Q UANTITATIVE TEXTURE OF OF FERROELECTRIC MODIFIED LEAD ED LEAD - PowerPoint PPT Presentation

Q UANTITATIVE TEXTURE Q UANTITATIVE TEXTURE OF OF FERROELECTRIC MODIFIED LEAD ED LEAD FERROELECTRIC MODIFI TITANATE THIN FILMS TITANATE THIN FILMS J. Ricote, D. Chateigner, L. Pardo † , M. Algueró ‡ , J. Mendiola † , M.L. Calzada † Laboratoire de Physique de l'Etat Condensé. Université du Maine-Le Mans (FRANCE) † Instituto de Ciencia de Materiales de Madrid. CSIC. (SPAIN) ‡ Queen Mary and Westfield College, Univ. of London (UK)

WHY TEXTURE STUDIES IN FERROELECTRIC THIN WHY TEXTURE STUDIES IN FERROELECTRIC THIN FILMS ? FILMS ? F ERROELECTRICS : ❖ Polar materials ❖ Characterised by a spontaneous electric polarisation ❖ The polarisation can be inverted by the application of an electric field ð H YSTERESIS LOOPS . P OLAR AXIS for tetragonal compositions is [001] In polycrystalline materials a P OLING PROCESS (application of a strong electric field) is required to obtain spontaneous polarisation.

Polycrystalline thin films showing a preferred orientation with the polar axis perpendicular to the film: Ä DO NOT REQUIRE THE POLING PROCESS Ä HAVE AN IMPROVED FERROELECTRIC BEHAVIOUR F F A THOROUGH TEXTURE CHARACTERISATION IS NEEDED : FOR THE CONTROL AND OPTIMISATION OF THE PREPARATION PROCESS OF HIGHLY ORIENTED FERROELECTRIC THIN FILMS REQUIRED FOR APPLICATIONS . F F U NTIL RECENTLY QTA HAVE NOT BEEN CARRIED OUT IN THIN FILMS OF FERROELECTRICS Chateigner et al., Int. Ferro. 19, 1998, 121 Bornand et al., Int. Ferro. 19, 1998, 1 CORRELATION WITH PHYSICAL PROPERTIES NEITHER

MODIFIED LEAD TITANATE THIN FILMS MODIFIED LEAD TITANATE THIN FILMS - interesting piezo and pyroelectric properties - suitable for infrared sensors and electromechanical applications. F ILMS WERE OBTAINED BY SOL - GEL PROCESSING AND DEPOSITION BY SPIN - COATING PTL ð Pb 0.92 La 0.08 TiO 3 (20% PbO excess) PTC ð Pb 0.76 Ca 0.24 TiO 3 (10% PbO excess) S AMPLE S UBSTRATE N UMBER A NNEALING CONDITIONS LAYERS H EAT . RATE T EMP -- TIME PTL-4c Pt/TiO 2 /Si (100) 4 10ºC/min 650°C--12 min PTL-1 to 5 Pt/TiO 2 /Si (100) 1-5 >500°C/min 650°C--12 min PTC-Si Pt/TiO 2 /Si (100) 1 30ºC/s 650°C--50 s PTC-Mg Pt/MgO (100) 2 30ºC/s 700°C--50 s PTC-Sr Pt/SrTiO 3 (100) 2 30ºC/s 700°C--50 s

QUANTITATIVE TEXTURE ANALYSIS QUANTITATIVE TEXTURE ANALYSIS ❖ Experimental X-ray POLE FIGURES : - asymmetric reflection mode (Cu K α radiation) - 4-circle goniometer with Euler cradle + PSD CPS-120

Intensity ❖ Corrections for asymmetry, volume/absorption ❖ Scans: 5° x 5° grid, at incidence ω = 11°: sum diagrams 1000 2000 3000 4000 5000 6000 0 20 001/100 25 TiO 2 30 101/110 35 111 + 111-Pt 40 45 Heizmann et al., J. Appl. Cryst. 19 (1986) 467 002/200 +200-Pt + 220 Si 50 102/201/210 2 θ ( 0 ) 55 112/211 + 311-Si 60 65 202/220 +220-Pt + 400-Si 70 75 PTC ω = 11 0 PTC-452 103/301 + 311-Si 80 113/311 + 311-Pt 85 222 + 222-Pt 90 Intensity (a.u.) 1000 2000 3000 4000 5000 6000 0 20 001/100 25 TiO 2 30 101/110 35 111 + 111-Pt 40 45 002/200 +200-Pt + 220-Si 50 102/201/210 2 θ (°) 55 112/211 + 311-Si 60 65 400-Si + 220-Pt 70 221/103/300 PTL PTL-3 75 310 + 331-Si 80 311 + 311-Pt 85 222 + 222-Pt 90

❖ Cyclic integration of {100/001}, {101/110}, {102/201/210}, {112/211} and {221/103/300} overlaps ❖ Corrections for location Bunge et al., Text. Micr. 5 (1982) 153 ❖ ORIENTATION DISTRIBUTION (OD) refinement - WIMV algorithm (BEARTEX) - crystal symmetry: tetragonal, PTL: 3.93 Å x 4Å sample triclinic PTC: 3.89 Å x 4.04 Å - pole figure ranges: 100/001: 0-70 101/110: 5-70 102/201/210: 15-70 112/211: 35-70 (substrate) 221/103/300: 25-70 - Overlaps: as for the powder

OD- -RELIABILITY RELIABILITY OD Separation of tetragonal peaks appeared better than a cubic perovskite-like refinement 23 Hopefully !! ⇓ {100} vs {001} separation 1 mrd ⇓ polarisation 0.3

PTL S INVERSE POLE FIGURE ( NORMAL TO THE FILM ) PTL S INVERSE POLE FIGURE ( NORMAL TO THE FILM ) PTL-4c PTL-1 PTL-2 PTL-3 PTL-4 PTL-5

PTL S INVERSE POLE FIGURE ( NORMAL TO THE FILM ) PTL S INVERSE POLE FIGURE ( NORMAL TO THE FILM ) PTC-Si PTC-Mg 49.16 1 mrd PTC-Sr 0

RESULTS FOR PTL THIN FILMS FILMS RESULTS FOR PTL THIN Sample Thickness Components Texture RP0 RP1 Remanent of texture index polarisation (m.r.d. 2 ) ( µ C cm -2 ) (nm) (%) (%) ð low heat. PTL-4c 485 weak <110>,<101> 1.3 14 10 13.8 rate PTL-1 130 <001>,<100> 10.5 36 16 -- ð affected by leakage weak <221> PTL-2 210 <001>,<100> 7.5 24 19 7.4 weak <221> PTL-3 420 <001>,<100> 6.2 22 16 27 F 2 ➘ weak <221> with PTL-4 500 <001>,<100> 4.6 18 12 19 thickness weak <221> . PTL-5 560 <001>,<100> 3.9 19 15 10.7 weak <221> High heating rates results in stronger textures .

RESULTS FOR PTC THIN FILMS FILMS RESULTS FOR PTC THIN Sample Components Texture RP0 RP1 Pyroelectric coef. of texture index γ S γ P (m.r.d. 2 ) (%) (%) spontaneous poled (8V) (10 -8 C cm -2 K -1 ) Less PTC-Si weak <001>,<100> 2.4 23 12 0.28 2.5 textured <110>,<101> Pt/TiO 2 /Si films are easier to PTC-Mg <001>,<100> 5.2 22 10 1.5 3.2 pole Pt/MgO weak <111> PTC-Sr <001>,<100> 32.1 26 24 1.1 2.0 Pt/SrTiO 3 Poled values are not ò Substrate: determinant retained with time components factor of the final texture . contribution

RECALCULATED POLE FIGURES OF PTL THIN GURES OF PTL THIN RECALCULATED POLE FI FILMS FILMS 25.72 Similar contribution 1 m.r.d. of texture components 0 Thickness é Texture index ê Polar axis contribution ê PTL-1 PTL-3 PTL-5 (<001>) Spontaneous polarisation ê

RECALCULATED POLE FIGURES OF PTC THIN GURES OF PTC THIN RECALCULATED POLE FI FILMS FILMS PTC-Mg 50.77 1 m.r.d. PTC-Sr 0 <100> contribution é Strong contribution ( ⊥ polar axis and of <100> non –contributing to component polarisation) Spontaneous polarisation ê Spontaneous pyroelectricity ê

THICKNESS EFFECTS ON TEXTURE AND THICKNESS EFFECTS ON TEXTURE AND PROPERTIES PROPERTIES T HE USE OF A HIGH HEATING RATE ENHANCES THE APPEARANCE OF PREFERENTIAL ORIENTATION . During slow heating several variants of texture can be nucleated, resulting in a random thin film. A S THE NUMBER OF LAYERS INCREASES THE TEXTURE INDEX DECREASES . T HIS SEEMS TO INDICATE HETEROGENEOUS NUCLEATION , I . E ., WHEN CRYSTALS NUCLEATE IN SITES THROUGHOUT THE THICKNESS OF THE FILM . Except for the very thin PTL-1 and PTL-2 films, which present high leakage currents, the remanent polarisation decreases as the texture strength of the film decreases. An increase of the texture index involves in this case a higher fraction of crystals oriented with the polar axis, <001>, perpendicular to the film surface. The largest contribution to the net polarisation comes from these crystals. Therefore, there is an increase of the polarisation of the film.

T HE INCREASE OF THE FILM THICKNESS BY INCREASING THE NUMBER OF DEPOSITED LAYERS LEADS TO A DECREASE OF THE TEXTURE STRENGTH THAT RESULTS IN A REDUCTION OF THE REMANENT POLARISATION .

SUBSTRATE EFFECTS ON TEXTURE AND TEXTURE AND SUBSTRATE EFFECTS ON PROPERTIES PROPERTIES - DIFFERENT SUBSTRATES LEAD TO : DIFFERENT OVERALL DEGREE OF ORIENTATION DIFFERENT CONTRIBUTIONS FROM <100> AND <001> COMPONENTS U SE OF THE TRADITIONAL P T /T I O 2 /S I SUBSTRATE RESULTS IN : WEAKLY TEXTURED FILMS LOW SPONTANEOUS PYROELECTRIC COEFFICIENTS - PTC-Sr: largest <100> ⊥ component: lowest PYROELECTRIC COEFFICIENTS T HE USE OF A P T /S R T I O 3 SUBSTRATE , COMPARED TO P T /M G O, PRODUCES HIGHLY TEXTURED FILMS , BUT WITH A STRONG CONTRIBUTION FROM CRYSTALS WHOSE POLAR AXIS IS IN THE PLANE OF THE FILM , WHICH REDUCES THE SPONTANEOUS PYROELECTRIC COEFFICIENTS OF THE FILMS .

SUMMARY SUMMARY A QUANTITATIVE TEXTURE ANALYSIS HAS BEEN CARRIED OUT FOR THE FIRST TIME ON SEVERAL FERROELECTRIC THIN FILMS OF MODIFIED LEAD TITANATE . The effects of different processing parameters on the final texture have been studied: F rapid thermal annealing gives highly texture films F heterogeneous nucleation of the perovskite: for higher thicknesses, lower texture strengths F choice of substrate: determinant factor for texture, then properties Remanent polarisation and pyroelectric coefficients can be correlated to the texture strength and the contribution of the different texture components. A CKNOWLEDGEMENTS A CKNOWLEDGEMENTS Grant awarded by the regional government of Pays de Loire and the French Education Ministry Work financed by projects MAT98-1068 (Spanish CICYT) and COPERNICUS CIPA-CT94-0236 Declared of technological interest by the EU COST514 action on ferroelectric thin films.