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We present a study of Ge segregation at the surface of highly germanium-doped gallium oxide (2.5 × 1020 cm−3 nominal doping level) grown by molecular beam epitaxy. We probed the dopant concentration as a function of depth by hard x-ray photoelectron spectroscopy and standard laboratory photoemission spectroscopy. We notably found that there is germanium segregation within the top 2 nm where its concentration is 3 times the nominal doping level. This increased dopant concentration leads to a threefold enhancement of surface conductivity. The results suggest a reliable method for delta doping for power electronics applications. [ABSTRACT FROM AUTHOR]

We report the surface tension γ of Cu64Zr36 metallic glass liquid nanoparticles calculated using molecular dynamics simulation. We carried out a systematic investigation to obtain the dependence of γ on particle size and temperature. The values of γ are between 0.85 and 1.25 N/m, with a negative coefficient of temperature dependence that covers both the liquid and (amorphous) solid state. The particle size dependence is extracted through the Tolman length δ, which takes the values between 0.53 and 0.83 Å. The surface tension is influenced by temperature, sphere size, and surface chemical segregation. [ABSTRACT FROM AUTHOR]

We report on the low-temperature metalorganic vapor-phase epitaxy (MOVPE) growth of silicon delta-doped β-Ga2O3 films with a low full width at half maximum (FWHM). The as-grown films are characterized using secondary-ion mass spectroscopy, capacitance–voltage, and Hall techniques. Secondary ion mass spectroscopy measurements show that surface segregation is the chief cause of a large FWHM in MOVPE-grown films. The surface segregation coefficient (R) is observed to reduce with reduction in the growth temperature. Films grown at 600 °C show an electron concentration of 9.7 × 1012 cm−2 and a FWHM of 3.2 nm. High resolution scanning/transmission electron microscopy of the epitaxial film did not reveal any observable degradation in the crystal quality of the delta sheet and surrounding regions. Hall measurements of the delta-doped film on the Fe-doped substrate showed a sheet charge density of 6.1 × 1012 cm−2 and a carrier mobility of 83 cm2/V s. Realization of sharp delta doping profiles in MOVPE-grown β-Ga2O3 is promising for high performance device applications. [ABSTRACT FROM AUTHOR]

The ferromagnetic metal La2/3Sr1/3MnO3 (LSMO) is an important correlated oxide material for oxide spintronics. However, the existence of a dead layer not only degrades device performance but also hinders the ultimate miniaturization of devices. Intense research has been made to eliminate the dead layer and understand its origin. Here, we have investigated the dead layer of (110)-oriented LSMO films on SrTiO3 (STO) substrates in which there is no polar discontinuity across the interface, in contrast to the (001)-oriented LSMO/STO interface. Our results reveal an 8 unit cell (u.c.) dead layer for (110) LSMO. Angle resolved x-ray photoemission demonstrating Sr segregation at the surface. Additionally, the broken symmetry at LSMO/vacuum interfaces also degrades the transport property. By introducing a capping layer, the film conductivity is enhanced. Finally, an ultrathin limit dead layer of 5 u.c. (=1.36 nm) has been obtained by introducing both the LaMnO3 capping layer and the buffer layer to compensate the excess holes at both LSMO/STO and Vacuum/LSMO interfaces and also to reduce the structural distortion at the top surface of LSMO. Our work paves the way toward eliminating the LSMO dead layer for spintronics application and provides a route to engineer the intriguing physical properties of oxide heterointerfaces. [ABSTRACT FROM AUTHOR]

The surface diffusion kinetics has been measured on an Au60Cu15.5Ag7.5Si17 metallic glass using the method of surface grating decay from room temperature up to 20 K below the glass transition temperature (Tg). In the early stage of grating decay, the surface diffusion coefficients were estimated as varying in the range of 10−20 to 10−18 m2/s and the corresponding average activation energy is about 0.67 eV. During longer annealing, times the surface grating decay rate slowed down significantly. The slowing of the initial decay was a result of the surface dynamics and influenced by the surface segregation of silicon at a long time, which induced a surface crystallization of a silicon-rich phase. [ABSTRACT FROM AUTHOR]

Transition metals, such as Fe, are commonly used in either layers or substrates to serve as deep intentional acceptors to realize semi-insulating substrates, regrowth interfaces, or buffer layers. The unintentional incorporation of the compensating acceptor in subsequent layers is a major concern in epitaxial growth. In this paper, we report on unintentional Fe incorporation for the homoepitaxial growth of (010) β-Ga2O3 by plasma-assisted molecular beam epitaxy on (010) Fe-doped β-Ga2O3 substrates. Fe was found to incorporate heavily into films grown at 500 °C, while growth temperatures of 650 °C and higher showed a significantly longer tail of Fe in the films. This Fe tail was determined to be a result of surface riding during growth rather than diffusion. The total surface riding concentration of Fe was found to be approximately 3 × 1012 cm−2 from a typical Fe-doped (010) β-Ga2O3 substrate. Surface segregation coefficients of 0.982 and 0.993 were calculated for growth temperatures of 500 °C and 700 °C, respectively. Furthermore, growth temperatures of 500 °C–700 °C demonstrated high crystalline quality and smooth surface morphology. [ABSTRACT FROM AUTHOR]

Surfactants in heteroepitaxy are catalytic elements that float up to the surface during growth to control the energetics/kinetics of adatoms. “Autosurfactants” are exceptional in that the surfactant action is self-contained without foreign species. So far, autosurfactants as surface smootheners are known. Here, we demonstrate a different class of autosurfactants as surface-segregation quenchers: Bi, a dopant with a strong surface-segregation tendency in Si, is utilized to lock otherwise elusive Bi adatoms themselves to the Si lattice underneath during molecular beam epitaxy. Quasi-1D δ-doping of Bi in Si up to 4x1020 cm-3 in terms of volume concentration is achieved. [ABSTRACT FROM AUTHOR]

We demonstrate the formation of abrupt phosphorus (P) δ-doping profiles in germanium (Ge) by the insertion of ultra-thin silicon (Si) layers. The Si layers at the δ-doping region significantly suppress the surface segregation of P during the molecular beam epitaxial growth of Ge and high-concentration active P donors are confined within a few nm of the initial doping position. The current-voltage characteristics of the P d-doped layers with Si insertion show excellent Ohmic behaviors with low enough resistivity for ultra-shallow Ohmic contacts on n-type Ge. [ABSTRACT FROM AUTHOR]

The mechanism of Sn surface segregation during the epitaxial growth of GeSn on Si (001) substrates was investigated by Auger electron spectroscopy and energy dispersive X-ray spectroscopy. Sn surface segregation depends on the growth temperature and Sn content of GeSn layers. During Sn surface segregation, Sn-rich nanoparticles form and move on the surface during the deposition, which results in a rough surface owing to facet formation. The Sn-rich nanoparticles moving on the surface during the deposition absorb Sn from the periphery and yield a lower Sn content, not on the surface but within the layer, because the Sn surface segregation and the GeSn deposition occur simultaneously. Sn surface segregation can occur at a lower temperature during the deposition compared with that during postannealing. This suggests that the Sn surface segregation during the deposition is strongly promoted by the migration of deposited Ge and Sn adatoms on the surface originating from the thermal effect of substrate temperature, which also suggests that limiting the migration of deposited Ge and Sn adatoms can reduce the Sn surface segregation and improve the crystallinity of GeSn layers. [ABSTRACT FROM AUTHOR]

TiFe, a potential candidate for solid-state hydrogen storage, does not absorb hydrogen without a sophisticated activation process because of severe oxidation. This study shows that nanostructured TiFe becomes active by high-pressure torsion (HPT) and is not deactivated even after storage for several hundred days in the air. Surface segregation and formation of Fe-rich islands and cracks occur after HPT. The Fe-rich islands are suggested to act as catalysts for hydrogen dissociation and cracks and nanograin boundaries act as pathways to transport hydrogen through the oxide layer. Rapid atomic diffusion by HPT is responsible for enhanced surface segregation and hydrogen transportation. [ABSTRACT FROM AUTHOR]