass system. The structural arrangement of these glasses is studied as a function of the composition in the ternary glass-forming region. The vibrational assignment is made mainly in terms of bridging and non-bridging sulphur in the vitreous network. The tendency of gallium atoms to form GaS4 tetrahedra forces germanium atoms to induce the formation of S3GeGeS3 structural groups due to the shortage of sulphur atoms. When Ag2S is associated with Ge2S4Ga2S3 glasses, its defavors the edge-shared GeS4 tetrahedra and contributes to the cohesion of corner-shared tetrahedra. When Ag2S content increases, we observe a band located at low frequency that we have attributed to charge-coupled silver cation motion. A slight distortion of the tetrahedral groups is also seen from the vibrational features.

Diamond, diamond-like and titanium nitride biocompatible coatings for human body parts

Abstract: A new approach is proposed for fabricating human body parts that last longer and are more biocompatible than those presently available. In this approach, bulk material is chosen that has desirable mechanical properties (low modulus, high strength, high ductility and high fatigue strength) and then this material is coated with highly corrosion- and erosion-resistant and totally biocompatible layers. As an example, we have investigated diamond, TiN, diamond/diamond-like, and diamond/TiN coatings on Ti−6wt.%Al−4wt.%V alloy used for hip prosthesis. This alloy has desirable mechanical properties but the toxicity of vanadium and the neurological disorders associated with aluminum have raised some concerns. To overcome this problem, we have developed a laser physical vapor deposition method to form TiN and diamond-like coatings, and a hot-filament-assisted chemical vapor desposition method to form diamond layers. Cementless diamond-coated hip prostheses of titanium alloys are expected to last approximately ten times longer or more compared with the polymethylmethacrylate-cement-coated CoCr prostheses used at present. The microstructure of diamond films can be controlled by substrate and deposition variables. The microstructures of these films have been investigated using optical and scanning electron microscopy, chemical composition by Auger electron spectroscopy, structure by X-ray diffraction, and atomic arrangements (lattice vibration) characteristics by Raman spectroscopy. The average grain size of diamond films varied from 0.5 to 2.0 μm, and the diamond-like films were amorphous. The average grain size of TiN films was found to vary from 10 to 20 nm. The diamond films showed characteristics Raman peak at 1332 cm−1 (sp3 bonding), and diamond-like films contained 1350 and 1580 cm−1 Raman peaks (a mixture of sp2 and sp3 bonding). The mechanical properties and adhesion characteristics of these films together with biocompatibility issues are discussed for titanium alloy hip prosthesis.

Ion-assisted laser processing of CNx films

Abstract: We have produced CNx thin films with x = 0–0.70 by using an ion-assisted pulsed laser deposition method. In this method a graphite target was ablated while simultaneously bombarding the substrate with nitrogen ions. The deposition rate, ion current, substrate temperature and substrate bias were varied to enhance nitrogen incorporation and to optimize the properties of the thin films. The films were characterized using Rutherford backscattering-channelling. Auger electron spectroscopy, electron energy loss spectroscopy, transmission and scanning electron microscopy, Raman spectroscopy and nano-indentation hardness measurements. The average nitrogen concentration was found to vary (N/C atomic ratio) in the range 0–0.70. The transmission electron microscopy studies showed that these films were polycrystalline with a fine-grain equiaxed structure (average size 10 nm) above a substrate temperature of 200°C. The sp3/sp2 ratio was determined to be 25%–30% using electron energy loss spectroscopy. The Raman studies showed two peaks at 1285 and 1575 cm−1 corresponding to sp3 and sp2 bonding respectively. The films exhibited qualitatively very high hardness and thermal annealing resistance.

Selective epitaxial growth of SiGe alloys—influence of growth parameters on film properties

Abstract: In this paper the selective growth of SiGe by low-pressure chemical vapor deposition is discussed on the basis of the present understanding of the selective growth process, with emphasis on the Si-Ge-Cl-H system and SiO 2 masks. The selectivity of growth is a function of total pressure, temperature, Cl/H ratio and partial pressures of the reactive species. These factors, among others, influence strongly the chemistry of the deposition reaction. Their role must therefore be clearly understood, interpreted and predicted by thermochemical and kinetic analysis. An attempt will be made to determine the most significant parameters and to find selection criteria. The growth rate and composition dependence is affected by the pad size; this dependence is weaker at low-pressure than at atmospheric-pressure conditions. The evolution of relaxation the pad size is demonstrated for 16%Ge. Finally, intense photoluminescence observed in dots 500 nm wide with multiple quantum wells is presented.

Epitaxial ferroelectric thin films for memory applications

Abstract: Epitaxial ferroelectric Pb0.9La0.1Zr0.2Ti0.8O3 (PLZT) thin film capacitors with perovskite metal oxide electrodes (such as YBa2Cu3O7) were grown on single-crystal LaAlO3 and on buffered [100] Si. Structural studies using X-ray diffraction and transmission electron microscopy show that the PLZT layer is free of large-angle grain boundaries (i.e. it is single crystal like). Chemical analysis by Rutherford backscattering shows the composition of the PLZT layer to be close to that of the target. Ferroelectric hysteresis measurements using both pulsed measurements and a variable frequency Sawyer-Tower circuit yield remnant polarization values (at 5 V) in the range 10–30 μC cm−2 (depending on deposition conditions) with a coercive field in the range 35–80 kV cm−1. These heterostructures also show excellent resistance to bipolar fatigue, aging and logic state retention characteristics.

Optical and recording properties of copper phthalocyanine films

Abstract: Vacuum-sublimed films of copper phthalocyanine (CuPc) were prepared. The absorption spectrum and optical constants of the film were studied. It was found that there is a comparatively large absorption region at wavelength of 550 – 750 nm. A static optical recording test on a CuPc film on a glass substrate indicated that low power HeNe laser irradiation could initiate a relatively large reflectivity change. It has been observed that the introduction of a metallic reflective layer on the recording layer will raise the writing threshold energy and the maximum reflectivity contrast.

The Czochralski growth of SrLaGa3O7 single crystals and their optical and lasing properties

Abstract: Single crystals of SrLaGa 3 O 7 undoped and doped with 5 and 10 at.% of neodymium were grown by the Czochralski technique. Their composition and dopant distribution were checked by X-ray microprobe. The Nd-doped crystals were investigated for their lasing and spectral properties. Absorption spectra in the range 180–8000 nm and the luminescence spectra in the range 200–800 nm were measured. Throughout the whole spectrum differential changes of the absorption coefficient were found. Optical transition corresponding to this effect are indicated.

Stability of cubic ZrO2 (10 mol.% Y2O3) when alloyed with NiO, Al2O3 or TiO2: implications to solid electrolytes and cermets

Abstract: This work uses room temperature X-ray diffraction and electron microscopy to investigate the solution of NiO, Al2O3 or TiO3 in cubic (c-) ZrO2 (with 10 mol.% Y2O3, designated as yttria fully stabilized zirconia (Y-FSZ)) powders and the consequent destabilization, if any, of Y-FSZ. After sintering at 1600 for 4 h, the dissolution of the additives (the solubilities are 15 mol.% for TiO2, ca. 5–10 mol.% for NiO and rather limited for Al2O3) caused a smaller cell volume of c-ZrO2. Among the additives, only TiO2 causes the precipitation of tetragonal (t-) ZrO2 in c-ZrO2 matrix. The increased solution of TiO2 in Y-FSZ results in a larger tetragonality of t-ZrO2, but the t-ZrO2 precipitates remain as twin-related colony structure with {101} habit plane, the same as for the ZrO2-Y2O3 system. Due to oversaturation, the added NiO or Al2O3 appears as second phase particles which hindered the grain growth of c-ZrO2. The implication of these results to zirconia electrolyte cell and metal/ceramic joints such as thermal barrier coating (TBC) are discussed.

Preparation and characterization of semi-insulating undoped indium phosphide

Abstract: We show, for the first time, that undoped InP samples annealed in vacuum turn out to be semi-insulating if the cooling after the high temperature treatment (around 900 °C) is sufficiently slow (around 30 °C h−1). However, the best semi-insulating properties ( ϱ > 10 7 Ω cm ; μ ≈ 4000 cm 2 V −1 s −1 ) are achieved with a double anneal under vacuum of InP with residual carrier concentration less than 4 × 10 15 cm −3 . It is also shown that InP specimens with n ≈ 10 16 cm −3 can exhibit high resistivity, but lower mobility, after sequential thermal treatments. The conductivity conversion is ascribed to the overlap of two phenomena: (i) in-diffusion of deep impurities and (ii) generation-incorporation of “shallow” acceptors. It was also observed that annealed semi-insulating samples resumed the semi-conducting behavior after a second thermal treatment and quenching. This suggests that the concentration of both types of annealing-related levels are dependent on the cooling rate.

Mapping of defects and their recombination strenght by a light-beam-induced current in silicon wafers

Abstract: Knowledge of the correlation between the nature of recombining defects (extended crystallographic defects, impurities etc.) and the local electrical properties of wafers are necessary for the control of semiconductor materials. The control of the evolution of the electrical properties during the processing steps used to manufacture a device is of a great interest provided that the investigation technique is non-destructive. The light-beam-induced current (LBIC) mapping technique is able (1) to recognize and reveal the existence of extended crystallographic defects, such as grain boundaries, dislocations and stacking faults, when they recombine minority carriers; (2) to evaluate the recombination strength of the preceding defects and (3) to suggest the existence of impurity clouds or precipitates in the homogeneous regions of the wafers. In silicon wafers and devices, LBIC mapping can reveal bulk and surface defects, by changing the wavelength of the light only. The LBIC map can be transformed into a quantitative minority-carrier diffusion length L map in silicon, because the light beam excitation is compatible with the determination of L from the spectral variations in the photocurrent and in the light absorption coefficient in the near IR. Finally, LBIC maps can also be used to detect and evaluate any improvements associated with treatments such as hydrogenation, internal and external getterings.

Dielectric relaxation of paired defects in perovskite-type oxides

Abstract: A number of Debye-type dielectric relaxation peaks have been observed for acceptor-doped KTaO3 and CaTiO3, both of which have the perovskite structure. In the case of KTaO3, the dopants are divalent and trivalent cations of the iron group, while CaTuO3 is doped with Al3+. These peaks occur at relatively low temperatures and have activation energies in the range 0.1–0.4 eV, much lower than the activation energy for the conductivity. They are attributed to M-Vo pairs, where M represents the dopant ion and Vo the oxygen ion vacancy. The possibility that the peaks are due to partial reorientation of the pairs in “off-center” configurations is ruled out by electron paramagnetic resonance work on Mn-doped KTaO3 by Geifman and coworkers (I.P. Bykov, I.N. Geifman, M.D. Glinchuk and B.K. Krulikovskii, Sov. Phys. Solid State, 22 (1980) 1248). They must therefore be due to full reorientation, and possible reasons for the low activation energy are discussed. A study of the peak heights shows that a large correction for internal field effects is necessary for KTaO3, but this correction is not nearly so large as that given by the Lorentz factor.

Rapid consolidation of BiPbSrCaCuO powders by a plasma activated sintering process

Abstract: A new rapid powder consolidation technique—plasma activated sintering (PAS)—was used to obtain high density disk-shaped bulk superconductor samples in the BiPbSrCaCuO system. Different powder precursors prepared by the glass-ceramic route, sol-gel processing, and solid state reactions were investigated. Results of density, hardness, and critical superconducting transition temperature were compared after PAS processing of powder precursors. Microstructures showing oriented grains of the 2223 phase were obtained as a result of PAS densification in very short time frames ( ).

Selectivity in low pressure chemical vapor deposition of copper from hexafluoroacetylacetonate-copper(I)-trimethylvinylsilane in the presence of water

Abstract: Electronic device-quality Cu was deposited on Pt-patterned oxidized Si wafers from hexafluoroacetylacetonate-copper(I)-trimethylvinylsilane by using low-pressure chem. vapor deposition at 150-250 Deg in He carrier gas. Smooth Cu films ?0.8 mm thick have been deposited with an elec. resistivity close to that of bulk Cu. From the temp. dependence of the mean deposition rate, an apparent activation energy of 53-62 kJ/mol was detd. for the overall deposition process. Monitoring of the deposition process by reflection of diode laser beams allowed an early detection of nucleation and film growth on both metal-seeded and unseeded surface sites. By the addn. of water to the reaction mixt., the nucleation on Pt-seeded SiO2 was accelerated, whereas the nucleation on unseeded SiO2 was delayed. Compared to deposition without water, the overall deposition rate on Pt-seeded SiO2 was slightly increased. However, the deposits showed a rough surface structure, and the film resistivity increased to >=20 mW-cm.

Imaging of high field regions in semi-insulating GaAs under bias

Abstract: The electric field within high energy particle detectors fabricated from semi-insulating GaAs is non-uniform. Two methods which map the electric field within these detectors are compared. The data obtained by these techniques are self consistent and their application should prove useful in assessing the properties of these and similar detectors.

Spray pyrolysis deposition for indium oxide doped with different impurities

Abstract: Spray pyrolysis including the doping process of impurities of different valence for preparation of transparent conductive indium oxide thin films is investigated. In this work, the oxidized Si wafers and pyrex glass slides are used as substrates. Low concentration InCl 3 -methanol solution is the main source chemical with ZnCl 2 , SbCl 3 and TeCl 4 as additives for dopants. The deposition process and film properties are characterized as functions of various process parameters including the substrate temperature, the flow rate and the concentrations of reactants. In general, the deposition rates are optimized to reach 40 to 50 nm/min at substrate temperature between 300 °C and 325 °C. All the films are smooth and transparent in the visible and IR range of radiation with transparency greater than 90%. The refractive index is between 1.8 and 2.1. The resistivity of the In 2 O 3 films without doping is in the range of 2 × 10 −3 to 8 × 10 −3 Ω-cm. It increases with Zn doping by an order of magnitude while decreases with Sb and Te as dopants. In this work, the lowest resistivity from Te doping reaches to 5 × 10 −4 Ω-cm.

Resonant raman scattering studies of SnS2 crystals

Abstract: Details of resonant Raman scattering (RRS) in SnS 2 single crystals are presented A detailed investigation of some newly observed features has been carried out using far-IR (FIR) absorption spectroscopy. A comparison of RRS and FIR data shows that the new features are IR active modes, and second-order or combinations modes which become allowed under the resonant Raman condition. The resonant behaviour of non-polar modes is explained using a quasi-two-dimensional approach. Our experimental data provide a good fit to the theory of RRS at the M 1 critical edge.

Copper diffusivity in silicon: A re-examination

Abstract: Recent studies dealing with copper in silicon have shown large disparities in diffusivity data. We show that copper, as a positively charged diffusing species, is susceptible to many possible reactions. In particular, it strongly reacts with acceptors and oxygen. Moreover, a large distribution gradient of this impurity induces a local electric field, considerably enhancing its mobility. We show that even partial ignorance of these effects leads to the observed disparities. From our studies we determine the intrinsic diffusion coefficient of copper over the range 180–1200 K.

Influence of temperature on the properties of SiCxNy:H films prepared by plasma-enhanced chemical vapour deposition

Abstract: SiCxNy:H films were prepared via plasma-enhanced chemical vapour deposition using a mixture of SiH4, CH4, and NH3 at substrate temperatures between 493 and 773 K. The influence of substrate and annealing temperatures on film properties was investigated with Rutherford backscattering spectrometry, secondary ion mass spectrometry, X-ray diffraction, transmission electron microscopy etc. The results reveal that the substrate temperature does not significantly affect the Si, C, and N compositions but influences the H content which decreases with increasing temperature. The substrate temperature influences the film deposition rate. SiCxNy:H films are thermally and electrically stable at and below 873 K. Above this temperature, SiH and NH bonds begin to break, and hydrogen evolution occurs. Both as-deposited and after-annealed SiCxNy:H films are of amorphous structure. The refractive index of SiCxNy:H can be adjusted from 1.78 of Six:H to 2.57 of SiCx:H. Therefore, SiCxNy:H can be among its many applications, used as an optical thin film with variable optical constants between those of SiCx:H and SiNx:H.

Optical characterization of As2S3 and As2Se3 semiconducting glass films of non-uniform thickness from transmission measurements

Abstract: Based on the interference fringes in transmission spectra at normal incidence of thin films of As2S3 and As2Se3 semiconducting glass compositions deposited by thermal evaporation, a method is applied that makes it possible to obtain a parameter indicating the degree of film thickness uniformity, and to accurately determine the refractive index subsequently used to derive the average film thickness. Thickness measurements have been performed with a surface profiling stylus to check the results from the transmission spectra, showing a very good agreement between both measurements. The dispersion of n is discussed in terms of the single-oscillator Wemple and DiDomenico model. The optical absorption edges are described using the non-direct transition model proposed by Tauc. The optical energy gaps were calculated using Tauc's extrapolation, resulting in values of 2.37 and 1.80 eV respectively. Finally, the optical band gap is interpreted in terms of the bond-strengths of the chemical bonds present in the glass compositions under study.

Anomalous thermionic-field emission in epitaxial Al/n-AlGaAs junctions

Abstract: We present a method for the evaluation of the Schottky barrier height φ b 0 , Richardson constant A ∗ , characteristic energy E 00 and bias dependence of the barrier height β from the temperature-dependent current-voltage characteristics of Schottky junctions using the thermionic-field emission )TFE) theory. The application of this method to experimental current-voltage characteristics of epitaxial Al/n-Al0.25Ga0.75As ( N = 1.4 × 10 17 cm −3 ) barriers shows that the current flow through these junctions is dominated by TFE with anomalously high E 00 . We conclude that this anomaly may be partly connected with the electric field enhancement at the periphery of the diodes, and with the multistep tunnelling through deep levels.

Spectroscopic investigation of deep levels related to the compensation mechanism of nominally undoped semi-insulating InP

Abstract: Measurements were performed by deep level transient spectroscopy (DLTS) and by the thermally stimulated current (TSC) technique on annealed InP samples to investigate the compensation mechanism which causes the semi-insulating (s.i.) behaviour of nominally undoped annealed InP. The results show that the electron traps detected by DLTS in conducting InP can also be found in TSC spectra of s.i. annealed samples. Additionally another deep level is detected, which can then be assigned to a hole trap. The currently used evaluation schemes of TSC measurements had to be reconsidered in favour of a variation of the heating rate to eliminate inherent errors.

Fabrication of photonic integrated circuits using quantum well intermixing

Abstract: Intermixing the wells and barriers of quantum well structures generally results in an increase in the band gap and is accompanied by changes in the refractive index. A range of techniques, based on impurity diffusion, dielectric capping and laser annealing, have been developed to enhance the quantum well intermixing (QWI) rate in selected areas of a wafer. Such process offer the prospect of a powerful and relatively simple fabrication route for integrating optoelectronic devices and for forming photonic integrated circuits (PICs). Recent progress in QWI techniques is reviewed, concertrating on processes which are compatible with PIC applications, and illustrated with device demonstrators.

Two types of electron-beam-induced current behaviour of misfit dislocations in si(ge) : experimental observations and modelling

Abstract: The paper reports measurements of electron-beam-induced current contrast c vs. temperature T and beam current Ib for two misfit dislocations in a p-type Si/SiGe sample. It is demonstrated that the different behaviour observed can be explained on the basis of the Shockley-Read-Hall recombination theory. The activity of the dislocation with dc(T)/dT > 0 is controlled by deep centres while that of the other dislocation having dc(T)/dT < 0 indicates shallow centres lying about 0.075 eV from the band edge.

Influence of carbon on the phase transformation kinetics and magnetic properties of MnAl alloys

Abstract: Magnetic measurement, optical microscopy and X-ray diffraction were used to investigate the phase transformation kinetics in pure MnAl and carbon-doped MnAl alloys. The results indicate that the transformation from e-phase to τ-phase is inhibited when C doping exceeds its solubility limit in the alloy. A small amount of C addition greatly increases the stability of the τ-phase, and the most stable τ-phase is obtained at 0.6–0.8 wt.% C content. The reason why C addition increases the stability of τ-phases is that the interstitial C atoms inhibit the diffusion of Mn and Al atoms. The magnetic properties of the C-doped MnAl alloy are also studied.

Lithium insertion in indium selenide films: application to microbatteries

Abstract: Indium selenide films are formed on silica slides and silicon wafers using a flash evaporation technique in which the material source stoichiometry is modified to obtain InpSeq films with various compositions. The structural and optical properties of indium selenide films are reported. These characterizations have shown that, using different growth conditions, either single phases, such as InSe, In4Se3 or In2Se3, are formed or a mixture of these compounds is present in the film structure. The products are examined by X-ray diffraction, and Raman and IR spectroscopies. The electrochemical properties of lithium-intercalated films are presented. It is observed that the morphology and stoichiometry play an important role in the lithium insertion process. The thermodynamics and kinetics of the insertion reaction are reported. Li/Li+ -borate glass/InSe microbatteries have been built, and their characteristics are reported and discussed using a Butler-Volmer relationship.

γRRadiation-hydrothermal synthesis and characterization of nanocrystalline copper powders

Abstract: γ Radiation combined with hydrothermal treatment has been successfully used to prepare pure nanocrystalline copper powders from a solution of Cu(II)-EDTA complex. The average particle size of the copper powder was 16 nm. The growth process of nanocrystalline copper particles into a single-crystalline form induced by electron irradiation, investigated by electron microscoopy, is reported.

Study of the growth rate of diamond film by hot-filament CVD

Abstract: In synthesis of diamond thin film by HF-CVD, many factors, such as CH 4 H 2 ratio, gases pressure, flow rate, pretreatment of substrate surface, substrate temperature, filament temperature, distance between filament and substrate etc., can affect the growth rate of diamond film. Adequate pretreatment of substrate surface, high filament temperature, high CH 4 /H 2 ratio (0.1%–2%), small distance between filament and substrate, adequate substrate temperature (800–900°C) and gases pressure (5–15 kpa) are beneficial to the promotion of diamond growth rate.

Growth, laser and magneto-optic properties of Nd-doped Bi4Ge3O12 crystals

Abstract: A series of Nd-doped Bi4Ge3O12 (Nd:BGO) crystals have been grown with an improved Bridgman-Stockbarger method; the mechanism of formation of different kinds of defects introduced during growth was studied, and the methods for removing these defects were also discussed. Using our improved growth technology, Nd:BGO crystals with an Nd2O3 doping level as high as 1.5 wt.% were successfully grown. The crystals have good optical quality and a uniformly distributed Nd:ion concentration. The Nd:BGO crystals exhibit medium laser amplification. Compared with Nd:YAG, the Nd:BGO has a broad absorption band at about 800 nm, so they could absorb the pump light of a diode laser more effectively. The Verdet constant of a pure BGO crystal at room temperature was measured with high power laser system to be 0.033 min cm−1 G−1, 27% higher than that of FR-4 magneto-optic glass. Thus Nd:BGO may be a promising laser material that combines the laser properties of Nd ions and the magneto-optic properties of the BGO crystal. A prototype diode laser pumped elf -Q- switched Nd:BGO laser has been demonstrated for the first time by the large Faraday effect in BGO crystals.

Preparation and characterization of In4Se3 films

Abstract: Thin films of In4Se3 were obtained by vacuum evaporation of polycrystalline materials onto substrates at different temperatures T s . Systematic X-ray analysis was carried out and revealed that the flash evaporation technique can produce stoichiometric films for depositing polycrystalline materials of composition In1.11Se0.89. Optical and electrical characterizations are reported. The energy gap of In4Se3 films is located between 1.41 and 1.48 eV depending on the substrate temperature. A.c. and d.c. conductivities were measured as a function of T s . Amorphous films exhibit a T − 1 4 dependence of the conductivity which fits well with the Mott model with a density of localized states N(E F ) = 8.5 × 10 18 cm −3 eV −1 . The effect of annealing temperature was also investigated. Hall measurement as a function of temperature show that the predominant conduction mechanism is scattering by grain boundaries in polycrystalline In4Se3 films.