Stability of coherently strained semiconductor superlattices

Quantification of Material Gradients in Core/Shell Nanocrystals Using EXAFS Spectroscopy

Core/shell nanocrystals with a graded interface between core and shell exhibit improved optoelectronic properties compared with particles with an abrupt, sharp interface. Material gradients mitigate interfacial defects and define the shape of the confinement potential. So far, few works exist that allow to quantify the width of the gradient. In this study, ZnSe/CdS nanocrystals with graded shells made at different temperatures are characterized using extended X-ray absorption fine structure (EXAFS) and Raman spectroscopies. The average coordination number of the probed element with respect to the two possible counterions is fit to a simple, geometric model. It is shown that at the lower temperature limit for shell growth (260 °C), substantial interfacial alloying can be attributed mainly to cation migration. At higher temperature (290 °C), strain minimization leads to atomic ordering of the metal ions and an anomalously low degree of phase mixing.

Influence of defect distribution on the thermoelectric properties of FeNbSb based materials

Cooperative effects of a solid solution and phase separation could strongly scatter phonons and improve the performance of thermoelectric materials.

Stability, Electronic Structure, and Phase Diagrams of Novel Inter- Semiconductor Compounds

High-technology electronic devices are based on highly specialized core materials that make their operation pos sible : semiconductors. Unfortunately, the range of mate rial properties that make high-technology devices work is extremely narrow, since the sheer number of useful semiconductors is small. Hence, a major challenge has been to predict and develop new, potentially useful semiconductors. We describe here how the use of state- of-the-art techniques in both quantum and statistical mechanics can lead to predictions of new, stable, and ordered semiconductor alloys. A number of laboratories have already grown experimentally these new materials; efforts to characterize their useful material properties are ongoing in the United States, Japan, and Europe. This work describes the theoretical methodologies of our ap proach, and shows how supercomputers make possible the quantum-mechanical architectural analysis of new materials.

Theoretical prediction and experimental confirmation of unusual ternary ordered semiconductor compounds in Sr-Pb-S system

We examine the thermodynamics of phase separation and ordering in the ternary Ca(x)Pb(1-x)S and Sr(x)Pb(1-x)S systems by density-functional theory combined with a cluster expansion and Monte Carlo simulations. Similar to most other ternary III-V or IV-VI semiconductor alloys, we find that bulk phase separation is thermodynamically preferred for PbS-CaS. However, we predict the surprising existence of stable, ordered ternary compounds in the PbS-SrS system. These phases are previously unreported ordered rocksalt-based compounds: SrPb3S4, SrPbS2, and Sr3PbS4. The stability of these predicted ordered phases is confirmed by transmission electron microscopy observations and band gap measurements. We believe this work paves the way for a combined theory-experiment approach to decipher complex phase relations in multicomponent chalcogenide systems.

Thermodynamic theory of epitaxial alloys: first-principles mixed-basis cluster expansion of (In, Ga)N alloy film

Epitaxial growth of semiconductor alloys onto a fixed substrate has become the method of choice to make high quality crystals. In the coherent epitaxial growth, the lattice mismatch between the alloy film and the substrate induces a particular form of strain, adding a strain energy term into the free energy of the alloy system. Such epitaxial strain energy can alter the thermodynamics of the alloy, leading to a different phase diagram and different atomic microstructures. In this paper, we present a general-purpose mixed-basis cluster expansion method to describe the thermodynamics of an epitaxial alloy, where the formation energy of a structure is expressed in terms of pair and many-body interactions. With a finite number of first-principles calculation inputs, our method can predict the energies of various atomic structures with an accuracy comparable to that of first-principles calculations themselves. Epitaxial (In, Ga)N zinc-blende alloy grown on GaN(001) substrate is taken as an example to demonstrate the details of the method. Two (210) superlattice structures, (InN)(2)/(GaN)(2) (at x = 0.50) and (InN)(4)/(GaN)(1) (at x = 0.80), are identified as the ground state structures, in contrast to the phase-separation behavior of the bulk alloy.

Strain-minimizing tetrahedral networks of semiconductor alloys

The atomic size mismatch between different binary semiconductors has been long known to limit their mutual solubility, leading instead to phase separation into incoherent phases, forming inhomogeneous mixtures that severely limit technological applications that rely on carrier transport. We show here that this atomic size mismatch can lead, under coherent conditions, to the formation of a homogeneous alloy with characteristic (201) two-monolayer ordering. This occurs because such specific layer arrangement corresponds to a unique strain-minimizing network in tetrahedral systems.