Sensitive Etching of Semiconductors

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. 

Qualitative Picture of Processes Taking Place at a Semiconductor-Electrolyte Interface under Its Nonuniform Illumination 

Let us distinguish between the three most important cases. a. Illumination Under Anodic Polarization 

Here corrosion occurs even in darkness. In the simplest case where the partial cathodic reaction proceeds exclusively through the conduction band and the anodic reaction through the valence band, the corrosion rate is limited, as was shown in Section 8, by the supply of minority carriers to the surface, irrespective of the type of sample conductivity. Therefore, in darkness the corrosion rate is low. Illumination accelerates corrosion. This case is similar to case (a), but with the difference that the role of anodic polarization is played by chemical polarization with the help of an oxidizer introduced into the solution (see Section 13 for examples). 

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. 

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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). 

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. 

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... 

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... 

Customarily, semiconductor surfaces are chemically or physically prepared to optimize their chemical and/or electro-optical properties. For chemical sensing applications, a freshly etched surface often provides greater chemical sensitivity. A Br2/MeOH etch of n-CdSe, for example, has typically yielded larger luminescence responses to analytes than have polished samples. Additionally, transducing films have been used to modify semiconductor surfaces to enhance the selectivity of CdSe for particular analytes [2]. 

The analysis of semiconductor materials is illustrated in Tables II and III. Table II shows comparison of the Impurity content of a series of silicon samples. These materials are received as chunks (in the case of starting materials) or slices from a crystal. They are cut to the appropriate size and etched for cleaning with high purity acid. The figure Illustrates high sensitivity analysis of three polycry stall ine silicon samples with the -20 material showing natch higher aluminum content than the others. SSMS performs well in this type of comparative analysis. 

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. 

Figure 18. Schematic diagram of the illumination of a surface in laser light-Semiconductor sensitive etching.

Perhaps the simplest of these techniques are the potentiostatic photocurrent transients (79) that were shown to be sensitive to the semiconductor electrodes down to 1 ns. (80) and below (81). Often the time resolution is limited by the RC of the system and the technique is most valuable in the longer time scales for identification of intermediates and products of photo redox reactions (79). The interpretation of the data follows the routine in some of the methods that we have explored to interpret impedance data, i.e., assume an equivalent circuit and analyze the decay as a superposition of exponential decays where the time constants are correlated with the elements of the equivalent circuit (79)(80)(82). The time constant that was associated with the space charge layer was in reasonable agreement with the Mott-Schottky data (79)(80). The time-scale of the predicted response (83) is much faster than the one observed by the authors of Ref. 79, but the much faster resolution reported in Ref. 81 was in agreement with the time-dependent version of Gartner s model. Etching wasfoundtohavealargeeffecton the amplitude and decay time of the transients (82). This method was also applied to the study of dye sensitization and the role of a super sensitizers in these systems (84). 

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