Study Of Growth And Doping Of ZnO Films By Atmospheric Pressure-MOCVD

Zinc Oxide (ZnO) is one kind of important compound semiconductor photoelectric materials. It has a bandgap of 3.37 eV at room temperature (RT) and a large exciton binding energy of 60 meV, which provides an attractive prospect to highly efficient UV light emitters, light emitting diodes (LEDs) and low-threshold excitonic laser diodes (LDs). The greatest obstacle to harvest these advantages in real devices is posed by the realization of low resistant and reliable p-type ZnO. Although massive progress in growth of p type ZnO films has been made in recent years, the applicable ZnO light emitting devices have not been fabricated yet. Especially, it needs more research in the growth of ZnO and p-ZnO films by metal organic chemistry vapor deposition (MOCVD), otherwise it will be difficult to realize the bulk production of ZnO. Under such background, the first object of the dissertation is to grow device-quality ZnO films and the doping of ZnO so that pave the way for high performance ZnO light emitting devices.In this thesis, a home-built vertical atmospheric pressure-MOCVD(AP-MOCVD) system was used for the growth of the undoped ZnO, Al doped ZnO and NH₃ doped ZnO films using 6N-purity diethylZinc (DEZn) as Zn precursor, deionized water (H₂O) as O-precursors, 7N-purity nitrogen as the carrier gas, and two inch c-plane sapphire(c-Al₂O₃) as the substrate. Finally, the realization of ZnO thin film with p-type conductivity makes it successfully to fabricate of ZnO p-n homojunction. New results are as follows:1. A method was proposed to enhance the crystal quality of ZnO films by adding a small corrosive gas into the growth ambient of AP-MOCVD system. The effects of adding small H₂ and NH₃ gas into the growth ambient on the film properties have been studied. Experimental results showed that compared to the effect of small H₂, it was more effective on the improvements of the surface morphology, crystalline structure, and optical quality of ZnO epilayers by adding small NH₃ gas into the growth ambient. The full widths at half maximum (FWHMs) of the (0002) and (10-12) double crystal X-ray diffraction (DCXRD)ω-rocking curves of the ZnO film grown by adding NH₃ gas into the growth ambient were 152 arcsec and 253 arcsec, respectively, indicating the small mosaicity and low dislocation density of the film. It was very effective on the improvement of surface morphology of ZnO films by adding NH₃ gas into the growth ambient, which eventually leaded to a smooth surface with larger hexagonal grains (20μm). To the best of our knowledge, this is largest grain size report of ZnO film grown on c-Al₂O₃ by MOCVD, which was also just to manifest the characteristic of obtaining the big crystal grain size by APMOCVD.2. The effects of the thickness of high temperature buffer layer, the growth rate of epitaxial layer and the rich Zn environment on the properties of ZnO films grown by AP-MOCVD were investigated. By the optimized condition, the high growth rate of 4.2μm/h has been obtained, which is the highest value for ZnO films grown by MOCVD compared to literatures. It will be extremely advantageous to the next scale production of ZnO films due to the quick growth speed as well as the high crystallization quality.3. The high quality, the high transmittance of visible light and low resistance of Al doped ZnO(AZO) films was successfully prepared by AP-MOCVD on Al₂O₃(0001) substrate. The FWHM of AZO (0002) plane DCXRD omega rocking curve is 289", and (10-12) plane is 406". This is the first report of AZO film having the skew symmetry (10-12) plane DCXRD rocking curve result. The transmittance of visible light of the AZO film surpasses 90%, and the resistivity was about 9.72×10^-4Ω.cm. These parameters were equal to the best results of the AZO reported by the present literatures.4. The effects of the doped temperature, the doped mode and the doped flux of NH₃ on the properties of ZnO films grown by AP-MOCVD were investigated, which were considered to the important parameters. By the optimized condition, the p type ZnO has been obtained under the suitable doped temperature and flux. The experimental results of the metal-phase interference microscope and the DCXRD indicated that the p-ZnO film was the crystallization performance, as well as smooth surface. The FWHMs of the (0002) and (10-12) DCXRDω-rocking curves of p-ZnO were 246" and 368", respectively. The extremely strong DAP peak of the p-ZnO film could be observed in both the 10K PL and room temperature PL. The room temperature Hall measurement result indicated that the hole carrier concentration of p-ZnO film was 10¹⁶～10¹⁷cm^-3, and the mobility of hole was 3～10cm²/V.S.5. With the use of Ti/Au and the Ni/Au alloy as n and p type ZnO thin film ohm contact, respectively, ZnO homojunction has been fabricated by AP-MOCVD on the sapphire substrate by using the Al and N as the doping of the n and p type ZnO thin film, respectively. A typical ZnO homojunction shows rectifying behavior with a turn-on voltage of about 5 V, and the reverse voltage reaches as high as 20V, which obviously preceded the result reported by Japanese and the American researchers. To the best of our knowledge, this is best report of ZnO homojunction of the 20V reverse voltage according to literature. By the stabile test experiment of ZnO homojunction, it indicated that the p type ZnO doped with N was reliable.From the material growth method angle of AP-MOCVD system, the above results are the first reported.This work was supported by the 863 project (contract No. 2003AA302160) and China foundation for development of electronic information technology (contract No. 2004-125).