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Thin Film Substrates from the Raman spectroscopy point of view

We have investigated ten standard single crystal substrates of complex oxides on the account of their applicability in the Raman spectroscopy based thin film research. In this study we suggest a spectra normalization procedure that utilizes a comparison of the substrate Raman spectra to those of well-established Raman reference materials. We demonstrate that MgO, LaGaO3, (LaAlO3)_0.3(Sr2AlTaO6)_0.7 (LSAT), DyScO3, YAlO3, and LaAlO3 can be of potential use for a Raman based thin film research. At the same time TiO2 (rutile), NdGaO3, SrLaAlO4, and SrTiO3 single crystals exhibit multiple phonon modes accompanied by strong Raman background that substantially hinder the Raman based thin film experiments.


INTRODUCTION
Thin film research is an integral part of modern materials science. Investigation of the artificial heterostructures consisting of correlated oxide materials represents one branch of such studies. This research gives insight into complex interactions taking place in the correlated oxides and phenomena associated with the stress present on the interface of the oxide and the substrate. Artificial heterostructures have a potential to yield a number of electronic applications. [1][2][3] .
Optical spectroscopy in general and Raman spectroscopy in particular provide a versatile non-destructive tool to investigate thin films. For instance, Raman spectroscopy allows one to probe low frequency elementary excitations such as phonons, magnons and some electronic excitations as well as complex interplay between them. Many ccorrelated phenomena such as metal-insulator transition, orbital ordering, and charge ordering are manifested through changes in structural symmetry [4][5][6][7] . These changes can be probed by Raman technique. Furthermore Raman spectroscopy has become one of the leading tools to investigate oxide heterostructures as the substrate induced strain effect causes additional structural changes. [8][9][10] All this makes Raman spectroscopy a valuable tool to investigate correlated phenomena in thin films.
A choice of a thin film substrate is dictated by minimisation of the mismatch between the film and the substrate's unit cell. However such choice often overlooks Raman properties of the substrate and may unintentionally render such substrate useless for a Raman-based experiment particularly when the substrate exhibits rich and intensive Raman spectrum. Establishing a procedure for selection of a good Raman substrate is one of the goals of this study.
In a Raman experiment the laser light is shone on the film and scattered light is analysed.
Typically the film is too thin to prevent the light from penetrating into the substrate. As a consequence the Raman spectra of both the film and the substrate are detected in the experiment.
In majority of the situations only the spectrum of the film is of interest to a researcher. Hence it is necessary to separate the spectrum of the film from that of the substrate.
A vast majority of the film's low energy excitations is typically located below eight to nine hundred wavenumbers (<100 meV) Raman shift. A substrate that has no Raman modes within this frequency range and exhibits low Raman background will not mask any excitations that come from the film. Such substrate would be considered a good candidate for a Raman based thin film experiment. The goal of this study is to select such substrate. This goal can be achieved through comparison of the intensities and the frequencies of the substrate's modes.
Comparing the intensity of Raman signal of different materials is often a challenge. The use of an absolute scattering cross-section in Raman spectroscopy is rare. In most of the situations the researchers are interested in the Raman shift of a certain mode but not in its absolute cross-section. 11 As a consequence the Raman intensity is routinely reported in arbitrary units. Different sensitivity of the detectors, different collection optics, and different configurations of experiment performed by different research groups make it practically impossible to make direct comparison of the raw spectra reported in the literature. However, evaluation of the normalised Raman intensity provides a sensible alternative to the absolute scattering cross-section measurements. One of the oldest methods that have been used is the sample substitution method 11,12 : during the experiment the sample is replaced by a standard by "means of a lateral displacement" 11,12 and spectra of both the sample and the reference are recorded and compared. The wide gap insulators with charge gap exceeding the energy of the visible light photon are of particular use as the references. In particular, CaF 2 (E g =12 eV) 13 , BaF 2 (E g =11eV) 13 , and diamond (E g =5.5eV) 14 can be used for this purpose. 12 In this short paper we suggest a modified sample replacement procedure that allows one to make a consistent comparison of the Raman intensities of different materials. We employed this procedure to analyse different single crystal complex oxide substrates and found that LaAlO 3 , YAlO 3 , LaGaO 3 , DyScO 3 , (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 (LSAT), MgO substrates can be used in a Raman spectroscopy based thin film study while NdGaO 3 , SrLaAlO 4 , SrTiO 3 , and TiO 2 (rutile) display strong background signal and/or too many phonon modes to be of practical use in such Raman experiment.

II. EXPERIMENTAL
Un-polarized Raman spectra of commercially available substrates (Crystec, Germany) have been measured at room temperature using 514 nm line of Coherent Innova-70 Argon Ion Laser with the laser power not exceeding 10 mW when focused on the sample. The spectra were collected in the backscattering geometry using microscope attachment of the Horiba/Jobin Yvon T64000 Raman spectrometer operated in the double subtraction mode. All the measurements were performed with Olympus × 100 microscope objectives with numerical aperture of 0.9. The objective produced a 2 µm laser spot on the sample. Corresponding laser intensity did not exceed 2.5 GW/m 2 or 250 kW/cm 2 .

III. RESULTS AND DISCUSSION
In the experiment we compare the Raman intensity of a sample to that of CaF 2 and Si.
CaF 2 displays a well pronounced Raman mode at 321 cm -1 . The CaF 2 charge gap 13 of 12 eV is significantly larger than the visible photon's energy. This fact assures that no resonance effects will affect the Raman scattering in the CaF 2 . Si is a very common reference material in practically every Raman laboratory. This material displays a well pronounced single mode at 520 cm -1 which is routinely used for a Raman setup alignment. This is the reason to provide the spectra normalised to the Si in addition to those normalised to CaF 2 even though Si has a charge gap in the infrared range (1.1 eV) 15  We would like to note that the use of the slopes of the peak intensity vs. incident laser intensity eliminates a potential effect of a non-Raman background. In other words, if there is an offset in the Raman spectrum due to a flat non-Raman background then such offset will not affect the slope of this graph but will show up as a non-zero intercept on the peak intensity vs.
laser intensity graph. wavenumbers would be critical for any substrate to be used in the Raman base thin film measurements. We consider materials with broad FSR and weak Raman background in FSR to be potentially useful Raman substrates. Table I  The goal of the analysis is to find which substrate may be of potential use for a thin film research involving Raman technique. We define a good Raman substrate as one that has an FSR broader than 200 cm -1 and with the normalised (to CaF 2 ) intensity in FSR not higher than 0.65.
The later number is the FSR Raman intensity observed in Si.
Based on these criteria we found magnesium oxide (MgO) to be the best Raman substrate among those that we have measured. This material does not display any strong Raman modes between 200 and 900 wavenumbers and the relative (to CaF 2 ) background intensity in the FSR is about 0.24 as opposed to 0.65 in Si or 2.9 in strontium titanate ( Table I).
Aside of MgO all other substrates that we measured display a number of phonon modes and/or broad Raman background. Some of these substrates display FSR broader than 200cm -1 with low Raman background intensity. Those are LaGaO 3 , LSAT, DyScO 3 , YAlO 3 , and LaAlO 3 .
These substrates may have a potential use for the Raman based thin film research. However actual use of any of these substrates depends on the particular frequency of the spectral features of interest in the spectrum of the film. If these features fall within the FSR of the substrate then the substrate will be of use. Table I may be of help when deciding on a particular substrate.
In the situation when reasonable FSR is present the Raman intensity within the FSR seems to correlate with the extent of the substrate's charge gap (E g ). Table I  When number of phonon modes in the spectrum is large the FSR becomes small (<200 cm -1  Based on our analysis MgO is the best Raman substrate with largest free spectra range and fairly low Raman background. LaGaO 3 , LSAT, DyScO 3 , YAlO 3 , LaAlO 3 can be considered as good Raman substrates since they display a reasonably wide spectral range that is free of the phonon modes together with low intensity flat Raman background within this range. Finally TiO 2 rutile, NdGaO 3 , SrLaAlO 4 , and SrTiO 3 display a very little of free spectral range and thus the interpretation of Raman spectra with those substrates is rather complex and may dramatically hinder the physics associated with the films of interest. The strontium titanate (STO) is by far exhibits the strongest Raman response among those that we measured.
Specifically, it has practically no FSR and displays high background intensity over 100-900 wavenumbers.   as measured by the CCD detector, I max is the peak intensity of the 155 cm -1 mode, m 155 is the slope of the graph in the inset of this figure, m Si is that of the Si, Fig.1(a) inset. Note that the spectral range between 570 and 900 wavenumber is free of any strong Raman modes with the normalised Raman background intensity around 0.1.