The growth of high-quality InN and indium rich group III-nitride alloys are of crucial importance for the development of high-efficient energy conversion systems, THz emitters and detectors structures, as well as for high-speed linear/nonlinear optoelectronic elements. However, the fabrication of such device structures requires the development of growth systems with overlapping processing windows in order to construct high-quality monolithic integrated device structures. While gallium and aluminum rich group III-nitrides are being successfully grown by organometallic chemical vapor deposition (OMCVD), the growth of indium rich group III-nitrides presents a challenge due to the high volatility of atomic nitrogen compared to indium. In order to suppress the thermal decomposition at optimum processing temperatures, a new, unique high-pressure chemical vapor deposition (HPCVD) system has been developed, allowing the growth of InN at temperatures close to those used for gallium/aluminum-nitride alloys. The properties of InN layers grown in the laminar flow regime with reactor pressures up to 15 bar, are reported. Real-time optical characterization techniques have been applied to analyze gas phase species and are highly sensitive the InN nucleation and steady state growth, allowing the characterization of surface chemistry at a sub-monolayer level. The ex-situ analysis of the InN layers shows that the absorption edge in the InN shifts below 0.7 eV as the ammonia to TMI precursor flow ratio is lowered below 200. The results indicate that the absorption edge shift in InN is closely related to the In:N stoichiometry.

In search of novel approaches to produce new materials for electro-optic technologies, advances have been achieved in the development of computer models for vapor deposition reactors in space. Numerical simulations are invaluable tools for costly and difficult processes, such as those experiments designed for high pressures and microgravity conditions. Indium nitride is a candidate compound for high-speed laser and photo diodes for optical communication system, as well as for semiconductor lasers operating into the blue and ultraviolet regions. But InN and other nitride compounds exhibit large thermal decomposition at its optimum growth temperature. In addition, epitaxy at lower temperatures and subatmospheric pressures incorporates indium droplets into the InN films. However, surface stabilization data indicate that InN could be grown at 900 K in high nitrogen pressures, and microgravity could provide laminar flow conditions. Numerical models for chemical vapor deposition have been developed, coupling complex chemical kinetics with fluid dynamic properties.

In the course of a search for new thermostable acentric nonlinear optical crystalline materials, several heterocyclic imine derivatives were designed, with the general structure D-pi-A(D'). Introduction of a donor amino group (U) into the acceptor moiety was expected to bring H-bonds into their crystal structures, and so to elevate their melting points and assist in an acentric molecular packing. Six heterocycle-containing compounds of this type were prepared, single crystals were grown for five of them, and these crystals were characterized by X-ray analysis. A significant melting temperature elevation was found for all of the synthesized compounds. Three of the compounds were also found to crystallize in acentric space groups. One of the acentric compounds is built as a three-dimensional H-bonded molecular network. In the other two compounds, with very similar molecular structure, the molecules form one-dimensional H-bonded head-to-head associates (chains). These chains are parallel in two different crystallographic directions and form very unusual interpenetrating chain patterns in an acentric crystal. Two of the compounds crystallized with centrosymmetric molecular packing.

A procedure for calculating homolytic dissociation rate constants is reported for modeling organometallic vapor-phase epitaxy (OMVPE) of III-V compounds for all pressure regimes. Reaction rate constants were predicted following a semiclassical approach based on quantum mechanical calculations and transition-state theory. The critical configuration was determined using linear interpolations for the geometry of the intermediate structures, Morse potentials for the intermediate electronic energies, and Hase's relationship for the vibrational frequencies that become annihilated. Low-pressure rate constants were calculated from Rice-Ramsperger-Kassel-Marcus (RRKM) theory following the Troe approach. The calculations were compared with experimental values for the dissociation of one methyl radical from the closed-shell molecules Al(CH3)(3), Ga(CH3)(3), and In(CH3)(3) and the radical molecules Ga(CH3)(2) and In(CH3) and for the dissociation of one hydrogen atom from NH3, PH3, and AsH3. A simplified system of reactions for the homolytic dissociation of In(CH3)(3) was modeled in an OMV reactor designed for the pressure range 10(-2) to 10(2) atm using computational fluid dynamics coupled with chemical kinetics. The steady-state simulations were carried out at 1000 K and at N-2 pressures of 1 and 20 atm.

Molecular and crystal structure of a series of derivatives of N,N-dimethyl-4-nitroaniline has been studied by both X-ray diffraction method and high-level ab initio calculations. According to these data, the dimethylamino groups were found to have a trigonal-pyramidal configuration and are considerably turned with respect to the ring plane in all molecules having a substituent in the ortho-position; on the contrary, this group is planar in the meta-substituted molecules. Topological analysis of the electron density function for all molecules studied within the framework of Bader's ‘atoms in molecules’ (AIM) theory revealed that introduction of a substituent into the ortho- or meta-position of the ring results in increasing of the contribution of the resonance forms different from the quinoid one. Contribution of the latter form is predominant for the structure of N,N-dimethyl-4-nitroaniline (1). Topological analysis of the electron density distribution was used to explain a decreasing of the molecular hyperpolarisabilites of the ortho- and meta-substituted compounds as compared with those for 1.

The structural, electronic, and thermochemical properties of indium compounds which are of interest in halide transport and organometallic chemical vapor deposition processes have been studied by ab initio and statistical thermodynamic methods. The compounds reported include: indium halides and hydrides (InF, InCl, InCl3, InH, InH2, InH3); indium clusters (In-2, In-3); methylindium, dimethylindium, and their hydrogen derivatives [In(CH3), In(CH3)H, In(CH3)H-2, In(CH3)(2), In(CH3)(2)H]; dimethylindium dimer [In-2(CH3)(4)] and trimethylindium [In(CH3)(3)]; dehydrogenated methyl-, dimethyl-, and trimethylindium [In(CH3)(2)CH2, In(CH3)CH2. In(CH2)]; trimethylindium adducts with ammonia, trimethylamine and hydrazine [(CH3)(3)In:NH3, (CH3)(3)In:N(CH3)(3), (CH3)(3)In:N(H-2)N(H-2)]; dimethylamino-indium and methylimino-indium [In(CH3)(2)(NH2). In(CH3)(NH)]; indium nitride and indium nitride dimer (InN, In2N2); indium phosphide, -arsenide, and -antimonide (InP, InAs, InSb). The predicted electronic properties are based on density functional theory calculations; the calculated thermodynamic properties are reported following the format of the JANAF (Joint Army, Navy, NASA, Air Force) Tables. Equilibrium compositions at two temperatures (298 and 1000K) have been analyzed for groups of competing simultaneous reactions.

A search for potential non-linear optical (NLO) compounds has been performed using the Cambridge Structural Database and molecular modeling. We have studied a series of monosubstituted derivatives of dicyanovinylbenzene as the NLO properties of one of its derivatives (o-methoxy-dicyanovinylbenzene, DIVA) were described earlier. The molecular geometry in the series of the compounds studied was investigated with an X-ray analysis and discussed along with results of molecular mechanics and ab initio quantum chemical calculations. The influence of crystal packing on the molecular planarity has been revealed. Two new compounds from the series studied were found to be active in the second harmonic generation (SHG) in the powder state. The measurements of the SHG efficiency have shown that the o-F-, and p-Cl-derivatives of dicyanovinylbenzene are about 10 and 20 times more active than urea, respectively. The peculiarities of crystal structure formation in the framework of balance between the van der Waals and electrostatic interactions have been discussed. The crystal morphology of DIVA and two new SHG-active compounds have been calculated on the basis of the known crystal structures.

An advanced computational model is being developed to predict the formation of indium nitride (InN) film from the reaction of trimethylindium (In(CH 3 ) 3 ) with ammonia (NH 3 ). The components are introduced into the reactor in the gas phase within a background of molecular nitrogen (N 2 ). Organometallic chemical vapor deposition occurs on a heated sapphire surface. The model simulates heat and mass transport with gas and surface chemistry under steady state and pulsed conditions. The development and validation of an accurate model for the interactions between the diffusion of gas phase species and surface kinetics is essential to enable the regulation of the process in order to produce a low defect material. The validation of the model will be performed in concert with a NASA-North Carolina State University project.

The purpose of this paper is to review modeling and real-time monitoring by robust methods of reflectance spectroscopy of organometallic chemical vapor deposition processes in extreme regimes of pressure. The merits of p-polarized reflectance spectroscopy under the conditions of chemical beam epitaxy (CBE) and of internal transmission spectroscopy and principal angle spectroscopy at high pressure are assessed. In order to extend OMCVD to materials that exhibit large thermal decomposition pressure at their optimum growth temperature we have designed and built a differentially-pressure-controlled (DCP) OMCVD reactor for use at pressures ≤ 6 atm. We also describe a compact hard-shell (CHS) reactor for extending the pressure range to 100 atm. At such very high pressure the decomposition of source vapors occurs in the vapor phase, and is coupled to flow dynamics and transport. Rate constants for homogeneous gas phase reactions can be predicted based on a combination of first principles and semi-empirical calculations. The pressure dependence of unimolecular rate constants is described by RRKM theory, but requires variational and anharmonicity corrections not included in presently available calculations with the exception of ammonia decomposition. Commercial codes that include chemical reactions and transport exist, but do not adequately cover at present the kinetics of heteroepitaxial crystal growth.

The static third-order polarizabilities (gamma) of C-60, C-70, five isomers of C-78 and two isomers of C-84 were analyzed in terms of three properties, from a geometric point of view: symmetry; aromaticity; and size. The polarizability values were based on the finite field approximation using a semiempirical Hamiltonian (AMI) and applied to molecular structures obtained from density functional theory calculations. Symmetry was characterized by the molecular group order. The selection of six-member rings as aromatic was determined from an analysis of bond lengths. Maximum interatomic distance and surface area were the parameters considered with respect to size. Based on triple linear regression analysis, it was found that the static linear polarizability (alpha) and gamma in these molecules respond differently to geometrical properties: alpha depends almost exclusively on surface area while gamma is affected by a combination of number of aromatic rings, length and group order, in decreasing importance. In the case of alpha, valence electron contributions provide the same information as all-electron estimates. For gamma, the best correlation coefficients are obtained when all-electron estimates are used and when the dependent parameter is ln(gamma) instead of gamma. Published by Elsevier Science B.V.

Geometry optimizations were performed for singlet, triplet, and quintet states on the planar structures (in C-2h,, and C-2v, symmetries) of the diacetylene dimer, using restricted open-shell Hartree-Fock (ROHF), unrestricted Hartree-Fock (UHF), and unrestricted hybrid density functional theory (UB3LYP) methods, with 6-31G(d) and 6-311G(d, p) basis sets. The (1)A(g) state of the planar van der Waals dimer is lower in energy than are any covalently bonded dimers. At our best B3LYP/6-311G(d, p) level, the most stable covalently bonded diacetylene dimer is the B-3(u) State in C-2h, symmetry, 11 kcal mol(-1) above the van der Waals dimer, followed by the B-3(2) state in C-2v, symmetry with 13 kcal mol(-1) above the van der Waals dimer. Both structures were confirmed to be local minima. The two diacetylene monomers of these structures are bridged through a single bond and they exhibit a small bend at the neighboring carbons to the bridge, trans to the hydrogens. The B-1(u), and (5)A(g) states in C-2h,, and the B-1(2) and (5)A(1) states in C-2v are between 39 and 43 kcal mol(-1) above the van der Waals dimer. (C) 1998 John Wiley & Sons, Inc.

X-ray single-crystal study, molecular mechanics calculations, and calculations of the static nonlinear polarizabilities (beta and gamma) were performed for dicyanovinylbenzene and series of its mono- and dimethoxy-substituted derivatives. X-ray analysis has been done for dicyanovinylbenzene, its o- and p-methoxy- and 2,4-dimethoxy-substituted derivatives together with corresponding EFISH measurements of the beta values for these compounds. Nonlinear polarizabilities were calculated for all series of the mono- and disubstituted methoxy derivatives of the parent compound with modified finite-field formalism that included calculation of polarization values versus static electric fields using semiempirical Hamiltonian, polynomial fit of all tensor elements of beta and gamma on the former data, and evaluation of the numerical instability of the calculations. All calculations were performed with optimized molecular geometries taken from X-ray data, molecular mechanics, nb initio, and semiempirical quantum chemical results. Good correlation was found between the predicted static and experimental molecular values of beta. A factor of 2.0 was found to be a probable adjustment parameter to account for the solvent (1,4-dioxane) and dispersion (at 1064 nm) effects. Crystal packing analysis of the X-ray structures studied together with energetic calculations revealed the factors responsible for formation of centrosymmetric crystals. Only o-methoxydicyanovinylbenzene (also known as DIVA) forms acentric crystals (space group P2(1)) among the experimentally studied compounds, and it was found that its molecular dipole moment orientation with respect to the polar crystal axis is close to the "optimal" for a manifestation of the high NLO responses.

We have implemented a procedure to calculate static electronic molecular third-order polarizabilities for large molecules. The property is obtained semiempirically, using the finite-field formalism and either external or implicit fields, based on structures optimized by semiempirical, ab initio, or molecular mechanics methods. The numerical instability in the property is estimated, and various parameters can be modified to improve the uncertainty. The procedure involves first the calculation of the valence electron contribution to the property and then includes an estimate of the contribution from the electron cores.

A series of twenty-four Schiff's bases was synthesized and nonresonant static molecular second order polarizabilities (β) of these compounds were theoretically calculated and compared with experimental values. The computational method employed obtained: (a) values of polarization versus static electric fields using a semiempirical Hamiltonian; (b) all tensor elements of β by performing polynomial fits of the former data, within the finite-field approach. The experimental values were obtained using a modified electric field induced second harmonic generation (EFISH) experiment with 1,4-dioxane as the solvent. The measured quantities were the projection of β on μ (the permanent dipole moment), relative to MNA (2-methyl-4-nitroaniline). The correlation between the predicted static molecular quantities and their corresponding experimental values was 0.95 (based on a simple least-squares regression forced through the origin). A factor of 8.7 ± 0.3 was determined to be the adjustment parameter for Schiff's bases to account for the solvent and dispersion effects at the fundamental wavelength of 1064 nm.

This investigation explores the effect that aromatic subgroups have on the nonlinear optical properties of highly conjugated multi-dimensional molecules. In particular, carbon-cage fullerenes, porphyrins and phthalocyanines have been studied. The optimized geometries were determined from all-electron ab-initio calculations. The nonlinear properties were obtained using the finite field approximation. Data of polarization versus static electric field was obtained from valence-electron semi-empirical calculations using the AM1 Hamiltonian. The static electric fields were created using a variety of conditions. Polynomial fits were performed with 14 to 400 data points. The nonlinear properties were extracted from expansions of order four to sixteen. These last three conditions allowed estimation and minimization of the uncertainty in the results. Aromaticity was evaluated by analyzing the molecular geometry.