Oxygen vacancy induced conductivity in Sr n+1Ti n O3n+1/SrTiO3 heterostructures

SrTiO3-based heterostructures have attracted much attention due to the abundant properties compared to single components. Here, we fabricate oxide heterostructure of layed perovskite/perovskite Sr n+1Ti n O3n+1/SrTiO3 and investigate the n value-dependent and thickness-dependent conductivity. X-ray diffraction peaks and reflective high energy electron diffraction indicate good film quality. For films of n=6, the heterostructures are conductive, and the conductivity is better for thicker film. On the contrary, heterostructures with films of n=1, 2, 3, 4, ∞ are insulating. Conductive atomic force microscopy results and surface conductivity tests manifest that the oxygen vacancy induced conductive layer exists near the interface between film and substrate. This work provides feasible method to modulate the transport properties of functional transition metal oxides.


Background
In recent years, transition metal oxide has become one of the most intriguing field in condensed matter physics due to the exotic properties such as superconductivity [1][2] , ferromagnetism [3] , piezoelectricity [4] , and ferroelectricity [5] .Among the large number of transition metal oxides, SrTiO3 attracts much attention for more than 50 years [6][7][8][9][10] .On one hand, SrTiO3 itself can exhibit ferromagnetism [11] , high electron mobility [12][13] , and superconductivity with low carrier density [14][15] .On the other hand, SrTiO3 has proper lattice constant, stable chemical properties and excellent physical characteristics.Hence, SrTiO3 is a good candidate to serve as the substrate to grow numerous materials and show abundant properties compared to single component.In 2004, H. Y. Hwang reported the conductive interface with high electron mobility exceeding 10000 cm 2 V -1 s -1 when polar LaAlO3 film was deposited on nonpolar SrTiO3 substrate [16] .Since then, a variety of SrTiO3-based heterostructures were synthesized to investigate the mechanism of interfacial conductivity.

Research gap
The conductivity of some SrTiO3-based heterostructures originates from the creation of oxygen vacancies near the interface, and a typical example is γ-Al2O3/SrTiO3 [17][18][19] .Redox reaction occurs between deposited γ-Al2O3 film and substrate, forming lots of oxygen vacancies at SrTiO3 side.Y. Z. Chen detected the presence of Ti 3+ signal by X-ray photoelectron spectroscopy, and they found that Ti 3+ signal disappeared when the sample was annealed at 200 ℃ under 1 bar oxygen atmosphere.Furthermore, quantum oscillation phenomenon manifested the two-dimensional conductivity [17] .However, the elemental components are different for such heterostructures, which might cause elemental mixing and influence the sample quality.

Our work
In this paper, we grow a series of Ruddlesden-Popper Srn+1TinO3n+1 films with various n values on TiO2 terminated SrTiO3 substrates.Film growth processes are introduced in detail and the relationships of n value, film thickness and conductive behavior are investigated.The elemental components of these films are Sr, Ti and O, which are identical to those of the substrates.Therefore, the film quality is good because of the absence of elemental mixing.Experimental results suggest that only the heterostructures with n=6 films show conductive behavior, and the conductivity is thicknessdependent.

Method
Ruddlesden-Popper perovskite Srn+1TinO3n+1 films with different n values are deposited on (001) SrTiO3 substrates by molecular beam epitaxy method.We etched the purchased substrates in HF acid for 60 seconds to remove the surface SrO layers [20] .Then the substrates were annealed in O2 atmosphere at 1000 ℃ for 80 minutes to obtain the TiO2 terminated surfaces.Single TiO2 termination of the substrate is an important factor to ensure the crystalline quality of Srn+1TinO3n+1/SrTiO3 heterostructure.In Srn+1TinO3n+1, the n value corresponds to the number of SrTiO3 layers sandwiched between SrO-SrO layers.The crystal structures of SrTiO3, Sr2TiO4 (n=1), and Sr3Ti2O7 (n=2) are illustrated in figure 1.

Experimental results
We adjust the temperature of Sr and Ti sources to ensure that the flux ratio is 1:1.Reflective high energy electron diffraction (RHEED) was used to monitor the flux change during the calibration process and the oscillation curves are shown in the figure 2(a).Then the films are deposited by specific sequences according to the n values.The theoretical results of J. H. Lee indicate that the second layer SrO and the third layer TiO2 will overturn when the deposition sequence is SrO-SrO-TiO2 [21] .Besides, the third layer SrO and the fourth layer TiO2 will overturn when the deposition sequence is SrO-SrO-SrO-TiO2.As a result, the growth sequence for Sr2TiO4 (n=1) is two SrO layers and one TiO2 layer, while the growth sequence for Sr3Ti2O7 (n=2) is three SrO layers and two TiO2 layers.The sequences of Srn+1TinO3n+1 films with various n values and fixed thickness of 25 nm are defined according to the above theory, and the growth condition is 780℃ ， 1×10 -6 Torr O2.For Sr2TiO4 film, the half-order peaks exist along [110] azimuthal since the termination is SrO layer.XRD data (measured by Bruker D8 Discover) of n=1, 2, 3, 4 are shown in figure 2(b).The diffraction peaks are obvious even for the high n value films with weak peak intensity, suggesting the good film quality which is comparable to the previous literature [22] .After growth, the electrodes of these heterostructures are fabricated by ultrasonic aluminum wirebonding method, which can let the aluminum wires penetrate through the film and reach SrTiO3 substrate.Heterostructures of n=1, 2, 3, 4 are insulating, suggesting the absence of conducting layer.Next, we grow 5u.c., 7u.c., and 9u.c.Sr7Ti6O19 (n=6) films on SrTiO3 substrates to investigate the effects of film thickness on the conductivity of these heterostructures.The film thicknesses are 25 nm, 35 nm, and 45 nm, respectively.For XRD data in figure 3(a), the asterisks indicate the diffraction peaks of SrTiO3 substrates.All of them show characteristic peaks of Sr7Ti6O19, and the peaks for 5u.c.Sr7Ti6O19 are not obvious since the diffraction intensity is weak for thinner film.After film deposition, we fabricate electrodes by ultrasonic aluminum wirebonding to measure the resistance and find that the heterostructures are conductive.Then physical property measurement system (PPMS) experiments are performed to investigate the thickness-dependent electricity.According to the resistance-temperature results in figure 3(b), heterostructures with thicker films exhibit lower resistance values, suggesting better conductivity.In addition, heterostructure with 7u.c.Sr7Ti6O19 film is attached to conductive silicon wafer.Then the sample is polished to sectional specimen and scanned by conductive atomic force microscopy.In figure 4, the left panel is the topography image while the right panel is the current mapping.The applied voltage during scanning process is 1V.Furthermore, the sample surface is insulating when the electrodes are made by indium pressing.We can infer from the above data that the conductive layer exists near the interface between the film and substrate rather than the whole heterostructure.Besides, the conductive layer disappears when the sectional specimen is placed in ambient

Discussion
Compared to LaAlO3/SrTiO3, the elemental components of film and substrate are identical in our heterostructure.Hence, the conductivity can not from the polar catastrophe induced by the atomically abrupt interface [23] .Srn+1TinO3n+1 films with lower n values exhibit higher oxygen vacancy formation energy, so they can hardly provide channels for oxygen vacancies in SrTiO3 substrates to escape [24] .As a consequence, heterostructures with films of n=1, 2, 3, 4 are insulating.In our assumption, oxygen vacancies are created in the whole heterostructure of n=6 during the film deposition process since the growth temperature is relatively high.Upon cooling, a part of interfacial oxygen vacancies at SrTiO3 side are protected by Sr7Ti6O19 film, while oxygen vacancies in other positions are filled by oxygen atoms in surrounding atmosphere.In addition, we grow SrTiO3 (n=∞) films on SrTiO3 substrates and find that the heterostructures are insulating.According to the DFT calculation results in our previous work, the oxygen vacancy formation energy and diffusion barrier for SrTiO3 are relatively low [24] .As a result, the created oxygen vacancies during the film deposition process are filled by surrounding oxygen atoms during cooling process, leading to the insulating behavior.

Conclusion
In summary, we grow layered perovskite Srn+1TinO3n+1 films with different n values on perovskite SrTiO3 substrates and modulate the n value-dependent and thickness-dependent conductivity.Heterostructures with films of n=1, 2, 3, 4, ∞ are insulating while films with n=6 are conductive, and the conductivity is better for thicker film.Conductive atomic force microscopy results and surface conductivity tests manifest that the oxygen vacancy induced conductive layer exists near the interface between film and substrate.Our findings provide a promising platform to fabricate electronic devices related to transition metal oxide heterostructures.

Figure 2 .
Figure 2. (a) RHEED patterns for Sr2TiO4/SrTiO3 and RHEED oscillations during calibration process.Red and blue curves correspond to the dashed boxes in RHEED patterns.(b) XRD peaks of heterostructures with n=1, 2, 3, 4 films.The peaks marked by asterisks are from SrTiO3 substrates.

Figure 3 .
Figure 3. (a) XRD peaks of heterostructures with n=6 films for different thicknesses.(b) Resistancetemperature curves of heterostructures with n=6 films for different thicknesses.