Conducting LaAlO3/SrTiO3 heterointerfaces on atomically-flat substrates prepared by deionized-water

We have investigated how the recently-developed water-leaching method for atomically-flat SrTiO3 (STO) substrates affects the transport properties of LaAlO3 (LAO) and STO heterointerfaces. Using pulsed laser deposition at identical growth conditions, we have synthesized epitaxial LAO thin-films on two different STO substrates, which are prepared by water-leaching and buffered hydrofluoric acid (BHF) etching methods. The structural, transport, and optical properties of LAO/STO heterostructures grown on water-leached substrates show the same high-quality as the samples grown on BHF-etched substrates. These results indicate that the water-leaching method can be used to grow complex oxide heterostructures with atomically well-defined heterointerfaces without safety concerns.

is employed to ensure the formation of single-terminated atomically-flat substrates before deposition and to confirm film surface quality after deposition. Epitaxial LAO thin-films of various thickness (5-60 unit-cells) are deposited on the STO substrates using pulsed laser deposition with a laser fluence (KrF excimer, λ = 248 nm) of 1.6 J/cm 2 , a substrate temperature of 700 °C, and pO 2 of 10 −6 Torr. In situ reflection high energy electron diffraction (RHEED) is utilized to monitor the number of unit cells of LAO deposited. The grown samples are cooled naturally for 2 hours to room temperature at a higher oxygen partial pressure (10 mTorr) so that the films have proper oxygen stoichiometry. There is no clear systematic thickness dependence of LAO thin-films on their transport properties, as reported previously 28 . Thus, here we focus our discussion on the results obtained from the 5, 25, and 30 unit-cell LAO samples. Structural quality of the films is characterized using X-ray diffractometry (Bruker D8 Advance). Optical transmission spectra is taken at room temperature using a Fourier-transform infrared spectrometer (FT-IR) (for spectra regions between 50 meV and 0.6 eV) and a grating-type spectrophotometer (for spectra regions between 0.5 and 6 eV). Transport properties are measured using a Physical Property Measurement System (Quantum Design) with conventional four-probe and Hall geometries. Hall measurements are taken at various temperatures at a maximum magnetic field of 9 T. Electrical contacts are made using aluminum wire attached with indium solder, which gives access to the 2DEG present at the heterointerface.

Results and Discussion
LAO thin-films deposited on water-leached STO substrates show the same film quality as BHF-etched substrates. Figure 1(a) depicts a 3 × 3 μm 2 atomic force microscopy (AFM) topography scan of a water-leached STO substrate with respective line profile below. As indicated in the line profile, the substrate has a step height of 3.9 Å, which is the lattice constant of cubic STO. Figure 1(b) displays the same sample as in (a) after deposition of a 30 unit-cell LAO film. Both images show single-terminated atomically-flat step terraces before and after deposition. Figure 1(c) displays the RHEED intensity oscillations for the 5 unit-cell thick LAO film deposited on the water-leached substrate. The insets show the RHEED patterns at the beginning and end of film deposition, which, other than a change in intensity, do not display any noticeable changes. The high quality of the LAO films is confirmed further by the X-ray θ-2θ scans, as shown in Fig. 1(d) for the 30 unit-cell thick films. The peak position of the (220)-LAO plane does not depend on substrate preparation method. The X-ray reciprocal space maps near the (114)-STO reflection show that both LAO thin-films exhibit coherent in-plane tensile strain with no evidence of strain relaxation, as shown in Fig. 1(e) for water-leached and 1 (f) for BHF-etched samples.
The optical transmission spectra of both heterointerfaces show little difference in the range of 0.2-3.2 eV, demonstrating that their optical properties and electronic structures are quite similar regardless of substrate preparation method. Figure 2 illustrates the optical transmittance spectra of the 25 unit-cell LAO/STO grown on water-leached and BHF-etched substrates. Both spectra demonstrate clear Drude absorption due to conducting carriers, i.e. the decrease of optical transmittance, below about 1.5 eV. These transmittance spectra are consistent with the optical properties of LAO/STO heterointerfaces, reported in ref. 5. The three dip structures near 1.7, 2.4, and 2.9 eV are commonly observed in LAO/STO heterostructures and reduced STO crystals. The absorption at 1.7 eV increases as STO crystals are reduced, hence it is related to the oxygen vacancy level 29 . The dip structures at 2.4 eV and 2.9 eV are observed regardless of free carrier concentration, and they may originate from the excitation of electrons trapped by oxygen vacancies, i.e. F 1 centers 30 .
The sheet resistance of both heterointerfaces has similar behavior down to low temperatures, regardless of substrate preparation method. Figure 3 shows the sheet resistance as a function of temperature for the LAO/ STO heterointerfaces for the 5 unit-cell and 30 unit-cell LAO layers. The sheet resistance of the same LAO thickness is qualitatively identical despite the use of two different methods of substrate preparation. It is noteworthy that the 30 unit-cell LAO/STO samples display metal-insulator transitions at around 40 K while the 5 unit-cell LAO/STO samples are overall metallic. This behavior has been reported previously: the resistivity of LAO/STO

and 30 unit-cell LAO thin-films grown on water-leached (blue) and BHF-etched (red) STO substrates.
heterointerfaces with thicker LAO layers can be larger than that of thinner samples, which may be due to structural reconstructions at the LAO/STO interface 28 .
The heterointerfaces also have comparable carrier concentrations and mobilities. The results of the Hall measurements for the metallic 5 unit-cell LAO/STO heterointerfaces are displayed in Fig. 4(a, b). Sheet carrier concentration (n s ) and mobility (μ) of the heterointerfaces prepared on the two kinds of STO substrates are similar regardless of preparation method. The values of n s and μ compare well to those of other conducting LAO/STO 2DEG's where similar deposition conditions were used 1,3,4,28 . Further, room temperature n s exceeding or near 10 13 -10 14 cm −2 are observed in most LAO/STO 2DEG's when the pO 2 of deposition is below 10 −5 Torr 1,3,4,28 . Thus, as in most low-pO 2 LAO/STO heterointerfaces, oxygen vacancies play a role in the heterointerfacial conductivity. It is noteworthy that below 100 K we observe the non-linear Hall effect due to multi-channel electron conduction as shown in the inset of Fig. 4(b) as has been seen previously [6][7][8] . This effect can be fitted by a two-band model, assuming the same sign for the charge carriers 31  , where μ * and R ∞ are fitting parameters with R 0 being R H (B = 0). Using the zero field resistivity, R XX = (en 1 μ 1 + en 2 μ 2 ) −1 , we can find the low-density-high-mobility (LDHM) (n 2 and μ 2 ) and high-density-low-mobility (HDLM) (n 1 and μ 1 ) carriers using , and = + ∞ n C eR C 2 (1 ) [33][34][35] .
The model fits at 50 and 2 K are shown by the black lines in the inset of Fig. 4(b). As stated above, neither the LDHM nor the HDLM display any differences based on substrate preparation. According to ref. 24, the BHF-etching method might result in a few percent of fluorine doping into STO, which can provide 4 × 10 13 cm −2 to 1 × 10 14 cm −2 extra carriers 24 . Figure 4(a), however, shows that n s for both samples is very similar in the whole measurement temperature range. As for changes in μ, ref. 24 also suggests that the fluorine atoms, acting as impurity sites, would increase the scattering rate, thereby reducing the overall μ of any heterointerface. However, the μ of the two kinds of 5 unit-cell 2DEG samples shows little or no difference, as displayed in Fig. 4(b). Thus, fluorine doping does not appear to alter the electronic properties of oxygen-deficient conducting LAO/STO heterointerfaces.  , and NdGaO 3 /SrTiO 3 41,42 have demonstrated intriguing electronic reconstructions, interfacial superconductivity, and magnetic ordering. Hence, the use of the water-leaching method promotes research on future oxide electronics by providing a safe way to prepare atomically-flat complex-oxide substrates.