Enhanced electrical and magnetic properties in La0.7Sr0.3MnO3 thin films deposited on CaTiO3-buffered silicon substrates

We investigate the suitability of an epitaxial CaTiO3 buffer layer deposited onto (100) Si by reactive molecular-beam epitaxy (MBE) for the epitaxial integration of the colossal magnetoresistive material La0.7Sr0.3MnO3 with silicon. The magnetic and electrical properties of La0.7Sr0.3MnO3 films deposited by MBE on CaTiO3-buffered silicon (CaTiO3/Si) are compared with those deposited on SrTiO3-buffered silicon (SrTiO3/Si). In addition to possessing a higher Curie temperature and a higher metal-to-insulator transition temperature, the electrical resistivity and 1/f noise level at 300 K are reduced by a factor of two in the heterostructure with the CaTiO3 buffer layer. These results are relevant to device applications of La0.7Sr0.3MnO3 thin films on silicon substrates.

We investigate the suitability of an epitaxial CaTiO 3 buffer layer deposited onto (100) Si by reactive molecular-beam epitaxy (MBE) for the epitaxial integration of the colossal magnetoresistive material La 0.7 Sr 0.3 MnO 3 with silicon. The magnetic and electrical properties of La 0.7 Sr 0.3 MnO 3 films deposited by MBE on CaTiO 3 -buffered silicon (CaTiO 3 /Si) are compared with those deposited on SrTiO 3 -buffered silicon (SrTiO 3 /Si). In addition to possessing a higher Curie temperature and a higher metal-to-insulator transition temperature, the electrical resistivity and 1/ f noise level at 300 K are reduced by a factor of two in the heterostructure with the CaTiO 3 buffer layer. These results are relevant to device applications of La 0.7 Sr 0.3 MnO 3 thin films on silicon substrates. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. The full spectrum of electronic, optical, and magnetic properties-e.g., insulating, semiconducting, superconducting, ferroelectric, and ferromagnetic effects-is found within the structurally compatible family of perovskite oxides. The integration of these epitaxial functional oxides with silicon substrates offers significant opportunities for applications. [1][2][3][4][5][6][7][8] Among them are micro-electromechanical systems (MEMS) based on epitaxial piezoelectric layers [9][10][11][12][13] and suspended bolometers based on epitaxial La 0.7 Sr 0.3 MnO 3 thin films. 14 The use of silicon substrates greatly facilitates the fabrication of MEMS and suspended bolometers, where three-dimensional structures can be efficiently realized by volume silicon micromachining using conventional techniques such as isotropic etching in alkaline solutions (KOH, TMAH, etc.) or reactive ion etching. 15 for such silicon-based heterostructures is one in which the functional oxide film can be epitaxially grown directly on silicon. Such direct integration is, however, complicated by the high reactivity of silicon with oxygen and the disruption of epitaxy that results from the presence of the resulting amorphous silicon oxide layer at the surface of silicon wafers. An epitaxial buffer layer is, therefore, a general prerequisite to the growth of epitaxial functional oxides on silicon having high structural perfection. An additional challenge is the large difference in the thermal expansion coefficients between silicon and these functional oxides; the ratio of thermal expansion coefficients is about a factor of three between room temperature and growth temperature.
To fabricate epitaxial structures in which the properties of the underlying silicon and the overlying film both achieve their full potential, control of the silicon-oxide interface is critical. Fundamental considerations that must be taken into account in the selection of appropriate epitaxial buffer layers include chemical and structural compatibilities. 17 For the case of silicon, a comprehensive analysis of its thermodynamics stability in contact with binary oxides has been performed. 18 Reactions leading to the formation of interfacial silicide, silicate, or SiO 2 layers have been reported when these oxides are exposed to high temperatures during device processing. 19 20 In terms of structural compatibility, few oxides are well lattice matched to (100) Si. The lattice match of the small number of oxides with the perovskite structure that has been epitaxially integrated with (100) Si using a thin (as thin as a single monolayer (ML)) binary oxide buffer layer is shown in Fig. 1 28 and SrHfO 3 . 29,30 The latter two, SrZrO 3 and SrHfO 3 , with pseudocubic lattice constants of 4.101 Å and 4.070 Å at room temperature, respectively, lie off the top of Fig. 1.
Among these materials, SrTiO 3 (cubic cell with c = 3.905 Å) has been the most widely pursued perovskite buffer layer, but due to the large lattice mismatch with Si (1.7%), SrTiO 3 begins to relax for thickness beyond a few nanometers in thickness, drastically degrading the crystalline quality of the SrTiO 3 buffer layer. 31  reported, 21,22 the potential of CaTiO 3 as a buffer layer to transition from (100) Si to functional oxides with the perovskite structure has been largely ignored.
In this paper, we show that the mineral perovskite, CaTiO 3 , can be used as a buffer layer in the epitaxial transition from (100) Si substrates to perovskite functional oxides, such as La 0.7 Sr 0.3 MnO 3 . We demonstrate here that La 0.7 Sr 0.3 MnO 3 films deposited on CaTiO 3 /Si indeed showed enhanced electrical properties (e.g., lower electrical resistivity and 1/ f noise, and high temperature of the metal-to-insulator transition) as well as enhanced magnetic properties (higher Curie temperature) compared to other epitaxial La 0.7 Sr 0.3 MnO 3 thin films deposited on buffered silicon substrates. [33][34][35][36][37][38][39] We grew epitaxial CaTiO 3 thin films on (100) Si by reactive molecular-beam epitaxy (MBE). The native SiO 2 layer was removed from the (100) Si substrate using a strontium assisted process. 40 Two monolayers of strontium metal (corresponding to 1.2 × 10 15 atoms/cm 2 ) were deposited at a substrate temperature of T = 600 • C. Then, the substrate temperature was increased to T = 800 • C. At this temperature, the silicon dioxide layer was removed by the formation and evaporation of SiO x , 40 and a single crystalline reflection high-energy electron diffraction (RHEED) pattern with a double-domain 2 × 1 (100) Si reconstruction was observed.
CaTiO 3 films were grown using a codeposition technique in a manner analogous to the leading technique for producing the highest quality SrTiO 3 /Si films. 25,31 Calcium was evaporated from an effusion cell and titanium from a Ti-Ball™ sublimation source. 41 The fluxes of the constituent elements, calcium and titanium, were measured using a quartz crystal monitor and typical values for each element were around 1 × 10 13 atoms/cm 2 s. A substrate temperature of 330 • C was used to grow the first 2.5 MLs at a background partial pressure of molecular oxygen of 7 × 10 −9 Torr. vacuum, to 580 • C for 15 min (as shown in Fig. 2(b)). Next, the sample was cooled down to 330 • C to grow a second 2.5 MLs of CaTiO 3 under the same growth conditions as the first 2.5 MLs. Again the sample was annealed in vacuum at T = 580 • C for 8 min. At this temperature, the calcium, titanium, and oxygen shutters were opened simultaneously, and with the oxygen background pressure at 1 × 10 −7 Torr, the thickness of the epitaxial CaTiO 3 layer was grown to 20-40 nm. Figure 2(c) shows the RHEED pattern at the end of the growth of the 20-nm thick CaTiO 3 layer. Rutherford backscattering spectrometry/channeling (RBS/C) utilizing He + ions with an energy of 1.4 MeV was applied to investigate the composition and crystalline quality of the films. The computer software RUMP was employed to analyze the RBS data. 42 The films have a Ca:Ti composition ratio of 1.05 ± 0.05. A RBS/C minimum yield χ min = 12% was observed.
Following the growth of the CaTiO 3 film, 50 nm of La 0.7 Sr 0.3 MnO 3 was deposited on it. The La 0.7 Sr 0.3 MnO 3 film was grown at a substrate temperature of 670 • C by codeposition in a distilled ozone background pressure of 5 × 10 −7 Torr. 43 The RHEED pattern at the completion of the 50-nm thick La 0.7 Sr 0.3 MnO 3 layer completing the La 0.7 Sr 0.3 MnO 3 /CaTiO 3 /Si heterostructure is shown in Fig. 2(d).
Film structural properties and morphology were investigated by X-ray diffraction (XRD) and by atomic force microscopy (AFM) in tapping mode (Digital Instruments-Nanoscope III). Cross sectional transmission electron microscopy (TEM) specimens were prepared by mechanical grinding and polishing, followed by argon ion milling (Gatan model 691 Precision Ion Polishing System) to electron transparency. The samples were examined using a JEOL 3011 high resolution TEM, operated at 300 kV.
In Fig. 3(a), a θ-2θ x-ray diffraction scan of the La 0.7 Sr 0.3 MnO 3 /CaTiO 3 /Si heterostructure is shown. The labels indicate the h00 p series of CaTiO 3 and La 0.7 Sr 0.3 MnO 3 peaks. As expected, the La 0.7 Sr 0.3 MnO 3 film grows under compressive strain which results in a larger out-of-plane lattice constant c = 3.905 Å compared to bulk La 0.7 Sr 0.3 MnO 3 , which has a pseudocubic lattice spacing of 3.876 Å. Figure 3 Fig. 3(b). The peak has a FWHM of 0.86 • in φ. A smooth surface with a root mean square (RMS) roughness of 0.5 nm was measured over a 1 µm × 1 µm region by AFM (see Fig. 4). This is the lowest value reported for manganite films of comparable thickness grown on silicon substrates. 39,[44][45][46][47] The microstructure of the La 0.7 Sr 0.3 MnO 3 /CaTiO 3 /Si epitaxial heterostructure was studied by TEM. Figure 5(a) shows the overall microstructure of the specimen, including an amorphous SiO x layer, which formed in situ by the diffusion of oxygen through the growing epitaxial film and oxidation of the underlying silicon substrate during the film growth. The formation of SiO x layers during growth is common in epitaxial oxide on silicon systems when the oxygen partial pressure is high and at the same time, the silicon substrate is hot. 48,49 The out-of-plane linear defects evident in Fig. 5(a) are rotation domain boundaries, common to CaTiO 3 . A typical region of the La 0.7 Sr 0.3 MnO 3 /CaTiO 3 interface, exhibiting good epitaxy between the two layers, is shown in Fig. 5(b). A selected-area electron diffraction (SAED) pattern, taken from both film layers simultaneously, is shown in Fig. 5(c). No spot-splitting is evident, consistent with good epitaxy between the two layers.
We have performed electrical and magnetic measurements on this same sample. Electrical resistivity measurements as a function of temperature were performed on unpatterned films by the standard four-probe technique. Magnetization was measured by a superconducting quantum interference device (SQUID) magnetometer. Figure 6(b) reports the magnetization as a function of temperature in a field of 0.01 T. The zero-field-cooled (not shown) and field-cooled magnetizations have been measured. The curve has been fitted by a theoretical standard static Brillouin magnetization function. The La 0.7 Sr 0.3 MnO 3 shows a rapid increase of the magnetic moment below the Curie temperature T C = 360 K, and the saturated magnetization value at low temperature is 3.5 µ B /Mn. These values are similar to the values of bulk La 0.7 Sr 0.3 MnO 3 50 and the highest reported for thin films grown on silicon substrates. 44,47,51 The metal-insulator transition temperature is consistent with the magnetic properties. The temperature dependence of the electrical resistivity is shown in Fig. 6(a). The film shows a metal-insulator transition temperature, T MI , higher than 400 K. This is the highest value reported for La 0.7 Sr 0.3 MnO 3 grown with or without a buffer layer on silicon substrates. [36][37][38]44,47,[51][52][53][54][55] Moreover, the electrical resistivity value at 300 K is 1.5 mΩ cm, which is quite similar to bulk La 0.7 Sr 0.3 MnO 3 , 56  heterostructures. The inset of Fig. 6(a) shows the electrical resistance as a function of temperature for a bare 20-nm thick CaTiO 3 film on a (100) Si substrate. Insulating behavior over the whole temperature range is observed. This leads us to conclude that both the electrical and magnetic properties are enhanced compared to those reported in high epitaxial quality La 0.7 Sr 0.3 MnO 3 films deposited on SrTiO 3 buffered Si substrates, 39 where T MI and T C values were 350 K and 330 K, respectively. We ascribe the enhancement of these properties to the fact that the La 0.7 Sr 0.3 MnO 3 cell is under compressive in-plane strain on CaTiO 3 /Si as predicted by Millis et al. 57 and also experimentally observed in Ref. 43. Electrical low-frequency noise measurements were performed at 300 K in the same way as previously described. 39,58,59 The La 0.7 Sr 0.3 MnO 3 /CaTiO 3 /Si thin film was patterned by UV photolithography and argon ion etching to form a 50 µm wide and 150 µm long strip, which includes two gold pads for supplying the current and two gold pads at which the voltage was measured in a four-probe geometry. Figure 7 presents the voltage noise spectral density measured at various bias currents. We can clearly observe both Johnson (or thermal) noise at high frequency and 1/ f (or flicker) noise at low frequency. In contrast to Johnson noise, which depends neither on bias current nor on frequency, the latter gives a frequency and bias current dependent contribution to noise, which gives an indication of the material quality. 60 This 1/ f noise is usually described by the Hooge empirical relation, which does not have any physical basis, but has been shown to agree well with experimental observations for homogeneous samples. This relation is given by the following general formula: 61 where S V is the voltage noise spectral density (V 2 Hz −1 ), V is the sample voltage (V), α H is the Hooge parameter (dimensionless), n is the charge carrier density (m −3 ) , Ω is the sample volume (m 3 ), and f is the measuring frequency (Hz). It is very useful to compare the 1/ f noise magnitude in different materials independent of the sample volume and the bias conditions. In order to estimate the voltage noise spectral density of the material, the noise of the electronic readout and the noise of the voltage contacts were removed. As presented in the inset of Fig. 7, the quadratic dependence of the voltage noise spectral density at 1 Hz and at 300 K versus the sample voltage was verified within experimental error bars as expected from Eq. (1), thus enabling a correct estimation of α H /n values. The normalized Hooge parameters α H /n was then measured to be (4.2 ± 0.6) × 10 −31 m 3 at 300 K, which is about two times lower than the one measured in La 0.7 Sr 0.3 MnO 3 films of comparable thickness deposited on SrTiO 3 /Si with α H /n values of (9.8 ± 0.6) × 10 −31 m 3 at 300 K and only two times higher than the one measured in La 0.7 Sr 0.3 MnO 3 films of comparable thickness deposited on SrTiO 3 single crystal substrates with α H /n values of (2.47 ± 0.6) × 10 −31 m 3 . 39,58,59 In conclusion, we have shown the promise of epitaxial (100) p -oriented CaTiO 3 as a buffer layer for the integration of functional oxides having the perovskite structure with silicon. Specifically, we have grown high-quality epitaxial La 0.7 Sr 0.3 MnO 3 films on CaTiO 3 /Si. Both the Curie and the metal-to-transition temperatures are the highest reported values for La 0.7 Sr 0.3 MnO 3 thin films deposited on buffered silicon substrates. The corresponding electrical resistivity and the 1/ f noise level are decreased by a factor of two compared to those measured in high-quality epitaxial La 0.7 Sr 0.3 MnO 3 thin films of comparable thickness deposited on SrTiO 3 /Si. These films exhibit magnetic and electrical properties comparable with bulk La 0.7 Sr 0.3 MnO 3 , making them of interest for room temperature applications on silicon substrates.
We gratefully acknowledge the financial support of Intel and the National Science Foundation through the MRSEC program (Grant Nos. DMR-1120296 and DMR-1420620), and Grant Nos. ECCS-0708759 and DMR-0315633. This work was performed in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation (Grant ECCS-0335765).