BFO_ICMAB_poster_Z_MC_1 (1)

BFO_ICMAB_poster_Z_MC_1 (1)

• 1.
  Phase purity andcrystalline structure: X-ray Diffraction, Transmission Electron Microscopy BFO STO As-deposited Post annealed @650ºC 30 min air Post annealed @650ºC 60 min O2 Post annealed @750ºC 60 min air 2 2 2  a BFO exp = 3.98Å a BFO teor = 3.96Å 2 Growth and Characterization of Atomic Layer Deposited BiFeO3 for Energy Applications Zakariya Khayat, Mariona Coll, Jaume Gazquez, Ignasi Fina, Xavier Obradors, Teresa Puig Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 ,Barcelona [email protected] Chemical composition/Stoichiometry: X-ray Photoelectron Spectroscopy Study Surface morphology and roughness: Atomic Force Microscopy Rms =3.2 nm Rms= 40 nm As-deposited Post annealed @650ºC 30 min air Post annealed @650ºC 60 min O2 Post annealed @750ºC 60 min air Rms =0.3 nm Rms =2.3 nm Piezoelectric response : Piezoelectric Force Microscopy d33 = 15  5 pm/V Out-of-plane (phase) Optical properties: Eg, n k. Spectroscopic Ellipsometry Si SiO2 BFO Air 1Linear and nonlinear optical properties of BiFeO3. A. Kumar, APL. 2008 Tauc-Lorentz (TL) dispersion model Spectra collected at room temperature at three different angles of incidence : 50º, 60º, 70º Annalyser: 45º Eg1: 2.17 eV Eg : 2.81 eV Eg1: 1.84 eV Eg : 2.81 eV From literature source1 Nb-STO Conclusions -We have demonstrated the formation of BFO thin films by ALD at 250ºC - Smooth and homogeneous surface is obtained in as-deposited and post-annealed films at 650ºC. Post annealing at 750ºC leads to inhomogeneous surface morphology. -As-deposited films are nanocrystalline whereas two preferred crystalline orientations are identified after post-annealing treatment. Importantly no secondary phases are observed . -PFM analysis evidences the field-induced FE polarization switching in the as-deposited stage -We have been able to extract the optical n, k, Eg values from ellipsometric measurements in two different systems BFO/SiO2/Si and BFO/Nb-STO. Eg obtained are 2.81-2.68 eV consistent with the values reported for this system. Perovskite crystal structure with polarization moment along the 111 direction. • Multiferroic material that is ferroelectric and antiferromagnetic. • Has lowest known band gap for a ferroelectric oxide. Due to its ferroelectric behavior, separation of charge carriers happens via polarization induced internal electric fields. • Narrow band gap allows absorption closer to visible spectrum Properties • Ferroelectric • TC ~ 1083K • Antiferromagnetic • TN ~ 625K • Piezoelectric • Photovoltaic • Spontaneous polarization ~ 100 µC/cm2 • Direct band gap ~2.67 – 2.81 eV Applications to use this material include: • Memory electronics • Thin film optoelectronics (solar cells) • Electronic circuitry Bismuth Ferrite BiFeO3 (BFO) Atomic Layer Deposition (ALD) • Precise thickness control • Excellent repeatability • Surface-controlled nature allows for substrates of various sizes and geometries to be conformaly coated Advantages Disadvantages • Slow deposition time for films greater than hundreds of nanometers • Material deposition dictated by precursor chemistry –Availability of suitable precursors is critical … surface self-limiting reaction, excellent uniformity, conformity (3D), atomic layer control of film thickness and stoichiometry. Precursors (gas phase) Alternate Pulses of the precursor gas n cycles As-deposited Amorphous/ Crystalline films Thermal treatment: optional 1 2 34 Precursor 1 Purge Purge Precursor 2 ConsoliderConsolider ALD deposition conditions for BiFeO3 •Equipment: Savannah 100 Cambridge Nanotech •Deposition Temperature :250ºC •Exposure mode •Precursor pulse ratio Bi 2: Fe1 •Substrates: SrTiO3 and SiO2/Si •Growth rate: 0.03 nm/cycle •Film thickness: 13 nm O3  BFO/STO =-1.4%STO (100) BFO STO (100) BFO Post-annealed @650ºC •No BFO phase is identified in the as- deposited film indicating an amorphous or nanocrystalline structure •Post annealing at 650ºC lead to BFO films with two preferred crystalline orientations (100) (110). •No secondary phases are observed •Post annealed films are fully relaxed •XPS analysis confirms the achievement of stoichiometric Bi:Fe ratio. •The high resolution Fe(2p) spectrum reveals the presence of Fe3+ species. •As deposited films show an ultrasmooth surface, characteristic of the surface self-limiting ALD deposition process. •Post annealed films at 650ºC present a granular surface morphology preserving low surface roughness. •Post annealed films at 750ºC lead to rough film surface with islands outgrowths and some incipient dewetting phenomena inidcating that post anneling conditions can be further opitimized Film thickness: 13 nm , from X-ray reflectometry Index of refraction (n) and extinction coefficient (k) Optical Bandgap Eg : •The extracted n and k coefficients and Eg are in good agreement with the values obtained in the literature Aim: ALD is a unique chemical gas-phase deposition technique that has become the mainstream technology for the production of thin coatings with high degree of composition and thickness control and a well-defined chemical reaction without non-volatile side products. It is unique from other deposition methods because of its low temperature (T) (<300ºC), low vacuum deposition conditions (0.1 Torr) and excellent conformality at nanometer-scale level. We want to take advantage of the characteristics of this deposition technique to prepare ferroelectric perovskite oxides such as BiFeO3 which, among many potential applications, has recently emerged as an attractive alternative to conventional PV technologies. Generation of the n k for bare Nb-STO Using a three multilayer architecture and the Tauc-Lorentz (TL) dispersion model, n, k and Eg can be extracted Nb-STO BFO Air n and k curves are very close to those obtained on the SiO2/Si system. Slight differences are identified in the Eg absolute value which could be due to differences in crystalline quality. Further work is needed. Sweep -8V, +8V in DC and 3 V in AC 13 nm BFO on Nb-STO Nb-STO BFO The PFM image shows a clear evidance of the field-induced ferroelectric polarization switching at RT in the 13 nm as-deposited films STO STO STO STO STO STO STO STO BFO BFO BFO BFO BFO BFO BFO BFO900 750 600 450 300 150 0 Ti3s Sr3d Bi4f C1s Sr3p Bi4d Ti2p O1s Bi4p3/2 BFO/Nb-STO Intensity(A.U.) Binding Energy (eV) (x10) Fe2p Nb 168 162 156 Bi(4f) Intensity(A.U.) Binding Energy (eV) 740 730 720 710 700 Fe(2p) Intensity(A.U.) Binding Energy (eV) 20 30 40 50 60 70 80 PA @ 750ºC PA @650ºC as-deposit BFO(003) BFO(001) BFO(002) STO STO BFO(220) BFO(110) Intensity(a.u.) 2 (degrees) STO -10 -8 -6 -4 -2 0 2 4 6 8 10 0 20 40 60 80 100 120 140 160 180 Phase(deg) Voltage (V) -12 -8 -4 0 4 8 12 0.00 0.02 0.04 0.06 0.08 0.10 0.12 Amplitude(V) Voltage (V) 1 2 3 4 5 2 (deg) Int(A.U.) 200 400 600 800 1000 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Wavelength (nm) n k BFO/SiO2 /Si 2.0 2.2 2.4 2.6 2.8 3.0 0.00 0.05 Energy (eV) (E)^(1/2) (E)^(2)Eg1 =2.17 eV Eg= 2.81eV 2 3 4 5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Energy (eV) n k Nb-STO bare substrate 300 400 500 600 700 800 900 1000 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 k Wavelength (nm) BFO on Nb-STO n 2.0 2.2 2.4 2.6 2.8 3.0 0.0 0.5 1.0 1.5 Energy (eV) BFO on Nb/STO Eg =2.68 eV (E)^(2) (E)^(1/2)