Fabrication And Properties Investigation Of The ZnO-based Thin Films

Zinc oxide with the hexagonal wurtzite crystal structure is the II–VI group compound semiconductor functional material. It has attracted much attention for its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV) at room temperature, which are higher than some traditional semiconductor materials. Combining the superior chemical stability, anti-oxidation, and resistance to the elevated temperatures properties, ZnO nanomaterial is regarded as one of the most promising materials for fabricating efficient ultraviolet (UV) light emitting. Combining the superior electrical and optoelectronic properties with large surface area and high orientation, ZnO nanomaterials especially in the form of well-aligned nanorod/nanowire arrays show extensive applications in luminescence, catalysts, piezoelectric transducers, gas sensors, and solar cells.Under proper fabrication condition and doped contents, ZnO exhibits excellent electro-conductive properties. Al-doped ZnO (ZAO) thin films, especially the highly (002)-oriented ZAO films, are emerging as alternative candidates for ITO films recently not only because of their comparable optical and electrical properties to ITO films, but also because of their higher thermal and chemical stability under the exposure to hydrogen plasma. It has been wildly used in solar cells, sensors, piezoelectric devices, and panel displays.The research works and innovations are as follows:1, Fabrication and the luminescence properties research of the well-aligned ZnO nanorod arrays(1) The uniform c-axis-oriented ZnO thin films were obtained on the glass substrates by a sol-gel technique. We studied the effect of the solution concentration, spin coating condition, and the thermal process (pre-heated and post-annealed temperature) on the structure of ZnO films. The average size of the grains and the thickness of the (002)-preferred oriented ZnO film were 50nm and 40nm, respectively. The best process conditions were as follows: the solution concentration (0.75 mol/L), the spin rate (4500 r/min), the pre-annealed treatment (250℃for 10 min) and the post-annealed temperature (500℃for 1 h).(2) Well aligned ZnO nanorods (nanowires) were synthesized via an improved hydrothermal technique. The polyethyleneimine (PEI), which is a non-polar polymer with a large amount of side amino-groups (-NH2), was innovatively added to the growth solution. The influences of the growth solution concentration, the sort and amount of additive as well as the growth time on the morphologies of the ZnO nanorods (the diameters, size distribution and the length) were studied. The high (002)-oriented ZnO nanorods with the diameters of 40nm and the length of 2.5μm were obtained in the 60 ml growth solution of 0.05M with the addition of 5 ml PEI. PEI plays a great effect on the improvement of the morphologies of the ZnO nanorods. We contribute this to the polar nature of ZnO crystal and the selective adsorption of PEI on various facets of ZnO single crystal nanorods. The positively charged PEI molecules should be adsorbed on the lateral facets of the ZnO nanorods due to the electrostatic affinity. Thus, the lateral growth of the nanorods can be largely limited.(3) The effects of the PEI and the thermal treatments on the PL emission of ZnO nanorod arrays were investigated. The changes of the intensities of the UV emission peak and the visible spectrum as well as the possible reason of the UV luminescence were discussed in details. The results show that the best annealing temperature was 350℃as annealed in the air. The UV emission intensity would increase as increasing the temperature bellow 350℃. This is due to O-H and N-H contained in the samples are decomposed and vaporized, and the influence of the N-H on the PL emission became bigger. The UV emission intensity would decrease with increasing the temperature higher than 350℃, because of the different intrinsic defects formed in different annealing temperature.2, Fabrication and the electric properties research of the Al doped ZnO (ZAO) thin films(1) High (002)-oriented ZAO thin films with low resistivities were fabricated on the glass substrates by a sol-gel technique. XRD, SEM, AFM and four point resistivity test system were used to characterize the structure and the resistivity of the ZAO thin films.(2) The process conditions (such as the Al3+ dopant concentration, the annealed treatment and the thickness of the film) for fabricating the ZAO films were optimized. Moreover, the influences of the structure of the ZAO films on the resistivity were investigated. The (002)-preferred oriented ZAO film with the resistivity as low as 8×10-2 ? cm was obtained by the process conditions as follows: solution concentration (0.3 mol/L), the Al3+ dopant concentration (2 at%), the annealed treatment (550℃), and the number of the layers (13 layers). The influences of the processing parameters and electro-conductive mechanism of the ZAO film were studied.