Ch. Kleint

Surface diffusion model of adsorption-induced field emission flicker noise: I. Theory

Abstract For the interpretation of flicker noise in field emission occurring by surface diffusion of adsorbed molecules an adsorption model is analyzed for a homogeneous surface. Beside the weaker binding adsorption sites of the undisturbed surface a second stronger binding sort of sites is assumed taking thus into account the well-known structure irregularities of annealed emitters. The admolecule transition rate K between these sites derived from statistical and kinetic arguments determines essentially the noise by modulation of the microscopic work function; therefore, the K-dependence on temperature and on the degree of coverage is presented and discussed. The spectral density function is determined by use of the Bess formalism originally developed for a semiconductor current noise. The frequency and current dependences of the resulting flicker noise are analyzed. The model is extended to the case of surface diffusion noise at desorption-adsorption equilibrium.

Electron emission noise

Abstract Traced back to the early discoveries by Schottky and Johnson, a review will be given on the present state of electron emission noise phenomena and related fluctuations. Special emphasis is laid upon adsorbate induced noise under UHV conditions which can be investigated, at least in principle, down to single adparticle fluctuations. On the other hand, with increasing submonolayer coverage, adparticle interactions can play an important role which might even lead to collective phenomena. Different noise sources and models to describe them will be discussed. The examples for illustration are mainly taken from field emission investigations and will cover shot noise, prediffusion adparticle flip-flop, Brownian motion or concentration fluctuations in a patch, tunneling diffusion, trap and surface state governed emission noise and indications of phase transitions and soliton or domain wall movements. Some relations to current noise in solid state devices and to other noise sources and their characteristics will be mentioned.

Surface diffusion model of adsorption-induced field emission flicker noise: II. Experiments

Abstract Field emission measurements of the diffusion-induced flicker noise of adsorbate covered tungsten emitters are described with special emphasis on spectral density functions and dependence of the noise-power on emission current, degree of coverage, and tip temperature. Beside potassium and barium, the investigations included also N2, CO, and H2 as adsorbates. The results are discussed in terms of the theory presented in part I. The main noise properties are describable by the surface-diffusion model of flicker noise; the diffusion and desorption energies derived are not in contradiction with other investigations. The patch model by Timm and Van der Ziel is also discussed, the required frequency dependence of the spectral density function could not be observed in most cases, however.

Photo field-emission spectroscopy of optical transitions in the band structure of rhenium

Photo field-emission currents of clean and barium-covered tungsten tips in a sligthly modified FEM-configuration have been measured under UHV-conditions by modulating monochromatic mercury arc radiation and phase-sensitive detection. From Fowler-Nordheim plots of the field-emission currents field strength and work function were determined as usual. The photo field-emission current—voltage characteristics show various slopes and shoulders which are dependent on excitation energy, work function, and applied field. The shoulders are interpreted to indicate optical transitions, whose final and initial energies are evaluated from the excitation energy and the Schottky lowering of the barrier maximum. A comparison with the band structure of tungsten as calculated by Mattheiss or Christensen and Feuerbacher demonstrates that apart from single nondirect transitions from or to band extrema direct (k-conserving) transitions are prevailing. Some properties of this new method for the investigation of band structure details are given.

Coverage dependence of fe flicker noise: Spectral density functions due to potassium on W(112) and W(111)☆

Abstract Field emission flicker noise spectral density functions W (ƒ) have been determined for potassium on W(112) and W(111) single planes. The coverage dependence of the spectral densities W (ƒ j ) shows pronounced maxima and minima, whereas the slopes ϵ obtained from double logarithmic plots of W (ƒ) ~ ƒ −ϵ vary considerably. Minima and maxima of W (ƒ j ) are assumed to be due to coherent and disordered adlayers, respectively, and the behaviour of the exponents ϵ supports further the proposed observation of order-disorder transitions of the potassium adsorbate. LEED results for W(112)K and W(111)K are in fair agreement with the corresponding coverages from noise measurements.

Cross-correlation function of field emission flicker noise from K/W(110): Interpretation by adparticle surface diffusion

Abstract The cross-correlation function of field emission current fluctuations from the (110) vicinals of a tungsten emitter with adsorbed potassium has been measured for coverages θ within 0.1⩽ θ ⩽1.0 and temperatures T between 290 and 430 K. Gomers autocorrelation analysis for the determination of surface diffusion coefficient is adapted to the interpretation of the cross-correlation function. Diffusion coefficients and activation energies Ed of potassium diffusion were computed with this model. The maximum of Ed is 0.58 eV at θ = 0.15 and Ed then decreases with θ to an asymptotic value of 0.3 eV at θ = 0.5 . “Velocities of diffusion” are calculated for some experiments. They cover a wide range from 3 × 10−3 to 3 cm s−1 in this work and between 10−8 and 10−5 cm s−1, elsewhere. However, they cannot be interpreted by velocities of elastic waves which were evaluated for several approximations of a two-dimensional gas and are between 105 and 106 cm s−1.

Activation energy of field emission flicker noise power due to adsorbed potassium on tungsten

Abstract The temperature dependence of the field emission flicker noise spectral density functions has been investigated for potassium adsorbed on tungsten (112) planes by a probe hole technique. By integration of the spectral density functions W(ƒ) = B i ƒ −ge i the noise power (δn 2 Δƒ for different frequency intervals Δƒ is obtained. From the exponential temperature dependence of (δn 2 Δƒ noise power “activation energies” qΔƒ are determined. Plots of these energies versus coverage show a similar “oscillating” behaviour as recently found for W(ƒ j ) or (δn 2 Δƒ j which indicates phase transitions of the adsorbed potassium submonolayers. The noise activation energies are discussed in terms of existing models and a comparison is made between the experimental q values and surface diffusion energies Ed as determined by conventional methods.

Comparison of experimental noise spectral densities of K-covered tungsten emitters with field-emission flicker-noise theories

Abstract Spectral-density functions W(f) (f is the frequency) of the field-emission flicker noise were investigated for various emission currents with sealed-off field-emission microscopes of the Muller type. The tungsten emitters were covered with potassium in order to observe the surface diffusion-induced flicker noise. The constancy of the coverage for each set of measurements was secured with special precautions. The main object of the experiments is the exponent e of the spectral density W(f) ∼ f-e. An f-1.5 dependence as required by the theory of Timm and Van der Ziel and an increase of e with the emission current could not be detected. Therefore, the experimental results seem to be in better agreement with the surface-diffusion model of diffusion-induced field-emission flicker noise as suggested by Kleint.

On the history of photoemission

While field emission proves to be the oldest surface science, photoemission is certainly one of its most important branches. We will describe, in some detail, the early development of this field from the scarcely understood photoelectric observations to the commonly accepted and applied photoemission process, in the course of which very important fundamental physical discoveries were involved. Since the early sixties, research contributions have multiplied and photoemission spectroscopy has become a major investigative technique in solid state and surface physics as well as chemistry. Because of its recent rapid growth, the latter aspect is only discussed in the final overview.

Thermal desorption spectroscopy on silicon

Abstract Traced back to the early work on Thermal Desorption Spectroscopy (TDS) a short introductory survey is given on the experimental method, basic relations and the further development of evaluation procedures. Ad- and desorption of hydrogen on silicon as a seemingly simple system is discussed chiefly in terms of TDS results. Though the main H 2 desorption phase is apparently of 2nd order the expected peak temperature ( T p ) shift is often missing and in addition minor phases appear. Ethene adsorption also reveals the main H 2 phase from a monohydride state with a similar T p behaviour and similar desorption energies from Arrhenius plots. For the more complex methanol adsorption, various spectra are given and discussed in view of the aforesaid systems. By means of deuterated methanol adsorption the evolution of the differently bonded “hydrogen” is studied. A simple one-phase hydrogen desorption cannot explain the obtained parameters and features.