Study Of Growth And Properties Of ZnO Films And MgZnO Alloy Films

ZnO, a wide direct-gap semiconductor, attracts as much attention as GaN in optoelectronics research field. Especially in 1996, the upsurge in ZnO research is coming due to the realization of the light pumped excition emitting. It has great uses in this information age. It can be used to fabricate display devices, high frequency filters, emitting diode, lasers and high speed optical switch. Therefore, it has great uses in both civil and military fields. In order to fabricate the above-mentioned devices and realize its uses, what we should do firstly is to get high quality ZnO film. A lot of methods have been used to deposit ZnO films such as Molecure Beam Epitaxy (MBE), Metal-organic Chemical Vapor Deposition (MOCVD), Pulsed Laser Deposition (PLD), Atomic Layer Epitaxy (ALE), Sputtering and Thermal Evaporation. At present, MOCVD, MBE, PLD and Sputtering are used by many research groups. Among them, MOCVD offers the advantages of industrial production, such as high growth rate and growth efficiency, large area uniformity and different doping processes. And Sputtering can deposit large area films of well controlled compositions economically and low enough for ultrathin films by changing the sputtering rate. In this thesis, high quality ZnO films are deposited on sapphire and Si substrates by MOCVD and Sputtering, and the properties of the samples are studied. N-ZnO/p-Si heterojunction photodiodes are fabricated and their characteristics are investigated. P-ZnO thin films are studied preliminary. The two different methods of doping N into ZnO are used to prepare p-ZnO films. MgxZn1-xO alloy thin films were successfully grown on c-plane sapphire substrates by MOCVD. The structural and optical properties of MgxZn1-xO films were studied. ZnO films are grown by MOCVD on C-plane sapphire substrates. The effects of the growth temperature (Tg) on the properties of the films are analysed and the optimized growth conditions are obtained. The results show that the ZnO film deposited at 600°C has the best crystallinity, surface morphology and optical property. In order to obtain ZnO films with bigger grain or monocrystal ZnO films, increasing the Tg and decreasing the growth rate properly are necessary. A suitable Tg facilitates the coalescence among different grains and the formation of the even films. The higher or lower Tg will cause the crystalline quality to descend. Because of the difference among the grain sizes at different Tg, the ultraviolet peaks of PL spectra shift to a length wavelength. At the same time, it is also observed that the absorption edge of the transmission spectra shifted to a length wavelength. All films are highly transparent in the visible range, and the transmission in the visible region reaches 90% for the ZnO film deposited at 600°C. ZnO films are grown by MOCVD on C-plane sapphire substrates with the buffer layer. It is found that the properties of the films depend on the Tg and thickness of the buffer layer. The optimized Tg of the buffer layer is 300°C. After the optimized Tg is obtained, the effects of the buffer layer thickness on the properties of the films are further studied. It is found that ZnO film deposited on a 15nm thick buffer layer has a good surface morphology, the structural and optical properties. The results can help to obtain the high-quality ZnO films by a proper thickness buffer layer deposition [Journal of Crystal Growth, 262 (2004) 456-460]. ZnO films are grown on C-plane and R-plane sapphire substrates bylow-pressure MOCVD. The epitaxial relationships for ZnO films grown on C and R sapphire substrates are (0002) ║(0006) Al2O3, [1010] ZnO║[112 0] Al2O3 and (112 0) ZnO║(0112) Al2O3 , [0001]ZnO║[0111] Al2O3, respectively. ZnO film grown on C sapphire has a columnar morphology, while the surface of the film grown on R sapphire is smooth and dense. ZnO film grown on C substrate has a better optical quality. The two ZnO films are highly transparent with a transmission ratio larger than 80% in the visible range, and the transmission in the visible region reaches 90% for the ZnO film grown on C sapphire[Optical materials, 27 (2004) 399-402]. ZnO films are deposited on Si(001) substrates by r.f. magnetron sputtering . It is found that all ZnO films have strongly c-oriented and the samples have better crystal quality when the r.f. power is increased and the value of k is changed. When k=3:1 and the r.f. power is 600w, the FWHM of ZnO(002) diffraction peak is as small as 0.3°. It is also found that the crystallinity of the film is good when the sputtering rate is low with the same r.f. power. The comparison among ZnO films deposited under different sputtering conditions indicates that crystalline quality is improved through changing the r.f. power and gas flux ratio of O2 to Ar. In the experiment, we firstly make the profile of depth, measure ZnO films in the various depths and do qualitative and quantitative calculation. The results indicate that the samples are uniform phase consistent with the positive stoichiometric ratio before etch reaching the interface, which shows this method can be used to deposit films well. The bonds of Si-O-Zn exist on the interface and n-Si is changed into p-Si on the interface. [Journal of Crystal Growth, 243 (2002) 439-443;Journal of Jilin University, 41, 493-496 (2003)]. ZnO thin films are grown on p-Si substrates at the temperature range of580-620℃by low-pressure MOCVD. The properties of the films are dependent on the depositon temperature. All films are strongly c-axis oriented, while the ZnO film deposited at 610°C has the best crystallinity. The about 755nm thick film is observed using a SEM. By means of XPS, it is confirmed that a thin SiO2 layer is formed on the Si substrate. The room temperature PL spectra of all films exhibit a strong NBE peak at 375nm corresponding to the band edge emission of ZnO[The 3rd International Workshop on ZnO and Related Materials, 116, Sendai, Japan, October 5-8, 2004]. N-ZnO/p-Si heterojunction photodiodes are firstly fabricated by MOCVD deposition of n-ZnO films on p-Si substrates in China. Ohmic contacts to the n-ZnO are made by applying indium (In) dots with a soldering iron, and to the p-Si by evaporating of aluminium (Al) and gold (Au). Typical rectifying behaviors are observed in all the n-ZnO/ p-Si heterojunctions. The forward currents of all heterojunctions increase rapidly with forward bias voltages. The turn-on voltage is above 1.5V. The reverse current increases linearly with the increase of the reverse bias voltage. This can be explained by the generation current in the depletion region. Their photoelectric effects from the diodes have been observed using a red light of 650 nm. Leakage current may be caused mainly by the thin SiO2 layer between n-ZnO and p-Si. The preparation of p-ZnO is studied preliminarily and the two different methods of doping N into ZnO films are used in our experiments. One method is that N2 and NH3 as doping source are used to dope N into ZnO films on C-sapphire substrates by the plasma operation equipment. The results show p-ZnO films are obtain by ionizing N2. The carrier concentration is 5.5×1015cm-3, the resistivity is 65.4?·cm, and the mobility is 17.3cm2/Vs. Only high resistivity ZnO films are obtained by ionizing NH3 due to the effect of H atom. Another method is...