Analysis And Numerical Simulation Of Chemical Reaction Kinetics In GaN MOCVD

Metalorganic chemical vapor depositon (MOCVD) is a key technique for fabricating GaN thin film structures of light emitting and semiconductor laser diodes.As a general precursor, Ga (CH3)3 and NH3 caused complex gas-phase and surface reactions under high temperature, especially the pre-reaction which formed the Lewis acid-base adducts and polymer. These material condenses on the cold wall, caused the formation of the nanoparticles, reduced the quality of films, causing the precursor loss. Due to the complex chemical reaction has a great influence on MOCVD/MOVPE reactor design. Therefore, the full understanding of the GaN growth process about the reaction path has the significance for the reactor design,optimization, and the film quality.In this paper, on the basis of previous work, it is pointed out that different gas mixing and a heating caused different reaction paths. On this basis, analyzes the impact of the thermophoretic force on reactant concentration and the chemical reaction, the operating parameters and geometric parameters on the chemical reaction path. Then we gained some significance results:1.By examining carefully the experimental conditions by previous researchers, we found that, although similar pressure and temperature conditions were used (mostly in the range of MOCVD growth conditions), the methods of gas mixing and heating for TMG/NH3 in the MOCVD reactor inlets were different in different experiments, which provide some clues for solving the above controversies. The controversies about gas phase reactions in GaN MOCVD process are investigated with emphasis on the reactions after adduct formation. Different gas mixing and heating approaches in the reactor inlet are proposed to be the cause that lead to three different reaction paths:if mixing occurs at room temperature and heating is gradual (long residence time), the reversible dissociation of adduct TMG:NH3 into TMG and NH3 will dominate, followed by TMGa pyrolysis when further heated (Path 1); if mixing occurs at "warm" temperature (200～500℃)and heating is rather fast (short residence time), the irreversible adduct decomposition to form amide DMGNH2 or amide-derived particles will occur (Path 2); if mixing occurs at "hot" temperature (>500℃), and heating is very fast (short residence time), direct TMG pyrolysis will be the dominant reaction path (Path 3). 2. Numerical simulations for typical MOCVD reactors have verified that, in the vertical reactor with long reactor height the reactions will follow Path 1; in the premixed horizontal reactor both Path 1 and Path 2 exist, with Path 1 dominating the vicinity of susceptor and Path 2 dominating the vicinity of ceiling; and in the close spaced showerhead reactor the dominant reaction path will be Path 3.3. We firstly derived the expressions of thermophoretic force and thermophoretic velocity for GaN MOCVD growth. Then we derived the balance equation between thermophoretic velocity and diffusion velocity for TMGa molecules in a horizontal and vertical MOCVD reactor. From the calculation, at the same temperature gradient, the thermophoretic force of TMGa molecule is about 3.5 times of NH3 molecule, and 5.6 times of H2 molecule. The thermophoretic velocity and diffusion velocity are in the same order of magnitude, about 10-2～10-1 m/s, but in opposite direction. It was determined that most TMGa molecules reside at the position between the temperature of 500K and 800K. Numerical simulations are performed considering the case of gas transport only and the case with chemical reactions. The influences of thermophoretic force on deposition rate and concentration distribution are obtained for varying ceiling temperatures. Good correspondence is obtained by comparison of simulation results with experimental literature values. The results show that due to the influence of thermophoretic force, the concentration boundary layer of reactant particles in the horizontal reactor is about 3/4 of the reactor height. At last, a similar simulation is performed for a vertical MOCVD reactor. By comparison of the results for vertical reactor and horizontal reactor, we conclude that the derived basic principle of thermophoretic effects can be applied to the vertical reactor as well.4. Through changing the reactor geometry (such as height, the reactor diameter), control parameters (such as the inlet flow rate, pressure), analysis the impact reactor in chemical reaction path by geometrical size and control parameters. The numerical simulation results from the fluent are compared with experiment in the literature. We piont that reactor inlet flow increase and smaller reactor height, approaching pyrolysis reaction path; pressure is greater, approaches the adducts formation path.