Etching electrolytic, semiconductors

Electrolytic etching of semiconductors is used to remove damaged surface layers on single crystal material and/or shape... 

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. 

If the whole semiconductor/electrolyte interface is illuminated uniformly, both conjugate reactions proceed at the same rate over the same areas on the interface. The stationary potential of an illuminated semiconductor is thus a mixed potential. If the surface of a semiconductor, homogeneous in its composition and properties, is illuminated nonuniformly, in the illuminated and nonillumi-nated areas conditions will not be identical for electrochemical reactions. Here the conjugate reactions appear to be spatially separated, so that we can speak about local anodes and cathodes. This situation is deliberately created, for example, for selective light-sensitive etching of semiconductors (see Section V.2). 

D. R. Turner, Electrolytic Etching of Semiconductors, Symposium of the Electrochemical Society, Columbus, Ohio, 1959. 

In practice, the electrochemical behavior of semiconductor-electrolyte interfaces is far more complex than that described above (for a good review, see Boddy [1965]). One of the complications arises because the semiconductor surface at the electrolyte-semiconductor interface is not equivalent to that in the bulk. In particular, the energy states localized at the surface for holes and/or electrons are different than those present in the bulk. These surface states may arise in several ways—for example through pretreatment (etching, polishing, etc.) of the semiconductor surface before immersion in the electrolyte. The surface states can be detrimental to the PESC efhciency if they increase the recombination of the electron-hole pairs in the semiconductor, thus reducing the number of holes (electrons for p-type material) available for chemical reaction with the redox species in solution. 

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. 

Here n0) = ntp + inis the complex refractive index of the medium (n(/ 2 > 0) and j takes two values j = 1 corresponds to the electrolyte solution, = 2 to the semiconductor. The solution of Eq. (51) together with matching conditions at the interface completely describes the distribution of U). The change in the shape of the surface relief X(y,z,t) due to etching alters this distribu-... 

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

If the hole concent ration in the semiconductor is relatively low, as in low resistivity n-type germanium or silicon, the available holes in the surface region are used up at low current densities and the etch rate is slow. The anodic current under these conditions can be increased by providing additional holes at the surface. Holes produced as a result of illuminating the semiconductor give uniform electrolytic etching on n-type semiconductors. Germanium is electro-lytically etched in several electrolytes while silicon can only be dissolved anodically in fluoride solutions. A thick film of amorphous silicon forms on silicon anodes in acid fluoride solutions below a critical current density. 

Germanium and silicon are electrolytically etched at about the same rate, about 3x10" 5 cm 3/coulomb. Thus at a current density of 500 ma/crn, Ge and Si are dissolved at the rate of about 1.7x10 cm/sec (0.0004 in /min). In order to electrolytically etch n-type semiconductors at a reasonable rate, some means must be found to increase the hole concentration at... 

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