Annealing In Ultrahigh Vacuum Research Articles

J211016 FIB加工シリコンナノワイヤの機械特性に及ぼす超高真空アニールの影響([J211-01]"壊れない"マイクロシステムのためのナノ力学・ナノ計測(1))

J211016 FIB加工シリコンナノワイヤの機械特性に及ぼす超高真空アニールの影響([J211-01]"壊れない"マイクロシステムのためのナノ力学・ナノ計測(1))

Formation and thermal stability of 2D ordered SiC/Si(001) nanodots

Abstract

Growth of stoichiometric TiO2 thin films on Au(100) substrates by molecular beam epitaxy

The present study reports on the growth of thin TiO2 films onto Au(100) single crystals by Ti evaporation in a reactive O2 atmosphere at two different substrate temperatures: room temperature (RT) and 300°C. The growth of the oxide films was monitored by means of X-ray photoemission spectroscopy, while the valence and conduction band electronic structure was investigated by UV and inverse photoemission spectroscopy, respectively.The TiO2 film grows epitaxially on the Au(100) substrate at 300°C exhibiting the rutile (100) surface. The evolution of the Ti 2p lineshape with the oxide coverage shows the presence of reduced oxide species (characterized by Ti3+ ions) at the Au(100) interface. A crystalline and stoichiometric TiO2 oxide is produced at high substrate temperature, while growth at RT gives a measurable concentration of defects. Post growth annealing in ultra-high vacuum of the RT grown film increases this concentration, while subsequent annealing in O2 atmosphere restores the sample to the as-grown conditions.

A high-reflectivity, ambient-stable graphene mirror for neutral atomic and molecular beams

We report a He and H2 diffraction study of graphene-terminated Ru(0001) thin films grown epitaxially on c-axis sapphire. Even for samples exposed for several weeks to ambient conditions, brief annealing in ultrahigh vacuum restored extraordinarily high specular reflectivities for He and H2 beams (23% and 7% of the incident beam, respectively). The quality of the angular distributions recorded with both probes exceeds the one obtained from in-situ prepared graphene on Ru(0001) single crystals. Our results for graphene-terminated Ru thin films represent a significant step toward ambient tolerant, high-reflectivity curved surface mirrors for He-atom microscopy.

Ba and BaOx surface structures on Au(111)

Scanning tunneling microscopy (STM) is used to investigate surface structures of barium and barium oxide grown on (22×3)-reconstructed Au(111) surfaces. Evaporated Ba attaches to the Au herringbones after ultrahigh vacuum (UHV) annealing, forming a stripe pattern. Oxidation of the Ba stripes gives rise to a BaOx structure pinned to the Au herringbones. Conversely, well-ordered epitaxial BaOx structures can be obtained with oxidation of the deposited Ba by residual oxygen and subsequent UHV annealing. The two ordered BaOx phases, a (6×6) and a (23×23)-R30° structure, coexist on the surface and lift the Au reconstruction.

Polymer Material as a Gate Dielectric for Graphene Field-Effect-Transistor Applications

Epitaxial-graphene field-effect transistor (EG-FET) with a polymer gate dielectric was fabricated and their electrical characteristics were investigated. The epitaxial graphene layer was formed on a semi-insulating 6H-SiC substrate by a high-temperature annealing in ultrahigh vacuum. The formation of graphene was confirmed by low-energy electron diffraction (LEED), Raman-scattering spectroscopy and X-ray photoelectron spectroscopy (XPS). The polymer gate dielectric (ZEP520a) layer was formed by spin coating, which exhibits good dielectric properties without noticeable structural degradation of the graphene layer. The EG-FETs with this polymer gate dielectric shows an n-type characteristic, with the field-effect mobility of 580 cm2 V-1 s-1.

Temperature-Programmed Desorption Observation of Graphene-on-Silicon Process

With its industrial adaptability, graphene-on-silicon (GOS), formed by ultrahigh-vacuum annealing of a SiC thin film on a silicon substrate, is attracting recent attention. Little is known, however, about the growth mechanism of GOS. We demonstrate in this paper that temperature-programmed-desorption spectroscopy of deuterium (D2-TPD) can be a powerful in-situ probe to investigate the surface chemistry during formation of epitaxial graphene (EG) on SiC crystals. Using the D2-TPD, the surface stoichiometry and the back-bonds of the surface atoms, including their dependence on the crystallographic orientations [Si(111), Si(100), and Si(110)] can be obtained. Difference in the growth mechanism of GOS among the orientations is discussed based on the results.

Ultrahigh-Vacuum-Assisted Control of Metal Nanoparticles for Horizontally Aligned Single-Walled Carbon Nanotubes with Extraordinary Uniform Diameters

Uniform diameter, horizontally aligned single-walled carbon nanotubes (SWNTs) are grown on sapphire substrate through the control of Fe nanoparticle size by annealing in ultrahigh vacuum (UHV). We find that thermal annealing in UHV reduces the Fe nanoparticle size and significantly narrows their size distribution. These size-controlled Fe nanoparticles enable the catalytic growth of uniform-diameter SWNTs while maintaining their horizontal alignment. Systematic analyses of Raman radial breathing modes with three laser wavelengths indicate that ∼76% of the horizontally aligned SWNTs have diameters in the range of 1.3–1.4 nm under optimized annealing conditions. Longer UHV annealing induces the increase in the Fe nanoparticle size, giving large-diameter SWNTs. Precise correlation between the metal nanoparticle size and SWNT diameter is demonstrated. Effects of UHV annealing on the nanoparticle size are discussed in terms of evaporation, subsurface diffusion, and surface precipitation of catalyst metals.

Influence of CrN surface compound on the initial stages of high temperature oxidation of ferritic stainless steel

Segregation phenomena and formation of surface compounds on Fe–17Cr (ferritic stainless steel) were studied at temperatures up to 800 °C upon annealing and kinetically controlled oxidation by photoelectron spectroscopy and inelastic electron background analysis. The results revealed the formation of a chromium nitride surface compound during annealing in ultrahigh vacuum at temperatures exceeding 527 °C. Surface enrichment of P, As, and other trace elements became more prominent at higher temperatures approaching 800 °C. It was found that nitrogen was buried below the surface oxide layer during oxygen exposure, yet it had little effect on the rate of oxidation. However, the formation of CrN surface compound promoted the selective oxidation of Cr initially, which is of great importance in processing and application environments involving high temperature and controlled atmosphere conditions.

Probing the thermal decomposition behaviors of ultrathinHfO2 films by an in situ high temperature scanning tunneling microscope

The thermal decomposition of ultrathinHfO2 films(∼0.6–1.2 nm) on Si by ultrahigh vacuum annealing (25–800 °C) is investigated in situ in real time by scanning tunneling microscopy. Twodistinct thickness-dependent decomposition behaviors are observed. When theHfO2 thicknessis ∼ 0.6 nm, no discernible morphological changes are found below ∼ 700 °C. Then an abruptreaction occurs at 750 °C with crystalline hafnium silicide nanostructures formed instantaneously.However, when the thickness is about 1.2 nm, the decomposition proceedsgradually with the creation and growth of two-dimensional voids at800 °C. The observed thickness-dependent behavior is closely related to the SiO desorption, whichis believed to be the rate-limiting step of the decomposition process.

Atomic and electronic surface structures of dopants in oxides: STM and XPS of Nb- and La-doped SrTiO3(001)

High resolution X-ray photoelectron spectroscopy (XPS) was performed on the (001) surfaces of Nb-doped and La-doped single crystals of SrTiO${}_{3}$. For the Nb-doped samples the XPS results demonstrate that the fraction of Ti${}^{3+}$ ions increases with increasing Nb dopant concentration. The Nb dopants are shown to be in a $5+$ ionization state. Atomic resolution scanning tunneling microscopy (STM) images of the 0.7 at.% Nb-doped SrTiO${}_{3}$(001) surface show that Nb-dopant atoms can be imaged on the surface as bright four-point square clusters. No evidence of Nb segregation to the (001) surface was found under ultrahigh vacuum (UHV) annealing. However, UHV annealing of the 0.7 at.% La-doped SrTiO${}_{3}$(001) samples at 1400$ {}^{\ifmmode^\circ\else\textdegree\fi{}}\mathrm{C}$ for 1 h resulted in La surface segregation as determined by Auger electron spectroscopy. STM on these samples shows that the segregated La results in La-rich linear surface structures.

Desorption kinetics of GeO from GeO2/Ge structure

High-κ dielectrics on Ge have recently attracted much attention as a potential candidate to replace planar silicon transistors for sub-32-nm generations. However, the instability of the high-κ/Ge interface, especially the desorption of germanium monoxide (GeO), hampers the development of Ge-based devices. Therefore, the typical GeO2/Ge structure was chosen to investigate GeO desorption. In this contribution, we describe the desorption kinetics of GeO, including Ge/GeO2 interface reaction, the diffusion process during GeO desorption, the desorption activation energy of GeO, the different mechanisms of GeO desorption, and the active oxidation of Ge. Through annealing GeO2/Ge in an ultrahigh vacuum (UHV), direct evidence for the consumption of Ge substrate has been shown by atomic force microscopy (AFM) measurements of the consumption depth. By using thermal desorption spectroscopy (TDS) measurements and studying oxygen-18 isotope tracing, we have clarified that the GeO desorption is not caused by the GeO direct-diffusion process. Isothermal TDS measurements and morphology investigation have revealed a transition from a uniform desorption mechanism to a nonuniform one as GeO2 thickness was reduced. On the basis of the experimental results in this study, a GeO desorption model has been developed. For GeO desorption, about 2 eV of activation energy has been obtained from kinetic calculation on the basis of TDS analysis. Different from the UHV annealing, the active oxidation of Ge has been observed by AFM measurements under low oxygen partial pressures.

Formation of pyramid-like nanostructures in MBE-grown Si films on Si(001)

The growth of Si homoepitaxial layers on Si(001) substrates by molecular beam epitaxy is analyzed for a set of growth conditions in which diverse nanometer-scale features develop. Using Si substrates prepared by exposure to HF vapor and annealing in ultra-high vacuum, a rich variety of surface morphologies is found for different deposited layer thicknesses and substrate temperatures in a reproducible way, showing a critical dependence on both. Arrays of 3D islands (truncated pyramids), percolated ridge networks, and square pit (inverted pyramid) distributions are observed. We analyze the obtained arrangements and find remarkable similarities to other semiconductor though heteroepitaxial systems. The nanoscale entities (islands or pits) display certain self assembly and ordering, concerning size, shape, and spacing. Film growth sequence follows the ‘islands–coalescence–2D growth’ pathway, eventually leading to optimum flat morphologies for high enough thickness and temperature.

Epitaxial graphene field-effect transistors on silicon substrates

We have fabricated and characterized the field effect transistors having an epitaxial-graphene channel grown on Si substrates. Epitaxial graphene is usually formed on SiC substrates by ultrahigh-vacuum (UHV) annealing. We used an approach to grow 3C–SiC layer on Si substrates and subsequently to anneal them in UHV to make few layers of graphene on the sample surface. Backgate transistors were able to be formed by using the SiC layer as a gate insulator. Although the gate-leakage current is not negligible, the drain current modulation by means of the gate voltage is confirmed by extracting the channel current from the total drain current. A new evaluation method of the area contact resistance between graphene and metal is also proposed.

Thermal instability of silicon-on-insulator thin films measured by low-energy electron microscopy

Using low-energy electron microscopy (LEEM), we investigate the ultrahigh vacuum annealing of silicon-on-insulator (SOI) samples capped by a chemically-prepared oxide layer. Consistent with previous reports: (1) for T > 750°C, the capping-oxide decomposes by void nucleation and growth, then (2) for T > 850°C, the Si thin-film dewets from the SiO2 substrate. Here, we show that the morphological evolution of the surface during the dewetting process is dependent on the preparation of the SOI surface. Two dewetting pathways are evident in recent literature, we find that one evolution is characteristic of clean Si(100)−2 × 1 surfaces, while the other is correlated with surface contamination.

Frequency modulation atomic force microscope observation of TiO2(110) surfaces in water

Rutile titanium dioxide (TiO2) (110) surfaces were examined in water using a frequency modulation atomic force microscope. On the surfaces cleaned by Ar+ sputtering and annealing in ultrahigh vacuum, step-terrace structure was observed. The inlets at the steps and the pits on the terraces indicated erosion of the surface in water. Strings extended to the [001] direction were occasionally observed in the topography images and assigned to the clusters of the H2O molecules. The tip experienced a repulsive force when the vertical tip position of z was less than 6 nm from the surface, and the force oscillated at z at less than 2 nm. The repulsive force originated from the disruption of the hydrogen bonding network of H2O molecules formed on the hydrophilic sputter-annealed surface. The oscillatory force arose from structural alternate order-disorder transitions of the H2O molecules at the gap between the tip and the TiO2 surfaces. On the TiO2 surface annealed in air, no strings were observed in the topography images. The tip experienced an attractive force before experiencing a repulsive force in its approach to the surface. Oscillatory behavior was not observed in the force curve. The air-annealed TiO2 and tip surfaces were both hydrophobic and attracted to each other to expel the H2O molecules from their gap. Ordering the H2O molecules at the gap between the two hydrophobic surfaces was entropically unfavorable.

Chemical etchant dependence of surface structure and morphology on GaN(0001) substrates

We studied the procedure of cleaning GaN(0001) substrate surfaces by wet etching and subsequent annealing in ultrahigh vacuum for two different types of freestanding GaN wafers: hydride vapor phase epitaxy (HVPE) crystal and Na flux liquid phase epitaxy (LPE) crystal wafers. A flat surface containing GaN(0001)2 × 2 reconstruction was successfully achieved on both HVPE and LPE surfaces by etching in HF and subsequent annealing at ∼ 550 °C but was not achieved by etching in HCl, NaOH, and HNO 3.

STM–STS investigation of vacuum annealed ZnO nanoribbons

AbstractWe present a scanning tunneling microscopy and spectroscopy (STM–STS) investigation of the effects of ultra high vacuum annealing and oxygen exposure onto ZnO nanoribbons synthesised by chemical vapour deposition. STM imaging revealed a width to height ratio for the nanoribbons between 2:1 and 3:1 and average width and height of 90 and 40 nm, respectively. Imaging before annealing showed the presence of surface contaminants, which were removed after annealing at 800 °C. Analysis of the STS data before annealing shows n‐type behaviour with a band gap of 3.4 eV and an upward band bending of 0.9 eV. Annealing up to 700 °C induced a reduction in surface band bending, towards a near flat band behaviour. After the 800 °C anneal the surface electronic properties were significantly altered, with a large increase of tunnelling current at negative sample bias leading to a narrowing of the apparent surface band gap and a mid gap Fermi level. This change was attributed to a loss of surface lattice oxygen and was found to be reversible upon O2 exposure at room temperature. The anneal/O2 exposure process could lead to a way of cleaning the ZnO nanoribbons without using ion bombardment techniques.

In Situ Ambient Pressure Studies of the Chemistry of NO2 and Water on Rutile TiO2(110)

The adsorption of NO(2) on the rutile TiO(2)(110) surface has been studied at room temperature in the pressure range from approximately 10(-8) torr to 200 mtorr using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Atomic nitrogen, chemisorbed NO(2), and NO(3) were formed, each of which saturates at pressures below approximately 10(-6) torr NO(2). Atomic nitrogen originates from decomposition of the NO(x) species. For pressures of up to 10(-3) torr, no significant change in the NO(x) surface species occurred, suggesting that environmentally relevant conditions with typical NO(2) partial pressures in the 1-100 ppb range can be modeled by ultrahigh vacuum (UHV) studies. The chemisorbed surface species can be removed by in situ annealing in UHV: all of the NO(x) species disappear around 400 K, whereas the N 1s signal associated with atomic nitrogen diminishes around 580 K. At higher pressures of NO(2) (p(NO(2)) > or = 10(-6) torr), physisorbed NO(2) and adsorbed water, which was likely due to displacement from the chamber walls, appeared. The water coverage grew significantly above approximately 10(-3) torr. Concurrently with co-condensation of water and NO(2), the population of NO(3) species grew strongly. From this, we conclude that the presence of NO(2) and water leads to the formation of multilayers of nitric acid. In contrast, pure water exposure after saturation of the surface with 200 mtorr NO(2) did not lead to a growth of the NO(3) signals, implying that HNO(3) formation requires weakly adsorbed NO(2) species. These findings have important implications for environmental processes, since they confirm that oxides may facilitate nitric acid formation under ambient humidity conditions encountered in the atmosphere.

Spatial structures formed during high-temperature vacuum annealing of diamond-like film deposited on a silicon substrate

The process of an ultrahigh vacuum annealing of a diamond-like carbon film deposited on a silicon substrate has been studied in situ using a photoemission microscope mounted at the MAX-lab synchrotron’s radiation source. After the annealing the film was examined ex situ using an atomic force microscope. It is revealed that the graphitization of the film occurs and round formations of several microns in size with a core of ∼0.2 µm in size appear on its surface at the temperature exceeding 1000°C. Photoemission images of these formations have shown the presence of silicon on their surface. Apparently, the interaction between the carbon film and silicon substrate begins in the middle of the formation and spreads to the adjacent region at the following stage.