Interface layer thickness effect on the photocurrent of Pt sandwiched polycrystalline ferroelectric

Interface layer thickness effect on the photocurrent of Pt sandwiched polycrystallineferroelectric Pb ( Zr , Ti ) O 3 filmsDawei Cao, Hui Zhang, Liang Fang, Wen Dong, Fengang Zheng, and Mingrong ShenCitation: Applied Physics Letters 97, 102104 (2010); doi: 10.1063/1.3488829View online: /10.1063/1.3488829View Table of Contents: /content/aip/journal/apl/97/10?ver=pdfcovPublished by the AIP PublishingArticles you may be interested inInterface effect on the photocurrent: A comparative study on Pt sandwiched ( Bi 3.7 Nd 0.3 ) Ti 3 O 12 and Pb( Zr 0.2 Ti 0.8 ) O 3 filmsAppl. Phys. Lett. 96, 192101 (2010); 10.1063/1.3427500First-principles simulations on bulk Ta 2 O 5 and Cu / Ta 2 O 5 / Pt heterojunction: Electronic structures andtransport propertiesJ. Appl. Phys. 106, 103713 (2009); 10.1063/1.3260244Photovoltaic characteristics in polycrystalline and epitaxial ( Pb 0.97 La 0.03 ) ( Zr 0.52 Ti 0.48 ) O 3ferroelectric thin films sandwiched between different top and bottom electrodesJ. Appl. 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Downloaded to IP:Interface layer thickness effect on the photocurrent of Pt sandwiched polycrystalline ferroelectric Pb…Zr,Ti…O3filmsDawei Cao,Hui Zhang,Liang Fang,Wen Dong,Fengang Zheng,a͒and Mingrong Shen b͒Department of Physics and Jiangsu Key Laboratory of Thin Films,Soochow University,Suzhou215006,People’s Republic of China͑Received16June2010;accepted20August2010;published online10September2010͒Based on the analysis of the photocurrent behavior of Pt sandwiched Pb͑Zr0.2Ti0.8͒O3͑PZT͒films,the experimental evidence of top Pt/PZT interface layer thickness effect on the photocurrent isreported.It was well established before that the photocurrent of metal/ferroelectricfilm is attributedto the height of Schottky contact barrier.However,our results suggest that the photocurrent ofPt/PZT interface contact is determined not only by the barrier height but also by the interface layerthickness,namely,by the built-in electricalfield at the interface layer.The mechanism behind suchphotocurrent phenomenon is proposed.©2010American Institute of Physics.͓doi:10.1063/1.3488829͔Recently,the photovoltaic effect of ferroelectric materi-als,such as Pb͑Zr,Ti͒O3͑PZT͒,1–7͑Pb,La͒͑Zr,Ti͒O3,8–10 and BiFeO3,11–14has become a research focus for their po-tential applications for the UV detection,1nondestructive op-tical reading,15and photovoltaic devices.16,17It is well estab-lished that the“bulk photovoltaic effect”is mainly originated from the remnant polarization in bulk ferroelectric materials, and the photocurrent is in proportion to the magnitude of remnant polarization.13In metal/ferroelectric/metal structure, however,the photovoltaic effect comes partly from the rem-nant polarization same as the bulk photovoltaic effect,5,14and partly from the metal/ferroelectric-film interface effect, which produces the photocurrent determined by the barrier heights of metal/ferroelectric-film interfaces.8,12In this pa-per,we report the experimental evidence of top Pt/PZT in-terface layer thickness effect on the photocurrent of ferro-electric polycrystallinefilm,which manifests that the photocurrent of Pt/PZT interface contact is determined by the built-in electricalfield in the interface layer.First,a PZTfilm,deposited on Pt/Ti/SiO2/Si substrate by a sol-gel method,was crystallized at600°C in air for 2h.Then,Pt dots as top electrode,with an area of7.88ϫ10−4cm2,were deposited by sputtering at room tempera-ture.In the upper of Fig.1,two-and three-dimensional atomic force microscopy͑AFM͒pictures show that the av-erage particle size is about50nm and the surface roughness is about20nm.The lower of Fig.1,the cross-section scan-ning electron microscope͑SEM͒image of Pt/PZT/Pt struc-ture,indicates that the thickness of PZT and Ptfilm is around 270nm and35nm,respectively.The Pt/PZT/Pt sample was cut into four parts,labeled as PZT0,PZT1,PZT10,and PZT30,respectively.PZT0was the sample in which the top Pt/PZT interface did not undergo a high temperature annealing process;PZT1,PZT10,and PZT30were the samples in which the top Pt/PZT interfaces were annealed at600°C in air for1,10min and30min, respectively.The thickness of top Pt/PZT interface layer is expected to increase with the increase in annealing time.The polarization and short-circuit photocurrent measure-ments are similar to that reported in our previous works.5–7 Figure2͑a͒shows the polarization hysteresis loops͑P-V͒in-dicating almost the same spontaneous polarization for all four samples.All samples except PZT0have an almost per-fect symmetrical P-V loop along both the voltage axes and the polarization axes.The larger coercivefield of PZT0than that of other samples implies that there are more defects or charges in top Pt/PZT interface,which is consistent with pre-viously reported results.18Consequently,the polarization re-versal needs a larger external electricfield in PZT0than that in others.Figure2͑b͒present the photocurrent versus poling volt-age͑PC-V͒loops of all PZT paring Figs.2͑a͒and2͑b͒,we canfind that the photocurrent changes greatly at the coercivefiled,similar to the polarization occurring in the P-V loops,which implies the happening of polarization re-versal.However,the symmetrical center͑PC0͒of vertical photocurrent axis in the PC-V loop is+2.29nA,Ϫ0.91nA,Ϫ0.27nA,andϪ0.07nA for PZT0,PZT1,PZT10,andPZT30,respectively.The value of PC0has been verified to be relative to the asymmetry Schottky contact between top Pt/PZT interface and bottom one.5,8a͒Electronic mail:zhfg@. b͒Electronic mail:mrshen@.FIG.1.͑Color online͒AFM surface morphology of PZTfilm and SEMcross-section picture of Pt/PZT/Pt capacitor.APPLIED PHYSICS LETTERS97,102104͑2010͒0003-6951/2010/97͑10͒/102104/3/$30.00©2010American Institute of Physics97,102104-1This article is copyrighted as indicated in the article. 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Downloaded to IP:In order to figure out the difference between top Schottky barrier height ͑␾top ͒and bottom one ͑␾bottom ͒,we measured the leakage current of all samples at 400–440K temperatures.The measurement details can be found elsewhere.7The determined values of ␾bottom are almostsame for all samples ͑ϳ0.79eV ͒,while that of ␾top is 0.31,0.45,0.64,and 0.73eV for PZT0,PZT1,PZT10,and PZT30,respectively.It can be concluded that ␾top is always lower than ␾bottom ,and the difference between ␾bottom and ␾top͑⌬␾=␾bottom −␾top ͒decreases monotonously with the in-crease in postannealing time.However,the PC 0values donot change monotonously with the increase in postannealingtime.Undoubtedly,the variation in PC 0is not consistent withthat of ⌬␾,so it cannot be simply attributed to the variationin ⌬␾in Pt/PZT/Pt capacitor.8In this case,we suggest that the postannealing not only causes the changing of Pt/PZT contact barrier but also may change the thickness of Pt/PZT interface region.In order to check this point,we performed the x-ray photoemission spectroscopy ͑XPS ͒depth profile of all Pt/PZT/Pt capacitors with an etching rate about 0.1nm/s.The results are shown in Fig.3.Here,the Pt/PZT interface layer thickness ͑d ͒is de-fined as the depth of Pt element diffusing into the PZT film͑Our reason for doing this will be discussed later ͒,and thecalculated thickness of top Pt/PZT interface layer ͑d top ͒is24.1,24.1,33.9,and 39.5nm for PZT0,PZT1,PZT10,andPZT30,respectively.It can be observed that the content ofoxygen element is enhanced at top Pt/PZT interface alongwith the decreasing Pt element content.Higher temperatureprocessing may efficiently reduce the defects,especially theoxygen vacancy at Pt/PZT interface,and result in the raisingof barrier height.6,18It is interesting that the content of Pb element increasesslightly at top interfaces of PZT films,especially in PZT10and PZT30samples,which may be due to the evaporation ofPbO during the annealing process.The presence of PbO,as asemiconductor,will result in a lower barrier height,whichmay be one of the reasons that the values of ␾top are alwayslower than that of ␾bottom .4,7As a witness,the XPS depth profile of another Pt/PZT/Pt sample annealed at 600°C in air for 60min is shown in Fig.3͑e ͒.It can be found that d top is almost the same as the thickness of bottom PZT/Pt interface layer ͑d bottom ͒,while ␾top is still a little lower than ␾bottom .Note that the d bottom of all samples are about 44.0nm.We believe that PC 0is strongly relative to Pt/PZT inter-face layer thickness.For an ideal Pt/PZT/Pt structure,the PC 0is supposed to be zero because the electric field at top interface cancel out that at bottom one due to the symmetri-cal back-to-back Schottky contacts.However,the PC 0in an actual Pt/PZT/Pt capacitor usually is not zero because of the asymmetry between top Pt/PZT interface and bottom one under different annealing conditions.The Pt/PZT interface is a disturbed layer due to the interdiffusion of Pb,Ti,Zr,O,and Pt ions.References 18and 19reported that the small amount of Pt element diffusion into the ferroelectric film will change notably the surface Fermi level at the Pt/film interface.Reference 20reported that a very thin layer ͑ϳ10nm ͒near the metallic electrode had a large concentra-tion of trapped charge ͑about 1020cm −3͒,which is two or-ders higher than that in the depletion region.20,21This layer is thinner than the depletion region and is probably responsible for the pinning of Fermi level at the metal/PZT interface.Both large carrier concentration and pinning of Fermi level result in a much larger electric field at Pt/PZT interface layer than that in the depletion region near the center of PZT film.20Figures 4͑a ͒–4͑c ͒schematically indicate the asymme-try of barrier height,electric field and interface layer thick-ness near top and bottom electrodes in Pt/PZT/Ptstructure.FIG.2.͑Color online ͒Polarization and photocurrent loops of PZTfilms.FIG.3.͑Color online ͒XPS depth profiles for ͑a ͒PZT0,͑b ͒PZT1,͑c ͒PZT10,͑d ͒PZT30,and ͑e ͒a witness Pt/PZT/Pt sample,in which top Pt/PZT interface layer was annealed at 600°C in air for 60min. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: /termsconditions. Downloaded to IP:In Fig.4͑b ͒,the red line indicates the electric field distribu-tion of Pt/PZT/Pt capacitor without the interface layer,whilethe blue line gives the one with the interface layer,where the field is mainly bounded.For discussion efficiently,we introduce a factor f =␾/d =qE eV /␮m to describe the electric field at top and bottom interfaces,and plot ␾top and d top in Fig.5͑a ͒,along with ␾bottom ͑E is the field of Schottky contact at the interface,and f is the electric force of the electron or hole in the filed ͒.The bottom electric field is almost unchanged because d bottom and ␾bottom are almost constant for all samples.Therefore,it is reasonable that the variation of PC 0is mainly attributed to top electric field,which is manipulated by top interface layer thickness.Although ␾top is lower than ␾bottom ,the electrical field at top interface layer may be greater than that at bottomone if d top is far thinner than d bottom ,as shown in Figs.4͑a ͒and 4͑b ͒.Generally,the greater the field is,the more the electron or hole excited by the light can be separated by the field,resulting in a larger photocurrent.For PZT0sample in Fig.5͑b ͒,f bottom is much larger than f top ,which can lead to a large positive PC 0.It is indicated that the contribution of bottom Pt/PZT interface to the photocurrent is more than that of top Pt/PZT interface.On the contrary,f bottom is lower than f top in PZT1,PZT10,and PZT30samples,implying that top Pt/PZT interface has a dominating influence on the photocur-rent,so a series of negative PC 0occur.Furthermore,the difference ͑⌬f =f bottom −f top ͒between f bottom and f top de-creases with the increase in postannealing time,which can lead to a lower PC 0.In Fig.5͑c ͒,it is clear that the tendency of PC 0is almost consistent with that of ⌬f .Note that the observed tendency of PC 0can be reproduced completely.In summary,the photocurrent originated from the inter-face Schottky contact effect in Pt/PZT/Pt structure is found to be determined not only by the barrier height but also by the interface layer thickness,namely by the built-in electrical field at the interface layer.We believe that those interesting phenomena on the photocurrent can help us to understand better the photovoltaic of the ferroelectric,and to present a method that the photocurrent in metal/ferroelectric/metal structure could be manipulated effectively by controlling the thickness of metal/ferroelectric interface layer.This work was supported by Grant ͑Grant No.50702036͒from National Natural Science Foundation of China.1L.Pintilie,M.Alexe,A.Pignolet,and D.Hesse,Appl.Phys.Lett.73,342͑1998͒.2L.Pintilie and M.Alexe,J.Appl.Phys.98,124103͑2005͒.3L.Pintilie,I.Vrejoiu,G.L.Rhun,and M.Alexe,J.Appl.Phys.101,064109͑2007͒.4L.Pintilie,I.Vrejoiu,D.Hesse,G.L.Rhun,and M.Alexe,Phys.Rev.B 75,104103͑2007͒.5F.G.Zheng,J.Xu,L.Fang,M.R.Shen,and X.L.Wu,Appl.Phys.Lett.93,172101͑2008͒.6J.Xu,D.W.Cao,L.Fang,F.G.Zheng,M.R.Shen,and X.L.Wu,J.Appl.Phys.106,113705͑2009͒.7D.W.Cao,J.Xu,L.Fang,W.Dong,F.G.Zheng,and M.R.Shen,Appl.Phys.Lett.96,192101͑2010͒.8M.Qin,K.Yao,Y .C.Liang,and B.K.Gan,Appl.Phys.Lett.91,092904͑2007͒.9M.Qin,K.Yao,and Y .C.Liang,Appl.Phys.Lett.93,122904͑2008͒.10M.Qin,K.Yao,and Y .C.Liang,Appl.Phys.Lett.95,022912͑2009͒.11S.R.Basu,L.W.Martin,Y .H.Chu,M.Gajek,R.Ramesh,R.C.Rai,X.Xu,and J.L.Musfeldt,Appl.Phys.Lett.92,091905͑2008͒.12S.Y .Yang,L.W.Martin,S.J.Byrnes,T.E.Conry,S.R.Basu,D.Paran,L.Reichertz,J.Ihlefeld,C.Adamo,A.Melville,Y .H.Chu,C.H.Yang,J.L.Musfeldt,D.G.Schlom,J.W.Ager,and R.Ramesh,Appl.Phys.Lett.95,062909͑2009͒.13T.Choi,S.Lee,Y .J.Choi,V .Kiryukhin,and S.W.Cheong,Science 324,63͑2009͒.14W.Ji,K.Yao,and Y .C.Liang,Adv.Mater.22,1763͑2010͒.15S.Thakoor,E.Olson,and R.H.Nixon,Integr.Ferroelectr.4,257͑1994͒.16M.Ichiki,R.Maeda,Y .Morikawa,Y .Mabune,T.Nakada,and K.Nonaka,Appl.Phys.Lett.84,395͑2004͒.17A.Kholkin,O.Boiarkine,and N.Setter,Appl.Phys.Lett.72,130͑1998͒.18R.Schafranek,S.Payan,M.Maglione,and A.Klein,Phys.Rev.B 77,195310͑2008͒.19S.Takatani,H.Miki,K.K.Abdelghafar,and K.Torii,J.Appl.Phys.85,7784͑1999͒.20I.Boerasu,L.Pintilie,M.Pereira,M.I.Vasilevskiy,and M.J.M.Gomes,J.Appl.Phys.93,4776͑2003͒.21L.Pintilie,M.Lisca,and M.Alexe,Appl.Phys.Lett.86,192902͑2005͒.FIG.4.͑Color online ͒Schematic illustrations of ͑a ͒asymmetric Schottky barriers,͑b ͒built-in field distributions:red ͑or blue ͒line indicates the elec-tric field without ͑or with ͒the interface layer and ͑c ͒top and bottom inter-face layers in Pt/PZT/Ptcapacitor.FIG.5.͑Color online ͒Top Pt/PZT interface annealing time dependent ͑a ͒barrier height ͑␾top and ␾bottom ͒and top interface layer thickness ͑d top ͒,͑b ͒top and bottom factor f =␾/d ͑f top and f bottom ͒,and ͑c ͒difference betweenf top and f bottom ͑⌬f =f bottom −f top ͒and photocurrent ͑PC 0͒. 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