Search

This work reports the growth of crystalline SrHfxTi1-xO3 (SHTO) films on Ge (001) substrates by atomic layer deposition. Samples were prepared with different Hf content x to explore if strain, from tensile (x = 0) to compressive (x = 1), affected film crystallization temperature and how composition affected properties. Amorphous films grew at 225 °C and crystallized into epitaxial layers at annealing temperatures that varied monotonically with composition from -530 °C (x= 0) to -660 °C (x= 1). Transmission electron microscopy revealed abrupt interfaces. Electrical measurements revealed 0.1 A/cm2 leakage current at 1 MV/cm for x= 0.55.

The integration of dissimilar materials is highly desirable for many different types of device applications but often challenging to achieve in practice. The unrivalled imaging capabilities of the aberration-corrected electron microscope enable enhanced insights to be gained into the atomic arrangements across heterostructured interfaces. This paper provides an overview of our recent observations of oxide-semiconductor heterostructures using aberration-corrected high-angle annular-dark-field and large-angle bright-field imaging modes. The perovskite oxides studied include strontium titanate, barium titanate, and strontium hafnate, which were grown on Si(001) and/or Ge(001) substrates using the techniques of molecular-beam epitaxy or atomic-layer deposition. The oxide layers displayed excellent crystallinity and sharp, abrupt interfaces were observed with no sign of any amorphous interfacial layers. The Ge(001) substrate surfaces invariably showed both 1× and 2× periodicity consistent with preservation of the 2 × 1 surface reconstruction following oxide growth. Overall, the results augur well for the future development of functional oxide-based devices integrated on semiconductor substrates.

Display Omitted High crystallinity ferroelectric BaTiO3 with out of plane polarization integrated on Si by MBE.High crystallinity ferroelectric BaTiO3 with in plane and out of plane polarization integrated on Ge by MBE.High crystallinity SrTiO3 is grown on Si and Ge by low temperature ALD.High crystallinity BaTiO3 is grown on Si by ALD. The recently developed ability to grow layers of transition metal oxides with atomic precision by means of physical vapor deposition has opened up a possibility of monolithic integration of these oxides on semiconductors. Here we review the recent progress in integrating ferroelectric films with Si and Ge, and their potential applications in electronics and nanophotonics. Perovskite films described in the talk were grown by molecular beam epitaxy (MBE) and, when possible, chemical routes were tested via atomic layer deposition (ALD). Design of the structures and analysis of the experimental results were aided by density functional theory (DFT).

The rapidly increasing global demand for energy combined with the environmental impact of fossil fuels has spurred the search for alternative sources of clean energy. One promising approach is to convert solar energy into hydrogen fuel using photoelectrochemical cells. However, the semiconducting photoelectrodes used in these cells typically have low efficiencies and/or stabilities. Here we show that a silicon-based photocathode with a capping epitaxial oxide layer can provide efficient and stable hydrogen production from water. In particular, a thin epitaxial layer of strontium titanate (SrTiO3) was grown directly on Si(001) by molecular beam epitaxy. Photogenerated electrons can be transported easily through this layer because of the conduction-band alignment and lattice match between single-crystalline SrTiO3 and silicon. The approach was used to create a metal–insulator–semiconductor photocathode that, under a broad-spectrum illumination at 100 mW cm−2, exhibits a maximum photocurrent density of 35 mA cm−2 and an open circuit potential of 450 mV; there was no observable decrease in performance after 35 hours of operation in 0.5 M H2SO4. The performance of the photocathode was also found to be highly dependent on the size and spacing of the structured metal catalyst. Therefore, mesh-like Ti/Pt nanostructured catalysts were created using a nanosphere lithography lift-off process and an applied-bias photon-to-current efficiency of 4.9% was achieved. A silicon-based photocathode with an epitaxial strontium titanate protection layer and a mesh-like nanostructured catalyst can provide an applied bias photon-to-current efficiency of 4.9% for water reduction.

We report pulsed laser deposition (PLD) synthesis of epitaxial and polycrystalline monoclinic bismuth vanadate (BiVO4, BVO) thin films. X-ray diffraction (XRD), atomic force microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy were used to characterize the samples. Epitaxial, c-axis oriented growth was achieved using single crystal yttria-stabilized zirconia (100), a substrate temperature of 575–600 °C, and an oxygen pressure of 7.8 mTorr. The volatility of Bi necessitated a large excess (Bi:V = ∼6:1) of this element in the ceramic targets to obtain stoichiometric films. XRD confirmed a BVO (001)||YSZ (001) and BVO [100]||YSZ [100] epitaxial relationship. Film growth was 3-D, and the morphology was discontinuous, consisting of irregular, smooth grains. Additionally, dense, continuous polycrystalline films were deposited on fluorine-doped tin oxide (FTO) on glass substrates at room temperature with stoichiometric targets and postdeposition annealing in air. Evaluation of these sam...

Resistive switching in single crystal anatase TiO2 grown by atomic layer deposition on Si (001) with an epitaxial single crystal SrTiO3 buffer layer is characterized and analyzed. Although switching occurs via a valence-change mechanism involving motion of oxygen vacancies, electrical characteristics highly reminiscent of electrochemical metallization memories – low leakage current in the high-resistance state, highly linear current-voltage characteristics in the low-resistance state, and a very high on/off current ratio – are observed. We postulate that these characteristics may be associated with formation of Ti n O2n-1 Magnéli or Magnéli-like nanophases in the low-resistance state. In addition, quantized conductance is observed in the low-resistance state, with the number of conducting ballistic channels being highly controllable via application of different compliance currents during the memory SET process. For compliance currents of 10-200µA, low resistance state conductances corresponding to 1 to 4 conductance channels, controllable to within a single quantum of conductance, are observed.

Integration of functional oxides with semiconductors opens ways of creating novel hybrid architectures. However, epitaxy of these different types of materials brings challenges unlike those encountered in traditional semiconductor epitaxy. In this paper we will discuss our recent work on monolithic integration of perovskite oxides SrTiO3 and BaTiO3 on Si(001) and Ge(001), and the use of these pseudosubstrates to integrate other functional oxides using molecular beam epitaxy (MBE) and atomic layer deposition (ALD).

Lanthanum aluminate (LAO) films were grown epitaxially on Si (001) by atomic layer deposition (ALD) using a buffer layer of strontium titanate (STO) grown by molecular beam epitaxy. The ALD growth of LAO was done at 250 °C by using tris(N,N′-diisopropylformamidinate)–lanthanum, trimethylaluminum, and water as co-reactants. Reflection high-energy electron diffraction, X-ray diffraction and transmission electron microscopy were used to determine film crystallinity. The as-deposited LAO films were amorphous and became crystalline after vacuum annealing at 600 °C for 2 h. In-situ X-ray photoelectron spectroscopy (XPS) was used to characterize the LAO/STO/Si interfaces at various stages throughout the growth and annealing process. XPS analysis showed minimal SiO bonding at the STO/Si interface after the ALD process and after post-deposition annealing at 600 °C for 2 h. The results demonstrate a method to integrate epitaxial LAO films on Si (001) substrates by ALD.

Germanium (Ge)-based metal-oxide-semiconductor field-effect transistors are a promising candidate for high performance, low power electronics at the 7 nm technology node and beyond. However, the availability of high quality gate oxide/Ge interfaces that provide low leakage current density and equivalent oxide thickness (EOT), robust scalability, and acceptable interface state density (D(it)) has emerged as one of the most challenging hurdles in the development of such devices. Here we demonstrate and present detailed electrical characterization of a high-κ epitaxial oxide gate stack based on crystalline SrHfO3 grown on Ge (001) by atomic layer deposition. Metal-oxide-Ge capacitor structures show extremely low gate leakage, small and scalable EOT, and good and reducible D(it). Detailed growth strategies and postgrowth annealing schemes are demonstrated to reduce Dit. The physical mechanisms behind these phenomena are studied and suggest approaches for further reduction of D(it).

Epitaxial anatase titanium dioxide (TiO2) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer grown by molecular beam epitaxy (MBE) to serve as a surface template. The growth of TiO2 was achieved using titanium isopropoxide and water as the co-reactants at a substrate temperature of 225–250 °C. To preserve the quality of the MBE-grown STO, the samples were transferred in-situ from the MBE chamber to the ALD chamber. After ALD growth, the samples were annealed in-situ at 600 °C in vacuum (10− 7 Pa) for 1–2 h. Reflection high-energy electron diffraction was performed during the MBE growth of STO on Si(001), as well as after deposition of TiO2 by ALD. The ALD films were shown to be highly ordered with the substrate. At least four unit cells of STO must be present to create a stable template on the Si(001) substrate for epitaxial anatase TiO2 growth. X-ray diffraction revealed that the TiO2 films were anatase with only the (004) reflection present at 2θ = 38.2°, indicating that the c-axis is slightly reduced from that of anatase powder (2θ = 37.9°). Anatase TiO2 films up to 100 nm thick have been grown that remain highly ordered in the (001) direction on STO-buffered Si(001) substrates.

We report the epitaxial growth of γ-Al2O3 on SrTiO3 (STO) substrates by atomic layer deposition (ALD). The ALD growth of γ-Al2O3 on STO(001) single crystal substrates was performed at a temperature of 345 °C. Trimethylaluminum and water were used as co-reactants. In-situ reflection high-energy electron diffraction and ex-situ x-ray diffraction were used to determine the crystallinity of the Al2O3 films. In-situ x-ray photoelectron spectroscopy was used to characterize the Al2O3/STO heterointerface. The formation of a Ti3+ feature is observed in the Ti 2p spectrum of STO after the first few ALD cycles of Al2O3 and even after exposure of the STO substrate to trimethylaluminum alone at 345 °C. The presence of a Ti3+ feature is a direct indication of oxygen vacancies at the Al2O3/STO heterointerface, which provide the carriers for the quasi-two dimensional electron gas at the interface.

TiO2 has been investigated extensively as an active resistive switching (RS) material for resistive random access memory. In this letter, single-crystal anatase- TiO2 thin films fabricated on silicon by atomic layer deposition are used to realize highly stable and clean bipolar RS behavior with a record high ON/OFF ratio (~107) and low leakage current in the high-resistance state. The switching characteristics resemble those of electrochemical memories via formation and dissolution of conductive filaments (CFs) composed of oxygen vacancies, and small numbers of quantized channels are reproducibly observed in the low-resistance state, consistent with quantized conductance (QC) found in conventional electrolytic systems and indicating its potential for forming ultrathin CF amenable to device scaling. A detailed analysis of QC and contact resistance is presented. The emergence of QC is believed to be related to the single-crystal nature of the TiO2 thin films.

In this work, hydrogen production from water is demonstrated via a p-type silicon photocathode with a thin epitaxial strontium titanate, SrTiO3 (STO), as capping layer by molecular beam epitaxy. The advantages of using STO are the ideal conduction band alignment and perfect lattice match between single crystalline SrTiO3 and Si, so the photogenerated electrons can transport through the capping layer with a reduced recombination rate. The STO/p-Si photocathode exhibited a maximum photocurrent density and open circuit potential of 35 mA/cm2 and 450 mV, respectively. There was no observable decrease in performance after 10 hr operation in 0.5M H 2 SO 4 . We found the efficiency and performance were highly dependent on the size and spacing of the structured metal catalyst. Scaled down the metal catalysts feature size into nanometer region can greatly improve the efficiency. In addition, samples with graphene (Grahene/p-Si) as the lateral transport channel and capping layer shown an enhanced fill factor compared with that of STO/p-Si.

TiO2 is being widely explored as an active resistive switching (RS) material for resistive random access memory. We report a detailed analysis of the RS characteristics of single-crystal anatase-TiO2 thin films epitaxially grown on silicon by atomic layer deposition. We demonstrate that although the valence change mechanism is responsible for the observed RS, single-crystal anatase-TiO2 thin films show electrical characteristics that are very different from the usual switching behaviors observed for polycrystalline or amorphous TiO2 and instead very similar to those found in electrochemical metallization memory. In addition, we demonstrate highly stable and reproducible quantized conductance that is well controlled by application of a compliance current and that suggests the localized formation of conducting Magneli-like nanophases. The quantized conductance observed results in multiple well-defined resistance states suitable for implementation of multilevel memory cells.

Crystalline lanthanum aluminate (LAO) films were grown epitaxially on SrTiO 3 (001) and on Si(001) with a buffer layer of four unit cells of SrTiO 3 by atomic layer deposition. The SrTiO 3 buffer layer was grown by molecular beam epitaxy. Tris(N,N’-diisopropylformamidinate)-lanthanum, trimethylaluminum, and water as co-reactants were employed at 250 °C for atomic layer deposition. Films were characterized using ex-situ reflection high-energy electron diffraction, X-ray diffraction and in-situ X-ray photoelectron spectroscopy. The as-deposited LAO films were amorphous. Different annealing conditions were necessary to realize crystalline films because of different degrees of tensile strain between crystalline LAO and the SrTiO 3 or the Si(001) substrate. When grown on SrTiO 3 (001), with a lattice mismatch of 2.9%, annealing temperatures of 750 °C for 2 h were necessary. Crystalline films were realized at 600 °C under vacuum at 2 h for SrTiO 3 -buffered Si(001), with a lattice mismatch of 1.3%. By keeping the annealing temperature relatively low (2 h at 600 °C under vacuum), the interfacial amorphous layer at the STO/Si interface was minimized to about one monolayer and an abrupt interface between SrTiO 3 and LAO was maintained.

First-principle calculations are used to model the adsorption and hydration of strontium bis(cyclopentadienyl) [Sr(Cp)2] on TiO2-terminated strontium titanate, SrTiO3 (STO), for the deposition of strontium oxide, SrO, by atomic layer deposition (ALD). The Sr(Cp)2 precursor is shown to adsorb on the TiO2-terminated surface, with the Sr atom assuming essentially the bulk position in STO. The C–Sr bonds are weaker than in the free molecule, with a Ti atom at the surface bonding to one of the C atoms in the cyclopentadienyl rings. The surface does not need to be hydrogenated for precursor adsorption. The calculations are compared with experimental observations for a related Sr cyclopentadienyl precursor, strontium bis(triisopropylcyclopentadienyl) [Sr(iPr3Cp)2], adsorbed on TiO2-terminated STO. High-resolution x-ray photoelectron spectroscopy and low-energy ion scattering spectroscopy show adsorption of the Sr precursor on the TiO2-terminated STO after a single precursor dose. This study suggests that ALD growth from the strontium precursors featuring cyclopentadienyl ligands, such as Sr(Cp)2, may initiate film growth on non-hydroxylated surfaces.

Atomic layer deposition (ALD) is a proven technique for the conformal deposition of oxide thin films with nanoscale thickness control. Most successful industrial applications have been with binary oxides, such as Al2O3 and HfO2. However, there has been much effort to deposit ternary oxides, such as perovskites (ABO3), with desirable properties for advanced thin film applications. Distinct challenges are presented by the deposition of multi-component oxides using ALD. This review is intended to highlight the research of the many groups that have deposited perovskite oxides by ALD methods. Several commonalities between the studies are discussed. Special emphasis is put on precursor selection, deposition temperatures, and specific property performance (high-k, ferroelectric, ferromagnetic, etc.). Finally, the monolithic integration of perovskite oxides with semiconductors by ALD is reviewed. High-quality epitaxial growth of oxide thin films has traditionally been limited to physical vapor deposition techniques (e.g., molecular beam epitaxy). However, recent studies have demonstrated that epitaxial oxide thin films may be deposited on semiconductor substrates using ALD. This presents an exciting opportunity to integrate functional perovskite oxides for advanced semiconductor applications in a process that is economical and scalable.

We report the formation of a quasi-two-dimensional electron gas (2-DEG) at the interface of γ-Al2O3/TiO2-terminated SrTiO3 (STO) grown by atomic layer deposition (ALD). The ALD growth of Al2O3 on STO(001) single crystal substrates was performed at temperatures in the range of 200–345 °C. Trimethylaluminum and water were used as co-reactants. In situ reflection high energy electron diffraction, ex situ x-ray diffraction, and ex situ cross-sectional transmission electron microscopy were used to determine the crystallinity of the Al2O3 films. As-deposited Al2O3 films grown above 300 °C were crystalline with the γ-Al2O3 phase. In situ x-ray photoelectron spectroscopy was used to characterize the Al2O3/STO interface, indicating that a Ti3+ feature in the Ti 2p spectrum of STO was formed after 2–3 ALD cycles of Al2O3 at 345 °C and even after the exposure to trimethylaluminum alone at 300 and 345 °C. The interface quasi-2-DEG is metallic and exhibits mobility values of ∼4 and 3000 cm2 V−1 s−1 at room temperature...

The current work explores the crystalline perovskite oxide, strontium hafnate, as a potential high-k gate dielectric for Ge-based transistors. SrHfO3 (SHO) is grown directly on Ge by atomic layer deposition and becomes crystalline with epitaxial registry after post-deposition vacuum annealing at ∼700 °C for 5 min. The 2 × 1 reconstructed, clean Ge (001) surface is a necessary template to achieve crystalline films upon annealing. The SHO films exhibit excellent crystallinity, as shown by x-ray diffraction and transmission electron microscopy. The SHO films have favorable electronic properties for consideration as a high-k gate dielectric on Ge, with satisfactory band offsets (>2 eV), low leakage current (

Resistive switching in TiO2 has been studied extensively for resistive random access memory and other applications in nanoelectronics; nearly all of these studies have employed polycrystalline or amorphous TiO2. We have used atomic layer deposition to grow single-crystal anatase TiO2 thin films on Si (001) substrates, with single-crystal SrTiO3 grown by molecular beam epitaxy serving as a buffer layer between the TiO2 and the Si substrate. Resistive switching devices are then fabricated with the bottom electrode consisting of the SrTiO3/Si substrate, and the top electrode formed by deposition of 5-8nm Ti followed by 160nm Au [Figure 1(a)-(b)]. Highly stable bipolar resistive switching and very linear current-voltage characteristics in the on (low-resistance) state are consistently observed [Figure 1(c)-(d)]. For a subset of devices studied, the single-crystal TiO2 was annealed after 3nm deposition, resulting in formation of oxygen vacancies in that layer. In all the devices, deposition of Ti for the top electrode also creates oxygen vacancies near the top of the TiO2 layer. The oxygen vacancies, created either by annealing or by Ti deposition, play a key role in the bipolar resistive switching processes observed. Detailed analysis of electrical behavior in these devices reveals that switching between the high-resistance and low-resistance states occurs via an electrochemical metallization-like mechanism in which oxygen vacancies form a highly conductive filament during the device SET process, which then dissolves during the RESET process. Extremely high resistance ratios between the off (high-resistance) and on (low-resistance) states – as high as 107 – are regularly observed, which we attribute to the highly insulating nature of the single-crystal TiO2 layer. In addition, we observe that the on-state conductance can be controlled quite precisely and reproducibly via the compliance current imposed during the measurement, and assumes values that are integer multiples of G 0 = 2e 2/h, the fundamental quantum of conductance for a single-mode ballistic conductor. At compliance currents of 10-200µA, on-state conductance can be controlled to within a single quantum of conductance by imposing a given compliance current during the device SET process (Figure 2). These results demonstrate substantial advantages of single-crystal TiO2 as a material for resistive switching devices, and suggest a variety of avenues for integration into high-density memory arrays and elucidation of issues relevant to scaling potential.

Strontium titanate, SrTiO3 (STO), thin films incorporated with lanthanum are grown on Si (001) substrates at a thickness range of 5–25 nm. Atomic layer deposition (ALD) is used to grow the LaxSr1−xTiO3 (La:STO) films after buffering the Si (001) substrate with four-unit-cells of STO deposited by molecular beam epitaxy. The crystalline structure and orientation of the La:STO films are confirmed via reflection high-energy electron diffraction, X-ray diffraction, and cross-sectional transmission electron microscopy. The low temperature ALD growth (∼225 °C) and post-deposition annealing at 550 °C for 5 min maintains an abrupt interface between Si (001) and the crystalline oxide. Higher annealing temperatures (650 °C) show more complete La activation with film resistivities of ∼2.0 × 10−2 Ω cm for 20-nm-thick La:STO (x ∼ 0.15); however, the STO-Si interface is slightly degraded due to the increased annealing temperature. To demonstrate the selective incorporation of lanthanum by ALD, a layered heterostructure ...

This work demonstrates the growth of crystalline SrTiO 3 (STO) directly on germanium via a chemical method. After thermal deoxidation, the Ge substrate is transferred in vacuo to the deposition chamber where a thin fi lm of STO (2 nm) is deposited by atomic layer deposition (ALD) at 225 °C. Following post-deposition annealing at 650 °C for 5 min, the STO fi lm becomes crystalline with epitaxial registry to the underlying Ge (001) substrate. Thicker STO fi lms (up to 15 nm) are then grown on the crystalline STO seed layer. The crystalline structure and orientation are confi rmed via refl ection high-energy electron diffraction, X-ray diffraction, and transmission electron microscopy. Electrical measurements of a 15-nm thick epitaxial STO fi lm on Ge show a large dielectric constant ( k ≈ 90), but relatively high leakage current of ≈10 A/cm 2 for an applied fi eld of 0.7 MV/cm. To suppress the leakage current, an aluminum precursor is cycled during ALD growth to grow crystalline Al-doped STO (SrTi 1−x Al x O 3−δ ) fi lms. With suffi cient Al doping (≈13%), the leakage current decreases by two orders of magnitude for an 8-nm thick fi lm. The current work demonstrates the potential of ALD-grown crystalline oxides to be explored for advanced electronic applications, including highmobility Ge-based transistors.

Atomic layer deposition (ALD) of epitaxial c-axis oriented BaTiO3 (BTO) on Si(001) using a thin (1.6 nm) buffer layer of SrTiO3 (STO) grown by molecular beam epitaxy is reported. The ALD growth of crystalline BTO films at 225 °C used barium bis(triisopropylcyclopentadienyl), titanium tetraisopropoxide, and water as co-reactants. X-ray diffraction (XRD) reveals a high degree of crystallinity and c-axis orientation of as-deposited BTO films. Crystallinity is improved after vacuum annealing at 600 °C. Two-dimensional XRD confirms the tetragonal structure and orientation of 7–20-nm thick films. The effect of the annealing process on the BTO structure is discussed. A clean STO/Si interface is found using in-situ X-ray photoelectron spectroscopy and confirmed by cross-sectional scanning transmission electron microscopy. The capacitance-voltage characteristics of 7–20 nm-thick BTO films are examined and show an effective dielectric constant of ∼660 for the heterostructure.

Cobalt oxide (CoO) films are grown epitaxially on Si(001) by atomic layer deposition (ALD) using a thin (1.6 nm) buffer layer of strontium titanate (STO) grown by molecular beam epitaxy. The ALD growth of CoO films is done at low temperature (170–180 °C), using cobalt bis(diisopropylacetamidinate) and water as co-reactants. Reflection high-energy electron diffraction, X-ray diffraction, and cross-sectional scanning transmission electron microscopy are performed to characterize the crystalline structure of the films. The CoO films are found to be crystalline as-deposited even at the low growth temperature with no evidence of Co diffusion into Si. The STO-buffered Si (001) is used as a template for ALD growth of relatively thicker epitaxial STO and TiO2 films. Epitaxial and polycrystalline CoO films are then grown by ALD on the STO and TiO2 layers, respectively, creating thin-film heterostructures for photoelectrochemical testing. Both types of heterostructures, CoO/STO/Si and CoO/TiO2/STO/Si, demonstrate water photooxidation activity under visible light illumination. In-situ X-ray photoelectron spectroscopy is used to measure the band alignment of the two heterojunctions, CoO/STO and CoO/TiO2. The experimental band alignment is compared to electronic structure calculations using density functional theory.

Epitaxial strontium titanate (STO) films have been grown by atomic layer deposition (ALD) on Si(001) substrates with a thin STO buffer layer grown by molecular beam epitaxy (MBE). Four unit cells of STO grown by MBE serve as the surface template for ALD growth. The STO films grown by ALD are crystalline as-deposited with minimal, if any, amorphous SiOx layer at the STO-Si interface. The growth of STO was achieved using bis(triisopropylcyclopentadienyl)-strontium, titanium tetraisopropoxide, and water as the coreactants at a substrate temperature of 250 °C. In situ x-ray photoelectron spectroscopy (XPS) analysis revealed that the ALD process did not induce additional Si–O bonding at the STO-Si interface. Postdeposition XPS analysis also revealed sporadic carbon incorporation in the as-deposited films. However, annealing at a temperature of 250 °C for 30 min in moderate to high vacuum (10−6–10−9 Torr) removed the carbon species. Higher annealing temperatures (>275 °C) gave rise to a small increase in Si–O bonding, as indicated by XPS, but no reduced Ti species were observed. X-ray diffraction revealed that the as-deposited STO films were c-axis oriented and fully crystalline. A rocking curve around the STO(002) reflection gave a full width at half maximum of 0.30° ± 0.06° for film thicknesses ranging from 5 to 25 nm. Cross-sectional transmission electron microscopy revealed that the STO films were continuous with conformal growth to the substrate and smooth interfaces between the ALD- and MBE-grown STO. Overall, the results indicate that thick, crystalline STO can be grown on Si(001) substrates by ALD with minimal formation of an amorphous SiOx layer using a four-unit-cell STO buffer layer grown by MBE to serve as the surface template.

Epitaxial anatase titanium dioxide (TiO2) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer without any amorphous SiOx layer at the STO–Si interface. Four unit cells of STO grown by molecular beam epitaxy (MBE) serve as the surface template for ALD growth. To preserve the quality of the MBE-grown STO, the samples were transferred in situ from the MBE chamber to the ALD chamber. The growth of TiO2 was achieved using titanium isopropoxide and water as the coreactants at a substrate temperature of 250 °C. In situ x-ray photoelectron spectroscopy analysis revealed that the ALD process did not induce Si–O bonding at the STO–Si interface. Slight improvement in crystallinity of the TiO2 film was achieved through in situ annealing under vacuum (10−9 Torr) at 450–600 °C. However, the amount of Si–O bonding increased following annealing at temperatures greater than 250 °C. X-ray diffraction revealed that TiO2 films annealed at a temperature of 250 °C in vacuum (10−9 Torr) for 1 h were the anatase phase and well crystallized. The results indicate that careful consideration of growth temperature and annealing conditions may allow epitaxial oxide films to be grown by ALD on STO-buffered Si(001) substrates without formation of an amorphous SiOx layer.Epitaxial anatase titanium dioxide (TiO2) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer without any amorphous SiOx layer at the STO–Si interface. Four unit cells of STO grown by molecular beam epitaxy (MBE) serve as the surface template for ALD growth. To preserve the quality of the MBE-grown STO, the samples were transferred in situ from the MBE chamber to the ALD chamber. The growth of TiO2 was achieved using titanium isopropoxide and water as the coreactants at a substrate temperature of 250 °C. In situ x-ray photoelectron spectroscopy analysis revealed that the ALD process did not induce Si–O bonding at the STO–Si interface. Slight improvement in crystallinity of the TiO2 film was achieved through in situ annealing under vacuum (10−9 Torr) at 450–600 °C. However, the amount of Si–O bonding increased following annealing at temperatures greater than 250 °C. X-ray diffraction revealed that TiO2 films annealed at a temperature of...