Photoelectrochemical Surface Processing

Woodall et al. (44) determined that photochemical oxidation of GeiAs immersed in 1 1 HCI/HjO resulted in only As surface films, about 5 nm thick, if Oj was excluded and if the H O was deionized (18 MO) without these precautions, a mixture of As and Ga oxides was formed. The As further prevented oxide formation on the surface, and could be removed by a relatively mild heat treatment at 290°C in vacuo. The surfaces so-produced were superior to those formed by much higher temperature heat treatments in vacuo that were necessary to clean typical oxide-coated 

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The chemical, electrochemical, and photoelectrochemical etching processes by which microelectronic components are made are controlled by electrochemical potentials of surfaces in contact with electrolytes. They are therefore dependent on the specific crystal face exposed to the solution, on the doping levels, on the solution s redox potential, on the specific interfacial chemistry, on ion adsorption, and on transport to and from the interface. Better understanding of these processes will make it possible to manufacture more precisely defined microelectronic devices. It is important to realize that in dry (plasma) processes many of the controlling elements are identical to those in wet processes. 

Control and observation of electrochemical and photoelectrodiemical reactions at semiconductor electrodes are very important in establishing the electrochemical/ photoelectrochemical etching processes and stable photoelectrochemical cells (1,2). To understand the mechanism of the electrochemical and photoelectrochemical reactions, in situ information of morphological and electronic structures of semiconductor electrode surfaces with atomic resolution is essential. Although techniques such as electron microscopy and optical microscopy have been applied to examine the morphology of the surface of solid substrates, the former can be used only for ex situ examination and the latter has poor resolution (3,4),... 

The present technique enables light-induced redox reaction UV light-induced oxidative dissolution and visible light-induced reductive deposition of silver nanoparticles. Reversible control of the particle size is therefore possible in principle. The reversible redox process can be applied to surface patterning and a photoelectrochemical actuator, besides the multicolor photochromism. 

GaAs, CuInS2, CuInSe2- Semiconductor electrodes have received increasing attention as a consequence of their potential application in photoelectrochemical energy conversion devices. In order to achieve optimum efficiency, the knowledge of the surface composition plays a crucial role. Surface modifications may occur during operation of the photo electrode, or may be the result of a chemical or electrochemical treatment process prior to operation. 

The synthesis of catalytic photocathodes for H2 evolution provides evidence that deliberate surface modification can significantly improve the overall efficiency. However, the synthesis of rugged, very active catalytic surfaces remains a challenge. The results so far establish that it is possible, by rational means, to synthesize a desired photosensitive interface and to prove the gross structure. Continued improvements in photoelectrochemical H2 evolution efficiently can be expected, while new surface catalysts are needed for N2 and CO2 reduction processes. 

Carbon is widely used in the catalytic processes of the chemical industry due to its unique characteristics, such as chemical inertness, high surface area and porosity, good mechanical properties and low cost. It is used for the production of chlorine and aluminum, in metal refining (gold, silver, and grain refinement of Mg-Al alloys) as well as for the electrolytic production of hydrogen peroxide and photoelectrochemical water splitting. 

Surface treatments of CD CdSe films deposited from selenosulphate/NTA solutions have a pronounced effect on various optical, electrical, and optoelectronic properties of the films, due to interaction with or modification of such surface states. Mild etching (dilute HCl) of the films reverses the direction of current flow both in CdSe/polysulphide photoelectrochemical cells [108] and in Kelvin probe surface photovoltage (SPV) measurements in air [109], These studies are discussed in more detail in Chapter 9, in Section 9.2 on photoelectrochemical cells. At this point, it is sufficient to state that the effect is believed to be due to preferential trapping of either electrons or holes at surface states that are modified by the etching process. 

Photoelectrochemical processes may proceed in quite different regimes, depending on the relative magnitudes of the depth of light penetration into a semiconductor, the diffusion length and the thickness of the space-charge region, and also between the rates of electrode process and carrier supply to the surface. Nevertheless, in important particular cases relatively simple (but in no way trivial) relations can be obtained, which characterize a photoprocess, and the theory can be compared with experiment. 

The above results indicate that a requirement for water photolysis by Pt/Ti02 is to prevent the reverse reaction on Pt sites. Wagner and Somoijai8) successfully carried out gas-phase water photolysis by Pt/SrTi03-crystal coated with deliquescent basic materials. Their method is reasonable to suppress the reverse reaction, because a deliquescent material coated on a substrate absorbs a large amount of water to form a thin film of its aqueous solution. The film inhibits the reaction products to readsorb directly on the catalyst, while the products on the catalyst can escape to the gas phase by diffusion, it is very important that H2 and 02 can desorb from the catalyst surface to the gas phase without making bubbles, because if they desorb as bubbles then they would inevitably mix with each other in the growing process of bubbles and recombine on Pt sites. In addition, an aqueous basic solution would work as an electrolyte which enhances ion transfer in photoelectrochemical reactions. 

It is well known that the surface orientation of crystals and imperfections in the surface, like grain boundaries or dislocations, affect largely the reaction rates at electrodes made of metals or semiconductors. Such effects are most pronounced in those reactions where atoms leave their position in a crystal lattice or have to be incorporated into such one. These processes are connected with activation barriers which are particularly high for semiconductors where the chemical bonds between the components of the crystal lattice are highly directed and localized. If we consider photoelectrochemical reactions at semiconductors we have additionally to discuss the influence of these factors on light absorption and its consequences. 

Besides such electronic surface states which can interact either with electrons in the bulk of the semiconductor or with a redox system in the electrolyte, we have to consider another type of excess charge at the surface. This stems from adsorbed ions or from ionic groups attached to the surface of the semiconductor. This is well known from the pH dependence of the flat band potential of semiconducting oxides (8) or the dependence of the flat band potential of sulfides on the sulfide concentration in solution (9). Since surfaces of different orientation will interact differently with such ionic charge, this again will affect the photoelectrochemical processes via the different barrier heights at different surface orientation. 

N-type semiconductors can be used as photoanodes in electrochemical cells Q., 2, 3), but photoanodic decomposition of the photoelectrode often competes with the desired anodic process (1 4 5). When photoanodic decomposition of the electrode does compete, the utility of the photoelectrochemical device is limited by the photoelectrode decomposition. In a number of instances redox additives, A, have proven to be photooxidized at n-type semiconductors with essentially 100% current efficiency (1, 2, 3, 6>, ], 8, 9). Research in this laboratory has shown that immobilization of A onto the photoanode surface may be an approach to stabilization of the photoanode when the desired chemistry is photooxidation of a solution species B, where oxidation of B is not able to directly compete with the anodic decomposition of the "naked" (non-derivatized) photoanode (10, 11, 12). Photoanodes derivatized with a redox reagent A can effect oxidation of solution species B according to the sequence represented by equations (1) - (3) (10-15). 

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