Volume 20, Issue 4
July 1997, pages 377-622
pp 377-377 July 1997
pp 379-384 July 1997
The electronic structure of hydrogenated amorphous silicon (a-Si:H) is in a state of metastable equilibrium and can change upon application of external stimuli. We study the effect of thermal quenching and light soaking in lithium-doped a-Si:H, on its conductivity and thermopower. We present evidence showing that the metastable state obtained after fast quenching is different than that obtained after light exposure. Experiments on chalcogenides show that they are not affected by thermal quenching although they change upon light soaking. This is in contrast with lithium doped a-Si:H in which both effects are observed. Our experiments suggest that hydrogen present in a-Si:H plays an important role by controlling heterogeneities and potential fluctuations in a-Si:H. Light soaking appears to enhance these potential flucutations, whereas fast cooling seems to have little effect on them.
pp 385-389 July 1997
Samples of crystalline silicon, porous silicon, gallium arsenide and silicon diodes were exposed to 50–80 MeV silicon and oxygen ions in the fluence range of the order of 1013 to 1014 ions/cm2. The irradiated samples were characterized to obtain information on the relative concentration and depth distribution of the induced defects. For comparison a few silicon diodes and crystalline silicon samples were also exposed to 6 MeV electrons. The main techniques used for the analysis of silicon samples were low angle X-ray diffraction, photo-luminescence spectroscopy and lifetime of minority carriers, whereas diodes were characterized on the basis of switching parameters. It is observed that a large number of defects are produced in the surface region of each of the irradiated semiconductor sample though the energy deposited in the surface region through electronic loss is three orders of magnitude greater than that of nuclear collisions.
pp 391-399 July 1997
Positron annihilation spectroscopy (PAS) is a powerful probe to study open-volume defects in solids. Its success is due to the propensity of positrons to seek out low-density regions of a solid, such as vacancies and voids, and the emissions of gamma rays from their annihilations that carry information about the local electronic environment. The development of low-energy positron beams allows probing of defects to depths of few microns, and can successfully characterize defects in the near-surface and interface regions of several technologically important systems. This review focuses on recent studies conducted on semiconductor-based systems.
pp 401-407 July 1997
Photoluminescence has been studied in Cu diffusedn andp-InP. Inp-InP a Cu related photoluminescence (PL) band was observed at 1·216eV. The temperature dependence of line-width was studied and line-shape and line-width analysis carried out. The configuration coordinate diagram of the band was calculated which showed a small lattice relaxation of 0·079 Å.
Inn-InP two PL bands at 1·20 and 1·01 eV were found at 10 K. The former was similar to the 1·216eV band inp-InP. The PL of the 1·01 eV band was also studied in detail and the corresponding configuration coordinate diagram derived.
pp 409-416 July 1997
In this paper we discuss howab initio local density electronic structure calculations can be used to investigate extended defects such as interfaces and polytypes. LMTO-supercell calculations have been performed to understand the nature of bonding in epitaxial metal/ceramic interfaces such as Ag/MgO(001) and Ti/MgO(001). Cohesive and electronic properties of hexagonal polytypes of diamond, with different stacking sequences, have been predicted for the first time and compared with the available experimental data. The relative stabilities of 4H, 6H and 8H diamond polytypes have been calculated using a generalized version of force theorem.
pp 417-421 July 1997
An isothermal spectroscopic technique called time analyzed transient spectroscopy (TATS) in the constant capacitance (CC) mode has been used to characterize electrically active defects in the MeV Ar+ implanted silicon. The problems associated with high defect density and the presence of damaged region in the as-implanted material are overcome by CC-TATS method. The CC-TATS spectra of the as-implanted sample shows two positive peaks and an attendant negative peak. Two distinct traps have also been identified using thermally stimulated capacitance method modified to operate in constant capacitance mode. Variable pulse width measurements using CC-TATS show exponential capture kinetics in contrast to extremely slow capture observed in conventional deep level transient spectroscopy (DLTS) experiment. The results indicate that trapping behaviour is due to point-like defects associated with extended defects such as dislocation and stacking fault.
pp 423-427 July 1997
The copper germanide phase Cu3Ge which is emerging as an alternative material for making contacts and interconnects for semiconductor industry has been produced across the interface of Cu/Ge bilayers by ion beam mixing at room temperature using 1 MeV Ar ions. The dose dependence of the thickness of the mixed region shows a diffusion controlled mixing process. The experimental mixing rate and efficiency for this phase are 5·35 nm4 and 10·85 nm5/keV respectively. At doses above 8 × 1015 Ar/cm2 the formation and growth of another copper rich phase Cu5Ge has been observed. The present theoretical models are inadequate to explain the observed experimental mixing rate.
pp 429-433 July 1997
We have studied structural properties of amorphous hydrogenated silicon usingab initio molecular dynamics simulations. A sample was generated by simulated annealing using periodic boundary conditions with a supercell containing 64 silicon and 8 hydrogen atoms. The radial pair distribution functions for Si-Si, Si-H and H-H have been studied at 300 K and are found to be in good agreement with experimental data. Our results show that hydrogen saturates the dangling bonds and reduces bond strain. We also report existence of Si-H-Si bridge sites which are likely to play an important role in understanding the light induced metastability in this material.
pp 435-440 July 1997
Silver halide crystals AgCl and AgBr exhibit all the three types of intrinsic point defects, namely, cationic Frenkel disorder, Schottky disorder and the bound vacancy pair disorder. The ionic conductivity anomaly at high temperatures, the unusual behaviour in respect of self- and impurity diffusion and the contrasting features in the anion self-diffusion between AgCl and AgBr do not all satisfactorily fit into the prevailing models of ionic transport. We review these features critically. We also discuss the potential and polarization models used in the theoretical evaluation of the enthalpies of formation of the intrinsic defects in these crystals. Results of our recent investigations on these questions will be presented to highlight their implications to the defect physics of these materials.
pp 441-449 July 1997
The rotational excitations of NH4+ ions in dilute solution in metal alkali halide lattices have been studied at 4K by inelastic neutron scattering technique. It is found that the ions exist in matrix isolation at very low ammonium ion concentration,c (c → 0), and it transforms to orientational glass phase on increase ofc. It is shown that the onset of the orientational glass phase is moderated through the strain field generated by the substituted impurity in the lattice. The variation of the intensity profiles with increase ofc have been successfully explained by a phenomenological model based on gaussian distributed heights.
pp 451-454 July 1997
We present new findings on the enthalpies of formation of vacancies and interstitial ion defects on either sublattice in the alkali halide crystals MX (X=F, Cl, Br and M=Li, Na) using the MPPI model. The model uses a minimum fit to crystal properties and is fully consistent with the dielectric response. The role of the non-Coulombic short range forcesvis-a-vis polarization fields in the stabilization of the defect formation process is examined and discussed in detail. It is shown that it is important to treat the electrostatic polarization caused by the point defect carefully and variation in potential modelling has a comparatively minor effect on the calculations of enthalpies of formation.
pp 455-459 July 1997
In LiF, the photoluminescence emission ofF2 andF3+ centres are produced on irradiation peaks in the red (660 nm) and green (520 nm) region respectively. In this work, their relative sensitivities for thermal neutrons and gamma rays are compared.
pp 461-465 July 1997
Here we present the electronic band structure calculations of CaFCl, SrFCl, BaFCl, BaFBr and BaFI performed using tight binding linear muffin-tin orbital method. The calculated ground state properties namely the lattice parameters, bulk moduli and the pressure-volume relations were found to be in good agreement with the experimental values. The electronic band structure of these compounds were calculated at ambient as well as in the high pressure region. The band gap values of these insulating systems were calculated and compared with available experimental values. At high pressure these compounds exhibit the interesting phenomenon of band overlap metallization.
pp 467-474 July 1997
Small angle neutron scattering (SANS) has been widely used in investigating defects in metals, and in particular, to characterize the helium bubble population in implanted materials. The main advantage in using SANS is the non-destructive feature of the tests and the quantitative results obtained by averaging over a large sample volume. SANS is a powerful technique, very sensitive to microstructural changes and its use was of fundamental importance to show the bimodal distribution of the bubble population: in the vicinity of grain boundaries and free surface and inside grains, respectively. Here the most important applications of the SANS technique to the study of the helium bubbles in implanted materials are reviewed. Most of the work has been done on nickel samples, but also a ternary alloy Fe-Ni 15%wt-Cr 15%wt and a steel (MANET) with a more complicated structure have been successfully investigated. Different annealing treatments, isothermal and isochronal, were investigated in order to determine the active mechanisms of the bubble coarsening and their activation energies. From the SANS data the bubble size distributions have been determined, from which parameters such as mean radius and density of the bubble population have been calculated. The gas pressure inside the bubbles was also determined by the contrast variation technique in SANS and by a computational procedure, and an excellent agreement was found between the results. These results show a marked overpressure inside the bubbles as compared to thermodynamical equilibrium values of about 3 GPa. A comparison with results obtained by other techniques confirms the validity of SANS, which has to be considered as a complementary technique for its indirect image of the sample.
pp 475-479 July 1997
Time differential perturbed angular correlation (TDPAC) measurement performed on the 482 keV level of181Ta probe nuclei inp-implantedNbHf sample shows that hydrogen decorated vacancy complexes are trapped by 6% of probe nuclei experiencing a Lorentzian distribution of quadrupole frequencies with a width of 32 MHz having a mean value of 545 ± 10 MHz with an asymmetry parameter 0·32. Measurements at RT following isochronal annealing treatment of the sample shows that the onset of detrapping of H-V complexes from probe nuclei occurs at 373 K and gets completed at 573 K. Hydrogen atoms stabilized vacancy clusters are found to be trapped by probe nuclei. The role played by other gaseous impurities in the sample in stabilizing the lattice defects bound to probe nuclei is discussed, to have a complete understanding of the recovery of defects in the sample.
pp 481-489 July 1997
Monte-Carlo simulations are described using the Kawasaki model of oxygen (dis)ordering in the basal plane of YBa2Cu3O7−δ(YBCO). In this model the presence or absence of an interstitial oxygen defect is mapped into a pseudo Ising spins=+1 or − 1. We study the rate law and the relaxation time of the order parameter and most importantly, the diffusion of oxygen in the basal plane across the structural phase transition in this system. In a separate investigation we have carried out numerical analysis of a mean-field dynamical model (with Kawasaki kinetics) to examine phase ordering dynamics and pattern formation, for δ=0.
pp 491-497 July 1997
Isothermal resistivity measurements have been carried out to study the dynamics of oxygen out- and in-diffusion in thin films of YBa2Cu3O7−x in the temperature range 648–773 K. The activation energies for the out- and in-diffusion were determined to be 1·36 and 0·7 eV respectively. We have modelled the resistance-time plots for the oxygen in-diffusion using an equation for one dimensional diffusion into a plane. The microstructural defects such as low angle grain boundaries associated with the c-axis oriented grains are believed to provide the required diffusion paths in thin films of YBCO.
pp 499-502 July 1997
In the high temperature superconducting material yttrium barium copper oxide (YBCO) the basal plane oxygen plays a dominant role in determining the superconducting properties of the material. We have carried outin situ high temperature measurements of (a) electrical resistivity and (b) X-ray diffraction in vacuum as well as in helium atmospheres to probe the nature of ordering of oxygen atoms. While published phase diagram of temperature vs oxygen concentration in different partial pressures allow only a tetragonal phase for the annealing temperatures we have investigated, the experiments point to the coexistence of orthorhombic (OII) and tetragonal phases.
pp 503-507 July 1997
The structural instability in Ti rich compositions of Nb-Ti alloy system, which leads to various anomalies in the normal and superconducting state properties, has been extensively studied in bulk samples in the past. In this paper we report the formation of thin films of several compositions in the Ti rich region of this alloy system by RF magnetron sputtering and investigation of their electrical properties. Compositional analysis of two representative films was carried out by the RBS technique and the compositions agreed to within 2% of the targeted values. The anomalous variations in the electrical properties characteristic of the bulk, which can be ascribed to the structural instability related to the formation of athermal ω-phase, are also observed in thin films.
pp 509-517 July 1997
The transformation between quasicrystals and related crystals, the so-called approximant phases, appears as a major point in the understanding of quasicrystal stability. Structural defects of approximant phases seem to be involved in the mechanism of crystal/quasicrystal transformation. Theoretical works as well as observations are supporting this point of view. We here report observations made by transmission electron microscopy on two systems which provide relevant examples for two types of mechanisms. In both cases, the approximant defects are identified as antiphase boundaries. In the first system (Al-Li-Cu), the transformation is due to a progressive organization of the approximant phase defects. In the second system (Fe-Cr-Mo), the vertex of intersecting defects exhibits key structural feature for the transformation in a quasicrystal.
pp 519-523 July 1997
The study of interfaces in quasicrystalline alloys is relatively new. Apart from the change in orientation, symmetry and chemistry which can occur across homophase and heterophase boundaries in crystalline materials, we have the additional, exciting possibility of an interface between quasicrystalline and its rational approximant. High resolution electron microscopy is a powerful technique to study the structural details of such interfaces. We report the results of a HREM study of the interface between the icosahedral phase and the related Al13Fe4 type monoclinic phase in melt spun and annealed Al65Cu20Fe15 alloy.
pp 525-530 July 1997
The strip projection method has been used here to establish the structural relationships between various two dimensional quasiperiodic structures and certain well known periodic structures. These periodic structures, designated as unmodulated approximants, can transform into their quasiperiodic counterparts in a continuous fashion. The early stages of such transformations are characterized by the occurrence of faults or APBs. There is considerable high resolution electron microscopy (HREM) and electron diffraction evidence in favour of this in the case of the β-Mn to octagonal quasicrystal, the σ-phase to dodecagonal quasicrystal and the Al5Ti3 to quasiperiodic superlattice structure transformations.
pp 531-534 July 1997
We report measurement of the electrical resistivityρ(T) and point contact differential conductance (dI/dV) for icosahedral Al70Pd30 −x (x=7,9 and 11) at low temperatures. Below 10 K, forx=11,ρ(T) follows a logT dependence. All the three compositions show a sharp minimum in the differential conductance near zero bias. Magnitude of the dip in the point contact conductance increases as the Mn concentration increases. The dip near zero bias also follows a logV dependence forV < 30 mV.
pp 535-540 July 1997
Al61·4Cu22·7Fe15·9, Al63·2Cu21·8Fe15 and Al71Cu14Fe15 have been studied by Mössbauer, positron annihilation and X-ray diffraction techniques. The positron lifetime, measured as a function of temperature, in Al61·4Cu22·7Fe15·9 shows a two-step increase. This is found to be due to the dynamics of phasons associated with Al and Cu, having activation energies of 0·43 eV and 0·78 eV respectively. In the case of Al71Cu14Fe15, a phase transformation from the quasicrystalline-crystalline phase at ≈ 550°C is observed.
pp 541-548 July 1997
In this paper, we have examined different types of interfaces that occur between orientational/translational variants generated during the ordering process. This has been illustrated citing examples of ordering of the FCC structure into DO22, Dla and Pt2Mo type structures in some nickel base alloys. Microstructures consisting of more than one ordered structure have also been investigated. Superlattice domains of DO22 and Pt2Mo type structures have also been found to coexist in a microscopic scale of mixing in Ni-V alloys while mixed domains of Dla and Pt2Mo type structures on a much finer scale have been observed in Ni-Mo alloys. The formation of different variants (rotational and translational) of ordered structure(s) from the disordered lattice has been explained on the basis of group theoretical and symmetry considerations.
pp 549-555 July 1997
This paper deals with the development of an effective interionic interaction for transition metals and alloys in the spirit of a hybridized pseudopotential tight-binding formalism, developed among others by Wills and Harrison. Stated briefly, the recipe consists in writing the total interionic interaction as a sum of contributions arising from nearly free s-electrons and that of tightly bound d-electrons. The s-d hybridization is also accounted for by allowing for the relative change in the occupancy of s and d electron counts. This potential, although not taking into account the magnetic contribution, contains otherwise all the essentials of a physically meaningful interatomic interaction. In this paper, a modified version of the Wills-Harrison model is used to calculate the structural and cohesive properties of transition metals at 0 K. The calculated values are in reasonable agreement with the experimental ones. This method is also used to derive an effective interatomic interaction for γ-TiAl. The use of this potential in obtaining an estimate of point defect energetics in TiAl is discussed.
pp 557-563 July 1997
The experimental methods of small-angle scattering of X-rays, neutrons and light have been established as excellent tools for investigating particulate inhomogeneities in condensed matter media in the size range of 10 Å-10000 Å. In this paper we bring out the usefulness of small-angle X-ray scattering studies of particulate microstructure in condensed matter. Notwithstanding the extreme extensivity of the field of its applicability, we shall discuss in detail only one field, namely, precipitation of second phase particles in phase separating alloys. Specifically, we shall analyse the example ofγ’ precipitation in Ni0·8Al0·2 alloy system in order to demonstrate the efficacy of the small-angle scattering techniques. We shall touch upon the maximum entropy technique, a relatively new and efficient method of SAXS data analysis, used to derive material data with excellent certainty and statistics from these experiments.
pp 565-572 July 1997
Colloidal crystal of polystyrene spheres of 0·53 µm diameter dispersed in water is prepared in a quartz container. Optical microscope, video camera and image processor are used to investigate the defects, their dynamics and the effect of shear in this colloidal crystal. Six layers of the crystal from the surface of the container are imaged sequentially, one after another, and the crystal is reconstructed. FCC structure with (111) planes parallel to the container surface is stable at high volume fraction whereas, at low volume fraction (100) planes also coexist. Vacancies, dislocations, grain boundaries, voids and particle aggregates are identified. The vibration of aggregates is sluggish as compared to that of a single particle. The nearest layer of particles to the container surface is better ordered than those in the interior layers. The container wall provides a stabilizing potential to the nearby layers of particles. The 2-D crystalline-like characteristics of the nearby layers are probed. The colloidal crystal is shear-melted by shaking the container. On resting it, the crystal-flow slows down. The flow is constrained to be parallel to the container surface and occurs along the layer in quantum jumps. During the flow, both (111) and (100) planes coexist in the layer. Flow at low rates anneals the defects created by the shear-melting.
pp 573-578 July 1997
Pure and yttria, calcia doped zirconia powders were prepared in nanostructured form by the method of co-precipitation by hydrolysis. As prepared and heat treated powders were characterized by X-ray diffraction and transmission electron microscopy. Features and variations in crystallite/particle size measurements by both XRD and TEM are discussed.
pp 579-582 July 1997
Perfluoroethylene sulfonic acid polymer (NAFION) films are subjected to ion exchange in the medium of aqueous solutions of cadmium acetate, followed by ammonia passivation. The films are then treated with hydrogen sulfide gas for prescribed times. X-ray powder diffraction data of these samples have been analyzed for estimating the sizes of the nanocrystallites. The optical absorption spectra of the samples show an absorption edge beginning at 525 nm for the largest size clusters. A broad absorption band appears with a maximum around 410 nm–440 nm in the smaller size clusters.
pp 583-589 July 1997
Amorphization is often observed during irradiation of intermetallic compounds with energetic charged particles or neutrons. This paper discusses various mechanisms of radiation induced amorphization and also presents the results of amorphization in Al-Mn alloys.
pp 591-596 July 1997
Kanzaki lattice static method is generalized to evaluate impurity-induced strain field in the dilute alloys. The displacements of nearest neighbours of impurity are found positive and negative, however, their magnitudes are found decreasing away from the impurity.
pp 597-600 July 1997
The high-pressure behaviour of thorium monopnictides is of considerable interest as these systems exhibit structural phase transitions under pressure. At ambient conditions these compounds crystallize in the NaCl-type (B1) structure. Experiments show that with the application of pressure these compounds transform to the CsCl-type (B2) structure. ThSb and ThAs are found to exhibit B1–B2 transition in the pressure range between 9–12 GPa and 1826 GPa respectively. In this work, we present the electronic and high-pressure behaviour of ThAs and ThSb performed using the tight-binding linear muffin-tin orbital method. The total energies within the atomic sphere approximation were calculated as a function of volume for both the B1 and B2 structures. The total energy calculations reveal that both ThSb and ThAs are stable in the B1 structure at ambient conditions and undergo structural transition to the B2 structure at pressures 78 and 240 kbar respectively, which are in good agreement with the experimental values. The calculated values of equilibrium lattice parameter and the transition pressure are found to be in good agreement with the experimental results.
pp 601-605 July 1997
An analytic expression for the force between two parallel screw dislocations, derived earlier on the basis of the gauge theory of dislocations, has been used to investigate the static distribution of a given numberN of parallel screw dislocations confined between two immobile dislocation obstacles. It is shown that in the limit of a continuous distribution of dislocations the equilibrium condition leads to a Fredholm integral equation of first type which does not admit any nontrivial solution. Implication of this result is discussed. For a finite number of dislocations, the ratio (η) of the obstacle separation to the core radius is an important parameter governing the nature of solution of the discrete equation. It is found that for a givenN, there is a critical valueηc below which there does not exist any solution.
pp 607-611 July 1997
The dynamics and energetics of a light impurity (hydrogen) embedded in metals (Al, Cu, and Ni) are studied using a phenomenological ‘local’ oscillator potential experienced by it as a result of its screened interaction with the host ions. Calculations of the migration energy of hydrogen in the above mentioned metals are reported.
pp 613-622 July 1997
From the detailed electronic structure studies on intermetallic compounds, it has been found that these materials have low heat of formation and large glass-forming ability, if the Fermi level falls on the peak in the density of states (DOS) curve. On the other hand, if theEF falls on the pseudogap in the DOS curve, the ordering energy will be larger and the system prefers to form an ordered alloy. The first principles electronic structure calculations performed on Ni3Al show that it is possible to vary the filling of bonding, nonbonding and antibonding states in the DOS curve and this in turn shows gradual structural transformation as well as formation of multiphases by ternary alloying. Possibilities of tailoring the properties of materials by tuning the Fermi level is discussed in this paper.
Volume 42 | Issue 6
Click here for Editorial Note on CAP Mode