Research On Manufacture Of AZO Transparent Conductive Oxide Targets And Its Films

In the recent years, transparent conductive films had became one of materials inthe development of flat-panel display, plasma display panel, touch panel, OLEDdisplay, thin film solar cells and the like. Since the most important commercialmaterial for transparent conducting films nowadays is Sn-doped In₂O₃ (ITO) but themetal indium is rare resources, so the research and development of non-In materialshave attracted considerable attention in scientific research and in technologicalapplications. Recently, ZnO films have attracted interest as a transparent conductivecoating material, because the materials are consist of cheap and abundant element,non-toxic and lots of excellent characteristics. Thereby, there were aspects studied byus as follow: the ultra-density ZnO-base ceramic target were researched and developedin a low cost course of colloidal processing and pressureless sintering; the effect ofprocessing parameters on films were studied which were prepared by radio frequence(rf) with our ultra-density targets; bi-layer transparent conductive films were preparedand studied tentatively. The main concerned results and conclusions are as follow:The influences of mass fraction of additive, solid volume fraction, pH value andmilling time on the stability and fluidity of ZnO-Al₂O₃ powders aqueous suspensionwere investigated by experiments of zeta potential, viscosities and sedimentation etc. Itis found that ZnO and Al₂O₃ powders had lower zeta potential than -45 mV commonlyat pH 8-10.3 with polyacrylic acid (PAA) added. The zeta potential of ZnO and Al₂O₃had not research obviously changes adding polyethylene glycol (PEG) simultaneously,compared with only PAA added. At 0.2wt% PAA, the viscosity is lower and the slurriesare stabiler due to the saturated adsorbed amount of PAA is 0.2wt%. The lowestviscosity, the best stability and the 55% (vol. %) solids of the suspension has beenobtained when the pH value is about 9 and the mass fraction of polyacrylic acid is0.2%. The quantity of adding polyethylene glycol is increased, the viscosity ofsuspension is increased and the stability of suspension is deceased. Milling time of 40his selected as optimum in this test. XRD analysis shows that theα-Al₂O₃ particles aredistributed homogeneously among ZnO particles and are stable during process timeTo optimize the conditions of ZnO-Al₂O₃ aqueous suspensions and slip casting inorder to obtain dense green bodies and further obtain high density ZnO-Al₂O₃compound ceramics. The influences of mass fraction of additives on the fluidity ofslurries and the density and strength of green and sintered bodies were investigated.The maximum density of green bodies was 66.6% of theoretical density with compactand homogeneous green particles used 30 vol% ZnO-Al₂O₃ slurries at pH9 added0.2wt% ployelectrolytes PAA and 0.2wt% binder PEG. Since 0.2 wt% PAA could equal to the saturated adsorption amount for the ZnO-Al₂O₃ mixture powders which resultsin well dispersing slurry. On the other hand, it is advantageous to subsequent processcompare to without binders that the radial crushing strength of cast samples increasedto 12.5 MPa with 0.2% PEG added. After pressureless sintering at 1 400℃for 2 h, analmost full density body (＞99.6% TD) can be obtained at the conductions of pH9, 0.2wt% PAA and 0.2 wt% PEG. 0.3%PAA (slightly execess of PAA) added improved thestrength and density of sintered bodyies.The Al₂O₃ reacted with ZnO to form theZnAl₂O₄ spinel phase during pressureless sintering. The microstructure observationsreveal that very homogeneous materials are obtained without abnormal grain growthand free of defects and with homogeneous distribution of Al species.In the processing of film prepared, the increasing substrate temperature couldimprove conductivity and transmittance evidently due to increase the diffusivity ofadsorbed atoms to improve crystalline quality, preferred orientation and doped-level,However the temperature of glass substrate is not suitable above 450℃. When thesubstrate temperature reached 400℃, the film resistivity reached 10^-4 levels andtransmittance was above 90%. The band gaps of the AZO films are larger than theun-doped ZnO film (3.2eV) owing to Burstein-Moss effect. With the film thicknessincreasing, the gains sizes and the preferred orientation increased, the crystalline qualityand conductivity improved, but the transmittance decreased according to Lambert law.When film thickness is thinner, the band gap decreased with film thickness increasingmight be due to quanta-size effect. With the distance between target and substrate (TS)increased the resistivity of AZO film increased, which gives no evident influence oftransmittance. The change of transmittance mainly owe to the thickness effect of film.With the sputter power increased up to 400W, the resistivity of AZO filmmonotonously decreasing and film thickness increasing due to improve sputteringefficiency and the decrease of transmittance mainly owe to the thickness effect of film.Increase of sputter power improved crystalline quality and doped-level. In our study, at180W the inflexion of the minimum resistivity point is not found which might becontributed to the use of our ultra-density target. In the range of work pressure from 0.1 to1.0 Pa, the minimum resistivity of film was appeared at various work pressure withsputtering power changed which might be duo to inter-correspond between the mean freedistance of sputtered ions and the mean atom distance of plasma gas. The influence ofwork pressure on the transmittance and band gap of film is not evident in our workpressure range. The lowest resistivity of 5.3×10^-4Ω.cm and 1.48×10^-4Ω.cm could beobtained at 200℃and 400℃of substrate temperature respectively. At higher oxidepartial pressure the film resistivity increased in level of quantity due to oxide vacancydecreasing. The oxide partial pressure was improving the film transmittance to visiblelight which is relative to crystalline quality improved and the concentration of defectdecreased under oxide pressure. However the lower oxide partial pressure (0.1%O₂) is notbenefit to (002) preferred orientation. The changes of band gap of AZO film at various oxide partial pressures might be duo to the change of film stress.Adding ZnO suffer layer could improve crystalline quality and (002) preferredorientation of AZO film and depress their residual stress. The mean transmittance ofbi-layer film is about 90%, since the transmittance is not remarkably effected by ZnOsuffer. The ITO/AZO bi-layer film had higher reflectivity (up to 51%) with lowerresistivity (10^-4) or higher transmittance (87%, at below 500nm of AZO film thickness)with higher resistivity (10^-3) at 40 mm or 60 mm of TS when sputtering AZO film on250℃glass substrate respectively. The ITO/AZO bi-layer film had higher transmittance(85～90%) and lower resistivity (10^-4) when sputtering AZO film at 60 mm of TS and 400℃of glass substrate. The lowest resistivity of 4.89×10^-4Ω.cm could be obtained at 1:9 ofthe ITO/AZO thickness ratio.