Craig E. Moore

Growth of bulk single crystals of organic materials for nonlinear optical devices - An overview

Abstract Highly perfect single crystals of nonlinear optical organic materials are required for use in optical devices. An overview of the bulk crystal growth of these materials by melt, vapor, and solution processes is presented. Additionally, methods that may be used to purify starting materials, detect impurities at low levels, screen materials for crystal growth, and process grown crystals are discussed.

Advanced computational modeling for growing III-V materials in a high-pressure chemical vapor-deposition reactor

A numerical model was developed to simulate vapor deposition in high-pressure chemical vapor-deposition reactors, under different conditions of pressure, temperature, and flow rates. The model solved for steady-state gas-phase and heterogeneous chemical kinetic equations coupled with fluid dynamic equations within a three-dimensional grid simulating the actual reactor. The study was applied to indium nitride (InN) epitaxial growth. The steady-state model showed that at 1050-1290 K average substrate temperatures and 10 atm of total pressure, atomic indium (In) and monomethylindium [In(CH3)] were the main group III gaseous species, and undissociated ammonia (NH3) and amidogen (NH2) the main group V gaseous species. The results from numerical models with an inlet mixture of 0.73:0.04:0.23 mass fraction ratios for nitrogen gas (N2), NH3 and trimethylindium [In(CH3)3], respectively, and an initial flow rate of 0.17 m s-1, were compared with experimental values. Using a simple four-path surface reaction scheme, the numerical models yielded a growth rate of InN film of 0.027 μm per hour when the average substrate temperature was 1050 K and 0.094 μm per hour when the average substrate temperature was 1290 K. The experimental growth rate under similar flow ratios and reactor pressure, with a reactor temperature between 800 and 1150 K yielded an average growth rate of 0.081 μm per hour, comparing very well with the computed values.

Development of an advanced computational model for OMCVD of indium nitride

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.

Aromaticity and conjugation effects on the nonlinear optical properties of multidimensional molecules

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 AMI 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.

The growth of InN and related alloys by high-pressure CVD

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.