Phase Miscibility Gap Research Articles

The coherent {1 0 0} and {1 1 0} interfaces between rocksalt-PbTe and zincblende-CdTe

The formation of PbTe quantum dots (QDs) in a crystalline CdTe host matrix is observed after annealing of a coherent, heteroepitaxial PbTe layer clad between CdTe layers. The two tellurides possess almost identical lattice constants, but differ fundamentally in their lattice structure. The large solid phase miscibility gap leads to phase separation near thermodynamic equilibrium resulting in coherently embedded, centrosymmetric PbTe QDs with almost defect-free {1 0 0}, {1 1 0} and {1 1 1} interfaces. We compare high resolution transmission electron microscopy (HRTEM) with first-principles total-energy calculations in the repeated-slab approximation. For the two polar (0 0 1) interfaces, significantly different lattice plane spacings are observed, depending on whether the polar CdTe (0 0 1) face is cation-(Cd)- or anion-(Te)-terminated. The most drastic effect occurs at the electrostatically neutral (1 1 0) interface, where we find a lateral spatial offset between the two crystal halves. The agreement between the first-principles calculations and the HRTEM data is excellent for both interface classes.

Phase separation and glass-forming abilities of ternary alloys

The liquid phase miscibility gap was calculated in ternary glass-forming systems using the sub-regular solution model. We found that liquid miscibility gaps exist at low temperature in most of the reported ternary bulk-forming metallic glass systems even if enthalpies of mixing of constituent elements are all negative. The glass-forming ranges do not overlap with the miscibility gaps except in the Cu–Ti–Zr system, suggesting that glass-forming ability is low in the phase separating compositions.

Influence of the addition of PVP on the morphology of asymmetric polyimide phase inversion membranes: effect of PVP molecular weight

A macrovoid-free, porous membrane with a sponge-like structure was obtained by phase inversion of a polyimide (PI)/γ-butyrolactone (GBL)/water system, whereas a membrane with a finger-like structure was obtained from a PI/ N-methyl-2-pyrrolidinone (NMP)/water system. In this study, the effect of the molecular weight of poly(vinyl pyrrolidone) (PVP) additive ( M w: 360k and 40k) in two systems on membrane morphology has been investigated by the differences in phase separation rates and miscibility gaps. On addition of PVP into the PI casting solutions, the two systems exhibited significantly different membrane morphologies; the former augmented macrovoid formation while the latter suppressed formation of finger-like structures. The membrane morphology and the phase separation rate significantly depend on the molecular weight of PVP in PI/NMP/water system whereas those do insignificantly in PI/GBL/water system. The cloud points representing the miscibility gap do not depend on the molecular weight of PVP in both systems very much. Therefore, it is suggested that the morphology change in the PI/NMP/water system is mostly due to the change in the phase separation rate caused by the change in the solution viscosity upon addition of PVP. However, in the PI/GBL/water system, the molecular weight effect of PVP is not as significant as in the PI/NMP/water system, and the morphology change seems to be primarily due to the change in the miscibility gap.

Application of Metastable Phase Diagrams to Silicate Thin Films for Alternative Gate Dielectrics

Using the concept of metastable phase diagrams, we discuss the microstructure evolution during annealing of amorphous ZrO2–SiO2 and HfO2–SiO2 thin films for gate dielectric applications. These systems are characterized by a low solid solubility, a liquid miscibility gap and a kinetic barrier to the formation of the complex, crystalline silicate. We show that phase partitioning is expected for most compositions. Compositions within the metastable extensions of the spinodal are unstable and will spontaneously unmix in the amorphous phase upon heating. Hafnia- or zirconia-rich phases will subsequently crystallize to form HfO2 or ZrO2. Most compositions outside the metastable extensions of the liquid phase miscibility gap must phase separate above the crystallization temperature by nucleation of crystalline HfO2 or ZrO2 out of an amorphous silica-rich matrix. We present calculations of the metastable extensions of the miscibility gap and spinodal. The calculations predict that SiO2-rich compositions, investigated for gate dielectric applications, will show spinodal decomposition if they contain less than ∼90 mol% SiO2 at the typical device processing temperature of 1000°C. Experimental studies of Hf-silicate films with three different compositions, between ∼40 and 80 mol% HfO2 that lie inside and outside the miscibility gap, respectively, are presented. All three compositions show phase separation. Despite the different pathways of microstructure evolution, the final phase separated microstructures are similar. Experimental verification of the pathways that lead to these microstructures requires further studies.

Evaluation of thermodynamic properties from alloy phase diagram with miscibility gap using non-random two-liquid equation

The non-random two-liquid equation has been applied to evaluate the thermodynamic properties of the liquid solution at elevated temperatures in a binary alloy system with a liquid phase miscibility gap. Only upon making use of the phase equilibrium data at the critical and monotectic points of the miscibility gap from a T-X phase diagram and thermochemical data, the parameters needed for the evaluation, i. e., ( g 12 - g 22), g 21 - ( g 11) and a of the non-random two-liquid solution approach, can be determined. The evaluation of thermodynamic properties was carried out numerically for three binary alloy systems, i. e., Al-Pb, Zn-Pb and Ga-Hg systems. The application of the non-random two-liquid equation to these three binary alloy systems shows that the evaluated results are close to the available experimental measurements.

Phase separation and gap bowing in zinc-blende InGaN, InAlN, BGaN, and BAlN alloy layers

We present first-principles calculations of the thermodynamic and electronic properties of the zinc-blende ternary In x Ga 1− x N, In x Al 1− x N, B x Ga 1− x N, and B x Al 1− x N alloys. They are based on a generalized quasi-chemical approximation and a pseudopotential-plane-wave method. T– x phase diagrams for the alloys are obtained. We show that due to the large difference in interatomic distances between the binary compounds a significant phase miscibility gap for the alloys is found. In particular for the In x Ga 1− x N alloy, we show also experimental results obtained from X-ray and resonant Raman scattering measurements, which indicate the presence of an In-rich phase with x≈0.8. For the boron-containing alloy layers we found a very high value for the critical temperature for miscibility, ∼9000 K , providing an explanation for the difficulties encountered to grow these materials with higher boron content. The influence of a biaxial strain on phase diagrams, energy gaps and gap bowing of these alloys is also discussed.

Solid phase immiscibility in GaInN

The large difference in interatomic spacing between GaN and InN is found to give rise to a solid phase miscibility gap. The temperature dependence of the binodal and spinodal lines in the Ga1−xInxN system was calculated using a modified valence-force-field model where the lattice is allowed to relax beyond the first nearest neighbor. The strain energy is found to decrease until approximately the sixth nearest neighbor, but this approximation is suitable only in the dilute limit. Assuming a symmetric, regular-solutionlike composition dependence of the enthalpy of mixing yields an interaction parameter of 5.98 kcal/mole. At a typical growth temperature of 800 °C, the solubility of In in GaN is calculated to be less than 6%. The miscibility gap is expected to represent a significant problem for the epitaxial growth of these alloys.

Liquid phase epitaxy and characterization of InAs 1- x - ySb x P y on (100) InAs

The solid phase miscibility gap of the In-As-Sb-P pseudo-ternary system has been studied in the 560–600°C temperature range. InAs 1- x - y Sb xP y epilayers were grown by liquid phase epitaxy (LPE) on (100) oriented InAs substrates. We show th at the LPE growth inside the metastable region of the phase diagram is possible. The quality of the epilayers was investigated by low temperature photoluminescence experiments. The band-gap and spin-orbit splitting energies of the layers were measured by the electroreflectance technique. We have established the composition dependence of these parameters. The InAs-lattice-matched, most phosphorus-rich, stable and metastable alloys are InAs 0.45Sb 0.17P 0.38 and InAs 0.35Sb 0.20P 0.45, respectively. Thus, the ro temperature cut-off wavelength, for our LPE grown samples, is shown to be limited at λ c ≥2.6 or 2.45 μm, depending on the chosen limit.

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

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.

High temperature liquid phase epitaxy of (100) oriented GaInAsSb near the miscibility gap boundary

The Ga 1- x In x As y Sb 1- y quaternary alloy has been grown by liquid phase epitaxy at high temperature ( T = 600–615° C) on (100) oriented GaSb substrate. The highest indium concentration in the solid phase that could be achieved for perfect GaSb-lattice-matched layers was x = 0.23, which corresponds to a band-gap cut off wavelength λ = 2.39 μm at room temperature. Thermodynamical calculations show this composition ( x = 0.23 y = 0.20) is situated at the boundary of the solid phase miscibility gap (binodal curve). Mismatched quaternary layers with more indium content (up to x = 0.52) could be grown outside the miscibility gap, but their morphology was bad.

The Hydride Phase Miscibility Gap in Palladium-Rare Earth Alloys

Recent years have witnessed a very substantial increase in the body of data concerning the effects of various alloying elements on the phase relationships of the palladium-hydrogen system.

Phase separation reactions in Ti–50V alloys

Phase transformations in Ti–50V alloys containing oxygen in the range <0·095–0·36 wt-% have been studied at 750 and 400°C for times up to 1000 h. Results of electron microscopy, X-ray diffraction, and hardness testing indicate that oxygen widens the α+β phase field at 750°C and the metastable β phase miscibility gap at 400°C. The evidence is consistent with a phase diagram for high purity Ti–V alloys containing a stable α+β phase field with a β transus decreasing with increasing vanadium content. No evidence was obtained consistent with a β monotectoid form of diagram which has recently been proposed.MST/1039

Effects of Crucible Wetting During Solidification of Immiscible Pb-Zn Alloys

Many industrial uses for liquid phase miscibility gap alloys are proposed. However, the commercial production of these alloys into useful ingots with a reasonable amount of homogeneity is arduous because of their immiscibility in the liquid state. In the low-g environment of space gravitational settling forces are abated, thus solidification of an immiscible alloy with a uniform distribution of phases becomes feasible. Elimination of gravitational settling and coalescence processes in low-g also makes possible the study of other separation and coarsening mechanisms. Even with gravitational separation forces reduced, many low-g experiments have resulted in severely segregated structures. The segregation in many cases was due to preferential wetting of the crucible by one of the immiscible liquids. The objective was to analyze the wetting behavior of Pb-Zn alloys on various crucible materials in an effort to identify a crucible in which the fluid flow induced by preferential wetting is minimized. It is proposed that by choosing the crucible for a particular alloy so that the difference in surface energy between the solid and two liquid phases is minimized, the effects of preferential wetting can be diminished and possibly avoided. Qualitative experiments were conducted and have shown the competitive wetting behavior of the immiscible Pb-Zn system and 13 different crucible materials.

Low temperature phase diagram of the Ga1−xInxAsySb1−y system

The phase diagram of the Ga 1− x In x As y Sb 1− y quaternary alloy was calculated in the range 400–800°C by a progressive thermodynamic analysis combining binary data and generating ternary and quaternary systems; the mixing functions of the liquid phase are expressed in Redlich-Kister polynomial form, whereas the solid phase is described by a short-range order and substrate-induced strain in the delta lattice parameter (DLP) model. Our calculation give a solid phase miscibility gap smaller than predicted by previous calculations. A comparison with published data concerning the liquid phase epitaxy of GaInAsSb shows that, in fact, no growth was performed inside the miscibility gap of this alloy by LPE.

Thermodynamic aspects of OMVPE

Thermodynamic factors are important for determining several aspects of the overall organometallic vapor phase epitaxial (OMVPE) growth process including the maximum growth rate, the occurence of parasitic homogeneous gas phase reactions, the phase diagram, i.e. the conditions necessary for the growth of a single phase III/V semiconductor without the presence of other liquid or solid phases, and the solid III/V alloy composition. Each of these is discussed in turn. A simple model is developed which assumes thermodynamic equilibrium to be established at the solid/vapor interface. This model can be used to understand the growth rate limiting process, the conditions necessary to grow a single phase III/V material, and the factors determining solid composition. Systems with mixing on the group V sublattice such as InAsSb and GaAsSb have been studied in detail. The thermodynamic calculations predict accurately the effects of the ratio of As/Sb and V/III ratio in the vapor phase on solid composition. An additional aspect of the thermodynamic analysis is that it predicts the occurrence of solid phase miscibility gaps in many III/V ternary and quaternary systems. OMVPE is found capable of growing alloys within the range of solid immiscibility. The growth and properties of the model alloy GaAsSb in the range of solid immiscibility will be discussed briefly.

GaAs1−xSbx growth by OMVPE

The system GaAs1−xSbx has a solid phase miscibility gap ranging from x = 0.2 to x = 0.8 at the typical growth temperature of 600 C; however, metastable alloys covering this entire composition range have been grown by organometallic vapor phase epitaxy (OMVPE) using trimethylgallium, antimony and -arsenic as the source materials. The solid composition is studied for various values of growth temperature, the ratio of Sb to total group V elements in the vapor phase and the ratio of group III to group V elements in the vapor phase. The fraction of trimethylarsenic (TMAs) pyrolyzed in the vapor phase is found to be < 1 and to vary with temperature. Taking into account the incomplete pyrolysis of TMAs, solid composition is found to be controlled by thermodynamics. The effects of temperature and vapor composition are all accurately predicted using a simple thermodynamic model assuming equilibrium to be established at the solid-vapor interface. The properties of these metastable GaAs1−x Sbx alloys were explored using interference contrast microscopy, room temperature and 4 K photoluminescence, Hall effect and conductivity measurements. The alloy GaAs0.5Sb0.5 grown lattice matched to the InP substrate has excellent surface morphology, is p-type, and the photoluminescence spectrum consists of a single, intense, fairly broad (23.5 meV half width) peak at a wavelength of 1.54 Μm.

Organometallic vapor phase epitaxial growth of GaAs0.5Sb0.5

The pseudobinary III/V system GaAs1−ySby is well known to have a solid phase miscibility gap with a critical temperature of 751 °C. We have succeeded in growing epitaxial layers of GaAs0.5Sb0.5 lattice matched to InP at temperatures of 600 and 630 °C using the organometallic vapor phase epitaxy technique. The key requirement is a III/V ratio of greater than unity. This leads to the incorporation of all As and Sb reaching the interface and the ability to grow metastable alloys. The epitaxial GaAs0.5Sb0.5 layers have excellent surface morphology and efficient photoluminescence at a wavelength of 1.6 μm.

Immiscibility and spinodal decomposition in III/V alloys

Many advanced semiconductor devices depend on the use of quaternary III/V alloys for the fabrication of lattice-matched heterostructures. Most III/V quaternary alloys have solid phase miscibility gaps which may limit the composition range over which device quality epitaxial layers can be fabricated. In this paper the miscibility gaps and associated spinodal decomposition are discussed. The major topics include thermodynamics, the role of growth technique in determining the range of solid compositions which can be produced, the stability of quaternary alloys and the consequences of the miscibility gap and spinodal decomposition on electrical and optical properties of the alloys and the potential of these alloys for devices.

OMVPE growth of GaAs 1- xSb x: solid composition

III/V ratio is found to have a major effect on the Sb distribution coefficient for OMVPE growth of GaAs 1- x Sb x . Experimental results are described well by thermodynamic calculations. For III/V > 1, essentially all As and Sb in the vapor are incorporated into the solid allowing the growth of GaAs 0.5Sb 0.5 right in the center of the solid phase miscibility gap at 600°C.

AlxGa1− xAsySb1− y phase diagram

The phase diagram of the important quaternary III/V system Al x Ga 1- x As y Sb 1- y has been experimentally explored by analyzing the solid compositions grown by liquid phase epitaxy. Data are presented for the Sb and Al distribution coefficients as a function of melt composition at 700°C. The extent of the solid phase miscibility gap is also reported for the GaAs y Sb 1- y system at 650 and 700°C and for the Al x Ga 1- x As y Sb 1- y system at 700°C. The binodal data for both the GaAs y Sb 1- y and Al x Ga 1- x As y Sb 1- y systems are found to be in agreement with the calculated binodal curves. The calculation uses the DLP model with the value of K taken to be 1.37 × 10 7 cal/mole Å 2.5.