Cathodic bias

Hole Injection under Cathodic Bias without Illumination... 

A p-type electrode is in depletion if a cathodic bias is applied. Illumination generates one electron per absorbed photon, which is collected by the SCR and transferred to the electrolyte. It requires two electrons to form one hydrogen molecule. If the photocurrent at this electrode is compared to that obtained by a silicon photodiode of the same size the quantum efficiencies are observed to be the same for the solid-state contact and the electrolyte contact, as shown in Fig. 4.13. If losses by reflection or recombination in the bulk are neglected the quantum efficiency of the electrode is 1. 

A sufficiently anodic bias and the availability of holes are the two necessary conditions for the dissolution of silicon aqueous HF. In this case the Si dissolution rate is proportional to the current density divided by the dissolution valence. In all other cases silicon is passivated in HF this is the case under OCP, or under cathodic conditions, or under anodic conditions if the sample is moderately n-type doped and kept in the dark. If an oxidizing agent like HN03 is added silicon will already dissolve at OCP, but the dissolution rate remains bias dependent. If an anodic bias is applied the dissolution rate will be enhanced, whereas a cathodic bias effectively decreases the rate of dissolution. 

For micro PS a decrease in the specific resistivity by two or three orders of magnitude is observed if the dry material is exposed to humid air [Ma8] or vapors of polar solvents, e.g. methanol [Be6]. This sensitivity of PS to polar vapors can be used to design PS-based gas sensors, as discussed in Section 10.4. This change in resistivity with pore surface condition becomes dramatic if the pores are filled with an electrolyte. From the strong EL observed under low anodic as well as low cathodic bias in an electrolyte it can be concluded that micro PS shows a conductivity comparable to that of the bulk substrate under wet conditions [Ge8]. Diffusion doping has been found to reduce the PS resistivity by more than five orders of magnitude, without affecting the PL intensity [Ell]. 

Fig. 10.6 A p-type Si wafer with a 20 nm thick thermal oxide has been contaminated by scratching the backside with metal wires (Ni, Cu, Fe), according to the pattern shown in (a) and later annealed at 1200°C for 30 s. (e) Under cathodic bias in acetic acid, oxide defects become decorated by hydrogen bubbles. (c, d) After oxide removal junction defects caused by metal precipitates are decorated by hydrogen bubbles, if sufficient catho...

Figure 17. Degree of isotropy observed in silicon etching in CI2/CIF3 plasma as a function of CIF3 content of the feed. Conditions are 5 seem total flow, 0.02 Torr pressure, 100 W rf with 30 V cathode bias at a frequency of 13.6 MHz. (Reproduced with permission from Ref. 39J...

As shown in Fig. 5, the current density for HER on p-Si, in the dark, at cathodic bias voltage is almost equal to zero, in fact it hardly reaches a limiting value near RiSO nA cm even at —l.OVvs. SCE. It is interesting to recall that such a small value for p-Si will provide the favorable conditions for the measurement of electrochemical impedance diagrams. Naturally, this limiting current is classically attributed to the limiting diffusion flux of the mobile electron minority carriers in the space charge layer [6]. 

Fig. 43. Double-logarithmic plot of the electrode polarization resistance versus the microelectrode diameter measured with impedance spectroscopy (ca. 800 °C) at (a) a cathodic dc bias of -300 mV, and (b) at an anodic dc bias of +300 mV. In (b) the first data point of the 20-pm microelectrode is not included in the fit. (c) Sketch illustrating the path of the oxygen reduction reaction for cathodic bias, (d) Path of the electrochemical reaction under anodic bias the rate-determining step occurs close to the three-phase boundary.

Bias-dependent measurements were performed in order to check to what extent the mechanism depends on the electrical operation conditions. Fig. 43 shows double-logarithmic plots of the electrode polarization resistance (determined from the arc in the impedance spectrum) versus the microelectrode diameter observed at a cathodic bias of —300 mV and at an anodic bias of +300 mV respectively. In the cathodic case the electrode polarization resistance again scales with the inverse of the electrode area, whereas in the anodic case it scales with the inverse of the microelectrode diameter. These findings are supported by I-V measurements on LSM microelectrodes with diameters ranging from 30-80 pm the differential resistance is proportional to the inverse microelectrode area in the cathodic regime and comes close to an inverse linear relationship with the three-phase boundary (3PB) length in the anodic regime [161]. 

Under cathodic bias, the chemical dissolution is accompanied by electrochemical reactions, which reduce Si dissolution. Under slightly anodic potentials, the... 

An example of the size of the impurity effects that may arise is shown in Fig. 1, which gives the electrode kinetics for the ferro-ferricyanide reaction on three different zinc oxide single crystals of varying conductivity. Each of the crystals was in excess of 99.999% pure. As can be seen, each crystal gives a linear Tafel plot under cathodic bias. However, the exchange currents, i.e, the extrapolations back to the reversible potential (+. 19 volts), differ by a factor of about 1000 and... 

Fig. 7. The hole current (Ip) as a function of the total current across a germanium electrode under cathodic bias.

Fig. 5. Typical /- U curves of an n-type semiconductor electrode in darkness and under illumination. The band diagrams illustrate the shift of the band edges with illumination (right) and at cathodic bias (left).

Fig. 6. Comparison between tunneling conditions on a semiconductor in vacuum (top) and in the electrolytic environment (bottom). In vacuum empty states (a) and occupied states (b) are imaged with a negative and positive tip respectively. In the liquid the position of band edges is fixed with respect to the tip Fermi level a cathodic bias stabilizes the tip above the n-type electrodes and occupied states are imaged in (c). Under depletion the tip comes into contact (d). Arrows refer to the direction of tunneling electrons.

By AFM, Uosaki et al. have imaged p-InSe in acidic medium [22] and in air [167]. Figure 35 shows that the topmost layer of atoms (presumably Se) can be resolved at potentials cathodic relative to the rest potential, just as in air. When the potential is anodic of the open circuit, images become noisier by formation of insoluble oxidized Se layer (In is dissolved as InOH " ) [22]. Returning the potential to negative values restores atomic resolution. Prolonged anodic polarization makes however the Se layer too thick to be dissolved at cathodic bias and atoms are not seen any more. 

Fig. 67. Bands for the Si-H stretching vibration in FTIR spectra of an n-Si(lll) surface at cathodic bias (200/iA cm ). The oxidized surface, where the Si-H band is absent, was taken as a reference, (a) Spectra in 0.1 M NaF, pH 4.0 adjusted with HF, (b) after exchange of this solution by 0.5M NaOH. (After [181]). Reprinted by permission of the Electrochemical Society.

Figure 1. Faradaic efficiency 77 (CO) for CO and 77 (Hj) for Hj on an Au electrode in pulsed reduction at a cathodic bias of V= -1.5V vs. Ag/AgQ, as a function of anodic bias with and without the addition of ImM halogen ion.

It has been reported that the concentration of proton and adsorbed hydrogen can be controlled by adjusting the anodic and cathodic bias in the pulsed method [7]. The hydrogen adsorbed on the electrode surface seems to interrupt the reaction for the electrochemical reduaion of COj. The CO2 coverage on the electrode surface may be increased by the elimination of adsorbed hydrogen during anodic period. In the subsequent cathodic period, the electron transfer to CO2 was promoted, yielding CO2 radical anions. The selectivity of products for the electrochemical reduction of CO2 was determined in association with electrode material and CO2 radical anion [10,11]. CO is intermediate species in the reaction process of hydrocarbonization [8]. 

The difference between n-Si and p-Si at the cathodic potentials indicates the effect of carriers on the etching process. The hydrogen evolution may either obtain the electron directly from the dissolving surface silicon atom or from the semiconductor. For n-Si, cathodic bias provides a high concentration of surface electrons for the hydrogen reaction resulting in a decrease of the dissolution rate. However, for p-Si electrons are the minority carriers, which are not available at a cathodic bias, and the etch rate remains more or less constant at cathodic potentials. 

L. T. Canham, W. Y. Leong, M. I. J. Beale, T. I. Cox, and L. Taylor, Efficient visible electroluminescence from highly porous silicon under cathodic bias, Appl. Phys. Lett. 61(21), 2563, 1992. 

H. S. Park, T. S. Kang, and K. J. Kim, Influence of etching current density on visible electroluminescence from porous n-Si under cathodic bias, J. Electrochem. Soc. 146, 1991, 1999. 

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