This award to Spelman College by the Solid State Materials Chemistry program in the Division of Materials Research is to model high pressure chemical vapor deposition aimed at the growth of indium and other group III nitride alloys. Computational models that couple complex chemical kinetics systems with fluid dynamics will be used to simulate the chemical vapor deposition of indium nitride and indium-rich group III-nitride at high pressures. The goal is to determine the optimal design reactor parameters and ambient conditions, and to understand the complex heterogeneous kinetics. A novel combination of theory, modeling and experiments will be used to develop and understand fundamental gas phase and interfacial reaction constants. The results coming out of the simulation experiments will be beneficial in interpreting the experimental results, and the potential pay-offs in basic science and in technology could be great. This research will be carried out in collaboration with Georgia Tech and Georgia State University. The proposal is aimed directly at areas of group III-nitride growth which are currently not understood and which are fertile ground both for good, solid basic science and for technological development.
This multi-disciplinary project will be an excellent training ground for undergraduate students at Spelman College in demonstrating the successful modeling of a complex growth process and experimental approaches. This close coupling of theory, modeling and experiment approaches could be appealing from an educational perspective. This project will expose undergraduate students early in their careers to the idea that theory, modeling and experiment can and should be done in unison. Interactions among students and faculty members at Spelman College, Georgia State University and Georgia Tech would be an added benefit to all involved in the project.
This project is of importance to the semiconductor industry, as well as to advance knowledge regarding the chemical mechanisms occurring during growth of semiconductor materials. The investigation generated theoretical reaction rate constants for adsorption of group III and group IV source materials onto gallium nitride and indium nitride substrates and their subsequent dissociation. The selected source materials are those commonly used to produce indium gallium nitride semiconductors. Indium gallium nitride (InGaN) semiconductors consist of alloys of gallium and indium nitride. Gallium nitride has a band gap of 3.4 eV and indium nitride of 0.7 eV and, by changing the ratio of indium to gallium, the band gap of the semiconductors can be tuned to different wavelengths, from the near ultraviolet to the infrared. InGaN semiconductors with low indium ratios and high band gap are utilized in commercial devices such as light-emitting diodes, laser diodes and photo detectors. High-quality crystal films can be obtained by molecular beam epitaxy (MBE) or by organometallic chemical vapor deposition (OMCVD). As the indium content in InGaN film increases, the growth temperature must be lowered because of the lower decomposition temperature of indium nitride compared to that of gallium nitride. However, growth of indium-rich InGaN has been achieved at higher growth temperatures by increasing the nitrogen pressure above the growth surface, in a process called high-pressure organometallic chemical vapor deposition (HP-OMCVD). Higher temperature leads to crystal films of better quality, and is more compatible to the temperature needed for processing complex heterostructures for devices. To optimize film quality and other properties during HP-OMCVD it is necessary to fine tune several parameters, such as temperature, pressure, and flow rate of the reactants and carrier gases. Modeling of the processes occurring in HP-OMCVD reactors requires knowledge of the reaction rate constants for all possible chemical reactions, including adsorption of source materials onto the substrate and dissociation of the source materials, both in the gas phase as well as adsorbed, under specific temperature and pressure conditions. This research has generated values for the adsorption rate constants of trimethylgallium, trimethylindium, and ammonia derivatives onto model gallium nitride and indium nitride substrates. In addition, rate constants have been predicted for the methyl and hydrogen dissociation of those three source materials, in the gas phase as well as adsorbed onto the model substrates. The predictions were done for pressures between 1 and 100 atm, and for temperatures ranging from 300 to 1400K. The model substrates were simulated using a quantum mechanical semiempirical approach; the free gaseous species were studied using density functional theory (DFT); and the adsorbate-substrate systems were investigated using a hybrid semiempirical/DFT approach. The reaction rate constants were obtained using a semiclassical approach that combines quantum mechanics and transition-state theory. The results of this investigation are contained in two articles of the Journal of Physical Chemistry C of the American Chemical Society (in 2009 and 2011). Four students from Spelman College participated in this project, a historical college for African-American women. These students learned how to perform calculations to predict important structural and thermodynamic properties of molecules, as well as chemical kinetic properties of reactions. Their studies were summarized in seven presentations at the National Conference of Undergraduate Research and seven papers of the Proceedings for the 2009, 2010 and 2011 years.
