Surface Reaction Study On GaN MOVPE Growth

Gallium nitride（Ga N） is a kind of very important III-V direct band gap semiconductor material, which has advantages of wide bandgap, high breakdown electric field, high electron transfer rate and strong ability of anti radiation etc. Ga N is widely used in the production of LED, semiconductor laser and high-frequency and high-power microelectronic devices. MOVPE is the principal method used to grow thin film of Ga N. In the process of MOVPE grown Ga N films, H2 and N2 are used as carrier gases, and TMG and NH₃ are used as source gases. The source gas are carried into the reaction chamber by carrier gas, where the gas phase reactions and the surface reactions occur. The growth rate and uniformity of the films are determined by the gas phase reactions, while the surface morphology and growth quality of the films are determined by the surface reactions. For the surface reaction, the previous studies mainly aimed at c-plane Ga N MOVPE growth, Ga atoms and NH₃ were considered as adsorbed particles of the surface reaction. The adsorption of MMG and NH₃ molecules on m-plane or a-plane Ga N surface, and the mechanism are still not clear.The CASTEP module based on DFT theory was used to construct the super cell modle of m-plane and a-plane Ga N surface, and the adsorption of surface reaction precursor on the surface was optimized.Based on the calculation results, the characteristics of adsorption and bonding were analyzed. Based on the calculation results and analysis, the growth mechanism of Ga N thin film is proposed. In view of c-plane Ga N surface growth model and the surface of the island, the model is established, to analysis reasons of the island, in addition to analyze the impact of carrier gas on the surface morphology.The main work of this paper is as follows:（1）For the initial adsorption sites for Ga CH3 and NH₃ on m-plain Ga N surface, the optimized adsorption energy, atomic distance, density of states, charge density and electron population were all obtained.In 1×1 periodic, MMG and NH₃ have a optimal adsorption position. It was proved by the charge distribution and population that Ga atom of MMG and N atom of the adjacent surface, Ga atom of MMG and Ga atom of the adjacent surface formed Ga-N, Ga-Ga covalent bond respectively, while N atom of NH₃ and Ga atom of adjacent surface formed N-Ga covalent bond. The.H atoms are adsorbed on the N atoms of the Ga N surface, thus the N-H bond is formed. The adsorbed NH₃ tends to be dissociated from the energy point of view.（2）For the initial adsorption sites for Ga CH3 and NH₃ on a-plain Ga N surface, the optimized adsorption energy, atomic distance, density of states, charge density and electron population were all obtained. In 1×1 periodic, MMG and NH₃ both have two optimal adsorption position. Bonding between the particles and the surface atoms is the same as that of the m-plane Ga N.（3） Based on the adsorption characteristics and the bonding condition of MMG and NH₃ on a-plane Ga N surface,a-plane Ga N surface reaction grouped into four reaction equation was proposed, and a-plane Ga N films growth mechanism was proposed. A-plane Ga N surface was covered with H atoms, H atoms which adsorpted on the N atoms. NH₃ adsorpted onto the optimal sites, and then dissociated into NH2 and H atom which formed H2 away from the surface. MMG adsorbed on the surface, and then bonded with N atom of the surface, while H atoms adsorbed on N atoms and then formed H2 away from surface. MMG bonded with adjacent NH2 with the release of CH4. As a result,a new layer of atomic layer was grown, and the surface structure was the same as the initial.（4） For the island on c-plane Ga N surface,it was simplified as a step model. It was found that the total energy of particles adsorbed on the step or under the step was different. With higher step, the total energy difference increases. When the total energy difference increases to a certain extent, the particles no longer stable adsorption on the step, Ga N surface island size is no longer increasing. Under different growth conditions, sizes of islands are different. The proportion of H2 in carrier gas affects the surface reaction of H atom, affects particles onto adsorption surface, affects surface island size and surface roughness.