Study on anisotropies and momentum densities in AlN, GaN and InN by positron annihilation

The independent particle model (IPM) coupled with empirical pseudopotential method (EPM) was used to compute the thermalized positron charge densities in specific family of binary tetrahedrally coordinated crystals of formula ANB8-N. Initial results show a clear asymmetrical positron charge distribution relative to the bond center. It is observed that the positron density is maximum in the open interstices and is excluded not only, from the ion cores but also to a considerable degree from the valence bonds. Electron-positron momentum densities are calculated for the (001,110) planes. The results are used to analyze the positron effects in AlN, GaN and InN compounds. Our computational technique provides the theoretical means of interpreting the k-space densities obtained experimentally using the twodimensional angular correlation of annihilation radiation (2D-ACAR).

Theoretical Studies of Structure and Doping of Hydrogenated Amorphous Silicon

ABSTRACTIn a-Si:H, large concentrations of B or P (of order 1%) are required to dope the material, suggesting that doping mechanisms are very different than for the crystal for which much smaller concentrations are required. In this paper, we report simulations on B and P introduced into realistic models of a-Si:H and a-Si, with concentrations ranging from 1.6% to 12.5% of B or P in the amorphous host. The results indicate that tetrahedral B and P are effective doping configurations in a-Si, but high impurity concentrations introduce many defect states. For a-Si:H, we report that both B(3,1) and P(3,1) (B or P atom bonded with three Si atoms and one H atom) are effective doping configurations. We investigate H passivation in both cases. For both B and P, there exists a “hydrogen poison range” of order 6 Å for which H in a bond-center site can suppress doping. For B doping, nearby H prefers to stay at the bond-center of Si-Si, leaves B four-fold and neutralizes the doping configuration; for P doping, nearby H spoils the doping by inducing a reconstruction rendering initially tetrahedral P three-fold.

STUDY ON MOMENTUM DENSITY IN SEMICONDUCTOR ALLOYS GeC and SnC BY POSITRON ANNIHILATION

The independent particle model (IPM) coupled with empirical pseudopotential method (EPM) was used to compute the thermalized positron charge densities in specific family of binary tetrahedrally coordinated crystals of formula ANB8-N . Initial results show a clear asymmetrical positron charge distribution relative to the bond center. It is observed that the positron density is maximum in the open interstices and is excluded not only from the ion cores but also to a considerable degree from the valence bonds. Electron-positron momentum densities are calculated for the (001, 110) planes. The results are used to analyze the positron effects in GeC and SnC . Our computational technique provides the theoretical means of interpreting the k-space densities obtained experimentally using the two-dimensional angular correlation of annihilation radiation (2D-ACAR).

Fracture Study of Ultrasonic Aluminum Wire Wedge Bonding on Copper Substrate with Mechanical Peeling-Off Method

In this paper, the characteristics of bond interface and bonding mechanism were investigated with peeling-off method. The fracture was observed and interfacial composition was certified by map scanning of EDX (Energy dispersive X-ray analysis). Based on the features of interfacial characters, the actual joining area mainly distributed at bond periphery; non-bonded at bond center. When the bonding time was lower, the ratio of the bond length to its width was larger and elemental aluminum distributed discontinuously on the bond fracture, primarily at the periphery. After aging, the fractures were also analyzed and Cu2Al3 intermetallic compound (IMC) was identified. The phenomena of bond interfacial tracings were analyzed, and the bonding mechanism was ascribed to plastic flow analyzed by finite element method based on the contact issues.

Comparative Analysis of Process of Diffusion of Interstitial Oxygen Atoms and Interstitial Hydrogen Molecules in Silicon and Germanium Crystals: Quantumchemical Simulation

A theoretical modeling and comparative analysis of the process of diffusion of the interstitial oxygen atoms and interstitial hydrogen molecules (H2) in silicon and germanium crystals at normal and hydrostatic pressure (HP) have been performed. The process of diffusion of particle with a strong interaction with a crystal lattice (interstitial oxygen atom) is a cooperative process. Three nearest Si (Ge) atoms of crystal lattice are involved in an elementary oxygen jump from a bond-center site to another bond-center site along a path in the (110) plane. It is precisely their optimum position (corresponding to a local minimum of the crystal total energy) determines the value of the diffusion parameters of an interstitial oxygen atom in silicon and germanium crystals. In a sense, the diffusion process may be considered as a diffusion process of qwasiparticle – (Oi+3Si). In the case of a particle weakly interacting with a crystal lattice (interstitial hydrogen molecules) we come up against the opposite case – the diffusion of H2 is not a cooperative process. The calculated values of the activation energy and pre-exponential factor for an interstitial oxygen atom δE(Si) = 2.59 eV, δE(Ge) = 2.05 eV, D0 (Si)= 0.28 cm2 s−1, D0 (Ge)= 0.39 cm2 s−1 and interstitial hydrogen molecule δE(Si) = 0.79 – 0.83 eV, δE(Ge) = 0.58 – 0.62 eV D0 (Si)= 7.4 10−4 cm2 s−1, D0 (Ge)= 6.510−4 cm2 s−1 are in an excellent agreement with experimental ones and for the first time describe perfectly an experimental temperature dependence of an interstitial oxygen atom and hydrogen molecules diffusion constant in Si crystals.