Study On Transport-Reaction Model For GaN MOCVD Growth

Metalorganic chemical vapor deposition (MOCVD) is a key technique for fabricating GaN-based semiconductor optoelectronic devices. The research on the modeling GaN-MOCVD process involves establishment of appropriate transport-reaction model, calculation and analysis of gas flow, temperature distribution, component concentration distribution, gas-phase reaction pathways in reaction chamber, and prediction of growth rate and uniformity of epitaxial layer. By using the results from modeling research, the reactor geometry and operating parameters can be optimized so as to improve the characteristics of epitaxial layer.This thesis focuses on the development and application of a transport-reaction model for GaN-MOCVD process. First, on the basis of comprehensive analysis of literatures, a complex transport-reaction model is proposed and validated; then the model is applied to simulate and analyze the gas-phase reaction pathways, film growth rate, TMG precursor’s utilization rate and parasitic reaction degree in MOCVD reactor. In addition, analytical models for TMG precursor’s utilization rates are established in horizontal and vertical MOCVD reactors, respectively, and the gas residence time in vertical reactor is derived.The major works completed in this thesis are as follows:1. A transport-reaction model for GaN-MOCVD process is proposed. First, a general transport model for MOCVD process is established, and then a kinetic mechanism involving6gas-phase reactions and5surface reactions is proposed based on literatures. The reaction model (i.e. kinetics mechanism) together with the transport model constitutes a complete GaN-MOCVD transport-reaction model. Trimethylgallium (TMG) and ammonia (NH3) as general precursors will cause complex gas-phase pre-reactions and form Lewis type acid-base adduct or polymer which will lead to the formation of nanoparticles having no contribution to the film growth. For this reason, the establishment of the chemical kinetic model is the keypoint and the difficulty in the whole modeling process. The innovation of the model lies in the polymer forming chemical reaction, which is a direct source of the nanoparticle’s nucleation in MOCVD reactor.2The growth rates of GaN film and reaction pathways in both rotating disk reactor (RDR) and horizontal reactor are simulated and analyzed based on the established transport-reaction model. By comparing the predicted growth rate with the experimental data it is found that they are in good agreement with each other (the prediction error for RDR is only4%), which proved the effectiveness of this model. In RDR, trimer concentration in gas phase is not neglectable; the reactions mainly follow the pathway of adduct’s reversible decomposition and TMG’s pyrolysis; the change of operating parameters such as pressure, substrate rotation speed, height of the reaction chamber, inlet velocity will not alter the dominance of the pyrolysis reaction pathway, but reduction of substrate temperature will lead to increased role of adduct reaction pathway. When the growth uniformity is ensured and an adequate supply of active N atoms is provided, it is found that the growth rate of GaN film can be improved by increasing pressure, elevating rotation speed of substrate, changing inlet type (annular separated inlets adopted as an example); while reduction of flow rate and chamber height helps to improve growth uniformity though. In horizontal reactor, trimer concentration in gas phase is so small that its impact is minimal;gas-phase reactions follows the same pathway of reversible adduct decomposition and TMG pyrolysis reaction as in RDR. Changing pressure does not change the dominance of the pyrolysis pathway, but increasing inlet velocity and reducing substrate temperature will lead to the increased role of adduct reaction pathway; the growth rate of GaN film can be improved by increasing flow rate.3The utilization rates of TMG precursor are calculated in both horizontal and rotating disk reactor based on the established transport-reaction model, and the influence of various operating parameters in both reactors are analyzed; analytical models of MO utilization rate combined with boundary layer theory for the two type reactors are established. The results show that, to improve TMG utilization in vertical reactor, the following methods should be taken:increasing pressure, elevating rotation speed of substrate, increasing the ratio of substrate to chamber diameter, reducing inlet velocity and improving inlet design (to make TMG source concentrated directly above the substrate). To improve TMG utilization in horizontal reactor, the following ways should be followed:minimizing inlet velocity, reducing chamber height, increasing the ratio of substrate to chamber width, increasing TMG precursor’s diffusion coefficient and increasing substrate length (or diameter).4The influence of various operating parameters on trimer formation and parasitic reaction in RDR is studied based on the proposed transport-reaction model; and the formula of gas residence time, which has a significant impact on the nanoparticles’growth in RDR, is derived. Simulation based on the model shows that the measures, including reducing pressure, increasing rotation speed of substrate, reducing chamber height, and separating inlets, are beneficial to inhibit parasitic reaction and to reduce the degree of trimer formation, thereby reducing nanoparticles’ nucleation. Calculation of residence time shows that the measures, including lowering pressure, increasing substrate temperature, lowering chamber height, increasing inlet velocity, and improving rotation speed of substrate, help to reduce gas residence time in the reactor, thereby inhibiting nanoparticles’growth.The key innovation points of this work are:(1) introducing trimer-forming reaction into the gas-phase reaction model to reflect the effect of nanoparticles on film growth;(2) establishing the analytical model of TMG precursor utilization for horizontal and vertical reactors to examine the impact of various factors on MO source utilization;(3) deriving the detailed formula for gas residence time in vertical reactor. The outcome of this work will provide references for optimization of MOCVD reactor’s design and operation.