BaTiO3 Thin Films from Atomic Layer Deposition ... - ACS Publications

Article pubs.acs.org/JPCC

BaTiO3 Thin Films from Atomic Layer Deposition: A Superlattice Approach Matthias Falmbigl,† Irina S. Golovina,†,∥ Aleksandr V. Plokhikh,† Dominic Imbrenda,‡ Adrian Podpirka,† Christopher J. Hawley,† Geoffrey Xiao,† Alejandro Gutierrez-Perez,† Igor A. Karateev,⊥ Alexander L. Vasiliev,⊥ Thomas C. Parker,# and Jonathan E. Spanier*,†,‡,§ †

Department of Materials Science & Engineering, ‡Department of Electrical & Computer Engineering, and §Department of Physics, Drexel University, Philadelphia, Pennsylvania 19104, United States ∥ Institute of Semiconductor Physics, NAS of Ukraine, Pr. Nauki 41, Kiev 03028, Ukraine ⊥ National Research Center, “Kurchatov Institute”, Kurchatov Square 1, Moscow 123182, Russia # US Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, United States S Supporting Information *

ABSTRACT: A superlattice approach for the atomic layer deposition of polycrystalline BaTiO3 thin films is presented as an example for an effective route to produce high-quality complex oxide films with excellent thickness and compositional control. This method effectively mitigates any undesirable reactions between the different precursors and allows an individual optimization of the reaction conditions for the Ba−O and the Ti−O subcycles. By growth of nanometer thick alternating Ba(OH)2 and TiO2 layers, the advantages of binary oxide atomic layer deposition are transferred into the synthesis of ternary compounds, permitting extremely high control of the cation ratio and superior uniformity. Whereas the Ba(OH)2 layers are partially crystalline after the deposition, the TiO2 layers remain mostly amorphous. The layers react to polycrystalline, polymorph BaTiO3 above 500 °C, releasing H2O. This solid-state reaction is accompanied by an abrupt decrease in film thickness. Transmission electron microscopy and Raman spectroscopy reveal the presence of hexagonal BaTiO3 in addition to the perovskite phase in the annealed films. The microstructure with relatively small grains of ∼70 Å and different phases is a direct consequence of the abrupt formation reaction. The electrical properties transition from the initially highly insulating dielectric semiamorphous superlattice into a polycrystalline BaTiO3 thin film with a dielectric constant of 117 and a dielectric loss of 0.001 at 1 MHz after annealing at 600 °C in air, which, together with the suppression of ferroelectricity at room temperature, are very appealing properties for voltage tunable devices.

■

extending this method from binary to multinary films bears severe challenges as it demands the compatibility of different precursor chemistries in one process. Possible interactions and reactions among these precursors and with the substrate have to be considered and controlled.7−10 One of the most intensely studied ternary oxides is the perovskite BaTiO3 (BTO). Efforts to grow high quality thin films of this material originate from a plethora of appealing properties exhibited by BTO, such as high dielectric constant coupled with low dielectric losses in its paraelectric state, ferroelectricity, piezoelectricity, and large electro-optic coefficients.11 Also, its nontoxic nature and high chemical stability are important factors. The successful growth of BTO utilizing

INTRODUCTION In the pursuit to synthesize complex functional oxide thin films, the utilization of a deposition method that offers scalability, and at the same time uniformity, and high compositional and thickness control during the growth process is highly desirable. Atomic layer deposition (ALD) unifies all these requirements and is to date frequently used for the growth of binary oxide films in industrial applications.1−5 Recently, strides toward extending this deposition technique to higher order oxides and their epitaxial integration with semiconductor substrates have been made.6,7 The unique competitive advantage of ALD over other thin film deposition techniques originates from the growth mechanism based on sequential self-limiting chemical reactions between molecules in the gas phase and active species at the film surface. However, these chemical reactions also introduce a high inherent complexity to the deposition process with numerous parameters to be optimized. In particular, © XXXX American Chemical Society

Received: June 12, 2017 Revised: June 29, 2017

A

DOI: 10.1021/acs.jpcc.7b05633 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

(TRC) of 290 °C and base pressure of 2−5 hPa. All precursors were transported into the reaction chamber in separate lines. The pulse and purge times were 1.6/6 s for Ba(iPr3Cp)2 and 0.1/10 s for H2O for the Ba−O subcycle and 0.3/1 s for TTIP and 1/3 s for H2O for the Ti−O subcycle, respectively. For all films an initial 120 Å thick layer of TiO2 was deposited to ensure uniform coverage of the substrates using the deposition parameters described above. Pt top electrodes of ∼80 nm thickness and a square base area of 90 × 90 μm2 were deposited using photolithography and sputtering at room temperature for three 1100 Å thick films grown on Pt(111)/Ti/SiO2/Si(100) substrates prior to annealing. Ex situ annealing conditions for two of the samples were 500 and 600 °C for 1 h in air followed by cooling to room temperature at a cooling rate of 1 °C/min. Structural and Property Characterization. X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GI-XRD) measurements were performed using a Rigaku Smartlab equipped with a Cu source. Film thicknesses were extracted from XRR data by least-squares fits to the modified Bragg equation or by fitting the data to a model using the Motofit package.18 Lattice parameters were calculated using the WinCSD program package.19 In situ XRD experiments were conducted utilizing a domed hot stage (Anton Paar DHS 1100) under a low vacuum of 10−1 mbar. Surface morphology and cation ratio of the films were investigated on a Zeiss Supra 50VP scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) detector (Oxford Inst.). Raman spectra were collected in backscattering configuration using a single monochromator (XploRA, Horiba Jobin-Yvon, Edison, NJ) and a laser (4 mW, λ = 532 nm) focused through a 50× objective to a spot diameter of ≈10 μm at an intensity of 1.6 × 103 W/cm2. Light is dispersed using a 2400 grooves/mm grating and collected using a thermoelectrically cooled array detector. A cross section of a 1100 Å thick as grown film on (100)Si for scanning transmission electron microscopy and transmission electron microscopy (STEM/TEM) was prepared in a Helios (FEI, USA) scanning electron microscope (SEM)/focus ion beam (FIB) dual beam system equipped with gas injectors for W and Pt deposition and an Omniprobe micromanipulator (Omniprobe, USA). First, a 2 μm thick protective Pt layer was deposited on the sample surface. Subsequent FIB milling using 30 keV Ga+ ions resulted in a cross-section area of 5 × 5 μm2, which was polished with 5 and 2 keV Ga+ ion beams, respectively. These specimens were investigated using a Titan 80-300 operated at 300 kV, which is equipped with a high-angle annular dark-field (HAADF) detector (Fischione, USA), a spherical aberration (Cs) probe corrector, and a postcolumn Gatan image filter (GIF). Digital Micrograph (Gatan, USA) and Tecnai (FEI, USA) Imaging and Analysis software were used for the image processing. The exact cation ratio was determined from Rutherford backscattering spectrometry (RBS). The backscattered α particle spectrometry (BAPS) data were measured utilizing an NEC Pelletron model 5SDH-2 accelerator located at the U.S. Army Research Laboratory, Aberdeen Proving Ground, MD. All measurements were conducted using a 2 MeV He+ beam with a beam energy spread of ±1.75 keV. The sample beam current was nominally 50 nA, and the total integrated beam current for each sample was about 10 μC. The RBS chamber was operated at a base-pressure of 3 × 10−8 Torr. The SIMNRA software package version 6.06 was used to generate

ALD was already demonstrated highlighting (i) challenges associated with controlling the reaction of the Ba precursor,12 (ii) the influence of plasma treatment on the properties,13 (iii) the epitaxial integration on SrTiO3-buffered Si,14 and (iv) the plasma-enhanced synthesis of amorphous and semiamorphous BTO films.15,16 In all cases intimate mixing of the Ba−O and Ti−O subcycles was utilized, which is the commonly used method to grow higher order oxides by ALD.6 However, this approach naturally limits the ability to precisely adjust the composition. Furthermore, intimate mixing demands a compromise between the optimal parameters and conditions for each cation precursor and several times causes a reduction of the growth rate and conformity of the films; e.g., the nonuniformity for BTO films on 200 mm wafers and for binary BaO on 100 mm wafers remained above 2%, which can already have detrimental effects on the resulting properties,15,17 and is considerably increased compared to values below 1% typically observed for the growth of binary oxides. Herein, we use the growth of BTO thin films as an example to demonstrate that an approach that splits the deposition process into extensive binary subcycles of Ba−O and Ti−O results in the growth of superlattice (SL) thin films enabling superior uniformity and compositional control. In addition, the growth rates for the binary oxides are maintained. This is achieved by disentangling the complex growth process of a ternary compound into two independent subcycles mimicking ALD of individual binary oxides. Hereby, the conditions for the growth reactions can be optimized separately, and any unfavorable precursor interactions are effectively mitigated. The deposition results in a SL thin film consisting of alternating amorphous TiO2 and partially crystalline Ba(OH)2 layers. The solid state reaction between these layers to form polycrystalline BTO upon annealing is investigated by in situ X-ray diffraction, transmission electron microscopy, and Raman scattering spectroscopy. The results reveal an abrupt formation of polycrystalline BaTiO3 above 500 °C accompanied by a 10% decrease in film thickness. This reaction pathway results in nanocrystalline thin films containing both BTO polymorphs, the perovskite (t-BTO) and hexagonal phase (h-BTO), as confirmed by Raman spectroscopy and TEM. The dielectric properties evolve from a highly insulating superlattice with a field-independent dielectric constant of 14 into polycrystalline BTO films with higher dielectric constant and tunability as the annealing temperature is increased. As expected, no hysteretic behavior and extremely low dielectric losses are observed for the nanostructured thin film annealed at 600 °C. The excellent growth characteristics and resulting properties demonstrate that this superlattice approach is a suitable method to synthesize BTO thin films via ALD.

■

EXPERIMENTAL SECTION Sample Preparation. Atomic layer depositions of Ba−Ti− O thin films were carried out in a Picosun R200 Advanced Reactor on (100) oriented silicon substrates with native oxide layer and Pt(111)/Ti/SiO2/Si(100) substrates (Gmek Inc.). 6 N purity N2 gas was used as carrier gas, and Absolut Ba (Air Liquide, bis(1,2,4 triisopropylcyclopentadienyl)Ba, Ba(iPr3Cp)2, titanium isopropoxide (Alfa Aesar, Ti(iOPr)4, TTIP), and deionized H2O served as precursors for Ba, Ti, and O, respectively. The source cylinders for Ba(iPr3Cp)2 and and TTIP were heated to 200 and 115 °C, while the water source was kept at room temperature. The deposition experiments were conducted at a reaction chamber temperature B


Article


Figure 1. (a) Schematic of the ALD-grown thin film superlattices, (b) XRR scans of as-grown films, (TTIP−H2O) × 40 + [(4 × Ba(iPr3Cp)2−H2O) × a + (TTIP−H2O) × b] × y, with varying Ba−O and Ti−O subcycle repeats, a and b, and a total repeat number of y = 6. The dashed black lines represent fits using a superlattice model.

atomic layer deposition of SrTiO323 and BiFeO324,25 that high quality thin films can evolve from ALD-grown superlattices. Initially, the TiO2 growth using TTIP and H2O at TRC = 290 °C was established with a saturated growth rate (GPC, growth per cycle) of 0.35 Å, consistent with previous literature reports for this well-investigated binary system (see Supporting Information for details).26−29 The more challenging part in terms of reaction control is the Ba(iPr3Cp)2−H2O subcycle as it was already shown that this precursor combination suffers from hydration,12 which in turn leads to uncontrolled reactions between the gas phase and the film surface according to

RBS simulations, and the Rutherford cross sections were utilized in the simulations. The electrical properties were measured in a metal− insulator−metal (MIM) configuration on the samples grown on Pt(111)/Ti/SiO2/Si(100) substrates. The bottom electrode was contacted using Ag paste. The MIM-structured samples were placed in a probe station (Lakeshore Cryotronics TTP4) and measured in air at room temperature utilizing a Keithley SCS-4200 electrometer.

■

RESULTS AND DISCUSSION Thin Film Growth. One of the biggest challenges for extending ALD from binary oxides to ternary ABO3 perovskites containing alkaline and/or alkaline earth metals in the A-site is the control of the A−O deposition cycle, in particular when water is used as reactant.9,12,20 Until now, syntheses of BTO thin films utilizing ALD overcame this issue by thorough mixing of the A−O and B−O subcycles12−15,17,21,22 or the use of plasma-enhanced O2 as a reactant for both Ba and Ti precursors.16 However, keeping the repeat number for each subcycle below five, as needed for the intermixing, naturally limits the composition control to certain ratios, and therefore fine-tuning of the cation ratio is impossible. Furthermore, the optimization of a full deposition cycle is realized by a compromise between the ideal conditions for each individual subcycle. These shortcomings can be overcome by utilizing a superlattice approach, where alternating stacks of A−O and B− O layers are grown, and the parameters for each subcycle can be optimized similar to the growth of binary oxides. The complete deposition sequence of the SL films can be described as [(k × Ba(iPr3Cp)2−H2O) × a + (TTIP−H2O) × b] × y, where y is the number of repeats of the SL, a is the number of Ba(iPr3Cp)2−H2O subcycles, b is the number of TTIP−H2O subcycles, and k is the number of consecutive Ba(iPr3Cp)2 pulses (see Figure 1a). This approach permits the utilization of two major advantages of the ALD process: (i) an extremely high uniformity over large areas and (ii) a superior control of the cation stoichiometry in the thin films, as both a and b are numbers >20. Hereby a very fine adjustment of the cation ratio is possible. It was already demonstrated for the

[...−O−Ba]a −O−Ba(OH) ·nH 2O(s) + n Ba(iPr3Cp)2 (g) ⇒ [...−O−Ba]a + n −O−Ba(OH)(s) + 2nH iPr3Cp(g) (1)

In order to minimize the hydrate incorporation, consecutive Ba(iPr3Cp)2 pulses (k) were introduced as additional parameter to the deposition sequence, [(k × Ba(iPr3Cp)2−H2O) × a + (TTIP−H2O) × b] × y. The influence of k was investigated directly for superlattice films and showed that reaction 1 indeed is present at TRC= 290 °C. Increasing k from 1 to 6 constantly decreases the growth per Ba(iPr3Cp)2 pulse from 0.7 to 0.31 Å. The increase of the film thickness as well as the Ba/Ti ratio with increasing k clearly demonstrates that the Ba−O layer retains a significant amount of hydrate (Ba(OH)2·nH2O)30 on the growing surface. Importantly, this pulsing sequence not only mitigates uncontrolled reactions and the incorporation of H2O into the film, but also improves the uniformity from 95% for k = 1 to 99% for k = 4. Based on these results, k = 4 was selected as for deposition of the superlattice thin films (for details see the Supporting Information). The individual growth rates of each binary sequence can be extracted from the change in total film thickness and SL period from two series of samples, where one repeat number of the two subcycles (a, b) is kept constant while the other one is varied (Figure 1a, for varying b and constant a). The XRR scans of three representative films (25,50; 25,100; 30,50) and corresponding fits using a superlattice model are depicted in Figure 1b. The excellent agreement between the data and the C


Article


Figure 2. (a) Total film and superlattice thicknesses as a function of consecutive Ba−O subcycles (a, square) or Ti−O subcycles (b, circles) for a general deposition sequence, (TTIP−H2O) × 40 + [(k × Ba(iPr3Cp)2−H2O) × a + (TTIP−H2O) × b] × y. (b) Ba/Ti ratio measured by EDS as a function of the Ba/Ti pulse ratio.

Figure 3. (a) XRR scans of a 1100 Å thick film with Ba/Ti ratio of 1 deposited on a Si substrate as a function of temperature. (b) GI-XRD scans of the same film as a function temperature. The Bragg indices correspond to TiO2 (blue, anatase-phase), α-Ba(OH)233 (pink), and cubic BTO (black).

the deposition of binary Ba−O and Ti−O subunits in a regime of constant growth rates. A layer-by-layer growth is frequently stated as one of the key benefits of ALD; however, this strictly applies only to a larger number of repetitions as for low repeat cycles the substrate or top surface and other factors significantly influence the growth reactions.1,2,8,9,31 In particular for the ALD growth of ternary compounds, reactions involving different ligands can change the stoichiometry in the case of an intimate mixing of the cation subcycles.32 All these disadvantages, which reduce the reproducibility and control of the deposition process, can be significantly reduced by this superlattice approach, where most deposition cycles take place in the same defined environment as for binary oxides. This improvement is also reflected in the extremely low nonuniformity of 0.2 and 1.2% for diameters of 100 and 200 mm, respectively (for details see the Supporting Information). This nonuniformity is smaller than previously reported values of 2.7−6% for ALD-grown BTO thin films using an intimate mixing approach.17 Crystallization. In order to optimize the annealing conditions for these thin films, the crystallization pathway

model showcases the high reproducibility and control of the superlattice growth. The presence of Kiessig fringes implies uniform coverage and very smooth surfaces and interfaces of the thin films. Based on these two sets of samples, the GPC for each individual subunit grown on the other one can be calculated from linear fits to the total thickness and the SL repeat length. The fits displayed in Figure 2a for a varying number of Ba(iPr3Cp)2−H2O pulses (a) result in growth rates of 0.38(2) Å (film) and 0.40(2) Å (SL) for the Ba−O subcycle. Linear fits for a varying number of TTIP−H2O pulses (b) reveal GPCs of 0.30(2) Å (film) and 0.28 Å (SL) for the Ti−O subcycle, respectively (Figure 2a). The Ba/Ti ratio of the films as a function of the pulse ratio exhibits a strictly linear dependence in the investigated compositional range (Ba/Ti ratio) from 0.2(1) to 1.8(1) independent of varying a, b, or y in the deposition sequence, [(4 × Ba(iPr3Cp)2−H2O) × a + (TTIP− H2O) × b] × y, as depicted in Figure 2b. This behavior together with the growth rate evaluation clearly demonstrates that keeping the individual subcycle repeats (a, b) > 20 allows D


Article


polymorph of BTO, but also indicates the presence of a tetragonal distortion in the perovskite structure. Therefore, Raman spectra of three different samples were collected to gather complementary information on the phases and symmetry present in the thin films (Figure 4). Raman modes

from the as-grown superlattice to the polycrystalline BTO thin film has to be thoroughly understood. Therefore, in situ XRR and XRD measurements as well as in situ TEM investigations were conducted between 200 and 700 °C. These experiments were performed on a 1100 Å thick film with Ba/Ti ratio of 1 using a deposition sequence, (TTIP−H2O) × 40 + [(4 × Ba(iPr3Cp)2−H2O) × 25 + (TTIP−H2O) × 50] × 20. In Figure 3a, XRR scans at different temperatures are displayed. Up to 500 °C the interferences at 1.7°, 3.3°, and 4.9° in 2θ imply the presence of a superlattice. The existence of 18 Kiessig fringes (y − 2) between these Bragg interferences confirms a layering scheme in agreement with the deposition sequence, and the XRR data collected at 200 °C can be reproduced by a superlattice model, where the subunit thicknesses coincide with the established growth rates (see Figure S4b). At 550 °C the Bragg interferences vanish, and the total film thickness decreases abruptly by 10% and subsequently remains constant until 700 °C (see also Figure S4a). These observations suggest cation interdiffusion between the two layers and the transformation into a more dense structure. Indeed, from the difference in average density between the initial constituents TiO2 (ρ = 3.88 Mg m−3) and BaO (ρ = 5.99 Mg m−3) or α-Ba(OH)2 (ρ = 4.4 Mg m−3)33 to BaTiO3 (ρ = 6.05 Mg m−3) even higher thickness changes between 18 and 32% would be expected. The considerably lower change points toward porosity within the bulk of the film, which is confirmed by TEM images in Figure 6e and Figure S7a. The GI-XRD pattern of the as-grown film (200 °C) (Figure 3b) reveals two very broad peaks indicating low crystallinity in the superlattice after the ALD process. The peak positions can be indexed to the most intense peaks of TiO2 in the anatase phase and α-Ba(OH)2.33 The observation of weakly crystalline α-Ba(OH)2 at a deposition temperature of 290 °C is consistent with a previous study using very similar precursors for the Ba− O subcycle.12 Up to 500 °C no changes are observed. Interestingly, at 500 °C only the (220)PC peak of BaTiO3 emerges as additional reflection (scan duration of 1 h). This indicates that the formation of BTO is already slowly progressing during the data collection and together with the vanishing of superlattice interferences in the XRR scan implies that the cation diffusion and the crystallization of BTO occur almost simultaneously. In consistence with the extinction of the superlattice interferences and the abrupt decrease of the film thickness the initial peaks vanish between 500 and 550 °C, and all new Bragg peaks can unambiguously be indexed to cubic BTO. Therefore, the following formation reaction of BTO from the superlattice constituents occurs between 500 and 550 °C: Ba(OH)2 (s) + TiO2 (s) ⇒ BaTiO3(s) + H 2O(g)

Figure 4. Raman spectra of 1100 Å thick BTO films grown on Pt(111)/Ti/SiO2/Si(100) substrates after the deposition and annealing at 500 and 600 °C, respectively. + denotes signature modes of the h-BTO,35 and # denotes signature modes of t-BTO.

expected for TiO2 in anatase structure are not visible for the asgrown film, most likely due to the superlattice structure and weak crystallinity in the TiO2 layers consistent with the observations from in situ XRD. For the annealed films the typical features for cubic BTO are observed, and weak additional modes corresponding to tetragonal BTO are present, confirming a mix of the ferro- and paraelectric phase in the thin films.36,37 The fraction of the t-BTO increases slightly from 500 to 600 °C. In addition, modes at 155 and 640 cm−1 are observed, which can be attributed to h-BTO.35 Modes around 640 cm−1 in the Raman spectra of polycrystalline BTO thin films are frequently observed independent of the synthesis method38−41 and are sometimes even present in epitaxial films.42 Thorough investigations of these vibrations in polycrystalline ceramics including pressure dependence and thin films revealed that these peaks arise from strain localized in the grain boundary regions and disordered grain boundaries of submicrometer size grains.41,43 Considering the expected small grain sizes in thin films and the presence of strain initiated by the abrupt thickness decrease of the film during the formation of BTO and the thermal expansion mismatch between the film and substrate, it is possible that the high temperature hexagonal phase, which is in bulk only stable above 1460 °C,44 could form already at much lower temperatures. To locally examine the morphology and crystallization behavior of these films, an in situ TEM investigation was performed. The HAADF-STEM image in Figure 5a clearly reveals the presence of a superlattice in the as-grown film. The contrast between the different layers, which are separated by abrupt interfaces, confirms the segregation of the cations. In total, 20 repeats of the superlattice plus one additional TiO2 layer at the substrate surface can be identified, and the film and individual layer thicknesses of 1110(5) Å, 14(2) Å (Ti−O), and 36(2) Å (Ba−O), respectively, are in excellent agreement with the deposition sequence and XRR results. A closer inspection of a BF-TEM image reveals the presence of crystalline areas (see

(2)

The release of H2O during the formation most likely causes the unexpectedly low thickness change and presence of pores in the polycrystalline film. Above 550 °C the intensities of the BTO reflections increase, and the peak widths decrease slightly. The lattice parameter, a = 4.053(4) Å at 550 °C, constantly increases by 7 × 10−6 K−1 as expected from thermal expansion of BTO.34 The change in peak width corresponds to an increase in grain size by 34% from 500 to 700 °C as estimated by the Scherrer equation. RBS measurements, conducted before and after the in situ XRD experiment, reveal no changes in the Ba/Ti ratio (= 1) within error (see Figure S5). As the lattice vibrational modes of BTO strongly alter with crystal symmetry,36,37 Raman spectroscopy not only allows a clear distinction between the perovskite and the hexagonal E


Article


Figure 5. (a) HAADF-STEM image of an as-grown 1100 Å thick film on Si. (b) BF-TEM image revealing crystallized areas in the Ba−O layer. (c) SAED of the as-grown film. Debye−Scherrer rings attributed to polycrystalline α-Ba(OH)2 are observed besides the Si substrate.33

Figure 6. (a) BF-TEM image of the film at 500 °C. (b) High-resolution TEM image at 500 °C revealing crystalline BTO with different orientations (highlighted by blue lines). (c) SAED at 500 °C with Debye−Scherrer rings indexed to cubic BTO. (d) High-magnification image at 575 °C revealing crystalline BTO with different orientations (highlighted by blue lines). (e) BF-TEM image of the film after annealing at 600 °C. (f) Magnified area from (e) with FFT patterns from two adjacent grains revealing hexagonal and tetragonal BTO.

same time the film thickness shrinks to 1030(5) Å (Figure 6a). Randomly oriented crystallites with average sizes below 70 Å appear, demonstrating that nucleation does not occur at preferred sites (see Figure 6b). The SAED at 500 °C, displayed in Figure 6c, reveals that the crystallites are BTO in the perovskite phase. A close inspection of the (110) ring reveals some splitting, which implies the joint crystallization of different polymorphs, e.g., h-BTO and/or t-BTO together with the cubic phase. The individual grains increase in size at higher temperatures, and at 575 °C some grains are larger than 500 Å in one direction (see Figure 6d). The BTO diffraction

Figure 5b), which are strictly confined to the Ba−O layers, whereas the Ti−O layers appear mostly amorphous. The selected area diffraction (SAED) of the as-grown film shows Debye−Scherrer rings closely resembling d-spacings of αBa(OH)2 and peaks from the Si substrate (Figure 5c). The microstructure of the film remains unchanged until 450 °C, and the unchanged SAED confirms that no new crystalline phases form (see Figure S6). At 500 °C, however, the scenario changes significantly, corroborating the XRR and XRD results: the superlattice structure vanishes due to intermixing of the cations, and at the F


Article


Figure 7. (a) J−E characteristics for the BTO film before and after annealing at 500 and 600 °C. (b) Schottky emission fits. (c) Poole−Frenkel (PF) fits for the annealed films. In all cases the slope was defined by using a refractive index, n = 2, for the BTO films.45

rings present at 500 °C transition slowly into defined spots at 650 °C, emphasizing the crystallite growth at higher temperatures (see Figure S7c). In order to further investigate the presence of polymorphism in the annealed films, one specimen was investigated after ex situ annealing at 600 °C (Figure 6e). Grains of various sizes and shapes with diameters ranging from tens to several hundreds of angstroms are present. A closer inspection of individual grains using fast Fourier transformation (FFT) indeed unambiguously confirms the presence of BTO grains in the hexagonal and tetragonal phase adjacent to each other. Although a conclusive examination of the distribution of h-BTO in the film is not possible due to overlap of grains, it is obvious from various inspected areas that the h-BTO grains have a similar size as tBTO grains and appear to be randomly distributed throughout the film and not agglomerated at interfaces and/or grain boundaries. Composition measurements in different areas of this TEM specimen confirm a uniform distribution of the cations (see Figure S7a). Physical Properties. To investigate the evolution of the physical properties, current density vs electric field (J−E) dependences of the as-grown film and annealed films (500 and 600 °C for 1 h in air) were measured and are displayed in Figure 7a. Interestingly, the evolution starting from the highly insulating as-grown film does not follow a constant trend upon annealing, and the film annealed at 500 °C exhibits the highest leakage current (see also Table 1). This is attributed to higher

and typically encompass contact effects such as Schottky emission46 as well as the bulk-limited Poole−Frenkel effect47 and space charge limited conduction48 among various others.49−55 The leakage current of the as-grown film does not match any of the considered mechanisms. This is not surprising considering the complex morphology of this film consisting of amorphous and crystalline areas arranged in a layered structure. For the annealed films, however, an analysis was possible (Figure 7b,c). In order to ascribe the observed field dependence to a conduction model, the slope can be defined using the refractive index (n) of the material under investigation.49,51 For BTO thin films the refractive index depends slightly on the crystallinity and n = 2 was used for our analysis.45 As shown in Figure 7b, at electric fields between 100 and 225 kV cm−1 a good agreement with Schottky emission is observed for the film annealed at 500 °C. For the film annealed at 600 °C Schottky emission can describe the observed dependence between 100 and 400 kV cm−1 fairly well (Figure 7b). Poole−Frenkel conduction provides a good fit over a similar voltage range for both films, and the quality of the fit is slightly better than for the Schottky emission. The dependence of ln(J/E) vs E1/2 also accounts for a modified Schottky emission model, which applies to insulators where the electronic mean-free path is less than the insulator thickness.55 However, the symmetric J−E dependence for applying positive and negative bias found for both films points toward the bulk limited Poole−Frenkel mechanism.11 The very steep increase found for the film annealed at 500 °C above 225 kV cm−1 is similar to the observations for epitaxial Bi0.5Na0.5TiO3, where this behavior is attributed to the trap-field limit.56 The space charge limited conduction model48 does not describe the field dependence of J for any of the three films within the measured voltage range. The dielectric constant, ε, and dissipation factor, tan δ, for all films were measured as a function of frequency between 10 kHz and 1 MHz and as a function of electric field at 100 kHz. The data are displayed in Figure 8. All films show rather weak frequency dependence of ε, and as expected, the dielectric constant increases with annealing temperature (Figure 8a). The dielectric constant of the amorphous film is slightly smaller than reported for partially crystalline ALD-grown BTO films,12,15,57 and the dielectric constant of 117 for the film annealed at 600 °C is even significantly higher than 70 reported for a 320 Å thick BTO film grown by intimate mixing using comparable precursors and similar annealing conditions.12 This demon-

Table 1. Lattice Parameter (a), Current Density at E = 200 kV cm−1 (J200), Dielectric Constant (ε), and Loss Tangent (tan δ) at E = 0 and 100 kHz for the As-Grown Film and after Annealing at 500 and 600 °C for 1 h in Air sample as-grown 500 °C 600 °C

a (Å) n.a. 4.028(3) 4.012(2)

J200 (A cm−2) −9

8.0 × 10 9.1 × 10−6 1.7 × 10−7

ε

tan δ

14 85 117

0.016 0.038 0.007

oxygen vacancy content as implied by the larger lattice parameter for this film (see Table 1). However, for all films the leakage current remains very low and is within the range of other ALD-grown BTO films.12,13,15,16,21 In order to gain a deeper insight into the conduction mechanisms in these films, different models can be considered. These mechanisms lead to an enhancement of the electronic conductivity at high fields G


Article


Figure 8. (a) Frequency dependence of the dielectric constant (ε) and the dielectric loss (tan δ) and (b) electric field dependence of the normalized dielectric constant and tan δ of the as-grown and annealed films at 100 kHz.

strates that films grown by the superlattice approach exhibit similar or even enhanced dielectric properties compared to films grown using intimate mixing. The dielectric losses are lowest for the film annealed at 600 °C and decrease with increasing frequency for both annealed films (Figure 8a). The dissipation factors (tan δ) of 0.007 at 100 kHz and 0.001 at 1 MHz for the film annealed at 600 °C are lower than for polycrystalline BTO films synthesized by various other methods with tan δ ranging between 0.02 and 90 at 1 MHz.40,58−62 The dielectric loss is also by an order of magnitude lower than 0.04 (at 1 kHz) and 0.05 (at 100 kHz) reported for epitaxial BTO thin films,63,64 and comparable to BST films, which are currently considered as most promising candidates for voltage tunable devices.65 The as-grown film exhibits typical dielectric behavior, with no change as a function of applied field (Figure 8b). The field dependence of the dielectric constant increases with annealing temperature, whereas the dielectric losses decrease in accordance with increasing grain size and a larger fraction of t-BTO in the films. Dielectric breakdown occurs at an electric field of 1 MV cm−1 applying a negative bias for the film annealed at 600 °C. As expected, for mixed phase BTO thin films with grain sizes in the range of 70 Å (Figure 6d,e) the ferroelectric properties are strongly suppressed.66 Together wit the almost frequency-independent dielectric constant of 117 and dielectric losses of 0.001−0.007 for the film annealed at 600 °C, it is demonstrated that this synthesis method comprises the capability to produce technologically promising thin films.67 An optimization of the annealing conditions should result in even higher quality MIM capacitors with improved functionality for voltage tunable components.

This growth method produces semiamorphous superlattices consisting of alternating TiO2 and Ba(OH)2 layers with defined constituent thicknesses and abrupt interlayer interfaces. The evolution into polycrystalline BaTiO3 thin films is investigated by in situ XRD, TEM, and Raman measurements. These methods reveal a high thermal stability of the as-grown superlattice up to 450 °C. At 500 °C an abrupt formation of BaTiO3 is accompanied by a thickness reduction of 10%. This reaction mechanism results in thin films with randomly distributed nanocrystalline grains of the hexagonal and perovskite polymorph. Upon annealing, the initially highly insulating dielectric superlattice transitions into films with increasing dielectric constant exhibiting typical features for polycrystalline BaTiO3 thin films. The structural characteristics are reflected in the properties resulting in low leakage current of 1.7 × 10−7 A cm−2 at a field of 200 kV cm−1 and low dielectric loss of 0.001 together with a dielectric constant of 117 at 1 MHz for the film annealed at 600 °C in air. Ferroelectric properties are effectively suppressed, rendering these films an appealing alternate to BST for voltage tunable capacitors at room temperature. Overall, the results demonstrate that this superlattice approach opens a pathway to retain the high uniformity and thickness control of atomic layer deposition when expanded to ternary oxides and that the resulting properties of the BaTiO3 thin films show potential for voltage tunable components.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05633. Additional detailed information on the atomic layer deposition characteristics, temperature dependence of the film thickness, Rutherford backscattering spectroscopy data, transmission electron microscopy images, and Xray diffraction data including Figures S1−S8 (PDF)

■

CONCLUSIONS We present an ALD method utilizing a superlattice approach which promises unprecedented uniformity and stoichiometry control for ternary BTO thin films. These key factors of the ALD process are achieved by disentangling the complex balancing of chemical reactions required for the growth of ternary oxides into separate deposition subcycles for each cation. Hereby, an individual optimization of the Ba−O subcycle, namely the effective reduction of hydrate incorporation, is permitted.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.E.S.). H


Article

The Journal of Physical Chemistry C ORCID

Titanate Thin Films Using a Novel Pyrrole Based Precursor. J. Mater. Chem. C 2016, 4 (10), 1945−1952. (16) Schindler, P.; Kim, Y.; Thian, D.; An, J.; Prinz, F. B. PlasmaEnhanced Atomic Layer Deposition of BaTiO3. Scr. Mater. 2016, 111, 106−109. (17) Matero, R.; Rahtu, A.; Tuominen, M.; Vehkamäki, M.; Hatanpäa,̈ T.; Ritala, M.; Leskelä, M. Scale-up of the Barium Titanate Atomic Layer Deposition Process onto 200mm Wafer. ECS J. Solid State Sci. Technol. 2006, 1 (10), 137−141. (18) Nelson, A. Co-Refinement of Multiple-Contrast neutron/X-Ray Reflectivity Data Using MOTOFIT. J. Appl. Crystallogr. 2006, 39, 273−276. (19) Akselrud, L.; Grin, Y. WinCSD: Software Package for Crystallographic Calculations. J. Appl. Crystallogr. 2014, 47, 803−805. (20) Østreng, E.; Sønsteby, H. H.; Sajavaara, T.; Nilsen, O.; Fjellvåg, H. Atomic Layer Deposition of Ferroelectric LiNbO3. J. Mater. Chem. C 2013, 1 (27), 4283. (21) Vehkamäki, M.; Hatanpäa,̈ T.; Hänninen, T.; Ritala, M.; Leskelä, M. Growth of SrTiO3 and BaTiO3 Thin Films by Atomic Layer Deposition. Electrochem. Solid-State Lett. 1999, 2 (10), 504−506. (22) Hatanpäa,̈ T.; Vehkamäki, M.; Mutikainen, I.; Kansikas, J.; Ritala, M.; Leskelä, M. Synthesis and Characterisation of Cyclopentadienyl Complexes of Barium: Precursors for Atomic Layer Deposition of BaTiO3. Dalt. Trans. 2004, 1181−1188. (23) Riedel, S.; Neidhardt, J.; Jansen, S.; Wilde, L.; Sundqvist, J.; Erben, E.; Teichert, S.; Michaelis, A. Synthesis of SrTiO3 by Crystallization of SrO/TiO2 Superlattices Prepared by Atomic Layer Deposition. J. Appl. Phys. 2011, 109 (9), 094101. (24) Marchand, B.; Jalkanen, P.; Tuboltsev, V.; Vehkamäki, M.; Puttaswamy, M.; Kemell, M.; Mizohata, K.; Hatanpäa,̈ T.; Savin, A.; Räisänen, J.; et al. Electric and Magnetic Properties of ALD-Grown BiFeO3 Films. J. Phys. Chem. C 2016, 120, 7313−7322. (25) Puttaswamy, M.; Vehkamäki, M.; Kukli, K.; Dimri, M. C.; Kemell, M.; Hatanpäa,̈ T.; Heikkilä, M. J.; Mizohata, K.; Stern, R.; Ritala, M.; et al. Bismuth Iron Oxide Thin Films Using Atomic Layer Deposition of Alternating Bismuth Oxide and Iron Oxide Layers. Thin Solid Films 2016, 611, 78−87. (26) Ritala, M.; Leskela, M.; Niinisto, L.; Haussalo, P.; Niinist, L. Titanium Isopropoxide as a Precursor in Atomic Layer Epitaxy of Titanium Dioxide Thin Films. Chem. Mater. 1993, 5 (8), 1174−1181. (27) Matero, R.; Rahtu, A.; Ritala, M.; Leskelä, M.; Sajavaara, T. Effect of Water Dose on the Atomic Layer Deposition Rate of Oxide Thin Films. Thin Solid Films 2000, 368 (2000), 1−7. (28) Aarik, J.; Aidla, A.; Sammelselg, V.; Uustare, T.; Ritala, M.; Leskelä, M. Titanium Isopropoxide as a Precursor for Atomic Layer Deposition: Characterization of Titanium Dioxide Growth Process. Appl. Surf. Sci. 2000, 161, 385−395. (29) Rahtu, A.; Ritala, M. Reaction Mechanism Studies on Titanium Isopropoxide−Water Atomic Layer Deposition Process. Chem. Vap. Deposition 2002, 8 (1), 21. (30) Lutz, H. D.; Eckers, W.; Christian, H.; Engelen, B. Hydrates of Barium Hydroxide. Preparation, Thermal Decomposition and X-Ray Data. Thermochim. Acta 1981, 44, 337−343. (31) Zaera, F. Mechanisms of Surface Reactions in Thin Solid Film Chemical Deposition Processes. Coord. Chem. Rev. 2013, 257 (23− 24), 3177−3191. (32) Elliott, S. D.; Nilsen, O. Reaction Mechanisms in ALD of Ternary Oxides. ECS Trans. 2011, 41 (2), 175−183. (33) Buck, P.; Bärnighausen, H. Kristalldaten von α-Ba(OH)2. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1968, 24 (12), 1705−1706. (34) Bland, J. A. The Thermal Expansion of Cubic Barium Titanate BaTiO3 from 350°C to 1050°C. Can. J. Phys. 1959, 37 (4), 417−421. (35) Yamaguchi, H.; Uwe, H.; Sakudo, T.; Sawaguchi, E. RamanScattering Study of the Soft Phonon Modes in Hexagonal Barium Titanate. J. Phys. Soc. Jpn. 1987, 56 (2), 589−595. (36) DiDomenico, M.; Wemple, S. H.; Porto, S. P. S.; Bauman, R. P. Raman Spectrum of Single-Domain BaTiO3. Phys. Rev. 1968, 174 (2), 522−530.

Jonathan E. Spanier: 0000-0002-3096-2644 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Work at Drexel University was supported primarily by Office of Naval Research under grant N00014-15-11-2170. I.G. was supported by National Science Foundation (NSF) under grant IIP 1403463. C.J.H., G. X. and A.G.-P. acknowledge support of NSF DMR under grant no. DMR 1608887. The experiments were partially conducted using the equipment of the Resource Center of Probe and Electron Microscopy (Kurchatov Complex of NBICS-Technologies, NRC “Kurchatov Institute”). We acknowledge the core shared user facilities at Drexel University for access to XRD (NSF DMR 1040166) and Picosun Oy (Finland) for support.

■

REFERENCES

(1) Miikkulainen, V.; Leskelä, M.; Ritala, M.; Puurunen, R. L. Crystallinity of Inorganic Films Grown by Atomic Layer Deposition: Overview and General Trends. J. Appl. Phys. 2013, 113, 021301. (2) George, S. M. Atomic Layer Deposition: An Overview. Chem. Rev. 2010, 110, 111−131. (3) Parsons, G. N.; George, S. M.; Knez, M. Progress and Future Directions for Atomic Layer Deposition and ALD-Based Chemistry. MRS Bull. 2011, 36, 865−871. (4) Devi, A. “Old Chemistries” for New Applications: Perspectives for Development of Precursors for MOCVD and ALD Applications. Coord. Chem. Rev. 2013, 257 (23−24), 3332−3384. (5) Johnson, R. W.; Hultqvist, A.; Bent, S. F. A Brief Review of Atomic Layer Deposition: From Fundamentals to Applications. Mater. Today 2014, 17 (5), 236−246. (6) Sønsteby, H. H.; Fjellvåg, H.; Nilsen, O. Functional Perovskites by Atomic Layer Deposition - An Overview. Adv. Mater. Interfaces 2017, 4, 1600903. (7) McDaniel, M. D.; Ngo, T. Q.; Hu, S.; Posadas, A.; Demkov, A. A.; Ekerdt, J. G. Atomic Layer Deposition of Perovskite Oxides and Their Epitaxial Integration with Si, Ge, and Other Semiconductors. Appl. Phys. Rev. 2015, 2 (4), 041301. (8) Zaera, F. The Surface Chemistry of Atomic Layer Depositions of Solid Thin Films. J. Phys. Chem. Lett. 2012, 3 (10), 1301−1309. (9) Lee, S. W.; Choi, B. J.; Eom, T.; Han, J. H.; Kim, S. K.; Song, S. J.; Lee, W.; Hwang, C. S. Influences of Metal, Non-Metal Precursors, and Substrates on Atomic Layer Deposition Processes for the Growth of Selected Functional Electronic Materials. Coord. Chem. Rev. 2013, 257 (23−24), 3154−3176. (10) Kim, J.-H.; Kim, J.-Y.; Kang, S.-W. Film Growth Model of Atomic Layer Deposition for Multicomponent Thin Films. J. Appl. Phys. 2005, 97 (9), 093505. (11) Dawber, M.; Rabe, K. M.; Scott, J. F. Physics of Thin-Film Ferroelectric Oxides. Rev. Mod. Phys. 2005, 77, 1083−1130. (12) Vehkamäki, M.; Hatanpäa,̈ T.; Ritala, M.; Leskelä, M.; Väyrynen, S.; Rauhala, E. Atomic Layer Deposition of BaTiO3 Thin Films - Effect of Barium Hydroxide Formation. Chem. Vap. Deposition 2007, 13 (5), 239−246. (13) An, J.; Usui, T.; Logar, M.; Park, J.; Thian, D.; Kim, S.; Kim, K.; Prinz, F. B. Plasma Processing for Crystallization and Densification of ALD BaTiO3 Thin Films. ACS Appl. Mater. Interfaces 2014, 6, 10656− 10660. (14) Ngo, T. Q.; Posadas, A. B.; McDaniel, M. D.; Hu, C.; Bruley, J.; Yu, E. T.; Demkov, A. A.; Ekerdt, J. G. Epitaxial c-Axis Oriented BaTiO3 Thin Films on SrTiO3-Buffered Si(001) by Atomic Layer Deposition. Appl. Phys. Express 2014, 82910, 16−19. (15) Acharya, S.; Torgersen, J.; Kim, Y.; Park, J.; Schindler, P.; Dadlani, A. L.; Winterkorn, M.; Xu, S.; Walch, S. P.; Usui, T.; et al. Self-Limiting Atomic Layer Deposition of Barium Oxide and Barium I


Article


(58) Kamalasanan, M. N.; Kumar, N. D.; Chandra, S. Dielectric and Ferroelectric Properties of BaTiO3 Thin Films Grown by the Sol-Gel Process. J. Appl. Phys. 1993, 74 (9), 5679−5686. (59) Joshi, P. C.; Desu, S. B. Structural, Electrical, and Optical Studies on Rapidly Thermally Processed Ferroelectric BaTiO3 Thin Films Prepared by Metallo-Organic Solution Deposition Technique. Thin Solid Films 1997, 300, 289−294. (60) Zeng, J. M.; Wang, H.; Wang, M.; Shang, S. X.; Wang, Z.; Lin, C. L. Structural and Electrical Characteristics of Thin Films of BaTiO3 Prepared by Atmospheric-Pressure Metal-Organic Chemical Vapour Deposition. J. Phys. D: Appl. Phys. 1998, 31 (19), 2416−2420. (61) Du, H.; Liang, W.; Li, Y.; Gao, M.; Zhang, Y.; Chen, C.; Lin, Y. Leakage Properties of BaTiO3 Thin Films on Polycrystalline Ni Substrates Grown by Polymer-Assisted Deposition with Two-Step Annealing. J. Alloys Compd. 2015, 642, 166−171. (62) Lee, E. J. H.; Pontes, D. S. L.; Leite, E. R.; Longo, E.; Varela, J. A.; Araujo, E. B.; Eiras, J. A. Preparation and Properties of Ferroelectric BaTiO3 Thin Films Produced by the Polymeric Precursor Method. J. Mater. Sci. Lett. 2000, 19, 1457−1459. (63) Hoerman, B. H.; Ford, G. M.; Kaufmann, L. D.; Wessels, B. W. Dielectric Properties of Epitaxial BaTiO3 Thin Films. Appl. Phys. Lett. 1998, 73 (16), 2248−2250. (64) Li, C. L.; Chen, Z. H.; Zhou, Y. L.; Cui, D. F. Effect of Oxygen Content on the Dielectric and Ferroelectric Properties of LaserDeposited BaTiO3 Thin Films. J. Phys.: Condens. Matter 2001, 13, 5261−5268. (65) Im, J.; Auciello, O.; Baumann, P. K.; Streiffer, S. K.; Kaufman, D. Y.; Krauss, A. R. Composition-Control of Magnetron-SputterDeposited (BaxSr1‑x)Ti1+yO3+z Thin Films for Voltage Tunable Devices. Appl. Phys. Lett. 2000, 76 (5), 625−627. (66) Ihlefeld, J. F.; Harris, D. T.; Keech, R.; Jones, J. L.; Maria, J. P.; Trolier-McKinstry, S. Scaling Effects in Perovskite Ferroelectrics: Fundamental Limits and Process-Structure-Property Relations. J. Am. Ceram. Soc. 2016, 99 (8), 2537−2557. (67) Tagantsev, A. K.; Sherman, V. O.; Astafiev, K. F.; Venkatesh, J.; Setter, N. Ferroelectric Materials for Microwave Tunable Applications. J. Electroceram. 2003, 11 (1−2), 5−66.

(37) Maslova, O. A.; Shirokov, F. V.; Yuzyuk, Y. I.; El Marssi, M.; Jain, M.; Ortega, N.; Katiyar, R. S. Raman Spectroscopy Study of Lattice Dynamics of Macro-, Micro-, and Nanostructured Barium Titanates. Phys. Solid State 2014, 56 (2), 310−316. (38) Tsuzuku, K.; Couzi, M. In Situ Investigation of Chemical Reactions between BaCO3 and Anatase or Rutile TiO2. J. Mater. Sci. 2012, 47, 4481−4487. (39) Reddy, Y. K. V.; Mergel, D.; Reuter, S.; Buck, V.; Sulkowski, M. Structural and Optical Properties of BaTiO3 Thin Films Prepared by Radio-Frequency Magnetron Sputtering at Various Substrate Temperatures. J. Phys. D: Appl. Phys. 2006, 39 (6), 1161−1168. (40) Silva, J. P. B.; Sekhar, K. C.; Almeida, A.; Moreira, J. A.; Pereira, M.; Gomes, M. J. M. Influence of Laser Repetition Rate on Ferroelectric Properties of Pulsed Laser Deposited BaTiO3 Films on Platinized Silicon Substrate. Appl. Phys. A: Mater. Sci. Process. 2013, 113 (2), 379−384. (41) Naik, R.; Nazarko, J. J.; Naik, V. M.; Mohammed, M. S.; Auner, G. W.; Mantese, J. V.; Schubring, N. W.; Micheli, A. L.; Catalan, A. B. Temperature Dependence of the Raman Spectra of Polycrystalline Ba1‑xSrxTiO3. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61 (17), 11367−11372. (42) Colder, H.; Domengès, B.; Jorel, C.; Marie, P.; Boisserie, M.; Guillon, S.; Nicu, L.; Galdi, A.; Méchin, L. Structural Characterisation of BaTiO3 Thin Films Deposited on SrRuO3/YSZ Buffered Silicon Substrates and Silicon Microcantilevers. J. Appl. Phys. 2014, 115 (5), 053506. (43) Venkateswaran, U. D.; Naik, V. M.; Naik, R. High-Pressure Raman Studies of Polycrystalline BaTiO3. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58 (21), 14256. (44) Lee, S.; Randall, C. A.; Liu, Z. K. Modified Phase Diagram for the Barium Oxide-Titanium Dioxide System for the Ferroelectric Barium Titanate. J. Am. Ceram. Soc. 2007, 90 (8), 2589−2594. (45) Woehlecke, M.; Marrello, V.; Onton, A. Refractive Index of BaTiO3 and SrTiO3 Films. J. Appl. Phys. 1977, 48 (4), 1748−1750. (46) Schottky, W. Halbleitertheorie der Sperrschicht. Naturwissenschaften 1938, 26 (52), 843. (47) Frenkel, J. On Pre-Breakdown Phenomena in Insulators and Electronic Semi-Conductors. Phys. Rev. 1938, 54 (8), 647. (48) Lampert, M. A.; Mark, P. Current Injection in Solids; Academic Press: New York, 1970. (49) Pabst, G. W.; Martin, L. W.; Chu, Y.-H.; Ramesh, R. Leakage Mechanisms in BiFeO3 Thin Films. Appl. Phys. Lett. 2007, 90, 072902. (50) Wang, J.; Zhang, T.; Xiang, J.; Li, W.; Duo, S.; Li, M. Studies of BaTiO3 Thin Films on Different Bottom Electrode. J. Mater. Sci.: Mater. Electron. 2009, 20 (1), 44−48. (51) Jong-Min, O.; Hyung-Jun, K.; Song-Min, N. Characterization of Leakage Current Mechanisms for Aerosol-Deposited BaTiO3 Thin Films at Room Temperature. J. Korean Phys. Soc. 2010, 57 (41), 1096. (52) Raj, P. M.; Lee, B.-W.; Balaraman, D.; Tummala, R. R. Leakage Current Analysis of Hydrothermal BaTiO3 Thin Films. J. Electroceram. 2011, 27 (3−4), 169−175. (53) Dedon, L. R.; Saremi, S.; Chen, Z.; Damodaran, A. R.; Apgar, B. A.; Gao, R.; Martin, L. W. Nonstoichiometry, Structure, and Properties of BiFeO3 Films. Chem. Mater. 2016, 28, 5952−5961. (54) Waser, R.; Klee, M. Theory of Conduction and Breakdown in Perovskite Thin Films. Integr. Ferroelectr. 1992, 2, 23−40. (55) Zafar, S.; Jones, R. E.; Jiang, B.; White, B.; Kaushik, V.; Gillespie, S. The Electronic Conduction Mechanism in Barium Strontium Titanate Thin Films. Appl. Phys. Lett. 1998, 73 (24), 3533. (56) Hejazi, M. M.; Safari, A. Temperature-Dependent Leakage Current Behavior of Epitaxial Bi0.5Na0.5TiO3-Based Thin Films Made by Pulsed Laser Deposition. J. Appl. Phys. 2011, 110 (10), 103710. (57) Torgersen, J.; Acharya, S.; Dadlani, A. L.; Petousis, I.; Kim, Y.; Nordlund, D.; Prinz, F. B.; Trejo, O.; Nordlund, D.; Prinz, F. B. Relating Electronic and Geometric Structure of Atomic Layer Deposited BaTiO3 to Its Electrical Properties. J. Phys. Chem. Lett. 2016, 7, 1428−1433. J


BaTiO3 Thin Films from Atomic Layer Deposition ... - ACS Publications

Recommend Documents