Preparation Of High Quality AZO Films By Magnetron Sputtering Without External Substrate Heating

The negative ions are commonly found to be formed at the target surface and accelerated into the plasma by the cathode fall,during the deposition of oxide thin films.They can reach essentially a very high energy(say several hundreds eV).When a growing film in sputter deposition is bombarded by these highly energetic negative ions,resputtering effects lead to significant damage such as uneven thinning,compositional changes,and/or excessive structural damage.For transparent conductive oxides(TCOs)thin films,the electrical properties are usually undermined by these damages since they are susceptible to the integrity of the crystal structure.TCOs are widely applied in thin film solar cells,panel display,and touch screen.Aluminum-doped zinc oxide(AZO)is one of important family of TCOs,and though to be the promising replacement for indium-doped tin oxide(ITO),due to their excellent optoelectronic properties,etching properties,and abundant raw materials.However,for the AZO films prepared by magnetron sputtering,energetic negative ions(>100 eV)have been identified to deteriorate the structure and the resistivity,which result in unstable processes and difficulties in preparation of large-area uniform films.To minimize such negative effects,considerable attention has been devoted to lowering the discharge voltage |V_d| : indeed,a smaller |V_d| –– from 430 V to 90 V –– led to an increased Hall mobility when other deposition parameters were fixed.Although the negative ions with reduced energies are expected to alleviate the collisionally induced damage,residual radiation damage still be noticeable considering the defect formation energies in ZnO(50~60 eV),especially at the low growth temperature.Here we systematically investigated the relationship between the process,the structure,and the optoelectronic properties of the AZO films deposited at low |V_d| values down to 40 V.The depositions were performed under conditions such that dense plasmas were deliberately excited by superimposing a very-high-frequency(VHF)RF power –– in this case 81 MHz –– onto a DC power,which is the more efficient power coupling.The main findings and conclusions are as follows:(i)The results revealed a dual role played by the negative ions.The positive role(say densification of the thin film)largely overlooked in previous studies,in magnetron sputter deposition of TCO and other oxide films,was firstly reported at |V_d| <60 V region.Negative effects of the negative ions on the film growth were identified,including point defects(such as pairs of interstitials-vacancies),lower grain size,and non-(002)texture under the conditions of |V_d| >100 V.(ii)We found a lower carrier concentration in the samples prepared by magnetron sputtering,compared with that deposited by PLD.This phenomenon was interpreted by the inactive Al dopants in the magnetron sputtering.(iii)High quality AZO films shared some key structural features such as a complete(002)texture,a strain free lattice,and a high crystallinity.(iv)We found that the energy of negative ions has the most effect on the structure of AZO films,followed by the flux of negative ions,then the energy of positive ions,through adjusting |V_d|,substrate bias,sputtering pressure,and target-to-substrate distance.High quality AZO films,with a ~2.6×10-4 ? cm resistivity and ~37 cm2/(V s)Hall mobility,can be deposited on the no intentionally heated glasses with large areas.Our results have a very important scientific and engineering significance for the low temperature deposition of large-scale TCO thin films by magnetron sputtering.And it is a significant reference for the preparation of other oxides.