2.4.1 X-Ray Diffraction and Reflectivity
Thin film structure and thickness were analyzed using X-ray diffraction (XRD) and X-ray reflectivity (XRR) of thin film samples. The measurements were carried out in a 4-circle X-ray diffractometer (Rigaku Smartlab and Bruker D8 Advance, 40 kV, 44 mA, Cu Kα) equipped with a double (220)Ge monochromator in a parallel beam geometry. Fitting of XRR data was performed using Motofit analysis package [81]. Reciprocal space mapping was carried out the Panalytical Empyrean X-ray diffractometer (2x(220)Ge monochromator, 45 kV, 40 mA) for each of the (103) peaks of the perovskite substrates and the films. XRR data was fitted using the Motofit package written by A. Nelson [81] and implemented in Igor Pro.
In some XRD patterns due to a very high intensity of the (002) SrTiO3 reflection a very small Cu Kβ peak was not removed completely by the monochromator used in the measurements.
XRD with in situ annealing was performed in the Rigaku Smartlab with a temperature step of 25°C and 40 min. of the total measurement time at each temperature point (heating rate was ~5°C/min). A commercial domed hot stage (Anton Paar DHS 1100) was used for these experiments. The sample was placed on the ceramic hot plate under the vacuum (10-1 mbar).
2.4.2 Scanning Electron Microscopy
Compositional analysis was collected within a dual-beam scanning electron- focused ion beam microscope (SEM FEI Strata DB235) equipped with an X-ray
fluorescence spectroscopy source and detector (iXRF), and within an electron microscope (Zeiss Supra 50VP) equipped with an energy-dispersive X-ray spectroscopy system (EDS).
2.4.3 Transmission Electron Microscopy
Specimen preparation for the transmission electron microscopy (TEM) was done via two possible routes:
1. Mechanical polishing and subsequent ion milling (Fishione 1010 Low-angle ion mill). 2. Focused Ion Beam (FIB) milling of the cross-sectional specimen followed by the lift- out process. A dual-beam focused ion beam SEM (FEI Strata DB235) was used in this case.
Bright-field imaging was performed using TEM (JEOL JEM2100) operated at 200 kV. High-angular annular dark-field scanning TEM (STEM) was carried out at Oak Ridge National Laboratory using Nion UltraSTEM 200 operated at 200 kV and equipped with the electron energy loss spectrometer (EELS). Fourier filtering of the TEM images was done using Gatan DigitalMicrograph software; Wiener filtering was done using a script “HRTEM filters” written by D.R.G. Mitchell [82].
2.4.4 Atomic Force Microscopy and Piezoresponse Force Microscopy
Topographic height, local ferroelectric piezoelectric hystereses and piezoresponse force microscopy (PFM) were collected using an atomic force microscope, the latter two data types collected using dual ac resonance tracking (DARTTM) as implemented on an atomic force microscope (Cypher and MFP-3D, Asylum Research/Oxford Instruments,
Santa Barbara CA) using a Pt-coated cantilever (Olympus AC240TM, with nominal stiffness of 2 N/m and DART frequency of ~300 kHz). The presented phase and amplitude loops of the local ferroelectric piezoelectric signal represent an average of four full cycles, and are representative of phase switching and amplitude variation obtained in the films.
A separate set of measurements was carried out using ultrahigh vacuum variable- temperature AFM/STM (Omicron) located in the Center for Nanophase Materials Sciences (CNMS) at the Oak Ridge National Laboratory. The chamber pressure was 10-11 Torr, the temperature was varied from 110 K to 325 K. The band-excitation PFM was operated using the National Instruments PXI-based electronics. Numerical analysis was done using Matlab home-made scripts.
2.4.5 X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (Physical Electronics VersaProbe 5000) was carried out under ~8x10-9 Torr base pressure using the incident photon energy of 1486.6 eV (Al Kα line) with the irradiation power of 100 W over thr 100x100 µm2 area on the film. In situ XPS was collected from 250°C to 400°C with the step of 25°C, from 400°C to 600°C with the step of 20°C, and from 600°C to 700°C the step was 25°C. The heating rate was ~10°C/min, and acquisition time at each temperature was ~10 min.
2.4.6 Spectroscopic Ellipsometry
Spectroscopic ellipsometry was performed in the angle range of 65° to 75° using a dedicated variable-angle instrument (J.A. Woollam M2000). The signal acquisition was
carried out in the wavelength range from 247 to 1000 nm with a step of 1.6 nm. Fitting of the data was carried out in the native V.A.S.E. software using the Levenberg-Marquardt optimization method for minimizing the mean squared error between the model parameters and measured data. Four Tauc-Lorentz oscillators were used in the fitting procedure to properly assess the absorption coming from the electronic states below the conduction band of the parent ferroelectric (Eg<3.2 eV for BTNNO) [83]. This model is typically used for the amorphous materials, where broad defect-mediated absorption should be taken into account. The coefficients of the Tauc-Lorentz oscillators derived in the fitting are given in the Appendix.
2.4.7 Ferroelectric and photovoltaic measurements
The electrical measurements were carried out using a Lakeshore Cryotronics probe station (TTP4) equipped with the tungsten probes. Switching of ferroelectric polarization was measured with the commercial tester (Radiant Technologies, Inc) either in a high vacuum (10-6 torr) or in air. In each measurement two loops were collected at different frequencies and the data was processed using Time-Dependent Charge Compensation function as implemented by Radiant Technologies, Inc., following the procedure described in [84]. This approach is designed to reduce the contribution from leakage current in leaky ferroelectric thin films.
The I-V characteristics and other electric measurements were performed using a picoammeter (Keithley, model 6487) and Keithley 4200 Semiconductor Characterization System. The light excitation was done using 405-, 450- and 532-nm lasers (commercial laser pointers with a high beam power). The intensity of the laser beam was varied by
inserting specific neutral density filters in the beam path.
2.4.8 Magnetic measurements
The magnetization measurements of the ~30 nm BiFeO3/SrTiO3 thin film were performed using PPMS instrument (Quantum Design). The film was attached to the non- magnetic holder using tape. Magnetic field was applied in-plane, for ZFC/FC curves the field was 1 T. Because the substrate is diamagnetic, its contribution to the total signal of the substrate is observed. The M(H) curves and a hysteresis are given without diamagnetic contribution, which was subtracted via conventional data processing of M(H).
2.4.9 Rutherford Backscattering Spectroscopy
Rutherford Backscattering Spectroscopy (RBS) was conducted at the Army Research Lab, APG.
CHAPTER 3: GROWTH OF EPITAXIAL FILMS USING ALD