Epitaxial barium titanate (BTO) thin films are grown on strontium titanate-buffered Si(001) using atomic layer deposition (ALD) at 225 °C. X-ray diffraction confirms compressive strain in BTO films after the low temperature growth for films as thick as 66 nm, with the BTO c-axis oriented in the out-of-plane direction. Postdeposition annealing above 650 °C leads to an in-plane c-axis orientation. Piezoresponse force microscopy was used to verify the ferroelectric switching behavior of ALD-grown films. Electrical and electro-optic measurements confirm BTO film ferroelectric behavior in out-of-plane and in-plane directions, respectively, at the micrometer scale.

The use of ferroelectric materials for microelectronics and photonics applications has captured the attention of numerous researchers. Integration of ferroelectric materials on semiconductors enables novel device designs in memory,1,2 transistors,3–6 and photonics.7–9 In particular, BaTiO3 (BTO)—a ferroelectric perovskite oxide—emerges as a suitable candidate for integration with silicon-based micro- and nanofabrication processes. Lattice parameters of perovskite oxides are comparable to the Si-Si interatomic distance along the [110] direction, which fosters the ability to grow perovskite oxides epitaxially on Si(001)10,11 using growth methods such as atomic layer deposition (ALD)12,13 and molecular beam epitaxy (MBE).14–16 There are numerous reports integrating epitaxial BTO with silicon substrates for both electronic and photonic devices.4,8,9,12,17

One important challenge with incorporating BTO into devices is to reliably stabilize ferroelectricity in BTO thin films, in relevant microstructured devices and on relevant substrates. The appropriate BTO microstructure is dictated by the intended application. For example, BTO films need to be single-domain ferroelectric with polarization oriented normal to the substrates for some transistor applications;3,5,14 on the other hand, in-plane polarized BTO is often desired for optical applications.7,9

Several BTO film growth techniques have demonstrated films featuring ferroelectric behavior. Choi et al. reported strain enhancement of ferroelectricity for epitaxial BTO thin films grown by pulsed laser deposition (PLD) and MBE on substrates with different lattice spacing.18 They reported BTO films up to 200 nm thick that showed no relaxation to bulk BTO parameters when compressively strained on GdScO3(110) (−1.0% strain) and only relaxed partially on DyScO3(110) (−1.7%). Ferroelectricity was confirmed via second harmonic generation and polarization-electric field measurement on BTO thin film capacitors.18 

Kormondy et al. reported several growth techniques to fabricate electro-optically active epitaxial BTO thin films;19 epitaxial BTO thin films were grown on SrTiO3-buffered (STO-buffered) Si(001) substrates by MBE, PLD, radio-frequency sputtering, and chemical vapor deposition (CVD). By comparing the microstructure, ferroelectricity, and electro-optic responses of BTO films grown by different growth methods, MBE emerged as the technique that produced BTO films with the least defects and strongest electro-optic response.19 This work also demonstrated clearly that the ferroelectric and electro-optic responses of BTO thin films are largely influenced by their microstructure and defect density.

While MBE has so far been the predominant growth method for producing high-quality BTO films,4,7,15,20–22 ALD has also been demonstrated as a promising technique to grow crystalline perovskites on Si(001).12,23,24 ALD has many potential advantages over MBE for the production of perovskite thin films, including low growth temperatures and superior scalability. Although many reports in the literature document the ALD process to grow BTO films,12,25–31 the demonstration of robust ferroelectric behavior for ALD-grown BTO has remained elusive despite the fact that structural data have conclusively shown that epitaxial BTO films are c-axis out-of-plane oriented by virtue of being strained to Si(001).12 

Several factors may account for the inability to measure ferroelectricity in BTO thin films. The structural and electrical properties can depend on the composition of the BTO film.28,29 Deviation in Ba, Ti, and O compositions can all lead to a change in electrical properties of the film. As an example, oxygen vacancies within perovskite films can result in significant leakage current,21 thus impeding the observation of ferroelectric behavior. The lack of a practical conduction band offset between BTO and STO, and silicon also leads to additional leakage current.21,32 Previous study on Al-doped STO and BTO using ALD demonstrated the improvements in leakage current.29,33 Incorporating Al into STO and BTO crystal structures can increase the conduction band between them and silicon.34 Furthermore, the very low ferroelectric coercive field (EC) of bulk BTO, at 500 V/cm,35,36 translates to a hysteresis width of 0.75 mV in 15 nm BTO thin film capacitance vs voltage (CV) measurements.12 Herein, we report ferroelectricity in ALD-grown BTO films, the necessary steps needed to enable such observations, and some additional considerations for device applications of crystalline BTO thin films.

The epitaxial BTO films are grown on STO-buffered Si(001) substrates. Both p- and n-doped Si(001) substrates were used for electrical measurements. Wafers were diced into 20 × 18 mm2 rectangles for film growth. The n-doped substrates (Virginia Semiconductor, Fredericksburg, VA) are As-doped with a dopant concentration of 1019 cm−3 and a resistivity of <0.005 Ω cm. The p-doped substrates (MTI Corporation, Richmond, CA) are B-doped, with resistivity specified as 0.1–1.0 Ω cm. 2-in. undoped double-side-polished Si(001) wafers (Virginia Semiconductor, Fredericksburg, VA) with resistivity greater than 10 kΩ cm were used to grow BTO films for optical measurements.

The vacuum system used for substrate preparation and film growth is comprised of a DCA 600 MBE chamber, a custom-built ALD chamber, and an analysis chamber capable of performing X-ray photoelectron spectroscopy (XPS) connected to a common ultrahigh vacuum transfer line. The system details are documented elsewhere.37 STO-buffered Si(001) substrate preparation is described in detail by Choi et al.38 Briefly, the cleaned Si(001) substrate is introduced into the MBE chamber to remove the native oxide layer via Sr-assisted deoxidation as well as to deposit the STO buffer layer. Sr, Ti, and O2 are codelivered to the deoxidized Si(001) surface at 200 °C. The target thickness for the buffer layer is between 2.0 and 2.8 nm, or 5–7 unit cells of STO. Once the desired thickness is reached, the sample is annealed under high vacuum at 600 °C to crystallize the STO buffer layer. The sample is then transferred in vacuo to the ALD reactor for BTO film growth.

The ALD reactor is a stainless steel, hot-wall reactor that can process samples with sizes up to 2 in. diameter. Ar is used as the carrier and purge gas. A rotary vane pump is used to evacuate the ALD reactor and maintain the pressure of 1 Torr during operation. The precursors are introduced into the ALD reactor via a gas manifold. The BTO growth follows the procedures reported previously,12,27 which comprises Ba-O and Ti-O unit cycles. Each unit cycle has 4 phases: precursor dose, precursor purge, coreactant dose, and coreactant purge. Barium bis(triisopropylcclopentadienyl) {Ba[(iPr)3Cp]2, sold as HyperBa by Air Liquide} and titanium triisopropoxide (TTIP, purchased from Sigma-Aldrich) are used as the precursors for Ba and Ti, respectively. Water vapor is used as the coreactant for both Ba and Ti unit cycles. Before deposition, Ba[(iPr)3Cp]2 precursor is heated to 148 ± 3 °C, and TTIP to 38 ± 3 °C. Deionized water is at room temperature. The substrate is brought to thermal equilibrium with the hot-wall ALD reactor, with a substrate temperature of 225 °C. Both Ba and Ti unit cycles comprise of four parts: 2 s dose of the precursor {Ba[(iPr)3Cp]2 for Ba and TTIP for Ti}, 15 s of Ar purge, 1 s of water dose, and 15 s of Ar purge. The number of Ba and Ti unit cycles was adjusted to 3:4 to ensure a near-stoichiometric composition, where [Ba]:[Ti] is as close to 50:50 as possible. The deposited BTO film is analyzed in vacuo as-is, without any vacuum postdeposition annealing process.

In situ reflective high-energy electron diffraction (RHEED) and XPS were used throughout the film fabrication process to monitor the surface order and composition of the samples without risk of air exposure. The XPS system uses a VG Scienta R3000 analyzer and a monochromated Al X-ray source with a photon energy of 1486.6 eV. Film composition is determined by the relative area of each elemental X-ray photoelectron spectrum feature, with appropriate relative sensitivity factors determined using a BTO single crystal. Once the BTO film is deposited, the BTO/STO/Si samples are taken out of the vacuum system without any in vacuo postdeposition processes.

In many cases, the BTO/STO/Si samples are subjected to postdeposition ex vacuo annealing prior to additional characterization. Two annealing recipes were used: one for electrical measurements and the other for optical measurements. The samples used for electrical measurements were annealed in air at 300 °C for 30 min, with a temperature ramp rate of ≤5 °C/min, to ensure that the films remained strained to the Si(001) substrate. The samples for optical characterization were annealed under an O2 flow at 650 °C for 30 min, with a temperature ramp rate of ≥10 °C/min, to relax the films from the Si(001) substrate.

The BTO/STO/Si samples were characterized ex vacuo via X-ray diffraction (XRD) and X-ray reflectivity (XRR) using a Rigaku Ultima IV diffractometer with thin film attachment and a Cu X-ray source. XRD was used to determine the lattice spacing of the epitaxial BTO films. BTO film thickness was determined by fitting the XRR data using GenX,39 where the thickness of the STO buffer layer deposited by MBE is associated with the number of STO unit cells.

Au electrodes were sputter-deposited on the BTO/STO/Si samples for leakage current (C-V) measurements; Ni electrodes with a Au capping layer were deposited by electron beam evaporation. The electrode dimensions were defined by a shadow mask, with electrode sizes ranging from 25 × 25 μm2 to 200 × 200 μm2. Once the electrodes were deposited, the backside of the Si substrate was scratched with a scalpel and immediately placed on a scalpel-scratched copper plate with a eutectic liquid In-Ga alloy sandwiched in between. The copper plate, electrically connected to the Si substrate, served as the bottom contact. Current-voltage (I-V) and C-V measurements were performed using a Keithley 4200 Semiconductor Characterization System with Keithley 590 C-V Analyzer. A cat-whisker tungsten probe was used to contact the top Ni/Au electrode to avoid film puncture during electrical measurements.

Piezoresponse force microscopy (PFM) measurements were performed on a Park XE70 atomic force microscope. The measurements were performed with commercially-available Pt-coated Si tips (AppNano ANSCM-PC, 30-nm tip radius of curvature) driven at 200 kHz. For a typical scan rate of 4–8 μm/s, the BTO samples are electrically switched (poled) at tip biases of −8 V and 6 V. PFM measurements are conducted at a tip bias of 2 V. Data were acquired at room temperature under ambient conditions, and all PFM measurements were made within a few minutes after the domains were created by electrical switching.

For the electro-optical measurements, the ALD-grown epitaxial BTO/STO/Si samples were annealed to ensure relaxation of the BTO layer from the Si(001) substrate, such that the epitaxial BTO film is c-axis in-plane oriented.40 Additionally, annealing heals the possible oxygen vacancies in the BTO film, which is important in reducing the leakage current and allowing for applying bias voltages used in electro-optic measurements.15 The annealed sample was patterned with W electrodes via e-beam evaporation, with the patterns defined by photolithography. The measurement devices were fabricated such that the distance between each pair of top electrodes is 5 μm. The gaps between electrodes are oriented such that the applied electric fields are at various directions with respect to the [100] direction of the BTO film. To measure the electro-optic effect of a sample, a linearly polarized infrared laser (λ = 1550 nm) is focused at the electrode gap, with the laser beam directed orthogonally to the sample surface. The light transmitted across the BTO film becomes slightly elliptically polarized as a result of BTO birefringence. The transmitted, elliptically-polarized light can then be linearly polarized again using a quarter waveplate, which results in a beam that is rotated with respect to the incident light. The optical rotation (δ), as a result of BTO birefringence and the optics used, is measured with respect to the applied DC electric field (Eoff). For each measurement, the Eoff across the electrode gap is applied via an offset DC voltage. An AC field at 17.3 kHz is applied on top of the DC bias to modulate δ by Δδ and create a power modulation on the detector. A lock-in technique was used to determine Δδ. More details of device fabrication, measurement setup, and data collection are reported elsewhere.7,15

The BTO films prepared for electrical measurements reported herein were grown by using a Ba:Ti ALD unit cycle ratio of 3:4. The growth per unit cycle is between 0.6 and 0.7 Å. The Ti-half cycle of 1 s TTIP dose, 10 s Ar purge, 1 s water dose, and 10 s Ar purge was shown previously to match ALD growth rates for TiO2.37 The BTO growth rate did not increase for Ba[(iPr)3Cp]2 dose times of 2, 3, or 4. A 2 s dose was adopted to ensure surface saturation. XPS analyses determine that the crystalline as-grown BTO films have the Ba:Ti ratios between 50:50 and 55:45. The XPS-measured Ba:Ti ratio of each film undergoing electrical measurements will be specified individually throughout this section.

In situ RHEED was used to monitor the surface order of BTO/STO/Si samples. Figure 1 shows typical RHEED images taken after the deposition of an STO buffer layer via MBE and a BTO thin film via ALD. The RHEED image demonstrates that the as-grown BTO film retains the square surface symmetry of the STO buffer layer, which indicates epitaxial registry between the BTO film and the underlying STO. Figure 2 shows the typical X-ray photoelectron spectra of a crystalline BTO film, which is used to determine the [Ba]:[Ti] ratios in this study.

FIG. 1.

RHEED patterns of a BTO/STO/Si sample after deposition of STO buffer layer and BTO film. Both diffractions were performed at the [110] azimuth. The 15 nm ALD-grown BTO film is crystalline as-deposited—with a [Ba]:[Ti] ratio of 55:45, as shown by the similarity of RHEED patterns and symmetry between the two surfaces.

FIG. 1.

RHEED patterns of a BTO/STO/Si sample after deposition of STO buffer layer and BTO film. Both diffractions were performed at the [110] azimuth. The 15 nm ALD-grown BTO film is crystalline as-deposited—with a [Ba]:[Ti] ratio of 55:45, as shown by the similarity of RHEED patterns and symmetry between the two surfaces.

Close modal
FIG. 2.

(a) Ba 3d and (b) Ti 2p X-ray photoelectron spectra of a BTO film grown on STO/Si. The measured [Ba]:[Ti] ratio is 52:48.

FIG. 2.

(a) Ba 3d and (b) Ti 2p X-ray photoelectron spectra of a BTO film grown on STO/Si. The measured [Ba]:[Ti] ratio is 52:48.

Close modal

Figure 3(a) presents the XRD data for a BTO/STO/Si sample used in C-V and I-V measurements. As-deposited BTO samples are c-axis out-of-plane oriented. Using a Gaussian fit, the peak location of BTO(002) translates to an out-of-plane lattice spacing of 4.022 Å. Although the out-of-plane c-axis orientation makes the BTO/STO/Si sample suitable for transistor-type applications,3,5 the as-deposited BTO films are too conductive. The leakage current density is approximately 103 A/cm2 at 500 kV/cm. Annealing in air decreases the leakage current by three orders of magnitude (Fig. 4). Furthermore, the as-deposited BTO films cannot be poled and measured by PFM. The leakage current was too high to apply enough voltage across the film (data not shown).

FIG. 3.

(a) Out-of-plane XRD of a c-axis out-of-plane oriented 15 nm BTO film after deposition and air annealing at 300 °C for 30 min. Inset is the ω scan about the BTO(002) peak. The BTO orientation remained unchanged after the annealing process. (b) XRD scans for out-of-plane and in-plane lattice spacing of a 66 nm BTO film fabricated for optical measurement. Dashed lines are the lattice spacing of bulk BTO crystals.40 (c) Out-of-plane XRD scans of the same sample in (b) show the relaxation of the film after O2-annealing the sample at 650 °C for 30 min.

FIG. 3.

(a) Out-of-plane XRD of a c-axis out-of-plane oriented 15 nm BTO film after deposition and air annealing at 300 °C for 30 min. Inset is the ω scan about the BTO(002) peak. The BTO orientation remained unchanged after the annealing process. (b) XRD scans for out-of-plane and in-plane lattice spacing of a 66 nm BTO film fabricated for optical measurement. Dashed lines are the lattice spacing of bulk BTO crystals.40 (c) Out-of-plane XRD scans of the same sample in (b) show the relaxation of the film after O2-annealing the sample at 650 °C for 30 min.

Close modal
FIG. 4.

I-V measurements of a 15 nm c-axis out-of-plane BTO film on n-doped Si before and after air annealing. The [Ba]:[Ti] ratio is measured to be 52:48.

FIG. 4.

I-V measurements of a 15 nm c-axis out-of-plane BTO film on n-doped Si before and after air annealing. The [Ba]:[Ti] ratio is measured to be 52:48.

Close modal

Due to the large mismatch in the thermal expansion coefficient between BTO and Si, postdeposition annealing processes near 650 °C lead to relaxation of BTO films.12,15,40 Relaxation of BTO is not always desirable.3,4 A postdeposition annealing recipe that allows for strain retention of a 15-nm BTO film involves annealing the films at 300 °C for 30 min in air, with a temperature ramp rate of ≤5 °C/min. As the symmetric 2θ-ω scans and the rocking curves about BTO(002) indicate [Fig. 3(a)], the out-of-plane lattice spacing of the 15 nm BTO film did not change after the annealing treatment.

Figure 3(b) shows the symmetric in-plane and out-of-plane XRD scans of a 66 nm ALD-grown BTO film. The peak locations are 4.025 and 4.033 Å for in-plane and out-of-plane BTO lattice spacing, respectively. Partial relaxation from the Si(001) substrate and the resulting mixed orientation occur during the ALD process. As the XRD measurement of the as-deposited 66 nm film [Figs. 3(b) and 3(c)] shows, the out-of-plane lattice constant is still near the bulk c-axis value of BTO. While the out-of-plane peak coincides with the bulk c-axis value well, the broad in-plane BTO peak covers both the a-axis and c-axis 2θ values. The broad peak-top of the in-plane BTO peak indicates the presence of both BTO(200) (a-axis) and BTO(002) (c-axis) reflections, which support the interpretation of mixed orientations within the film.

There are several reports on epitaxial BTO films grown by MBE in which researchers observed mixed orientations using XRD, scanning transmission electron microscopy, and geometric phase analysis.14,15 In addition, both Ngo et al. and McDaniel et al. reported the relaxation of ALD-grown epitaxial BTO and STO films, respectively, on STO-buffered Si with similar thicknesses.12,41 The 4.033 Å out-of-plane lattice spacing suggests that a significant part of the ALD-grown BTO film remains strained by the underlying STO crystalline template, even with a thickness over 60 nm. Annealing the sample under the O2 atmosphere at 650 °C for 30 min can relax the strain on BTO films, which is consistent with the literature.12,15 As shown in Fig. 3(c), the out-of-plane lattice spacing of the BTO film changed from 4.033 Å to 4.007 Å, indicating BTO film relaxation and change in the BTO c-axis orientation to in-plane.

A 15 nm BTO film deposited on STO-buffered, heavily n-doped Si was used for leakage current and capacitance measurements. Figure 4 shows the I-V characteristics of the BTO/STO/Si sample before and after the 300 °C air annealing with the 25 × 25 μm2 Au top electrodes. The as-deposited sample exhibits ohmic behavior, with a leakage current density of ∼103 A/cm2 at an electric field of 500 kV/cm. Annealing the sample to 300 °C reduces the leakage current by approximately three orders of magnitude. The film also exhibits rectifying behavior after the annealing process (Fig. 4), which is consistent with a previous report on BTO-based I-V measurements.20 The relatively low annealing temperature used here also allows for minimizing the interfacial SiO2 formation in addition to retaining strain on BTO. This annealing process did not change the out-of-plane lattice spacing, as presented in Fig. 3(a).

The reduction of leakage current with annealing indicates that oxygen vacancy states within the BTO films play a primary role in enabling leakage current across the BTO film, and the as-grown BTO films contain oxygen vacancies. The post-air-annealing sample is still exhibiting a relatively large leakage current density compared to other BTO films grown by ALD. This can be attributed to the lack of conduction band offset between BTO and Si, the substrate and bottom electrode. In addition, the 300 °C air-annealing process cannot eliminate oxygen vacancies within the film. Studies on electro-optic measurements of BTO all used higher annealing temperature and greater oxygen concentration to minimize leakage current through the BTO film.15,17 Previous studies showed that BTO grown by plasma-enhanced ALD achieved a lower leakage current density.28 However, the use of oxygen plasma also carries the risk of oxidizing the oxide-silicon interface. Further optimizations on annealing conditions, substrate preparation, and growth process can improve the electrical properties of the BTO/Si structures while preserving the oxide-silicon interface. Alternatives on precursor selection31 and growth process30 offer additional avenues to improve the electrical performance of the BTO films in general. Nonetheless, the results presented in Figs. 3(a) and 4 demonstrate the use of the low temperature, air-annealing process at 300 °C to heal oxygen vacancies within the film, while retaining the strain on BTO by the underlying template.

Metal-ferroelectric-semiconductor (MFS) capacitors were fabricated from BTO/STO/Si. Figure 5 presents representative C-V results measured from 25 × 25 μm2 Ni/Au top electrodes and the p-doped substrate for as-deposited and air-annealed BTO films. Both capacitance and conductance diverge at <−1 V for the C-V measurement on the as-deposited BTO film [Fig. 5(a)]. After air annealing, the C-V curves show both the depletion and accumulation regions of the Si, which are delineated by the capacitance change near a gate voltage of −1.5 V [Fig. 5(b)]. It is worth noting that the capacitance and conductance measurements are done on the sample (i.e., the BTO/STO/Si stack directly), and the measurements made no distinction between the BTO and STO oxide layers. Rather, the oxide layers are treated as a single dielectric layer of a capacitor. The conductance maxima can be observed at the voltages of capacitance change, indicating charge movement within the MFS capacitor at such gate voltages. The decrease in sample conductance after air annealing corroborates the observations made in I-V measurements. The counterclockwise hysteresis on a p-doped Si substrate is the result of charge trapping within the oxide layer, as opposed to ferroelectric swtiching.42 In addition, the hysteresis width of the C-V curves decreases from 0.4 V for the as-deposited BTO film to 0.1 V for the air-annealed BTO film. The decrease in hysteresis width indicates the reduction in trap density as a result of annealing. Nevertheless, no ferroelectric hysteresis is observed. While the precise origin of the defects causing charge trapping is currently unknown, it may be related to oxygen vacancy states in the BTO layer.

FIG. 5.

C-V measurements of a c-axis out-of-plane (a) as-deposited and (b) air-annealed BTO film. The films were deposited on a p-doped Si with the STO buffer layer annealed in oxygen, with a [Ba]:[Ti] ratio of 52:48. Arrows indicate the voltage sweep direction.

FIG. 5.

C-V measurements of a c-axis out-of-plane (a) as-deposited and (b) air-annealed BTO film. The films were deposited on a p-doped Si with the STO buffer layer annealed in oxygen, with a [Ba]:[Ti] ratio of 52:48. Arrows indicate the voltage sweep direction.

Close modal

To explore the possible presence of oxygen vacancies within the STO template layer and their impact on the C-V measurements, a different set of BTO/STO/Si structures were prepared for which the STO was deliberately over-oxidized. This is achieved by annealing the STO template layer at 600 °C at an O2 partial pressure of 5 × 10−7 Torr instead of under high vacuum. Annealing STO under vacuum leads to the presence of Ti3+ as seen in the Ti 2p XPS results in Fig. 6(a). The Ti3+ XPS signal is reduced to below the XPS detection limit for the film annealed at 600 °C under an O2 partial pressure of 5 × 10−7 Torr [Fig. 6(a)]. The modified annealing procedure (5 × 10−7 Torr of O2) also led to formation of an amorphous SiO2/SiOx layer, which is observed in the XPS scan of the Si 2p spectral feature [Fig. 6(b)].

FIG. 6.

XPS scans of (a) Ti 2p and (b) Si 2p signals of STO buffer layers crystallized at different oxygen partial pressures.

FIG. 6.

XPS scans of (a) Ti 2p and (b) Si 2p signals of STO buffer layers crystallized at different oxygen partial pressures.

Close modal

Figure 5(b) shows that the counterclockwise hysteresis remains observable after the modification of STO annealing procedure. Such hysteresis effects due to trapped charges are also observable for epitaxial BTO films grown on STO/Si prepared in accordance with the previously reported methods (i.e., annealing the STO-buffered Si in high vacuum), which prevent amorphous interfacial layer formation between the perovskite and Si layers.12,41 The C-V measurement results for BTO films grown on vacuum-annealed STO-buffered Si show no difference from the results presented in Fig. 5 (data not shown). The hysteresis observed in Fig. 5 suggests that oxygen vacancies and/or other defect states remain within the BTO layer.

Even without the charge effect seen in Fig. 5, ferroelectric hysteresis behavior is likely not observable in C-V measurements of very thin films. Compressively-strained BTO films exhibit increased ferroelectric polarization and decreased coercivity.18 This exacerbates the difficulty in using C-V measurements to observe ferroelectric hysteresis of epitaxial, c-axis out-of-plane BTO films grown on Si(001). Both the higher film thickness (90 nm) and the SiO2 layer formed during growth contributed to the observation of clockwise (i.e., ferroelectric) C-V hysteresis for a BTO-based MFS capacitor grown by PLD under high oxygen pressure (0.12 Torr).20 By using the 10 kV/cm coercivity estimated from electro-optic measurement (see below), the 15 nm BTO films studied herein should result in a hysteresis window of approximately 15 mV, which would be obscured by the trapped charge-induced hysteresis.

To characterize the polarization behavior of ALD-grown BTO films, PFM measurements were performed on air-annealed BTO/STO/Si samples, where the n-type Si (As-doped, σ < 0.005 Ω cm) served as the bottom electrode. Figure 7 presents a typical topography and PFM scan result. A 10 × 10 μm2 square was poled on a 15 nm BTO/STO/Si sample surface with a tip bias of −8 V, follow by an inner 6 × 6 μm2 square poled with a tip bias of +6 V. The PFM phase image after poling shows a clear phase contrast between the two oppositely poled regions, with 180° of PFM phase difference. Phase boundaries between the oppositely poled regions are also clearly visible from the PFM-amplitude scan. The phase contrast measured by PFM demonstrated a switchable polarization of the sample, which confirms the ferroelectric behavior of the ALD-grown BTO film.

FIG. 7.

Topography scan and PFM-amplitude and phase responses of a 15 nm c-axis out-of-plane BTO film on STO/Si after poling. The BTO film has a [Ba]:[Ti] ratio of 52:48.

FIG. 7.

Topography scan and PFM-amplitude and phase responses of a 15 nm c-axis out-of-plane BTO film on STO/Si after poling. The BTO film has a [Ba]:[Ti] ratio of 52:48.

Close modal

A BTO/STO/Si sample with a 40-nm BTO thickness was measured optically to characterize the ferroelectric and electro-optic behavior of ALD-grown epitaxial BTO. The BTO film has an in-plane c-axis orientation after the annealing step in oxygen at 650 °C. This is to ensure the BTO c-axis will be in-plane oriented and suited for the measurement geometry. As a result, the annealed film has a lattice constant of 3.978 Å and an in-plane constant of 4.023 Å (data not shown). Figure 8 summarizes the electro-optic measurement results for a sample with a 5 μm electrode gap, with the electric field applied along the BTO[110] direction. The measurement result is reported in field- and thickness-normalized optical rotation δ′ = δ/EACt,7,15 where t is the BTO film thickness and EAC is the AC field used for the measurement. The result shows the change in optical rotation with respect to the DC electric field applied on the measurement device. Based on the distance between the two sweep directions, the coercivity is on the order of 10 kV/cm. However, the noisiness of the electro-optic data prevents an accurate estimation of coercivity. The hysteretic behavior observed in the voltage sweep is consistent with the ferroelectric switching behavior of BTO thin films. This electric field-dependent behavior confirms the electro-optic behavior of the BTO film and presents the possibility of using ALD to fabricate electro-optically active thin films for photonic applications.7,15,17,43 In addition, the hysteresis behavior indicates the flipping of polarization domains of the BTO film, which is characteristic of ferroelectric materials. Using data presented in Fig. 8 and the equation reported previously,7 the effective Pockels coefficient of the material is estimated to be 26 ± 6 pm/V.

FIG. 8.

(a) Schematic of the electro-optic measurement geometry: An IR laser is focused at the gap between the W electrodes, where light transmission of the exposed BTO/STO/Si can be observed. Bias is applied on the W electrodes to provide electric field on the BTO film. (b) Result of the electro-optic measurement. Polarization rotation (δ′) is changing with respect to the electric field on the BTO film. Arrows indicates electric field sweep directions. The BTO film used for electro-optic measurement has a [Ba]:[Ti] ratio of 52:48.

FIG. 8.

(a) Schematic of the electro-optic measurement geometry: An IR laser is focused at the gap between the W electrodes, where light transmission of the exposed BTO/STO/Si can be observed. Bias is applied on the W electrodes to provide electric field on the BTO film. (b) Result of the electro-optic measurement. Polarization rotation (δ′) is changing with respect to the electric field on the BTO film. Arrows indicates electric field sweep directions. The BTO film used for electro-optic measurement has a [Ba]:[Ti] ratio of 52:48.

Close modal

The comparison between the ALD-grown BTO films and BTO films grown by other techniques may also shed some light on the microstructure of the ALD-grown BTO films, as the effective Pockels coefficient depends on the defect density of the BTO films.19 The ALD-grown BTO effective Pockels coefficient of 26 pm/V is comparable to BTO films fabricated by other techniques, reported in Ref. 19, where the range of effective Pockels coefficients of epitaxial BTO films on Si(001) was reported to be between 7 pm/V (by CVD) and 140 pm/V (by MBE). The defect nature of the films correlates with the Pockels coefficient19 and ALD films can likely be improved by postgrowth annealing at higher temperatures than employed herein and under higher O2 partial pressures or using atomic oxygen to heal possible oxygen defects. The electro-optic measurement further suggests the ferroelectric behavior of ALD-grown BTO films.

Both PFM and electro-optic measurements clearly indicate the presence of switchable, remnant polarization within the ALD-grown BTO films, which is characteristic of ferroelectric behavior. The ferroelectric polarization direction is constrained by the c-axis orientation of the BTO film—a function of film thickness and postdeposition annealing. The agreement between the ferroelectric polarization direction, based on measurement geometry, and the BTO film c-axis orientation, on XRD, shows that the ferroelectric behavior is from the epitaxial BTO films.

In summary, epitaxial BTO films were grown on STO-buffered Si(001). Postdeposition treatment can exert control over the polarization directions of ALD-grown epitaxial BTO films as thick as 66 nm. Without postdeposition thermal treatments, strain can still be observed from the as-deposited films. BTO as thick as 66 nm still exhibits a mixed c-axis orientation. It is possible to preserve strain on the BTO film when the samples are heated up and cooled down slowly (≤5 °C/min) and annealed at up to 300 °C.

Ferroelectric behavior of the ALD-grown epitaxial BTO films was observed by using both PFM and electro-optical measurements. While defects such as oxygen vacancies are readily addressable, film thickness and other defects causing trapped charges prevent the observation of ferroelectric behavior in C-V measurements. Overall, this work demonstrates definitive observation of ferroelectricity on ALD-grown epitaxial BTO as well as the effect on expression of such ferroelectric behaviors as a result of BTO film microstructure.

This work was supported by Air Force Office of Scientific Research (Grant Nos. FA9550-14-1-0090 and FA9550-18-1-0053) and the National Science Foundation IRES (Award No. 1358111). L.Z. and K.L. were supported by the National Science Foundation (NSF) (No. DMR-1707372). J.E.O. is grateful for the generous support of the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1610403. S.A. and J.F. acknowledge funding from the European Commission under Grant Agreement Nos. H2020-ICT-2015-25-688579 (PHRESCO) and H2020-ICT-2017-1-780997 (plaCMOS) and from the Swiss State Secretariat for Education, Research and Innovation under Contract No. 15.0285.

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