Atomic layer deposition of environmentally benign SnTiOx as a potential ferroelectric material

Inspired by the need to discover environmentally friendly, lead-free ferroelectric materials, here the authors report the atomic layer deposition of tin titanate (SnTiOx) aiming to obtain the theoretically predicted perovskite structure that possesses ferroelectricity. In order to establish the growth conditions and probe the film structure and ferroelectric behavior, the authors grew SnTiOx films on the commonly used Si(100) substrate. Thin films of SnTiOx have been successfully grown at a deposition temperature of 200 C, with a Sn/Ti atomic layer deposition (ALD) cycle ratio of 2:3 and postdeposition heat treatments under different conditions. X-ray photoelectron spectroscopy revealed excellent composition tunability of ALD. X-ray diffraction spectra suggested anatase phase for all films annealed at 650 and 350 C, with peak positions shifted toward lower 2-theta angles indicating enlarged unit cell volume. The film annealed in O2 at 350 C exhibited piezoresponse amplitude and phase hysteresis loops, indicative of the existence of switchable polarization. VC 2015 American Vacuum Society. [http://dx.doi.org/10.1116/1.4935650]


I. INTRODUCTION
Perovskite-based oxides that possess ferroelectric properties, such as BaTiO 3 and PbTiO 3 , have long been the focus of intensive research for decades because of their strong polar, piezoelectric and dielectric properties that make them useful for a wide variety of technological applications.2][3][4][5][6] Since the behavior of both ions is governed by the electron lone-pair phenomena, SnTiO 3 is supposed to display properties similar to those of its "isoelectronic relative." 4 recent theoretical study utilizing first principle calculations has identified perovskite as the most stable polymorph of SnTiO 3 (P-SNO). 42][3][4][5] Yet, the actual synthesis of bulk P-SNO by conventional methods, such as sintering, remains unsuccessful due to the easy disproportionation of Sn 2þ into Sn 4þ and Sn metal at high temperatures. 1,3he loss of the Sn 2þ oxidation state results in the disappearance of the (stereochemically active) electron lone-pair resulting in a nonpolar material.Nevertheless, thin film deposition techniques provide an alternative route to bypass the limitations imposed by traditional bulk synthesis-e.g., they can stabilize the film structure through misfit strain introduced from the lattice mismatch between the film and the substrate. 7owever, in a recent attempt to synthesize P-SNO via pulsed laser deposition, a growth of nonpolar, ilmenite SnTiO x thin film on sapphire and perovskite substrates from SnO 2 and TiO 2 targets was reported, 8 suggesting that stoichiometric growth of the SnTiO 3 phase exhibiting lone-pair activity was not achieved.This motivated us to conduct a further investigation into the issue of the stabilization of P-SNO in the proper oxidation state using thin film deposition techniques.
Atomic layer deposition (ALD) is a well-established, gasphase thin film deposition technique that is used to fabricate a variety of thin films that are utilized in many fields such as semiconductor processing, energy harvesting, and catalyst synthesis and design.Its advantages include conformal coating, excellent thickness control, large-area uniformity, and good compositional tunability.ALD can be performed at lower deposition temperatures compared to chemical vapor deposition or pulsed laser deposition, which not only reduces the thermal budget, but also allows the production of films that are unstable at high temperatures.0][11][12] Indeed, ALD is a suitable technique to tackle the challenge of growing P-SNO.
In this work, we report the preparation of low temperature (200 C), as-deposited and annealed SnTiO x thin films on p-type Si substrates using tetrakis(diethylamino)titanium (TDEAT), tin(II)acetylacetonate [Sn(acac) 2 ] and ozone as the reactants.Postdeposition annealing at 350 and 650 C in N 2 , 4% H 2 and O 2 ambient gases was performed to obtain better film a) Electronic mail: takoudis@uic.educrystallinity, which is, presumably, also supposed to improve the sample polar properties.X-ray photoelectron spectroscopy (XPS) was used to probe film stoichiometry and ALD compositional tunability.Film structure was studied by x-ray diffraction (XRD), and a pattern corresponding to anatase phase was found for most of the films.Piezoresponse force microscopy (PFM) revealed that the film annealed at 350 C in O 2 exhibited piezoresponse hysteresis loop, which is an indication of switchable polarization.

II. EXPERIMENT
ALD of tin titanate was performed in our custom-built reactor, using TDEAT and Sn(acac) 2 as the metal precursors, and ozone as the oxidant.The reactor is typically operated at $500 mTorr and has a base pressure of $20 mTorr.Both precursors were kept in stainless steel bubbler and maintained at 65 and 70 C, respectively, during deposition.N 2 (99.998%) was used as the carrier gas for both precursors as well as the purging gas to clean the chamber after each reactant's pulse.P-type Si(100) substrates (resistivity: 1-10 XÁcm, 20 Â 20 mm 2 ) were cleaned according to Radio Corporation of America (RCA) standard cleaning procedure (SC-1), followed by an HF treatment to reduce the native oxide, and finally rinsed by DI water and dried by N 2 blow.Before the codeposition on Si(100), optimization of ALD growth parameters for both oxides were carried out.Precursor pulses for TDEAT and Sn(acac) 2 were set to 6 and 8 s, respectively, to saturate the surface.
Thicknesses of the films were measured with a spectroscopic ellipsometer (J.A. Woollam Co., Inc., model M44).Postdeposition annealing was carried out in a preheated quartz horizontal furnace (Lindberg Blue three-zone furnace) under N 2 , O 2 , and 4% H 2 environment for 5 min at 350 and 650 C.
Elemental compositions of the films were studied using a high resolution x-ray photoelectron spectrometer (Kratos AXIS-165, Kratoz Analytical, Ltd., United Kingdom) equipped with a monochromatic Al Ka (1486.6 eV) x-ray source operated at 15 kV and 10 mA.Grazing incidence xray diffraction (GIXRD) diffractograms were obtained for as-deposited and annealed films using a high resolution xray diffractometer (Philips X'pert) configured with a 0.1542 nm x-ray emission line of Cu.
PFM measurements were carried out in a dual AC resonance tracking PFM (DART-PFM) mode (Asylum research, MFP-3D, Santa Barbara, CA, USA) using Pt/Ir coated silicon cantilevers (PPP-EFM, Nanosensors) with tip radius of 30 nm. 13 Vertical piezoresponse hysteresis loops were measured at the contact resonance frequency in DART-PFM mode at three randomly selected points on sample surface.Four measurements were taken at each arbitrary point and averaged to give final results.

A. Film growth characteristics
It is crucial to have a sufficient overlap of the constituent binary oxides temperature window before attempting the ternary oxide ALD, in order to easily select the codeposition temperature as well as reproduce the results.Figure 1 shows the temperature dependence of growth rates of both TiO 2 and SnO x .The region where growth rate is independent of temperature is taken as the temperature window.It can be seen that a wide, overlapping ALD temperature window of 175-275 C has been obtained.The increased growth rates of both oxides found below the temperature window were attributed to precursors condensation, whereas the decreased TiO 2 growth rate at temperatures above the window resulted from reduced adsorption of precursors onto the substrate surface, and the increased growth rate of SnO x in this region was likely due to the Sn(acac) 2 decomposition.Within this common temperature window suitable for SnTiO x deposition, the reaction temperatures of each metal oxide as well as the mixed oxide were set to 200 C.
Figure 2 shows the dependence of SnTiO x , TiO 2 , and SnO x film thicknesses on total number of ALD cycles.Excellent linearity of the curve indicates precise thickness control of the ALD processes that corroborates its selflimiting nature.A TiO 2 film growth rate of 0.05 nm/cycle, and SnO x film growth rate of 0.1 nm/cycle were found after linear regression analyses (inset).The growth rate of TiO 2 agrees well with that reported earlier by our group using the same precursor but in a different system. 14SnO x ALD growth conditions used in this work and its film characterization have been studied by our group and reported elsewhere. 15Film thicknesses of SnTiO x (main) also linearly depended on the total number of ALD cycles.It is found that at an ALD cycle ratio (ALDCR) of Sn/Ti ¼ 2:3, the growth rate was 0.07 nm/cycle, indicating a simple linear combination of binary oxide growth rates and a clean codeposition process with no cross-contamination of precursors.

B. Compositional and structural analysis of SnTiO x thin films
Controlling the composition of each element is another key step in growing targeted ternary oxides.In ALD, this is achieved by adjusting the ALDCR of metal precursors.A series of seven ALD reactions of SnTiO x with different ALDCR was performed in order to get the optimal ratio that most likely results in the stoichiometry of SnTiO 3 .The atomic ratio [Sn/(Sn þ Ti)] in the resultant films as a function of normalized ALD cycle ratio was examined by XPS, and shown in Fig. 3.By varying the ALD cycle ratio of SnO x and TiO 2 , Sn content in the films, which spanned from 30% to 80% across different samples, can be effectively controlled.A normalized cycle ratio [SnO x /(SnO x þ TiO 2 )] of 0.4 represents two cycles of SnO x alternating with three cycles of TiO 2 .With this ALDCR, we achieve the atomic ratio of Sn/(Sn þ Ti) $ 0.5, indicating a SnTiO x stoichiometry.However, excess oxygen was confirmed from XPS elemental analysis, suggesting an x value slightly greater than three.This can be attributed to the sensitivity of XPS to the surface absorbed, oxygen-containing species, e.g., moisture from ambient exposure, or the use of strong oxidizer such as ozone.About 4 at.% of carbon contamination was detected after 10 min Ar þ sputtering, which has been reported in thin films grown using b-diketonate precursors. 16ue to the metastable nature of Sn 2þ , the annealing conditions are chosen such that films go through neutral (N 2 ), oxidizing (O 2 ), and reducing (4% H 2 ) heat treatment environments for better comparison.Two annealing temperatures were selected: a lower 350 C to prevent disproportionation of Sn(II), and 650 C in the hope of getting better film crystallization.Structures of as-deposited and annealed SnTiO x films were studied by grazing incidence XRD (GIXRD), as presented in Fig. 4. All films examined were $40 nm-thick.As-deposited films show diffraction features around 25 , 54 , and 55 , which are assigned to TiO 2 anatase phase with crystal planes of (101), (105), and (211), respectively, signifying film crystallization at deposition temperature (200 C).Anatase formation at this temperature has been reported in ALD of TiO 2 , 17,18 as well as in Sn-doped TiO 2 films grown using spin coating technique. 19The films annealed at 650 C [Fig.4(a)] and 350 C [Fig.4(b)] under different ambient gases exhibited similar XRD patterns as those of asdeposited films, suggesting anatase as the major phase after the heat treatments.However, the positions of anatase peaks of all the samples shifted to lower angles without peak broadening, especially for the one annealed in O 2 at 350 C.This is a clear indication of unit cell enlargement.Since the radius of Sn ion is slightly larger than that of Ti, it is likely that Sn atom gets incorporated into TiO 2 lattice by taking over the position of Ti, thus inducing a lattice distortion which leads to the expansion of the cell volume.As for the film annealed in O 2 at 350 C that showed the largest peak  shift, its new peak position corresponds to that of (010) orientation of perovskite SnTiO 3 ; however, lack of other evidential peaks from GIXRD pattern prevents drawing definite conclusions on the final structure.Also, the broadened doublet peak at $54 may indicate the presence of a rutile phase, but its characteristic peak at 27.5 is not observed.Therefore, a further structural investigation, i.e., through TEM, is needed to obtain a better insight into the crystal structure of these thin films.

C. PFM characterization
Surface topography, piezoresponse amplitude and phase images of as-deposited and annealed SnTiO x films, which were fabricated at various temperatures and gas conditions, were taken on each sample by piezoresponse force microscopy.All films exhibited relatively smooth surfaces with surface rms roughness of below 1.25 nm, as shown in Figs.5(a) and 5(b).After annealing at higher temperature (650 C), the particle size increased comparing to the as-deposited films, the surface roughness also increased slightly, which can be explained by the heat-induced particle growth commonly seen in thin films.In order to confirm the presence of piezoelectric and ferroelectric properties in as-deposited and annealed SnTiO x films, the averaged piezoresponse hysteresis loops were measured from randomly selected regions on each film.Among the samples, only the SnTiO x film annealed in O 2 at 350 C showed better-defined piezoresponse amplitude and phase hysteresis loops with coercive voltage of 3.04 V, as shown in Fig. 6.The other films processed under different annealing conditions did not show any meaningful piezoresponse (not shown), ruling out possible ferroelectric properties from PFM data.[22][23] IV.SUMMARY AND CONCLUSIONS ALD synthesis of SnTiO x thin films was successfully carried out on Si substrates by combining two binary oxide ALD processes, with the utilization of tetrakis(diethylamino)titanium and tin(II)acetylacetonate as metal precursors, and ozone as the oxidant.The growth rates of TiO 2 and SnO x after optimization were found to be 0.05 and 0.1 nm/ cycle, respectively.A wide overlapping ALD temperature window of constant growth rates ranging from 175 to 275 C was obtained.Sn/(Sn þ Ti) atomic ratio in SnTiO x films increased monotonically with increasing normalized SnO x / (TiO 2 þ SnO x ) ALD cycle ratio, indicating the effective composition tunability of the ALD process.Excess oxygen was found by XPS, possibly resulting from surface adsorbed moisture.Film crystallization at deposition temperature (200 C) was confirmed by GIXRD, and anatase was found to be the major phase for all as-deposited and annealed films.Both as-deposited and annealed SnTiO x films exhibited relatively smooth surfaces with an rms roughness value of 1.25 nm.The SnTiO x film annealed in O 2 at 350 C exhibited a piezoresponse hysteresis loop with a coercive voltage of 3.04 V, which indicates the existence of ferroelectric property.Although the atomic-scale nature of the polarization present in this sample is yet unclear, our study suggests that SnTiO x indeed has the potential as a ferroelectric material, even when grown on a Si substrate (as opposed, e.g., to a perovskite one, such as SrTiO 3 ).With further structural characterizations underway, we believe these findings will provide useful insights into fabricating perovskite SnTiO 3 films with good polar and piezoelectric properties, facilitating further research on lead-free ferroelectric materials.

FIG. 1
FIG. 1. (Color online) ALD temperature dependence of growth rates of TiO 2 and SnO x .Error bars indicate film uniformity across the deposition area.Each data point represents the averaged film thicknesses taken at different locations on the sample.

FIG. 2 .
FIG. 2. (Color online) Thicknesses of SnTiO x thin films with ALDCR of SnO x /TiO 2 ¼ 2:3 dependent on the total number of ALD cycles (main).Film thicknesses of TiO 2 and SnO x as functions of total ALD cycles are shown in the inset.All ALD reactions were carried out at 200 C.

FIG. 5 . 4 J
FIG. 5. (Color online) (a) PFM images and (b) surface rms roughness of as-deposited and annealed SnTiO x films.All measured samples are 40 nm-thick.