Simulation And Optimization Of ZnO MOCVD Growth

Zinc Oxide(ZnO) is a II-VI compound direct-gap semiconductor with wurtzite structure. Because of its wide band gap of3.37eV at room temperature and large exciting binding energy of60meV, ZnO is a promising material for efficiency short-wavelength optoelectronic light-emitting devices and detectors. Metal-organic chemical vapor deposition (MOCVD) is a promising method to grow high-quality ZnO films nowadays, because of its large production scale, reproducibility, accurate control of components and high deposition rate. But lots of complicated physical and chemical changes occur in MOCVD system, and are still hard to understand for us. In order to grow high quality ZnO films through MOCVD, every physical and chemical process should be well studied.In this thesis, gas phase pre-reactions, which commonly exist in ZnO MOCVD process and deteriorate the structure and properties of films, has been studied from simulations and experiments in details. Solutions to suppress the gas phase pre-reaction and so to improve the quality of films have been proposed. Systematical Growth of ZnO buffer layers on sapphire substarte have been studied. Optimized growth parameters have been finally suggested for high qualiy ZnO growth. The results include:1.3D model of ZnO MOCVD has been established to investigate the behavior of the commonly existed gas phase pre-reactions, which are supposed to depend on the gas temperature and mixing content of reaction gases. Studies revealed that the flow rate of gas, the gap between the shower head and substrate, substrate temperature and convective heat transfer coefficient of cooling water have great impacts on the temperature distribution, which furtherly influences the pyrolysis of metal-oganics. When the gas temperature is high enough with the reaction gases fully mixed, the gas phase pre-reactions will occur before the reaction gases reach the substrate. Through the research of MOCVD parameters and conditions, larger gas flow rate, shortened distance between shower head and substrate, and increased cooling water flow rate are found to be effective ways to suppress the gas phase pre-reactions.2. Comparisons between DMZn and DEZn are made to determine a suitable Zn precursor for ZnO growth by MOCVD. Through in-situ quadrupole mass spectrometer mounted on MOCVD system, the dissociation behavior of the reaction precursors were studied at different substrate temperatures. The extent of gas phase pre-reactions are further studied under certain temperature between two groups of precursors, one using DEZn and O2, the other using DMZn and t-BuOH, as the zinc and oxygen precursors, respectively. The former group reacts violently because of the strong oxidability of O2, while the later group reacts moderately. The later group is then considered to be the preference precursors to grow high quality ZnO films with the gas phase pre-reactions suppressed.3. Using DMZn and t-BuOH as precursors, systematic ZnO growth by MOCVD method has been especially investigated. Growth experiments show hydrogen addition should be kept at a low content due to its strong etching effect on ZnO, although it could reduce carbon incorporation in ZnO films. Enhanced reactor pressure in the system may lead a smooth surface morphology and high quality of the film in unitary but the growth temperature should be carefully controlled. A proper growth temperature is found to be suitable for high quality ZnO growth with larger migration length of atoms on the growth surface and fully suppression of the gas phase pre-reactions. A lower growth rate is preferred to obtain smooth surface morphology, while a higher growth rate may be helpful for crystal quality, and may be of benefit to subsequent high temperature epilayer growth.