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Laboratory studies have been made on molecules of astrophysical interest such as AlO, CO, CrO, SiS, NH+ and OH. Vibrational and rotational constants have been determined more accurately in the various electronic states.
Computational materials science has evolved in recent years into a reliable theory capable of predicting not only idealized materials and device performance properties, but also those that apply to practical engineering developments. The codes run on workstations and even now are fast enough to be useful design tools. A review will be presented of the current status of this rapidly advancing field. Examples of the accuracy of the codes are displayed by comparing the predicted atomic volumes, and cohesive and excess energies of several materials with experiment. As a further demonstration of the methods, the band structures of AlN, GaN, and InN in wurtzite and zinc blende structures will be presented. There are anomalies in the conduction and valence bands of these materials. Some consequences on light emitting and power devices made from these materials will be examined.
Nucleation and growth mechanisms in the formation of heteroepitaxial films are reviewed. The various processes can be incorporated into rate equations to model the number density and size distribution of clusters.Recent work on extensions of this approach to include the effect of surface steps and other surface imperfections is highlighted.
The most important processes are being studied experimentally using a combination of surface-analytic and -microscopic techniques, based on SEM and STEM instrumentation. Recent examples are given in which nucleation densities, surface diffusion lengths and the effects of steps have been studied in the systems Ag/Si(lll) and Ge/Si(100).
The cuprate superconductor problem is approached from the conventional metallic standpoint, valid at dopings near the Tc maximum and beyond. There is strong evidence that the Tc maximum corresponds to EF lying at a van Hove singularity, a special situation leading to Marginal Fermi Liquid behavior, and also to a minimum in the isotope shift, as observed. The superconducting properties are discussed in the light of phononic and electronic pairing models.
We have demonstrated that the atomic distribution of constituents in semiconductor alloys is never truly random. There are always interactions causing correlations; the degree and nature of the correlations depend on which interactions dominate and on the growth conditions. While we have identified most of the interactions which are expected to cause correlations, not all of them have been treated completely to date. Therefore, some details remain unclear, but the principal effects can now be appreciated in broad terms.
Thermotropic chiral nematic copolymers containing (S)–(–)–1–phenylethyl alcohol and (R)–(–)–methyl mandelate with methacrylate backbone and those containing (S)–(–)–1–phenylethylamine and cholesterol with acrylate/methacrylate mixed backbone structures have been synthesized and characterized. It is found that the helical sense is dictated by chiral/nematic molecular interactions of a steric nature, and that the helical twisting power (HTP) is favored by the structural similarity between the chiral and nematic components. Furthermore, it is shown that HTP can be optimized with a suitable nematogenic comonomer by regulating the backbone flexibility within the acrylate/methacrylate series, which facilitates supramolecular arrangement without affecting the HTP.
A study of the molecular beam epitaxial (MBE) growth on singular and vicinal (110) surfaces of GaAs is presented. Quantum well structures and tilted superlattices (TSL) were grown on substrates misoriented 0.5°-2° towards the nearest  and A azimuths at growth temperatures ranging from 450° C to 600° C under different growth conditions. The structures were characterized by Nomarski optical microscopy, transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy.
Two types of faceting were observed on the surfaces. The structures grown at temperatures above 540°C and As beam fluxes below l×10-5 torr showed V-shaped facets pointing in the  direction and are attributed to As deficient island growth. Lower temperatures and higher As beam fluxes lead to surfaces with microfacets that are elongated along the respective step directions on the vicinal surface and are due to step bunching during growth. Their density and height decrease with decreasing vicinal angle and they disappear on the singular (110) surface. The photoluminescence of the GaAs quantum wells grown on these samples is redshifted with respect to that of the quantum wells grown on the flat surface. This is being ascribed to the fact that on the vicinal surface, the recombination takes place at the facets where the quantum wells are wider.
The contrast in the TEM images of the TSL show for the samples misoriented towards  that the lateral segregation to the step edges on this surface is appreciable. The TSL spacing and the tilt however show that during growth the vicinal surfaces tend towards a surface with smaller miscut.
Molecular beam epitaxial growth of Ge on Si(110) surfaces reveals interesting aspects of the heterogeneous nucleation of coherent Ge islands. Cleaning of the Si substrate by desorption of a passivating oxide layer at high temperature creates surface pits. Two sets of experiments, including deposition of Ge on as-cleaned substrates, and surfaces with a thin Si buffer layer are compared to illustrate the nucleation behavior of Ge. Typical Ge deposition temperatures range from 600°C to 725°C.
For Ge deposition on as-cleaned surfaces, the faceted edges of pits serve as preferential sites for the heterogeneous nucleation of coherent Ge islands. Experiments were also performed on surfaces with thin (˜20nm) Si buffer layers grown on the as-cleaned surface. Though the faceted pits have not been completely covered by the Si buffer layer, they have decreased in lateral size. In addition, the Si(110) surface shows ledges that are formed along specific crystallographic directions. Ge deposited on the Si buffer nucleates first at the corners of the pits, in an interesting dipole orientation, as well as along the ledges on the surface.
Step bunching during epitaxial growth results in the transformation of a vicinal surface into a periodic array of micro-facets. Molecular beam epitaxial growth on the vicinal GaAs (110) surface exhibits this phenomenon which has primarily been characterized by electron microscopy. GaAs quantum wells with AlAs barriers were grown on GaAs(110) substrates vicinal 0.5-2· towards . The faceting on the vicinal surface creates two distinct quantum well thicknesses. While most of the quantum well is 96Å thick, it broadens at the faceted regions. This thickness modulation produces two distinct luminescence peaks. By using temperature dependent photoluminescence, we have observed trends in exciton mobility. The exciton mobility decreases at low temperatures for the 1.0° and 2.0° samples, indicating a scattering mechanism. We will discuss interface roughness and other sources of scattering.
Computational materials science has evolved in recent years into a reliable theory capable of predicting not only idealized materials and device performance properties, but also those that apply to practical engineering developments. The codes run on workstations and even now are fast enough to be useful design tools. A review will be presented of the current status of this rapidly advancing field. Examples of the accuracy of the codes are displayed by comparing the predicted atomic volumes, and cohesive and excess energies of several materials with experiment. As a further demonstration of the methods, the band structures of AIN, GaN, and InN in wurtzite and zinc blende structures will be presented. There are anomalies in the conduction and valence bands of these materials. Some consequences on light emitting and power devices made from these materials will be examined.
We report on the formation of lateral superlattices in short period vertical GaAs/AlAs superlattices. To explain the observed self-organized phase separation, we propose a model of vertical intermixing, driven by the exchange of Ga on the surface with impinging Al atoms. The model correctly describes the formation of lateral superlattices for both integer and fractional monolayer deposition. It also predicts a far-reaching intermixing at GaAs-AlAs interfaces. Insitu RHEED studies of the initial growth stage of both GaAs-AlAs and AlAs-GaAs interfaces support the assumption of an asymmetric exchange at the growing surface and confirm the longrange Ga migration predicted by the model.
Molecular beam epitaxial growth on GaAs(110) vicinal surfaces results in the formation of periodic micro-facets. We compare experimental results with computer simulations of a simple one dimensional step-flow growth model. The simulations show that preferential adatom attachment to the down step in a step array results in the destruction of step uniformity. A kinetic limitation due to adatom diffusion length along the terraces leads to stabilization of a periodic array of step-bunches. We extend our simulations to show the effects of the attachment and diffusion parameters on the dynamics of facet evolution.
We report on the differences in the epitaxial growth mechanisms between Ge1−xCx (O<x<0.1) and Ge1−x−ySixCy (x=0.2, 0<y<0.05) alloys grown on Si(100) using low temperature( 200°C) molecular beam epitaxy. Thin films (50˜65nm) were characterized in situ by RHEED and ex situ by transmission electron microscopy and x-ray diffraction. With increasing C concentration, the microstructure of both Ge and GeSi alloys changes from 2D layer growth to 3D islanding. The d400 spacing of the relaxed alloys decreases marginally, with a maximum of 1at.% C being substitutionally incorporated. Ge-C films with higher C content have a high density of planar defects, typically twins and stacking faults. The addition of 20% Si does not appear to increase the amount of substitutional C in the films. Rather, the additions of 20% Si to Ge-C alloys somehow seems to enhance the tendency for the formation of planar defects.
We report on the room temperature and the low temperature ( 77 K ) study of highly Si doped MBE grown bulk Al.2Ga.8As layers by Photoluminescence ( PL ), Hall, X-Ray and Electrochemical Voltage Profiler ( ECV ) technique. The room temperature PL spectra shows the presence of an asymmetric broad peak in the low energy side whose Full Width at Half Maxima (FWHM) and intensity increases with the doping level. The broad peak has been found to be a combination of two individual peaks, one arising out of the DX center related recombinations and the other due to some Silicon related complex acceptor sites. The low temperature PL also shows a ‘strange peak’ near the Al2Gas8As band edge luminescence peak for wafers grown at 600°C. This peak which increases in intensity at a very rapid rate with falling temperature and shift very rapidly with temperature can be attributed to some AIGaAs growth defect related recombination center, depending on the growth temperature.
We have performed calculations of Sn deposition on Cu(111) and Cu(100) surfaces. The atomic interactions are described by modified embedded atom method (MEAM) potentials. This is a modification of the embedded atom method (EAM) to include higher moments in the electron density. We find the at low coverages Sn deposited on Cu(111) leads to the formation of a two-dimensional (2D) alloy phase with a p (√3 × √3)-R 30° structure which is stable up to temperatures of 900K. These results are in agreement with ion-scattering experiments of thin films of Sn on Cu(111). For deposition of Sn on Cu(100), a 0.25 monolayer (ML) coverage results in the formation of a stable 2D alloy phase with a p(2 × 2) structure. This result is also in agreement with LEED measurements.
We present an investigation on the spatial compositional variation in InGaP layers grown by chemical beam epitaxy (CBE) on pre-patterned substrates. Neighboring regions with 190 meV bandgap discontinuity are observed with the growth at 500°C. The development of laser structures on V-grooves that incorporate these lateral heterostructures is achieved by controlling the growth temperature during growth