Light-sensitive etching

Let us now briefly outline the structure of this review. The next section contains information concerning the fundamentals of the electrochemistry of semiconductors. Part III considers the theory of processes based on the effect of photoexcitation of the electron ensemble in a semiconductor, and Parts IV and V deal with the phenomena of photocorrosion and light-sensitive etching caused by those processes. Photoexcitation of reactants in a solution and the related photosensitization of semiconductors are the subjects of Part VI. Finally, Part VII considers in brief some important photoelectrochemical phenomena, such as photoelectron emission, electrogenerated luminescence, and electroreflection. Thus, our main objective is to reveal various photo-electrochemical effects occurring in semiconductors and to establish relationships among them. 

On the other hand, processes of photocorrosion nature form the basis of light-sensitive etching used for the treatment of semiconductor surfaces, both in laboratory practice and in industry. 

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. 

Out of the above three methods of light-sensitive etching, the one with polarization from an external source [case (a)], called photoanodic etching, and etching in oxidizing solution [case (c)], called photochemical etching, are now of the greatest practical importance. 

Proceeding from Eqs. (59) and (60), one may formulate conditions that are imposed on the characteristic parameters of a semiconductor and a solution in order to enhance the resolution. It follows from the definition of that ( 9C 1VD) 1/2. This relation shows that frequencies, at which light-sensitive etching is possible, are the higher... 

Characteristics of Diffraction Gratings Obtained by the Light-Sensitive Etching Technique... 

The second area of activity also includes some problems in laser electrochemistry of semiconductors, which are in no way confined to the above-considered light-sensitive etching. First of all, it is threshold electrochemical reactions stimulated by intensive laser radiation. Such reactions may proceed via new routes, because both highly excited solution... 

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). 

Light-sensitive etching with polarization from an external source is called photoanodic etching, and that in an oxidizing solution, photochemical etching. 

Light-sensitive etching is based on the change, due to illumination, in the minority-carrier concentration, which determines the rate of anodic dissolution and corrosion of semiconductors. For example, under illumination of an n-type semiconductor in the anodic polarization regime, the etching rate can be limited by the rate of hole supply to the electrode surface. In darkness, a certain. 

Thus, the resolution of light-sensitive etching, i.e., the maximal value d attained, is not determined, in accordance with the above consideration, by the diffusion length Lp, as might be expected a priori (a — Lp), but is much higher. 

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