Anodic dissolution semiconductors

Bard AJ, Wrighton MS (1977) Thermodynamic potential forthe anodic dissolution of n-type semiconductors - A crucial factor controlling durability and efficiency in photoelectrochem-ical cells and an important criterion in the selection of new electrode/electrolyte systems. J Electrochem Soc 124 1706-1710... 

A typical featnre of reactions involving the minority carriers are the limiting currents developing when the snrface concentration of these carriers has dropped to zero and they mnst be snpplied by slow dilfnsion from the bulk of the semiconductor. A reaction of this type, which has been stndied in detail, is the anodic dissolution of germanium. Holes are involved in the first step of this reaction Ge — Ge(II), and electrons in the second Ge(ll) —> Ge(IV). The overall reaction equation can be written as... 

Formation of porous silicon is an anodic dissolution process, which consists of carrier transport in the semiconductor, electrochemical reactions at the interface, and mass transport of the reactants and reaction products in the electrolyte. There are a... 

Fig. 9-7. Ionization of surface at oms followed by ion tnnsfer across an electrode interface in anodic dissolution of covalent semiconductor S = covalently bonded atom in semiconductor S. = surface atom of semiconductor s = surface radical = surfisce ion 825 = hydrated ion OHP = outer Helmholtz plane.

In the anodic dissolution of covalent semiconductors, the transfer of surface ions across the compact layer (Helmholtz la r) occurs following the ionization of surface atoms S, illustrated in Eqn. 9-33, as described in Sec. 9.2.1 ... 

Fig. 9-10. Polarization curves of anodic dissolution and cathodic deposition of n-type and p-type covalent semiconductor electrodes n-SC (p-SC) = n-type (p-type) semiconductor electrode i (i ) = anodic dissolution (cathodic deposition) current Cp = Fermi level.

Fig. 9-11. Polamation curves observed for anodic dissolution of n- pe and p-type semiconductor electrodes of germanium in 0.05 M NaOH solution = current of...

The same disciission may apply to the anodic dissolution of semiconductor electrodes of covalently bonded compounds such as gallium arsenide. In general, covalent compoimd semiconductors contain varying ionic polarity, in which the component atoms of positive polarity re likely to become surface cations and the component atoms of negative polarity are likely to become surface radicals. For such compound semiconductors in anodic dissolution, the valence band mechanism predominates over the conduction band mechanism with increasing band gap and increasing polarity of the compounds. 

Pig. 10-18. (a) PolarizatioD curves of anodic dissolution and (b) Mott-Schottky plots of an n-type semiconductor electrode of molybdenum selenide in the dark and in a photo-excited state in an acidic solution C = electrode capacity (iph) = anodic dissolution current immediately after photoexdtation (dashed curve) ipb = anodic dissolution current in a photostationary state (solid curve) luph) = flat band potential in a photostationary state. [From McEv( -Etman-Memming, 1985.]... 

Pig. 10-19. (a) Capture of photogenerated holes in surface states to form siuface ions and (b) anodic dissolution of surface ions to form hydrated ions on an n-type semiconductor electrode Oj = rate of hole capture in surface states oqx = rate of anodic dissolution of surface ions Cn = surface state level S, = surface atom of semiconductor electrode h(vs) = hole in the valence band h(n> = hole captured in smface states h(soH-) = hole in dissolved ions. 

Fourth, the favoured anodic reaction at the semiconductor must be O2 evolution from water, rather than some anodic dissolution process in which the semiconductor breaks down, as happens with Ge 9), GaP (14,15,16) CdS or even ZnO (13). 

Among the methods of anodic and chemical etching of semiconductors, widely used both in the production of semiconductor devices and in investigations (see, for example, Schnable and Schmidt, 1976 Turner and Pankove, 1978), the so-called light-sensitive etching is of great importance. It is based on the variation, under illumination, of the concentration of minority carriers, which often determines, as was shown above, the rate of anodic dissolution and corrosion of semiconductors. 

Surface states on a semiconductor in a vacuum can sometimes be explained by means of the spare bonds that dangle from atoms on surfaces, or defects associated with dislocations. Neither of these mechanisms works at the semicon-ductor/solution interface. The dangling bonds will be expunged by adsorbed water, etc. Experiment shows that the concentration of surface states on semiconductors in solution is strongly potential dependent, and that defects in the crystal structure would not be potential dependent, at least until anodic dissolution of the substrate itself began. 

Germanium — (Ge, atomic number 32) is a lustrous, hard, silver-white metalloid (m.p. 938 °C), chemically similar to tin. Ge is a low-band-gap - semiconductor that, in its pure state, is crystalline (with the same crystal structure as diamond), brittle, and retains its luster in air at room temperature. Anodic dissolution of the material occurs at potentials more positive than ca. -0.2 V vs SCE. Peaks in the voltammograms of germanium in acidic electrolyte are ascribed to a back-and-forth change between hydrogenated and hydroxy-lated surfaces [i]. Studies are often conducted at p-doped and n-doped Ge electrodes [ii] or at Ge alloys (e.g., GeSe) where photoelectrochemical properties have been of considerable interest [iii]. 

The expected Tafel slope of 60mV/decade is not always found. There are a number of reasons for this, aside from kinetic effects in the bulk of the semiconductor. The kinetic effects associated with faradaically active surface states is of considerable significance, as shown below, but another common problem is that part of the potential change may appear across the Helmholtz layer rather than across the depletion layer. A well-known case in point is germanium, for which the surface is slowly converted from "hydride to "hydroxylic forms as the potential is ramped anodically. This conversion gives rise to a change in the surface dipole and hence Aij/ AT. In fact, the anodic dissolution of p-germanium is found to follow a law [106]... 

The study of the anodic dissolution of semiconductors has played an important role in clarifying the nature of faradaic processes occurring at semiconductor surfaces [112, 113]. In principle, anodic dissolution on such semiconductors as Ge, GaAs, and GaP might proceed either by hole capture from the VB or electron injection into the CB. For Ge, for example... 

ANODIC DISSOLUTION REACTIONS at semiconductor electrodes require electron holes. 

The electrochemistry of the anodic etching of semiconductors is similar in most respects to the anodic dissolution of metals. The main difference in the electrolytic behavior of metals and semiconductors is in the electrode material itself. 

During anodic dissolution, the applied potential is partitioned between the space charge layer in the semiconductor, C/jc and the Helmholtz double layer, C/h ... 

The chapters in this volume address challenging problems associated with the observation and interpretation of anodic dissolution of semiconductors, electrode reactions in nonaqueous solvents, and charge-transfer across the interface between two immiscible electrolytes. In-situ FTIR spectroscopy of surface reactions, and a review of electrochemical methods of pollution abatement complete the range of timely topics included. 

On the basis of the experimental data summarized above, and taking into account the chemistry of the elements involved in strongly acidic and alkaline media [15], the following overall reaction equations can be proposed for the anodic dissolution of the III-V semiconductors under consideration ... 

Several conclusions may be drawn from the results discussed in this section. Firstly, it appears that in almost all cases studied, the stabilization reaction involves decomposition intermediates instead of free holes. We will not comment on this point here (for a discussion, see ref. [52]). Similarly, we will not enlarge on the observation that in certain cases, Xj and in other cases X2 intermediates are involved, as these problems are beyond the scope of the present paper, which essentially pertains to anodic dissolution and etching. As far as this subject is concerned, two important points emerge, i. e., the fact that, due to the interconnection between stabilization and dissolution, the latter reaction tends to dominate at sufficiently high current densities, and the fact that, depending on the semiconductor and on the circumstances, dissolution either occurs by the DH or by the DX mechanism. In what follows, independent information on the latter point will be gathered, and the factors which determine the dissolution mechanism will be investigated. 

It was pointed out many years ago [3, 27] that the anodic dissolution of III-V semiconductors in aqueous media not only consists of electrochemical steps, but must also involve chemical steps in which H2O molecules or OH ions participate. This follows from mere consideration of the overall reaction Eqs. (9) to (14) and, as... 

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