Field emission spectroscopy of gold on the (110) and (211) planes of tungsten

Adsorption of Au up to monolayer amounts on the (110) plane of a tungsten field emitter leads to a decrease in work function, and also to a decrease in effective emitting area. Energy distributions of field emitted electrons show no structure attributable to Au. These results can be explained by postulating that there is no tunneling from Au atoms or from beneath Au atoms. The most probable reason for this is that in (110) tunneling matrix elements from outward normal tungsten d-orbitals are stronger than the tunneling matrix element from the non-directional 6s Au orbital. Experiments on the (211) plane show behavior similar to that previously found on (111), i.e. a work function increase but strong emission from Au atoms.     

Growth and desorption of indium epitaxial layers investigated by high field microscopy

Growth of indium single crystals on tungsten field emission tips was carried out by deposition of indium from vapour in ultra high vacuum, using substrate temperatures in the range of 293–420 K. Two different tungsten tips were used as the substrate: a perfect W single crystal in one case and a bi-crystal with a distinct grain boundary in the other. No influence of the grain boundary on the epitaxial growth was found. Two orientation relationships were observed mostly: {111}In ∥ {110}W with 〈110〉In ∥ 〈111〉W and {111}In ∥ {100}W with 〈110〉In ∥ 〈110〉W. In the first case the growth was initiated by the indium nucleus created on the ledges of the {110}W plane. A field strength of 0.9 V/Å was found for the evaporation field of indium. The field strength of the desorption of In-W interfacial layer atoms was found to be 4.4–5.2 V/Å. A mechanism of the growth of indium crystals has been proposed.     

Direct observation of adatom-substitutional atom interaction on dilute metal alloy surfaces

The field ion microscope is used to study interactions of a migrating tungsten adatom with substitutional rhenium atoms in the (110) plane of a W-3%Re alloy. By observing about 300 migration periods on a single (110) surface plane of a small field ion emittier, each position of the adatom with respect to deduced Re atom locations can be identified. The interaction is found to be attractive with a strength of 90 ± 7 meV at the closest equilibrium separation, and repulsive with a strength of at least 80 meV at the second closest separation. No interaction could be observed for larger separations indicating a strength of less than 10 meV. Results of a control experiment, diffusion of single W adatoms on the (110) plane of pure W, are also presented for comparison.     

Diffusion of nickel on the (110) region of a tungsten field emitter

The diffusion of nickel atoms on the (110) plane of a thermally cleaned tungsten crystal has been investigated by a field emission technique. The measurements were carried out for low nickel coverage and substrate temperatures T390 K when the nickel atoms escape from the (110) plane to its surroundings. the value of the activation energy obtained, Q=0.96 eV, is discussed taking into account the conditions under which diffusion occurs.     

Measurement of the barrier height of the reflective W {110} plane boundaries in surface diffusion of single atoms

Plane boundaries are in general reflective to diffusing atoms. From about 2000 heating periods of direct field ion microscope observations we have studied adatom plane edge interaction and determined the barrier heights of the W {110} plane boundaries in single atom diffusion of W, Re, and Ir. Using equations we have derived recently, the barrier heights ΔE_b are found to be about 200 meV for these adatoms.     

Binding states of Ga and Sn on W and Mo: Structures,evaporation field and its temperature dependence

Binding states of deposited Ga and Sn on the W and Mo substrates were studied by measuring the evaporation fields of the deposited metal atoms at various temperatures. Ga and Sn form two layers: an overlayer and an underneath pseudomorphic layer which directly contacts the substrate surface. The evaporation fieds of the overlayer Ga and Sn atoms were found to be the same on various crystal planes and significantly lower than that of the pseudomorphic Ga and Sn atoms, indicating that the binding between the overlayer atoms and the pseudomorphic atoms is much weaker than that between the pseudomorphic atoms and the substrate W and Mo atoms. The finding that the evaporation field of the pseudomorphic atoms on W is very close to that on Mo while the evaporation field of W is significantly higher than that of Mo was unexpected. Another interesting finding is that the field emission current from the (011) plane is not noticeably affected by the coverage of the mono-atomic Ga layer arranged in the superstructure while the current from other crystal planes covered by the pseudomorphic layer having the same atomic arrangement with the substrate surface is minimized. Furthermore, it has been found that the evaporation field of the Ga overlayer is lower than that of Sn, while the Ga pseudomorph is more stable than Sn pseudomorph on W surface at temperatures below about 100 K. The temperature dependence of the evaporation field was also examined and compared with existing models.     

Direct observation of the atomic structure of W {100} surfaces

Fully resolved atomic images of the W {100} plane are obtained with the field ion microscope at 21°K. The image structure is consistent with the (1 × 1) structure. If shifts of atoms in the 〈110〉 directions parallel to the plane have occurred as derived from LEED investigations in the temperature range of 100–370°K, then the shifts at 21°K are too small to be observed in the FIM.     

A study of local density of states of atom clusters on the W(110) surface

The kinetic energy distributions of field desorbed He ions from tungsten clusters of one to five atoms on a W(110) surface are measured using a high resolution pulsed-laser time-of-flight atom probe. The He field ion energy distribution from the single W adatom shows an extra peak-like feature centered at 2.7 eV above the Fermi level. It has a full width at half maximum (fwhm) of 2.3 eV. The data from two tungsten adatoms separated by two lattice constants have nearly the same feature with the extra peak located at 2.5 eV above the Fermi level. These peaks arise from resonance tunneling with the adatom local density of states (LDOS). The He ion energy distribution of a tungsten dimer has an extra peak centered at 1.5 eV above the Fermi level. The fwhm is about 4 eV. The spectra from four and five tungsten adatom clusters show only one peak each, similar to that from a flat plane.     

A calculation of surface diffusion coefficients of adsorbates on the (110) plane of tungsten

An attempt to calculate the prefactors of the diffusion coefficients of tungsten, xenon and oxygen atoms on the W(110) plane in the zero-coverage limit was made, using a Fokker-Planck formalism. Pairwise substrate-substrate and adsorbate-substrate atom interactions were assumed and expressed by appropriate Morse potentials. The vibrations of the substrate were approximated by an Einstein model of independent oscillators. When Morse potentials were so adjusted as to give good agreement between experimental and calculated activation energies, a prefactor in excellent agreement with experiment was calculated for W/W(110). For Xe/W(110) and O/W(110) the calculated values were ～10^?4 cm²/s, i.e. essentially “normal”, while the experimental values are much lower. Possible reasons for the discrepancies are discussed.     

Layer-by-layer analysis of charge state distribution of field evaporated ions from the W(011) plane

A charge state distribution of the field evaporated ions is layer-by-layer analyzed in a W(011) plane by using an atom-probe field ion microscope. A specimen temperature is varied from ～ 20 to ～ 250 K. The results indicate that at any temperature, the surface tungsten atoms field evaporate as triply and quadruply ions from the (011) plane, and the quadruply charged ions are always detected during the final collapse of the plane. They are also discussed on the basis of the post-ionization model.     

Diffusion of hydrogen into the (110) plane of a tungsten field emitter at low temperatures

Experiments taking advantage of the very small dissociative sticking coefficient of H₂ on the (110) plane of a tungsten field emitter show that diffusion into, rather than on the plane limits entry of H into the plane at low temperatures, i.e. diffusion on the plane is much faster than entry into the plane. The barrier to entry of H is nearly independent of temperature from 130 to 27 K and is independent of applied field. Over the limited range experimentally accessible tunneling diffusion on the (110) plane is independent of field emission current.     

Field-induced transfer of lithium in a scanning tunneling microscope

Field-induced transfer of Li atoms from a [110]-oriented W tip to a Pt sample is realized in the scanning tunneling microscope (STM) under UHV conditions, using a working principle of the new solid state Li surface diffusion metal ion source (Li-SDMIS), in which the supply of Li to the apex of the tip occurs via surface diffusion. By applying 5–10 μs voltage pulses to the Li-covered W tip placed within tunneling range, single Li hillocks (400 × 400 Ų) were formed on the chosen area of the Pt(110) sample in a well-rep way. The pulse magnitude necessary for Li emission displays a distinct threshold character. A comparative analysis of the energetics of Li field desorption in the STM and Li-FDM (lithium field desorption microscope) modes indicates chemically-assisted field desorption of Li as the field-induced transfer mechanism.     

氦泡在bcc钨中晶界处成核长大的分子动力学模拟

钨(W)是潜在的聚变堆面向等离子体材料.聚变反应中产生的氦(He)不溶于金属W,并在其中易聚集形成He泡,使W基体发生脆化,从而导致W基体的性能发生退化.在前人工作的基础上,本文采用分子动力学研究了He泡在单晶bcc-W中以及bcc-W中∑3[211](110)和∑9[110](411)晶界处He泡形核长大初期的演化过程.结果发现,晶界处He泡的长大机制和单晶W中有所不同.单晶W中He泡通过挤出位错环促进长大.而He泡在∑3[211](110)晶界处的长大机制为:首先挤出并发射少量自间隙W原子,而后挤出1/2⟨111⟩位错线,随后,该位错线会沿晶界面上[111]方向迁移出去;在∑9[110](411)晶界处,He泡在我们的模拟时间尺度范围内没有观察到W自间隙子的发射和位错的挤出.     

The work function of perfect W(110) planes: Fowler-Nordheim studies

Fowler-Nordheim measurements have been carried out on tungsten specimens field evaporated at low temperature in an endeavor to resolve conflicting values of the work function for the W(110) plane. Interpretation of such measurements has in the past been difficult, because of unknown contributions from plane edges and patch fields. These contributions have been isolated by taking advantage of the unusually low reactivity of W(110) toward molecular hydrogen. Observations on perfect W(110) planes yield a work function of 5.47 eV.     

ToF atom-probe fim investigation of surface segregation in dilute alloys

A time-of-flight atom-probe field ion microscope was successfully employed for the first time to investigate surface segregation in dilute alloys. We were able to achieve a single atomic layer resolution and obtained the absolute concentrations of alloy species of each surface layer. Cr atoms are found to segregate to the surface of Stainless Steel 410 whereas no segregation of the minority species was found for Pt-8% W and Pt-5% Ru. The first layer concentration of Cr in the {110} plane of Stainless Steel 410 at 500°C was found to be 38.5 ± 12.5% and 63.4 ± 10.2% respectively for samples with near surface layers Cr average concentration of 6.3 ± 2.1% and 11.9 ± 2.5%. The heat of segregation of Cr to the {110} plane of Stainless 410 was found to be 3.43 and 3.92 kcal/mole respectively from the two sets of data. The data also gives the difference in surface tensions between iron and chromium at this plane to be 269 and 282 erg/cm² respectively. Segregation studies on the {012} plane as well as on a grain boundary of Stainless Steel 410 were also done. In some cases, though the first surface layer is enriched with Cr in Stainless Steel 410, the near surface layers show a depletion of Cr. In Pt-8% W and Pt-5% Ru, our concentration depth profiles with a single atomic layer resolution show no segregation or depletion of the minority species either for the top layers or for the near surface layers.     

过渡元素掺杂对Mo力学性能的第一性原理研究

基于密度泛函理论,采用第一性原理方法计算了在Mo中掺杂摩尔百分比分别为2.08%和4.17%的过渡金属元素W,Ti,Cu和Fe后,体系在[111](110)滑移系统上的广义层错能以及解理能,并研究了掺杂元素对Mo的剪切形变以及脆性一韧性的影响,研究发现,掺杂W和Ti原子会使体系剪切形变的发生变得困难,并使Mo材料变脆;而掺杂Cu和Fe原子则会使体系剪切形变的发生变得相对容易,并使Mo材料的韧性增强,此外,随着掺杂浓度的增加,掺杂W会使体系剪切形变的发生变得更为困难,并使Mo材料脆性更强;而掺杂Fe则会使体系剪切形变的发生变得更为容易,并使Mo材料的韧性更强。     

Work function in field emission — the (110) plane of tungsten

Field emission for the (110) plane of field evaporated and thermally annealed tungsten has been restudied with particular reference to the significance of interpretation of the work function. It is concluded that the results obtained in field emission are consistent with results from non-field emission methods where the important variable of the local field is adequately considered. Possible contribution of other factors to measurement of the clean surface work function is also discussed.     

The interaction of ultrathin films of Ni and Pd with W(110): an XPS study

The interaction of ultrathin films of Ni and Pd with W(110) has been examined using X-ray photoelectron spectroscopy (XPS) and the effects of annealing temperature and adsorbate coverage (film thickness) are investigated. The XPS data show that the atoms in a monolayer of Pd or Ni supported on W(110) are electronically perturbed with respect to the surface atoms of Pd(100) and Ni(100). The magnitude of the electronic perturbations is larger for Pd than for Ni adatoms. Our results indicate that the difference in Pd(3d_5/2) XPS binding energies between a pseudomorphic monolayer of Pd on W(110) and the surface atoms of Pd(100) correlates with the variations observed for the desorption temperature of CO (i.e., the strength of the Pd---CO bond) on these surfaces. A similar correlation is seen for the Ni(2p_3/2) XPS binding energies of Ni/W(110) and Ni(100) and the CO desorption temperatures from the surfaces. The shifts in XPS binding energies and CO desorption temperatures can be explained in terms of: (1) variations that occur in the Ni---Ni and Pd---Pd interactions when Ni and Pd adopt the lattice parameters of W(110) in a pseudomorphic adlayer; and (2) transfer of electron density from the metal overlayer to the W(110) substrate upon adsorption. Measurements of the Pd(3d_5/2) XP binding energy of Pd/W(110) as a function of film thickness indicate that the Pd---W interaction affects the electronic properties of several layers of Pd atoms.     

A very low energy electron reflection study of hydrogen adsorption on W(100) and W(110) surfaces

Experimental results of the elastic backscattering of electrons with energies between 0 and 20 eV from the surface systems H/W(100) and H/W(110) are reported and interpreted on the basis of the Darwin model. On W(100) the adatom distance from the reconstructed substrate is found to be different from that on the relaxed surface. Large vertical displacements of the substrate atoms during reconstruction can be excluded. On W(110) the results indicate two different binding states which are occupied sequentially and which differ significantly in their distances from the surface. Work function change data are also reported for both systems.     

不同晶面的氢终端单晶金刚石场效应晶体管特性

通过微波等离子体化学气相淀积技术生长单晶金刚石并切割得到(110)和(111)晶面金刚石片,以同批器件工艺制备两种晶面上栅长为6μm的氢终端单晶金刚石场效应管,从材料和器件特性两方面对两种晶面金刚石进行对比分析.(110)面和(111)面金刚石的表面形貌在氢终端处理后显著不同,光学性质则彼此相似.VGS=–4 V时,(111)金刚石器件获得的最大饱和电流为80.41 m A/mm,约为(110)金刚石器件的1.4倍;其导通电阻为48.51 W·mm,只有(110)金刚石器件导通电阻的67%.通过对器件电容-电压特性曲线的分析得到,(111)金刚石器件沟道中最大载流子密度与(110)金刚石器件差异不大.分析认为,(111)金刚石器件获得更高饱和电流和更低导通电阻,应归因于较低的方阻.