SECOND HARMONIC GENERATION OF Bi 4 Ti 3 O 12 FILMS IN-SITU PROBING OF DOMAIN POLING IN Bi 4 Ti 3 O 12 THIN FILMS BY OPTICAL SECOND HARMONIC GENERATION YANIV BARAD, VENKATRAMAN GOPALAN Mterils Reserh Lortory nd Dept. of Mterils Siene nd Engineering, Pennsylvni Stte University, University Prk, PA 16802; We quntittively trk in rel-time, the hnges in domin sttistis with eletri field poling, of ferroeletri Bi 4 Ti 3 O 12 thin film using optil seond hrmoni genertion s proe. The ferroeletri hysteresis loop is extrted from these optil mesurements using theoretil model. The model lso yields rtios of intrinsi nonliner oeffiients suh s d 11 /d 12 =-3.54±0.31, d 26 /d 11 =0.4±0.03, nd optil irefringene n -n =0.079±0.015. Keywords: Feroeletris, Bismuth titnte, Thin films, Seond Hrmoni Genertion INTRODUCTION Ferroeletri Bismuth Titnte, Bi 4 Ti 3 O 12, whih elongs to the Aurivillius phses, is of interest in nonvoltile memory due to exellent ftigue resistne during repeted polriztion reversls with eletri field. [1],[2] The spontneous polriztion in monolini unit of Bi 4 Ti 3 O 12 hs omponents long oth the - nd - rystllogrphi diretions where - forms the mirror plne (010). Both - nd - omponents of the polriztion n e independently reversed, thus
Y. BARAD ET. AL. resulting in four different lsses of domin wlls nd 18 wll onfigurtions. [3] All these onfigurtions re not redily distinguishle in thin film y onventionl x-ry diffrtion or trnsmission eletron mirosopy. In reent pper, we showed how proing the seond hrmoni genertion (SHG) response of Bi 4 Ti 3 O 12 film with omplex domin mirostruture n provide mny of these domin distintions in quntittive mnner. [4] We present here the results of eletri field poling of Bi 4 Ti 3 O 12 thin film on SrTiO 3 sustrte. The ferroeletri hysteresis loop n e diretly extrted from the optil mesurements. As we show here, the poling experiments revel not only the mgnitude of domin mirostruturl is ut lso the sign of the is. DOMAIN STRUCTURE IN THE FILM The Bi 4 Ti 3 O 12 thin film studied here ws grown on SrTiO 3 (001) sustrte using moleulr em epitxy (MBE) s previously reported in detil. [5] The lttie prmeters of the ui SrTiO 3 (001) sustrte, = 3.9050 Å losely mth long its digonls of 2, with the lttie prmeters = 5.4500 Å nd = 5.4059 Å of the monolini Bi 4 Ti 3 O 12. (The other lttie prmeters re = 32.832 Å, nd β = 90.00 ). [6] The lttie plnes -, -, nd - of Bi 4 Ti 3 O 12 re respetively denoted s (100), (010) nd the (001) plnes. As reported efore, the epitxil reltionship is SrTiO 3 (001)[110]//Bi 4 Ti 3 O 12 (001)[100]. In this onfigurtion, there re eight possile domin onfigurtions of the Bi 4 Ti 3 O 12 s shown shemtilly in Fig. 1. Eh of the possile domins hs monolini unit ell, whih devites only slightly from the orthorhomi unit ell. The polriztion xis forms n ngle of ~ 4.5 from the rystllogrphi -xis in the - (010) plne. The film thikness is ~0.1µm.
SECOND HARMONIC GENERATION OF Bi 4 Ti 3 O 12 FILMS Bsed on the ove disussion, we define four lsses of domin vrints X+, X-, Y+ nd Y- ording to whether the -omponent of polriztion points in the SrTiO 3 [110], [1 10], [110], or [110] diretions, or lterntively (+x, -x, +y, -y) diretions in Fig. 1, respetively. In the following setions, we desrie how the hnges in the domin sttistis of X+, X-, Y+, nd Y- domins n e distinguished y seond hrmoni genertion mesurements. Domin Vrints in Bi 4 Ti 3 O 12 (001) film on SrTiO 3 (001) sustrte Y+ X- X+ - z Y- y, SrTiO 3 [110] x, SrTiO 3 [110] FIGURE 1. Ferroeletri domins vrints in the Bi 4 Ti 3 O 12 film on SrTiO 3 (001) sustrte. SECOND HARMONIC GENERATION MEASUREMENTS Detils of the experimentl setup hve een disussed in n erlier pulition. [4] The fundmentl em from 10 Hz Q-swithed Nd:YAG lser (λ = 1064 nm), nd pproximtely 1.2 mm is pssed through the film in norml inidene. The input polriztion of infrred light is rotted using hlf-wve plte. At the output, the seond hrmoni genertion (SHG) signl (λ=532nm) is seprted nd deteted for polriztions prllel to two in-
Y. BARAD ET. AL. plne SrTiO 3 <110> diretions in the sustrte (x- or y- xes in Fig. 1). An externl eletri field is pplied to the smple using gold eletrodes sputtered on the smple surfe with gp of ~ 3 mm etween them, to ssure of enough lerne for the em. The SHG signl I 2ω is mesured s funtion of input polriztion ngle, θ of the fundmentl. In generl, different re on the film gives slightly different polr plots. However, ll these plots n e nlyzed within the sme theoretil frmework given in our previous work. [4] Figure 2 presents results of typil SHG mesurement. y y I 2ω x FIGURE 2. The polr plots of SHG intensity, I 2ω, s funtion of input polriztion ngle θ of the fundmentl, in Bi 4 Ti 3 O 12 thin film on SrTiO 3 (001) sustrte. () y-polrized SHG, nd () x-polrized SHG, where x,y xes re shown in Fig. 1. Cirles re experiment nd solid line is the theoretil fit sed on Eq. 1. The experimentl results of Fig. 2 n e fit to the following theoretil eqution:
SECOND HARMONIC GENERATION OF Bi 4 Ti 3 O 12 FILMS I j 2ω = K 1, j (sin 2 θ + K 2,j os 2 θ ) 2 + K 3, j sin 2 2θ + K 4,j (sin 2 θ + K 2,j os 2 θ )sin 2θ (1) where j=x,y denote the output polriztion of the mesured SHG signl, nd the eight K i,j prmeters (i=1,2,3,4) re experimentlly determined from nonliner urve fitting s shown in Fig. 2. These prmeters re then used to lulte the following intrinsi mteril prmeters, s desried in Ref. [4]: d 11 /d 12 =-3.54±0.31, d 26 /d 11 =0.4±0.03, nd n -n =0.079±0.015, where d ij re seond order nonliner optil oeffiients for seond hrmoni genertion, nd n nd n re refrtive indies long rystllogrphi nd xes of Bi 4 Ti 3 O 12. In ddition, new mirostruturl informtion n e extrted from the mesured K ij prmeters. If the light therefore psses through thikness frtion t X+ of the X+ domin nd t X- of the X- domin, in n re da X of the X+/- domins, then the net thikness frtion, δa x, defined s A x = A X 0 s A y = (t X+ t X )da X. Similrly, onsidering the Y+ nd Y- domins in n re da y of the growth plne, the net thikness frtion, δa y, is defined A Y 0 (t Y + t Y )da Y. In words, δa x is proportionl to how muh more (or less) frtion of X+ domin exists in the proe re versus the X- domins. Similrly, δa y is proportionl to how muh more (or less) frtion of Y+ domin exists in the proe re versus the Y- domins. These reltionships reflets the net destrutive interferene of the seond hrmoni eletri fields reted y X+ (Y+) nd X-(Y-) domins due to the π phse shift etween the two fields. This nlysis therefore ssumes omplete phse orreltion, whih implies tht the seond hrmoni response of ll domin vrints re phse orrelted. This ssumption is justified in our present se sine the domin sizes of X+ nd X- vrints re of the order of
100-200 nm in the film growth plne, whih is less thn the wvelength of light. [4]
Y. BARAD ET. AL. From the K ij prmeters mesured from Fig. 2, we n lso extrt the rtio, δa y /δa x =2.25±0.05. In ddition, using stndrd rystl of LiTO 3 whose solute d ij oeffiients re known, we n lulte the solute mgnitudes of (δa y d 11 ) 2 ~10-3 nd (δa x d 11 ) 2 ~4.4x10-4. If the solute vlue of d 11 were known for Bi 4 Ti 3 O 12 (whih it isn t), one ould then lulte the tul mirostruturl ises δa y nd δa x. ELECTRIC FIELD POLING OF Bi 4 Ti 3 O 12 THIN FILMS Figure 3 shows the vrition of the mirostruturl is (δa y d 11 ) 2 s funtion of pplied voltge. (Note tht this proe re of the film ws different from tht proed in Fig. 2). FIGURE 3. The vrition of mirostruturl is (δa y d 11 ) 2 (see text for definition) (extrted from SHG mesurements) s funtion of eletri field pplied long the ±y xis in Fig. 1.
SECOND HARMONIC GENERATION OF Bi 4 Ti 3 O 12 FILMS The ove in-situ eletri field poling ws performed y pplying slow tringulr voltge wveform of 100 mhz frequeny ross two surfe gold eletrode pds, 3mm prt. The eletri field diretion ws long the y- xis with referene to Fig. 1. Sine δa y is proportionl to the net frtion of Y+ to Y- domins in the proe re, it mkes sense tht pplying negtive voltge (field in y diretion) inreses the frtion of Y- domins, nd positive voltge inreses the frtion of Y+ domins. At the minimum, the two domin types re pproximtely equl in mgnitude. One lso noties n symmetry in domin poling etween positive nd negtive field diretions, inditing tht n intrinsi domin is exists where Y- domins re more undnt thn Y+ domins t zero-field. This mirostruturl is is not unique to ll of the film re, ut is rther very lol feture within the proe re of the em, inditing the sensitivity of the SHG mesurements to the lol domin mirostruture. In onlusion, we hve developed seond hrmoni genertion s sensitive nd quntittive proe of the lol domin sttistis within the proe re. It n distinguish ntiprllel domins (180 domins) whih is diffiult y onventionl X-ry nd eletron mirosopy. The SHG mesurements n e performed in-situ during eletri field poling of the film. The ferroeletri hysteresis loop n then e diretly extrted from the study giving quntittive estimtes for re frtions of different domins vrints in the proe re of the film. Aknowledgements This work ws supported y the NSF wrds 9984691 nd 9988685. Referenes [1] C. A-Pz de rujo, J. D. Cuhiro, L. D. MMilln, M. C. Sott, nd J. F. Sott, Nture (London) 374, 627 (1995).
Y. BARAD ET. AL. [2] R. E. Newnhm, R. W. Wolfe, nd J. F. Dorrin, Mter. Res. Bull. 6, 1029 (1971). [3] S. E. Cummins, nd L. E. Cross, J. Appl. Phys., 39, 2268 (1968). [4] Y. Brd, J. Lettieri, C. D. Theis, D. G. Shlom, V. Gopln J. C. Jing, X. Q. Pn, J. Appl. Phys. 89, 1387 (2001). [5] C. D. Theis, J. Yeh, D. G. Shlom, M. E. Hwley, G. W. Brown, J. C. Jing, nd X. Q. Pn, Appl. Phys. Lett. 72, 2817 (1998). [6] A. D. Re, At Cryst. B46, 474 (1990).