Inverse metal-assisted chemical etching of germanium with gold and hydrogen peroxide.

Metal-assisted chemical etching (MACE) is a flexible technique for texturing the surface of semiconductors. In this work, we study the spatial variation of the etch profile, the effect of angular orientation relative to the crystallographic planes, and the effect of doping type. We employ gold in direct contact with germanium as the metal catalyst, and dilute hydrogen peroxide solution as the chemical etchant. With this catalyst-etchant combination, we observe inverse-MACE, where the area directly under gold is not etched, but the neighboring, exposed germanium experiences enhanced etching. This enhancement in etching decays exponentially with the lateral distance from the gold structure. An empirical formula for the gold-enhanced etching depth as a function of lateral distance from the edge of the gold film is extracted from the experimentally measured etch profiles. The lateral range of enhanced etching is approximately 10-20µm and is independent of etchant concentration. At length scales beyond a few microns, the etching enhancement is independent of the orientation with respect to the germanium crystallographic planes. The etch rate as a function of etchant concentration follows a power law with exponent smaller than 1. The observed etch rates and profiles are independent of whether the germanium substrate is n-type, p-type, or nearly intrinsic.

Weak anti-localization of two-dimensional holes in germanium beyond the diffusive regime.

Gate-controllable spin-orbit coupling is often one requisite for spintronic devices. For practical spin field-effect transistors, another essential requirement is ballistic spin transport, where the spin precession length is shorter than the mean free path such that the gate-controlled spin precession is not randomized by disorder. In this letter, we report the observation of a gate-induced crossover from weak localization to weak anti-localization in the magneto-resistance of a high-mobility two-dimensional hole gas in a strained germanium quantum well. From the magneto-resistance, we extract the phase-coherence time, spin-orbit precession time, spin-orbit energy splitting, and cubic Rashba coefficient over a wide density range. The mobility and the mean free path increase with increasing hole density, while the spin precession length decreases due to increasingly stronger spin-orbit coupling. As the density becomes larger than ∼6 × 1011 cm-2, the spin precession length becomes shorter than the mean free path, and the system enters the ballistic spin transport regime. We also report here the numerical methods and code developed for calculating the magneto-resistance in the ballistic regime, where the commonly used HLN and ILP models for analyzing weak localization and anti-localization are not valid. These results pave the way toward silicon-compatible spintronic devices.

Density-controlled quantum Hall ferromagnetic transition in a two-dimensional hole system.

Quantum Hall ferromagnetic transitions are typically achieved by increasing the Zeeman energy through in-situ sample rotation, while transitions in systems with pseudo-spin indices can be induced by gate control. We report here a gate-controlled quantum Hall ferromagnetic transition between two real spin states in a conventional two-dimensional system without any in-plane magnetic field. We show that the ratio of the Zeeman splitting to the cyclotron gap in a Ge two-dimensional hole system increases with decreasing density owing to inter-carrier interactions. Below a critical density of ~2.4 × 1010 cm-2, this ratio grows greater than 1, resulting in a ferromagnetic ground state at filling factor ν = 2. At the critical density, a resistance peak due to the formation of microscopic domains of opposite spin orientations is observed. Such gate-controlled spin-polarizations in the quantum Hall regime opens the door to realizing Majorana modes using two-dimensional systems in conventional, low-spin-orbit-coupling semiconductors.

Reflection high-energy electron diffraction measurements of reciprocal space structure of 2D materials.

Knowledge on the symmetry and perfection of a 2D material deposited or transferred to a surface is very important and valuable. We demonstrate a method to map the reciprocal space structure of 2D materials using reflection high energy diffraction (RHEED). RHEED from a 2D material gives rise to 'streaks' patterns. It is shown that from these streaks patterns at different azimuthal rotation angles that the reciprocal space intensity distribution can be constructed as a function of momentum transfer parallel to the surface. To illustrate the principle, we experimentally constructed the reciprocal space structure of a commercial graphene/SiO2/Si sample in which the graphene layer was transferred to the SiO2/Si substrate after it was deposited on a Cu foil by chemical vapor deposition. The result reveals a 12-fold symmetry of the graphene layer which is a result of two dominant orientation domains with 30° rotation relative to each other. We show that the graphene can serve as a template to grow other materials such as a SnS film that follows the symmetry of graphene.

Correlation between IL-6 and invasiveness of ectoderm cells of embryo in early pregnancy.

This study aimed to explore the correlation between Interleukin-6 (IL-6) and invasiveness of ectoderm cells of embryo in early pregnancy, in order to further discuss whether IL-6 can enhance invasiveness of ectoderm cells. The study lays the foundation for determination of pathogenesis of some gestation period-related diseases. Differences in mRNA and protein expression of trophoblastic cell line JEG-3 cells in IL-6, matrix metalloproteinase-2 (MMP-2) and MMP-9 were analyzed; the regulating effect of different concentrations of IL-6 on invasive ability of trophoblast cells was studied by Transwell assay; the effect of IL-6 on proliferation of ectodermal cell line JEG-3 of embryo was analyzed by methyl thiazolyl tetrazolium (MTT) assay. The invasive number of JEG-3 cells incubated by IL-6 (10 ng/ml) was higher than that of the control group, and the difference had statistical significance (p < 0.05). Results of using MMT assay to detect the effect of IL-6 on proliferation of trophoblastic cell line JEG-3 showed that JEG-3 cells before and after processing had no significant difference from the control group (p >0.05). Therefore, IL-6 can enhance invasiveness of ectoderm cells of embryo through activation of MMP-2.

High-mobility capacitively-induced two-dimensional electrons in a lateral superlattice potential.

In the presence of a lateral periodic potential modulation, two-dimensional electrons may exhibit interesting phenomena, such as a graphene-like energy-momentum dispersion, Bloch oscillations, or the Hofstadter butterfly band structure. To create a sufficiently strong potential modulation using conventional semiconductor heterostructures, aggressive device processing is often required, unfortunately resulting in strong disorder that masks the sought-after effects. Here, we report a novel fabrication process flow for imposing a strong lateral potential modulation onto a capacitively induced two-dimensional electron system, while preserving the host material quality. Using this process flow, the electron density in a patterned Si/SiGe heterostructure can be tuned over a wide range, from 4.4 × 10(10) cm(-2) to 1.8 × 10(11) cm(-2), with a peak mobility of 6.4 × 10(5) cm(2)/V·s. The wide density tunability and high electron mobility allow us to observe sequential emergence of commensurability oscillations as the density, the mobility, and in turn the mean free path, increase. Magnetic-field-periodic quantum oscillations associated with various closed orbits also emerge sequentially with increasing density. We show that, from the density dependence of the quantum oscillations, one can directly extract the steepness of the imposed superlattice potential. This result is then compared to a conventional lateral superlattice model potential.

Long-term results of coronary artery bypass grafting in patients with dialysis-dependent renal failure.

AIM: Coronary artery disease is the main cause of mortality and morbidity in dialysis-dependent renal failure patients. Both the prevalence and incidence of renal failure are high in Taiwan. However, there were few reports exploring the outcome of coronary aortic bypass grafting (CABG) in these patients. The aim of this study was to determine the survival outcome and risk factors for mortality from CABG in this population. METHODS: The operative, early postoperative and late results of 170 dialysis patients undergoing isolated coronary artery bypass grafting from January, 2000 to January, 2012 were retrospectively reviewed. Operative mortality, long-term survival, and risk factors were analyzed. RESULTS: One hundred and seventeen patients (68.8%) were male, and the mean age was 61.5±10.3 years (range, 34-86 years). Follow-up was 40.3±32.1 months. Operative mortality was 8.2%. Actuarial survival, including operative mortality, was 81±3% at 1 year, 68±4% at 3 years, 58±5% at 5 years and 49±6% at 10 years, better than the natural course of dialysis-dependent renal failure patients. Age, emergent operation, postoperative ventricular tachycardia or fibrillation, postoperative intra-aortic balloon pump insertion, gastrointestinal bleeding, and left internal mammary artery graft were significant predictors of operative or long term mortality. Most causes of late death were due to infection or cardiac events. CONCLUSION: CABG in dialysis patients is associated with a higher incidence of complications, but has acceptable mortality. CABG is beneficial in this population. Internal mammary artery grafting may provide more favorable long term outcomes.

Activation of transient receptor potential vanilloid 1 decreases endothelial nitric oxide synthase phosphorylation at Thr497 by protein phosphatase 2B-dependent dephosphorylation of protein kinase C.

AIMS: We investigated the effects and underlying molecular mechanism of transient receptor potential vanilloid 1 (TRPV1), a calcium (Ca(2+) )-permeable non-selective cation channel, on phosphorylation of endothelial nitric oxide synthase (eNOS) at threonine 497 (Thr497) in bovine aortic endothelial cells (BAECs) and in mice. METHODS: Western blotting and immunoprecipitation were used for the evaluation of protein phosphorylation; protein phosphatase 2B (PP2B) activity was assessed by convention kit; Griess assay was for NO production; tube formation and Matrigel plug assay were used for angiogenesis. RESULTS: In BAECs, treatment with the TRPV1 ligand evodiamine decreased the phosphorylation of eNOS at Thr497, protein kinase Cα (PKCα) at Serine 657 (Ser657) and PKCß2 at Ser660. Evodiamine increased protein phosphatase 2B (PP2B) activity and promoted the formation of a PP2B-PKC complex. Inhibition of TRPV1 activation by the pharmacological antagonists, removal of extracellular Ca(2+) or pharmacological inhibition of PI3K/Akt/calmodulin-dependent protein kinase II/AMP-activated protein kinase signalling pathway abolished the evodiamine-induced alterations in phosphorylation of eNOS at Thr497, PKCα at Ser657, PKCß2 at Ser660 and PP2B activity, as well as the formation of a PP2B-PKC complex. Inhibition of PP2B activation partially reduced the evodiamine-induced NO bioavailability and tube formation in endothelial cells (ECs) and angiogenesis in mice. Moreover, evodiamine decreased the phosphorylation of eNOS at Thr497, PKCα at Ser657 and PKCß2 at Ser660 in apolipoprotein E (ApoE)-deficient mouse aortas but not TRPV1-deficient or ApoE/TRPV1 double-knockout mice. CONCLUSION: TRPV1 activation in ECs may elicit a Ca(2+) -dependent effect on PP2B-PKC signalling, which leads to dephosphorylation of eNOS at Thr497 in ECs and in mice.

Biaxially textured Mo films with diverse morphologies by substrate-flipping rotation.

A class of nanostructured Mo thin films was grown by DC magnetron sputtering using a robust substrate rotation mode called 'flipping rotation'. In this rotation mode, the substrate is arranged to rotate continuously at a fixed speed around an axis lying within and parallel to the substrate. The incident flux is perpendicular to the rotational axis, and the incident flux angle changes continuously. Mo nanostructured films, grown under different rotation speeds with three orders of magnitude spread (ranging from 0.008 to 24 rotation min( - 1)), different flipping directions (clockwise and counter-clockwise), and different ending deposition angles, were characterized using scanning electron microscopy (SEM) and reflection high energy electron diffraction (RHEED) surface-pole-figure techniques. Despite their very different morphologies, such as 'C'-shaped, 'S'-shaped, and vertically aligned nanorods, the same [Formula: see text] biaxial texture with an average out-of-plane dispersion of ∼ 15° was observed. In contrast, we showed that only a fiber-textured Mo film was obtained by using the conventional rotation mode where the oblique incident flux angle was fixed with the substrate rotating around the surface normal.

Termination of two-dimensional metallic conduction near the metal-insulator transition in a Si/SiGe quantum well.

We report in this Letter our recent low-temperature transport results in a Si/SiGe quantum well with moderate peak mobility. An apparent metal-insulating transition is observed. Within a small range of densities near the transition, the conductivity σ displays a nonmonotonic temperature dependence. After an initial decrease at high temperatures, σ first increases with decreasing temperature T, showing a metallic behavior. As T continues decreasing, a downturn in σ is observed. This downturn shifts to a lower T at higher densities. More interestingly, the downturn temperature shows a power-law dependence on the mobility at the downturn position, suggesting that a similar downturn is also expected to occur deep in the apparent metallic regime at albeit experimentally inaccessible temperatures. This thus hints that the observed metallic phase in 2D systems might be a finite temperature effect.

Large artificial anisotropic growth rate in on-lattice simulation of obliquely deposited nanostructures.

On-lattice particle simulation is one of the most common types of Monte Carlo simulations used in studying the dynamics of film growth. We report the observation of a large artificial anisotropic growth rate variation owing to the fixed arrangement of particles in an on-lattice simulation of oblique angle deposition. This unexpectedly large anisotropy is not reported in previous literatures and substantially affects the simulation outcomes such as column angle and porosity, two of the most essential quantities in obliquely deposited nanostructures. The result of our finding is of interest to all on-lattice simulations in obliquely deposited films or nanostructures.

Morphology and texture evolution of nanostructured CaF2 films on amorphous substrates under oblique incidence flux.

The morphology and biaxial texture of vacuum evaporated CaF(2) films on amorphous substrates as a function of vapour incident angle, substrate temperature and film thickness were investigated by scanning electron microscopy, x-ray pole figure and reflection high energy electron diffraction surface pole figure analyses. Results show that an anomalous [220] out-of-plane texture was preferred in CaF(2) films deposited on Si substrates at < 200 °C with normal vapour incidence. With an increase of the vapour incident angle, the out-of-plane orientation changed from [220] to [111] at a substrate temperature of 100 °C. In films deposited with normal vapour incidence, the out-of-plane orientation changed from [220] at 100 °C to [111] at 400 °C. In films deposited with an oblique vapour incidence at 100 °C, the texture changed from random at small thickness (5 nm) to biaxial at larger thickness (20 nm or more). Using first principles density functional theory calculation, it was shown that [220] texture formation is a consequence of energetically favourable adsorption of CaF(2) molecules onto the CaF(2)(110) facet.

Texture evolution of vertically aligned biaxial tungsten nanorods using RHEED surface pole figure technique.

Vertically aligned biaxial tungsten nanorods with cubic A15 crystal structure were deposited by DC magnetron sputtering on native oxide covered Si(100) substrates with glancing angle flux incidence (theta approximately 85 degrees) and a two-step substrate rotation mode at room temperature. These vertical nanorods were grown to different thicknesses (10, 25, 50 and 100 nm) and analyzed for biaxial texture evolution using a highly surface sensitive reflection high-energy electron diffraction (RHEED) pole figure technique. The initial polycrystalline film begins to show the inception of biaxial texture with a fiber background between 10 and 25 nm. Biaxial texture development is eventually completed between 50 and 100 nm thicknesses of the film. The out-of-plane crystallographic direction is [002] and the in-plane texture is selected so as to obtain maximum capture area. In a comparison with 100 nm thick inclined tungsten nanorods deposited at 85 degrees without substrate rotation, it is found that the selection of in-plane texture does not maintain maximum in-plane capture area. This anomalous behavior is observed when the [002] texture axis is tilted approximately 17 degrees from the substrate normal in the direction towards the glancing incident flux.

The formation of vertically aligned biaxial tungsten nanorods using a novel shadowing growth technique.

Biaxially textured tungsten nanorods (A15 crystal structure) have been grown by oblique angle DC magnetron sputtering using a novel rotation mode called 'two-step rotation'. In this mode, the substrate is given a fast rotation through 180 degrees at 90 rpm and this is followed by a rest period of 30 s. These nanorods are vertically aligned and have a [100] texture normal to the substrate along with preferential in-plane texture as shown by x-ray pole figure analysis. In contrast, the tungsten nanorods obtained without substrate rotation are slanted at an angle of approximately 45 degrees and have a [100] texture tilted 16 degrees with respect to the substrate normal. The flux is incident from two diametrically opposite points on the sample at an oblique angle, averaging out the growth into vertical columns that retain the in-plane texture. Scanning electron microscopy shows that the tungsten nanorods have a mixture of {211} and {421} crystal habits; these planes are both minimum surface energy planes for a cubic A15 crystal structure.

Size control of Cu nanorods through oxygen-mediated growth and low temperature sintering.

Control of the size of Cu nanorods vapor-deposited at an oblique angle (approximately 85 degrees) by oxygen-mediated growth was investigated using scanning electron microscopy (SEM) and x-ray diffraction (XRD). It was observed that exposure of Cu nanorods to the oxygen ambient periodically resulted in a reduction in the diameter of the nanorods as well as an increase in the areal density of the nanorods. This oxygen-induced modification to the nanorod growth is attributed to the higher energy barrier for Cu adatom migration on the oxide surface at room temperature; this reduces the rod diameter. At a low annealing temperature of approximately 300 degrees C, the SEM images show that the nanorods have densified and formed a continuous film structure, which is consistent with the sintering phenomenon. The XRD and SEM analyses show that the coalescent/grain growth rate for Cu nanorods with smaller diameters is enhanced due to the size effect. This low temperature sintering characteristic of the Cu nanorod array has great potential for being utilized in wafer bonding for three-dimensional integration of devices.

Influence of nanotips on the hydrophilicity of metallic nanorod surfaces.

The hydrophilicity of vertically aligned metal nanorods with sharp nanotips were investigated experimentally. Ruthenium and platinum nanorod arrays were deposited on flat silicon substrates using oblique angle sputter deposition. We show that the effects of nanotips on nanorods should be considered in the "hemiwicking" model for hydrophilic metallic samples. With the influence of nanotips, we successfully explained the experimental contact angles of water sessile drops on metallic nanorod surfaces. Our experiments confirm that the shape of the nanorods is an important parameter in determining the hydrophilicity of the nanostructured surfaces.

The morphology and texture of Cu nanorod films grown by controlling the directional flux in physical vapor deposition.

We report the creation of unusual biaxial textures in Cu nanorod films, through the control of the incident vapor flux during oblique angle deposition. High-density twin boundaries were formed using a periodic azimuthal swing rotation of the substrate while the incident angle of the Cu flux was fixed at 85° with respect to the surface normal. In contrast, depositions on stationary substrates resulted in nanorod films with a much lower density of twinned crystals. From transmission electron microscopy and x-ray pole figure analysis, the nanorod axis was shown to coincide approximately with the [Formula: see text] crystallographic directions. We also observed the branching of these nanostructures into 'nanotrees'. This branching was attributed to the creation of edge dislocations during the deposition and was particularly prevalent for the case of swing rotation. The mechanisms for the development of texture, twinning, and branching in these nanostructures are discussed.

The Pd catalyst effect on low temperature hydrogen desorption from hydrided ultrathin Mg nanoblades.

We present hydrogenation/de-hydrogenation properties of ultrathin Mg nanoblades coated with Pd as a catalyst. The hydrided Pd/Mg/Pd and Pd/Mg nanoblades were highly curved as observed by ex situ scanning electron microscopy. An in situ thermal desorption spectroscopy (TDS) study showed that the hydrided Pd/Mg/Pd nanostructure has a low hydrogen desorption temperature, at ∼365 K. Through a combinational microstructure and TDS analysis of hydrided Pd/Mg/Pd nanoblades as well as hydrided Pd/Mg/Pd nanoblades covered with an additional ultrathin Mg layer, we found that the effect of Pd catalyst on reducing the hydrogen desorption temperature is significantly stronger than the conventionally proposed mechanisms due to grain size and strain.

Unusual magnesium crystalline nanoblades grown by oblique angle vapor deposition.

We observed the growth of unusual Mg nanoblades by oblique angle deposition. Although the vapor flux is obliquely incident, these nanoblades stand vertically on the substrates. The thickness of the Mg nanoblades along the incident vapor direction is reduced to approximately 15 nm to -30 nm at a vapor incident angle approximately 75 degrees, while the width perpendicular to the incident vapor direction is as wide as a few hundred nm. In addition to the anisotropic blade morphology, a (1010) [0001] biaxial (II-O) texture was observed using in situ reflection high energy electron diffraction (RHEED). The tilt angles of the texture axis and the nanoblades are correlated with the high surface diffusion on the (0001) surface along the [2130] direction. We also propose that the observed very thin thickness of the nanoblade along the vapor flux direction is due to the appearance of the surface steps parallel to the [0110] direction and the low surface diffusion on the top surface of the nanoblades.

Preferred orientation in Ru nanocolumns induced by residual oxygen.

Ru nanocolumns were grown on a native oxide covered Si(100) substrate using an oblique angle sputter deposition technique with substrate rotation at room temperature. Scanning tunneling microscopy images of conventional Ru film show the presence of straight columnar features on the film surface, which are very different from the nearly circular features observed on the nanocolumns surface. X-ray diffraction spectra confirm that these nanocolumns have (100) as the preferred orientation instead of the (002) orientation observed for a conventional film. The oxygen to Ru atomic ratio was determined for both the nanocolumns and the conventional film by using X-ray photoelectron spectroscopy. The nanocolumns were observed to incorporate about 6 times more oxygen than the conventional film near the surface region. We argue that the oxygen segregates onto the high-density (002) plane whereas it permeates through comparatively open planes like (100) and (101). The adsorbed oxygen atoms serve as a diffusion barrier for the landing Ru adatoms and inhibit the growth of the (002) plane. This results in the absence of the (002) plane and development of (100) and (101) planes in the nanocolumns. The oxygen plays a decisive role in determining the crystallographic orientation and the feature size/shape over the nanocolumns and conventional film surfaces.