Influence Of ZnO Buffer Layer On Electrical And Optical Characteristic Of VO₂ Thin Films

With the development of economy and the shortage of energy,people expect for green building which is more environment-friendly rather than only for living.Energy cost by windows or doors is half of the total building energy cost.In addition,glass was used more and more frequently in modern buildings.Using“smart window”made of energy-saving glass is a considerable way to cut down the building energy cost.And VO₂ is an ideal material for smart window which exhibits phase transition at around68?.Below 68?,VO₂ is semiconductor with high IR transmittance and resistance.While above the transition temperature,VO₂ changes to metal with dramatically drop of IR transmittance and resistance.Base on this characteristic,smart window made by VO₂ can modulate IR transmittance so that to control room temperature.This paper focused on the influence of ZnO buffer layer,oxygen flow rate,the thickness of buffer layer,substrate temperature and negative bias voltage on optical and electrical features of VO₂ thin films,to reduce deposition temperature and improve solar-energy modulation ability.ZnO buffer layers with fine crystallization were fabricated by DC reactive magnetron sputtering on soda-lime glasses.Then VO₂ thin films were deposited on the top of ZnO buffer layers.The results exhibit that,during deposition process,low or high oxygen flow rate is not suitable to fabricate VO₂ thin films.We found out the optimum oxygen flow rate is 2.1sccm.Besides,samples with ZnO buffer layers showed higher visible transmittance and better transition characteristic.In order to investigate the influence of thickness of buffer layer on VO₂ thin films,we deposited VO₂ on Zn O buffer layers with different thickness.The results show that the optimum deposition time for buffer layer is between 30 and 50 minutes.Thin buffer layer is not beneficial to the growth of VO₂ thin films,while thick buffer layer will reduce visible transmittance.VO₂ thin films were fabricated at different substrate temperature.The results exhibit that,ZnO buffer layers can lower the crystal temperature of VO₂ because of lattice matching.VO₂/ZnO Sample deposited at 340?exhibits transition ability.Sample deposited at 380?shows the best optical characteristic,the visible transmittance is 31.2%,the maximum visible transmittance is 40%and the solar-energy modulation ability is 10.6%.Finally,VO₂ thin films were fabricated under different negative bias voltage.The results exhibit that VO₂ thin films with high performance were prepared on ZnO buffer layer under negative bias voltage of-150V and substrate temperature of 320?.The solar-energy modulation ability of the sample is 8.5%,and the transition temperature is approximately 40?.Bombardment cause by bias voltage provides extra energy for growing films to crystalize.Therefore,the crystal temperature of VO₂ reduced slightly.But once the bias voltage is too large,bombardment will lead to lattice defect,which is a disadvantage of the growth of VO₂ thin films.