G. Theodorou

Theory of electronic and optical properties of 3C-SiC

We study the electronic and optical properties of cubic (3C) SiC, using a combination of first-principles and tight-binding electronic structure calculations. We employ pseudopotential density functional theory calculations, with appropriate corrections to the energy of conduction bands, to investigate the band structure of this material and obtain band gaps that are in agreement with experimental results. The optical properties are then studied within the framework of the empirical tight-binding model, which is fitted to reproduce the first-principles calculations. This approach allows for a thorough investigation of the dielectric functions, the reflectivity, and the refractive index. Critical points are identified and connected to the appropriate transitions in the band structure. The results are in good agreement with available experimental data. In addition, we investigate spin splitting effects.

Optical transitions of infinite and finite strained Si/Ge superlattices

A systematic study of the optical transitions of pseudomorphically strained (Si)n/(Ge)n superlattices grown on Si1−xGex(001) buffers is presented. The influence of period (n+m), synthesis (n/m), and strain on the transition energies and transition probabilities at the Γ point is studied. This is performed with the use of a realistic tight‐binding model in the three‐center representation. The transition energies and probabilities for the finite superlattices (Si)4/(Ge)4 inside Si and (Si)5/(Ge)5 superlattice inside Ge are also studied. It is proposed that the most promising material for optoelectronic applications is the strain‐symmetrized (Si)4/(Ge)6 strained layer superlattice.

SI-BASED NANOSTRUCTURE WAVEGUIDES

We investigate the possibility of constructing nanostructure waveguides using Si‐based materials. For this purpose we evaluate the dielectric function of strain‐symmetrized (Si)10−n/(Ge)n strained‐layer superlattices (SLS’s) with n=3, 5 and 7, and (Si)6/(Ge)4 SLS coherently grown on a Si(001) surface, as well as that for bulk Si1−n/10 Gen/10 alloys with n=3, 5 and 7, and bulk Si. In particular we explore the possibility of constructing waveguides using the materials (Si)6/(Ge)4 SLS, coherently grown on a Si(001) surface, and bulk Si. We investigate the case of planar waveguide structure, giving results concerning the propagation and penetration of the transverse magnetic modes.

CALCULATIONS OF ELECTRONIC AND OPTICAL PROPERTIES OF SI/GE ALLOYS AND SUPERLATTICES : APPLICATION TO PLANAR WAVEGUIDES

A systematic comparative study between the electronic and optical properties of the strain layer superlattices (Si)10−n/(Ge)n coherently grown on a Si1−n/10Gen/10(001) alloy surface and those of the corresponding bulk alloys Si1−n/10Gen/10 is presented. We find that the superlattices have a smaller fundamental gap than the corresponding alloys; also for energies smaller than 1.5 eV and polarization along the growth plane, the real part of the dielectric function, e1, for the SLS is larger than that of the corresponding alloy, while for perpendicular polarization, the two dielectric functions practically coincide. The utilization of this property for the construction of planar waveguides is investigated. In particular, the transverse electric modes of a waveguide consisting of a finite thickness SLS (Si)6/(Ge)4, sandwiched between two layers of the alloy Si0.6Ge0.4, are studied. No transverse magnetic modes exist in this structure.

Bonding in cubic and rhombohedral GeTe

The authors have investigated the bonding in cubic and rhombohedral GeTe by studying the charge density of each individual valence band, using the Baldereschi method. The calculated charge densities can be interpreted in terms of atomic orbitals. The orbitals that participate in the bonding are s Ge, p Te and p Ge. In cubic GeTe there are p orbitals along the three orthogonal axes forming polar sigma -bonds between the Ge and Te atoms. There is a pronounced admixture of s Ge states with p states in the second and the last valence band. This admixture is responsible for the rise of the last valence band, which makes the GeTe a narrow-gap semiconductor. For rhombohedral GeTe the bonding can be described in terms of one p orbital along the (111) direction and the three p orbitals orthogonal to the (111) direction. These orbitals interact and form an admixture of polar sigma - and pi -bonds. The bonding charge is greater for rhombohedral GeTe, resulting in an increase of the bonding and a lowering of the electronic energy of rhombohedral GeTe with respect to that of cubic GeTe. By calculation they find the dipole moment of the rhombohedral phase to be 1.4 D along the (111) direction.

Band structure of cubic and rhombohedral GeTe

The pseudopotential method is used to obtain the band structure and electronic density of states for both cubic and rhombohedral GeTe. We find that the minimum energy gap occurs along the line LK. Our calculations also show that the rhombohedral distortion lowers the electronic energy. In addition the reflectance spectra are obtained for the cubic GeTe and compared with experimental data.

Comparative Evaluation of the Bioactive Behavior of HA and ZrO2 Reinforced Coatings

The clinical use of plasma-sprayed hydroxyapatite (HA) coatings on metal implants has been widely investigated as the HA coating can achieve the firm and direct biological fixation with the surrounding bone tissue. It is shown in previous studies that the mechanical properties of HA coatings are improved by the addition of ZrO2 particles during the deposition of the coating on the substrate. Subsequently, the cohesive and adhesive strengths of plasma-sprayed hydroxyapatite (HA) coatings were strengthened by the ZrO2 particles addition as a reinforcing agent in the HA coating (HA+ZrO2 composite coating). The aim of the present work is to investigate and evaluate the in vitro bioactivity assessment of HA and HA/ZrO2 coatings, on stainless steel substrate, soaked in c-SBF, in order to study and compare their biological responses. The coatings were produced using vapor plasma spraying (VPS). The characterization of the surface of the coatings before and after soaking in SBF solution was performed using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and X-ray Diffraction analysis (XRD). All samples were smoothed before insertion in the medium and the in vitro bioactivity of all coating samples was tested in conventional Simulated Body Fluid (c-SBF) solution for various immersion times.

Electronic and optical properties of strained semiconductor films of groups IV and III-V materials

Publisher Summary This chapter reviews the effect of homogeneous strain on the electronic and optical properties of group IV materials and III–V compounds. The deformation potential theory for diamond and zinc blend structures under uniaxial and biaxial strain has been reviewed in the chapter. One of the many methods that are capable of achieving a microscopic description is the “tight-binding model.” This model is suitable for efficiently describing systems with a large number of atoms per unit cell and for unveiling the underlying physics of the problem. Si is an abundant and cheap material that dominates the area of microelectronics. However, the creation of advanced integrated circuits based on Si revealed the existence of different efficiency problems. The development of Si-based photonic components and subsequent integration of optic and electronic components on the same substrate creates optoelectronic integrated circuits and “superchips” that perform much better than optical circuits. Si-based optoelectronic technology finds applications in fiber-optic transmitters and receivers, optical computer integrations, optical controllers, information display panels, and numerous other devices. A relatively small and indirect band gap and low cartier mobility makes it unsuitable for use in photonic devices. The development of epitaxial techniques—such as molecular beam epitaxy and special kinds of chemical vapor phase epitaxy—have enabled to control the growth of individual semiconductor layers on an atomic scale.