AlAs-in-AlSb digital alloy superlattice morphology versus growth temperature

We report a systematic study of how growth temperature affects the quality of AlAs-in-AlSb digital alloy superlattices grown by molecular beam epitaxy for barrier layers in type-II W-structure infrared lasers. Using cross-sectional scanning tunneling microscopy to characterize the atomic-scale structure of the material, we find substantial differences in the superlattice morphology for growth temperatures between 435 and 540 °C. At lower growth temperatures, the AlAs forms three-dimensional clusters, with continuous structures threading through multiple periods of the superlattice. With increasing temperature, the morphology of the digitally doped AlAs layers consistently improves, with nearly perfect delta doping observed at the highest temperatures studied. The changes in the superlattice structure can be attributed primarily to the known temperature dependence of the AlSb growth front morphology, with secondary effects associated with anion-exchange at the interfaces and the different surface reconstructions on the two growth surfaces.     

Some aspects on thermodynamic properties, phase diagram and alloy formation in the ternary system BAs–GaAs—Part II: BGaAs alloy formation

We present a detailed analysis of the thermodynamics of the BGaAs alloy formation taking into account the question of boron solubility and Gibbs free energy change. We also predict a Gibbs free energy change for the gas phase formation of BAs to be compared to for GaAs. We show that the experimental boron solubility is actually an open question both theoretically and experimentally, while the maximum concentration obtained up to date is reaching 6–7%. It thus follows that despite the low boron solubility, the Gibbs free energy change for the alloy formation is dominated by the chemical term over the mixing energy change. We could deduce order of magnitude for the ratio of the boron partial pressure and that of gallium. P_B could be estimated as being always much lower than P_Ga; it is shown that the alloy content is probably controlled by the Ga equilibrium partial pressure.     

Thermal annealing of a-Si/Au superlattice thin films

The superlattice films, which consist of amorphous silicon (a-Si) and amorphous gold (Au), were prepared by ultra-high vacuum evaporation system. The first layer was grown a-Si with a thickness of 4.2 nm and the second layer was grown Au with a thickness of 0.8 nm. Thermal annealing was performed at 473, 673, and 873 K, respectively. The structural properties of the films were investigated using transmission electron microscope (TEM), X-ray diffraction (XRD), and Raman scattering spectroscopy. The electrical property was assessed by the temperature dependence of electrical conductivity. A crystallization of Si and a forming of Au nanoparticles were observed in all of the annealing films. The crystalline volume fraction reached 70% by annealing time for 15 min. An average diameter of the Au nanoparticles embedded in Si matrix also increased with increasing the annealing temperature. At annealing temperature above 873 K, Au atoms migrated toward the film surface. It was observed that the electrical conductivity changed in several temperatures.     

Interdiffusion phenomena in InGaAs/GaAs superlattice structures

We have studied structural properties of InGaAs/GaAs superlattice sample prepared by Molecular Beam Epitaxy (MBE) using high resolution X‐ray diffractometer (HRXRD). Increasing strain relaxation and defect generations are observed with the increasing Rapid Thermal Annealing (RTA) temperature up to 775 °C. The higher temperatures bring out relaxation mechanisms; interdiffusion and favored migration. The defect structure and the defects which are observed with the increasing annealing temperature were analyzed. Firstly, the in‐plane and out‐of‐plane strains after the annealing of sample were found. Secondly, the structural defect properties such as the parallel X‐ray strain, perpendicular X‐ray strain, misfit, degree of relaxation, x composition, tilt angles and dislocation that are obtained from X‐ray diffraction (XRD) analysis were carried out at every temperature. As a result, we observed that the asymmetric peaks especially in asymmetric (224) plane was affected more than symmetric and asymmetric planes with lower polar or inclination angles due to c‐direction at low temperature. These structural properties exhibit different unfavorable behaviors for every reflection direction at the increasing temperatures. The reason is the relaxation which is caused by spatially inhomogeneous strain distribution with the increasing annealing temperature. In the InGaAs superlattice samples, this process enhances preferential migration of In atoms along the growth direction. Further increase in the annealing temperature leads to the deterioration of the abrupt interfaces in the superlattice and degradation in its structural properties. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Configuration of grain boundaries

In the present paper a model of the configuration of grain boundaries from a general point of view in the framework of the CSL concept has been developed. Rotations and rotations with translations were taken under consideration. The main result is the determination of the coincidence conditions for symmetrical and asymmetrical grain boundary facets between two and three grains. The facet angles of grain boundaries in any crystal surface are determined.     

The configurational entropy of glass

Configurational entropy is of fundamental importance to understanding the universal nature of the glassy state. According to the conventional view, the configurational entropy of a liquid is ‘frozen’ below the glass transition temperature (T_g) all the way to absolute zero. Contrary to this, we claim that: (i) there is an entropy loss associated with the liquid to glass transition, and (ii) the configurational entropy in the glassy state vanishes at absolute zero. This paper contrasts our arguments (called the ‘entropy loss’ view) and the conventional view.     

Vibrational entropy of substitutional solid solutions

Positive deviations of the entropy of mixing from that of the ideal case have been established basing upon the experimental study of ionic substitutional solid solutions, carried out with the help of various techniques. These deviations are attached to the vibrational entropy, corresponding to the negative deviations of Debye temperatures of solid solution from additivity. The existence of linear correlation between the enthalpy of mixing and vibrational contribution to entropy is confirmed. Theoretical basis of this correlation, established empirically for the alkali halogenides, is given. The question is discussed, dealing with the universality of the relations of that kind and independence of vibrational entropy on temperature.     

On the reality of the residual entropy of glasses and disordered crystals: The entropy of mixing

We have previously shown that the assumption that the configurational entropy of a supercooled liquid vanishes at T_g leads to a non-trivial violation of the second law. Here we consider the example of the entropy of mixing. We use as a model system two similar chemical substances which form an ideal solution in a mixed phase. We apply the reasoning of our earlier paper to show that this vanishing would lead to a dilemma; either it violates the second law of thermodynamics, or else it cannot be demonstrated by any conceivable experiment. We show further that the vanishing of the entropy of mixing on kinetic arrest leads to the counter-intuitive result that the chemical potential of each component in an infinitely dilute kinetically arrested (or glassy) solution can equal or the chemical potential of the pure component. The most parsimonious conclusion from these results is that residual entropies are real.     

Dielectric properties of ternary melt processed blends

Thermal, morphological and dielectric properties of conductive thermoplastic blends based on polystyrene, styrene–butadiene–styrene block copolymer and doped polyaniline have been investigated with special emphasis on the effect of doped polyaniline concentration. Blends were prepared by melt blending using an internal mixer. Morphological studies by scanning electron microscopy of cryofractured surfaces indicated that thermoplastic matrix had a better miscibility with the presence of copolymer as compatibilizer and also good dispersability and structuring of doped polyaniline particles within polystyrene. A reduction in impedance from 26 to 2 MΩ at 10 Hz, with the increasing polyaniline content is also observed. The blends can be used for antistatic applications. From the fit to the impedance spectroscopy data we obtained the relaxation strength, the depression angle and the relaxation.     

Communal entropy and glass transition

Using the partition function formulated by Kirkwood, the glass transition temperature is shown to be the temperature at which the communal entropy vanishes in a liquid system. The non-existence of T₂ is inferred from the theory in accordance with the experimental fact that all the glasses made so far have some residual entropy compared with the crystal state.     

Superconducting state parameters of ternary amorphous superconductors

The theoretical investigations of the superconducting state parameters (SSP) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ^?, transition temperature T_C, isotope effect exponent α and effective interaction strength N_OV of (Ni₃₃Zr₆₇)_1 − xM_x (x = 0, 0.05, 0.1, and 0.15, M = Cu) ternary amorphous superconductors have been reported using Ashcroft's empty core (EMC) model potential. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used in the present investigation to study the screening influence on the aforesaid properties. The T_C obtained from Sarkar et al. (S) local field correction function is found in excellent agreement with available theoretical data. Quadratic T_C equation has been proposed, which provides successfully the T_C values of ternary amorphous alloys under consideration. Also, the present results are found in qualitative agreement with other such earlier reported data, which confirm the superconducting phase in the superconductors.     

Characterization of InAs quantum dots on lattice-matched InAlGaAs/InP superlattice structures

Self-assembled InAs quantum dots (QDs) in an InAlGaAs matrix, lattice-matched to InP substrate, have been grown by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL) are used to study their structural and optical properties. In InAs/InAlGaAs/InP system, we propose that when the thickness of InAs layer deposited is small, the random strain distribution of the matrix layer results in the formation of tadpole-shaped QDs with tails towards random directions, while the QDs begin to turn into dome-shaped and then coalesce to form islands with larger size and lower density to release the increasing misfit strain with the continuous deposition of InAs. XRD rocking curves showing the reduced strain with increasing thickness of InAs layer may also support our notion. The results of PL measurements are in well agreement with that of TEM images.     

Continuous periodic to quasiperiodic structural transformations in crystalline and superlattice structures

Attention is focussed on the fact that the β—Mn, the σ—phase and the Al₅Ti₃ and the Al₁₁Ti₇ superlattice derivatives of the δ—AlTi phase all belong to a category of structures which comprises an identical periodic arrangement of square and rhombic tiles — the shapes of the rhombic tiles being different in each of these cases. While the first and the second structures constitute periodic approximants of the octagonal and the dodecagonal quasicrystals respectively, the last two (superlattice) structures are the approximants of a hypothetical quasiperiodic superlattice structure. The strip projection method has been used here to model the intermediate structures which arise during the continuous periodic to quasiperiodic transitions between the relevant structures and was found to be quite successful in explaining many of the related experimental findings.     

Management of highly-strained heterointerface in InGaAs/GaAsP strain-balanced superlattice for photovoltaic application

We report an implementation of a strain-balanced superlattice structure in a photovoltaic device. A thin-period structure is an appropriate design for a strain-balanced epitaxy allowing minimum stress accumulation in the highly strained quantum well (QW). However, a large number of interfaces cause a serious issue as critical as strain balancing. Such a structure suffers from degraded crystal quality with the increase of number of interfaces; and the atomic contents within the well or barrier also become difficult to control. With help from in situ reflectivity measurement and x-ray diffraction, it was deduced that such an interface issue could result from unpredictable interfacial strain relaxation and irregular interface morphology between strained well and barrier. According to this mechanism, we developed a smart interface management, insertion of a monolayer-thin strain-neutral GaAs, to avoid diffused interfaces and morphology degradation. The modified superlattice solar cell exhibits good performance.     

Methods for determination of effective diffusion coefficients in ternary alloys (II). Indirect methods of calculating ternary diffusion matrix

We summarised, in part one of this paper, procedures of measuring diffusion matrix in three-component single phase alloys direct from the concentration-distribution profiles in infinite diffusion couples. In this paper (Part II) some procedures have been outlined which enable calculate ternary diffusion matrix from easily measurable tracer coefficients in ideal and non-ideal single phase solid solutions. These procedures have been illustrated with examples, so that it becomes apparant that in many cases the experimental difficulties faced by the experimentalists in direct measurement of ternary diffusion coefficients can be reduced to a large extent.     

Apparent entropy,residual entropy,causality, metastability,constraints, and the glass transition

A thermodynamic framework for the investigation of ‘residual entropy’ and related phenomena is developed, expanding and clarifying the work of Kivelson and Reiss and the important work of several other investigators. The main difficulties encountered in the development of such a framework are the need to deal with constrained equilibrium as well as to include irreversible processes in the overall study. These challenges are met by using the device of auxiliary constraints and the equivalent equilibrated states that they produce. The importance of a thermodynamic framework is tied to the fact that evolving molecular theories of residual entropy, which also impact the glass transition, no matter how sophisticated, invariably contain approximations whose effects are hard to assess. Thus a thermodynamic framework provides a vehicle within which the internal consistency of a theory can be tested. Phenomena that are treated in such a framework, along with others, are those mentioned in the title of this paper. The concept of residual entropy, its reality or unreality, is often considered to be an unimportant issue. The findings of this paper, besides emphasizing the unreality of residual entropy, show that the question of its existence is a significant one. Among other things, the fundamental principle of causality is involved and we should not be so cavalier as to dispense with it. Among other rigorous analysis we present an argument involving an ideal binary solution whose behavior as the limiting behavior of a real solution provides access, in principle, to experiment. The argument does make the generally accepted assumption that, at equilibrium at 0 K, pure crystals of the binary system’s components have zero entropy. It strongly suggests that residual entropy, in general, is an impression that stems from the inclusion of an irreversible step in an experimental thermodynamic cycle.     

Zero-point entropy of glasses as physical reality

This paper presents an analysis of the physical nature of the author’s results obtained since 1995 in the field of thermodynamics of the vitreous state (classic approximation). Excess entropy and free energy at 0 K are considered as the measures of non-equilibrium extent of glass with respect to crystal. Experimental data for melts and glasses of various chemical natures are generalized. A general form of the relations between reversible changes in free energy, enthalpy and entropy and their frozen values at the formation of any glass is shown. The problem of functional dissimilarity of the heat capacity of glass and that of crystal has been solved within the framework of the principle of free energy minimization as a result of self-organization of the vibration spectrum according to the frozen structure. The complete stabilization of glass structure at Kauzmann’s temperature or below it is forbidden by topologic reasons and (or) by thermodynamics. The effects of pressure on the thermodynamic properties of glasses are shown. Possible particular applications of the analysis of self-organization in chaotic systems are exemplified by the modeling of the abilities of neural systems.