Photoelectrochemical etching electrolytes

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

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. 

Figure 1.12 I-V scans [34] on the (a) Si-face and (b) C-face of 6H-SiC in 10% HF, 5% ethanol electrolytic solution at 1 V s-1 scan rate. Note that in order to better distinguish the curves recorded with and without UV illumination, the 7-V curve without UV illumination is lowered by 100 mA cm-2. UV intensity is 600 mW cm-2. Reproduced from Y. Ke, R.P. Devaty and W.J. Choyke, Self-ordered nanocolumnar pore formation in the photoelectrochemical etching of 6H SiC, Electrochem. Solid-State Lett., 10(7), K24-K27 (2007). Copyright 2007, with permission from The Electrochemical Society...

Interest in CdTe for solar energy conversion has led to a number of studies of the CdTe/electrolyte interface (8,10-12), and development of photoelectrochemical etching (75-75). In general, the above studies focused on macroscopic etching as a microfabrication process or surface cleaning technique for CdTe. 

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... 

Reaction Velocities for Photoelectrochemical Etching of InP in Various Electrolytes (Ref. 7). 

Forrest et al. (27) described the application of photoelectrochemical etching to the fabrication of an annular photodiode, shown in Figure 12. The central hole was photoelectrochemically etched through photolithographically defined circular areas in the metallization. The process was carried out under an external bias using an electrolyte of 0.75 N KF/0.75 N HF. The photo-process continued until the p region was reached, after which it was completed by a dark chemical etch. 

The equipment and the experimental procedures using the C02-methanol medium have already been described in previous papers. . For the photoelectrochemical experiments, a stainless steel pressure vessel was equipped with a 2-cm thick quartz window for illumination, p-type InP and GaAs wafers were cut into ca. 0.4 cm x 0.5 cm electrodes and were mounted using epoxy resin. Ohmic contact was made with successive vapor deposition of Zn (30 mn) and Au (100 nm), which was annealed afterward at 425 C in Ar. A silver wire (0.8 mm dia) was used as a quasi-reference electrode (Ag-QRE, ca. +80 mV vs. SCE). A Pt wire (0.8 mm dia) was used as the counter electrode. The photocathode was etched in hot aqua regia for ca. 5 s before each experiment. The electrolyte solution [3 cm, 0.3 mol dm" tetrabutylammonium perchlorate (TBAP) in CH3OH] was placed in a glass cell liner in the stainless steel vessel. Gases were introduced into the pressure vessel and were left to equilibrate for one hour at the desired pressure (1 to 40 atm). 

Highlights of research results from the chemical derivatization of n-type semiconductors with (1,1 -ferrocenediyl)dimethylsilane, , and its dichloro analogue, II, and from the derivatization of p-type semiconductors with N,N -bis[3-trimethoxysilyl)-propyl]-4,4 -bipyridinium dibromide, III are presented. Research shows that molecular derivatization with II can be used to suppress photo-anodic corrosion of n-type Si derivatization of p-type Si with III can be used to improve photoreduction kinetics for horseheart ferricyto-chrome c derivatization of p-type Si with III followed by incorporation of Pt(0) improves photoelectrochemical H2 production efficiency. Strongly interacting reagents can alter semicon-ductor/electrolyte interface energetics and surface state distributions as illustrated by n-type WS2/I-interactions and by differing etch procedures for n-type CdTe. 

Several etching procedures were attempted for p-CdTe for the photoelectrochemical reduction of carbon dioxide. Etching with dilute thiosulfite or bromine in methanol did not result in better photocurrent-potential relationship. Hence, it was concluded that etching with aqua regia followed by rinsing with water is the best surface treatment for the photoelectrochemical reduction of carbon dioxide. All the impedance results described below were recorded using this surface in contact with electrolyte. 

When photoelectrochemical solar cells became popular in the 1970s, many reports appeared concerning the stability, dissolution, and flat-band potential of semiconductors in solutions. These papers investigated parameters such as the energy level of the band edges, which is critical for the thermodynamic stability of the semiconductor and how to determine the potential for the onset of the (photo) electrochemical etching [38-40]. The criterion for thermodynamic stability of a semiconductor electrode in an electrolyte solution is determined by the position of the Fermi level with respect to the decomposition potential of the electrode with either the conduction band electrons or valence band holes E. Under illumination, the quasi-Fermi level replaces the Fermi level. The Fermi level is usually found within the band gap of the semiconductor and its position is not easily evaluated (especially the quasi-Fermi level of minority carriers). Therefore it was found more practical to use the conduction band minimum (Eq) and valence band maximum (Ey) as criteria for electrode corrosion. Thus, a semiconductor will be corroded in a certain electrolyte by the conduction band electrons if its... 

Light-localized, wet processing of semiconductors which is not photoelectrochemical in nature is also known. Localized surface heating can reduce the overpotential for electroplating (2) or accelerate etching, for example. In addition, reactive intermediates can be produced by photolysis of electrolyte species (e.g., Brg), which then go on to react chemicaiiy with the semiconductor surface (3). These nonphotoelectro-chemicai approaches will not be dealt with further in this chapter. 

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. 

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