Contact Potential Measurements Research Articles

Computer simulation of actual and Kelvin-probe-measured potential profiles: Application to amorphous films

We have employed a first-principles computer program to simulate actual potential profiles and potential profiles obtained experimentally from the Kelvin probe technique. Our results show that the Kelvin probe technique can be used as an in situ method for qualitatively characterizing amorphous films and structures during deposition. However, without additional information about the density and energy distribution of localized states in the bulk and at the free surface, we find that it is difficult to obtain, from the Kelvin probe measurement alone, an accurate quantitative measure of the actual spatial dependence of band bending in films and device structures. In addition, this computer analysis shows that interpretation of the profiles obtained from the Kelvin probe contact potential method can lead to misleading conclusions about the density of states in these amorphous materials. We also show that the barrier height of Schottky contacts or the built-in potential of junctions of differently doped layers can be obtained directly from Kelvin probe contact potential measurements only for films whose free-surface band bending is known from other experimental data. We see that this surface band bending can be significant even for small free-surface-state densities and is more pronounced in higher-quality films.

Photohole induced adsorption of oxygen on CdS

In the absence of illumination, adsorption of oxygen on clean CdS surfaces is not detected by XPS, nor unequivocally by SIMS, although contact potential measurements indicate that reversible changes in work function do occur. When exposure to oxygen is combined with bandgap illumination however, adsorption takes place at a rate of at least 5 orders of magnitude faster and is accompanied by the formation of sulfur oxyanions. Since this effect is not dependent on dark electron densities, we propose that the availability of holes in these n-type crystal is the rate limiting step for adsorption to occur. This can be reconciled with models formulated in the context of electrical measurements involving the transfer of electrons to the adsorbate.

Photoemission yield spectroscopy of electronic surface states on germanium (111) surfaces

The electronic surface states of cleaved and annealed Ge(111) surfaces have been investigated by photoemission yield spectroscopy and contact potential measurements on a set of differently doped samples. On the 2 × 1 cleaved surface, a surface state band centered about 0.7 eV below the valence band maximum is found. The variations of the work function with the doping level show that an empty surface state band exists above the Fermi level. After annealing at temperatures of the order of 350°C, this surface exhibits a 2 × 8 superstructure. A new surface state band is then found closer to the valence band maximum. This variation of the surface state distribution is correlated to a change in the surface potential. The variation of the electronic characteristics upon oxygen adsorption are also reported and an evaluation of the sticking coefficient is made for both structures.

The room temperature oxidation of Cu (111): Pressure effects

Ellipsometry has been used to follow the oxidation of the (111) face of a copper single crystal at 21 °C for oxygen pressures from 0.03–760 Torr. The pressure dependence is complex, with the short term oxide film thickness increasing to a limiting value as pressure increases, whereas the long term thickness goes through a maximum as pressure increases. The low pressure growth kinetics, as well as previously published contact potential measurements, can be described using the Cabrera–Mott theory for the formation of very thin oxide films modified to include the effects of space charge in the oxide. The rate determining step is taken to be the formation of cation vacancies at the gas–oxide interface. It is suggested that a monolayer of CuO forms on top of the Cu2O film at high oxygen pressures with a concomitant chance in mechanism.

Characterization of oral in vivo films formed on different types of solid surfaces

Studies were made of oral films formed in vivo, which had been allowed to form on fused silica and Ge-prisms during periods between 2 s and 2 h using a variety of physico-chemical methods. To produce surfaces of different qualities the silica and Ge-prisms had either been detergent-washed, glow discharge treated or covered with polydimethylsiloxane. The following simultaneous analytical techniques were performed on the adsorbed films: a. internal reflection infrared spectroscopy, b. ellipsometry, c. contact potential measurements, d. contact angle measurements, e. scanning electron microscopy and f. energy-dispersive x-ray analysis. The results of these studies show that the formation of oral films proceeds at high speed and is of a certain qualitative selectivity. The formed films were found to be stable over long periods of time, and only showed patches of adhering micro-organisms on some of the prisms which had been exposed in the oral cavity for 2 h.

Piezoelectric driven Kelvin probe for contact potential difference studies

A convenient piezoelectric driven Kelvin probe for the measurement of work function changes is described. The probe has a simple construction, is small, bakeable, and can be mounted on any 35-mm-i.d. UHV flange. As the piezoelectric device is inside the vacuum chamber, only one electrical feedthrough is needed for operating the probe. The distance between reference electrode and sample can be varied within ±0.5 mm simply by applying a dc voltage to the piezoelectric device in addition to the ac oscillator-driving voltage, not requiring any mechanical driving and feedthrough provisions. The probe was tested studying the adsorption of O2 on Pt(111). The sensitivity for contact potential measurements is better than 0.1 mV at a response time of 10 sec.

Adsorption of hydrogen on a Pt(111) surface

The H 2/Pt(111) system has been studied with LEED, ELS, thermal desorption spectroscopy and contact potential measurements. At 150 K H 2 was found to adsorb with an initial sticking coefficient of about 0.1, yielding an atomic H:Pt ratio of about 0.8:1 at saturation. H 2/D 2 exchange experiments gave evidence that adsorption is completely dissociative. No exrea LEED spots due to adsorbed hydrogen were observed, but the adsorbate was found to strongly damp the secondary Bragg maxima in the I/ V spectrum of the specular beam. The primary Bragg maxima were slightly increased in intensity and shifted to somewhat lower energy. A new characteristic electron energy loss at −15.4 eV was recorded upon hydrogen adsorption. The thermal desorption spectra were characterized by a high temperature (β 2-) state desorbing with second order kinetics below 400 K and a low temperature (β 2-) state that fills up, in the main, after the first peak saturates. The β 2-state is associated with an activation energy for desorption E∗ of 9.5 kcal/mole. The decrease E∗ with increasing coverage and the formation of the β 1-state are interpreted in terms of a lateral interaction model. The anomalous structure in the thermal desorption spectra is attributed to domains of non-equilibrium configuration. The work function change Δϑ was found to have a small positive maximum (∼ 2 mV) at very low hydrogen doses (attributed to structural imperfections) and then to decrease continuously to a value of −230 mV at saturation. The variation of Δϑ with coverage is stronger than linear. The isosteric heats of adsorption as derived from adsorption isotherms recorded via Δϑ compared well with the results of the analysis of the thermal desorption spectra.

Contact potential measurements on thin SIO 2 films : M. Bess, R. Oswald and M. Ohring. Solid St. Electron.17, 813 (1974)

Contact potential measurements on thin SIO 2 films : M. Bess, R. Oswald and M. Ohring. Solid St. Electron.17, 813 (1974)

Contact potential measurements on thin SiO 2 films

Contact potential measurements by the Kelvin method were performed in vacuum on silicon wafers whose thermal oxide film was etched into the shape of a wedge. A given position along the oxide corresponded directly to a given depth and by scanning a reference electrode stripe across the wafer, information about the distribution of the oxide charge through the thickness was obtained. It was found that the oxide charge was positive and concentrated within a few hundred angstroms of the SiSiO 2 interface.

Adsorption of CO on Pd single crystal surfaces

Studies of CO adsorption on Pd(110), (210) and (311) surfaces as well as with a (111) plane with periodic step arrays were performed by means of LEED, contact potential and flash desorption measurements. Isosteric heats of adsorption were evaluated from adsorption isotherms. Earlier work with Pd(111) and Pd (100) surfaces is briefly reviewed, yielding the following general picture: The initial adsorption energies vary between 34 and 40 kcal mole and close similarities exist for the dipole moments, the maximum densities of adsorbed particles and for the adsorption kinetics. At low and medium coverage the adsorbed particles are located at highly symmetrical adsorption sites, whereas saturation is characterized by the tendency for formation of close-packed layers.

Adsorption of hydrogen on palladium single crystal surfaces

The adsorption of hydrogen on clean Pd(110) and Pd(111) surfaces as well as on a Pd(111) surface with regular step arrays was studied by means of LEED, thermal desorption spectroscopy and contact potential measurements. Absorption in the bulk plays an important role but could be separated from the surface processes. With Pd(110) an ordered 1 × 2 structure and with Pd(111) a 1 × 1 structure was formed. Maximum work function increases of 0.36, 0.18 and 0.23 eV were determined with Pd(110), Pd(111) and the stepped surface, respectively, this quantity being influenced only by adsorbed hydrogen under the chosen conditions. The adsorption isotherms derived from contact potential data revealed that at low coverages θ ∞ √ p H 2 , indicating atomic adsorption. Initial heats of H 2 adsorption of 24.4 kcal/mole for Pd(110) and of 20.8 kcal/mole for Pd(111) were derived, in both cases E ad being constant up to at least half the saturation coverage. With the stepped surface the adsorption energies coincide with those for Pd(111) at medium coverages, but increase with decreasing coverage by about 3 kcal/mole. D 2 is adsorbed on Pd(110) with an initial adsorption energy of 22.8 kcal/mole.

Sodium adsorption on aluminum (100) and (111) surfaces: ELEED, Auger, and contact potential measurements

The adsorption of Na on sputter-cleaned Al(100) and Al(111) single-crystal surfaces has been investigated under ultrahigh vacuum conditions. Sodium was deposited with an apertured ion source permitting quantitative dosage determination. Low energy electron diffraction patterns show well-ordered coherent structures corresponding to 1 2 monolayer on both surfaces, and 1 3 monolayer on Al(111). A hexagonal pattern corresponding to 7 8 monolayer occurs at a nonconforming dosage on Al(100), indicating a reduction in effective sticking coefficient. This result is verified by Auger intensity measurements, which show saturation beginning at 1 2 monolayer on both surfaces. Changes in contact potential with exposure were inferred from shifts in the low-energy cutoff of secondary electrons. The initial dipole moment is in good agreement with that on transition metal substrates, while the saturation value of the contact potential corresponds closely to the difference between reported work functions for bulk Al and bulk Na. A true minimum in the work function was not observed. Present results are compared with results obtained from transition metal substrates. Essential differences are attributed to the more metallic character of the bonding in the Al:Na system.

Abstract: Sodium adsorption on aluminum (100) and (111) surfaces: ELEED, Auger, and contact potential measurements

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Abstract: Contact potential measurement of the charge distribution in thin SiO2 films

Abstract: Contact potential measurement of the charge distribution in thin SiO2 films

The formation of very thin oxide films on metals: contact potential measurements during the oxidation of different crystallographic faces of copper: E Boggie, J Chem Phys, 57 (11), 1 st Dec 1972, 4738–4742

The formation of very thin oxide films on metals: contact potential measurements during the oxidation of different crystallographic faces of copper: E Boggie, J Chem Phys, 57 (11), 1 st Dec 1972, 4738–4742

On the Formation of Very Thin Oxide Films on Metals: Contact Potential Measurements During the Oxidation of Different Crystallographic Faces of Copper

The low temperature oxidation of copper is treated in terms of a model in which the surface reaction involving the formation of copper ion vacancies at the gas-oxide interface is rate controlling. The predictions of the model are compared with experimental observations of the change in contact potential difference when clean (100), (110), and (111) copper surfaces are oxidized at room temperature. A value of 0.75±0.03 eV for the activation energy for formation of a copper ion vacancy is derived from the process of fitting experiment to theory. The interpretation of the observed changes in contact potential difference is made in terms of surface states located at the metal-oxide interface and at the gas-oxide interface. A reaction rate anisotropy that is in agreement with the observations of Young, Cathcart, and Gwathmey follows naturally from the model.

Molecular beam study of the desorption of cesium ions from tungsten crystals

The rates of desorption of Cs + from W(100) and W(110) crystals have been measured by the modulated molecular beam method. The Cs coverage was extremely low (≲ 5 × 10 11 cm −2), thereby insuring that: (a) the desorption rate of Cs atoms was negligible relative to that of Cs ions; (b) the desorption rate depended primarily on the interaction of Cs with W rather than the interaction of Cs with Cs. Assuming that the dependence of the Cs + desorption rate on coverage n and temperature T may be represented by the form R + = nν + exp( − χ0 + kT ) , we determined ν + and χ0 + from the measured rates. For W(100) in the range 900–1250 K, ν + = (1.0 ± 0.2) × 10 13 sec −1 and χ0 + = 2.05 ± 0.02 eV; for W(110) in the range 880–1050 K, ν + = (1.7 ± 0.4) × 10 12 sec −1 and χ0 + = 1.77 ± 0.02 eV. The results are compared with existing experimental data and theoretical predictions. The predictions of the image potential model appear to be consistent both with the binding energies inferred from the present results and with the dipole moments inferred from previous contact-potential measurements for Cs on W(100) and W(110) crystals.

Adsorption of Oxygen on the (100) Plane of Tungsten

The effect of oxygen adsorption on the (100) tungsten surface has been investigated in a thermionic electron emission microscope and by means of contact potential measurements (Kelvin method). Surface contamination was closely controlled. The measurements indicate that the work function versus oxygen pressure curves agree with those of Batzies. The oxygen desorption energy decreases with increasing oxygen coverage. The initial sticking coefficient S0, slope (dΔφ/dn)0 and dipole moment μD deduced from the experimental results are S0=0.17, d(Δφ/dn)0=4.3×10−15 eV at−1 cm2, μD=2.2 D.

Contact potential measurements on “clean” CdS surfaces

Contact potential difference (cpd) measurements were carried out on {112̄} surfaces of high conductivity CdS cleaved in ultrahigh vacuum. A modified Kelvin method was employed. A change in cpd upon illumination with white light (surface photovoltage) was observed, indicating the presence of intrinsic surface states in these surfaces. Long exposure (about 24 hr) to water vapor (2 × 10 −8 torr) caused an increase of about 1.0 V in cpd and a reduction in surface photovoltage. This increase in cpd was attributed primarily to a decrease in the electron affinity of the surfaces brought about by the molecular dipole moment of physically adsorbed water molecules. The “clean” surfaces exhibited no appreciable affinitity for spectroscopically pure oxygen, except under white light illumination apparently due to the increase of the density free electrons at the surface.

Oxygen chemisorption on copper single crystals

Contact potential measurements for clean, single crystal surfaces of copper compared with a gold polycrystalline reference surface have shown that the values of (ø cu− ø au) for the (111), (100) and (110)Cu planes are +0.84±0.019, +0.455 ±0.054 and +0.22 ± 0.019 eV respectively. The interaction of oxygen with these planes, at room temperature, leads to a work function increase in all cases. These increases are 0.125, 0.39, 0.675 eV for the (111), (100) and (110) planes respectively. Oxygen adsorption at 77°K leads to values essentially similar to the room temperature values, except in the case of the (110) plane, where the work function increases by only 0.36 eV. Warming such a (110) surface to room temperature allows further oxygen adsorption and a final work function increase of 0.63 eV. There is evidence that oxygen is being incorporated in the (110) surface at room temperature. The behaviour of oxygen on these surfaces is reconciled by means of a “hole” theory. Adsorption of carbon monoxide on the (111), (100) and (110) planes leads to a work function increase of 0.01 eV, 0.11 eV and 0.02 eV respectively, with little difference between the low temperature and room, temperature values.