Heat Treatment In Ultrahigh Vacuum Research Articles

The oxidation behaviors of high-purity niobium for superconducting radio-frequency cavity application in vacuum heat treatment

To explore the impact mechanism of vacuum heat treatment on the performance of Superconducting Radio-Frequency (SRF) Cavities, especially in the medium temperature range (300–400 °C), the chemical composition on the surfaces and the impurities distribution along the depth of the high-purity niobium (Nb) samples heat-treated at 250–800 °C in Ultra High Vacuum (UHV) were investigated using X-ray photoelectron spectroscopy (XPS). The XPS results on the surface showed that the surface oxide composition varied with the different heat-treatment temperatures. Among the range of 250–600 °C, Nb2O5 gradually reduces and suboxide such as NbO, Nb2O, and NbOx gradually enhances with the increasing of heat-treatment temperature. However, an elevated proportion of Nb2O5 was shown on the sample heat-treated at 800 °C. The XPS depth profiles acquired by different times of Ar+ etching revealed that the oxygen concentration in depths increased after heat treatment in UHV and presented a temperature dependence, while the nitrogen was not precisely identified. This work might also contribute to understanding the effect of material properties on the RF performance and help to optimize the surface-treatment process for SRF Nb cavities.

XANES and XPS investigations of surface defects in wire-like SnO2 crystals

Wire-like SnO2 micro- and nanocrystals prepared by gas-transport synthesis have been studied by X-ray photoelectron spectroscopy and X-ray absorption near edge structure spectroscopy with the use of synchrotron radiation. It has been found that the heat treatment in ultrahigh vacuum affects the surface state and the vacancy formation in surface layers of the wire-like crystals.

Surface structures of [formula omitted]- FeSi2 formed by heat-treatment in ultra-high vacuum and their influence on homoepitaxial growth

We have studied the surface structures of single crystalline β- FeSi2 substrate and their influence on homoepitaxial growth. After heat-treatment in ultra-high vacuum above 750 °C, where the native oxide layer (SiOx) on the substrate was removed, characteristic structures depending on the surface orientation were formed on the substrate. On the β- FeSi2 (100) substrate, the round shaped dip structures with conical elevation appeared after heat treatment above 800 °C. The composition of the dip structures was partially Fe-rich as compared with nominal β- FeSi2 composition (Si/Fe=2). These characteristic structures significantly affected the crystalline quality. They remained in the epitaxial layer after thin film growth. We also achieved homoepitaxial β- FeSi2 films on β- FeSi2 (111) substrate at the growth temperature of 700 °C and 800 °C.

Interfacial defects controlled electrical and magnetotransport in Co/ZnO nanocomposites

This work demonstrates, by frequency-dependent electrical measurements, significant variation associated with interfacial defects in Co/ZnO nanocomposites (NCs) for electrical and magnetic transport. Radio-frequency impedance measurements showed that heat treatment in an ultrahigh vacuum (UHV) increased interfacial defect density. Interfacial defects-assisted transport reduced the magnetoresistance (MR) ratio from 5.04% for the as-grown sample to 0.17% following heat treatment in UHV. The evolution of the onset frequency of power-law dispersion conductivity revealed that the increase in the interfacial defect density in Co/ZnO NCs upon heat treatment was consistent with the change in MR.

Sprout-like growth of carbon nanowires on a carbon-doped Ni(1 1 1) surface

Sprout-like growth of carbon nanowires on single-crystal graphite (0 0 0 1) terraces on a carbon-doped Ni(1 1 1) substrate has been observed for the first time using only a heat treatment in ultrahigh vacuum. Nanometer-scale morphology and chemistry have been clarified by low-energy electron diffraction, Auger electron spectroscopy and scanning tunneling microscopy (STM). The growth mechanism is based on a bulk-to-surface precipitation process of internal carbon atoms that were doped in a high-purity Ni(1 1 1) substrate in advance. The observed carbon nanowires are single wires and their bundles, which have a metallic conductivity. The structures have some similarity to those of single-wall and multi-wall carbon nanotubes. Simple manipulation of a single carbon nanowire is demonstrated by STM.

Discovery of Carbon Nanowires Formed on a Carbon-Doped Ni(111) Substrate by a Bulk-to-Surface Precipitation Process

We have discovered a synthesis method for carbon nanowires on a flat single-crystal graphite (0001) surface on a carbon-doped Ni(111) substrate using only heat treatment in ultrahigh vacuum. It should be noted that this new carbon nanowire synthesis method requires no external carbon-containing source. The growth mechanism utilizes a bulk-to-surface precipitation process of internal carbon atoms that were doped in a pure Ni(111) substrate in advance. Nanometer-scale morphology and chemistry of the carbon nanowires have been clarified by low-energy electron diffraction combined with Auger electron spectroscopy (LEED/AES), scanning tunneling microscopy (STM) and field-emission scanning Auger microcopy (FE-SAM).

Competitive Surfache Growt of Carbon Nanowires and Graphite (0001) Terraces on a C-doped Ni(111) Substratex

Carbon nanowires and flat single-crystal graphite (0001) terraces have been competitively formed on a carbon-doped Ni(111) substrate, for the first time, only by a heat treatment in ultrahigh vacuum. The growth mechanism is interpreted in terms of a bulk-to-surface precipitation process of internal carbon atoms that were doped in a high-purity Ni(111) substrate in advance. The observed carbon nanowires are single wires or bundles. Nanometer-scale morphology and chemical properties of the wires have been clarified by low-energy electron diffraction, Auger electron spectroscopy and scanning tunneling microscopy (STM). A simple manipulation technique of a single carbon nanowire is demonstrated by STM.

Controlled tuning of periodic morphologies on vicinal Si(111) surfaces

Regular hole patterns have been produced on vicinal Si(111) surfaces by optical lithography. Under heat treatment in ultrahigh vacuum, morphological rearrangements of the patterned surfaces occur. In situ light diffraction experiments were carried out during heat treatment to monitor the formation of the desired morphologies by an observation of the diffraction pattern. The surface morphology was characterized by different microscopy techniques after the heat treatment. Structured surfaces with periodic two-dimensional step arrangements and one-dimensional structures of atomically flat terraces uniformly separated by straight steps were obtained. The periodic structures extend over several μm.

Thermal stability of silver in ion-exchanged soda lime glasses

X-ray photoelectron spectroscopy was used to study the thermal stability of silver in ion-exchanged sodium calcium silicate glasses during heat treatment in ultrahigh vacuum. The changes in concentration, in line shape, and in binding energy as functions of sample temperature show that silver diffuses toward the surface and the chemical structure of the sample surface changes. Judging from a comparison of the line shapes and the binding energies of the samples to those of pure silver and silica, the concentration variations in bridging and nonbridging oxygens, and the binding energy shifts of nonbridging oxygens, it is concluded that the SiO2 glass network starts breaking up during Ag+ diffusion at sample temperatures below 200 °C, metallic silver clusters form, and nonbridging oxygens originally surrounded by silver atoms reconnect to silicon atoms at sample temperatures above 200 °C. The relaxation of surface stress introduced by the exchange process and the net reduction of Gibbs free energy cause the surface diffusion and segregation of silver and the reconnection between oxygen and silicon. The estimated activation energy of the silver surface diffusion is 0.16 eV.

Thermal Stabilities of Cu/Co/Cu/NiFe Multilayers

Cu/Co/Cu/NiFe multilayer films prepared by evaporation deposition in an ultrahigh vacuum were heat-treated in air and in ultrahigh vacuum. Heat treatment in ultra-high vacuum brought about no changes in the MR ratio or MR slope for annealing temperatures less than 400C. The thermal stability in air was limited by gradual oxidation from the surface, and thermal stability up to 250°C was verified. Improved thermal stability was achieved by the use of thicker NiFe layers.

Ｃｕ／Ｃｏ／Ｃｕ／ＮｉＦｅ人工格子の熱安定性

Multilayers of Cu/Co/Cu/NiFe prepared by evaporation in ultrahigh vacuum were heat treated in air and ultrahigh vacuum. It was confirmed that the heat treatment in ultrahigh vacuum showed no change in the MR ratio and MR slope for temperatures less than 400°C. Thermal stabilities in air were restricted by gradual oxidation from the surface, and verified up to 250°C. Improved thermal stability was realized by the use of thicker NiFe layers.

Study of interface diffusion of Ti and TiN PVD layers by Bremsstrahlung‐induced AES

AbstractThe influence of heat treatment in ultrahigh vacuum (UHV) on Ti and TiN layers coated by physical vapour deposition (PVD) has been studied by AES, XPS and bremsstrahlung‐induced AES. It could be concluded that up to 500° C (1 h) the Ti layer does not change significantly. At the TiN/steel substrate interface, however, a counterdiffusion of nitrogen and adventitious oxygen takes place, resulting in partial nitridation of the steel substrate and oxidation of the coating.

The interaction of hydrogen with silicon surfaces: A field ion microscope and pulsed-laser atom-probe study

Field ion microscope and pulsed-laser atom-probe studies of silicon surfaces following various types of heat treatment in ultrahigh vacuum and in hydrogen have been carried out to identify any structural or compositional changes produced by the heating. In particular, the possibility of surface hydride formation due to hydrogen segregation from the bulk at moderate temperatures has been explored. Results from high purity whisker samples show no evidence of surface structural or compositional changes following vacuum heating up to temperatures of 625 K, unless gas phase hydrocarbons are present. At 700 K, thermal faceting, which indicates surface atom mobility, is observed. Results from samples prepared from 5-Ω-cm boules indicate that hydrogen segregation may occur along extended defects. Pulsed-laser heating of Si samples in molecular hydrogen is found to produce an amorphous overlayer on the surface, and the predominant species detected from this surface with the pulsed laser atom probe is SiH+3 . This observation suggests the presence of a silicon-trihydride surface phase.

Luminescence spectrum of undoped CdS platelets as a function of slight heat treatment in ultra-high vacuum

The UV-excited luminescence spectrum of CdS (sublimed and cooled (2 deg/min)in an N2-H2S(1%) atmosphere) was studied at 77°K between 470 and 560 nm. Characteristic emission groups near 480 and 493 nm, the edge emission (with associated phonon replicas), and the doublet near 547 nm were investigated after successive heat treatment between 50°C and 300°C. It was observed that at rather well defined treatment temperatures the intensity of most of the emissions decreased while that of other emissions increased (e.g., those at 487.2 nm and 547 nm). Concurrent with these changes, certain typical changes in the spectral dependence of photoconductivity were observed. These changes are attributable to the desorption of gases (predominantly oxygen) from the surface1); that is, trapped carriers near the surface are forced to redistribute because of thechanges in the space charge region. This information aids in further identifying luminescence centers by allowing electron- or hole-activated processes (excitors bound to charged centers) to be distinguished, and to give a model for type I and type II CdS photoconductivity behavior.

The effect of thermal desorption of water on surface states on germanium in ultrahigh vacuum

Large signal alternating current field effect experiments in the dark and under illumination were carried out on “real” germanium surfaces following a heat treatment in ultrahigh vacuum. After prolonged heating (about 10h at 520°K, the surfaces were slightly p-type, exhibiting 3 to 5 times higher density of traps than prior to heating. Higher temperatures (560°K, 600°K and 640°K) rendered the surfaces more p-type, but caused essentially no change in the density of traps. The observed changes of the electrical properties were correlated to the results of desorption experiments in which a mass spectrograph was employed. The total surface charge was found to increase linearly with the amount of desorbed water (2.5 × 10 −4 negative elementary charges per desorbed water molecule).