P. Masri

Surface phonons and surface reconstruction in calcium doped magnesium oxide

Static lattice calculation of the structure and energy of calcium doped magnesium oxide have indicated that the surface will be heavily segregated with calcium in accordance with experimental measurement. At moderate temperatures the enthalpy of segregation is sufficient to produce a monolayer coverage of impurity at equilibrium. The phonons for the segregated surface reported here show an instability that was not found in the static calculation. A phonon of imaginary frequency over much of the Brillouin zone indicates that a lower energy structure of larger periodicity should exist. We propose a self-consistent static-dynamic procedure that uses the phonon results to suggest a restructuring that is calculated in a static calculation. The phonon calculation is then repeated. When a lowest energy static calculation gives surface phonons with no softening, we can be confident of the predicted structure. In this case we have predicted a segregation induced surface restructuring. The restructured cell is c(√2 × √2)R45° with half the oxygen ions pushed high out of the surface. The small long wavelength phonon anomaly which remains suggests that there may also be a longer range rumpling of surface cations.

Electrical characterization of SiC/Si heterostructures with Ge-modified interfaces

Abstract The electrical properties of the n-3C–SiC/p-Si heterojunction grown by solid source molecular beam epitaxy on germanium-modified Si(1xa01xa01) substrates have been investigated. The current flow in the forward direction is determined by diffusion and recombination currents. The interface state density was determined to be not larger than 10 11 xa0cm −2 . The obtained interface state density is lower than in the case of 3C–SiC grown on Si(1xa01xa01) by chemical vapour deposition. Ge predeposition on silicon prior to silicon carbide is able to improve the ideality factor of the diode and decreases currents of the reverse biassed n-3C–SiC/p-Si heterojunction diode.

Simple calculation of the mean square displacements of volume and surface atoms of face-centered cubic crystals

Abstract We use a method based on the expansion of the dynamical matrix to determine the thermal dependence of the mean square displacement 〈 u 2 〉 of the atoms. The method is applied to evaluate 〈 u 2 〉 for volume atoms and (100), (110) and (111) surface atoms of face-centered cubic crystals as well as for surface steps and kinks. We plot our results as a function of the temperature for aluminium, nickel, copper, palladium, silver, gold and lead.

Localized surface modes and resonances for vicinal surfaces: The (117) face of fcc crystals

Abstract Considering the (117) face of an fcc crystal, the dispersion relations of localized surface modes and resonances have been calculated using the method of generating coefficients of Green functions. The force between atoms are limited to the central force between the nearest neighbours. In the direction of propagation orthogonal to a step, for each wavevector value, a localized surface mode and two resonances have been found. Their polarization amplitude and width have been calculated. It is shown that these results can be qualitatively understood by folding three times the dispersion relation of the (001) face which is the crystallographic plane of the terrace. In the direction parallel to the step, the dispersion relations are obtained by the superposition of the sagittal and transverse dispersion relation of the (001) face. The (117) configuration introduces a coupling between these two polarizations and some resonances appear. A general rule which enables one to predict qualitatively the shape of the dispersion relation for any vicinal surface of type (11 m ) is expressed.

Studies of interface phonons

The vibrational modes localized at the interface between two distinct crystals have been studied for a simple crystal model obeying all of the invariance conditions required for models used in studies of dynamical properties of crystal surfaces, and giving rise to Rayleigh surface waves. The two crystals are assumed to be semi-infinite simple cubic and to have the same lattice parameter a. They differ by their mass (M and MA) and the central force interactions between first (K and Ka) and second nearest neighbors 12K and 12KA. The interface is obtained by coupling the (001) free surfaces of these distinct crystals by central force intractions (K). We find that the variation of the interaction conditions (K) at the interface and of the (KM)(KAMA) parameter has the following qualitative effects on the properties of surface and bulk phonons. When (K) increases from zero to a finite value, the frequencies of the surface phonons increase and are splitted in the case of two identical crystals. One can say that the surface phonons are transformed into interface modes. For some values of KK and (KM)(KAMA) parameters these interface phonons may be admixed with bulk phonons and thus become virtual interface states.

Bulk and surface electronic states in semiconductor superlattices

Abstract We study the bulk and surface electronic states of semiconductor superlattices with the help of a simple two-band model. The superlattice has a larger periodicity in the direction perpendicular to the slabs and therefore many electronic branches in the folded Brillouin zone. In the gaps existing between the bulk branches appear the surface localized modes. The simplicity of the model used here allows one to obtain in closed form the bulk and (001) surface Green function for this superlattice. The analytic knowledge of these functions enables us to study easily the bulk and surface electronic bands of this semiconductor superlattice, which otherwise require important numerical calculations. We give here for the first time the analytic expression we obtained for the folded bulk electronic bands and also the expression that gives the surface electronic states. A few figures, especially for InAsue5f8GaSb and GaAsue5f8AlAs superlattices illustrate these results.

Entropie d'adsorption du xénon en épitaxie sur la face (0001) du graphite

Abstract A model is built up, describing the vibrational properties of an atomic layer of xenon adsorbed in epitaxy on the (0001) face of graphite. This model allows to calculate the adsorption entropy of this twodimensional phase. The obtained value has the same order of magnitude as the one measured by analyzing the adsorption isotherms determined by Auger spectroscopy. We deduce that the vibrations of the xenon monolayer are partly hindered in respect to those of this rare gas crystal.

Some aspects of the physics of man-made semiconductor heterosystems: superlattice electronic structure and related topics

Abstract In this review article, we discuss different aspects of the physics of semiconductor superlattices relevant to the electronic structure of these systems.

A calculation of defect gap states on the clean (110) surfaces of some III–V semiconductors

Abstract The gap electronic structure of various defects on the (110) faces of GaSb, GaAs, GaP, InAs and InP is studied. States associated with defects having one or two dangling bonds on either metallic and non-metallic atoms are considered. For interacting defects the barycenter of the states is calculated and their splitting is estimated. It is found that for GaSb, GaAs, GaP and InAs only two clearly distinct groups of levels can be identified. The upper one always corresponds to centers with one dangling bond on a metallic atom, while the lower one contains the levels associated to all the other possibilities we have investigated. For InP, levels associated with two dangling bonds on a metallic atom are raised from the lower group. This is in agreement with the other available theoretical results. A discussion of the experimental data in the light of our results is presented.

Interface response theory of N-layered metallic superlattices

Abstract A recent interface response theory is applied for the first time to superlattices formed out of a periodic repetition of N different discrete metallic slabs. General expressions for the response functions are given. A simple application to an exactly solvable model of a two- and three-layered metallic superlattice illustrates this new general theory.