Study Of Growth And Doping Of ZnO Films By Plasma Assisted MOCVD

ZnO, a wide direct-gap semiconductor material, has attracted great interest recently because of its wide band-gap (3.37eV) and relatively large exciton binding energy (60meV) at room temperature (RT) and the production cost will be reduced significantly compared with current technology. It has been regarded as one of the most promising candidates for the next generation of short-wavelength light emitting diodes (LEDs) and lasing devices (LDs). Especially in 1996, the upsurge in ZnO research is coming due to the realization of the light pumped exciton emitting. ZnO can be used to fabricate emitting diode, ultraviolet detector, surface acoustic wave device, transparent electrode, thin film transistor, gas sensor and nano structure device. In order to realize the above-mentioned devices, it is necessary to grow high quality ZnO film and p type ZnO film by proper doping. A lot of methods have been used to grow ZnO films such as Sputtering, Pulsed Laser Deposition (PLD), Molecular Beam Epitaxy (MBE), Metal Organic Chemical Vapor Deposition (MOCVD). Among them, MOCVD offers the advantage of industrial production, such as high growth rate and efficiency, large area uniformity, prefect composition control and sharp interface. In this thesis, we grow high quality ZnO film on sapphire, silicon and gallium nitride epi-layer by new-type self-designed plasma-assisted MOCVD, and find a way of p type doping by using N2O plasma source. Then we fabricate p-ZnO/n-Si and p-GaN/n-ZnO heterojunction devices based on high quality p and n type ZnO.ZnO thin films had been grown in sapphire firstly by using MOCVD system. According to the growth temperature and O2 flow rate on ZnO films characteristics, we get the optimized growth condition. Temperature is the most crucial factor in the ZnO film growth, and proper growth temperature is in favor of the grains merger and the formation of thin films. As the growth temperature of 550℃and the O2 flow rate of 200~300sccm, ZnO sample has the best crystal quality, surface morphology and optical quality. For ZnO grown on c-sapphire, the FWHM of (002) X-ray diffraction peak was 0.187°, transmission ratio was more than 90%, and the intensity ratio of PL near band edge UV emission and deep level emission was as large as 433:1,which was the world's best results reported.We study the step-by-step annealing growth of ZnO films on c-sapphire. Step-by-step annealing growth was, after a period of growth time, ZnO film growth was suspended, and ZnO was experienced a period of annealing, then growth was restored. The quality of ZnO films has been further improved by step-by-step annealing growth, the FWHM of (002) X-ray diffraction peak was as narrow as 0.1699°, and the smallest RMS (root mean square) roughness measured by AFM (atomic force microscope) was down to 4.696nm. Compare with the samples under normal growth, ZnO samples under step-by-step annealing growth has better crystal quality and optical quality.Based on the growth condition of ZnO films on c-sapphire, we study the N-doping condition of ZnO films using RF ionized N2O plasma. Only at the right flow and temperature condition high quality p type ZnO samples can be made. Temperature is also the most crucial factor in the p-ZnO film growth. Best p type ZnO thin film had been grown at 430℃on c-sapphire with the resistivity of 8.71Ω?cm, hole mobility of 2.09 cm2/V?s and hole concentration of 3.44×1017 cm-3. XPS (X-ray Photoelectron Spectroscopy) was used to analyze the chemical composition of ZnO film. The XPS data proved that N-Zn bond exist in the film. All results show that N2O plasma is a high efficient source of N doping.With reference to the growth and doping conditions of ZnO films on c-sapphire, we also study growth and doping of ZnO films on (001) Si substrates. High quality undoped and N-doped p type ZnO films have been made on Si substrates, and we fabricate p-ZnO/n-Si heterojunction device. For ZnO grown on Si substrates, the FWHM of (002) diffraction peak was as narrow as 0.1602°, the intensity ratio of PL near band edge UV emission and deep level emission was as large as 327:1, which was the world's best results reported about ZnO films grown on Si substrates. The p-ZnO/n-Si heterojunction device showed that the device has good characteristics of the diode rectifier and photoelectric response characteristics. Turn-on voltage of devices was about 4.7V, at reverse voltage of-6V there is no significant increase in the reverse current. When there was light, reverse current increase in the device under the same reverse voltage, which clearly showed that the device has good photoelectric response characteristics.Using both Luming group's industrialized MOCVD system and our independent intellectual property rights MOCVD system, n-ZnO/p-GaN heterojunction light emitting diode was grown on (0001) sapphire substrate. An Mg doped GaN layer with a hole concentration of ~1017 cm-3 was used for the p-type side, and a unintentionally doped n-type ZnO layer with an electron concentration of ~1018 cm-3 for the n-type side of the heterojunction device. Distinct blue-violet electroluminescence emission with a dominant emission peak at ~ 415 nm was observed at room temperature. The origins of the EL emissions are discussed in comparison with the photoluminescence spectra,and it was supposed to be attributed to a radiative recombination in both n-ZnO and p-GaN layers. These results represent meaningful steps for developing ZnO-based optoelectronics devices. It is the first report of n-ZnO/p-GaN heterojunction LED grown by MOCVD and the phenomenon about radiative recombination in both n-ZnO and p-GaN layers.