Session D9: Semiconductor Structure and Mechanical Properties

First-Principles determination of deformation potentials and band parameters in group-II oxide semiconductors

Group-II oxide semiconductors (MgO, ZnO, and CdO) are excellent candidates for use in solid state lighting. Due to the lattice mismatch between oxide epilayers and substrates, strain effects (described by deformation potential parameters) play a crucial role in the band structure. On the other hand, the band dispersion in the vicinity of the band extrema (described by effective masses and Luttinger parameters) is also critical in determining the optical properties of the oxide-based materials and devices. We employ the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional in density functional theory to overcome the limitations of local-density or generalized-gradient functionals (LDA and GGA) and produce highly accurate band structures. Using the k-p approach, we obtain consistent sets of band parameters (band gaps, crystal-field splittings, spin- orbit splittings, effective masses, and Luttinger parameters) for CdO, ZnO, and MgO in the wurtzite phase. We also report a comprehensive set of deformation potentials that describe band- structure modifications in the presence of strain. The results can be used for accurate modeling of device structures.   

First principles study of oxidized AlN and GaN nonpolar surfaces

We study the reconstructions of oxidized AlN and GaN nonpolar surfaces using first-principles density functional theory. The atomic structures of the surfaces are relaxed using the generalized gradient approximated exchange-correlation functional, and the electronic structures are calculated using the hybrid functional implemented in the Heyd-Scuseria-Ernzerhof (HSE) screened Coulomb potential framework. The stability of surface structures strongly depends on the chemical potentials of Al (Ga) and O. Under cation-rich conditions and in equilibrium with the oxide, the AlN $(10\bar{1}0)$ surface favors a high-density oxide structure formed by O substitution for N (O$_{\textrm{N}}$) and O occupation of interstitial sites, while the AlN $(11\bar{2}0)$ surface favors a low-density oxide structure formed by O$_{\textrm{N}}$ and Al vacancies on the surface. GaN surfaces behave differently, however: under similar conditions, the two GaN non-polar surfaces are oxidized to a structure that consists of only O$_{\textrm{N}}$ in the top two layers. Despite the substantial change in atomic structure of the surface, the oxidation of nonpolar surfaces does not significantly change the surface states in the gap compared with the bare surfaces, and therefore it does not have significant effect on the electronic properties.   

Understanding the glass transition in GeSbTe materials

Moore's law demands the continual reduction in size of the components of computers. One future direction for memory technology involves the use of phase change materials which can be switched by pulsed electrically heating from a conducting crystalline phase to an insulating amorphous phase. These materials are typically alloys of Germanium, Antimony and Tellurium (GST). In order to form multi-state bits, it is necessary to arrest the glass transition via varying annealing time such that differences in resistivity can be measured based. As might be expected, this process is hinder by ``creep'' of the glass towards higher resistance states after the quench is halted. In this lecture, simulation studies are employed to study the glass transition from the crystalline state and discern the mechanism for the gap opening. The nature of mid gaps states found from the simulated quenches gives insight into the mechanism of the creep and suggests ways in which the phenomena can be arrested.   

Order, miscibility and electronic structure of AgSbTe$_2$-based thermoelectrics: a first principles study

Using first-principles density-functional theory calculations and cluster expansion, we analyze the composition, atomic structure and electronic properties of AgSbTe$_2$-based thermoelectric alloys. In particular, for AgBiTe$_2$-AgSbTe$_2$ system, we predict that the alloys exhibit D4 cation order at all temperatures below melting, and are fully miscible down to the room temperature and below. The band structures of the AgBiTe$_2$ and AgSbTe$_2$ compounds are compared and the difference is found to originate in the spin-orbit interaction. We analyze the evolution of the Fermi surface topology at low hole dopings and use these results to refine the interpretation of the recent experimental measurements on naturally doped AgSbTe$_2$ samples.\footnote{V. Jovovic and J.P. Heremans, Phys. Rev. B {\bf77}, 245204 (2008).} We further discuss the energetics of native defects, and give a theoretical perspective on the miscibility and ordering in other quasi-ternary (Ag,Sb,X)Te alloys with rocksalt topology.   

Origin of the Anatase to Rutile Conversion of Metal-Doped TiO2

Extensive calculations using density functional theory enable us to explain the origin of the surprising room-temperature conversion of anatase to rutile phase of TiO2 when doped with Co and Ni, but not with Cu. Contrary to earlier suggestion, neither high spin nor strain of the transition metals is found to be responsible for this phase conversion. The driving mechanism, instead, is attributed to the increased interaction between Co and Ni atoms forming a linear chain in the rutile phase. We predict that Cr and Mn which have even larger spins than Co and Ni cannot induce this phase conversion.   

Studies of the atomic structure of the adsorption of B and Al on GaN

We perform first principles total energy calculations to study the adsorption and diffusion of B and Al on GaN (0001) and (000-1) surfaces, in the (2x2) and ($\surd $3x$\surd $3) periodicities. Studies are done within the periodic density functional theory. In the repeated slab geometry method, the slab of the GaN(0001) surface is formed by 4 and 5 bilayers, for the (2x2) and ($\surd $3x$\surd $3) periodicities, respectively. The slab of the GaN(000-1) surface is formed by 3 and 4 bilayers, plus a Ga monolayer for the (2x2) and ($\surd $3x$\surd $3) periodicities, respectively. Dangling bonds of the bottom surfaces are saturated by pseudo-hydrogen atoms. The two lowest bilayers and the hydrogen are frozen at their ideal positions to simulate the bulk like environment. The upper bilayers and the adsorbed atoms are set free to relax. For the high symmetry sites the most stable structure of the adsorption on the GaN(0001)-(2x2) surface is the T4-site. For the adsorption on the GaN(000-1)-(2x2) is H3.   

Density functional/molecular dynamics simulations of phase-change materials: Characterizing disordered phases

The technological importance of phase-change materials (PCM) is based on the rapid amorphous-to-crystalline transition and changes in optical (and electrical) properties. Our density functional (DF)/ MD simulations on Ge$_{2}$Sb$_{2}$Te$_{5}$ (GST-225) and Ge$_{x}$Te$_{1-x}$ alloys showed that atoms can be classified as A (Ge,Sb) and B (Te). ``ABAB squares'' are essential structural units, many ``tetrahedral'' and ``octahedral'' Ge atoms coexist, and Sb and Te coordination numbers deviate from the ``8-N rule''. Small cavities provide space to allow rapid ordering of disordered ABAB squares. We describe simulations of tellurium, which has an unusual density maximum near the melting point, and of two PCM: Ge$_{8}$Sb$_{2}$Te$_{11}$ (melt-quench of 630 atoms over 400 ps) and liquid Ag$_{3.5}$In$_{3.8}$Sb$_{75.0}$Te$_{17.7 }$ (AIST, 640 atoms at 850 K) alloys. The structures of amorphous GST-8,2,11 and GST-225 are similar. The structure factor and pair distribution function of liquid AIST agree well with HEXRD measurements, there is medium-range order, and Ag and In atoms prefer to be near Te atoms.   

ZnX (X = O, S, Se, Te) under uniaxial strain

We present a first principles density functional theory based study of the impact of uniaxial strain on the structural and electronic properties of bulk ZnX (X = O, S, Se, Te) in the wurtzite and zinc blende phases. The strain axis was chosen to be along the [0001] and [111] directions, respectively, for the wurtzite and zinc blende systems. For large uniaxial compressive strains, all systems undergo a transition from the equilibrium wurtzite or zinc blende phases (which display $sp^{3}$ hybridization) to a graphite-like phase (displaying $sp^{2}$ hybridization). Simultaneously, the band gap of the systems gradually drops to a small or zero value. Under large uniaxial tensile strains, all systems tend to form individual stoichiometric ZnX layers, also with small band gap values relative to the corresponding equilibrium ones. Although the range of strains considered here is enormous, appreciable changes (i.e, reductions) of the band gap may be accomplished for modest and realistic strains achievable in nanowires.   

Folding of Suspended Graphene Sheets

Graphene, like a piece of paper, folds under mechanical force, resulting in a straight folded edge. The folded graphene edges have a nanotube-like surface structure, which possess unusual electronic properties and mechanical stability from the van der Waals attraction of folded graphene. Here we report a first systematic study of the atomistic structure of folded graphene and the energetics of graphene folding. We use graphene suspended in solution and free folding is achieved under random ultrasonic stimulations. This allows a statistically investigation of free folding of suspended graphene in all directions. The structure of the folded graphene is determined by nanoarea electron diffraction. Through a statistic measurement of about 100 folded graphene edges, we found the graphene preferentially folds along the symmetric directions. This preference is explained by a examination of graphene folding energetic and atomic simulation.   

Generation of coherent pulses of sub-terahertz longitudinal acoustic phonons in n-i-p-i silicon doping superlattices

Intense pulses of coherent 246-GHz longitudinal acoustic phonons have been produced in n-i-p-i silicon doping superlattices by the resonant absorption of pulsed far-infrared (FIR) laser radiation. A niobium small-period grating-coupler has been used to convert the incident transverse electric field into an evanescent longitudinal field over the thickness of the superlattice. Si:B piezo-phonon spectroscopy is used, in conjunction with a fast granular aluminum/palladium microbolometer, to verify that the phonons exist in a narrow frequency band ($\sim $10-GHz) around the FIR laser frequency at 246-GHz (1.22 mm). Time-of-flight across the thin (0.5-mm) substrate is used to verify that the phonons are longitudinal. The laser radiation is coupled onto the grating-coupler via a corrugated waveguide and a hyper-hemispherical silicon lens. Potential applications include the development of a novel terahertz cryogenic acoustic microscope for sub-surface imaging and sub-nanometer lateral resolution.   

The local structure of transition metal doped semiconducting boron carbides

Transition metal doped boron carbides produced by plasma-enhanced chemical vapor deposition of orthocarborane (\textit{closo}-1,2-C$_{2}$B$_{10}$H$_{12})$ and metallocenes were investigated by performing K-edge extended X-ray absorption fine structure and X-ray absorption near edge structure measurements. The Mn, Fe and Co transition metal atoms dope boron carbide pairwise. The transition metal atom occupies one of icosahedral boron or carbon apical site atomic site within the icosahedral cage on adjacent icosahedral cages. There is good agreement between the experiment and theoretical modeling of the local structure two adjoined carborane cages each with a Mn, Fe and Co metal atom (forming the pair wise doping). The local spin configurations of transition metal doped boron carbides, from Ti to Cu, are compared.   

The transition towards chaos in a coupled nanoelectromechanical system

The coupling between electron transport and mechanical motion of nano-structures has been studied since the electron shuttling system was suggested by Gorelik. Here we demonstrate how coupled nanopillars can be driven into the limit of chaotic response. This novel structure is investigated under AC excitation superimposed on a DC bias. The linear mechanical response evolves into Arnold tongues under a strong AC drive. Beyond a critical point, we observe chaotic behavior in the response.   

Excitonic condensation in an electron-hole bilayer with density imbalance

There is a growing interest in the BCS-BEC crossover phenomenon because of experimental advances in ultracold atomic gases. In particular, the possibility of exotic phases such as gapless superfluid (Sarma phase) and Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in imbalanced two-species Fermi systems attracted considerable attention. In this work, we study the excitonic condensation in an electron-hole bilayer with unequal layer densities at zero temperature. Coulomb repulsion within each layer suppresses the phase separation and different electron and hole band masses and nonlocal nature of the electron-hole attraction both favor the exotic phases. We solve the mean-field BCS gap equations and investigate the effects of intra- and inter-layer interactions and analyze the stability of the Sarma phase by calculating the superfluid mass density. We find that with bare Coulomb interactions the Sarma phase is always locally stable. Introducing a simple model for screening we find that in the intermediate density a region of the phase diagram becomes unstable which we identify as the FFLO phase.   

Electronic structures and magnetic properties of La$_{2-x}$Sr$_x$MnNiO$_6$

We have investigated hole carrier doping effects in La$_{2}$MnNiO$_{6}$, which is getting attraction for being a high $T_C$ ferromagnetic insulator. Employing the {\it ab-initio} band structure method, we have examined the changes in the electronic structures and the valence states of Sr-doped La$_{2-x}$Sr$_x$MnNiO$_6$ with varying Sr doping ratio. Upon Sr doping, which corresponds to the effective hole doping, we have found a transition from a ferromagnetic insulating phase to a robust half-metallic phase. We have verified that the substantially weak x-ray magnetic circular dichroism (XMCD) signal observed for La$_{2-x}$Sr$_x$MnNiO$_6$, as compared to the undoped system, is caused by anti-site disorder at $B$-sites in a Sr-doped system.