Showing papers in "Materials Science and Engineering B-advanced Functional Solid-state Materials in 1991"

Cubic boron nitride: synthesis, physicochemical properties and applications

Abstract: A review, with 112 references, is given of the different ways to prepare cubic boron nitride (BN). The different crystal forms as well as the phase diagram of BN are described. The physical and chemical properties and the correlated industrial or potential applications are shown.

Processing and fabrication techniques for bulk superconductors: A critical review

Abstract: For major applications of the bulk high-T c superconductors, proper processing to obtain desirable superconducting properties and physical shapes is essential. In this paper, various processing and fabrication methods are critically reviewed with respect to the resultant properties and likelihood of them becoming commercially viable techniques. Remarkable progress has been made during the past few years; however, the present status in materials processing and fabrication is far from ideal, especially in obtaining high critical currents in strong magnetic fields. It is concluded that further major processing innovations are needed to improve the superconductor properties and to accelerate progress towards major applications, and that many challenges and research opportunities are waiting to be exploited.

A.c. electrical properties of composite solid electrolytes

Abstract: An improved effective-medium model to describe a.c. electrical properties of composite electrolytes is proposed in the light of its microstructure. Three characteristic volume fractions, two percolation thresholds ( ν 2 ′, ν 2 ″ ) for percolation of interface bondsand disruption of conducting paths, and ν 2 ∗ corresponding to the maximum enhancement in conductivity, are obtained from this model. The effect of microstructure parameters, such as grain size and size distribution, and grain volume fraction on the three characteristic volume fractions and a.c. electrical properties is discussed in detail. The results obtained by the theoretical model are consistent with the present experimental results.

Mechanism of formation of 60° and 90° misfit dislocations in semiconductor heterostructures

Abstract: A proposal is made and experimental evidence provided for a mechanism of formation of 60° glissile ( a 2 )〈101〉[111] (mixed) and 90° ( a 2 )〈110〉[001] (edge) misfit dislocations in semiconductor heterostructures such as Ge/Si and GaAs/Si. With increasing thickness of the epilayer the nature of dislocations was found to change from being predominantly 60° to mostly 90°. Calculations of atomic structures of dislocations using Stillinger-Weber interatomic potentials were done to show that it is energetically favorable for a 60° core to transform into a 90° core. Thus the strained layer system relaxes its energy by minimizing short-range core distortions as well as long-range elastic displacements. A distinguishing feature of the present mechanism is that the second 60° dislocation is nucleated near the surface in the appropriate glide plane to meet (primarily via a glide process and partly by a climb process) and react with the first 60° dislocation which is already at the interface. High resolution transmission electron microscopy studies have been performed to rationalize and confirm the proposed mechanism.

Selective area growth for opto-electronic integrated circuits (OEICs)

Abstract: Selective area epitaxy (SAE) is emerging as an important technology in the fabrication of optoelectronic integrated circuits. This paper reviews the current growth technologies for their applicability to the process of SAE. It discusses in detail the interaction on the masked area with the adjoining epitaxial window. The minimization of the features formed by this interaction whilst optimizing selectivity is seen as the main aim of the growth processes.

Electrical transport properties of manganese-magnesium mixed ferrites

Abstract: Thermoelectric power studies are made of MnMg mixed ferrites over the temperature range 300–700 K by the differential method. The Seebeck coefficient S is found to decrease while the carrier concentration n and the charge carrier mobility μ are found to increase with increasing temperature. On the basis of these results an explanation for the conduction mechanism in MnMg mixed ferrites is suggested.

Point defect thermal equilibria in GaAs

Abstract: The thermal equilibrium concentrations of the six electrically neutral single point defect species in GaAs are expressed as explicit functions of well-defined thermodynamic quantities. The difference between the Gibbs free energies of an arsenic atom in the interior of a GaAs crystal and in an arsenic vapor phase molecule is emphasized. Numerical values of the thermal equilibrium concentrations of the gallium and arsenic vacancies and the two antisite defects are estimated. It is of interest that the calculated thermal equilibrium concentration of the anion antisite defect As Ga 0 reaches a peak value of about 1 × 10 17 cm −3 and is almost temperature independent. This computed value for As Ga 0 is consistent with experimental findings.

Dry-etching techniques and chemistries for III–V semiconductors

Abstract: Dry etching of III–V materials using both chlorine-based (CCl2F2, SiCl4, BCl3 and Cl2) and CH4-H2 discharges is briefly reviewed. The etch rates using chlorine-based mixtures are generally faster than those utilizing CH4-H2, but the latter gives smoother surface morphologies for indium-containing compounds. The use of microwave (2.45 GHz) electron cyclotron resonance (ECR) discharges minimizes the depth of lattice disorder resulting from dry etching, relative to conventional r.f. (13.56 MHz) discharges. Recent results on the systematics of ECR plasma etching of both indium- and gallium-based III–V semiconductors using CCl2F2-O2 and CH4-H2 mixtures will be discussed, including the determination of the maximum self-biases allowable which do not induce near-surface damage in the semiconductor. A further key issue is the prevention of changes in the surface stoichiometry of materials such as InP, where the lattice constituents may have considerably different volatilities in the particular discharge.

Growth limitations by the miscibility gap in liquid phase epitaxy of Ga1−xInxAsySb1−y on GaSb

Abstract: The boundary line (binodal curve) of the solid phase miscibility gap of the Ga1−xInxAsySb1−y quaternary alloy has been calculated from the regular solution model, taking into account the latticemismatched strain energy induced by a GaSb substrate. A calculation suggests that the miscibility gap is reduced and epitaxial layers might be deposited over the entire range of lattice-matched compositions at 615°C. Ga1−xInxAsySb1−y epitaxial layers have been grown by liquid phase epitaxy on (100)- and (111)B- oriented GaSb substrates. The lattice-matched epitaxial layers which were richest in indium (x = 0.23) for the (100) orientation and x = 0.26 for the (111)B orientation have compositions located in the vicinity of the boundary of the miscibility gap calculated ignoring the strain energy, showing that stabilization by the substrate is far less effective than predicted.

Electrochemistry of nickel oxide films in aqueous and Li+ containing non-aqueous solutions: an application for a new lithium-based nickel oxide electrode exhibiting electrochromism by a reversible Li+ ion insertion mechanism

Abstract: The new composition Li 0.6 + Ni 0.70 II O 2− was prepared by room temperature electrochemical insertion of lithium into X-ray-amorphous nickel-oxide-based film. The film undergoes a reversible electrochemical Li + insertion process which is accompanied by a net electrochromic effect.

Critical thickness during two-dimensional and three-dimensional epitaxial growth in semiconductor heterostructures

Abstract: The homogeneous nucleation of misfit dislocations in two-dimensional and three-dimensional epitaxial structures on rigid substrates was analyzed. It is shown that nucleation and growth of a dislocation in the epilayer involves an activation energy barrier. We propose that this activation energy barrier can be overcome from the residual coherent strain energy in the film. A critical thickness of the epilayer is defined at which a dislocation is nucleated. The activation energy and the coherent strain energy were determined for several different configurations of 60° glide and 90° climb dislocations. The discrepancies that are associated with different formulations are pointed out. Specific numerical calculations were performed for dislocation nucleation in the GaAs/Si system. The results are given in terms of the various energy contributions responsible for nucleation of misfit dislocations, and the critical thickness of the epilayer is evaluated. The critical thickness of the epilayer under the two-dimensional and three-dimensional growth conditions are compared and the results described in terms of the mechanisms of dislocation nucleation.

SiGe alloys and superlattices for optoelectronics

Abstract: Important advances in materials science and technology are opening the field of optoelectronics to device structures based on silicon and germanium. Optoelectronics offers new functionality to silicon integrated circuits which previously were thought to be limited to strictly electronic applications. In this paper we review recent achievements in optical modulators, detectors and light emitters fabricated from silicon and germanium. These devices hail the entry of silicon technology into a niche previously reserved for compound semiconductors. Innovations in materials growth and processing of silicon-based heterostructures are the locomotive driving this important development into optoelectronic materials.

Size determination of semiconductor nanocrystallites in glasses by low frequency inelastic scattering (LOFIS)

Abstract: Low frequency inelastic scattering (LOFIS) concerns Raman scattering of light below 100 cm −1 ; in this frequency range the eigenmodes of small crystallites give rise to low frequency lines whose position depends on the crystallite sizes. LOFIS of various CdS x Se 1- x semiconductor-doped glasses was analysed and diameters between 2 and 10 nm were obtained. Resonance effects strongly enhanced the longitudinal optical (LO) phonons. The influence of confinement on the scattering of light by LO phonons is discussed.

Improvement in photocurrent with n-type niobium- and rhenium-doped molybdenum and tungsten diselenide single crystals

Abstract: In order to increase the photoconductivity of n-type semiconductors, we made a particular study of TSe2 ( T ≡ Mo, W ) transition dichalcogenides. Niobium- and rhenium-doped single crystals were obtained by chemical vapour transport from a 1% polycrystalline metal-doped solution, using iodine in the case of MoSe2, and SeCl4 in the case of WSe2, as transport agents. The best results were obtained for doped MoSe2 crystals. In the Mo 1−x Re x Se 2 phase, it is possible, when x = 6 × 10 −5 , to increase the photocurrent gain by a factor of 10 without any applied voltage, the electrical conductivity being at a maximum. The best saturation current was obtained for Mo 1−x Re x Se 2 when x = 1 × 10 −4 , reaching 550 A m−2. This value is the highest ever found among transition dichalcogenides.

The fabrication of a novel composite GaAs/SiO2 nucleation layer on silicon for heteroepitaxial overgrowth by molecular beam epitaxy

Abstract: We report on the fabrication of a composite GaAsSiO2 nucleation layer. The layer is formed by a deposition of a GaAs island layer by molecular beam epitaxy (MBE), followed by an oxidation step of the silicon regions surrounding the islands. In this way, small GaAs islands, for which the critical thickness for misfit dislocation generation is increased, are surrounded by a stable amorphous phase. Lateral overgrowth seeded by the individual GaAs islands might enhance the overall epilayer quality. We describe the fabrication and cleaning of such a composite GaAsSiOx nucleation layer that is compatible with the epitaxy process. Preliminary regrowth on a non-optimized composite surface resulted in GaAs-on-silicon quality equal to standard GaAs-on-silicon. Compared with GaAs epitaxy on porous silicon, another seeded growth technique, the composite surface technique has greater technological potential for the monolithic integration of GaAs and silicon electronics.

Making quantum wires and boxes for optoelectronic devices

Abstract: This paper will review current techniques used to fabricate low dimensional quantum structures for optoelectronic applications. Work has included the fabrication of quantum wire structures using molecular beam epitaxy (MBE) growth of tilted superlattices on vicinal substrates, observation and modeling of self-ordering effects that occur during this process, extension to antimony-containing III–V compounds, and quantum wire formation in “serpentine superlattices”. In addition, we will discuss results obtained using focused ion beam implantation techniques to “write” quantum wires and boxes. Direct growth by both MBE and metal-organic chemical vapor deposition (MOCVD) on lithographically patterned substrates, presently underway at Bellcore, will also be described, along with the electron-beam patterning and MOCVD regrowth of phosphide-based quantum wire and box configurations carried out at the Tokyo Institute of Technology. Application of these various fabrication techniques to semiconductor lasers and high-speed photodetectors will be discussed.

Lattice site of trivalent impurities in Mg-doped lithium niobate crystals

Abstract: The lattice site of trivalent impurities (lutetium, neodymium and indium) was determined in magnesium-doped LiNbO3 crystals by Rutherford backscattering and proton-induced X-ray emission channelling experiments. It was shown that lutetium and indium substitute mainly for lithium ions, while most of the neodymium ions occupy structural vacancy positions. It cannot be excluded, however, that a fraction of the impurities may replace niobium sites. In all of the three crystals an IR absorption band was detected which is attributed to OH− stretching vibrations in M3+ (niobium site)-OH−Mg2+ (lithium site) complex defects.

Deformation behavior of CdTe and (Cd, Zn)Te single crystals between 200 and 600 °C

Abstract: The deformation behavior of CdTe and (Cd,Zn)Te single crystals oriented for single slip has been investigated in the temperature range 200–600 °C. The addition of 4.5 at.% Zn increases the yield strength by about eight times over that of CdTe crystals. Further, in both cases the yield strengths are nearly temperature independent above 300 °C. Arguments have been developed to rationalize these observations. Deformed structures have been evaluated by transmission electron microscopy. Parallel sets of dislocations, dislocation dipoles, loops, tangles and networks are observed in the deformed samples. In addition, the (Cd,Zn)Te samples deformed at 200 °C show profuse twinning. The evolution of the deformed structures has been discussed in the light of earlier studies pertaining to the elemental and III–V compound semiconductors. Also, the formation of twins is rationalized.

Implications of the Y2O3BaOCuO liquidus for processing pure YBa2Cu3O7−x material

Abstract: Results relating to the liquids relationships in the CuO-rich corner of the 1 2 Y 2 O 3  BaO  CuO phase diagram were used to identify the partial meltings occurring in YBaCuO powders at temperatures below the peritectic decomposition of the YBa2Cu3O7−x compound The production of this compound in three atmospheres with different oxygen potentials was followed by X-ray diffraction and differential scanning calorimetry The origin of the partial meltings in these powders was found in the failure of the synthesis to be complete (ie to reach the equilibrium state) at temperatures below the solid-liquid phase transitions, implying the presence of the compounds YBa2Cu3O7−x, BaCuO2 and CuO This difficulty is overcome if the powders are annealed in an O2 atmosphere which allows treatment temperatures as high as 940°C Sintering of such powders up to 1000 °C is not promoted by a liquid phase Differential scanning calorimetry proved to be an easy and valuable method for checking the purity of the YBa2Cu3O7−x compound

Semiconductor doped glass as a nonlinear material

Abstract: Within the last few years there has been an increasing interest in materials exhibiting non-linear optical effects. Various semiconductors and crystals are known for large non-linear coefficients but the intense search for even more efficient and cost-effective materials has led to organic polymers and semiconductor-doped glasses. We describe the fabrication of semiconductor-doped glasses, their transmission and fluorescence spectra and some of their non-linear properties as well.

Laser-assisted atomic layer epitaxy

Abstract: Laser-assisted atomic layer epitaxy (laser ALE) is reviewed. The characteristics and the mechanism of laser ALE are discussed, with a brief description of possible applications. The mechanism of laser ALE seems to be atom-selective decomposition of trimethylgallium and triethylgallium at the surface of GaAs. This atom-selective decomposition is not a thermal effect but a photochemical effect on the GaAs surface.

Determination of size shape and concentration of spheroidal silver colloids embedded in glass by VIS-spectroscopy

Abstract: Spherical silver colloids (silver clusters about 10 nm in diameter) in glass show a strong bell-shaped extinction band in the visible region of light (peak at about 410 nm wavelength). In the case of spheroidal silver colloids this band splits into two. One band tends to shorter wavelengths due to the absorption of light polarized parallel to the short axis of the ellipsoid. The other band shifts to longer wavelengths due to the absorption of light polarized parallel to the long axis of the ellipsoid. Calculations show that the peak positions of these two bands only depend on the particle shape (eccentricity of the ellipsoid) but not on the particle size. In contrast, the half-bandwidths of the bands only depend on the particle size but not on its shape. A further result is that the volume concentration of spheroidal silver colloids in glass is proportional to the area beneath an extinction curve. Electron micrographs of prolate silver colloids, made by heating and stretching of glass containing spherical colloids, show very good agreement with geometrical data drawn by this spectroscopic method.

Realization and characterization of Er and Yb glasses obtained by the sol-gel method

Abstract: Glasses doped with erbium or ytterbium were realized using the sol-gel method, which allows a high rare earth concentration without segregation to be obtained. The realization of doped glasses by sol-gel is described. The glasses were analysed by secondary ion mass spectroscopy in order to compare the concentration in the initial liquid solution and in glass. No rare earth segragation seemed to occur. The optical absorption spectra show the typical peaks of the rare earth. In erbium-doped glasses a fluorescence can be oberved at about 520 nm.

Electroluminescence and photoluminescence from Si1−xGex alloys grown on (100) silicon by molecular beam epitaxy

Abstract: Photoluminescence has been observed from Si 1− x Ge x alloy layers, superlattices and non-periodic multilayers where x was varied from 0 to 0.6 A p-type Si 0.82 Ge 0.18 alloy layer 200 nm thick, grown by molecular beam epitaxy (MBE), has been fabricated into a mesa diode which operated as a light-emitting diode, emitting at 1.4 μm at temperatures up to 80 K. This diode was selected from a series of heterostructures which exhibited intense photoluminescence with internal quantum efficiencies in the range 1%–10% at low temperatures. Photoluminescence in the wavelength range 1.2–1.7 μm has been observed from thick (100–200 nm) Si 1− x Ge x alloys and Si 1− x −Si strained layer superlattices with a range of dimensions which was large compared with the unit cell; i.e. where Brillouin zone folding effects were negligible. The intense Si 1− x Ge x alloy photoluminescence peak had a halfwidth of about 80 meV and the peak energy was found to shift consistently and predictably with the germanium fraction. Photoluminescence peak energies at 4.2 K varied from 620 to 990 meV for germanium fractions where 0.53 > x > 0.06. The photoluminescence and electroluminescence peaks were consistently about 120 meV below the established bandgap for tetragonally strained Si 1− x Ge x . In general, Si 1− x Ge x strained layers grown at low temperatures typical of MBE (below 500°C) exhibited low photoluminescence intensity. However, post-growth annealing in the 500–700°C temperature range enhanced luminescence efficiency by up to two orders of magnitude. A comparison is also made of the broad intense photoluminescence observed from MBE material with the weak near-band-edge spectra obtained from Si 1− x Ge x -Si grown by low temperature chemical vapour deposition.

Atomic configurations of tip apexes and scanning tunnelling microscopy-spectroscopy

Abstract: This paper demonstrates the variation in the images depicted by scanning tunnelling microscopy (STM) with the number and arrangement of apex atoms and in the spectra obtained by scanning tunnelling spectroscopy (STS) with the apex composition and profile of a scanning tip. Thus the clarification of the state of the tip apex is the fundamental requirement for the acquisition of reliable and reproducible STM and STS data. A promising approach which enables us to examine the tip apexes in atomic dimension would be the study of the tip apex with a combined instrument of the atom probe and the field emission electron energy spectrometer.

Influence of cubic perovskite phase content on the positive temperature coefficient of resistance of n-type BaTiO3 ceramics

Abstract: The resistivity-temperature as well as the current-voltage ( I–V ) relations in donor-doped polycrystaline BaTiO 3 (tetragonal) ceramics are tremendously modified by the presence of cubic perovskites such as BaSnO 3 , BaZrO 3 , SrTiO 3 in minor quantities. The minor cubic phase contents arise out of the compositional inhomogeneity in large industrial batches of titanate solid solutions or in chemically prepared BaTiO 3 powders or because of the grinding of raw materials for extended periods of time. Maintaining the same processing parameters, chemical compositional factors and microstructural contributions, the shape of the positive temperature coefficient of resistance (PTCR) curves are found to be completely altered by the addition of less than 10% cubic phase. The resistivity continues to increase not only across the Curie point T C but also at higher temperature, without any apparent maximum even above 500 K. Thus the PTCR region is broadened and the ratio of resistivities across T C has decreased. This is in sharp contrast with the characteristics of homogeneous solid solutions having the same chemical compositions wherein ϱ max /ϱ min is over six orders of magnitude, with PTCR occurring in a narrow temperature range. The I–V curves of the mixed-phase ceramics show stable current-limiting characteristics at higher field strength without any current maximum. The electron paramagnetic resonance spectra indicate the contribution of the disorder component induced by the minor phase. The vibronic activation of acceptor states arising mostly from arium vacancies located at the grain boundary layers takes place over a broader temperature range, as a result of the disorder contribution and the consequent spread of energy values.

Emissivity of oxides: a microscopic approach to glass coatings

Abstract: Knowledge of the thermal radiation of a material requires investigation of its emissivity. This parameter depends on a number of factors such as wavelength, temperature and texture of the material. The emissivity is calculated via Kirchhoff's law from both reflection and transmission spectra. This method is reliable for homogeneous, non-diffusive, optically polished samples. It is applied to the study of glassy oxide coatings. The different excitations responsible for the absorption of electromagnetic radiation are reviewed. The phenomenon of emissivity of glass coatings is mainly controlled by the transmission and is strongly dependent on the thickness of the layer. In addition, the total emissivity is found to decrease with temperature because the maximum of the black-body radiation is shifted towards a transmission window at high temperature. We show that windows can be filled by suitable doping with transition metal oxides diluted in silicate glasses. This method may yield emissivities as high as 0.9 in the entire temperature range. To perform emissivity measurements on diffusive materials, an alternative method is discussed.

Crystal growth of yttrium and samarium tartrates from silica gels

Abstract: The results on the growth of hydrated crystals of rare earth tartrates R 2 (C 4 H 4 O 6 ) 3 , where RY, Sm from silica gels, in two different temperature ranges (25–30°C and 35–40°C), employing single-and double- diffusion techniques, are presented. The effects of various parameters such as gel pH, gel aging and concentration of reactants have been investigated for each system. We report a new kind of periodic zone, generally known as Liesegang rings, for yttrium tartrate crystals when grown in the temperature range 35–40°C. The periodicity of nucleation zones, however, is broken for samarium tartrate crystals. An attempt has been made to improve the size of spherulites by using the method of seeded growth.

Growth and characterization of metal-organic vapour phase epitaxial Ga1−xInxAsySb1−y quaternary layers

Abstract: The growth in the miscibility gap and the characterization of MOVPE Ga1−xInxAsySb1−y quaternary layers have been undertaken. The experimental conditions have been determined to obtain quaternary epilayers in the miscibility gap grown on GaSb with a good morphology even for lattice mismatched layers, in varying ΣPIII and DH2. The material quality has been assessed by single and double X-ray diffraction. The spectral responses of GaInAsSb(p)/GaSb(n) heterojunction are given. A spectral response obtained under illumination from the GaSb side, presents a good response at 2.75 μm.

Potentially and challenge of metal-organic molecular beam epitaxy

Abstract: The method of metal-organic molecular beam epitaxy (MOMBE, or chemical beam epitaxy (CBE)) is basically the metal-organic vapour phase epitaxy (MOVPE) process carried out at extremely low pressures. Layers and device structures of group III–V elements have been grown by MOMBE that are of excellent quality and uniformity with regard to layer thickness and material composition. A striking feature is the superior selectivity of the growth process, opening new possibilities for the integration of electronic and optoelectronic devices. However, since MOMBE is a very recent epitaxial method it needs further development in the area of doping and in tailoring of starting materials particularly designed for MOMBE applications.