ZnO was grown on sapphire substrate by metal–organic chemical vapor deposition using the diethylzinc (DEZn) and oxygen (O2) as source chemicals at 500 °C. Influences of the chamber pressure and O2/DEZn ratio on the ZnO structural properties were discussed. It was found that the chamber pressure has significant effects on the morphology of ZnO and could result in various structures of ZnO including pyramid-like, worm-like, and columnar grain. When the chamber pressure was kept at 10 Torr, the lowest full width at half-maximum of ZnO (002) of 175 arc second can be obtained. On the other hand, by lowering the DEZn flow rate, the crystal quality of ZnO can be improved. Under high DEZn flow rate, the ZnO nanowall-network structures were found to grow vertically on the sapphire substrate without using any metal catalysts. It suggests that higher DEZn flow rate promotes three-dimensional growth mode resulting in increased surface roughness. Therefore, some tip on the ZnO surface could act as nucleation site. In this work, the growth process of our ZnO nanowall networks is said to follow the self-catalyzed growth mechanism under high-DEZn flow rate.
KeywordsZnO Chamber pressure O2/DEZn ratio Nanowall networks Self-catalyzed
ZnO is an attractive direct wide band gap (E g ~3.36 eV at 300 K) semiconductor material for applications in the short wavelength light-emitting devices in the blue to ultraviolet (UV) region . The interest in ZnO is fueled and fanned by its excellent properties such as good piezoelectric characteristics, chemical stability, and biocompatibility; and its potential applications in optoelectronics switches, near-UV lasers, and complex three-dimensional (3D) nanoscale systems [2, 3]. On the other hand, ZnO is also useful for many types of device such as surface acoustic wave devices, hydrogen-storage devices, transparent electrodes, transparent thin-film transistors, solar cells, and sensors [4–7]. Different methods have been used to synthesize various kinds of ZnO structures, for example, molecular beam epitaxy , metal–organic chemical vapor deposition (MOCVD) [9–11], thermal evaporation , and solution-phase process . Among these techniques, MOCVD has been used for high quality epitaxial growth of various semiconductors and oxides, which is the ideal production technology for growth of epitaxial ZnO thin films. However, unlike the relatively mature MOCVD technique for III–V compound semiconductor growth, research into MOCVD growth of ZnO is still in its early stages. Since MOCVD is believed to be one of the complicated methods for the epitaxial growth of ZnO thin films, it will be necessary to investigate the effects of growth parameters, e.g., growth temperature, chamber pressure, and flow ratio of group VI source gas to group II source gas (VI/II). Effects of growth temperature have been discussed in our previous work . This research is carried out to understand the structure and characteristics of ZnO grown on sapphire substrates under different chamber pressures and VI/II ratios. It was found that the ZnO nanowall-network structures were found to grow vertically on the sapphire substrate without using any metal catalysts under high-DEZn flow rate. According to the energy spectrum analysis, the growth of ZnO nanowll networks is said to follow the self-catalyzed growth mechanism.
The growth of ZnO was carried out by the MOCVD system reconstructed from the Emcore D-180 system. ZnO structure was deposited directly on thec-plane sapphire substrate. Diethylzinc (DEZn) and high-purity oxygen (O2) gas were used as the zinc precursor and the oxidizer. DEZn vessel was immersed in the bubbler at 17 °C while the pressure of the DEZn source was kept at 350 Torr. The growth parameters include the chamber pressure and the ratio of oxygen and zinc. Ar gas was used as a carrier gas and the total gas flow including DEZn, O2, and Ar in the chamber was fixed at 3000 sccm. The base parameters: growth temperature was 500 °C, oxygen flow rate was 600 sccm, Ar gas flow rate through the DEZn vessel was 20 sccm, rotation of disk was 1000 rpm, and growth time was 60 min. First of all, we changed the chamber pressure from 10 to 60 Torr. Then, the ratio of oxygen and zinc was adjusted from 200 to 500. At last, the flow of Ar gas through the DEZn vessel was changed from 10 to 40 sccm with the oxygen gas fixed at 600 sccm. The thickness of ZnO structure was measured using a Tencor-KLA (P-10) profilometer. X-ray diffraction (XRD; X’Pert Pro MRD) in the θ–2θ and rocking curve mode was carried out to identify the crystal quality and orientation of ZnO structure. The Cu Kα line (λ = 1.541874 Å) was used as the source and Ge (220) was used as the monochromator. The cross-sectional morphologies of ZnO structure were observed by scanning electron microscopy (SEM; Hitachi S-300H). The energy spectra were observed by field emission scanning electron microscopy/energy-dispersive X-ray spectroscopy (FE-SEM/EDS; JEOL JSM-6700F/OXFORD INCA ENERGY 400). The surface roughness of ZnO was analyzed using atomic force microscopy (AFM; Agilent 5400 AFM/SPM) and the measurements were accomplished with a Si cantilever for contact AFM, and the scan speed and scan area were 0.5 μm/s and 5 × 5 μm2, respectively. The photoluminescence (PL) was measured at room temperature by 325 nm line of a He–Cd laser (8 mW) as the excitation source.
Results and Discussion
The structure and morphology of ZnO was found to vary with the chamber pressure and the ratio of zinc and oxygen. By lowering the chamber pressure to 10 Torr and the DEZn flow rate to 10 sccm, the crystal and optical properties of ZnO can be improved. Under high-DEZn flow rate, the ZnO nanowall-network structures were found to grow vertically on the sapphire substrates without using any metal catalysts. This indicates that the ZnO grows in a 3D growth mode under higher DEZn flow rate. The 3D growth mode causes increasing surface roughness and thus forming the vertical ZnO nanowall networks. The tip on the ZnO surface may act as “nucleation site” to form nanowall-network structures following the self-catalyzed growth mechanism. The formation mechanism of the ZnO nanowall networks is different from the previous reports which use metal as catalysts.
This work was supported by the National Science Council and Ministry of Economic Affairs, Taiwan, Republic of China, with Grant Nos. NSC95-2221-E-005-131-MY3 and 97-EC-17-A-07-S1-097, respectively.