Supplementary Materials.doc

Supplemental MaterialsPhase structure and field-dependent charge injectionBiFeO3 thin films with the thickness of 200 nm were deposited on (100) SrTiO3 single-crystal substrates at 700C using pulsed laser deposition. During deposition, the flowing oxygen pressure was adjusted to be 50mTorr, and the pulsed KrF excimer laser beam (wavelength 248 nm) was focused to reach a laser fluence of 1-2 J/cm2 on the ceramic target surface. For the investigation of the effect of the film leakage current on the preferred domain orientations, three Fe-enriched bismuth ferrite ceramic targets were prepared with nominal chemical compositions of BiFeO3, BiFe1.05O3 and BiFe1.1O3. With the increase in the Fe content, we can observe the reflection from an impurity phase of Bi2Fe4O9 in BiFe1.1O3.1 Before electrical measurements, the 150 nm-thick epitaxial LaNiO3 conductive layer was pre-grown as a lower electrode using ULVAC ACS-4000-C4 magnetron sputtering. The crystal structures were investigated by X-ray diffraction (Bruker X-ray Diffractometer D8 discover) with Cu Ka1 and Kb radiations, as shown in Fig. S1. The film surface was studied with a Veeco MultiMode-V atomic force microscopy (AFM) operated in tapping mode, as shown in Fig. S2. The plate-like grains change into big islands with much rougher film surface in releasing epitaxial stresses with the further increase in the film thickness.2Before measurements, each film is susceptible to the repetitive bipolar voltage stressing at 10 V/300 ns for 10-15 cycles to achieve a stable measurement. The upper circular Au electrodes with the diameters of 5-100 mm were deposited on the films through lithography. The input domain switching pulses are supplied by an Agilent 33250A arbitrary waveform generator with a rise time of 5 ns, and the switching current is monitored through an in-series LC WR 6200A oscilloscope with the internal resistance of 50 W.Fig. S1 XRD patterns of bismuth ferrite thin films with different Fe excesses.Fig. S2 AFM micrograph of a 200 nm-thick BiFe1.1O3 thin film.The process of charge compensation is discussed here. The accumulated opposite charges contradict initial immobile trapped charges with the density of s0, for example, the positively charge oxygen vacancies VO and negatively charged FeFe (Fe3+Fe2+) separately piled up near top and bottom electrodes with density distribution functions of rO(x) and rFe(x), respectively, as shown in Fig. S3(a), where s0 = = . These immobile charges can compensate for the domain boundary charges for the as-grown films, which become uncompensated to generate an internal field immediately after polarization reversal, as shown in Fig. S3(b). Under a negative field stressing of the film for time t, the two immobile charges can be compensated by two opposite injected charges of electrons and holes near top and bottom electrodes with density distribution functions of re(x) and rh(x), as shown in Fig. S3(c). In this case, Ei disappears as s0 = = . However, if the two distribution functions between injection charges and immobile charges cannot overlap together, and the small portion of the injected charges near the interfaces could be smoothed out of the film thickness after the removal of the stressing field due to the built-in heterogeneous interfacial field, as shown in Fig. S3(d).Fig. S3 The sketched boundary charge compensation of the preferred domains by the opposite immobile charges of VO and FeFe as well as deeply trapped electrons and holes near top (x=0) and bottom electrodes (x = d), respectively, for (a) the as-grown films with the two immobile charge distribution functions of rO(x) and rFe(x), (b) the film with the presence of an internal field due to the appearance of uncompensated immobile charges immediately after polarization reversal, (c) the film with one-side electron or hole injection at a negative basing voltage to compensate for the immobile charge with a density distribution function of re(x) or rh(x), and (d) the reduced injection charge densities of and near electrodes after removal of the biasing voltage.References1 Z. M. Tian, S. L. Yuan, X. L. Wang, X. F. Zheng, S. Y. Yin, C.