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Continuously Tuning Epitaxial Strains by Thermal Mismatch Lei Zhang,†,‡,¶ Yakun Yuan,†,‡ Jason Lapano,†,‡ Matthew Brahlek,†,‡ Shiming Lei,†,‡ Bernd Kabius,‡ Venkatraman Gopalan,†,‡,§ and Roman Engel-Herbert*,†,‡,§,∥ †
Department of Materials Science and Engineering, ‡Materials Research Institute, §Department of Physics, and ∥Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
ACS Nano 2018.12:1306-1312. Downloaded from pubs.acs.org by STOCKHOLM UNIV on 10/02/18. For personal use only.
S Supporting Information *
ABSTRACT: Strain engineering of thin films is a conventionally employed approach to enhance material properties and to energetically prefer ground states that would otherwise not be attainable. Controlling strain states in perovskite oxide thin films is usually accomplished through coherent epitaxy by using lattice-mismatched substrates with similar crystal structures. However, the limited choice of suitable oxide substrates makes certain strain states experimentally inaccessible and a continuous tuning impossible. Here, we report a strategy to continuously tune epitaxial strains in perovskite films grown on Si(001) by utilizing the large difference of thermal expansion coefficients between the film and the substrate. By establishing an adsorption-controlled growth window for SrTiO3 thin films on Si using hybrid molecular beam epitaxy, the magnitude of strain can be solely attributed to thermal expansion mismatch, which only depends on the difference between growth and room temperature. Second-harmonic generation measurements revealed that structure properties of SrTiO3 films could be tuned by this method using films with different strain states. Our work provides a strategy to generate continuous strain states in oxide/semiconductor pseudomorphic buffer structures that could help achieve desired material functionalities. KEYWORDS: functional oxides, molecular beam epitaxy, SrTiO3 thin films, oxide electronics, ferroelectrics, strain engineering
S
train engineering has been proven a powerful tuning knob to enhance or enable various functionalities in thin films, which typically emerges in the coherent epitaxy, where the in-plane film lattice parameter precisely matches that of the substrate. In the semiconductor industry, depositing strained Si films on SiGe buffer layers has led to an increase of carrier mobility up to 80% in CMOS devices.1 The field of functional oxides has benefited from the rapid development of high-quality oxide substrates, and biaxial strains up to ∼6% have been successfully applied to oxide thin films, far exceeding their brittle bulk counterparts.2,3 Strain engineering in oxides has been used to craft the magnonic and spintronic response in magnetic thin films,4 to increase the transition temperature in high-temperature superconductors,5 to manipulate domain structures and enhance the Curie temperature and polarization in ferroelectric thin films,6−9 to drive the coexistence of tetragonal- and rhombohedral-like phases and corresponding large electromechanical strains in multiferroic thin films,10,11 to stabilize a multiferroic phase with ferroelectric and ferromagnetic order,12 to induce spontaneous polarization in incipient ferroelectric thin films,13−16 or to suppress polar phases in layer oxide materials.17 © 2018 American Chemical Society
However, whether the desired strain state can be achieved in the targeted oxide thin film with a specific lattice parameter is dictated by the availability of oxide substrates. The limited choice of commercially available substrates, particularly those with large lattice parameters (>4.00 Å) and between 3.79 and 3.90 Å, prevents an experimental realization of specific strain states and therefore hampers the ability to take full advantage of thin film strain states.2 Several attempts aiming for a continuous strain tuning on a single substrate were made. Chen et al. demonstrated that in a vertically aligned heteroepitaxial nanoscaffolding film, La0.7Sr0.3MnO3:MgO, on SrTiO3 substrate, a continuous strain up to 2% could be induced by varying the nanoscaffold density (MgO volume).18 Janolin et al. reported that by varying the Ba0.8Sr0.2TiO3 film thickness from 6 to 1000 nm on MgO substrate, a continuous strain control from −0.66 to 0.44% was realized through partial relaxation by dislocations formed at the film/substrate interface.19 Received: October 24, 2017 Accepted: January 10, 2018 Published: January 10, 2018 1306
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growth temperature (2.25% at 850 °C and 2.06% at 600 °C, which is larger than the room temperature lattice mismatch of 1.66%, as seen in Figure 2a), SrTiO3 thin films started to relax and restored their unstrained bulk lattice parameters, and the critical thickness for relaxation was reported to be as thin as around 8 ML thick.38 Two mechanisms are responsible for the film relaxation: the formation of misfit dislocation and the formation of an amorphous SiOx at the SrTiO3/Si interface after the first few ML of deposition,38 as shown schematically in Figure 1b. HAADF-STEM image (Figure 1e) shows an amorphous layer formed between the crystalline SrTiO3 film and Si substrate for the film grown at 850 °C. This amorphous layer was formed by oxidation of Si to amorphous SiO2 at the SrTiO3/Si interface, which has occurred after the epitaxy was picked up by the first few layers during the growth initialization process. Electron energy loss near-edge structure (ELNES) analysis showed that the interface layer consisted of amorphous SiO2. The shift of the Si-L electron energy-loss spectra (EELS) and elemental maps at the SrTiO3/Si interface are shown in Figures S3 and S4 in Supporting Information. The formation of the interfacial layer between SrTiO3 film and Si substrate is enabled by the oxygen diffusion in SrTiO3 at elevated temperature. Note that the amorphous layer was unlikely to form before growth because reflection high-energy electron diffraction (RHEED) patterns showed a clean, crystalline Si surface. Such amorphous interlayers were also seen for other oxide thin films grown on Si.39−41 After the films relaxed at growth temperature, a residual tensile strain was built upon cooling to room temperature that originated from thermal expansion mismatch between the SrTiO3 film and Si substrate, as schematically shown in Figure 1c. The detailed discussion of the mechanism and the analysis of the thermally induced strain states are presented later. Achieving a cation stoichiometry during the growth is essential in this study as the cation nonstoichiometry will make strain analysis convoluted with defect-induced lattice expansion,15,32−34,42−45 and it could also change the ferroelectric ground states of thin films.32−35 Hybrid MBE that combines chemical beam epitaxy and solid source MBE can provide a precise control of the stoichiometry by enabling a self-regulated growth mode.46,47 To ensure that all SrTiO3 films on Si substrates were grown within a growth window, the growth conditions were mapped by growing films at different PTTIP for 10 ML, and each 10 ML was characterized by in situ RHEED. Figure 2a shows RHEED pattern captured for films grown at 850 °C as a function of PTTIP along [100], [210], and [110] azimuths. The characteristic surface reconstructions of RHEED patterns were used to monitor the cation stoichiometry at the growth front. At low TTIP pressure (PTTIP = 46−50 mTorr), that is, the blue-shaded region in Figure 2a, the existence of 2× reconstructions along [110] azimuth was found, indicative of a Sr-rich surface, similar to those reported elsewhere.38,48,49 No surface reconstructions were observed at PTTIP = 52−54 mTorr, as highlighted by the orange region in Figure 2a. The emergence of 2× reconstructions along [100] and [210] was found at PTTIP = 56 mTorr, and additional 4× reconstructions along [110] were found at PTTIP = 52−68 mTorr, as shown by the orange region in Figure 2a. Such a systematic change of RHEED reconstruction was first observed in homoepitaxy growth of SrTiO3 by hybrid MBE under stoichiometric conditions.49 The appearance of 2× reconstructions at PTTIP = 56 mTorr can be related to the expanded region of coherent TiO2 coverage from unreconstructed surface. Higher order
Here, we show that by using a simple epitaxial heterostructure of SrTiO3 films grown on Si (001), a continuously tunable tensile strain can be generated by varying growth temperatures. This effect is attributed to the large difference of the coefficients of thermal expansion (CTE) between film and substrate with dissimilar structure and chemical bonding. Although epitaxial perovskite oxides could be grown on other semiconductors, such as GaAs and Ge,20,21 Si was used here due to its dramatically different CTE of ∼2.6 × 10−6/°C compared to ∼8.7 × 10−6/°C of SrTiO3.22,23 SrTiO3 is an intriguing complex oxide material exhibiting superconductivity and quantum oscillations.24−28 Two-dimensional electron gases can also form when interfaced with LaAlO3 or GdTiO3, and the superconducting transition temperature of single-layer FeSe was found to increase by an order of magnitude when epitaxially grown on SrTiO3.29−31 Furthermore, SrTiO3 is an incipient ferroelectric with a paraelectric-to-ferroelectric transition that can be triggered by biaxial strain or cation nonstoichiometry.13−16,32−35 In this study, we demonstrate that the magnitude of biaxial strain and physical properties of SrTiO3 on Si(001) can be continuously controlled by simply changing the growth temperature. Using high-resolution X-ray diffraction (HRXRD), reciprocal space mapping (RSM), highresolution transmission electron microscopy (HRTEM), highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and second-harmonic generation (SHG) measurements, intrinsic lattice parameter and strain states SrTiO3 thin films on Si(001) were extracted and related to their nonlinear optical properties.
RESULTS AND DISCUSSION SrTiO3 thin films were grown on Si(001) using hybrid molecular beam epitaxy (hybrid MBE). During the initial growth process, SrTiO3 thin films were coherently strained to Si with an in-plane orientation of [100] SrTiO3||[110] Si, as shown schematically in Figure 1a.36−38 The in-plane orientation was confirmed by a 45° separation of the SrTiO3(101) and Si(202) reflection peaks in a XRD φ scan in Figure 1d. Due to the larger lattice mismatch between film and substrate at
Figure 1. Schematic of (a) coherently strained, (b) fully relaxed, and (c) cooled SrTiO3 film on the Si substrate. (d) XRD φ scans around SrTiO3(101) and Si(202) diffraction peaks. (e) Scanning transmission electron microscope image of SrTiO3/Si interface. The scale bar represents 5 nm. 1307
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Figure 2. (a) RHEED images captured along the [100], [210], and [110] azimuths for SrTiO3 films grown at 850 °C as a function of titanium tetraisopropoxide (TTIP) pressure. (b) Out-of-plane lattice parameters as a function of titanium tetraisopropoxide (TTIP) pressure for films grown at 850 °C. (c) Out-of-plane lattice parameters and fwhm of rocking curve scans Δω of SrTiO3(002) peaks as a function of film thickness for SrTiO3 films grown at 850 °C.
mTorr (Sr-rich conditions) and PTTIP = 74 mTorr (Ti-rich conditions). The XRD growth window was consistent with the results from the RHEED analysis shown in Figure 2a. Figure 2c shows the oop lattice parameter a⊥ and the full width at halfmaximum (fwhm) of rocking curve scans taken from the SrTiO3(002) reflection peak as a function of film thickness grown at 850 °C. Films were fully relaxed at a thickness of 15− 20 nm as there was no further decrease of the oop lattice parameters. The crystalline quality improved with increasing thickness, in excellent agreement with earlier results on 46 and 120 nm thick films.53 The fwhm of the XRD rocking curve of (002) reflection peak of the 46 nm thick film grown at 600 °C is 0.67°, which is similar to 0.70° of the 40 nm thick film grown at 850 °C, as seen in Figure 2c, indicating that films grown at different temperatures here have similar crystalline qualities.54 The mechanism of continuous strain tuning is discussed next. Figure 3a shows the temperature-dependent lattice parameters of bulk Si taken as twice the interplanar distance along the [110] direction and bulk SrTiO 3.22,23,55,56 Bulk SrTiO3 undergoes an antiferrodistortive transition from cubic phase to tetragonal phase at Tc = 105 K. Clearly, the thermal expansion of SrTiO3 is much more dramatic than that of Si. The film cooled to room temperature is expected to build tensile tension arising from the stress induced by the large CTE difference. The in-plane tensile strain can be calculated based on the lattice parameter difference between the SrTiO3 thin
reconstructions are an indicator of purely TiO2-terminated surfaces, where the growing surface is saturated with TiO2. Therefore, the RHEED growth window was found to span from 52 to 68 mTorr at 850 °C. It is important to form such TiO2saturated surfaces to bring the growth kinetics into an adsorption-controlled regime. 49 The study of RHEED reconstruction was also carried out on films grown at 600 and 725 °C, as shown in Figure 2a, with detailed RHEED images shown in Supporting Information (Figure S1). A reduction in width and shift of the RHEED window was found for films grown at 725 °C. All reconstructions inherent to hybrid MBE, such as 4× reconstructions along [110], were not observed at 600 °C. SrTiO3 free of any surface reconstruction was only found at 49.5 mTorr, indicating that a similar behavior to that of solid source MBE occurred at these growth temperatures.38 The shift and shrinking of the growth window with the decreased temperature was in good agreement with earlier studies.46 To confirm the findings from the RHEED window, a series of 50 nm thick films were grown at 850 °C with different PTTIP. Figure 2b shows the out-of-plane (oop) lattice parameters calculated from XRD 2θ−ω scans as a function of PTTIP (Figure S2 in Supporting Information). Lattice parameter is an indicator of defects in SrBO3 (B = Ti or V), where cation nonstoichiometry was found to cause a lattice expansion.42,50−52 The oop lattice parameters were around ∼3.890 Å at PTTIP = 53−68 mTorr and rapidly increased for PTTIP = 46 1308
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temperature. The growth temperature-dependent strain state and in-plane lattice parameter are shown by dashed line as a function of growth temperature in Figure 3b. Under these considerations, SrTiO3 films grown at 900 °C can acquire a 0.65% tensile strain at room temperature if the amorphous SiO2 layer at the interface was rigid and did not relax the film lattice during cooling. To prove this hypothesis, SrTiO3 thin films were grown at three different temperatures (600, 725, and 850 °C). Since the cation stoichiometry can be well maintained during the growth, the in-plane lattice parameters were calculated from the oop lattice parameter a⊥ measured by XRD 2θ−ω scans from the Poisson equation:57 a⊥ =
where a⊥ is the oop lattice parameter, a∥ the in-plane lattice parameter, aSTO the intrinsic lattice parameter of SrTiO3, and υ the Poisson ratio. A value of 0.233 for the Poisson’s ratio was used here, which was estimated from RSM results (see Supporting Information) assuming a lattice parameter of 3.905 Å for SrTiO3 at room temperature. Such a value is in excellent agreement of 0.232 reported elsewhere.58 As shown in Figure 3b, SrTiO3 thin films have tensile strain of 0.42, 0.51, and 0.62% for films grown at 600, 725, and 850 °C, respectively, which is in good agreement with the calculation by the dashed line. The error bar of the in-plane lattice parameter and tensile strain in Figure 3b originated from the possible error of the Poisson ratio of SrTiO3, which is discussed in detail in Supporting Information. To confirm the in-plane lattice parameter, RSM were also taken around the asymmetric (103) and (10̅ 3) Bragg reflection of tetragonally distorted SrTiO3 film grown at 850 °C. The in-plane (oop) lattice parameters a∥ (a⊥) of 3.928 ± 0.001 Å (3.889 ± 0.001 Å) were determined by projecting the intensity peak to the in-plane and oop reciprocal lattice axes H and L, as shown in Figure 3c,d. The in-plane lattice parameter matched the calculated values very well, which confirmed the assumption that the film was fully relaxed at growth temperature prior to cooling, and the amorphous SiO2 layer was ideally rigid and did not allow the tensile strain built up in the film during cool-down to relax. Because SrTiO3 is an incipient ferroelectric and the occurrence of ferroelectric ground state is directly related to its exact strain state,2,15 we performed temperature-dependent SHG measurements to determine the paraelectric-to-ferroelectric phase transition for films under different strain states. SHG is a nonlinear optical process that is extremely sensitive to any inversion symmetry breaking transition. Figure 4 shows the SHG intensity as a function of the growth temperature of SrTiO3 films, and SHG polarimetry measurements are shown in Figure S5 in Supporting Information. The SHG signals remained at the noise level irrespective of temperature for SrTiO3 grown at 600 °C. In comparison, for films grown at 850 and 725 °C, the Curie temperatures of 170 ± 5 and 70 ± 5 K were found, respectively, determined by the inflection point of temperature-dependent SHG signal where long-range ordering of polarization started to be established. The SHG signal above the Curie temperature was attributed to the presence of polar nanoregions (PNRs), forming at the Burns temperature. The Burns temperature was determined by the intersection of SHG signal with noise level. The Burns temperatures for films grown at 850 and 725 °C films were determined to be 270 ± 5 and 230 ± 10 K, respectively. It should be noted here that the strain
Figure 3. (a) Lattice parameters of bulk SrTiO323,56 (purple line) and Si22,55 (blue line) and calculated out-of plane (green line) and in-plane (red line) lattice parameters for SrTiO3 films on Si substrate. (b) Calculated in-plane strain (dashed line) and calculated in-plane lattice parameters (solid line) of SrTiO3 films and experimental results. Reciprocal spaces mappings around (c) asymmetric (103) and (d) (103) Bragg reflections of a 120 nm thick SrTiO3 film grown at 850 °C.
film on Si(001) and SrTiO3 bulk, as indicated by the curly bracket in Figure 3a: a
,STO(T2)
ε=
a
= aSTO(T1) − [aSi(T1) − aSi(T2)]
,STO(T2)
− aSTO(T2)
aSTO(T2)
× 100%
where T1 is growth temperature, T2 room temperature, aSTO(T1) the lattice parameter of SrTiO3 at the growth temperatures, and a∥,STO(T2) the in-plane lattice parameter of SrTiO3 film on Si substrate at room temperature, aSi(T1) − aSi(T2) the lattice parameter reduction of Si from growth to room temperature, and aSTO(T2) the bulk SrTiO3 lattice parameter at room temperature. The formula for in-plane lattice parameter a∥,STO(RT) and in-plane lattice expansion Δa∥,STO(RT) at room temperature can be rephrased using the TCE αSi of Si: a
,STO(RT )
Δa
= aSTO(T1) −
,STO(RT )
T1
∫RT αSi·aSi(T )dT
= aSTO(T1) − aSTO(RT ) −
T1
∫RT αSi·aSi(T )dT
Assuming the TCE of Si is constant, the above equations can be simplified as a
,STO(RT )
Δa
≈ aSTO(T1) − αSi·aSi(RT ) ·(T1 − RT )
,STO(RT )
υ+1 2υ a − aSTO υ−1 υ−1
≈ aSTO(T1) − aSTO(RT ) − αSi·aSi(RT ) · (T1 − RT )
The strain state induced by the thermal expansion mismatch was therefore only dependent on the difference of lattice parameters between the growth temperature and room 1309
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desorption process in the MBE chamber.62 First, a 5 monolayer (ML) and a 10 ML buffer layer were deposited by co-supplying Sr and TTIP without molecular oxygen at 400 and 600 °C, respectively. SrTiO3 films were subsequently grown at different temperatures (600, 725, and 850 °C) at an oxygen background pressure of 5 × 10−7 Torr using conditions discussed in detail elsewhere.53 High-Resolution Transmission Electron Microscopy Characterization. Focused ion beam has been used to obtain cross sectional samples suitable for TEM examination. Atomic resolution was achieved by HAADF-STEM mode with a double-corrected FEI Titan TEM operated at 300 kV. EELS were recorded with a Gatan Quantum ERS spectrometer at an energy resolution of 1.0 eV, which is sufficient to distinguish Si from SiO2 from the energy shift and the fine structure of ELNES of the Si-L EELS transition. Temperature-Dependent Second-Harmonic Generation Measurements. Temperature-dependent SHG measurements were carried out using a far-field reflection geometry with a fundamental wavelength of 800 nm, generated by a Ti:sapphire amplified laser system (∼100 fs, 1 kHz, Solstice Ace). The fundamental beam was focused onto the sample at incident angle of 45°, and the reflected SHG signal at 400 nm was first spectrally filtered and then detected by a photomultiplier tube using a lock-in method. The measurement temperature was varied using a helium-cooled Janis 300 cryostat.
Figure 4. Temperature-dependent SHG intensity for films grown at 600, 725, and 850 °C. The Curie temperature (TC) and Burns temperature (TB) are labeled for films grown at different temperature.
of films further increased in the SHG measurement temperature compared to the values measured at room temperature. The strain at 170 K (70 K) for the film grown at 850 °C (725 °C) was 0.73% (0.67%), considering additional strain films gained after cooling from room temperature to cryogenic temperature. While films grown at 725 and 850 °C showed the clear sign of structural transitions that involved breaking of inversion symmetry, no sign of the formation of a polar phase was found for the film grown at 600 °C down to 15 K. This result is in good agreement with prior findings that a ferroelectric phase could be stabilized at room temperature for a SrTiO3 film on DyScO3 with ∼1.00% tensile strain and predicted strain−temperature phase diagram.2,13 However, further electrical studies on SrTiO3 films on highly insulating Si substrates are needed to confirm the polarization switchability.2
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b07539. RHEED images taken to determine the growth window at 600 and 725 °C; X-ray scans of SrTiO3 films grown at 850 °C on Si as a function of TTIP pressure; electron energy loss spectroscopy data taken at the Si-L edge; electron energy loss spectroscopy maps taken across the SrTiO3/SiO2/Si interface for Si, Ti, and O; SHG polarimetry measurements of SrTiO3 on Si; error propagation of the Poisson’s ratio for SrTiO3 thin films on Si substrate; error propagation of the in-plane lattice parameter and tensile strain of SrTiO3 thin films on Si substrate (PDF)
CONCLUSIONS We have experimentally demonstrated that a continuous strain tuning can be achieved by using the mismatch of the thermal expansion coefficients of an oxide thin film SrTiO3 and a semiconductor substrate Si(001). The amount of tensile strain of SrTiO3 films on the Si(001) substrate can be continuously tuned by varying the growth temperature. SrTiO3 thin films with different tensile strain states also exhibited different functional properties at low temperature, as measured by second-harmonic generation measurements. Such a scheme can also be potentially extended to other material systems beyond SrTiO3 on Si(001) substrate, which could ideally provide a pseudomorphic virtual substrate with variable lattice parameters for thin film growth.
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Lei Zhang: 0000-0002-1559-8469 Present Address ¶
Department of Materials Science and Engineering, University of California, Berkeley, California 94720.
Author Contributions
L.Z. and R.E.-H. conceived the idea and designed the experiment. L.Z., J.L., and M.B. grew the films. L.Z. performed XRD and RSM measurements. Y.Y., S.L., and V.G. performed SHG measurements. B.K. performed TEM measurements. L.Z. and R.E.-H. wrote the manuscript, with feedback from all of the authors.
EXPERIMENTAL SECTION Hybrid Molecular Beam Epitaxy Growth. SrTiO3 thin films were grown on 3 in. Si(001) wafers (p-type, 1−10 Ohm × cm, 380 μm thick, University Wafer, Inc.) by hybrid molecular beam epitaxy (hybrid MBE, DCA Instruments) using the recipe developed by McKee and also used by other groups.38,54,59−61 Sr was supplied from a thermal effusion cell at a fixed flux of 2.5 × 1013 cm−2 s−1 calibrated by a quartz crystal microbalance. Ti was supplied in the form of metal−organic precursor, titanium tetraisopropoxide, from a gas injector without carrier gas. A variable leak valve equipped with a capacitance manometer was used to precisely control the pressure of TTIP (denoted as PTTIP). The growth reactor is equipped with RHEED. Si wafers were degreased by acetone and isopropyl alcohol and etched in 2% dilute hydrogen fluoride solution prior to growth, and the remaining native oxide layer was removed using a Sr-assisted
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We acknowledge the financial support by the National Science Foundation through Grant No. DMR-1352502 (L.Z. and R.E.H.) and the Penn State MRSEC Program DMR-1420620 (J.L., S.L. and V.G.), and the Department of Energy through Grant DE-SC0012375 (Y.Y. and M.B.). 1310
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