H. M. Polatoglou

A modified empirical potential for energetic calculations of planar defects in GaN

Abstract The Stillinger–Weber empirical potential was modified and its parameters were determined to achieve a realistic description of the microscopic structure and the energetics of different planar defects and their interactions in wurtzite GaN. The formulation was based on the adjustment of the parameters in order to represent the Ga–Ga, N–N and Ga–N bonds. The input data comprises of the different crystalline phases of gallium, nitrogen and GaN. A satisfactory agreement on the values of the energy versus atomic volume per atom was obtained compared to those derived by ab initio calculations and experimental data for all the cases studied. By employing the modified Stillinger–Weber potential the energy of translation domain boundaries, which have been observed experimentally in GaN thin films, was calculated providing results comparable with ab initio calculations.

Study of the optical transitions in poly- and micro-crystalline Si by spectroscopic ellipsometry

Abstract Ellipsometric measurements in the energy range 1.66–5.6 eV were performed on poly- and micro-crystalline Si. The samples contained crystallites in the range of 5–150 nm, and were prepared with different techniques. The energies and broadenings of the optical interband transitions E0″, E1, E2(Σ), and E2(X) are considered as a function of the average crystallite size. These quantities exhibit a non-monotonic behaviour as the average crystallite size changes. The effects which can be responsible for this behaviour are discussed.

Investigation of the electronic transitions of cubic SiC

In this work we perform measurements with spectroscopic ellipsometry on the dielectric function of cubic bulk SiC (SiC(3C)) in the VUV region and theoretical band structure calculations on the material with the LMTO method. This is the first direct observation of the dielectric function in this energy range. The observed features are analyzed with standard lineshapes, the existence of four critical points (CPs) is revealed at energies 6.4, 6.9, 7.1 and 7.6 eV, and their temperature dependence was found to be similar to that of diamond. The calculated reflectivity from the dielectric function is compared to results from literature and certain ambiguities regarding the position and absolute values of the observed peaks are discussed. The LMTO calculations indicate the indirect gap to run along the Γ-X axis, while the first direct gap seems to be due to an electronic transition at the X-point, in contradiction to most published results. Finally, some ambiguities concerning the assignment of the interband transition of SiC(3C) are also discussed and commented.

Comparison of the constrained and unconstrained Monte-Carlo method: The case of Cu3Au

Abstract The order-disorder transition of Cu3Au is studied using the constrained and unconstrained (constant temperature and pressure) method. Two constrains are applied, namely the constant volume and the fixed atomic positions. A N-body potential is utilized determined from the zero temperature structural properties of fcc Au, Cu and Cu3Au. The transition temperature determined using the present potential and the constrained Monte-Carlo method agrees well with the experimental one. Releasing the constant volume constrain we obtain a change in the lattice constant at the transition temperature which is twice as large as the experimental data indicate. The unconstrained Monte-Carlo method predicts very well the change of the lattice constant and the mean-square displacement at the order-disorder transition. Only the transition temperature is found to be 25 % lower. The results indicate that the relaxation effects and the chemical effects are properly accounted by the N-body potential.

Optical properties of H terminated Si quantum wires

Abstract Recent work on Si-based materials, like Si/Ge superlattices, has shown that it is possible to obtain accurate electronic and optical properties using the empirical tight-binding method in the three-center representation. In the present study we extend the method for the case of Si quantum wires and investigate in detail their properties. Si quantum wires are believed to model the physics of porous Si. The interaction of the bulk Si can be transferred to the case of Si quantum wires. In this way we obtain an upshift of the energy band gap for different Si quantum wires, which is in agreement with recent ab initio results. Detailed study of the dielectric function and of the density of states reveals the effects of confinement and the one-dimensional character of the electronic states.

Microstructure of planar defects and their interactions in wurtzite GaN films

Abstract A formulation of the Stillinger–Weber empirical potential is presented and employed for energetic calculations of various planar defects in GaN. The energies of inversion domain boundaries and of the I1 stacking fault on the basal plane were calculated and compared to those of ab initio calculations from the literature. Present modification of empirical potential yielded comparable results with ab initio calculations. The potential was then applied for relaxation of large supercells comprising junction lines between inversion domain boundaries and stacking faults. The relaxed structures were used for HRTEM image simulations, which were compared with the corresponding experimental observations.

Wetting at the Σ = 5[001] twist boundary of Cu3Au

Abstract The Σ = 5 [001] twist boundary of Cu3Ayu is studied using the constant temperature, pressure and chemical potential difference Monte Carlo method. A N-body potential and bulk-like boundary conditions are utlized. The properties studied include the stoichiometry profile, the structure factor and the average position of the atoms along [001]. Various instantaneous atomic configurations are plotted. From these properties we find that the boundary is wetted for temperatures lower than the temperature of the bulk phase transition. The strain at the boundary is about 7% and it vanishes very quickly in about three atomic layers, with no long range strains. The last finding remains true even after the transition temperature. We find that the boundary energy changes a lot as the system passes the critical temperature. No noticable segregation of either Cu and Au is observed. The last results is due to the geometrical constrains of the boundary. From the values of the order-parameter away from the boundary it is found that the boundary should affect the bulk order-bulk transition temperature weakly.

Atomic-scale models of interactions between inversion domain boundaries and intrinsic basal stacking faults in GaN

Interactions between inversion domain boundaries (IDBs) and intrinsic basal stacking faults (SFs) in wurtzite-structured GaN grown on (0001) sapphire were studied using high-resolution electron microscopy (HREM) observations and empirical potential calculations. In one type of interaction, an SF terminates on an IDB, thereby inducing a transformation from a low-energy and electrically non-active IDB structure to a high-energy, electrically active one. In a second interaction, the SF crosses the IDB without changing its structure. These configurations were topologically established a priori, and were confirmed by experimental observations. It was found that the first junction line exhibits partial dislocation character, while the second is defect-free. Atomic-scale periodic supercells of the interactions have been produced and the relative energy values are discussed.

Effect of the [001] surface of Cu3Au on the order-disorder transition

Abstract Atomistic simulations of the two possible [001] surfaces of Cu3Au are performed for temperatures below the transition temperature. The method apply the constant temperature, pressure and chemical potential difference Monte Carlo method, while the potential is the N-body tight-binding in the second-moment approximation. At T=0 K the two possible [001] surfaces have very different surface energies and the results agree with previous calculations. On the other hand the relaxation of the first layer for both cases is outwards and of similar magnitude. For temperatures 20% below the transition temperature we observe segregation of Au to the first surface atomic plane independent from the initial surface termination. Also, one can observe that the layers close to the surface exhibit below the transition temperature more disordering than the bulk material. The results are discussed in terms of known experimental results and theoretical results.

Energy spectrum and oscillator strengths for spherical, conical and cylindrical CdSe quantum dots

We theoretically explore spherical, cylindrical and conical CdSe quantum dots (QDs) embedded in ZnSe. The electronic states close to the fundamental gap are of specific interest and depend on the details of the structures. Therefore, there is a need to have a very flexible method to study QDs of different shapes and sizes. To solve the Schr?dinger equation in the envelope function approximation, we employ the Finite Element Method (FEM). This is a very convenient method, and can be used to study QDs of any geometry and size. We have also implemented a procedure to calculate the oscillator strengths of the inter-band transitions and in this work we report on the electronic states and the optical transitions. Systematic comparisons are made between the studied QDs as well as comparison with experimental and other calculated findings.