Optical properties of epitaxial BiFeO3 thin film grown on SrRuO3-buffered SrTiO3 substrate
© Xu et al.; licensee Springer. 2014
Received: 19 February 2014
Accepted: 11 April 2014
Published: 23 April 2014
The BiFeO3 (BFO) thin film was deposited by pulsed-laser deposition on SrRuO3 (SRO)-buffered (111) SrTiO3 (STO) substrate. X-ray diffraction pattern reveals a well-grown epitaxial BFO thin film. Atomic force microscopy study indicates that the BFO film is rather dense with a smooth surface. The ellipsometric spectra of the STO substrate, the SRO buffer layer, and the BFO thin film were measured, respectively, in the photon energy range 1.55 to 5.40 eV. Following the dielectric functions of STO and SRO, the ones of BFO described by the Lorentz model are received by fitting the spectra data to a five-medium optical model consisting of a semi-infinite STO substrate/SRO layer/BFO film/surface roughness/air ambient structure. The thickness and the optical constants of the BFO film are obtained. Then a direct bandgap is calculated at 2.68 eV, which is believed to be influenced by near-bandgap transitions. Compared to BFO films on other substrates, the dependence of the bandgap for the BFO thin film on in-plane compressive strain from epitaxial structure is received. Moreover, the bandgap and the transition revealed by the Lorentz model also provide a ground for the assessment of the bandgap for BFO single crystals.
PACS codes: 78.67.-n, 78.20.-e, 07.60.Fs
KeywordsBiFeO3 thin film Optical properties Spectroscopic ellipsometry Lorentz model Dielectric function
BiFeO3 (BFO) has attracted extensive research activities as an excellent multiferroic material. It simultaneously exhibits ferroelectricity with Curie temperature (TC = 1,103 K) as well as antiferromagnetism with Neel temperature (TN = 643 K), and the properties make BFO potential for applications in electronics, data storage, and spintronics [1, 2]. Especially, the BFO thin film is paid much attention due to its large spontaneous polarization, which is an order higher than its bulk counterpart , and then the BFO thin film combined with nanostructures could be a promising candidate in the above applications . In addition to its structural and electronic properties, optical properties of BFO thin films are focused on [5–9]. However, in the published literatures on optical studies, the BFO thin film is usually directly deposited on perovskite oxide SrTiO3 (STO) and DyScO3 (DSO) substrate for epitaxial growth. So far, there is no report on optical properties of the BFO thin film with an electrode structure in spite of the fact that the lower electrode is necessary for the study on electronic and ferroelectric properties of the BFO thin film as well as for its applications including nonvolatile memory devices . Since SrRuO3 (SRO) is often chosen as the lower electrode for the BFO thin film as well as for the buffer layer to control its nanoscale domain architecture , it is desirable to investigate the optical properties of the BFO thin film grown on SRO.
Spectroscopic ellipsometry (SE) is a widely used optical characterization method for materials and related systems at the nanoscale. It is based on the measuring the change in the polarization state of a linearly polarized light reflected from a sample surface which consists of Ψ, the amplitude ratio of reflected p-polarized light to s-polarized light and Δ, the phase shift difference between the both . The obtained ellipsometry spectra (Ψ and Δ at measured wavelength range) are fitted to the optical model for thin film nanostructure, and thus, rich information including surface roughness, film thickness, and optical constants of nanomaterials are revealed [13, 14]. Since SE allows various characterizations of the material, our group has studied some thin-film nanostructure using SE methods [15–18].
In this paper, we report the optical properties of epitaxial BFO thin film grown on SRO-buffered STO substrate prepared by pulsed-laser deposition (PLD) and measured by SE. The dielectric functions of STO, SRO, and BFO are extracted from the ellipsometric spectra, respectively. And the optical constants of the BFO thin film are obtained. The bandgap of 2.68 eV for the BFO thin film is also received and is compared to that for BFO thin film deposited on different substrate as well as BFO single crystals.
The epitaxial BFO thin film was deposited by PLD on SRO-buffered (111) STO single-crystal substrate. The SRO buffer layer was directly deposited on the STO substrate by PLD in advance. More details about the deposition process can be taken elsewhere . The crystal phases in the as-grown BFO thin film were identified by X-ray diffraction (XRD, Bruker X-ray Diffractometer D8, Madison, WI, USA). The surface morphologies of the BFO thin film were investigated by atomic force microscopy (AFM, Veeco Instruments Inc., Atomic Force Microscope System VT-1000, Plainview, NY, USA). Both XRD and AFM investigation are employed to show growth quality of the BFO thin film for further optical measurement and analysis.
Results and discussion
The model parameterization consists of four Lorentz oscillators sharing a high-frequency lattice dielectric constant (ϵ∞). The parameters corresponding to each oscillator include oscillator center energy Ecenter, oscillator amplitude A j (eV) and broadening parameter ν j (eV). This model yields thickness 105.15 nm for the SRO layer and the dielectric spectra displayed in Figure 3b. The center energy of the four oscillators is 0.95, 1.71, 3.18, and 9.89 eV, respectively, and is comparable to the reported optical transition for SRO at 1.0, 1.7, 3.0, and 10.0 eV [23, 24], which indicates that the extracted dielectric functions are reliable.
The bandgap of BFO on SRO is almost the same as that on DSO and is smaller than that on Nb-doped STO. It is noted that the in-plane (IP) pseudocubic lattice parameter for SRO and DSO is 3.923 and 3.946 Å , respectively, while STO has a cubic lattice parameter of 3.905 Å . Considering the IP pseudocubic lattice parameter 3.965 Å for BFO , the compressive strain for the BFO thin film deposited on STO substrate is larger than that on SRO and DSO. Thus, the more compressive strain imposed by the heteroepitaxial structure, the larger bandgap for the BFO thin film, which agrees with the past report .
The obtained direct bandgap 2.68 eV of the epitaxial BFO thin film is comparable to 2.74 eV reported in BFO nanocrystals  but is larger than the reported 2.5 eV for BFO single crystals . This can be understood because even for the epitaxial thin film, the existence of structural defect such as grain boundaries is evitable, which will result in an internal electric field and then widen the bandgap compared to single crystals. On the other hand, a bandgap of 3 eV for BFO single crystals through photoluminescence investigation is also reported . The broad and asymmetric emission peak at 3 eV in the photoluminescence spectra presented in  is attributed to the bandgap together with the near-bandgap transitions arising from oxygen vacancies in BFO. However, the Lorentz model employed to depict BFO optical response in our work reveals the existence of a 3.08-eV transition, which is the transition from the occupied O 2p to unoccupied Fe 3d states or the d-d transition between Fe 3d valence and conduction bands rather than the bandgap . Therefore, the broad and asymmetric peak is more likely to be explained as the overlap of the 3.08-eV transition and the bandgap transition with lower energy.
In summary, the optical properties of the epitaxial (111) BFO thin film grown on SRO-buffered STO substrate by PLD were investigated. The XRD and AFM analysis indicated that the BFO thin film sample is grown well with epitaxial structure and smooth surface. Then SE measurements were taken to get the ellipsometric spectra of the STO substrate, the SRO buffer layer and the BFO thin film, respectively, in the photon energy range 1.55 to 5.40 eV. The dielectric functions of STO, SRO, and BFO are obtained by fitting their spectra data to different models in which BFO corresponds to a five-medium optical model consisting of a semi-infinite STO substrate/SRO film/BFO film/surface roughness/air ambient structure. The BFO film and surface roughness thickness are identified as 99.19 and 0.71 nm, respectively. The optical constants of the BFO film are determined through the Lorentz model describing the optical response, and a direct bandgap at 2.68 eV is obtained which near-bandgap transitions could contribute to. Moreover, the gap value is compared to the BFO thin film with similar thickness deposited on various substrate prepared by PLD, indicating the dependence of the bandgap for the epitaxial BFO thin film on the in-plane compressive strain. In addition, the transition at 3.08 eV disclosed by the Lorentz model in our work suggests that the bandgap of BFO single crystals is less than 3 eV as previously reported. The results given in this work are helpful in understanding the optical properties of the BFO thin film and developing its application in optical field.
atomic force microscopy
root mean square error
This work has been financially supported by the National Natural Science Foundation of China (Nos. 11174058, 61275160, and 61222407), the No. 2 National Science and Technology Major Project of China (No. 2011ZX02109-004), and the STCSM project of China with Grant Nos. 12XD1420600 and 11DZ1121900.
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