Investigations On The Preparation And Properties Of Highly Transparent Conductive And Near-IR Reflective ZnO:Ga Films

Transparent conductive oxides are an important optoelectronic material for a variety of applications in the fabrication of various devices such as electro-optical devices,thin-film solar cell,surface acoustic wave devices,sensors,flat panel liquid crystal display devices,infrared(IR)reflector,etc.Among the different materials belonging to this category,zinc oxide(ZnO)is a n-type semiconductor material with a bandgap of 3.3 eV and a promising alternative to indium tin oxide(ITO)in TCO applications,due to its ease in doping,abundance in nature,low cost,and relatively low deposition temperature and stability in hydrogen plasma compared to ITO and SnO₂.By now many film growth techniques have been used to deposit transparent conductive doped-ZnO films.Among the different techniques used for the growth of ZnO films,DC reactive magnetron sputtering method is promising in preparing transparent conductive doped-ZnO thin films due to the simplicity of the growth process required and ease in operation.Compared to other film growth techniques, large-area films can be grown by this method,which will help to industrialized production and application in the future.Moreover,doped ZnO transparent conductive films with good IR reflective behavior will significantly contribute to the applications of near-IR reflecting mirrors and heat reflectors.Therefore,this work on highly transparent conductive and IR reflective doped-ZnO films is of great interests and potentials.In this paper highly transparent conductive and near-IR reflective ZnO:Ga thin films were grown on glass substrate by using DC reactive magnetron sputtering technique.The influence of Ga content,substrate temperature,deposition pressure, oxygen partial pressure,sputtering power,annealing temperature,etc on the film properties has been studied systemically.And the mechanisms of IR reflection are discussed detailedly.Besides,ZnMgO:Ga films with different Mg content have been prepared by DC reactive magnetron sputtering,and effects of Mg content on structural, electrical and optical properties of the films were also investigated in detail.The main results are obtained as follows:1.Highly transparent conductive and near-IR reflective ZnO:Ga thin films have been firstly prepared on glass substrate by DC reactive magnetron sputtering.All the ZnO:Ga films with the preferential orientation of(002)plane are polycrystalline and possess hexagonal wurtzite structure with their crystallographic c-axis perpendicular to the substrate.The grain size along the c-axis is found between 17 nm and 32 nm.2.Substrate temperature,deposition pressure and sputtering power have great influences on the film surface morphology.The surface of the films deposited at 300℃,1.0 Pa and 140 W is smooth and the grains are uniformly stacked up and entirely compact,so no grain boundaries are visibly found.3.All the films are degenerate doped n-type semiconductor.A lowest resistivity of 3.0×10^-4Ω·cm was obtained under the deposition conditions of the target with 2～4 at%Ga content,substrate temperature 300℃,deposition pressure1.0～1.5 Pa,oxygen partial pressure 0.2～0.3 Pa,sputtering power 140～160 W,and the electron carrier concentration reaches the value of 2.5×10²¹cm^-3.The contents of Zn,O as well as Ga uniformly distribute in the whole film,and only one chemical state of Ga is present in the ZnO:Ga films.4.The IR reflective behavior of ZnO:Ga films has been studied.The average optical transmittance in the visible range exceeds 90%for all the films.All the films have low transmittance and high reflectance in the near-IR region.The IR reflectance of the films shows an increase with increasing the electron concentration.The IR transmission cut-off wavelength of the films shifts toward the lower wavelength with the electron concentration increasing.The transmittance of the films deposited under the deposition conditions of the target with 2～3 at%Ga content,substrate temperature 300℃,deposition pressure1.0～1.5 Pa,oxygen partial pressure 0.2～0.3 Pa,sputtering power 140～160 W is below 10%when the wavelength is higher than 1500 nm and the reflectance reaches 72%at the wavelength of 2500 nm.And they have the largest band gap～3.76 eV and the best optical properties.5.The influence of post-annealing temperature on the properties of ZnO:Ga thin films has aslo been studied.The average crystallite dimension of the ZnO:Ga films becomes larger and larger with the increase of annealing temperature.A lowest resistivity of 2.6×10^-4Ω·cm was obtained at the annealing temperature of 400℃.As the annealing temperature further increases,the resistivity increases and reaches 4.39×10^-4Ω·cm at the annealing temperature of 550℃.However,Annealing is not useful to improve near-IR reflective behavior of ZnO:Ga films although the conductivity of the films can be increased after annealed at the higher temperature.6.The ZnMgO:Ga films were synthesized on glass substrate by DC reactive magnetron sputtering for the first time.A systematic study of structural,electrical,and optical properties of ZnMgO:Ga films deposited with different Mg content from 0 to 10 at%have been investigated.Only the peaks indexed to hexagonal(002)and(004) ZnO were observed.The distribution of Zn,Mg,O,and Ga is uniform in the whole film.The resistivity of these films obviously increases with the Mg concentration increasing,accompanied with a reduction in carrier concentration and Hall mobility.