Anharmonic Keating model for group-IV semiconductors with application to the lattice dynamics in alloys of Si, Ge, and C

Composition- and Temperature-Resolved Raman Shift of Silicon

We formulated the composition and temperature dependence of the Si and Si_1-xGe _x Raman shift from the perspectives of bond order-length-strength correlation and local bond average approach. It is verified that the Raman shift Δω varies in the form of Δω ∝ zE^1/2/ d, with inclusion of bond length d and energy E changing with temperature and composition. Numerical reproduction of the thermally induced Si_1-xGe _x phonon softening indicates that bond thermal expansion and energy loss dictate the frequency redshift, which resulted in quantitative information on the bond energy and the reference frequencies from which the Raman shifts proceed. Observations not only gain deeper insight into the mechanism of the Raman shift but also demonstrate the revealing power of Raman technique for the bonding thermodynamics.

Transformation of shock-compressed graphite to hexagonal diamond in nanoseconds

The graphite-to-diamond transformation under shock compression has been of broad scientific interest since 1961. The formation of hexagonal diamond (HD) is of particular interest because it is expected to be harder than cubic diamond and due to its use in terrestrial sciences as a marker at meteorite impact sites. However, the formation of diamond having a fully hexagonal structure continues to be questioned and remains unresolved. Using real-time (nanosecond), in situ x-ray diffraction measurements, we show unequivocally that highly oriented pyrolytic graphite, shock-compressed along the c axis to 50 GPa, transforms to highly oriented elastically strained HD with the (100)HD plane parallel to the graphite basal plane. These findings contradict recent molecular dynamics simulation results for the shock-induced graphite-to-diamond transformation and provide a benchmark for future theoretical simulations. Additionally, our results show that an earlier report of HD forming only above 170 GPa for shocked pyrolytic graphite may lead to incorrect interpretations of meteorite impact events.

Interface properties of atomic layer deposited TiO2/Al2O3 films on In(0.53)Ga(0.47)As/InP substrates

Electrical and interfacial properties of metal-oxide-semiconductor (MOS) capacitors fabricated using atomic layer deposited bilayer TiO2/Al2O3 films on In0.53Ga0.47As/InP substrates are reported. Vacuum annealing at 350 °C is shown to improve the interface quality. Capacitance-voltage (C-V) characteristics with higher accumulation capacitance, negligible frequency dispersion, small hysteresis and low interface state density (∼1.5 × 10(11) cm(-2) eV(-1)) have been observed for MOS capacitors. Low frequency (1/f) noise characterization and inelastic electron tunneling spectroscopy (IETS) studies have been performed to determine defects and interface traps and explain the lattice dynamics and trap state generation mechanisms. Both the IETS and 1/f noise studies reveal the spatial locations of the traps near the interface and also the nature of the traps. The IETS study further revealed the dynamic evolution of trap states related to low frequency noise sources in the deposited TiO2/Al2O3 stacks. It is shown that deposition of an ultrathin layer of TiO2 on Al2O3 can effectively control the diffusion of As in the dielectric and the oxidation states of In and Ga at the In0.53Ga0.47As surface.

Correlation between ionic charge and ground-state properties in rocksalt and zinc blende structured solids

In this paper we have evaluated the ground-state properties (i.e., bulk modulus and cohesive energy) of rocksalt and zinc blende structured solids. We have presented two expressions relating the bulk modulus B (GPa) for the alkali halides, alkaline-earth chalcogenides, transition metal nitrides, rare-earth {divalent (R(2+)X) and trivalent (R(3+)X) } monochalcogenides, group IV, III-V and II-VI semiconductors and the cohesive energy E(coh) (kcal mol(-1)) for the alkali halides and alkaline-earth chalcogenides with the product of ionic charges (Z(1)Z(2)) and nearest-neighbour distance d (Å). The bulk moduli and cohesive energy of rocksalt and zinc blende type structure compounds exhibit a linear relationship when plotted on a log-log scale against the nearest-neighbour distance d (Å), but fall on different straight lines according to the ionic charge product of the compounds. We have applied the modified relation on rocksalt and zinc blende structured solids and found a better agreement with experimental data as compared to the values evaluated by earlier researchers. The results for bulk modulus differ from experimental values by the following amounts: BaO-0%, LiCl-0%, LaS-0%, SmSe-0%, ZnS-0%, CdS-0%, GaP-0%, InP-0%, MgO-0.61%, CaO-0.89%, SmS-1.7%, YbSe-1.6%, UP-1.9%, EuSe-1.9%; and the results for cohesive energy differ from experimental values by the following amounts: LiCl-0.49%, KF-0.51%, RbF-0.54%, SrO-1.2%, NaCl-1.6%, NaF-1.8%, MgSe-1.9%.

Parametrization of an anharmonic Kirkwood-Keating potential for AlxGa1-xAs alloys

We introduce a simple semiempirical anharmonic Kirkwood-Keating potential to model A(x)B(1-x)C-type semiconductors. The potential consists of the Morse strain energy and Coulomb interaction terms. The optical constants of pure components, AB and BC, were employed to fit the potential parameters such as bond-stretching and -bending force constants, dimensionless anharmonicity parameter, and charges. We applied the potential to finite temperature molecular-dynamics simulations on Al(x)Ga(1-x)As for which there is no lattice mismatch. The results were compared with experimental data and those of harmonic Kirkwood-Keating model and of equation-of-motion molecular-dynamics technique. Since the Morse strain potential effectively describes finite temperature damping, we have been able to numerically reproduce experimentally obtained optical properties such as dielectric functions and reflectance. This potential model can be readily generalized for strained alloys.

Arrested cracks in nonlinear lattice models of brittle fracture

We generalize lattice models of brittle fracture to arbitrary nonlinear force laws and study the existence of arrested semi-infinite cracks. Unlike what is seen in the discontinuous case studied to date, the range in driving displacement for which these arrested cracks exist is very small. Also, our results indicate that small changes in the vicinity of the crack tip can have an extremely large effect on arrested cracks. Finally, we briefly discuss the possible relevance of our findings to recent experiments.