Studies On Growth And Doping Of ZnO Films

Currently thin films of zinc oxide (ZnO) have been widely used in various applications, such as UV detectors, LEDs, sensors, LDs and surface acoustic w ave devices because of their structural, optical and electrical properties. Zinc ox ide is a II -VI compound semiconductor with hexagonal wurtzite structure. Sim ilar to GaN, ZnO exhibits a hexagonal structure with a direct band gap of 3.37e V at room temperature. The exciton binding energy of ZnO (60meV) is much la rger than that of GaN (25meV) and the thermal energy at room temperature (26 meV) .These properties ensure an efficient exciton-involved lasing process at ro om temperature, which is expected to facilitate semiconductor lasers in the ultra violet spectral region with low-threshold current density and high efficiency.Many techniques have been used for the deposition of ZnO films, includin g vaporation, sputtering, pulsed-laser deposition (PLD), molecular beam epitax y (MBE), metal organic chemical vapor deposition (MOCVD) and spray pyroly sis. Magnetron sputtering has been the most popular technique for the growth o f many materials, especially for ZnO films. From a technological viewpoint, ma gnetron sputtering has several advantages over some other deposition technique s to grow ZnO films. In this work we report the preparation of pure and doped ZnO films by magnetron sputtering. N-doped films were prepared in O₂-N₂-Ar and O2-NH3-AJ atmosphere using Zinc as a target. We also report that Al-doped and N+Al-codoped and N+Fe-codoped ZnO films were grown by co-sputtering technique using Zinc and Aluminum (Ferrum) as targets. The structural and op tical properties of the ZnO films were described. In the meantime, we analyzed the properties of ZnO films and ZnO nanotubes grown by APCVD technology.The crystal structures were analyzed by XRD and AFM. The optical prope rties of ZnO Films were studied by Raman spectroscopy, Transmission and pho toluminescence (PL) spectra. The results of experiments demonstrate that high-quality films have been achieved by this technique.AFM showed the crystal quality of the thin films. Post-annealing of the fil m and improving the substrate temperature could lead to the remarkable improv ement of the morphology properties;The decrease of oxygen partial pressure an d the increase of sputtering time or power improved the dimension of ZnO film s;The films prepared by DC magnetron sputtering were with better C-axis orie ntation;The high-quality films could be achieved if the N2 partial pressure 0 wa s in reasonable range;N+Al-codoped samples showed a very uniform nanocryst alline structure. X-ray diffraction (XRD) pattern of the sample was shown. Onl y the (0002) diffraction peak at 34.42° appeared in the spectra, which indicate d that the film possess a c-axis orientation and hexagonal wurtzite crystal struct ure. It is believed that annealing temperature provides enough active thermal en ergy, promoting the formation of the larger hexagonal columnar grains along th e (0002) direction.Optical properties of these films were studied by absorption, excitation, ph otoluminescence (PL) and Raman spectroscopy. Absorption peak appeared at 3 60 and 363 nm for ZnO and doped films, respectively. The absorption edge shif ted to higher energies for the doped films. The samples by DC sputtering show ed stronger absorption in the UV region than the samples by RF sputtering. The transmission spectrum exhibited a high transmission and sharp absorption edge in the visible region. The transmission spectra revealed a characteristic absorpti on of excitons and a shift of the absorption edge for the doped ZnO films towar ds short wavelength for the Al-doped and N+Al-codoped samples. Different ga ses of N source and different ratios showed different influence on the transmitta nee. The absorption peak was assigned to the formation of excitons while the sh ifts of the absorption edge were attributed to the occupation of the energy levels near the bottom of conduction band and/or the top of valance band by electronsand/or holes due to doping. Photoluminescence peaks were observed at room t emperature, and we observed strong emission of free excitons. The N-doped sa mples showed stronger photoluminescence than the un-doped ones, but the ban d position of doped films shifted toward long wavelength. The PL peaks at 420 and 440 nm was obviously enhanced for the doped samples when excited at 36 0 nm. More PL bands appear and the strongest ones locate at 398 and 418 nm w ith 220 nm excitation. The peaks may well be assigned to the transitions of fre e- and trapped- excitons and 0 defect level to the defect level of Zn interstitials. The Raman spectrum of ZnO films by sputtering was similar to those of ZnO p ower and films by CVD.After optimizing the process of the sputtering, N+Fe-codoped ZnO films were grown by co-sputtering technique. The structure and optical properties of t he doped ZnO samples have been investigated. And the results showed that the films had good c-axis orientation and the optical properties such as the Raman peaks at about 437cm"1, and a high transmission and sharp absorption edge. Pos t-annealing could remarkably improve the properties of N+Fe-codoped ZnO fil ms.Nitrogen (N) had been regarded as the most promising and effective accept or dopant for ZnO films. After N-doping, we achieved high resistive films, the r esistivity was increased to 105 Q ? cm. Refractive index of ZnO samples were s tudied and showed the remarkable change for different doped films. N-doping i ncreased the refractive index of ZnO films. In the meantime, the doping media nisms were discussed. In the end, the ZnO-based semiconductor device, which was described in detail, was designed.