Instability And Abnormal Growth Behaviors In Reactive Magnetron Sputtering Deposition Of Metal Oxides

Reactive magnetron sputtering(RMS)is a low-cost and high-efficient method of fabricating compound thin films,and can meet the needs of large-scale industrial production.However,there are some unstable phenomena in the preparation of compound thin films,which limit it in practical application.Therefore,the development of new methods for monitoring and studying the stability in reactive sputtering has greatly important practical significance.In addition,reactive sputtering deposition of metal oxides will lead to abnormal growth behavior of thin films under some special conditions.The systematic study of these abnormal growth behaviors is of significant value for the preparation of new functional films with special structure and properties.In this thesis,we focused on exploring the process instability and abnormal growth behavior in reactive magnetron sputtering deposition of oxides,and studying the influence of atomic oxygen density in plasma on process instability.The abnormal oxidation of Ag films during reactive magnetron sputtering deposition of oxides was systematically studied.Meanwhile,we suggested the formation mechanism of porous AgO nanorod films and epitaxial growth of NiO films with nano-braided on high temperature MgO substrate.It laid a necessary experimental foundation for the preparation of porous nanostructured films by reactive magnetron sputtering method.The main results in this thesis are summarized as follows:(1)Magnetron sputtering is a technique for preparation of thin film based on gas discharge.The instability of reactive magnetron sputtering mainly originates the unstable oxidation of metal targets by active species in plasma.Therefore,we determine the atomic oxygen density,electron excited temperature and dissociation of oxygen in plasma over target surface using the plasma emission spectroscopy.The evolution of these plasma parameters from glow to stable operation during reactive sputtering deposition of NiO was also discussed.It is found that the heating substrates by plasma is one of the reasons affecting the plasma discharge instability.Meanwhile,the instability of plasma is not only affected by the discharge parameters,such as oxygen partial pressure,discharge power and working pressure,but also by the substrate temperature and target-substrate distance,which can change the work mode of reactive magnetron sputtering.When preparation of Al-doped zinc oxide(AZO)films by reactive magnetron sputtering,we found the quasi-periodic variation of atomic oxygen density under special conditions,which can led to the spontaneous conversion between metal and oxide sputtering mode,and thus could format multi-layer film with rich-/poor-oxygen periodic structure.It is found that the spontaneous transformation of sputtering mode format at the transition region of two modes,which is mainly affected to the magnetic field intensity on the surface of the target.Additionally,the special time-evolving reaction diffusion between the plasma and target may play an crucial role in the conversion of sputtering mode.(2)We found the abnormal oxidation phenomenon of Ag film and the formation of porous AgO nanorod film with hierarchical structure during reactive magnetron sputtering deposition of various oxides on Ag films.This work focused on the evolution and physical mechanism of oxidizing Ag film and formatting porous AgO nanostructure during reactive magnetron sputtering deposition of NiO.Meanwhile,the effects of deposition parameters,such as deposition rate,substrate temperature,target-substrate distance,working pressure and oxygen partial pressure,on Ag film oxidation and porous AgO nanostructure were also discussed.It is found that Ag films is oxidized into hexagonal Ag2O(h-Ag2O)layer by layer,and without formatting any oxides of nickel on the surface.The further deposition of NiO will lead to the phase transformation from h-Ag2O to cubic AgO(c-AgO)and the formation of porous c-AgO nanorod films after Ag films are completely oxidized.The nucleation of c-AgO nanorods may be related to the nucleation of deposited NiO on AgO surface.With the continuous deposition of NiO,AgO nanorods gradually grow up and extend to the bottom of the films along with AgO nanorods.When the connection among nanorods disappears,the deposited NiO will lead to the formation of nano-voids in AgO nanorods and keep the porous AgO nanorods continuously growing.The similar phenomena were found during reactive sputtering deposition of different oxides on Ag film.The porous AgO nanorod films with the thickness of more than?4 ?m could be prepared by reactive sputtering deposition of NiO on-300 nm thick Ag films.By analyzing the influence of atomic oxygen density,deposited elements and growth temperature in reactive magnetron sputtering environment,it was suggested that the atomic oxygen in plasma is the main cause in the abnormal oxidation of Ag films,and the formation of porous AgO nanorods is mainly attributed to the Kirkendall effect.In addition,the shadow effect,substrate temperature and atomic oxygen density play an important role in the formation of porous AgO nanostructure.(3)h-Ag2O,TiO2//h-Ag2O heterostructure and porous c-AgO nanorod films were prepared by controlling the process parameters of reactive magnetron sputtering deposition of oxides on Ag films,and the potential applications of these films in photocatalytic degradation of organic pollution and all-solid-state thin film batteries were studied.It was found that h-Ag2O,TiO2//h-Ag2O heterostructure and porous c-AgO nanorod films showed excellent photocatalytic activity better than that of crystalline and amorphous TiO2 films,and the porous c-AgO nanorod films exhibited the best photocatalytic activity.The results showed that hydroxyl radical(OH·)in aqueous solution generated by photocatalysis is an important reason for degradation of organic pollution.According to the results of optical energy band and the stability of silver oxides,it was proposed that photoinduced carriers and the decomposition of silver oxides were the main mechanism for the generation of OH·,and the enhanced photocatalytic activity of porous AgO nanorod films was also due to the porous nanostructures,which could improve light capture ability and provide large specific surface area.In addition,the flexible Zn-AgO all-solid-state thin film batteries were assembled using porous c-AgO nanorod films grown on Cu foil as cathode and Zn sheets as anode.The results show that the specific capacity of the porous c-AgO nanorod electrode is 390 mAh/g,which is very close to the theoretical value of 432 mAh/g.(4)The possibility for formation of nanostructured films during epitaxial growth of NiO by reactive magnetron sputtering was discussed based on the Kirkendall effect.It has been suggested that the high quality NiO(111)epitaxial films with a rocking curve full width at half maximum of 0.040°were grown on a-Al2O3(0001)substrate by reactive magnetron sputtering.There are two kinds of epitaxial relationships of[211]NiO//[1120]Al2O3?and[112]NiO//[1120]Al2O3 in-plane,i.e.,NiO has two groups orientation grains of[111]and[111],and the lattice mismatch between NiO and substrate is 7.5%.When the substrate temperature is higher than 700?,NiO epitaxial films with nano-braided structure were grown on MgO(001)single crystal substrates,which might be caused by the interdiffusion between the film and substrate.The morphology of epitaxial NiO film can be improved by treatment of MgO substrate in atmospheric annealing and plasma,but the crystallization quality is still poor,which may be related to the solid state reaction between them.