Etching photoelectrochemical

The surprising variety of photoelectrochemical effects observed at silicon electrodes anodized in HF is solely a consequence of the semiconducting nature of the electrode, because the electrolyte is not photoactive. 

Illumination is a relevant parameter in the electrochemistry of silicon because photogenerated carriers may initiate or contribute to the charge exchange at the electrolyte-silicon interface. If an electrode is illuminated, photogenerated electron-hole pairs are generated corresponding to the number of absorbed photons. This number depends on spectral distribution, total illumination intensity and losses due to optical reflection and transmission. The number of electron-hole 

Illumination-induced effects at silicon electrodes are used in different ways. For some of these applications illumination is a minor attribute. The sections in which these topics are discussed in detail are summarized below  

Under anodic conditions hole transfer to HF electrolytes is accompanied by electron injection that may lead to quantum efficiencies greater than 1. This effect is known as current multiplication and is discussed in Section 4.4. 

The dependence of photocurrent on bulk recombination rate allows us to realize an analytical tool for the characterization of silicon, as discussed in Section 10.3. 

It is also important to note that many cases may be cited of photoelectrochemical processing in which control is lacking, such as using a two electrode arrangement (no reference electrode), or a non-potentiostatic power supply. Similarly, many examples exist of photoelectrochemical processing (primarily etching) of semiconductors simply immersed in the electrolyte without external contact (1). Indeed, these may be practical solutions to photoelectrochemical processing once the systems have been characterized electrochemically. 

Photoelectrochemical etching occurs only when an optically produced minority carrier at the semiconductor-electrolyte interface is sufficiently energetic to induce a corrosion reaction. This condition is metfor valence band holes for virtually all semiconductors used in electronics in contact with aqueous electrolytes (4). In orderfor etching to occur at a sufficiently 

Conditions for Photoelectrochemical Etching Reported for Various Semiconductors. 

Ostermayer et al. (5) have reported the photoelectrochemical etching of p-type lll-V compounds. Under cathodic polarizatton, the photogenerated minority carriers (electrons) give rise to formation of the I i I eiement on the surface along with the evolution of the group V hydride (e g., Ga and AsHj). Subsequent anodic polarization leads to dissolution of the surface film by a majority carrier mediated reaction. As this process is repeated, the light-localized etched profile is developed. 

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Tenne R, Giriat W (1985) Controlled photoelectrochemical etching of CdSe and observation of a photocathodic effect. J Electroanal Chem 186 127-137... 

Tenne R, Theys B, Rioux J, Clement CL (1985) Improved performance of InSe-based photoelectrochemical cells by means of a selective (photoelectrochemical) etching. J Appl Phys 57 141-145... 

Another example comes from the field of semiconductor photoelectrochemistry. Semiconductors in contact with aqueous solutions can drive chemical reactions when irradiated. This is the basis for the photoelectrochemical etching of semiconductors in the electronic industry and for much research aiming at... 

An initially flat silicon electrode surface will develop a surface topography if the photocurrent varies locally. This variation can be caused by a lateral variation of the recombination rate or by a lateral variation of the illumination intensity. The photoelectrochemical etching of a silicon electrode is related to the etch-stop techniques discussed in Chapter 3. While different etching rates for different areas of the electrode may be obtained by electrical insulation or by a different doping density, the etch rate may also be altered by a difference in illumination intensity. Basically four photoelectrochemical etching modes are possible for homogeneously doped substrates ... 

Photoelectrochemical etching The dissolution of a semiconductor in an electrolytic solution upon exposure to Ught. Used in the photopatterning of semiconductor surfaces. 

Only a few studies [453,543-547] have been carried but on the photoelectrochemical behaviour of YBCO ceramics and single crystals. In nonaqueous media, in the absence of degradation, the data show that the usual concepts of photoelectrochemistry of p-type semiconductors apply to those systems. Such studies have not yet been actively developed, probably because of the high sensitivity of photoelectrochemical processes to the nature of materials. Reproducibility of data for such complex systems as HTSC oxides is an issue. At the same time, the photoassisted electrochemical processes on HTSC electrodes can lay the basis for certain effective technologies. This is especially true for the photoelectrochemical etching and metallization which prove to be extremely effective as applied to semiconductors [503,504]. 

C. L. Clement, A. Lagoubi, and M. Tomkiewicz, Morphology of porous n-type silicon obtained by photoelectrochemical etching, J. Electrochem. Soc. 141, 958, 1994. 

C. Levy-Clement, A. Lagoubi, R. Tenne, and M. Neumann-Spallart, Photoelectrochemical etching of silicon, Electrochim. Acta 31(5), 877, 1992. 

J.-L. Maurice, A. Riviere, A. Alapini, and C. Levy-Clement, Electron beam irradiation ofn-type porous silicon obtained by photoelectrochemical etching, Appl Phys. Lett. 66(13), 1665, 1995. 

Tenne R. and Hodes G. (1980), Improved efficiency of cadmium selenide photoanodes by photoelectrochemical etching , Appl. Phys. Lett. 37, 428-430. Tomkiewicz M., Ling 1., and Parsons W. S. (1982), Morphology, properties, and performance of electrodeposited normal CdSe in liquid-junction solar cells ,... 

Chaparro A. M., Salvador P. and Mir A. (1997), Localized photoelectrochemical etching with micrometric lateral resolution on transition metal diselenide photoelectrodes , J. Electroanal. Chem. 422, 35 4. 

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. 

By using light, it is possible to create an excess of electrons or holes locally in a doped semiconductor and thereby increase or decrease the rate of etching in either dry or wet processes. Structures that cannot be produced by any other means, such as narrow holes with extreme aspect ratio, have been produced by photoelectrochemical etching. Because not all of the comple interrelated heat, mass, and electron transport processes involved are as yet understood, the results are not always predictable. 

Ostermayer, Jr., F. W., P. A. Kohl, and R. H. Burton. Photoelectrochemical etching of integral lenses and InGaAsP/InP light-emitting... 

Figure 1.2 Plan view image of a 4H-SiC Si-face sample, off-cut 8° towards [1210], photoelectrochemically etched to obtain the triangular porous morphology. About 2 pm of material was removed by RIE prior to imaging. The exposed channels apparently propagate preferably along (1210) directions. Reproduced from Y. Shishkin etal.,J. Appl. Phys., 96(4), 2311-2322. Copyright (2004), the American Institute of Physics...

Figure 1.10 Planar and cross-sectional SEM images [34] of (a)-(d) C-face porous SiC in EXP. 1 (e)-(h) Si-face 6H porous SiC in EXP. 2 (i)-(l) C-face 6H porous SiC in EXP. 3. Reproduced from Y. Ke, R.P. Devaty and W.J. Choyke, Self-ordered nanocolumnar pore formation in the photoelectrochemical etching of 6H SiC, Elec-trochem. Solid-State Lett., 10(7), K24-K27 (2007). Copyright 2007, with permission from The Electrochemical Society...

J.S. Shor and R.M. Osgood, Jr, Broad-area photoelectrochemical etching of n-type beta-SiC,/. Electrocbem. Soc., 140, L123-L125 (1993). 

Y. Ke, R.P. Devaty and W.J. Choyke, Self-ordered nanocolumnar pore formation in the photoelectrochemical etching of 6H SiC, Electrocbem. Solid-State Lett., 10 (7), K24-K27 (2007). 

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