Synthesis And Optical Properties Of Zinc Oxide Thin Films And Nanorods

The semiconductor ZnO has a wide band gap energy (3.37 eV) and a high exciton binding energy (60 meV) at room temperature. The wide band gap is suitable for short wavelength optoelectronic applications, and the high exciton binding energy can ensure efficient excitonic emission and ultraviolet (UV) luminescence at room temperature. Except for the above two merits, there are still some advantages over other wide-gap semiconductor, such as GaN, ZnSe, and ZnS. It exhibits high-energy radiation stability and amenability to wet chemical etching. The high-energy radiation stability makes it a very suitable candidate for space applications. ZnO is easily etched in all acids and alkalis, and this provides an opportunity for fabrication of small-size devices. In addition, ZnO has the same crystal structure and close lattice parameters to that of GaN and as a result can be used as a substrate for epitaxial growth of high-quality GaN films. Furthermore, ZnO is transparent to visible light, and by controlling the doping energy its electrical properties can be changed from insulator through n-type semiconductor to metal while maintaining optical transparency that makes it useful for transparent electrodes in flat-panel displays and solar cells. Owing to these above advantages and applications, ZnO materials with different morphology and structures (including films and structures) have gained substantial interests, and the availability of high-quality and high emission performance is hot at present.Several methods are usually used to synthesize ZnO films, such as metal organic chemical vapour deposition (MOVCD), molecular beam epitaxy, radio frequency magnetron sputtering, pulsed laser deposition and electrochemical deposition. ZnO structures can be obtained by vapor-liquid-solid (VLS) process, template assistant method and wet chemical route. Among all these methods, electrochemical deposition and wet chemical method possess most significant meanings for the fabrication of high quality ZnO films and structures, because they provide several advantages like low cost, large-scale deposition, low-temperature process and easily growth on different substrates. In this present work, ZnO films and rods were fabricated on the ITO coating substrates and bare glass substrates through electrochemical deposition and wet chemical method, respecticely. For the deposition of ZnO films, we took a two-step electrolysis method, which consist of a seed layer deposition on the substrate by a galvanostatic process and the subsequent film growth by a potentiostatic process. The effects of seed layer on the structure and property of ZnO thin films were investigated in detail. For the deposition of ZnO rods, for the sake of the best deposition process, the influences of the experimental parameters (including the reactant concentration and the growth time) on the microstructures, morphologies and the optical properties of ZnO rods were investigated in detail. In order to enhance the application of ZnO in photoelectric devices, the arbitrarily grown ZnO nanorods and the well-aligned ZnO nanorod array were obtained by the additions of PVP and through a PLD assistant wet chemical method, respectively. A new method was proposed to control the grain size of the ZnO rods by adjusting the concentration of polyvinylpyrrolidone (PVP) additive in the growth solution. In addition, Annealing on different temperature was done in order to improve the optical quality of the ZnO nanorods. The X-ray photoelectron spectroscopy (XPS), the room temperature and the variable temperature PL spectra were measured to reveal the possible defect emitting mechanism before and after annealing. The effect of the current densities of seed layers on the ZnO thin film, the possible mechanism of PVP effect on the growth of ZnO rods and the possible visible emission mechanism were discussed deeply and the main results are shown as follow:1. For the ZnO films, the grain size decreases with the increasing current density of seed layer. The surface morphologies of the ZnO films on the seed layers obtained under isl between -1.68 to -2.72 mA/cm2 are compact and flat and show uniform or narrow nanocrystalline grain size distribution. The optical transmittance and the PL performance if ZnO thin films exhibit a trend of firstly increase and then decrease as the current density of seed layer increases. It appears that the films on the sedd layers obtained under the current density of -1.68 to -2.72 mA/cm2 show high transmittance larger than 90%. The highest PL intensity ratio is obtained at the isl of -1.68 mA/cm2. According to the morphologies, optical transmittance and the PL properties, the optimum optical properties including both PL emission and visible light optical transmittance are observed from the film deposited on the seed layer under isl of–1.68 mA/cm2, which exhibits a fractional of (002) orientation and uniform and compact nanocrystalline structure.2. According to the effects of the reactant concentration and growth time on the surface morphology, crystalline structure and the photoluminescence properties. The well-facet ZnO rods were obtained after the concentration larger than 0.03 M. For the same growth time, the average diameter exhibits a firstly decreasing and then increasing tend, and the average length decreases with the increasing concentration. The average diameter and the crystalline quality of ZnO rods increase with the increasing growth time at the same concentration.3. For the ZnO rods aggregate grown arbitrarily on the substrate, according to the results of FESEM and XRD, it can be observed that Polyvinylpyrrolidone was added to control the grain size, the growth orientation and the density of ZnO nonorods. As the PVP concentration increase, the grain size shows a firstly decrease and the increase trend; the growth orientation is from perpendicular to random and then perpendicular again to the sunstrate; the density of ZnO rods shows a firstly increase then decrease trend. There must be a critical PVP concentration, at which the ZnO rods possess lowest grain size and the largest density. In our present experiment, At the concentration of 1.0 mM, the ZnO rods possesses the lowest rod diameter in nanoscale (although the average diameter is about 130 nm), the largest ratio of length to diameter (11.1) and the largest density (5×109/cm2). According to the results of FT-IR and FESEM, we speculate the effect of PVP. PVP can physical adsorb preferentially on the (101? 0) plane of ZnO, which would passivate this crystallographic plane and facilitate the crystal growth along the c-axis. Furthermore, coordinating the water from the dehydration reaction of the ZnO precursor, PVP may accelerate ZnO nucleation and promote ZnO crystallization at a moderate range of concentration. However, the viscosity of solution increases strongly with the increasing PVP concentrations, which greatly restricts the diffusion and growth process associated with the nanocrystal formation. So the density of rods decreases when the PVP concentration is higher than 1.0 mM and the nucleation is severely restrained at the concentration of 5.0-10 mM. At last we calculate that the highly dense ZnO rods in nanoscale with good crystalline quality were obtained, which exhibit high PL performance, at the polyvinylpyrrolidone concentration of 1.0 mM.4. For the highly aligned ZnO nanorod array, annealing temperature affects its PL prpperties. ZnO nanorod array with good crystallography, low defects concentration and good optical property representing by large PL intensity ratio (66.4) and the XPS results is obtained by annealing at 700°C for 1 hour. The low temperature (81 K) PL spectrum shows that the UV emission is associated with the bound exciton emission and the free exciton emission accompanied by longitudinal optical phonon replica emissions. The free exciton emission is almost enshrouded. The intensity of bound exciton emission decreases gradually with the increasing temperature. At the temperature of 218 K, the bound exciton emission is vanished, and the free exciton emission dominates the UV emission. According to the XPS results, all the relative atoms concentration ratios of Oa+Ob/Zn are larger than 1.0, so all the nanorod arrays were oxygen-rich. The Oa/Ob and the relative concentration of Oa increase after annealing, which may mean the involvement of abundant of defects in the as-grown ZnO nanorods, like VZ n or VZ n complexes, the existence of and surface trapping by chemisorbed oxygen. As a result of the results of XPS and PL spectra, we conclude that the green emission component was associated with VOiO and some surface defects in ZnO nanorods; the yellow-green emission was attributed to and yellow emission was owing to the presence of Zn(OH)Oi2 on the surface and some defect complexes like VO Zni.5. To sum up the above arguments, three kinds of ZnO were obtained to satisfy different applications, which is nanocrystalline ZnO thin films, randomly grown ZnO nanorods aggregation, and ZnO nanorod arrays. For all the three kinds of ZnO, the seed layer, addition of PVP and annealing temperature can effectively enhance the crystalline quality and the optical properties.