Change in the emitter surface during high-current-density field emission from the 〈111〉 trihedral angle of a reconstructed tungsten tip

Changes in the shape and emission characteristics of the atomically sharp trihedral 〈111〉 angle of a tungsten tip reconstructed in an electric field are studied by continuous-mode field desorption microscopy during high-current-density field emission. The main changes in the tip shape and the slope of the Fowler-Nordheim characteristic occur at an emission current of 1–5 μA. At a current of 50–100 μA taken from the angle, the tip shape and emission characteristics stabilize and remain unchanged in the range 0–150 μA. The new tip shape is characterized by the widening of the angle edges; the appearance of {112} and {001} plane steps in them; a decrease in the sizes of the {011} planes forming the angle faces; and the appearance of steplike transition regions between {011}, {001}, and {112} faces. These changes in the tip shape are related to the fact that the emission field is weaker than the electric field used for preliminary tip reconstruction, the weakening of the field by the space charge of emitted electrons, and a nonuniform temperature distribution in the tip.     

Current fluctuations from various crystal faces of a clean tungsten field emitter

The temperature, frequency and current dependence of current fluctuations have been measured for the (310), (112) and (100) planes of a tungsten field emitter. The rms percent fluctuation of the current at 900 K was 8.7, 1.5 and 0.13% and the threshold temperature for current fluctuation was 300, 650 and 1000 K for the (310), (112) and (100) planes respectively. Based on the results reported here it is concluded that the current fluctuations arise from the generation of mobile atoms on net plane terraces.     

Dissociation of carbon monoxide on stepped nickel surfaces

Field emission microscopy with photometric measurements has been applied to investigate the adsorption of carbon monoxide on various crystal planes of a nickel field emitter. Upon heating the CO-covered surface, the work function drastically decreased during desorption of CO molecules into the gas phase and exhibited almost the value of the clean surface at 450 K. However, part of the CO molecules adsorbed on the stepped planes such as (510) and (310) were found to dissociate upon heating at 450–470 K, which was accompanied by an increase of the work function of ～0.2 eV.     

Bias field effects on the dielectric properties of 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 single crystals with different orientations

The domain states and phase transitions in 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ single crystals were investigated by studying their relative permittivity under various dc bias at constant heating and cooling rates. The orientation dependence of the bias field effect was revealed by examining the temperature dependence of relative permittivity as a function of crystal orientation (the 〈111〉, 〈011〉 and 〈001〉 directions) and dc bias field. The crystals basically have a macrodomain rhombohedral ferroelectric state in the ferroelectric phase under zero dc bias. External bias field could modulate the domain state and induce a stable macrodomain state in the crystals. Also, it is proposed that the dc bias applied along the 〈001〉 or 〈011〉 direction could induce a tetragonal ferroelectric phase or an orthorhombic ferroelectric phase, respectively, in an intermediate temperature range.     

Field emission and field ion microscope study of Ga,In and Sn on W: Structure,work function,diffusion and binding energy

Gallium, indium and tin were deposited on a tungsten tip by making a contact between the tip and these metals in the liquid state. The activation energies of diffusion of the adsorbates on tungsten were found to be 0.29 eV for Ga, 0.35 eV for In and 0.71 eV for Sn. The adsorbates were field-evaporated by gradually increasing a positive tip voltage by a small increment each time and the variation of the work function with the decreasing coverage was examined for each evaporation stage. The result indicates that the adatoms assume one of two different adsorption states. The adatoms bound as strongly as in a bulk crystal were field-evaporated at a low evaporation field. The remaining adatoms form a more strongly bound covering layer which maximizes the average work function of the covered surfaces, 4.75 eV for Ga, 4.63 eV for In and 5.10 eV for Sn, and are field-evaporated at a significantly higher field. The covering layer of the strongly bound adatoms were observed on the areas from the {001} to {114} planes and were hardly noticed on the {011} and 112 areas. The arrangement of the strongly bound adatoms, particularly on the {114} planes, is found to be a precise replica of the substrate arrangement. Thus, the surface density of the adatoms is exactly the surface density of the substrate atoms. The observed results suggest that an adatom occupying a tungsten lattice site and contacting four substrate tungsten atoms can establish unusually strong bonding with the substrate.     

Thermal faceting of the rutile TiO2(001) surface

Temperature dependent faceting of rutile TiO₂ surfaces cut to the (001) plane has been reported [Tait and Kasowski, Phys. Rev. B20 (1979) 5178]. By comparing LEED data to beam positions calculated for various sets of facet planes, the facet planes have been identified. The first ordered structure observed on annealing ion bombarded surfaces is composed of {011} facets with the facet planes in a (2 × 1) reconstruction. The high temperature structure produced on annealing above 1300K is best described as {114} facets; however, there are deviations of the observed LEED pattern from that calculated for {114} facets, possibly because of the presence of related planes. LEED data have now been obtained on the behavior of (110), (100), (011), (114), and (001) surfaces in UHV. The observed stability of TiO₂ surfaces can be related to the Ti ion coordination numbers in the surface plane as derived from stoichiometric terminations of the rutile lattice.     

Field desorption microscopy of the 〈111〉 trihedral angle of a reconstructed tungsten tip

The surface structure near the 〈111〉 trihedral angle, which forms in an electric-field-heated tungsten tip, is studied by field electron microscopy, continuous-mode field desorption microscopy, and high-temperature field evaporation microscopy. The shape and structure of the surface depend on the temperature, field, and time. The angle is formed by three {011} planes, with the (111) plane at its vertex being retained in the form of a triangle or a hexagon with randomly arranged atomic clusters. The edges between {011} faces represent long and narrow {112} planes having longitudinal or transverse steps. In the absence of field evaporation, the edges and angle sharpen, becoming monoatomic. Field evaporation from the angle or microprotrusions on the edges extends these edges and causes transverse steps to appear on them. The explanation of the changes in the shape and structure of the surface is based on considering the competition of surface diffusion, crystal growth in an electric field, and field evaporation.     

Thermal desorption spectroscopy of Ni,Cu, Ag and Au from W(110)

Ni, Cu, Ag and Au on W(110) in the submonolayer range are studied by thermal desorption spectroscopy with the goal of obtaining information on lateral interactions and on the phase state of the adsorption layer in the temperature range in which desorption occurs. Ni, Cu and Ag are found to desorb over a wide coverage range from the two-phase (vapor-condensate) region while Au desorbs only from the single-phase vapor region. Segments of the coexistence curve are determined. The desorption energies have the following limiting values: 4.35–4.95. 3.2–3.85, 2.8–3.55 and 3.3–4.1 eV for Ni, Cu, Ag and Au, respectively.     

Coverage dependent desorption measurements of COCu(001)

We report time-resolved electron energy loss spectroscopy measurements of the coverage-dependent kinetic parameters for the thermal desorption of CO from Cu(001). Large, parallel increases in the activation energy for desorption and desorption prefactor are observed as a function of coverage. The activation energy away from zero coverage differs significantly from the measured isosteric heat of adsorption. The significance of these results is discussed in terms of applicable statistical models. We deduce the existence of an unusual temperature dependence in the adsorbate configurational entropy.     

Work function of adsorption systems potassium on tantalum and on molybdenum

Work functions φ_hkl of thermally annealed and potassium covered tantalum and molybdenum as a function of potassium surface density on (011), (112), (100) and (111) planes of these metals have been measured using a field emission microscope. The measurements have been performed at liquid nitrogen temperature (immobile layers). The work function decreases linearly at first, then more slowly, passes through a minimum, and then attains a constant value. Quantitative data on the dependence of φ_hklon surface density of potassium, N_hkl, for tantalum, molybdenum and tungsten have been compared. The principal results of the observations are: (i) for K on Ta, Mo, and W the work function minimum exhibits no distinct dependence on the type of substrate, however, it proves to depend on crystallographic direction of the latter; (ii) the values of the high coverage limit work function are approximately equal for one type of metal planes; (iii) the values of the high coverage limit surface densities of potassium adsorbed on Ta(011), Mo(011) and W(011) surfaces are approximately equal to the surface density of the (011) plane of bulk potassium crystal.     

The work function reduction of the Pt field emitter

We have carried out the field emission experiments to measure the temperature dependence of the work function of Pt field emitter and found that the work function steeply decreases more than 2 eV by annealing at relatively low temperature above about 500 K in ultra high vacuum. The maximum reduction of the work function was 2.59 eV. The reduced work function was restored the original value of Pt clean surface by applying high voltage of only 20% of Pt evaporation field. The experimental results are tentatively interpreted in terms of the formation of complex cyanides on the emitter shank during the electrochemical etching in KCN solution and the surface diffusion of potassium atoms formed by the thermal decomposition of the complex cyanides to the emitter cap.     

Oxidation of tungsten single crystals in the presence of a high-electric field

The effects of high-electric fields on oxidation of tungsten single crystals in 6 × 10^?4 torr of oxygen at 1200–1500 °K were studied by field emission and transmission electron microscopy. Exposure of field emitters to oxygen in the absence of a field resulted in the build-up of emitter tips. Oxidation under the application of a negative or positive field, on the other hand, involved plane faceting and formation of oxide crystallites. Plane faceting was recognized to occur on the {111} and the {112} regions, showing the facetings of the {111} and the {112} planes into the {110} planes, whereas, crystallite formation seemed to take place selectively on the {100} regions. It was suggested by field emission microscopy that negative fields have an additional effect which causes the growth of an oxide crystal on the (110) plane. Transmission electron microscopy of an emitter oxidized in a negative field actually revealed a tiny oxide crystal with a size of ～ 300 Å grown on the developed (110) plane. The crystal exhibited a triangular shadow image strongly indicating an external pyramid-like form.     

Adsorption of silver on (001), (011) and (111) oriented tungsten single crystals

Work function changes caused by silver adsorption were measured on (001), (011) and (111) tungsten single crystal planes using electron beam method. The decrease of the work function was observed on all planes studied in the small coverage region. The result evidences that the increase previously observed for the average work function in this coverage region can not be connected with negative dipole formation with silver atoms.     

Coefficient of ultrasound transmission from liquid helium to aluminum in the intermediate state at <Emphasis Type="Italic">T</Emphasis> ≈ 0.1 K

The coefficient α(θ) of ultrasound transmission from liquid ⁴He to an aluminum single crystal in intermediate, superconducting, and normal states at a temperature T ≈ 0.1 K is measured as a function of the polar angle θ at the azimuthal angle φ = 0. The experimental technique is based on the measurement of the Kapitza temperature jump at the interface between two media. The dependences of the transmission coefficient for the Rayleigh modes on the sound frequency (in the range 13–194 MHz) and the magnetic field strength are determined. It is demonstrated that the integrated transmission coefficient of the aluminum single crystal in the intermediate state at an angle θ_R larger than the critical value increases with an increase in the magnetic field strength. The integrals of the transmission coefficient in magnetic fields close to the field H _c are independent of the frequency. In the vicinity of H = 0, the transmission coefficient at ν > 39 MHz increases only slightly with increasing frequency. At the lowest frequencies, the transmission coefficient increases anomalously as the frequency decreases. The experimental data are compared with the results obtained in the framework of the Andreev theory. Numerical calculations are performed and the dependences α(θ, φ) for bulk modes in the range corresponding to angles smaller than the critical value are constructed for the three principal planes of the crystal, i.e., the (001), (011), and (111) planes. The dependence α(θ) is obtained for the azimuthal angle φ = 0. The width of the Rayleigh peak is estimated.     

Real-time monitoring of the desorption process of chlorine-covered Si(111)-‘1×1’ surface with second-harmonic generation

To elucidate the fundamental process underlying the semiconductor surface fabrication, isothermal desorption from a Cl/Si(111)-‘1×1’ surface was monitored by means of second-harmonic generation with 1064 nm light. During the desorption, surface Cl-coverages were obtained in real time. The temperature dependence of the desorption rates revealed that the energy barrier against chloride desorption is 2.1 eV. A very slow second phase in the recovery of the second-harmonic intensity is associated with reconstruction of 7×7 DAS structure following the desorption. The activation energy for the reconstruction was 2.4 eV.     

Translational energy release in the recombinative desorption of H2 from Ag(111)

We have investigated the recombinative desorption of H₂ from Ag(111) using (2 + 1) REMPI to detect the desorbing molecules. We describe a method for determining the energy released into translational motion normal to the surface over a wide range of surface temperatures. This was achieved by using a weak electric field to disperse the ions, in contrast to the usual field-free method. Calibration and analysis methods are discussed and energy distributions P(E) are reported for H₂ (v=0). Desorption from a 295 K surface shows only a single low-energy peak ( ), whereas the translational energy release is bimodal for desorption from a 580 K surface, with mean desorption energies of approximately 140 meV and 1 eV for the two pathways. Sticking functions are calculated using detailed balance, revealing a large dependence on surface temperature.     

The field ionization characteristics of individual atomic planes

A detailed study of the field ionization characteristics of nine different planes of tungsten in the (001)-(011)-(111) standard triangle has been performed as function of tip temperature (T_T) between 11°K and 86°K, and local radius of curvature (rT). The measured helium ion current-voltage characteristic curve for each plane was found to consist of two distinct regimes. The first regime was linear on a log-log plot, and the slope varied from 27 to 41. A modified version of Gomer's model for the very low field ion current fitted the regime I data reasonably well. The second regime of each characteristic curve was quite complicated and exhibited several maxima and minima whose positions were functions of both T_T and crystallographic plane. A qualitative explanation for the behavior of the ion current in regime II was given in terms of a patch field model consisting of three dominant spatial regions on the surface of the field ion microscope specimen. In addition, the explanation also considered the role played by a lateral supply of gas atoms on the specimen's surface, and a slowly increasing field dependent radial supply function of gas atoms. It was also found that the probability of ionization was a strong function of both T_T, and crystallographic plane. An expression was derived for the temperature dependence of this effect which fitted the data for atomically smooth planes [e.g., the (011) plane]. Finally, the ion current from individual planes was proportional to a power of rT (at constant electric field) which varied between 2.3 and 2.9. This result was at variance with the existing theories of the supply function, and indicated that the shank of the specimen was a significant source of imaging gas atoms.     

Impact induced adsorption of C on silicon (001) surface

Molecular dynamics simulations were carried out to investigate the adsorption of a low-energy C₂₀ on a reconstructed silicon (001)-(2×12\times1) surface. The impact energies of the C₂₀ fullerene range from 1 eV/atom to 5 eV/atom. After impacting, the C₂₀ molecule is found to move along (011) direction and resides either in the trough or on the dimer at the end of our simulations. The lateral motion of C₂₀ on the surface is dependent on its incident energy. Chemical bonds are formed between C₂₀ and the surface. By the force field analysis, we show that the anisotropic molecule-surface interaction plays the leading role in the lateral motion of C₂₀ as well as its preferable adsorption sites on the dimerized Si surface. These findings are consistent with experimental observations of C₆₀ on Si (001) surface and small carbon clusters on solid surfaces.     

Optical second-harmonic investigations of the isothermal desorption of SiO from the Si(100) and Si(111) surfaces

The isothermal desorption of SiO from the Si(100) and Si(111) surfaces was investigated by means of optical second-harmonic generation (SHG). Due to the high adsorbate sensitivity of this method, desorption rates could be measured over a wide range from 10⁻¹ to 10⁻⁶ ML s⁻¹. From their temperature dependence between 780 and 1000 K, activation energies of E_A=3.4±0.2 eV and E_A=4.0±0.3 eV and pre-exponential factors of ν₀=10^16±1 s⁻¹ and ν₀=10^20±1 s⁻¹ for SiO desorption were obtained for Si(100) and Si(111), respectively. In the case of the Si(100) surface, a pronounced decrease of the first-order rate constants was observed upon increasing the initial coverage from 0.02 to 0.6 ML. The results are interpreted in terms of coverage-dependent oxygen-binding configurations, which influence the stability of the oxide layer.     

Effect of temperature on the laser initiation of pentaerythritol tetranitrate (PETN)

The temperature dependence of the probability of PETN explosion upon laser pulse initiation (1064 nm, 10 ns, 1–5 J) was examined. As the temperature increases from 393 K to 450 K, the initiation threshold lowers, with the initiation of open-surface samples occurring over the entire temperature range. It was concluded that the initiation mechanism involves thermal-and photoactivation steps. The activation energy for the thermal activation step was found to be 0.35 ± 0.05 eV.