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Thick epitaxial BaTiO3 films ranging from 120 nm to 1 μm were grown by off-axis RF magnetron sputtering on SrTiO3-templated silicon-on-insulator (SOI) substrates for use in electro-optic applications, where such large thicknesses are necessary. The films are of high quality, rivaling those grown by molecular beam epitaxy (MBE) in crystalline quality, but can be grown 10 times faster. Extraction of lattice parameters from geometric phase analysis of atomic-resolution scanning transmission electron microscopy images revealed how the in-plane and out-of-plane lattice spacings of sputtered BaTiO3 changes as a function of layer position within a thick film. Our results indicate that compared to molecular beam epitaxy, sputtered films retain their out-of-plane polarization (c-axis) orientation for larger thicknesses. We also find an unusual re-transition from in-plane polarization (a-axis) to out-of-plane polarization (c-axis), along with an anomalous lattice expansion, near the surface. We also studied a method of achieving 100% a-axis-oriented films using a two-step process involving amorphous growth and recrystallization of a seed layer followed by normal high temperature growth. While this method is successful in achieving full a-axis orientation even at low thicknesses, the resulting film has a large number of voids and misoriented grains. Electro-optic measurement using a transmission setup of a sputtered BTO film grown using the optimized conditions yields an effective Pockels coefficient as high as 183 pm/V. A Mach–Zehnder modulator fabricated on such films exhibits phase shifting with an equivalent Pockels coefficient of 157 pm/V. These results demonstrate that sputtered BTO thick films can be used for integrated electro-optic modulators for Si photonics.

Dating to the first reports of epitaxial oxide deposition on Si(001), the integration of complex oxides on silicon has been a fast-moving area of research, where fundamental materials physics is in...

The fabrication of epitaxial, ultra-thin SrTiO3 (STO) on thick SiO2 without the need for complicated wafer-bonding processes has been demonstrated. The resulting transition metal oxide (TMO)-on-glass layer stack is analogous to traditional silicon-on-insulator (SOI) wafers, where the crystalline device silicon layer of SOI has been replaced by a crystalline functional TMO layer. Fabrication starts with ultra-thin body SOI on which crystalline STO is grown epitaxially by molecular beam epitaxy. The device silicon layer is subsequently fully oxidized by ex situ high-temperature dry O2 annealing, as confirmed by X-ray photoelectron spectroscopy, X-ray reflectivity, and high-resolution electron microscopy. STO maintains its epitaxial registry to the carrier silicon substrate after annealing and no evidence for degradation of the STO crystalline quality as a result of the TMO-on-glass fabrication process is observed. The ease of fabricating the TMO-on-glass platform without the need for wafer bonding will enab...

$\mathrm{BaTi}{\mathrm{O}}_{3}$ (BTO) is an emergent material in the field of silicon-integrated photonics, as its thin films have been demonstrated to have a very large electro-optic (Pockels) coefficient that can be used for optical modulators. However, BTO grown directly on $\mathrm{SrTi}{\mathrm{O}}_{3}$ (STO)-buffered Si (STO is required for epitaxial growth) initially grows with a polarization direction not suitable for the geometries currently used in photonic devices. Here, we grow BTO on a $\mathrm{BaSn}{\mathrm{O}}_{3}$-buffered STO substrate to form orthorhombic $mm2$ BTO, which has in-plane polarization orientation needed for photonic devices. Extensive crystalline characterization is done to confirm the high quality of the films, along with electro-optic measurements. Theoretical simulations coupled with the experimental results provide a foundational understanding of the properties of strain-stabilized orthorhombic BTO. Large electro-optic coefficients of 121 pm/V are observed in films as thin as 40 nm.

The surface reconstructions and surface core level shifts of 0.25 monolayer (ML), 0.5 ML and 1.0 ML of europium (Eu) on the Ge(001) surface are investigated experimentally and theoretically, using molecular beam epitaxy (MBE), reflection high-energy electron diffraction (RHEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Starting with a clean Ge(001) surface we find that the surface reconstruction changes from the initial (2 × 1) to a (3 × 2), (8 × 1) and a final (3 × 1) pattern with increasing Eu coverage of 0.25 ML, 0.5 ML and 1.0 ML, respectively. XPS scans demonstrate that the Ge 3d core level shifts to higher binding energies with increasing Eu coverage. We find that Eu is in the 2+ oxidation state, transferring its charge to the Ge surface, resulting in the disappearance of the asymmetry of the dimer tilt at a coverage of 0.5 ML of Eu. From density functional calculations we find that the Ge dimers flatten out at 0.5 ML Eu coverage and partially break apart at 1.0 ML Eu.

This work reports the atomic layer deposition (ALD) and epitaxial growth of 10–12 nm BaxSr1−xTiO3 (x ∼ 0.5) thin films on Zintl-templated Ge (001) substrates, formed with 0.5 monolayers of Ba. The best thin films were grown using two steps. An amorphous 2–3 nm layer was deposited at 225 °C and then was converted to a crystalline film by annealing for 15 min at 650 °C under vacuum to generate the seed layer that had an abrupt interface with the Ge (001) substrate. A 7–10 nm layer was subsequently grown and crystallized at the same conditions as the seed layer. Metal-insulator-semiconductor structures that employed Au electrodes were used to assess leakage currents and dielectric properties. Interfacial effects inherent to the ∼10 nm BaxSr1−xTiO3 films affect the capacitance measurements leading to k of 87 and 140 for 10.9 and 14.6 nm films, respectively. The epitaxial films have high k in the bulk. Using capacitance measurements for BaxSr1-xTiO3 films that are 13–18.4 nm thick, a bulk k of ∼2000 and low interfacial capacitance density (C/A) of ∼100 fF/μm2 were extracted from thickness dependent relationships. The 10 nm films display a high leakage current (∼102 A/cm2 at −1.0 V) that can be reduced (to ∼10−1 A/cm2 at −1.0 V) by capping the film with ∼1 nm Al2O3 using ALD before depositing the Au electrode or by annealing in O2 at 300 °C for 15 min. The thin low-k Al2O3 layer or the GeOx formed during the O2 annealing step, while improving leakage current, introduces a capacitor in series that further reduces the film dielectric constant such that a 9.9 nm Al-capped film and a 9.8 nm O2-annealed film yield dielectric constants of 80 and 41, respectively.

Barium titanate BaTiO 3 (BTO) thin films were epitaxially grown at 225 °C on 2 × 1-reconstructed Ge(001) surfaces via atomic layer deposition (ALD). Approximately 2 nm of BTO film was grown directly on Ge(001) as an amorphous film. Electron diffraction confirmed the epitaxy of the BTO films after post-deposition annealing at 650 °C. Additional BTO layers grown on the crystalline BTO/Ge(001) film were crystalline as-deposited. X-ray diffraction indicated that the epitaxial BTO films had a c -axis out-of-plane orientation, and the abrupt BTO/Ge interface was preserved with no sign of any interfacial germanium oxide. Scanning transmission electron microscopy provided evidence of Ba atoms occupying the troughs of the dimer rows of the 2 × 1-reconstructed Ge(001) surface, as well as preservation of the 2 × 1-reconstructed Ge(001) surface. This study presents a low-temperature process to fabricate BTO/Ge heterostructures.

Combining first-principles calculations and experiment, we investigate the atomic and electronic structure of the Si/EuO interface. We consider the thermodynamic stability of interface structures with different levels of oxidation to identify the most probable configuration. By comparing the calculated band alignment and core-level shifts with measured values, we validate the theoretically constructed interface model. We find that the band offset can be tuned by altering the relative energy positions of the Si and EuO conduction bands via interface oxidation, which can be used to tune this materials system for specific applications in spintronics.

Using a combination of density functional theory and in situ x-ray photoelectron spectroscopy, we study the atomic and electronic structure of the Eu/EuO interface. Calculations predict that electrons transfer from Eu metal into EuO and induce an unexpected downward band bending at the interface. Accounting for spectral broadening and attenuation of the signal from subsurface layers, the calculated layer-resolved total density of states agrees well with experimental x-ray photoelectron spectroscopy in the valence-band region. The total $4f$ spectrum contains contributions from both Eu and EuO, with the latter component significantly broadened as a result of band bending. This bending and charge transfer originate from Eu Fermi-level pinning at the EuO charge neutrality level, which has been suggested to be located above the conduction-band bottom [Phys. Rev. B 86, 205310 (2012)].

Strontium barium niobate (Sr x Ba1–x Nb2O6, referred to as SBN) is a material of interest for making efficient optical modulators due to its very high electro‐optic (EO or Pockels) coefficient. SBN is a ferroelectric with a Curie temperature that varies between 20 and 80 °C, depending on its Sr/Ba ratio. Structurally, it belongs to the tetragonal tungsten bronze family. The successful integration of SBN on Si in textured epitaxial form using molecular beam epitaxy via a SrTiO3 (STO) buffer layer is reported. X‐ray diffraction and scanning transmission electron microscopy are used to analyze the crystalline quality, STO/SBN interface structure, and preferred orientations of the SBN film. Thin, highly oriented SBN films integrated on Si have potential applications in low power and small footprint integrated Si photonics.

BaTiO3 (BTO) is a highly promising material for the fabrication of electro-optic (EO) modulators due to the large effective Pockels coefficient of the material, particularly in an epitaxial form. It also has the added benefit of being readily integrated on a Si material platform via a SrTiO3 template. These two characteristics make epitaxial BTO ideal for use in next generation silicon photonics applications. Being a ferroelectric, BTO has a unique crystallographic direction in which the ferroelectric polarization points. For EO modulators, because the polarization direction controls the coupling between light and an external electric field, it is important to understand how different growth methods and subsequent processing affect the direction of the ferroelectric polarization. Certain electro-optic devices may require polarization to be in the plane of the film (in-plane switching liquid crystal devices), while other applications may require it to be normal to the plane of the film (Mach–Zehnder modulator). Here, we review the growth of epitaxial BTO on Si by a variety of deposition methods including molecular beam epitaxy, pulsed laser deposition, and RF sputtering. We summarize the resulting BTO film structure and quality based on the reported characterization results. We also discuss EO measurements of basic devices made from this material platform where such data are available.

β-Ga2O3 was deposited in thin film form by plasma-assisted molecular beam epitaxy at 670 °C and 630 °C onto a γ-Al2O3 (111) buffer layer grown at 840 °C by e-beam evaporation on a clean Si (001) surface. The β-Ga2O3 film was 66 nm thick, stoichiometric, and strongly textured, as determined by x-ray reflectivity, x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, x-ray diffraction, and transmission electron microscopy, with three basal growth planes (2̄01), (101), and {310}, including one twin variant {31̄0}. The observed basal growth planes correspond to the close-packing planes of the distorted face-centered cubic oxygen sublattice of β-Ga2O3. Local structural ordering can be thought to occur due to a continuation of the oxygen sublattice from the γ-alumina buffer layer into the β-gallia film. Each β-Ga2O3 growth plane further gives rise to 12 symmetry-derived rotational in-plane variants, resulting in a total of 48 domain variants. Atomistic models of possible gallia–alumina interfaces are presented.

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.

Different surface preparation methods for cleaning MgO, a widely used substrate in oxide epitaxy, are summarized and compared. We find that in situ surface preparation methods are preferable to ex situ preparation methods. We show that the complete removal of hydroxide, carbonate, and adventitious carbon from the MgO surface can be achieved via oxygen plasma exposure at 200 °C without high temperature annealing. Using this process, an atomically flat surface with root mean square roughness values of ∼0.1 nm is demonstrated. Surfaces treated thus also exhibit sharp RHEED streaks indicating good crystalline order of the surface. We also show that high temperature annealing of MgO, either by itself or following other ex situ cleaning methods, such as solvent cleaning, is a reasonably effective method for the removal of surface contaminants, enabling one to achieve a surface roughness of ∼0.2 nm. We show that wet etching or other ex situ cleaning methods alone without annealing cannot eliminate all surface contaminants and may even worsen the surface roughness significantly.

BaTiO3 (BTO) and LaxSr1 − xTiO3 (x ≤ 0.15) perovskite heterostructures are deposited epitaxially on SrTiO3 (STO)-buffered Si(001) via atomic layer deposition (ALD) to explore the formation of a quantum metal layer between a ferroelectric film and silicon. X-ray diffraction and scanning transmission electron microscopy show the crystallinity of the heterostructure deposited by ALD. After postdeposition annealing of the La-doped STO film in ultrahigh vacuum at 600 °C for 5 min, x-ray photoelectron spectra show the lack of La-dopant activation when the film is deposited on 10 nm-thick BTO. The same postdeposition annealing condition activates the La-dopant when LaxSr1 − xTiO3 films are deposited on STO-buffered Si(001) surfaces consisting of 2.8 nm of STO(001) on Si(001). Annealing of LaxSr1 − xTiO3 films sandwiched between BTO and STO-buffered Si(001) layers in air at temperatures ≤350 °C preserves the La-dopant activation. Piezoresponse force microscopy demonstrates the ferroelectric behavior of BTO films grown on LaxSr1 − xTiO3 surfaces. Sheet resistance and capacitance-voltage measurements further demonstrate the conductivity of the LaxSr1 − xTiO3 films sandwiched between the BTO film and the Si(001) substrate.

The Si-compatibility of perovskite heterostructures offers the intriguing possibility of producing oxide-based quantum well (QW) optoelectronic devices for use in Si photonics. While the SrTiO3/LaAlO3 (STO/LAO) system has been studied extensively in the hopes of using the interfacial 2-dimensional electron gas in Si-integrated electronics, the potential to exploit its giant 2.4 eV conduction band offset in oxide-based QW optoelectronic devices has so far been largely ignored. Here, we demonstrate room-temperature intersubband absorption in STO/LAO QW heterostructures at energies on the order of hundreds of meV, including at energies approaching the critically important telecom wavelength of 1.55 μm. We demonstrate the ability to control the absorption energy by changing the width of the STO well layers by a single unit cell and present theory showing good agreement with experiment. A detailed structural and chemical analysis of the samples via scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) is presented. This work represents an important proof-of-concept for the use of transition metal oxide QWs in Si-compatible optoelectronic devices.

The development of novel nano-oxide spintronic devices would benefit greatly from interfacing with emergent phenomena at oxide interfaces. In this paper, we integrate highly spin-split ferromagnetic semiconductor EuO onto perovskite SrTiO3 (001). A careful deposition of Eu metal by molecular beam epitaxy results in EuO growth via oxygen out-diffusion from SrTiO3. This in turn leaves behind a highly conductive interfacial layer through generation of oxygen vacancies. Below the Curie temperature of 70 K of EuO, this spin-polarized two-dimensional t 2g electron gas at the EuO/SrTiO3 interface displays very large positive linear magnetoresistance (MR). Soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) reveals the t 2g nature of the carriers. First principles calculations strongly suggest that Zeeman splitting, caused by proximity magnetism and oxygen vacancies in SrTiO3, is responsible for the MR. This system offers an as-yet-unexplored route to pursue proximity-induced effects in the oxide two-dimensional t 2g electron gas.

Display Omitted Highly crystalline BaTiO3 was integrated on Si using molecular beam epitaxy.Electro-optic response was evaluated for a-axis, c-axis, and mixed films.Increased a-axis fraction correlated with increased electro-optic response.Post-deposition oxygen anneal improved crystallinity and reduced leakage current. High-quality epitaxial BaTiO3 (BTO) on Si has emerged as a highly promising material for future electro-optic (EO) devices based on BTO's large effective Pockels coefficient. We report on the EO response of BTO films deposited on Si by molecular beam epitaxy (MBE), and characterize the structure of these films by reflection high-energy electron diffraction and X-ray diffraction. O2 rapid thermal anneal at 600?C for 30min ensures full oxidation of BTO for minimal leakage current with minimal change in crystalline structure.

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).

NbO2 has the potential for a variety of electronic applications due to its electrically induced insulator-to-metal transition (IMT) characteristic. In this study, we find that the IMT behavior of NbO2 follows the field-induced nucleation by investigating the delay time dependency at various voltages and temperatures. Based on the investigation, we reveal that the origin of leakage current in NbOx is partly due to insufficient Schottky barrier height originating from interface defects between the electrodes and NbOx layer. The leakage current problem can be addressed by inserting thin NiOy barrier layers. The NiOy inserted NbOx device is drift-free and exhibits high Ion/Ioff ratio (>5400), fast switching speed (453 K) characteristics which are highly suitable to selector application for x-point memory arrays. We show that NbOx device with NiOx interlayers in series with resistive random access memory (ReRAM) device demonstrates improved readout margin (>29 word lines) suitable for x-point memory array application.

Here we report the recent advances towards the ferroelectric field-effect on Ge(001). We will demonstrate carrier density modulation in the underlying Ge(001) substrate by switching the ferroelectric polarization in the epitaxial c-axis-oriented BaTiO 3 on Ge. Recent results of patterning BaTiO 3 for device applications and electrical properties of Pt/BTO/Ge heterostructures will be addressed.

The electronic structure of Eu sesquioxide (Eu2O3) presents a significant challenge to the electronic structure theory due to the presence of correlated Eu semicore 4f electrons. The bandgap values do not agree between computational methods, and even experimentally, there are discrepancies between reports. Eu2O3 was grown epitaxially in a thin film form on n-type GaN (0001) by molecular beam epitaxy. The film was analyzed using UV and x-ray photoemission spectroscopies as well as inverse photoelectron spectroscopy in order to characterize both occupied and unoccupied states. Signatures of Eu2+ are detected after annealing in UHV or after exposure to air, which can be removed by subsequent O2 annealing. The sample reduction is shown to strongly affect the electronic structure. The bandgap of 4.3 eV, electron affinity of 2.2 eV, and band alignment to the substrate with a valence band offset of 0.2 eV for a stoichiometric Eu2O3 film were extracted from the measurements of the occupied and unoccupied electronic states. The electronic structure is interpreted in view of recent theoretical models, and the energy band alignment across the Eu2O3/GaN interface is discussed.

Dry oxidation of Si (001) beneath a thin epitaxial SrTiO3 layer has been studied using furnace annealing in flowing oxygen. A 10-nm layer of SrTiO3 is epitaxially grown on Si with no SiO2 interlayer. For such a structure, an annealing temperature of 800 °C was found to be the limiting temperature to prevent silicate formation and disruption of the interface structure. The effect of annealing time on the thickness of the SiO2 layer was investigated. In situ x-ray photoelectron spectroscopy and reflection-high-energy electron diffraction were used to ensure that the quality of SrTiO3 is unchanged after the annealing process. The experimental annealing data are compared with a theoretical oxygen diffusion model based on that of Deal, Grove, and Massoud. The model fits the experimental data well, indicating that oxygen diffusion through the SrTiO3 layer is not the limiting factor. One can therefore readily control the thickness of the SiO2 interlayer by simply controlling the annealing time in flowing oxygen.

La2O3 has been reported as a good gate dielectric for GaN-based high electron mobility transistor and metal-oxide-semiconductor field effect transistor applications. In this study, atomic layer deposition (ALD) was selected as the deposition technique, and the growth behavior and structure of La2O3 films on GaN(0001) were investigated. Unlike La2O3 deposited by molecular beam epitaxy (MBE) at 450 °C that can form a two-dimensional film directly on GaN(0001), island growth mode was observed when a La2O3 film was deposited directly on GaN(0001) at 250 °C by ALD using tris(N,N′-diisopropylformamidinato)-lanthanum as the precursor and H2O as the coreactant. Only with the use of a template layer, a 2 nm-thick hexagonal La2O3 film deposited by MBE or a 3 nm-thick cubic Er2O3 film deposited by ALD, can a two-dimensional crystalline ALD-La2O3 thin film be formed on GaN(0001). Reflection high-energy electron diffraction, x-ray diffraction, and scanning transmission electron microscopy were conducted to verify the films’ crystalline nature and structure. The ALD-La2O3 film retains a cubic structure on ALD-Er2O3 templated-GaN(0001), while it transforms from the cubic phase to a mix of cubic and hexagonal phases on MBE-La2O3 templated-GaN(0001) when the film is thicker than 15 nm. The stabilization of a thin cubic La2O3 phase could be related to a difference in the surface energy and the ALD growth temperature.La2O3 has been reported as a good gate dielectric for GaN-based high electron mobility transistor and metal-oxide-semiconductor field effect transistor applications. In this study, atomic layer deposition (ALD) was selected as the deposition technique, and the growth behavior and structure of La2O3 films on GaN(0001) were investigated. Unlike La2O3 deposited by molecular beam epitaxy (MBE) at 450 °C that can form a two-dimensional film directly on GaN(0001), island growth mode was observed when a La2O3 film was deposited directly on GaN(0001) at 250 °C by ALD using tris(N,N′-diisopropylformamidinato)-lanthanum as the precursor and H2O as the coreactant. Only with the use of a template layer, a 2 nm-thick hexagonal La2O3 film deposited by MBE or a 3 nm-thick cubic Er2O3 film deposited by ALD, can a two-dimensional crystalline ALD-La2O3 thin film be formed on GaN(0001). Reflection high-energy electron diffraction, x-ray diffraction, and scanning transmission electron microscopy were conducted to verify the ...

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.

The authors report epitaxial BaSnO3(001) and SrSnO3(001) growth on SrTiO3(001) (STO) substrates via atomic layer deposition (ALD) at 180 °C, where the BaSnO3 growth rate is 0.46 ± 0.03 A and the SrSnO3 growth rate is 0.69 ± 0.04 A per ALD unit cycle. Reflection high-energy electron diffraction, x-ray diffraction (XRD), and rocking curve analyses verify the single crystalline nature of BaSnO3(001) and SrSnO3(001) thin films on STO(001), while the atomic force microscopy analyses reveal the smooth surface of the stannate films with rms roughness values of ∼0.4 nm, which is only slightly higher than the STO substrate roughness of 0.32 nm. Out-of-plane XRD and reciprocal space mapping show that both BaSnO3(001) and SrSnO3(001) (∼10 nm) are fully relaxed on STO(001), owing to the large lattice mismatches (5.1% for BaSnO3/STO and 3.2% for SrSnO3/STO). The visible light transmittance spectra indicate that ALD-grown BaSnO3 and SrSnO3 thin films have high transparency at 400–800 nm that matches the transparency of STO(001) substrates.The authors report epitaxial BaSnO3(001) and SrSnO3(001) growth on SrTiO3(001) (STO) substrates via atomic layer deposition (ALD) at 180 °C, where the BaSnO3 growth rate is 0.46 ± 0.03 A and the SrSnO3 growth rate is 0.69 ± 0.04 A per ALD unit cycle. Reflection high-energy electron diffraction, x-ray diffraction (XRD), and rocking curve analyses verify the single crystalline nature of BaSnO3(001) and SrSnO3(001) thin films on STO(001), while the atomic force microscopy analyses reveal the smooth surface of the stannate films with rms roughness values of ∼0.4 nm, which is only slightly higher than the STO substrate roughness of 0.32 nm. Out-of-plane XRD and reciprocal space mapping show that both BaSnO3(001) and SrSnO3(001) (∼10 nm) are fully relaxed on STO(001), owing to the large lattice mismatches (5.1% for BaSnO3/STO and 3.2% for SrSnO3/STO). The visible light transmittance spectra indicate that ALD-grown BaSnO3 and SrSnO3 thin films have high transparency at 400–800 nm that matches the transparency of...

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.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.

Epitaxial growth of SrTiO₃ on silicon by molecular beam epitaxy has opened up the route to the integration of functional complex oxides on a silicon platform. Chief among them is ferroelectric functionality using perovskite oxides such as BaTiO₃. However, it has remained a challenge to achieve ferroelectricity in epitaxial BaTiO₃ films with a polarization pointing perpendicular to the silicon substrate without a conducting bottom electrode. Here, we demonstrate ferroelectricity in such stacks. Synchrotron X-ray diffraction and high-resolution scanning transmission electron microscopy reveal the presence of crystalline domains with the long axis of the tetragonal structure oriented perpendicular to the substrate. Using piezoforce microscopy, polar domains can be written and read and are reversibly switched with a phase change of 180°. Open, saturated hysteresis loops are recorded. Thus, ferroelectric switching of 8- to 40-nm-thick BaTiO₃ films in metal-ferroelectric-semiconductor structures is realized, and field-effect devices using this epitaxial oxide stack can be envisaged.

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).

Using in situ X-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and density functional theory, we analyzed the surface core level shifts and surface structure during the initial growth of ABO3 perovskites on Ge (001) by atomic layer deposition, where A = Ba, Sr and B = Ti, Hf, Zr. We find that the initial dosing of the barium- or strontium-bis(triisopropylcyclopentadienyl) precursors on a clean Ge surface produces a surface phase that has the same chemical and structural properties as the 0.5-monolayer Ba Zintl layer formed when depositing Ba by molecular beam epitaxy. Similar binding energy shifts are found for Ba, Sr, and Ge when using either chemical or elemental metal sources. The observed germanium surface core level shifts are consistent with the flattening of the initially tilted Ge surface dimers using both molecular and atomic metal sources. Similar binding energy shifts and changes in dimer tilting with alkaline earth metal adsorption are found with density functional theory calculations. High angle angular dark field scanning transmission microscopy images of BaTiO3, SrZrO3, SrHfO3, and SrHf0.55Ti0.45O3 reveal the location of the Ba (or Sr) atomic columns between the Ge dimers. The results imply that the organic ligands dissociate from the precursor after precursor adsorption on the Ge surface, producing the same Zintl template critical for perovskite growth on Group IV semiconductors during molecular beam epitaxy.

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.

The growth of Co-substituted $\mathrm{BaTi}{\mathrm{O}}_{3}$ (BTO) films on Ge(001) substrates by molecular beam epitaxy is demonstrated. Energy-dispersive x-ray spectroscopy and transmission electron microscopy images confirm the uniform Co distribution. However, no evidence of magnetic ordering is observed in samples grown for Co concentrations between 2% and 40%. Piezoresponse force microscopy measurements show that a 5% Co-substituted BTO sample exhibits ferroelectric behavior. First-principles calculations indicate that while Co atoms couple ferromagnetically in the absence of oxygen vacancies, the occurrence of oxygen vacancies leads to locally antiferromagnetically coupled complexes with relatively strong spin coupling. The presence of a significant amount of oxygen vacancies is suggested by x-ray photoelectron spectroscopy measurements.

We present theoretical and experimental studies of three oxide heterostructures: LaAlO3/EuO heterojunction, Fe/BaTiO3/Pt superlattice and epitaxial LaCoO3 on (100)-oriented silicon. We use density functional theory (DFT) to predict the physical properties of the heterostucture and molecular beam epitaxy (MBE) to realize it in practice. We predict spin polarized two dimensional electron gas at the LaAlO3/EuO interface, linear magnetoelectric effect in a Fe/BaTiO3/Pt superlattice, and predict and demonstrate experimentally strain stabilized ferromagnetism in LaCoO3 on silicon.

The EuO/SrTiO3 heterojunction is a promising combination of a ferromagnetic material and a two-dimensional electron system. We explore the deposition of Eu metal on SrTiO3/Si pseudo-substrates, with varying SrTiO3 (STO) thickness, under ultrahigh vacuum conditions. By varying the thickness of the STO layer (2-10 nm) and the deposition temperature (20-300 °C), we investigate the process by which oxygen is scavenged from STO by Eu. In situ x-ray photoelectron spectroscopy is used to investigate the electronic structure of the nominal Eu/STO/Si stack. We find that as a result of Eu deposition, epitaxial EuO is formed on thick STO (6-10 nm), leaving behind a highly oxygen-deficient SrTiO3-δ layer of ∼4 nm in thickness. However, if the thickness of the STO layer is comparable to or less than the scavenging depth, the crystal structure of STO is disrupted and a solid state reaction between Eu, Si, and STO occurs when the deposition is done at a high temperature (300 °C). On the other hand, at a low temperature (20 °C), only a 1-2 nm-thick EuO interlayer is grown, on top of which the Eu metal appears to be stable. This study elucidates the growth process under different conditions and provides a better understanding and control of this system.

In this work, we report the formation of a conductive layer through oxygen vacancies in an underlying SrTiO3 (STO) layer due to the growth of LaTiO3 (LTO) and the resulting LTO thickness-dependent conductivity of the LTO/STO system. Crystalline LTO films were grown by molecular beam epitaxy on TiO2-terminated STO(001) single-crystal substrates and 8-unit-cell (u.c.) STO template layers grown on Ge(001), under partial pressures of molecular oxygen ranging from 10−10 to 10−7 Torr. Film crystallinity was studied by in situ reflection high-energy electron diffraction, ex situ X-ray diffraction, and ex situ transmission electron microscopy. Film composition and the existence of oxygen vacancies were confirmed by in situ X-ray photoelectron spectroscopy. LTO films grown on STO substrates at oxygen partial pressures of 10−10 Torr were optimally oxidized (1:1:3 La:Ti:O). However, LTO films grown on 8-u.c. templates of STO on Ge with oxygen partial pressures less than 10−7 Torr showed extensive reduction of the Ti oxide and desorption of Sr/SrO in the STO layer. LTO films began to over-oxidize when grown on STO single-crystal substrates at oxygen partial pressures greater than 10−10 Torr but were nearly optimally oxidized when grown on STO templates on Ge at oxygen partial pressures of 10−7 Torr. Electrical characterization showed a dependence of conductivity on the thickness of the LTO films, with sheet carrier densities reaching ∼5 × 1016 cm−2 for 20-u.c. (8-nm-thick) LTO/STO grown at 10−10 Torr of oxygen, suggesting that significant conduction occurred throughout the STO substrate due to the formation of oxygen vacancies.In this work, we report the formation of a conductive layer through oxygen vacancies in an underlying SrTiO3 (STO) layer due to the growth of LaTiO3 (LTO) and the resulting LTO thickness-dependent conductivity of the LTO/STO system. Crystalline LTO films were grown by molecular beam epitaxy on TiO2-terminated STO(001) single-crystal substrates and 8-unit-cell (u.c.) STO template layers grown on Ge(001), under partial pressures of molecular oxygen ranging from 10−10 to 10−7 Torr. Film crystallinity was studied by in situ reflection high-energy electron diffraction, ex situ X-ray diffraction, and ex situ transmission electron microscopy. Film composition and the existence of oxygen vacancies were confirmed by in situ X-ray photoelectron spectroscopy. LTO films grown on STO substrates at oxygen partial pressures of 10−10 Torr were optimally oxidized (1:1:3 La:Ti:O). However, LTO films grown on 8-u.c. templates of STO on Ge with oxygen partial pressures less than 10−7 Torr showed extensive reduction of the Ti...

We study the nonlinear optical response in a strained thin film ferroelectric oxide BaTiO3 using piezoelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (001) as a variable strain substrate and La-doped SrTiO3 as a conductive buffer layer. The rotation-anisotropic second harmonic intensity profile shows hysteretic modulation corresponding to the strain variation from the inverse piezoelectric response of the substrate. An enhancement of 15% is observed at 1.2 kV/cm, while a control sample shows negligible change as a function of piezovoltage. Reflection high-energy electron diffraction, x-ray photoelectron spectroscopy, and high-resolution scanning transmission electron microscopy reveal the epitaxial interface. X-ray diffraction and piezoresponse force microscopy confirm tetragonal distortion and ferroelectricity of the BaTiO3 overlayer. Our results suggest a promising route to enhance the performance of nonlinear optical oxides for the development of future nano-opto-mechanical devices.

Heteroepitaxial growth of crystalline SrZrO3 (SZO) on Ge (001) by atomic layer deposition is reported. Ge (001) surfaces are pretreated with 0.5-monolayers (ML) of Ba and an amorphous ∼3-nm SZO layer is grown from strontium bis(triisopropylcyclopentadienyl), tetrakis (dimethylamido) zirconium, and water at 225 °C. This ∼3-nm layer crystallizes at 590 °C and subsequent SZO growth at 225 °C leads to crystalline films that do not require further annealing. The film properties are investigated using X-ray photoelectron spectroscopy, x-ray diffraction, aberration-corrected electron microscopy, and capacitance-voltage measurements of metal-oxide semiconductor capacitor structures. Capacitance-voltage measurements of the SrZrO3/Ge heterojunctions reveal a dielectric constant of 30 for SrZrO3 and a leakage current density of 2.1 × 10−8 A/cm2 at 1 MV/cm with an equivalent oxide thickness of 0.8 nm. Oxygen plasma pretreatment of Ge (001), Zintl layer formation with 0.5 ML Ba, and atomic deuterium post-growth treatment were explored to lower interface trap density (Dit) and achieved a Dit of 8.56 × 1011 cm−2 eV−1.

Titanates exhibit electronic properties highly desirable for field effect transistors such as very high permittivity and ferroelectricity. However, the difficulty of chemically etching titanates hinders their commercial use in device manufacturing. Here, the authors report the selective area in finestra growth of highly crystalline BaTiO3 (BTO) within photolithographically defined openings of a sacrificial SiO2 layer on a Ge (001) wafer by molecular beam epitaxy. After the BaTiO3 deposition, the sacrificial SiO2 can be etched away, revealing isolated nanoscale gate stacks circumventing the need to etch the titanate thin film. Reflection high-energy electron diffraction in conjunction with scanning electron microscopy is carried out to confirm the crystallinity of the samples. X-ray diffraction is performed to determine the out-of-plane lattice constant and crystal quality of the BTO film. Electrical measurements are performed on electrically isolated Pt/BaTiO3/SrTiO3/Ge capacitor devices.Titanates exhibit electronic properties highly desirable for field effect transistors such as very high permittivity and ferroelectricity. However, the difficulty of chemically etching titanates hinders their commercial use in device manufacturing. Here, the authors report the selective area in finestra growth of highly crystalline BaTiO3 (BTO) within photolithographically defined openings of a sacrificial SiO2 layer on a Ge (001) wafer by molecular beam epitaxy. After the BaTiO3 deposition, the sacrificial SiO2 can be etched away, revealing isolated nanoscale gate stacks circumventing the need to etch the titanate thin film. Reflection high-energy electron diffraction in conjunction with scanning electron microscopy is carried out to confirm the crystallinity of the samples. X-ray diffraction is performed to determine the out-of-plane lattice constant and crystal quality of the BTO film. Electrical measurements are performed on electrically isolated Pt/BaTiO3/SrTiO3/Ge capacitor devices.

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).

We investigate the x-ray photoelectron spectroscopy (XPS) of nominally ${d}^{1}$ and $n$-doped ${d}^{0}$ transition-metal oxides including ${\mathrm{NbO}}_{2},\phantom{\rule{0.16em}{0ex}}{\mathrm{SrVO}}_{3}$, and ${\mathrm{LaTiO}}_{3}$ (nominally ${d}^{1}$), as well as $n$-doped ${\mathrm{SrTiO}}_{3}$ (nominally ${d}^{0}$). In the case of single phase ${d}^{1}$ oxides, we find that the XPS spectra (specifically photoelectrons from Nb $3d$, V $2p$, Ti $2p$ core levels) all display at least two, and sometimes three distinct components, which can be consistently identified as ${d}^{0},\phantom{\rule{0.16em}{0ex}}{d}^{1}$, and ${d}^{2}$ oxidation states (with decreasing order in binding energy). Electron doping increases the ${d}^{2}$ component but decreases the ${d}^{0}$ component, whereas hole doping reverses this trend; a single ${d}^{1}$ peak is never observed, and the ${d}^{0}$ peak is always present even in phase-pure samples. In the case of $n$-doped ${\mathrm{SrTiO}}_{3}$, the ${d}^{1}$ component appears as a weak shoulder with respect to the main ${d}^{0}$ peak. We argue that these multiple peaks should be understood as being due to the final-state effect and are intrinsic to the materials. Their presence does not necessarily imply the existence of spatially localized ions of different oxidation states nor of separate phases. A simple model is provided to illustrate this interpretation, and several experiments are discussed accordingly. The key parameter to determine the relative importance between the initial-state and final-state effects is also pointed out.

Growth of crystalline Er2O3, a rare earth sesquioxide, on GaN(0001) is described. Ex situ HCl and NH4OH solutions and an in situ N2 plasma are used to remove impurities on the GaN surface and result in a Ga/N stoichiometry of 1.02. Using atomic layer deposition with erbium tris(isopropylcyclopentadienyl) [Er(iPrCp)3] and water, crystalline cubic Er2O3 (C-Er2O3) is grown on GaN at 250 °C. The orientation relationships between the C-Er2O3 film and the GaN substrate are C-Er2O3(222) ǁ GaN(0001), C-Er2O3⟨-440⟩ ǁ GaN ⟨11‐20⟩, and C-Er2O3⟨-211⟩ ǁ GaN ⟨1‐100⟩. Scanning transmission electron microscopy and electron energy loss spectroscopy are used to examine the microstructure of C-Er2O3 and its interface with GaN. With post-deposition annealing at 600 °C, a thicker interfacial layer is observed, and two transition layers, crystalline GaNwOz and crystalline GaErxOy, are found between GaN and C-Er2O3. The tensile strain in the C-Er2O3 film is studied with x-ray diffraction by changes in both out-of-plane and in-p...

The authors investigate properties of ultrathin Pt as a function of coverage (up to 10 monolayers) on TiO2-terminated SrTiO3 (001) substrate at different temperatures (200–800 °C). In situ x-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy are used to observe the electronic structure and surface morphology evolution of Pt. The authors find that although Pt will not wet SrTiO3 in the thermodynamic sense, it forms a continuous film when deposited at 200 °C due to the low surface mobility. At 800 °C, even at very low coverage, Pt forms nanoclusters showing bulk-like metallic features in the photoemission spectra. The authors compare the observed electronic structure evolution of Pt and the different growth patterns at low and high temperatures with available theoretical calculations.

The high-pressure hexagonal phase of Eu2O3 has been grown epitaxially on C-plane GaN (0001) by molecular beam epitaxy. A structural phase transition from the hexagonal to the monoclinic phase is observed with increasing film thickness by ex-situ X-ray diffraction. The critical thickness for the structural transition is between 2 and 6 nm. The observed epitaxial relationships between the substrate and the film are GaN (0001) ǁ Eu2O3 (0001), GaN ⟨11 2¯0⟩ ǁ Eu2O3 ⟨11 2¯0⟩ for the hexagonal phase, and GaN (0001) ǁ Eu2O3 (20 1¯), GaN ⟨11 2¯0⟩ ǁ Eu2O3 [020] with six rotational domains for the monoclinic phase. The (0.8 ± 0.2) eV conduction band offset and bulk dielectric constant of ∼14 makes Eu2O3 a possible gate dielectric for a GaN-based field effect transistor.

In recent years, research on Ge nanodevices has experienced a renaissance, as Ge is being considered a possible high mobility channel material replacement for Si MOSFET devices. However, for reliable high performance devices, an atomically flat and perfectly clean Ge surface is of utmost importance. In this review, the existing methods for cleaning the Ge(001) surface are reviewed and compared for the first time. The review discusses three broad categories of cleaning techniques that have been successfully demonstrated to obtain a clean Ge surface. First, the use of ultraviolet light and/or oxygen plasma is discussed. Both techniques remove carbon contamination from the Ge surface and simultaneously form an oxide passivation layer. Second, in situ ion sputtering in combination with germanium regrowth, which can lead to extremely clean and well-ordered Ge surfaces, is discussed. Finally, various wet-etching recipes are summarized, with focus on hydrofluoric acid (HF), NH4OH, and HCl. Despite the success of...

SrTiO3 is a widely used substrate for the growth of other functional oxide thin films. The reactivity of the substrate with respect to the film during deposition, particularly with regard to redox reactions, has typically been glossed over. We demonstrate by depositing a variety of metals (Ti, Al, Nb, Pt, Eu, and Sr) and measuring the in situ core level spectra of both the metal and SrTiO3 that, depending on the oxide formation energy and work function of the metal, three distinct types of behavior occur in thin metal films on SrTiO3 (100). In many cases, there will be an interfacial layer of oxygen-deficient SrTiO3 formed at the interface with the overlying film. We discuss how this may affect the interpretation of the well-known two-dimensional electron gas present at the interface between SrTiO3 and various oxides.

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.