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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is analogous to traditional silicon-on-insulator (SOI) wafers, where the crystalline device silicon layer of SOI has been replaced by a crystalline functional TMO layer.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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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 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.

In this chapter, we focus on the specific features and challenges that set oxide-semiconductor heteroepitaxial systems apart from the more commonly discussed cases of semiconductor-semiconductor and oxide-oxide epitaxy. Here we focus on the fundamental issues of the oxide/semiconductor epitaxy, using SrTiO3 on Si and GaAs as examples. The fundamental difficulty of perovskite/semiconductor epitaxy lies in thermodynamics. To achieve layer-by-layer growth, the film should wet the substrate. Wetting is controlled at the microscopic level by the interatomic forces. Even if the issues concerning lattice matching and wetting have somehow been resolved, the success of an epitaxial growth process is still dependent on an even more basic issue: thermodynamic stability of the film when in contact with the substrate at the growth conditions. A good atomic-scale understanding of the materials system is shown to be necessary, which highlights the importance of the use of microscopic theory in this field.

In this chapter, we review selected key developments in the integration of functional complex oxides on silicon by means of an MBE-grown epitaxial SrTiO3 buffer. Materials that are high-k dielectric, ferroelectric, ferromagnetic, photocatalytic, and multiferroic have been integrated on silicon. The properties exhibited by these materials are similar in quality to bulk, showing the high degree of crystallinity and robustness of the STO/Si pseudo-substrate even under somewhat high temperatures and oxygen pressures needed to grow some of these materials. We focus on molecular beam epitaxial (MBE) growth and very briefly mention some recent work involving atomic layer deposition (ALD). Detailed discussion of the integration of anatase TiO2, strained LaCoO3, Co-doped SrTiO3, BaTiO3, LaAlO3, and BiFeO3 are included.

This chapter is focused on the details of the epitaxial integration of SrTiO3 on Si, the only widely utilized direct epitaxy of a perovskite oxide on Si. The key enabling feature is the deposition of 1/2 monolayer of Sr on clean Si(001) 2 × 1 as a Zintl template for SrTiO3 growth. We discuss the reconstruction of Sr on Si (001) for the coverage range from 1/6 to 1 monolayer at both high and low temperatures, including the electronic structure changes occurring during the Sr Zintl template formation on Si(001) as observed by the Si 2p surface core-level shifts as a function of Sr coverage. We also discuss the details of the various processes by which one can achieve epitaxial SrTiO3 on Si as well as the possibility of residual strain control in SrTiO3 by interface oxidation. Finally, we provide a detailed discussion of the atomic and electronic structure of the SrTiO3/Si interface.

The success of integrating perovskites on Si(001) has also stimulated work on developing growth processes for other oxide materials, other crystallographic orientations, and even other semiconductor substrates. A review of the current work is summarized in this chapter, including efforts at growing the opposite stack of semiconductors on oxide surfaces. Work on the epitaxial integration of oxides on Si(111), Ge, GaAs, GaN, SiC is described. Oxides other than SrTiO3 that have been grown on Si(100) are also reviewed. It is hoped that this short history of the major advances in oxide/semiconductor epitaxy can provide insight into the various substrate preparation and film deposition tricks and techniques that enable such epitaxial systems to be made.

Various techniques of materials characterization are also crucial in order to “see” what is happening during and after the growth, as well as to be able to analyze the properties of the resulting structure. In this chapter, we provide a brief overview of the more commonly used thin film characterization methods based on incident electrons, photons, ions, and scanning probe techniques. These techniques can be used to determine the crystalline structure, and electronic and chemical characteristics of epitaxial oxide systems. In most cases, a combination of one or more of these techniques is performed in order to gain a more complete picture of the sample being measured. The chapter is primarily designed to introduce thin film growth practitioners to the most common characterization methods available and what types of information can be obtained from them.

Epitaxial thin film deposition methods are the principal means by which functional oxide-on-semiconductor heterostructures are achieved. By adapting the concepts once limited only to the semiconductor field to oxide materials systems, new experimental platforms for integrating the two types of materials into a single structure with cooperative functionality are now starting to become routine. In this chapter, we give a brief outline of five different thin film deposition methods that have been demonstrated to be capable of growing epitaxial oxide thin films on a semiconductor substrate. We describe physical vapor deposition methods such as molecular beam epitaxy, pulsed laser deposition, and sputtering, as well as chemical vapor deposition methods, including metal-organic chemical vapor deposition and atomic layer deposition.

Epitaxial heterostructures of γ−Al2O3/SrTiO3, grown by atomic layer deposition (ALD) and molecular beam epitaxy, have been characterized by advanced electron microscopy techniques, including aberration-corrected negative-Cs imaging, electron-energy-loss near-edge fine-structure analysis, and off-axis electron holography. Analysis of two-dimensional spectrum maps from samples that previously showed highly conductive interfacial layers revealed partial reduction of the Ti oxidation state in the SrTiO3 layer from Ti4+ to Ti3+, which was confined to within ∼1–2 unit cells of the interface. Electron holography of an ALD-grown sample revealed a phase profile within the SrTiO3 layer that rose sharply over a distance of about 1 nm moving away from the interface. Taken together, these results suggest a strong connection between reduction of oxidation state, which could be caused by oxygen vacancies and the quasi-two-dimensional electron gas present at the γ−Al2O3/SrTiO3 interface.

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 ...

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.

Half-monolayer Sr on Si(001) is a Zintl template necessary for epitaxial growth of SrTiO3 on Si(001). The authors investigate the reconstruction in the atomic and electronic structure of Si(001) induced by sub-monolayer Sr deposition using in-situ x-ray/ultraviolet photoemission spectroscopy and density functional theory. Sub-monolayer Sr is deposited on Si(001) using molecular beam epitaxy and the structural evolution of the surface is monitored using reflection high-energy electron diffraction. Experimentally, the authors find that the ionization energy of Si(001) decreases as a function of Sr coverage from 4.82 eV for pure Si(001) to 3.97 eV for half-monolayer Sr on Si(001) due to charge transfer from Sr to Si. They calculate the ionization energy for sub-monolayer Sr on Si(001) by considering several atomistic models and find good agreement with experiment. The authors clearly establish the Zintl character of the template by measuring the surface core level shifts of Si(001) and half-monolayer Sr/Si(001). They analyze and interpret the results of these measurements using first-principles final state theory.

The formation of the half monolayer (ML) Sr Zintl template layer on Si(001) is investigated in a combined experimental and theoretical work consisting of in situ reflection high energy electron diffraction, in situ x-ray photoelectron spectroscopy (XPS), and density functional theory. Starting with clean 2 × 1 reconstructed Si(001), we demonstrate that Sr deposition leads to a charge transfer from the metal to the Si substrate resulting in the disappearance of the asymmetry of Si dimers—an essential structural change that enables direct perovskite epitaxy on Si, and likely, other semiconductors. XPS reveals an unexpected shift to higher binding energy of the Si 2p core-level components, including the bulk. This unusual behavior is attributed to final state effects using first principles calculations. As measured by ultraviolet photoelectron spectroscopy, the deposition of 0.5 ML of Sr lowers the work function of the system by 1.35 eV, and is in good agreement with our theoretical calculations.

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.