Research On Preparation Of Transparent And Conductive ZnO: Al Films By Radical Assisted Magnetron Sputtering

The demand of transparent and conductive oxide (TCO) films has been highly stimulated by the appearance of fast growing solar voltaic,flat panel display and touch panel techniques. Transparent and conductive ZnO:Al (AZO) films has attracted lots of research interests and is considered as one of the best candidates for substitution of the traditional In2O3:Sn (ITO) films because of its excellent electrical and optical properties and cheap,abundant and non-toxic raw materials. Practical utilization of AZO films on thin film solar cells together with the aggravation of supply deficiency of In, which is the main raw material of ITO films, have provided the best opportunity for the industrialization and commercialization of AZO films.ZnO and AZO films were prepared by a large radical assisted magnetron sputtering apparatus RAS-1100C (Shincron Co.,Ltd.) with AZO ceramic targets, metallic zinc targets and aluminum targets and zinc-aluminum alloy targets. The relation between physics properties of these films, such as electrical and optical properties, microstructures, film stress, energy bandgap and reflective index were systematically studied. In the sputtering process of AZO ceramic targets, several physics quantities including resistivity were found to exhibit significant spatial distributions. Detail works were conducted to disclose the origins of these spatial distributions and find out that a bombarding implantation effect by negative oxygen ions was mainly responsible for these spatial inhomogeneities. As for the situation of sputtering processes of metallic targets, a detail observation of the microstructures and crystallinity evolution of the deposited zinc oxide films related to the sputtering powers and oxygen flow rates were carried out with a extra comparative study on re-sputtering phenomena which was caused also by energetic negative oxygen ions bombardments. Besides, a discussion on the defects states, behaviors and their influences on the properties of deposited films were given base on comparative experimental results of post annealing treatments of the deposited ZnO and AZO films under air or hydrogen atmospheres. We endeavored to shed lights on the deep mechanisms on the influences of sputtering parameters and deposition mode, which are beneficial for improvements of the properties of sputtering prepared AZO films.The main results and conclusions in this thesis are:1,We obtained AZO films with resistivity of 2.4x 10-3Ω·cm, carrier concentration of 2.49×1020 cm-3, mobility of 10.5cm2V-1s-1 and transmittance (at wavelength of 550nm) of 85% through optimizing sputtering parameters with AZO ceramic targets;2,The resistivity, carrier concentration, mobility, energy bandgap and reflective index for AZO films deposited on substrates fixed in different regions in front of the AZO ceramic targets were found to exhibited significant spatial distributions. The resistivity of AZO films deposited on substrates facing the erosion areas of targets were found to be high with maximum resistivity of 2.2×10-20Ω·cm while those deposited in the regions facing the non-sputtered areas were relatively much lower, with a minimum resistivity of 4.9×10-4Ω·cm. The gap between the resistivity maximum and minimum was as high as 45 times or more.3,We confirmed implantation effect by the bombardments of energetic negative oxygen ions as the main cause of resistivity (and other physics quantities) spatial distribution. The negative oxygen ions originated from the liberation of targets'O anions in the form of negative ions and obtained high energy under the acceleration by electrical field in the cathode fall. The bombardment of O" brought damages to crystalline grains and deteriorated the (0002) preferred oriented growth and introduced lots of oxygen interstitial defects in the deposited AZO films. The oxygen interstitial defects was mainly responsible for the degradation of electrical properties as they can trapped free electron carriers and cause lattice distortions and increase the neutral impurity scattering in AZO films, which ultimately led to decrease in carrier concentration and mobility and thus increased the resistivity of AZO films.4,AZO films deposited on the drum of RAS possess average physics properties as compared to those deposited on fixed substrates in different regions in front of the targets. Properties of AZO films were influenced by sputtering parameters through affecting the degree of bombardments by energetic negative oxygen ions and through changing of the thin film crystallinity.5,We have studied the microstructure and crystallinity evolution of zinc oxide films in detail in the sputtering process of metallic zinc targets with observation of both forward and backward deposited films. Microstructure zone models for both forward and backward deposited films related to sputtering power and oxygen flow rate have been obtained, which helps to further understand what is going on in the sputtering zone in typical sputtering process of RAS-1100C and helps to determine the suitable working parameters area for preparation of AZO transparent and conductive films by sputtering of metallic targets.6,We comparatively studied re-sputtering effect caused by the generation of energetic negative oxygen ions in the sputtering zone through different sputtering processes with several kinds of metallic targets. It was pointed out that although the re-sputtering effect was caused from the energetic negative oxygen ions bombardments, the two effects was not equal, as the former was simultaneously affected by the oxidation degree of the thin layers deposited in sputtering zone. Re-sputtering effect lowers the targets utilization efficiency and affects the aluminum content in the deposited AZO films. It was suggested that the re-sputtering effect should be intentionally avoid.7,Investment of the defects states and the influence of their behavior on the properties of AZO films were carried out through comparative post annealing experiments of ZnO and AZO films both in air and hydrogen gas ambient. The experimental results supported the interpretation of implanted oxygen interstitial defects as the main factors which determined the properties of the as-deposited AZO films. Meanwhile the reason for the limitation of properties of AZO films prepared by co-sputtering process with metallic zinc and aluminum targets were analyzed and discussed.8,The properties (both resistivity and visible light transmittance) of the deposited AZO films were found to be further improved with post-annealing treatments in hydrogen ambient. AZO films with resistivity of 4.5×10-4Ω·cm, carrier concentration of 5.15×1020cm-3, mobility of 24.6 cm2V-1s-1 and transmittance (at wavelength of 550nm) of 89% was obtained through 500℃annealing treatment for 2h in hydrogen ambient. The improved properties of AZO films are stable under storage in air conditions and thus are valuable for practical applications.