Illumination anodic oxide formation

If the same experiment is performed with an n-type Si electrode under identical illumination intensity the anodic photocurrent is found to be larger than for the p-type electrode under cathodic conditions. This increase is small (about 10%) for current densities in excess of JPS. Figure 3.2 shows that in this anodic regime injected electrons are also detected at p-type electrodes. This allows us to interpret the 10% increase in photocurrent observed at n-type electrodes as electron injection during anodic oxide formation and dissolution. 

If an oxide-free, hydrogen-terminated, n-type electrode is anodized under illumination in an electrolyte free of HF (for example HC1), a quantum efficiency of close to 2 is observed for the initial contact of the electrode to the electrolyte. During the oxidation of the first hydrogenated monolayer the quantum efficiency decreases to 1 and remains at that value during the formation of the first few nanometers of anodic oxide, as indicated by filled triangles in Fig. 4.13. For a further increase of oxide thickness the quantum efficiency decreases to values significantly below 1 [Chl4]. 

A schematic view of the cold cathode fabrication process is shown in Fig. 10.18. The cold cathode is fabricated by low pressure chemical vapor deposition (LPCVD) of 1.5 pm of non-doped polysilicon on a silicon wafer or a metallized glass substrate. The topmost micrometer of polysilicon is then anodized (10 mA cnT2, 30 s) in ethanoic HF under illumination. This results in a porous layer with inclusions of larger silicon crystallites, due to faster pore formation along grain boundaries. After anodization the porous layer is oxidized (700 °C, 60 min) and a semi-transparent (10 nm) gold film is deposited as a top electrode. 

All the results discussed above strongly suggest that the exposure of the Ti02 (anat-ase) electrode immersed in an alkaline solution to the band-gap illumination, under anodic bias or at open circuit, leads in all cases to the formation of very similar (if not identical) species bound to the oxide surface. Such conclusion is supported by the similarity of the potential, shape and decay characteristics of the corresponding cathodic reduction peaks shown in Figs. 4,5,7 and 8. These species have been identified, in the course of earlier experiments performed with aqueous alkaline dispersions of Ti02 (anatase) particles irradiated with near-UV light, as surface-bonded, peroxo-titanium com-... 

Illumination with light having a wavelength larger than the band gap of silicon generates a photocurrent under an anodic potential on an n-Si electrode but has essm-tially no effect on p-Si, as would be expected from the basic theories of semiconductor electrochemistry. However, the photocurrent may not be sustained because of the formation of an oxide film, which passivates the silicon surface to various degrees depending on the electrolyte composition. In solutions without fluoride species, the photocurrent is only a transient phenomenon before the formation of the oxide film. In fluoride solutions, in which oxide film is dissolved, a sustained photocurrent can be obtained. 

Reaction paths (1) and (11) in Fig. 5.70 account for the anodic reactions onp-Si and illuminated n-Si in HF solutions at high light intensities. Path (1) is involved in the exponential region at an anodic potential much lower than Vp responsible for direct dissolution of silicon and dissolution valence of 2, while path (11) is involved at a potential above Vp responsible for the indirect dissolution of silicon through formation and dissolution of oxide and for the dissolution valence of 4. At a potential that is lower... 

Dissolution of PS. The dissolution of PS during PS formation may be due to two proeesses a proeess in the dark and a proeess under illumination. Both are essentially eorrosion proeesses by which the silicon in the PS is oxidized and dissolved with simultaneous reduction of the oxidizing species in the solution. The corrosion process is responsible for the formation of micro PS of certain thickness (stain film) as well as the dissolution of the existing PS. The material in the PS which is at a certain distance from the pore tips is little affected by the extanal bias due to the high resistivity of PS and is essentially at an open-circuit condition (OCP). This dissolution process, which is often referred to as chemical dissolution, is an electrochemical process because it involves charge transfer across the interface. The anodic and cathodic reactions in the microscopic corrosion cells depend on factors such as surface potential and carrier concentration on the surface which can be affected by illumination and the presence of oxidants in the solution. 

Concerning the potential dependence of the interfacial current under illumination, it is frequently useful to measure it in the presence of only one species of the redox couple, the reduced species for an anodic and the oxidized species for a cathodic reaction. Taking n-WSe2 as an example, then the current-potential curve under illumination, as measured in an aqueous solution free from any redox system, is presented in Fig. 7.26. The cathodic dark current which occurs cathodic of the flatband potential, is due to H2 formation (conduction band process). The anodic photocurrent which starts... 

In principle, another anodic reaction can take place instead of semiconductor decomposition (dissolution), for example, oxidation of dissolved substance or oxygen evolution from water. Apparently, in the latter case, the illumination of semiconductor leads to photoelectrolysis of water with the formation of hydrogen and oxygen, that is, conversion of the energy of light into chemical energy of the photoelectrolysis products. 

Next, photogeneration of electron-hole pairs leads to the formation of quasi-levels of minority and majority carriers, Fp and F , as shown in Fig. 12. Since, at the surface, Fp < Fs -/sl and F > Fs -/sl, illumination results in the acceleration of both forward and reverse reactions in a sulfide polysulfide couple. If the circuit is closed on an external load R, the anodic and cathodic reactions become separated the holes are transferred from the semiconductor photoanode to the solution, so that ions are oxidized to 82 , and the electrons are transferred through the external circuit to the metal counterelectrode (cathode) where they reduce S2 to The potential difference across a photocell is iphR, where iph is the photocurrent, and the power converted is equal to /phF. 

Similar to the formation of porous aluminum oxide a passivation - dissolution mechanism can be used to form nanopo-rous structures on InP. If (OOl)n-InP is polarized anodically under illumination in HCl solutions, nanoscaled pores are formed [117]. For potentials up to 1.2 V vs. SGE the main reaction is uniform anodic dissolution. Above this potential porous InP with a surface oxide is formed. The overpotential and anodizing time influence pore diameter (110-250 nm), wall thickness (16-50 nm) and pore length... 

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