Numerical Study And Optimization Of GaN Thin-film Growth By MOCVD Method

Gallium nitride (GaN) is a direct bandgap semiconductor widely used inhigh-efficiency optoelectronic devices and high-temperature high-power electronicdevices due to its excellent optical properties. Metal Organic Chemical Vapor Deposition(MOCVD) method is the most important technique to fabricate high-grade GaN thin filmsfor applications in bright light-emitting diodes (LEDs) and semiconductor lasers. Study ofthe principles and characteristics of MOCVD technique is significant for improving thequality of epitaxial wafers, as well as for reducing the production costs of LED and otherGaN-based devices.The reactor is the key component of MOCVD system for GaN thin film growth.Study of the transport process inside the reactor is a major task of the epitaxy growth. Inthis thesis, a systematic study has been performed to investigate and optimize the growthprocess in a typical close coupled showerhead (CCS) MOCVD reactor. First, relevanttheories for study of the process have been introduced. Then, the flow field andtemperature distribution inside the reactor are simulated. The effects of reactor geometryand process parameters on GaN deposition rate and growth uniformity have further beeninvestigated. It is found that GaN growth rate is mainly affected by the concentration of(CH3)3Ga:NH3on the susceptor, and can be improved by increasing the distance betweenthe gas inlet and the susceptor, gas inlet velocity, and susceptor rotation rate, or bydecreasing gas inlet temperature. Growth uniformity is mainly influenced by recirculatingflows above the susceptor caused by natural convection. Increase of the gas inlet size andgas inlet velocity, and decrease of the distance between the gas inlet and the susceptor arebeneficial for improving the growth uniformity. The effects of gas inlet temperature andsusceptor temperature on growth rate can be explained by two competing mechanisms.Increase of gas inlet temperature decreases the temperature gradients in the reactor and improves GaN growth rate; while high gas temperature leads to decomposition of(CH3)3Ga:NH3into Ga(CH3)3, thus lowering the growth rate. Increase of susceptortemperature enhances the temperature gradients in the reactor, and thus reduces the growthrate; yet high susceptor temperature is preferred for the fast reaction rates on the susceptorand result in high growth rate.Another task of this work is optimization of the reactor design. Higher growthefficiency can be achieved by optimizing the design of the reactor after introducing thenew conceptions of growth uniformity and growth efficiency. The optimized designs arefurther proposed. To verify the reliability of the results, three-dimensional simulation isalso carried out on the basis of two-dimensional simulation results.