Probe-hole Field Emission Microscope Research Articles

Field emission from a-GaN films deposited on Si (100)

Amorphous gallium nitride (a-GaN) films have been deposited on Si (100) substrates using ion-assisted deposition. The deposited films were characterised by X-ray diffraction (XRD) and atomic force microscopy (AFM). XRD confirms the amorphous nature of the films and AFM showed nanostructures in the films. The field electron emission from the film was obtained in a probe-hole field emission microscope, and the current–voltage ( I– V) characteristics were studied. The corresponding Fowler–Nordheim (F–N) plots showed a linear behaviour. A current density of 0.1 A/cm 2 has been obtained for 1.2 V/μm electric field. The field emission current–time ( I– t), curves were recorded at a current level of 500 nA for 3 h. The field emission behaviour is compared with that of crystalline GaN as reported in literature.

Adsorption studies of Cr on W ( [formula omitted]) plane by probe hole field emission microscopy

A probe hole field emission microscope has been used to study adsorption of chromium (Cr) on microscopic (1 1 0) plane of tungsten (W) tip. Adsorption is studied by measuring variation in workfunction ( Φ) with Cr deposition ( θ) at two temperatures, 722 and 300 K. For the 722 K tip, Φ decreases linearly by about 0.5 eV till completion of one monolayer and a very small increase is observed thereafter, indicating that the formation of Cr monolayer is by two-dimensional islanding. The average dipole moment associated with Cr–W pair is calculated to be 6.0×10 −31 C m. On a tip held at 300 K, Cr atom population on the (1 1 0) plane is low. Therefore, only a small (0.3 eV) decrease in Φ is observed. Also the ΔΦ– θ behaviour is found to be erratic, indicating crystallite formation on the (1 1 0) plane. These observations are compared with those of Berlowitz and Shinn [Surf. Sci. 209 (1989) 345], who used macroscopic W surfaces to study Cr/W system. The results are discussed in view of the existing literature.

Diffusion studies of nickel on the W(121) and the W(111) planes by probe-hole field emission microscopy

In a probe-hole field emission microscopy experiment, doses of nickel have been in situ deposited onto a thermally cleaned tungsten tip held at 300 K in clean ultra high vacuum conditions. The deposited doses of nickel were annealed successively at temperatures ranging from 340 to 2215 K to cause diffusion of the deposited amount on various tip surfaces. The changes in the work function (ΔΦ) and in the pre-exponential function (ln B) have been calculated from ten point Fowler–Nordheim (F-N) plots for the total tip surface, the W(121) and the W(111) surfaces at each successive stage of annealing. These variations are discussed in terms of diffusion of nickel atoms on the tungsten tip surface.

Adsorption of nickel on tungsten: a probe-hole field emission microscopy study

In a probe-hole field emission microscopy experiment, submonolayer amounts of high-purity nickel were deposited on to a tungsten tip in clean, ultra high vacuum conditions and observations were made on W(1 1 0), (1 2 1) and (1 1 1) planes. The changes in the work function, Δ φ, and corresponding pre-exponential factors, ln B, have been obtained by ten point Fowler–Nordheim plots. The results show that the change in average work function of nickel covered tungsten surface increases and attains a saturation value of +0.52 eV. This is accompanied by a decrease in the pre-exponential factor. Adsorption of nickel on the W(1 1 0) plane causes a decrease in Δ φ in the lower coverage region and then exhibits an abrupt change to reach a saturation value of +0.5 eV as the coverage increases. This has been attributed to surface reconstruction. In the case of the W(1 2 1) plane Δ φ decreases with the number of doses to a value of −0.45 eV. On the W(1 1 1) plane, the Δ φ increases and saturates to a value of +0.50 eV after a brief initial decrease in the low-coverage region. The changes in the work function are explained in terms of atomic and electronic structure of nickel overlayers on various single-crystal planes of tungsten. A comparison of nickel with iron and cobalt adsorption on tungsten shows that the behaviour of nickel is similar to iron.

Adsorption studies of cobalt on tungsten (110), (100) and (111) planes by probe-hole field emission microscopy

The changes in the workfunction, Δφ, and in the Fowler-Nordheim pre-exponential factor, ln B, have been investigated for cobalt adsorption on tungsten (110) (100) and (111) planes by probe-hole field emission microscopy. The workfunction variations have been found to be strongly dependent on the temperature in the range 300–580 K on these planes. Δφ decreases with submonolayer coverages of cobalt on the W(110) and W(100) planes. On the W(111) plane, Δφ initially decreases and then exhibits a jump towards positive values at relatively higher does. This abrupt increase in Δφ and the corresponding zero crossover of ln B have been attributed to surface reconstruction. The results are discussed in terms of the electronic structure and the degree of strain produced in the pseudomorphically grown cobalt layers on various crystallographic planes of tungsten.

Adsorption studies of iron on tungsten by probe-hole field emission microscopy

Probe-hole field emission microscopy has been used to study submonolayer and monolayer amounts of iron on various tungsten planes. The adsorption is found to be insensitive to the substrate deposition temperatures of 300, 600, and 1100 K as seen by the variation of the workfunction and the pre-exponential factor with the adsorbate doses. The workfunction of the adsorbate covered surface reduces in case of the (110), (100), and (112) tungsten planes, while it increases in case of the (111) and (116) planes, becoming coverage independent with a workfunction of 4.6 eV. This has been atributed to surface alloying of iron atoms with tungsten because of the structure of these two planes. The results are discussed in the light of the existing experimental results on metallic adsorbates on tungsten.

Oxygen-induced buildup of W(100) and W(111) planes

Adsorption and desorption of oxygen on the W (100) and the W (111) planes have been studied by using a probe-hole field emission microscope. When an emitter covered with adsorbed oxygen is gradually heated from 291K to 1800K, the minimum increments of work functions on the W(100) and W(111) planes are respectively ΔØ 100 min = -0.30 eV at 1750 K and ΔØ 111 min = -0.50 eV at 1700K. The experimental results on the W (100) and the W (111) planes are compared. And the physical explanation of the minimum work-function increments is given.

Field Emission Study of Germanium Thin Films on a Niobium Surface

The behaviour of germanium on atomically clean niobium single crystal planes has been studied by probe-hole field emission microscopy as well as by means of the spectral analysis of field emission current fluctuations. Variations of average work function, surface diffusion activation energies, and slopes of the spectral density function with different coverages and substrate temperatures are measured.

Probe-hole field emission microscope system controlled by computer

A probe-hole field emission microscope system, controlled by the Apple II computer, has been developed and operated successfully for measuring the work function of a single crystal plane. The detection screen, the phototube and the amplifier are combined for measuring the probe-hole current. The combination is calibrated and the calibrated data are used in the computer program. The high voltage on the viewing screen is adjusted by using a D/A converter. The total current and the probe-hole current are acquired by using an A/D converter. A program in BASIC is used for processing all data and the Fowler-Nordheim plot parameters are given. Work functions of single crystal planes can then be calculated; as examples, the work functions on the clean W(100) and W(111) planes are measured to be 4.67 eV and 4.45 eV, respectively.

Probe-Hole Field Emission Microscope System Controlled by Computer

A probe-hole field emission microscope system, controlled by an Apple II computer, has been developed and operated successfully for measuring the work function of a single crystal plane. The work functions on the clean W(100) and W(111) planes are measured to be 4.67 eV and 4.45 eV, respectively.

Adsorption and desorption of oxygen on the W(100) plane

Adsorption and desorption of oxygen on the W(100) plane has been studied by using a probe-hole field emission microscope with a computer system. When an emitter covered with adsorbed oxygen is gradually heated from 78 to 1800 K, the work function on the W(100) plane shows a minimum, 4.1 eV, at 1500–1600 K. This value is smaller than that of the clean W(100) plane (4.64 eV).

Field emission current fluctuations due to lithium adsorbed on the W(111) region

Field emission current fluctuations for lithium adsorbed on the tungsten (111) region were investigated by a probe-hole field emission microscope. The coverage dependence of the noise power as well as spectral density functions W(ƒ) at different temperatures were obtained. The spectral density functions have been analysed in terms of the Timm and van der Ziel concentration fluctuation model. Using Comer's method the surface diffusion energies and prefactors have been determined for submonolayer coverages of lithium. They are strongly coverage dependent. The activation energy varies non-monotonically between 0.41 and 0.53 eV and the prefactor between 2.4 × 10 −4 and 1.3 × 10 −2 cm 2/s in the Li surface concentration interval (0.5−3.7) × 10 14 cm −2. The results are compared with those obtained for the W(111)/K system.

Design and performance of a scanning probe-hole field emission microscope

A novel scanning field emission probe-hole microscope controlled by a computer is presented. Changes in work function can be measured continuously and simultaneously on the different single crystal surfaces present on a field emission tip under various experimental conditions such as temperature and gas pressure. Some tests experiments will be presented. The anisotropy of the work function over a rhodium field emitter is described. The particular attractiveness of the method is its unique ability to follow surface processes simultaneously on different surfaces with known surface structure. For example, the step density and the step structure can be varied systematically and their influence on processes like adsorption and reaction can be examined. Some possible applications are mentioned.

Adsorption of La on W using probe-hole field emission microscopy

The authors present results mainly on the variation of the work function phi ad, with the coverage theta of La on W, on the (011), (111) and (112) planes and the (161) region using probe-hole field emission microscopy. On the (011) plane, initially, phi ad decreases monotonically with an increase in the coverage theta , and attains the minimum at about theta =1. On the other planes, phi ad increases initially, and then decreases. Attempts have been made to estimate the adsorbate-substrate dipole moment and the charge transfer between them on the (011) plane. The results are discussed in the light of the geometrical structure of the adsurface.

Field emission current fluctuations due to lithium adsorbed on the W(111) region

Abstract Field emission current fluctuations for lithium adsorbed on the tungsten (111) region were investigated by a probe-hole field emission microscope. The coverage dependence of the noise power as well as spectral density functions W(ƒ) at different temperatures were obtained. The spectral density functions have been analysed in terms of the Timm and van der Ziel concentration fluctuation model. Using Comer's method the surface diffusion energies and prefactors have been determined for submonolayer coverages of lithium. They are strongly coverage dependent. The activation energy varies non-monotonically between 0.41 and 0.53 eV and the prefactor between 2.4 × 10−4 and 1.3 × 10−2 cm2/s in the Li surface concentration interval (0.5−3.7) × 1014 cm−2. The results are compared with those obtained for the W(111)/K system.

Design and performance of a scanning probe-hole field emission microscope

Design and performance of a scanning probe-hole field emission microscope

SURFACE DIFFUSION OF LITHIUM ON THE W(112) PLANE BY FIELD EMISSION SPECTRAL DENSITY ANALYSIS

The spectral density functions W(f) of the field-emission flicker noise of lithium adsorbed on the tungsten (112) plane were determined at different temperatures in a probe-hole field emission microscope. By an analysis in terms of the Timm and van der Ziel model the surface diffusion coefficients have been obtained and their temperature dependence reveals the surface diffusion energies. For the two coverages of c = 0.38 and 0.75 investigated their values could be determined to be Q = 0.56 and 1.08 eV, respectively. The diffusivity prefactors were also calculated. The results are compared with those obtained for the W(112)K system.

METAL-SEMICONDUCTOR INTERFACE (Al-Si)

The microscopic structure of metal-semiconductore interface and the behavior of Al on a p-Si (110) surface have been investigated using the probe-hole field emission microscope (FEM), the field desorption (FD) and the field emission retarding potential analyser (FPA). The Fowler-Nordheim (FN) plots for the Si tip prepared by electrolytic etching are classified into two groups, the curve and the line. The work function of Si surface by an adsorption of Al decrease with the θ (Al) and the value reaches 3.2 ± 0.2 eV at θ (Al) = 1. A surface resistance of giga ohms which is difficultto remove at usual FD field have been observed for the Si tip having the curved FN plot. This layer can easily be metallized by the interdiffusion of Al at room temperature.

Adsorption of sulfur on a molybdenum field emitter

The adsorption system molybdenum/sulfur has been studied with a probe-hole field emission microscope. Elemental sulfur (S 2) impinging onto a clean molybdenum surface at room temperature is captured with high efficiency in a mobile precursor state. During dosing spreading of sulfur is anisotropic, with the highest rate occurring near the 〈111〉 zones and on the {110} vicinals. The precursor is transformed into chemisorbed sulfur on clean molybdenum surfaces of any orientation. On the saturated chemisorption layer additional sulfur can be physisorbed. On several surface orientations reaction to molybdenum sulfide occurs at slightly elevated temperatures. Work function measurements of the clean molybdenum surface result in φ 110 = 5.0 ± 0.15 eV, φ 100 = 4.70 ±0.03 eV, φ 211 = 4.64 ± 0.03 eV, φ 111 = 4.18 ± 0.01 eV and φ 544 = 4.28 ± 0.04 eV based on an average work function φ = 4.2 eV. In general, adsorption of sulfur increases the work function. After heating to 1000 K which effects decomposition of molybdenum sulfide the saturated chemisorption layer increases the work function by Δφ = 0.8−1.2 eV depending on the orientation. On heating to higher temperatures, work function changes indicate substrate rearrangement on several orientations and gradual desorption. Complete desorption occurs between 1680 K ({111} face) and 1800 K ({110} and {100} faces). Diffusion of chemisorbed sulfur exhibits distinct face specifity. The temperatures where migration becomes observable range from 320 K at the 〈111〉 zones up to 500–600 K near the {100} faces. Activation energies of diffusion in the low-concentration chemisorption layer from 56 kJ/mol on the {110} vicinals up to 142 kJ/mol on the {100} faces have been determined.

Field emission spectroscopy of bismuth adsorbed on the W(100) surface

Adsorption of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe hole field emission microscopy and electron spectroscopy. Sub-monolayer bismuth forms a relatively strongly bound layer of bismuth-tungsten dipoles with dipole moment μ = (0.18 ± 0.02) × 10 −30C m and polarizability α = (6.3 ± 1.3) × 10 −40J V −2m 2. Changes in work function and their dependence on temperature closely parallel those produced by adsorbed lead which was shown by LEED to form c(2 × 2), p(2 × 2) and (1 × 1) structures. Bismuth is thought to behave in a similar way, but, unlike lead, forms a second monolayer which replicates the first. Electron spectroscopy shows that sub-monolayer bismuth removes the surface state (Swanson) peak and at monolayer coverage a new peak emerges and shifts with increasing coverage. Using Gadzuk's theory, this peak is tentatively attributed to the 2P level in bismuth adatoms which form a p(2 × 2) structure in the first and second monolayers. Its shift with coverage is ascribed to an increase in the local surface field. There remains the difficulty of reconciling the proposed occupation of the 2P level with the observed small positive charge on the bismuth adatom.