Nanoparticle semiconductor compositions in the miscibility gap

Abstract

When semiconductor alloys are used to engineer devices one may not be able to achieve compositions that could provide desirable properties such as a specific band gap because there exists an immiscibility of the alloys, particularly when compound semiconductors are involved. Bulk phase diagrams are used to predict the solubility limits as a function of temperature and modification of these limits can be achieved through strain of the alloy. Metastable alloys that are single phase can be prepared with compositions in the miscibility gap but these kinetically trapped alloys return to their equilibrium two‐phase state at elevated temperatures. An alternative approach to achieve compositions in the bulk miscibility gap is to prepare samples as nanoparticles with high surface curvature. These individual nanoparticles exhibit phase diagrams considerably different from those of the bulk and do not transform into a two‐phase state as long as the nanoparticles remain isolated. The purpose of this article is to show how phase diagrams in general can be expected to change with decreasing size and how the GaAs‐GaSb alloy in particular experiences changes from bulk behaviour when it is in the form of isolated nanoparticles with high surface curvature. In this paper is presented experimental characterization of GaAs‐GaSb alloy nanoparticles that are prepared by pulsed laser deposition and equilibrated in vacuum and analyzed in an analytical transmission electron microscope to show that compositions can be achieved in the bulk miscibility gap provided the nanoparticles are smaller than about 10 nm across. Further, the lattice parameter of nanoparticle alloys as a function of composition were found to deviate from Vegard's law by several percent in a positive sense, contrary to that for bulk alloys which show a deviation of a negative sign. Finally, the diffusivity of As in GaAsSb was estimated to be 1018 cm2/s, which is several orders of magnitude larger that known for bulk alloys. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characterization of GaAs‐GaSb III‐V pseudo‐binary nanoparticles

“…From an analysis of the diffraction pattern, the crystal structure is diamond cubic, which is expected as it is the same as both of the constituent components. In this particular example, the average composition maintained a 50 at% composition, but this was found to vary between 45% and 55% across the entire set of samples analyzed [5,8].…”

“…The present experiment is designed for the independent simultaneous ablation from targets of GaAs and GaSb so that the combination of these ablations plumes can be chosen to yield any composition x, within 0% ≤ x ≤ 50%, resulting in a cross-over region between the plumes of composition Ga 50 As x Sb 50-x . The present paper will describe the experimental details and provides some results of this work, and serves to compliment a second paper by the same authors within these proceedings [5].…”

Morphology Dependence of Ag-Ni Solid Solubility

“…Compositional identification from the STEM-EDS analysis revealed the presence of Ag and Ni atoms in both type A and solution arrangement, the crystallographic attributes (lattice parameter, interplanar spacing, interplanar spacing value ratios) of the solid solution phase can be different (a possible positive deviation from the Vegard's law 12 ) from the one expected if the system were ideal solutions and bulk type. 13,14 Although a detailed structural analysis has not been conducted at this stage, the presence of both Ag and Ni atoms and an absence of diffraction signatures corresponding to the pure Ag and Ni phases or their oxides strongly indicate the arrangement of the Ag and Ni atoms in a solid solution atomic configuration inside the representative type A feature.…”

“…It has been reported by researchers that an increase in the curvature or a decrease in the size promotes an increase in the extent of miscibility. 1,9,13,15 This effect is commonly known as the capillary effect or the Gibbs-Thompson effect. 16 An increase in curvature induced enhancement of miscibility manifests as an increase in the solute atom concentration in the solid solution structure made up of bulk immiscible component solute and solvent atoms.…”

Compositionally Graded Microstructure for Ag-Fe Nanoparticles

“…The growing interest is primarily due to two important incentives; (a) tunable functional properties and (b) realization of microstructures that cannot be obtained in the bulk systems with similar compositions and under similar conditions. With respect to the novel microstructures, one significant observation has been the formation of solid solution between atoms with a large difference in sizes (>14%) and a high positive enthalpy of mixing [1,2]. Such component atoms do not form a single phase solid solution in bulk for certain or overall composition ranges.…”