Session L9: Semicondutor Atomic Structure, Lattice Properties, and Phase Transitions

Structure and dynamics of CdTe studied by X-ray and neutron scattering

We present x-ray diffraction and inelastic neutron scattering studies of the structure and lattice dynamics of commercially available cadmium telluride. We also present complementary density functional theory calculations. The x-ray data show a subtle structural transition is present near 80 K, which manifests also in the measured phonon density of states. Refinement of the structure above and below the transition temperature shows no change to the long-range ordered structure. The inelastic neutron scattering studies were performed using an isotopically un-enriched sample of CdTe, which possesses a high cross section for thermal neutron absorption. The neutron portion of the study was performed with a thin-plate geometry in the reflection condition at the ARCS instrument at the SNS, showing the high flux of the instrument makes possible lattice dynamics studies of materials with high thermal neutron absorption. Single crystal and powder inelastic neutron scattering measurements will be presented. Current interpretation of the nature of the transition and future studies will be discussed.   

First-principles calculations of phonons and Raman spectra in monoclinic CsSnCl$_3$

Halide perovskites have recently attracted attention for photovoltaic applications. While CsSnCl$_3$ in the perovskite structure is less suitable for solar cells because of its higher band gap than the iodides, it is still of interest as the end member of mixed CsSn(I$_{1-x}$Cl$_x$)$_3$ and addition of Cl has been found to increase solar cell efficiencies. The other reason this material is interesting is that at 390 K it undergoes a phase transition to a monoclinic structure with even larger band gap, which differs from the yellow phase occuring for CsSnI$_3$. Understanding the various possible phase transitions and structures in the trihalides is important for the long-term stability of these materials in solar cells. Raman data exist on monoclinic CsSnCl$_3$ material since the late 80s but have in the past not been compared with first-principles calculations of the phonons in this material. We present calculations of the phonons at the $\Gamma$-point using density functional perturbation theory using the abinit program. A symmetry analysis is presented and the calculated phonon modes are compared with experimental data and previous attempts to classify the modes as internal to the SnCl$_3$ tetrahedra and lattice modes. Supported by DOE.   

Application of Quantum Monte Carlo Methods to Describe the Properties of Manganese Oxide Polymorphs

First-principles descriptions of the properties of correlated materials such as transition metal oxides has been a long-standing challenge. Manganese oxide is one such example: according to both conventional and hybrid functional density functional theory, the zinc blende polymorph is predicted to be lower in energy than the rock salt polymorph that occurs in nature. While the correct energy ordering can be obtained in density functional approaches by careful selection of modeling parameters, we present here an alternative approach based on quantum Monte Carlo methods, which are a suite of stochastic tools for solution of the many-body Schrodinger equation. Due to its direct treatment of electron correlation, the QMC method offers the possibility of parameter-free, high-accuracy, systematically improvable analysis. In manganese oxide, we find that the QMC methodology is able to accurately reproduce relative phase energies, lattice constants, and band gaps without the use of adjustable parameters. Additionally, statistical analysis of the many-body wave functions from QMC provides some diagnostic assessments to reveal the physics that may be missing from other modeling approaches.   

Ab Initio Study of Structural Stability and Phase Transition Properties of Phase Transition Material : GeTe

Using ab initio density functional calculations, we investigate the structural and electronic properties of the ordered crystalline and disordered amorphous phases of a GeTe material, which would be used for phase change random access memory. With suitable pseudopotentials selected for respective elements, we first explore the equilibrium configurations and electronic properties of their crystalline phases. Different amorphous phases are, then, modeled by performing molecular dynamics simulations, which are composed of various stages including thermal equilibrations, a high-temperature pre-melting process with a low gravimetric density, and a quenching process to room temperature. To identify the local structures in amorphous phases, we evaluate their radial distribution functions (RDFs) and order parameters (OPs). Our calculated OPs and RDFs are analyzed and compared to EXAFS data available. Finally, we estimate energy barriers not only between crystalline and one of amorphous phases, but also between different amorphous phases to explore their structural stability, using nudged elastic band method.   

Latent heat of magnetization for MnFeSi0.33P0.66

Magnetic refrigeration is a very promising environmental-friendly method to encounter the energy shortage of the world by implementing the magnetocaloric effect. MnFeSiP series of materials are distinguishable magnetocaloric meterial for the use of non-toxic, inexpensive elements as well as high efficiency. There are several ways to measure the efficiency of the MCE, viz.- measuring the adiabatic temperature change or measuring the entropy change at the transition. MnFeSiP materials show a first order magneto-elastic phase transition at the Curie temperature ($T_C$). This simultaneous occourance of the magnetic and elastic transition in this material account for a higher $\Delta T_{\rm ad}$ (or high entropy change), which is linearly proportional to the Latent heat (L) of magnetization. Experimentally L can be determined with techniques such as Differential Scanning Calorimetry. In our study we use VASP in addition to the Phonopy package, to determine the finite temperature properties of the system. Quasi Harmonic Approximation was applied successfully to determine the Gibbs free energy of MnFeSi$_{0.33}$ P$_{0.66}$. Hence we show a phase transition around 425 K. From the temperature derivative of G , the specific heat was obtained and finally the latent heat was obtained.   

Shear-Induced Phase Transformation: From Single-Crystal Silicon to Si-IV

Silicon has been recognized as one of the most important semiconductors in modern electronics industry. Investigations in the past decades have led to observation of more than 12 different polymorphs of silicon. Among these polymorphs, the wurtzite silicon (Si-IV) shows promising application potential. It has been widely accepted that Si-IV is a metastable phase of silicon forming from annealing Si-III at temperature range between 200 C and 600 C. Besides the annealing, the shear stress can also lead to the phase transition from Si-I into Si-IV. It has been confirmed that the mechanism of shear-induced phase transition is different from that observed from hydrostatic pressure-induced phase transition. However, this shear-induced phase transition has not been studied systematically, and further investigations are required to clarify this transition on silicon. In this work, we develop a new method to study the formation of Si-IV. Combining nanoscratching and micro-Raman spectroscopy, shear effect on Si-I to Si-IV phase transformation has been studied qualitatively and quantitatively. A clear evolution of phase transition of silicon has been recorded. The stability of Si-IV has been analyzed by applying an in-situ Raman measurement under various temperature.   

Phase Diagram and Electronic Structure of Praseodymium and Plutonium systems

We apply a new implementation of LDA$+$Gutzwiller to calculate the zero-temperature phase diagram and electronic structure of Pr and Pu. Our study of Pr indicates that its pressure-induced volume-collapse transition would not occur without change of lattice structure --- contrarily to Ce. Our study of Pu shows that the most important effect originating the differentiation between the equilibrium densities of its allotropes is the competition between the Peierls effect and the Madelung interaction. However, the proper treatment of electron correlation effects is crucial to reach good agreement with experiment. A similar interplay between correlation effects and bands structure is also displayed in Pr, and might emerge in even greater generality.   

Strain engineered optoelectronic properties of transition metal dichalcogenides lateral heterostructures

Most three-dimensional bulk-scale materials rarely survive beyond 1{\%} strain, while recently spotlighted two-dimensional (2-D) materials can sustain a high elastic strain (up to 10{\%}) to optimize optical quantities such as band gaps and absorption spectra governing optoelectronic device performance. Despite the enormous interest in strained 2-D materials, most researches are focused on single materials or vertical heterostructures where precise control of stacking orientation is challenging. Here, using first-principles density-functional calculations, we explore how uniaxial tensile strains modify overall electronic and optical properties of transition metal dichalcogenides lateral heterostructures, such as MoX$_{2}$/WX$_{2}$ (X$=$S, Se). Based on the detailed optoelectronic information, we predict the optimal strain condition for maximal power efficiency. Furthermore, we find that uniaxial tensile strain readily develops a continuously varying direct-bandgap across the lateral heterojunctions, which results in the broad range absorption of solar spectrum useful for future optoelectronic devices.   

Nonlocal Electron Coherence in MoS$_{2}$ Flakes Correlated through Spatial Self Phase Modulation

Electron coherence among different flake domains of MoS$_{2}$ has been generated using ultrafast or continuous wave laser beams. Such electron coherence generates characteristic far-field diffraction patterns through a purely coherent nonlinear optical effect---spatial self-phase modulation (SSPM). A wind-chime model is developed to describe the establishment of the electron coherence through correlating the photo-excited electrons among different flakes using coherent light. Owing to its finite gap band structure, we find different mechanisms, including two-photon processes, might be responsible for the SSPM in MoS$_{2}$ [with a large nonlinear dielectric susceptibility $\chi^{(3)}=$ 1.6 $\times $ 10$^{-9}$ e.s.u. (SI: 2.23 $\times $ 10$^{-17}$ m$^{2}$/V$^{2})$ per layer]. Finally, we realized all optical switching based on SSPM, demonstrating that the electron coherence generation we report here is a ubiquitous property of layered quantum materials, by which novel optical applications are accessible.   

Temperature Dependence of Brillouin Light Scattering Spectra of Acoustic Phonons in Silicon

Thermal management represents an outstanding challenge in many areas of technology. Electrons, optical phonons, and acoustic phonons are often driven out of local equilibrium in electronic devices or during laser-material interaction processes. Interest in non-equilibrium transport processes has motivated the development of Raman spectroscopy as a local temperature sensor of optical phonons and intermediate frequency acoustic phonons, whereas Brillouin light scattering (BLS) has recently been explored as a temperature sensor of low-frequency acoustic phonons. Here, we report temperature dependent BLS spectra of silicon, with Raman spectra taken simultaneously for comparison. The origins of the observed temperature dependence of the BLS peak position, linewidth, and intensity are examined in order to evaluate their potential use as temperature sensors for acoustic phonons. We determine that the integrated BLS intensity can be used measure the temperature of specific acoustic phonon modes.   

Diffuse X-ray Scattering as a Tool to Characterize Morphology of Multilayered Structures of Ultra-small (Submonolayer) Quantum Dots

Characterization of submonolayer quantum dots (QDs) (i.e., QDs formed from deposition of less than a monolayer of material) is challenging due to their small size and a low electron density contrast between the embedded QDs and the host. For example, our samples have ZnTe/ZnSe type-II submonolayer QDs grown via migration enhanced epitaxy, in which the contrast between Te- and Se-containing sublattices is very low. We have devised a systematic diffraction analysis to obtain quantitative structural information about such hard-to-image systems. In this procedure, a large portion of the reciprocal space is mapped to acquire both out-of-plane and in-plane information. Vertical self-ordering of QDs is determined from out-of-plane and non-specular reflectivity maps, while diffuse scattering analysis is used to check in-plane correlations. This work greatly enhances the potential for extracting structural information of complex embedded 3D QD structures.   

Brillouin Light Scattering study of patterned TiN/SiOC:H/Si structures

In order to improve device performance of interconnects, the microelectronics industry utilizes low-k dielectric technology in place of traditional SiO$_{2}$. Furthermore, titanium nitride (TiN) is being widely used as a hard mask to pattern low k materials such as SiOC:H into desired architectures with \textless 100 nm length scales. However, the high stress and stiffness of the TiN over-layer may adversely influence the delicate underlying patterns and affect device performance. In this study we utilize Brillouin light scattering (BLS) to probe the elastic properties of TiN/SiOC:H structures grown on Si that have been patterned into a series of parallel wires of rectangular cross-sections with sub 200 nm pitch and depths. In studying the influence of the hard mask on the mechanical properties of SiOC:H, BLS offers a non-invasive approach to detect acoustic excitations and to measure their mode dispersions for incident light with wave-vector components parallel or perpendicular to the TiN wires. The results of measurements performed on samples with a range of wire dimensions (width/depth) will be presented that include the dependence of the Brillouin peak intensities on the incident and scattered light polarization as well as a model of the mode profiles.