Reaction and transport processes in OMCVD : selective and group III-nitride growth
Author(s)Mihopoulos, Theodoros, 1969-
Reaction and transport processes in organometallic chemical vapor deposition
Klavs F. Jensen.
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Researchers have continued to explore light sources that are brighter, cheaper, more reliable, and emit light closer to natural sunlight than conventional incandescent and fluorescent lighting. The most recent advance in this direction is the fabrication of light emitting diodes (LEDs), and laser diodes that can emit in the short wavelength region of the visible spectrum (from green to violet). By using organometallic chemical vapor deposition (OMCVD) to fabricate thin films of the group III nitride materials (GaN, AIN, and InN), LEDs with lifetimes over 10,000 hours are now commercially available while rapid progress is being made towards a laser diode structure with a similar lifetime. These devices, coupled with the existing red, yellow, orange, and amber LEDs (based on AIInGaP, also grown by OMCVD) whose light-emission efficiency is already superior to incandescent lamps can lead to full color solid-sate light sources. OMCVD of AlGaInN involves complex chemistry and flow phenomena, which determine the quality of the deposited layers. Incorporation of significant concentrations of Al and In have proven difficult to achieve. The understanding of the dominant reaction pathways and their interaction with transport phenomena has been insufficient for design and optimization of nitride deposition processes. This thesis describes coupled finite element simulations of fluid flow, heat and mass transfer with emphasis on constructing kinetic mechanisms that incorporate all the chemistry information known by experimental studies and quantum chemistry calculations. A kinetic mechanism for GaN deposition is proposed. The model involves fast reaction between trimethylgallium and NH3 to form a Lewis type acid-base adduct which can dissociate or decompose at higher temperatures. The decomposition fragments can subsequently react to form dimer or trimer complexes in the gas phase containing multiple gallium-nitrogen bonds. Reaction rate parameters are obtained from quantum chemistry calculations in the literature and analysis of experimental data. The reaction mechanism is shown to be consistent with individual experimental observations, flow-tube decomposition studies, and growth rate temperature and pressure dependence in a horizontal OMCVD reactor as well as growth rate data in a close-spaced OMCVD reactor. The growth rate appears to be limited by GaN formation at low temperatures, mass transport at intermediate temperatures and GaN decomposition at temperatures higher than 1000°C. Dimer and trimer formation provide additional pathways of Ga supply to the surface at low temperatures and high pressures. In order to simulate nitride growth in a new OMCVD reactor with close-spaced-injector, a hierarchical simulation approach of fluid flow and mass transfer in such reactors was initially performed. Three-dimensional calculations establish that there is complete mixing in the gas phase, while the individual gas injectors dissipate within 5-8 mm from the reactor inlet under typical operating conditions. Two-dimensional parametric studies of growth rate and uniformity dependence on operating conditions and geometric factors were used to gain insight into the chamber performance. Regions of stagnation and rotating disk flows were delineated as a function of operating parameters. In the case of rotating disk flow, growth uniformity increases with pressure, contrary to the classical vertical rotating disk reactor response. A mechanism for AIN growth is also described. Formation of dimers and trimers in the gasphase is identified as the major pathway for decreased growth efficiency with decreasing pressure. An additional pathway involving nucleation and growth of oligomers from dimers and trimers, and ultimately particle formation, is consistent with decreased growth efficiency with increasing temperature. The kinetic model is consistent with experimental observations of temperature and pressure dependence of AIN growth rate in a horizontal hot-wall reactor and growth rate data for AlGaN in a close-spaced reactor. In agreement with experimental observations, the simulations predict AIN deposition in a close-spaced reactor under conditions that prohibit AIN growth in a horizontal reactor. Thin films of InxGalxN are used as the active region in the III-N devices. Thus, controlling the indium composition in a reproducible manner is imperative for III-N device fabrication. The solid indium mole fraction in InGaN is reported to independently depend on temperature, relative indium amount at the inlet, film growth rate, and carrier gas used. A simple trapping mechanism is proposed for InN growth in InGaN ternary alloys. In agreement with multiple experimental observations, the indium content appears to be controlled by competition between desorption kinetics and incorporation, the latter being determined by the GaN growth rate since InN is not stable under typical growth conditions. The effect of H2 carrier gas on indium mole fraction is also discussed. For laser diode fabrication in the III-Nitride system, selective area growth is used to deposit buffer layers with fewer dislocations. In addition to its recent use in the nitride system, selective area epitaxy has been pursued in OMCVD of III-V compound semiconductors in general. Quantitative understanding of selective epitaxy, in particular compositional variations arising in selective growth of ternary alloys such as InGaAs and InGaP that are currently not understood, is needed to realize advanced optoelectronic devices. A hierarchical modeling approach of selective area epitaxy is undertaken to identify the origins of growth rate enhancement and indium composition enrichment in the case of ternary InGa(As/P) growth. Simulations using the stagnant layer approach reveal that surface reaction rate differences give rise to the compositional modulation. A realistic fluid flow description in a vertical axisymmetric reactor is coupled with a simple kinetic mechanism for InGaAs/P deposition. Differences in homogeneous decomposition kinetics of In and Ga precursors give rise to different "effective" surface reaction rates that lead to the observed In-enrichment. The proposed model is in agreement with reports on the dependence of In-enrichment on operating parameters. Simulations show that, while the alloy deposition is limited by mass transport, differences in reaction rates are responsible for the composition variations in selective growth. Thus, the usefulness of reaction-transport models in elucidating the relative roles of different deposition pathways and gaining insight to the deposition process is demonstrated. Growth rate enhancement and In-enrichment model predictions are in excellent agreement with experimental data on lateral and axial dependence obtained in a horizontal reactor with a large masked area.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1999.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Department of Chemical Engineering
Massachusetts Institute of Technology