Tailoring The Optical And Electrical Properties Of ZnO:Al Thin Films Prepared By Magnetron Sputtering Through Defect Manipulation And The Physics Behind

ZnO-based thin films have attracted widespread attention.The extraordinary features of ZnO,including wide band gap,high exciton recombination energy,abundance and low cost,make it promising in many applications.The main factors limiting the application of ZnO-based thin films are:(1)how to control the defects and(2)how to prepare stable p-type ZnO films.The former factor influences the optical and electrical properties of ZnO-based thin films.For example,the carrier concentration and mobility of Al-doped ZnO films are still much lower than those of Sn-doped In2O3(ITO).The latter factor is important for the preparation of ZnO-based homojunctions.For now the performance of ZnO-based homojunction is still far from satisfactory.As a result,overcoming these two obstacles should be a key for the rapid development of application of ZnO-based thin films.Magnetron sputtering is an advanced technique to prepare thin films.It is quite suitable for industrial production because large-scale dense thin films with high adhersion and repeatability can be achieved.But on the other hand,the bombardment of highly energetic ions on the film will bring in multiple defects and influence the microstructures of films when preparing oxide films by magnetron sputtering.For ZnO-based semiconducting thin films,their optical and electrical properties are closely related to the defects and microstructures.Therefore,it has both scientific and technological significance to control the defects and microstructures in ZnO-based thin films to achieve higher performance of films.The background of this dissertation are transparent conducting oxide(TCO)films and p-type ZnO.We focused on Al-doped and Al-N co-doped ZnO thin films.Through tailoring the defects in these films,we investigated the main factors influencing the optical and electrical properties of ZnO-based thin films prepared by magnetron sputtering,as well as the physics behind.Our main findings include:1.We investigated the influences of defects evolvement on the properties of sputtering deposited ZnO:Al films upon hydrogen annealing.High resolution transmission electron microscopy observations reveal that annealing at-300℃induces the formation of partial dislocations(PD)and stacking faults(SF),which disrupt the lattice periodicity leading to decreased grain size.Annealing at temperatures above～500℃can remove the PD and SF,but large number of zinc vacancies will be generated.Our results show that when films are annealed at～500℃,the oxygen-related defects(interstitials Oi.etc.)in the as-grown films can be remarkably removed or converted,which lead to increments in the carrier concentration,mobility,and the transmittance in the visible range.At annealing temperatures above 550℃,the hydrogen etching effect becomes predominant,and Al donors are deactivated by zinc vacancies.We also find an abnormal endothermic process by thermal analysis and an abnormal increase in the resistivity during heating the sample under hydrogen atmosphere,based on which the interaction of Oi with the defects(mainly Al donors and PD)is discussed.It is also demonstrated that by annealing the as-grown AZO films at～500℃under hydrogen atmosphere,high performance TCO films with a low resistivity of 4.48×10-4Ωcm and high transmittance of above 90%in the visible light are obtained.Such AZO films have already met the criteria for some of the TCO applications.2.We proposed an Oi mediated doping effect of Al in ZnO:Al films.The concentration of oxygen interstitials(Oi)in the as-grown films was tailored by changing the oxygen partial pressure during sputtering.Although the Al donors were temporarily passivated by Oi,they were easily reactivated through the removal of Oi by post-annealing in hydrogen at 500℃.Our results show that the as-grown film which had the highest Oi concentration turned out to have the highest carrier concentration after hydrogen annealing,and the doping efficiency was increased by～10%because of the Oi mediated doping effect.We infer that although Oi deactivate Al donors in the as-grown films,they favor the distribution of the doped A1 at the atomic level and the formation of AI substituting at Zn sites(Alzn),and thus,increase the effective A1 donors after hydrogen annealing.The Oi mediated doping effect reported here should be also applicable for other oxide semiconductors,e.g.ITO.3.We investigated the feasibility of suppressing the influences of bombardment of energetic oxygen ions on AZO films through introducing H2 into the vacuum chamber during sputtering.Our results show that introducing H during sputtering suppress the formation of O-related defects(Oi,VO,OGB),weaken the bombardment of energetic oxygen ions and improve the spatial distribution of optical and electrical properties of AZO films.4.We developed the technique of preparing p-type Al-N co-doped ZnO thin films by radical-assisted magnetron sputtering.By adopting an independent RF source during sputtering to turn N2 into more active N radicals and precisely controlling the sputtering parameters,we successfully obtained p-type ZnO films with a mobility as high as 3.11 cm2/Vs.Moreover,the p-type conductivity was maintained after keeping the samples in atmosphere for one month.Thus it is proven feasible to prepare stable p-type ZnO-based thin films by magnetron sputtering.