The role of vibrational–rotational coupling in V–V and V–R,T energy transfer

The effect of neglecting vibrational–rotational coupling in energy transfer calculations is studied for collisions of HF (v=1–7) with HF (v=0). An analog of a "classical path" method is considered in which rigid-rotor trajectories are used to determine a time-dependent forcing term on the vibrational motion of each molecule. The results are compared with our quasiclassical calculations in which no such approximation was used. At higher vibrational states the rigid-rotor forced-oscillator model is found to predict substantially smaller V–R,T rate constants than those found in the exact study

Cross-correlation trajectory study of V-V energy transfer in HF-HF and DF-DF

Results of a fully three‐dimensional classical trajectory calculation of vibrational energy transfer are presented for the collision of HF(v=1) with HF(v=1) and its deuterium analog. A cross‐correlation method, together with quasiclassical trajectories, is introduced to relate the changes in vibrational states of the two molecules to probabilities and rate constants. Multiple collisions are found to make an important contribution to the vibrational energy transfer cross‐sections for the present potential surface. Vibrational anharmonicity is shown to decrease the energy transfer rate constant by a factor of ten, by causing the process to be further from exact resonance. Excellent agreement with experiment is obtained for the HF–HF and DF–DF systems

Cross‐correlation trajectory study of vibrational relaxation of HF (v=1–7) by HF (v=0)

Results are presented for a three‐dimensional quasiclassical trajectory study of the vibrational deactivation of vibrationally excited HF (v=1–7) by ground vibrational HF. A cross‐correlation method of analysis is used to calculate probabilities and rate constants for V–V and V–RT transitions using trajectory results. Comparisons are made of calculated total deactivation rate constants (V–V plus V–R T) with experimental values. The V–R T dominates the relaxation for higher v states, and increases particularly rapidly with increasing v. Comparisons are made with recent classical‐path calculations for this system, and in the use of Morse versus equivalent harmonic oscillator potentials

Cross-correlation trajectory study of vibrational relaxation of DF(v = 1 to 7) by DF(v = 0) and of HF by HF

Three-dimensional quasiclassical trajectories and a cross-correlation method of analysis are used to study the vibrational relaxation of DF(v1 = 1 to 7) by DF(v2 = 0). Rate constants are calculated for V–V and V–R, T energy transfer. As was seen in earlier studies on HF–HF, the V–R, T mechanism becomes increasingly important at higher initial v1, as the V–V transfer moves further off resonance and also becomes increasingly endothermic. Both factors contribute to the decrease of V–V transfer rates with increasing v1 for the higher values of v1. Comparisons are made with results of a classical path study of vibrational relaxation in DF–DF and with experiment where possible. New results on the HF–HF V–V transfer rates are presented

Transport, Growth Mechanisms, and Material Quality in GaN Epitaxial Lateral Overgrowth

Growth kinetics, mechanisms, and material quality in GaN epitaxial lateral over-growth (ELO) were examined using a single mask of systematically varied patterns. A 2-D gas phase reaction/diffusion model describes how transport of the Ga precursor to the growth surface enhances the lateral rate in the early stages of growth. In agreement with SEM studies of truncated growth runs, the model also predicts the dramatic decrease in the lateral rate that occurs as GaN over-growth reduces the exposed area of the mask. At the point of convergence, a step-flow coalescence mechanism is observed to fill in the area between lateral growth-fronts. This alternative growth mode in which a secondary growth of GaN is nucleated along a single convergence line, may be responsible for producing smooth films observed to have uniform cathodoluminescence (CL) when using 1{micro}m nucleation zones. Although emission is comprised of both UV ({approximately}365nm) and yellow ({approximately}550nm) components, the spectra suggest these films have reduced concentrations of threading dislocations normally associated with non-radiative recombination centers and defects known to accompany growth-front convergence lines

SURFACE CHEMKIN-III: A Fortran package for analyzing heterogeneous chemical kinetics at a solid-surface - gas-phase interface

This document is the user`s manual for the SURFACE CHEMKIN-III package. Together with CHEMKIN-III, this software facilitates the formation, solution, and interpretation of problems involving elementary heterogeneous and gas-phase chemical kinetics in the presence of a solid surface. The package consists of two major software components: an Interpreter and a Surface Subroutine Library. The Interpreter is a program that reads a symbolic description of a user-specified chemical reaction mechanism. One output from the Interpreter is a data file that forms a link to the Surface Subroutine Library, which is a collection of about seventy modular Fortran subroutines that may be called from a user`s application code to return information on chemical production rates and thermodynamic properties. This version of SURFACE CHEMKIN-III includes many modifications to allow treatment of multi-fluid plasma systems, for example modeling the reactions of highly energetic ionic species with a surface. Optional rate expressions allow reaction rates to depend upon ion energy rather than a single thermodynamic temperature. In addition, subroutines treat temperature as an array, allowing an application code to define a different temperature for each species. This version of SURFACE CHEMKIN-III allows use of real (non-integer) stoichiometric coefficients; the reaction order with respect to species concentrations can also be specified independent of the reaction`s stoichiometric coefficients. Several different reaction mechanisms can be specified in the Interpreter input file through the new construct of multiple materials

Low-dislocation-density GaN from a single growth on a textured substrate

The density of threading dislocations (TD) in GaN grown directly on flat sapphire substrates is typically greater than 10{sup 9}/cm{sup 2}. Such high dislocation densities degrade both the electronic and photonic properties of the material. The density of dislocations can be decreased by orders of magnitude using cantilever epitaxy (CE), which employs prepatterned sapphire substrates to provide reduced-dimension mesa regions for nucleation and etched trenches between them for suspended lateral growth of GaN or AlGaN. The substrate is prepatterned with narrow lines and etched to a depth that permits coalescence of laterally growing III-N nucleated on the mesa surfaces before vertical growth fills the etched trench. Low dislocation densities typical of epitaxial lateral overgrowth (ELO) are obtained in the cantilever regions and the TD density is also reduced up to 1 micrometer from the edge of the support regions

Transport, Growth Mechanisms, and Material Quality in GaN Epitaxial Lateral Overgrowth

Growth kinetics, mechanisms, and material quality in GaN epitaxial lateral over-growth (ELO) were examined using a single mask of systematically varied patterns. A 2-D gas phase reaction/diffusion model describes how transport of the Ga precursor to the growth surface enhances the lateral rate in the early stages of growth. In agreement with SEM studies of truncated growth runs, the model also predicts the dramatic decrease in the lateral rate that occurs as GaN over-growth reduces the exposed area of the mask. At the point of convergence, a step-flow coalescence mechanism is observed to fill in the area between lateral growth-fronts. This alternative growth mode in which a secondary growth of GaN is nucleated along a single convergence line, may be responsible for producing smooth films observed to have uniform cathodoluminescence (CL) when using 1{micro}m nucleation zones. Although emission is comprised of both UV ({approximately}365nm) and yellow ({approximately}550nm) components, the spectra suggest these films have reduced concentrations of threading dislocations normally associated with non-radiative recombination centers and defects known to accompany growth-front convergence lines