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Optimizing thermal anneals of Si-implanted β-Ga2O3 is critical for low resistance contacts and selective area doping. We report the impact of annealing ambient, temperature, and time on the activation of room temperature ion-implanted Si in β-Ga2O3 at concentrations from 5 × 1018 to 1 × 1020 cm−3, demonstrating full activation (>80% activation, mobilities >70 cm2/V s) with contact resistances below 0.29 Ω mm. Homoepitaxial β-Ga2O3 films, grown by plasma-assisted molecular beam epitaxy on Fe-doped (010) substrates, were implanted at multiple energies to yield 100 nm box profiles of 5 × 1018, 5 × 1019, and 1 × 1020 cm−3. Anneals were performed in an ultra-high vacuum-compatible quartz furnace at 1 bar with well-controlled gas compositions. To maintain β-Ga2O3 stability, pO2 must be greater than 10−9 bar. Anneals up to pO2 = 1 bar achieve full activation at 5 × 1018 cm−3, while 5 × 1019 cm−3 must be annealed with pO2 ≤ 10−4 bar, and 1 × 1020 cm−3 requires pO2 < 10−6 bar. Water vapor prevents activation and must be maintained below 10−8 bar. Activation is achieved for anneal temperatures as low as 850 °C with mobility increasing with anneal temperatures up to 1050 °C, though Si diffusion has been reported above 950 °C. At 950 °C, activation is maximized between 5 and 20 min with longer times resulting in decreased carrier activation (over-annealing). This over-annealing is significant for concentrations above 5 × 1019 cm−3 and occurs rapidly at 1 × 1020 cm−3. Rutherford backscattering spectrometry (channeling) suggests that damage recovery is seeded from remnant aligned β-Ga2O3 that remains after implantation; this conclusion is also supported by scanning transmission electron microscopy showing retention of the β-phase with inclusions that resemble the γ-phase. [ABSTRACT FROM AUTHOR]

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow α-(AlxGa1−x)2O3 films on (110) sapphire substrates over the 0 < x < 0.95 range of aluminum content. In S-MBE, 99.98% of the gallium-containing molecular beam arrives at the substrate in a preoxidized form as gallium suboxide (Ga2O). This bypasses the rate-limiting step of conventional MBE for the growth of gallium oxide (Ga2O3) from a gallium molecular beam and allows us to grow fully epitaxial α-(AlxGa1−x)2O3 films at growth rates exceeding 1 µm/h and relatively low substrate temperature (Tsub = 605 ± 15 °C). The ability to grow α-(AlxGa1−x)2O3 over the nominally full composition range is confirmed by Vegard's law applied to the x-ray diffraction data and by optical bandgap measurements with ultraviolet–visible spectroscopy. We show that S-MBE allows straightforward composition control and bandgap selection for α-(AlxGa1−x)2O3 films as the aluminum incorporation x in the film is linear with the relative flux ratio of aluminum to Ga2O. The films are characterized by atomic-force microscopy, x-ray diffraction, and scanning transmission electron microscopy (STEM). These α-(AlxGa1−x)2O3 films grown by S-MBE at record growth rates exhibit a rocking curve full width at half maximum of ≊ 12 arc secs, rms roughness <1 nm, and are fully commensurate for x ≥ 0.5 for 20–50 nm thick films. STEM imaging of the x = 0.78 sample reveals high structural quality and uniform composition. Despite the high structural quality of the films, our attempts at doping with silicon result in highly insulating films. [ABSTRACT FROM AUTHOR]

High-conductivity undoped GaN/AlN 2D hole gases (2DHGs), the p-type dual of the AlGaN/GaN 2D electron gases (2DEGs), have offered valuable insights into hole transport in GaN and enabled the first GaN GHz RF p-channel FETs. They are an important step toward high-speed and high-power complementary electronics with wide-bandgap semiconductors. These technologically and scientifically relevant 2D hole gases are perceived to be not as robust as the 2DEGs because structurally similar heterostructures exhibit wide variations of the hole density over Δ p s > --> 7 × 10 13 cm − 2 , and low mobilities. In this work, we uncover that the variations are tied to undesired dopant impurities such as silicon and oxygen floating up from the nucleation interface. By introducing impurity blocking layers (IBLs) in the AlN buffer layer, we eliminate the variability in 2D hole gas densities and transport properties, resulting in a much tighter control over the 2DHG density variations to Δ p s ≤ 1 × 10 13 cm − 2 across growths, and a 3 × boost in the Hall mobilities. These changes result in a 2–3 × increase in hole conductivity when compared to GaN/AlN structures without IBLs. [ABSTRACT FROM AUTHOR]

Magnetic skyrmions hold promise for spintronic devices. To explore the dynamical properties of skyrmions in devices, a nanoscale method to image spin textures in response to a stimulus is essential. Here, we apply a technique for operando electrical current pulsing of chiral magnetic devices in a Lorentz transmission electron microscope. In ferromagnetic multilayers with interfacial Dzyaloshinskii–Moriya interaction, we study the creation and annihilation of skyrmions localized by point-like pinning sites due to defects. Using a combination of experimental and micromagnetic techniques, we establish a thermal contribution for the creation and annihilation of skyrmions in our study. Our work reveals a mechanism for controlling skyrmion density, which enables an examination of skyrmion magnetic field stability as a function of density. We find that high-density skyrmion states are more stable than low-density states or isolated skyrmions resisting annihilation over a magnetic field range that increases monotonically with density. [ABSTRACT FROM AUTHOR]

Intermetallic Nb3Sn alloys have long been believed to form through Sn diffusion into Nb. However, our observations of significant oxygen content in Nb3Sn prompted an investigation of alternative formation mechanisms. Through experiments involving different oxide interfaces (clean HF-treated, native oxidized, and anodized), we demonstrate a thermodynamic route that fundamentally challenges the conventional Sn diffusion mechanism for Nb3Sn nucleation. Our results highlight the critical involvement of a SnOx intermediate phase. This new nucleation mechanism identifies the principles for growth optimization and new synthesis of high-quality Nb3Sn superconductors. [ABSTRACT FROM AUTHOR]

Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of incipient ferroelectrics. KTaO3 is near a polar instability and shows anisotropic superconductivity in electron-doped samples. Here, we demonstrate growth of high-quality KTaO3 thin films by molecular-beam epitaxy. Tantalum was provided by either a suboxide source emanating a TaO2 flux from Ta2O5 contained in a conventional effusion cell or an electron-beam-heated tantalum source. Excess potassium and a combination of ozone and oxygen (10% O3 + 90% O2) were simultaneously supplied with the TaO2 (or tantalum) molecular beams to grow the KTaO3 films. Laue fringes suggest that the films are smooth with an abrupt film/substrate interface. Cross-sectional scanning transmission electron microscopy does not show any extended defects and confirms that the films have an atomically abrupt interface with the substrate. Atomic force microscopy reveals atomic steps at the surface of the grown films. Reciprocal space mapping demonstrates that the films, when sufficiently thin, are coherently strained to the SrTiO3 (001) and GdScO3 (110) substrates. [ABSTRACT FROM AUTHOR]

Beta-phase gallium oxide (β -Ga2O3) is a promising semiconductor for high frequency, high temperature, and high voltage applications. In addition to the β -phase, numerous other polymorphs exist and understanding the competition between phases is critical to control practical devices. The phase formation sequence of Ga2O3, starting from amorphous thin films, was determined using lateral-gradient laser spike annealing at peak temperatures of 500–1400 °C on 400 μs to 10 ms timescales, with transformations characterized by optical microscopy, x-ray diffraction, and transmission electron microscopy (TEM). The resulting phase processing map showed the γ -phase, a defect-spinel structure, first nucleating under all annealing times for temperatures from 650 to 800 °C. The cross-sectional TEM at the onset of the γ -phase formation showed nucleation near the film center with no evidence of heterogeneous nucleation at the interfaces. For temperatures above 850 °C, the thermodynamically stable β -phase was observed. For anneals of 1–4 ms and temperatures below 1200 °C, small randomly oriented grains were observed. Large grains were observed for anneals below 1 ms and above 1200 °C, with anneals above 4 ms and 1200 °C resulting in textured films. The formation of the γ -phase prior to β -phase, coupled with the observed grain structure, suggests that the γ -phase is kinetically preferred during thermal annealing of amorphous films, with β -phase subsequently forming by nucleation at higher temperatures. The low surface energy of the γ -phase implied by these results suggests an explanation for the widely observed γ -phase inclusions in β -phase Ga2O3 films grown by a variety of synthesis methods. [ABSTRACT FROM AUTHOR]

Efficient manipulation of antiferromagnetically coupled materials that are integration-friendly and have strong perpendicular magnetic anisotropy (PMA) is of great interest for low-power, fast, dense magnetic storage and computing. Here, we report a distinct, giant bulk damping-like spin–orbit torque in strong-PMA ferrimagnetic Fe100−xTbx single layers that are integration-friendly (composition-uniform, amorphous, and sputter-deposited). For sufficiently thick layers, this bulk torque is constant in the efficiency per unit layer thickness, ξ DL j /t, with a record-high value of 0.036 ± 0.008 nm−1, and the damping-like torque efficiency ξ DL j achieves very large values for thick layers, up to 300% for 90 nm layers. This giant bulk torque by itself switches tens of nm thick Fe100−xTbx layers that have very strong PMA and high coercivity at current densities as low as a few MA/cm2. Surprisingly, for a given layer thickness, ξ DL j shows strong composition dependence and becomes negative for composition where the total angular momentum is oriented parallel to the magnetization rather than antiparallel. Our findings of giant bulk spin torque efficiency and intriguing torque-compensation correlation will stimulate study of such unique spin–orbit phenomena in a variety of ferrimagnetic hosts. This work paves a promising avenue for developing ultralow-power, fast, dense ferrimagnetic storage and computing devices. [ABSTRACT FROM AUTHOR]

We report the structural and electronic properties of NbN/GaN junctions grown by plasma-assisted molecular beam epitaxy. High crystal-quality NbN films grown on GaN exhibit superconducting critical temperatures in excess of 10 K for thicknesses as low as 3 nm. We observe that the NbN lattice adopts the stacking sequence of the underlying GaN and that domain boundaries in the NbN thereby occur at the site of atomic steps in the GaN surface. The electronic properties of the NbN/GaN junction are characterized using Schottky barrier diodes. Current–voltage–temperature and capacitance–voltage measurements are used to determine the Schottky barrier height of the NbN/GaN junction, which we conclude is ∼1.3 eV. [ABSTRACT FROM AUTHOR]

Conductive homoepitaxial Si-doped β-Ga2O3 films were fabricated by pulsed laser deposition with an as-deposited 2323 S cm−1 conductivity (resistivity = 4.3 × 10−4 Ω-cm, carrier concentration = 2.24 × 1020 cm−3, mobility = 64.5 cm2 V−1 s−1, and electrical activation efficiency = 77%). High quality homoepitaxial films deposited on commercial (010) Fe-compensated β-Ga2O substrates were determined by high-resolution transmission electron microscopy and x-ray diffraction. The β-Ga2O3 films have ∼70% transparency from 3.7 eV (335 nm) to 0.56 eV (2214 nm). The combination of high conductivity and transparency offers promise for numerous ultrawide bandgap electronics and optoelectronic applications. [ABSTRACT FROM AUTHOR]

Epitaxial multilayer heterostructures of ScxAl1−xN/GaN with Sc contents x = 0.11–0.45 are found to exhibit significant differences in structural quality, chemical impurity levels, and electronic properties depending on the starting Sc source impurity levels. A higher purity source leads to a 2–3 orders of magnitude reduction in the carbon, oxygen, and fluorine unintentional doping densities in MBE-grown ScxAl1−xN/GaN multilayers. Electrical measurements of ScxAl1−xN/n+GaN single heterostructure barriers show a 5–7 orders of magnitude reduction in the electrical leakage for films grown with a higher purity Sc source at most Sc contents. The measured chemical and electrical properties of epitaxial ScxAl1−xN highlight the importance of the starting Sc source material purity for epitaxial device applications that need these highly piezoelectric and/or ferroelectric transition-metal nitride alloys. [ABSTRACT FROM AUTHOR]

β-Ga2O3 is a promising ultra-wide bandgap semiconductor whose properties can be further enhanced by alloying with Al. Here, using atomic-resolution scanning transmission electron microscopy, we find the thermodynamically unstable γ-phase is a ubiquitous structural defect in both β-(AlxGa1−x)2O3 films and doped β-Ga2O3 films grown by molecular beam epitaxy. For undoped β-(AlxGa1−x)2O3 films, we observe γ-phase inclusions between nucleating islands of the β-phase at lower growth temperatures (∼500–600 °C). In doped β-Ga2O3, a thin layer of the γ-phase is observed on the surfaces of films grown with a wide range of n-type dopants and dopant concentrations. The thickness of the γ-phase layer was most strongly correlated with the growth temperature, peaking at about 600 °C. Ga interstitials are observed in the β-phase, especially near the interface with the γ-phase. By imaging the same region of the surface of a Sn-doped β-(AlxGa1−x)2O3 after ex situ heating up to 400 °C, a γ-phase region is observed to grow above the initial surface, accompanied by a decrease in Ga interstitials in the β-phase. This suggests that the diffusion of Ga interstitials toward the surface is likely the mechanism for growth of the surface γ-phase and more generally that the more-open γ-phase may offer diffusion pathways to be a kinetically favored and early forming phase in the growth of Ga2O3. However, more modeling and simulation of the γ-phase and the interstitials are needed to understand the energetics and kinetics, the impact on electronic properties, and how to control them. [ABSTRACT FROM AUTHOR]

Using epitaxial stabilization, we synthesized single-phase (001)-oriented thin films of DyFe2O4+x on (111) MgAl2O4 substrates by molecular-beam epitaxy. The metastable DyFe2O4 polymorph formed is isostructural to known trigonal ferrimagnetic RFe2O4 phases with space group R 3 ̄ m , where R = Ho to Lu. The epitaxial DyFe2O4 thin films have two in-plane orientation relationships: [100] DyFe2O4 || 2 11 ̄ MgAl2O4 plus a twin variant related by a 60° in-plane rotation. DyFe2O4 is not bulk stable and has never been synthesized before. Indeed, it has been predicted to be on the edge energetically of what may be possible to stabilize. The fact that the RFe2O4 phase is stable for all elements leading up to dysprosium (Ho–Lu) leads us to believe that DyFe2O4 could be a "remnant metastable phase," one which, given the right thermodynamic conditions, could become the lowest free energy phase. We find that although we are able to get structurally very close to R 3 ̄ m DyFe2O4, the films are not stoichiometric as they have an increased c lattice parameter, indicative of extra oxygen as is sometimes seen in other RFe2O4 phases. The unintended surplus oxygen opens questions regarding what may be achievable using such tricks as epitaxial stabilization to access metastable phases and whether this indeed constitutes "remnant metastability." [ABSTRACT FROM AUTHOR]

This paper introduces a growth method—suboxide molecular-beam epitaxy (S-MBE)—which enables a drastic enhancement in the growth rates of Ga2O3 and related materials to over 1 μ m h−1 in an adsorption-controlled regime, combined with excellent crystallinity. Using a Ga + Ga2O3 mixture with an oxygen mole fraction of x(O) = 0.4 as an MBE source, we overcome kinetic limits that had previously hampered the adsorption-controlled growth of Ga2O3 by MBE. We present growth rates up to 1.6 μ m h−1 and 1.5 μ m h−1 for Ga2O3/Al2O3 and Ga2O3/Ga2O3 structures, respectively, with very high crystalline quality at unparalleled low growth temperature for this level of perfection. We combine thermodynamic knowledge of how to create molecular beams of targeted suboxides with a kinetic model developed for the S-MBE of III–VI compounds to identify appropriate growth conditions. Using S-MBE, we demonstrate the growth of phase-pure, smooth, and high-purity homoepitaxial Ga2O3 films that are thicker than 4.5 μ m. With the high growth rate of S-MBE, we anticipate a significant improvement to vertical Ga2O3-based devices. We describe and demonstrate how this growth method can be applied to a wide range of oxides. With respect to growth rates and crystalline quality, S-MBE rivals leading synthesis methods currently used for the production of Ga2O3-based devices. [ABSTRACT FROM AUTHOR]

Homologous series are layered phases that can have a range of stoichiometries depending on an index n. Examples of perovskite-related homologous series include (ABO3)nAO Ruddlesden–Popper phases and (Bi2O2) (An−1BnO3n+1) Aurivillius phases. It is challenging to precisely control n because other members of the homologous series have similar stoichiometry and a phase with the desired n is degenerate in energy with syntactic intergrowths among similar n values; this challenge is amplified as n increases. To improve the ability to synthesize a targeted phase with precise control of the atomic layering, we apply the x-ray diffraction (XRD) approach developed for superlattices of III–V semiconductors to measure minute deviations from the ideal structure so that they can be quantitatively eradicated in subsequent films. We demonstrate the precision of this approach by improving the growth of known Ruddlesden–Popper phases and ultimately, by synthesizing an unprecedented n = 20 Ruddlesden–Popper phase, (ATiO3)20AO where the A-site occupancy is Ba0.6Sr0.4. We demonstrate the generality of this method by applying it to Aurivillius phases and the Bi2Sr2Can–1CunO2n+4 series of high-temperature superconducting phases. [ABSTRACT FROM AUTHOR]

ScxAl1−xN (x = 0.18–0.40) thin films of ∼28 nm thickness grown on metal polar GaN substrates by molecular beam epitaxy are found to exhibit smooth morphology with less than 0.5 nm roughness and predominantly single-phase wurtzite crystal structure throughout the composition range. Measurement of the piezoelectric d33 coefficient shows a 150% increase for lattice-matched Sc0.18Al0.82N relative to pure aluminum nitride, whereas higher Sc contents exhibit lower piezoelectric coefficients. The electromechanical response of the epitaxial films correlates with the crystal quality and the presence of zinc blende inclusions, as observed by high-resolution electron microscopy. It is further found that the polarity of the epitaxial ScxAl1−xN layers is locked to the underlying substrate. The measured electromechanical properties of epitaxial ScxAl1−xN, their relation to the atomic crystal structure and defects, and its crystal polarity provide useful guidance toward the applications of this material. [ABSTRACT FROM AUTHOR]

Interband Zener tunneling of electrons has been recently used in III-nitride semiconductor based light emitters to efficiently inject holes into p-cladding layers. Zener tunneling probabilities can be significantly enhanced if crystal symmetry-induced internal polarization fields assist the dopant-induced built-in electric fields of tunnel junctions because of the large reduction of the tunneling distance. In a metal-polar buried tunnel junction geometry, such electric field alignment needs an AlN interlayer at the tunnel junction. Because AlN is a larger bandgap semiconductor than GaN, it is not clear a priori if the net tunneling probability is reduced or enhanced compared to a homojunction. By combining theoretical modeling with experimental blue light emitting diodes, we find that the large tunneling enhancement due to the polarization field and band realignment overcome the reduction in tunneling due to the larger bandgap of AlN. Compared to a homojunction tunnel-junction, the inclusion of AlN in the tunnel junction is found to lower the turn-on and operating voltages and increase the wall-plug efficiency. This proves that polarization-induced AlN tunnel junctions are superior to homojunctions at low injection currents, resulting in higher optical emission intensity and superior uniformity. [ABSTRACT FROM AUTHOR]

We compare the effectiveness of in situ thermal cleaning with that of Al-assisted cleaning of native surface oxides of bulk AlN for homoepitaxial growth by molecular beam epitaxy. Thermal deoxidation performed at 1450 ° C in vacuum results in voids in the AlN substrate. On the other hand, Al-assisted deoxidation at ≈ 900 ° C results in high-quality AlN homoepitaxy, evidenced by clean and wide atomic terraces on the surface and no extended defects at the growth interface. This study shows that Al-assisted in situ deoxidation is effective in removing native oxides on AlN, providing a clean surface to enable homoepitaxial growth of AlN and its heterostructures; furthermore, it is more attractive over thermal deoxidation, which needs to be conducted at much higher temperatures due to the strong bonding strength of native oxides on AlN. [ABSTRACT FROM AUTHOR]

We report the in situ, direct epitaxial synthesis of (0001)-oriented PdCoO2 thin films on c-plane sapphire using ozone-assisted molecular-beam epitaxy. The resulting films have smoothness, structural perfection, and electrical characteristics that rival the best in situ grown PdCoO2 thin films in the literature. Metallic conductivity is observed in PdCoO2 films as thin as ∼2.0 nm. The PdCoO2 films contain 180° in-plane rotation twins. Scanning transmission electron microscopy reveals that the growth of PdCoO2 on the (0001) surface of Al2O3 begins with the CoO2 layer. [ABSTRACT FROM AUTHOR]

The contrast due to a strain field at an amorphous silicon/crystalline silicon (a-Si/c-Si) interface relative to the bulk crystal is studied with a scanning transmission electron microscope equipped with a low angle annular dark field (LAADF) detector and a high angle ADF (HAADF) detector. Experimental observations suggest that strain contrast depends closely on sample thickness and collection angle. For a thin sample (<100 Å) strain contrast is negative in both the LAADF and HAADF images. For a thick sample (>150 Å) strain contrast is positive in the LAADF image and negative in the HAADF image. Theoretical calculations of the effect of a random strain field are carried out. First, a simple model based on atomic scattering with an extra Debye–Waller factor is employed. It predicts a positive strain contrast in the LAADF image and no contrast in the HAADF image. The simple model fails for the HAADF contrast because it does not consider the propagation process of the electron beam inside the sample. Therefore multislice simulations including propagation details are carried out. The multislice simulation results agree well with the experimental observations. A measure of the strain amplitude is attained by comparing the experimental data with multislice simulation results. Quantitative measurements of strain contrast are only possible if the sample thickness is known. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

β-Ga2O3 has emerged as a promising candidate for electronic device applications because of its ultrawide bandgap, high breakdown electric field, and large-area affordable substrates grown from the melt. However, its thermal conductivity is at least one order of magnitude lower than that of other wide bandgap semiconductors such as SiC and GaN. Thermal dissipation in electronics made from β-Ga2O3 will be the bottleneck for real-world applications, especially for high power and high frequency devices. Similar to AlGaN/GaN interfaces, β-(AlxGa1−x)2O3/Ga2O3 heterogeneous structures have been used to form a high mobility two-dimensional electron gas where joule heating is localized. The thermal properties of β-(AlxGa1−x)2O3/Ga2O3 are the key for heat dissipation in these devices, while they have not been studied before. This work reports the temperature dependent thermal conductivity of β-(Al0.1Ga0.9)2O3/Ga2O3 superlattices from 80 K to 480 K. Its thermal conductivity is significantly reduced (5.7 times reduction) at room temperature compared to that of bulk Ga2O3. Additionally, the thermal conductivity of bulk Ga2O3 with (010) orientation is measured and found to be consistent with literature values regardless of Sn doping. We discuss the phonon scattering mechanism in these structures by calculating their inverse thermal diffusivity. By comparing the estimated thermal boundary conductance (TBC) of β-(Al0.1Ga0.9)2O3/Ga2O3 interfaces and Ga2O3 maximum TBC, we reveal that some phonons in the superlattices transmit through several interfaces before scattering with other phonons or structural imperfections. This study is not only important for Ga2O3 electronics applications, especially for high power and high frequency applications, but also for the fundamental thermal science of phonon transport across interfaces and in superlattices. [ABSTRACT FROM AUTHOR]

La-doped BaSnO3 has been epitaxially integrated with (001) Si using an SrTiO3 buffer layer via molecular-beam epitaxy (MBE). A 254 nm thick undoped BaSnO3 buffer layer was grown to enhance the mobility of the overlying La-doped BaSnO3 layer. The x-ray diffraction rocking curve of the BaSnO3 002 peak has a full width at half maximum of 0.02°. At room temperature, the resistivity of the La-doped BaSnO3 film is 3.6 × 10−4 Ω cm and the mobility is 128 cm2 V−1 s−1 at a carrier concentration of 1.4 × 1020 cm−3. These values compare favorably to those of La-doped BaSnO3 films grown by all techniques other than MBE on single-crystal oxide substrates. Our work opens an exciting arena for integrating hyper-functional oxide electronics that make use of high-mobility oxide films with the workhorse of the semiconductor industry, silicon. [ABSTRACT FROM AUTHOR]

We report the integration of SrRuO3, one of the most widely used oxide electrode materials in functional oxide heterostructures, with silicon using molecular-beam epitaxy and an SrTiO3 buffer layer. The resulting SrRuO3 film has a rocking curve full width at half maximum of 0.01°, a resistivity at room temperature of 250 μΩ cm, a residual resistivity ratio (ρ300 K ρ4 K) of 11, and a paramagnetic-to-ferromagnetic transition temperature of ∼160 K. These structural, electrical, and magnetic properties compare favorably to the best reported values for SrRuO3 films on silicon and rival those of epitaxial SrRuO3 films produced directly on SrTiO3 single crystals by thin film growth techniques other than molecular-beam epitaxy. These high quality SrRuO3 films with metallic conductivity on silicon are relevant to integrating multi-functional oxides with the workhorse of semiconductor technology, silicon. [ABSTRACT FROM AUTHOR]

Epitaxial La-doped BaSnO3 films were grown in an adsorption-controlled regime by molecular-beam epitaxy, where the excess volatile SnOx desorbs from the film surface. A film grown on a (001) DyScO3 substrate exhibited a mobility of 183 cm² V-1 s-1 at room temperature and 400 cm² V-1 s-1 at 10 K despite the high concentration (1.2 × 1011 cm-2) of threading dislocations present. In comparison to other reports, we observe a much lower concentration of (BaO)2 Ruddlesden-Popper crystallographic shear faults. This suggests that in addition to threading dislocations, other defects-- possibly (BaO)2 crystallographic shear defects or point defects--significantly reduce the electron mobility. [ABSTRACT FROM AUTHOR]

Selective capture of disease-related proteins in complex biological fluids and tissues is an important aim in developing sensitive protein biosensors for in vivo applications. Microprojection arrays are biomedical devices whose mechanical and chemical properties can be tuned to allow efficient penetration of skin, coupled with highly selective biomarker capture from the complex biological environment of skin tissue. Herein, the authors describe an improved surface modification strategy to produce amine-modified polycarbonate arrays, followed by the attachment of an antifouling poly(sulfobetaine-methacrylate) (pSBMA) polymer or a linear polyethylene glycol (PEG) polymer of comparative molecular weight and hydrodynamic radius. Using a "grafting to" approach, pSBMA and linear PEG coatings yielded comparative antifouling behavior in single protein solutions, diluted plasma, or when applied to mouse flank skin penetrating into the vascularized dermal tissue. Interestingly, the density of immobilized immunoglobulin G (IgG) or bovine serum albumin protein on pSBMA surfaces was significantly higher than that on the PEG surfaces, while the nonspecific adsorption was comparable for each protein. When incubated in buffer or plasma solutions containing dengue non-structural protein 1 (NS1), anti-NS1-IgG-coated pSBMA surfaces captured significantly more NS1 in comparison to PEG-coated devices. Similarly, when wearable microprojection arrays were applied to the skin of dengue-infected mice using the same coatings, the pSBMA-coated devices showed significantly higher capture efficiency (>2-fold increase in signal) than the PEG-coated substrates, which showed comparative signal when applied to naïve mice. In conclusion, zwitterionic pSBMA polymers (of equivalent hydrodynamic radii to PEG) allowed detection of dengue NS1 disease biomarker in a preclinical model of dengue infection, showing significantly higher signal-to-noise ratio in comparison to the PEG controls. The results of this study will be useful in the future development of a range of protein biosensors designed for use in vivo. [ABSTRACT FROM AUTHOR]

We report the growth of (001)-oriented VO2 films as thin as 1.5 nm with abrupt and reproducible metal-insulator transitions (MIT) without a capping layer. Limitations to the growth of thinner films with sharp MITs are discussed, including the Volmer-Weber type growth mode due to the high energy of the (001) VO2 surface. Another key limitation is interdiffusion with the (001) TiO2 substrate, which we quantify using low angle annular dark field scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy. We find that controlling island coalescence on the (001) surface and minimization of cation interdiffusion by using a low growth temperature followed by a brief anneal at higher temperature are crucial for realizing ultrathin VO2 films with abrupt MIT behavior. [ABSTRACT FROM AUTHOR]

We demonstrate the ability to tune the thermal conductivity of homoepitaxial SrTiO3 films deposited by reactive molecular-beam epitaxy by varying growth temperature, oxidation environment, and cation stoichiometry. Both point defects and planar defects decrease the longitudinal thermal conductivity (k33), with the greatest decrease in films of the same composition observed for films containing planar defects oriented perpendicular to the direction of heat flow. The longitudinal thermal conductivity can be modified by as much as 80%--from 11.5W m-1K-1 for stoichiometric homoepitaxial SrTiO3 to 2W m-1K-1 for strontium-rich homoepitaxial Sr1þdTiOx films--by incorporating (SrO)2 Ruddlesden-Popper planar defects. [ABSTRACT FROM AUTHOR]

We report a strong enhancement of the efficacy of the spin Hall effect (SHE) of Pt for exerting anti-damping spin torque on an adjacent ferromagnetic layer by the insertion of ≈0.5 nm layer of Hf between a Pt film and a thin, ≤2 nm, Fe60Co20B20 ferromagnetic layer. This enhancement is quantified by measurement of the switching current density when the ferromagnetic layer is the free electrode in a magnetic tunnel junction. The results are explained as the suppression of spin pumping through a substantial decrease in the effective spin-mixing conductance of the interface, but without a concomitant reduction of the ferromagnet's absorption of the SHE generated spin current. [ABSTRACT FROM AUTHOR]

Bi2Pt2O7 pyrochlore is thought to be one of the most promising oxide catalysts for application in fuel cell technology. Unfortunately, direct film growth of Bi2Pt2O7 has not yet been achieved, owing to the difficulty of oxidizing platinum metal in the precursor material to Pt4+. In this work, in order to induce oxidation of the platinum, we annealed pulsed laser deposited films consisting of epitaxial δ-Bi2O3 and co-deposited, comparatively disordered platinum. We present synchrotron x-ray diffraction results that show the nonuniform annealed films contain the first epitaxial crystals of Bi2Pt2O7. We also visualized the pyrochlore structure by scanning transmission electron microscopy, and observed ordered cation vacancies in the epitaxial crystals formed in a bismuth-rich film but not in those formed in a platinum-rich film. The similarity between the δ-Bi2O3 and Bi2Pt2O7 structures appears to facilitate the pyrochlore formation. These results provide the only route to date for the formation of epitaxial Bi2Pt2O7. [ABSTRACT FROM AUTHOR]

Crystallinity and texturing of RF sputtered c-axis aligned crystal InGaZnO4 (CAAC IGZO) thin films were quantified using X-ray diffraction techniques. Above 190°C, nanocrystalline films with an X-ray peak at 2h = 30° (009 planes) developed with increasing c-axis normal texturing up to 310°C. Under optimal conditions (310°C, 10% O2), films exhibited a c-axis texture full-width half-maximum of 20°. Cross-sectional high-resolution transmission electron microscopy confirmed these results, showing alignment variation of ±9° over a 15 x 15 nm field of view and indicating formation of much larger aligned domains than previously reported. At higher deposition temperatures, c-axis alignment was gradually lost as polycrystalline films developed. [ABSTRACT FROM AUTHOR]

Crystallinity and texturing of RF sputtered c-axis aligned crystal InGaZnO4 (CAAC IGZO) thin films were quantified using X-ray diffraction techniques. Above 190 °C, nanocrystalline films with an X-ray peak at 2θ = 30° (009 planes) developed with increasing c-axis normal texturing up to 310 °C. Under optimal conditions (310 °C, 10% O2), films exhibited a c-axis texture full-width half-maximum of 20°. Cross-sectional high-resolution transmission electron microscopy confirmed these results, showing alignment variation of ±9° over a 15 x 15 nm field of view and indicating formation of much larger aligned domains than previously reported. At higher deposition temperatures, c-axis alignment was gradually lost as polycrystalline films developed. [ABSTRACT FROM AUTHOR]

With nearly complete spin polarization, the ferromagnetic semiconductor europium monoxide could enable next-generation spintronic devices by providing efficient ohmic spin injection into silicon. Spin injection is greatly affected by the quality of the interface between the injector and silicon. Here, we use atomic-resolution scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy to directly image and chemically characterize a series of EuO|Si and EuO|YAlO3 interfaces fabricated using different growth conditions. We identify the presence of europium silicides and regions of disorder at the EuO|Si interfaces, imperfections that could significantly reduce spin injection efficiencies via spin-flip scattering. [ABSTRACT FROM AUTHOR]

The effect of dimensional confinement on the optical band gap of SrTiO3 is investigated by periodically introducing one extra SrO monolayer every n SrTiO3 layers. The result is the n = 1-5 and 10 members of the Srn+1TinO3n+1 Ruddlesden-Popper homologous series. Spectroscopic ellipsometry, optical transmission, and cathodoluminescence measurements reveal these Srn+1TinO3n+1 phases to have indirect optical band gaps at room temperature with values that decrease monotonically with increasing n. First-principles calculations suggest that as n increases and the TiO6 octahedra become connected for increasing distances along the c-axis, the band edge electronic states become less confined. This is responsible for the decrease in band gaps with increasing n (for finite n) among Srn+1TinO3n+1 phases. [ABSTRACT FROM AUTHOR]

Erratum: "Adsorption-controlled growth of Ga2O3 by suboxide molecular-beam epitaxy", [APL. REFERENCE 1 P. Vogt, F. V. E. Hensling, K. Azizie, C. S. Chang, D. Turner, J. Park, J. P. McCandless, H. Paik, B. J. Bocklund, G. Hoffman, O. Bierwagen, D. Jena, H. G. Xing, S. Mou, D. A. Muller, S.-L. Liu, and D. G. Schlom, APL Mater. [Extracted from the article]

We have fabricated MgO-barrier magnetic tunnel junctions with a Co/Ni switching layer to reduce the demagnetizing field via interface anisotropy. With a fcc-(111) oriented Co/Ni multilayer combined with an FeCoB insertion layer, the demagnetizing field is 2 kOe and the tunnel magnetoresistance can be as high as 106%. Room-temperature measurements of spin-torque switching are in good agreement with predictions for a reduced critical current associated with the small demagnetization for antiparallel-to-parallel switching. For parallel-to-antiparallel switching the small demagnetization field causes spatially nonuniform reversal nucleated at the sample ends, with a low energy barrier but a higher switching current. [ABSTRACT FROM AUTHOR]

The three-dimensional reconstruction of a porous low-dielectric constant film (κ=2.5), resolving pores as small as 1 nm, was achieved using annular dark-field scanning transmission electron tomography, enabling quantitative measurements of the pore morphologies and size distribution. Most large pores were elliptical. Together with log-normal pore-size distribution, this suggests pore coalescence during the material’s growth. Ellipsometric porosimetry indicates a high degree of interconnectivity between pores. Tomography shows the material exhibits little large-scale pore connectivity, thus placing an upper limit on the size of the interconnections at below 1 nm. Systematic errors in the tomographic and ellipsometric size distributions appear to be largely complementary. [ABSTRACT FROM AUTHOR]

The use of boron-alloyed electrodes with the radio frequency (rf) sputter deposition of MgO yields magnetic tunnel junctions (MTJs) with Mg–B–O tunnel barriers. After annealing, such MTJs can exhibit very high tunneling magnetoresistance (TMR) in the thin (∼1.0 nm) barrier regime. Scanning tunneling spectroscopy of Mg–B–O layers reveals a better defined, but smaller band gap in comparison to that of thin MgO. We produced Fe60Co20B20/Mg–B–O/Ni65Fe15B20 MTJs where after a 350 °C annealing the Ni–Fe–B free electrode crystallizes into a highly textured (001)-normal body centered cubic (bcc) crystal structure and the MTJs achieve 155% TMR. [ABSTRACT FROM AUTHOR]

The ability to detect individual impurity atoms has been greatly enhanced by the development of aberration-corrected electron microscopes. The reduced depth of focus potentially enables three-dimensional reconstructions of impurity atoms from through-focal series. We test the robustness of this depth-sectioning method for detecting impurity atoms in gate oxides using multislice simulations. For amorphous materials, dopants can be reliably imaged, and are accurately described by a simpler three-dimensional linear imaging model. For crystalline materials, however, channeling artifacts can render the signal uninterpretable. These artifacts can be eliminated by orienting the crystal slightly off the zone axis, which still preserves atomic resolution. [ABSTRACT FROM AUTHOR]

Electron energy-loss spectroscopy at subnanometer resolution is used to investigate CoFeB/MgO/CoFeB magnetic tunnel junctions grown by electron-beam evaporation and radio frequency (rf) sputtering before and after annealing. Gap states were observed in the MgO layer for both growth methods although the rf-sputtered MgO layer showed significantly more gap states. Asymmetry in oxygen bonding between the top and bottom CoFeB/MgO interfaces was also observed. Moreover, significant amounts of diffused B as BOx were observed in the rf-sputtered MgO layer. A Mg underlayer between the MgO layer and the bottom electrode greatly reduced BOx formation in the barrier upon annealing. [ABSTRACT FROM AUTHOR]

Silicon nanoparticles embedded in silica show promising optoelectronic properties, due to quantum confinement and/or radiative interface states that should correlate with the particles’ average size and shape. Here the authors report the combination of electron tomography with plasmon-filtered microscopy in order to reconstruct the three-dimensional morphology of silicon nanoparticles. They find that particles with complex morphologies and high surface to volume ratios are dominant, rather than the commonly assumed near-spherical structures. These results should affect quantum-confined excitons and the interface density of states. Their findings may help to explain the physical origin of the unusually broad photoluminescence bands and efficiencies. [ABSTRACT FROM AUTHOR]

As integrated circuits have shrunk, conventional electron microscopies have proven inadequate for imaging complicated interconnect structures due to the overlap of features in projection. These techniques produce transmission functions with a nonmonotonic dependence of intensity on thickness for common microelectronic materials, making them unsuitable for tomography. We report the use of an incoherent bright field imaging technique in a scanning transmission electron microscope optimized for the three-dimensional reconstruction of thick copper microelectronic structures. Predictable behavior of the signal in samples up to ∼1 μm thick allows us to reconstruct and quantify the shape and volume of stress voids within Ta-lined interconnects. [ABSTRACT FROM AUTHOR]

High-performance metal-oxide-semiconductor field effect transistors (MOSFETs) on III–V semiconductors have long proven elusive. High-permittivity (high-κ) gate dielectrics may enable their fabrication. We have studied hafnium oxide and aluminum oxide grown on gallium arsenide by atomic layer deposition. As-deposited films are continuous and predominantly amorphous. A native oxide remains intact underneath HfO2 during growth, while thinning occurs during Al2O3 deposition. Hydrofluoric acid etching prior to growth minimizes the final interlayer thickness. Thermal treatments at ∼600 °C decompose arsenic oxides and remove interfacial oxygen. These observations explain the improved electrical quality and increased gate stack capacitance after thermal treatments. [ABSTRACT FROM AUTHOR]

We have examined the interfaces in Zr-silicate gate dielectrics grown on Si substrates using electron energy loss spectroscopy. The Zr-silicate interface is found to be stable with the Si substrate and the polycrystalline silicon (poly-Si) electrode under annealing to 1050 °C. At this interface, a 0.35 nm wide Zr-free interface region is observed in the as-deposited film, and does not change on annealing. The Zr-free region is too thin to take on the bulk SiO[sub 2] electronic structure, and thus is unlikely to compromise the dielectric properties of the device. For films with an Al electrode, a 2 nm reaction layer forms at the Zr-silicate interface. © 2001 American Institute of Physics. [DOI: 10.1063/1.1426268]. [ABSTRACT FROM AUTHOR]

In this letter, we report on electrical and microstructural properties of ultrathin zirconia dielectrics grown on SiO[sub 2] by ultraviolet (UV) ozone oxidation and natural oxidation (no UV light). Capacitance–voltage (C–V) measurements were performed at multiple frequencies on capacitors fabricated from a ZrO[sub 2]–SiO[sub 2] stack. It was found that the C–V curves from samples grown by natural oxidation were distorted and showed severe frequency dependence while samples grown with UV light exposure under otherwise identical conditions had superior electrical behavior. Loss tangent measurements and detailed electron energy loss spectroscopy studies performed on the two samples revealed that the sample grown by natural oxidation was highly oxygen deficient, and this led to its poor electrical properties. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this letter, we report on the growth of ultrathin films of zirconia on silicon oxide and silicon oxynitride passivated (001) Si by ultraviolet ozone oxidation of Zr metal precursor layers. The oxidation kinetics has been measured using an accelerator-based nuclear reaction analysis. It was found that oxide films up to 55 Å could be grown at room temperature by oxidation at 600 Torr while oxidation at 80 mTorr is self-limiting at 20 Å. The interfaces between the dielectric and the substrate have been characterized by scanning transmission electron microscopy. The ZrO[sub 2] films were found to be crystalline as grown. Electrical measurements on capacitor structures with 30-Å-thick ZrO[sub 2] films grown on native oxide on silicon show a capacitance–voltage hysteresis of 15 mV and a capacitance-based equivalent oxide thickness of 17 Å at 100 kHz. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

(001)-oriented LaAlO3 films were grown epitaxially on (001) Si substrates utilizing an ultrathin 5 unit-cell-thick SrTiO3 buffer layer. The SrTiO3 layer was grown at ∼250 °C and annealed in vacuum at 550 °C, following an epitaxy-by-periodic-annealing procedure. Upon this buffer layer, the LaAlO3 layer was then grown by codeposition at 580 °C. The rocking curve of the as-grown LaAlO3 film exhibits a full width at half maximum value as small as 0.02°. Atomic force microscopy shows that the surface of the LaAlO3 film has a root-mean-square roughness of 1.3 Å. Scanning transmission electron microscopy reveals that the LaAlO3/SrTiO3 interface and the SrTiO3/Si interfaces are sharp. This high crystalline quality, twin-free, epitaxial LaAlO3 on SrTiO3 on silicon could be relevant to integrating oxides with multiple functionalities on silicon. [ABSTRACT FROM AUTHOR]