Anodic dissolution illumination

Anodic dissolution of n-Si can also proceed at a polarization under illumination. The maximum current is limited by illumination intensity when the saturation photo current density is lower than the critical current, Ji. The characteristics of i-V curves of n-Si under a high illumination intensity, when the reaction is no longer limited by the availability of photo generated carriers, is identical to that for p-Si. Similar also to p-Si, formation of PS on n-Si occurs only below the critical current, Jx 24... 

A nontrivial feature of a silicon electrode in alkaline aqueous solutions is its ability to pass reversibly, under illumination, from the passive state to the active one, and vice versa. For example, suppose that the initial state is actively dissolved silicon under illumination its potential spontaneously and sharply shifts (at a constant current) to more positive values, i.e., into the passive region, and self-dissolution ceases photopassivation occurs (Fig. 20a). In contrast, once silicon has already been anodically passivated, illumination shifts its potential to less positive values (Fig. 20b). In this case, the point on the dashed line, which characterizes the state of the system, passes from the descending branch to the ascending one, and active selfdissolution starts, i.e., photoactivation takes place. 

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. 

Lingier, S., Vanmaekelbergh, D. and Gomes, W.P. 1987. On the role of chemical steps in the anodic dissolution of illuminated n-type GaAs electrodes. J. Electroanal, Chem., 228. 77-88. 

Since holes are consumed at the surface during the anodic dissolution, the n-type samples show increasing differences between the measured and the calculated capacities with increasing rate of dissolution, i. e., with increasing anodic polarization. In this case d-c potential curves also show deviations from the initial exponential slope. At higher anodic potentials a saturation current occurs. Illumination compensates for or decreases the influence of the anodic current on the concentration cf holes. Fig. 11 shows schematically the influence of anodic dissolution and illumination. For p-type Ge the same effects occur, when electrons are consumed by the electrode reaction, i. e., in the cathodic region. 

As stated in the introduction, wet etching processes may proceed either with or without external current flow. In the former case, the semiconductor crystal is incorporated as an electrode in an electrochemical cell and polarized anodically under illumination or in darkness for n- and p-type samples, respectively, leading to dissolution of the sample (see Sec. 2). This is referred to as the (photo)anodic etching process. 

W. P. Gomes and H. H. Goossens analyze the kinetics of the electrochemical reactions used for etching gallium arsenide. The authors describe means by which information about the mechanisms may be obtained. Based on the understanding of the anodic dissolution of n-type and p-type GaAs, both in the dark and under illumination, etching mechanisms for different oxydants with and without current are derived. 

Porous silicon (PS) is a material that is formed by anodic dissolution of silicon in HF solutions. The formation of PS was first reported in the late 1950s in studies on electropolishing of silicon. Since then, particularly after 1990 when luminescence of PS was discovered, numerous investigations have been undertaken. These investigations have revealed that PS has extremely rich morphological features with properties that are very different from those of silicon and the formation process of PS is a very complex function of many factors such as HF concentration, type of silicon, current density, and illumination intensity. 

Another class of semiconductors comprises Il-VI compounds, mainly sulfides and oxides, which also undergo anodic decomposition. A typical example is CdS ( g = 2.5 eV only available as n-type material) which has been the subject of many investigations. As shown by Meissner et al., anodic dissolution can occur under illumination in two ways depending on the oxygen concentration in the electrolyte [45, 46]. In the presence of oxygen ... 

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. 

Similar to the formation of porous aluminum oxide a passivation - dissolution mechanism can be used to form nanopo-rous structures on InP. If (OOl)n-InP is polarized anodically under illumination in HCl solutions, nanoscaled pores are formed [117]. For potentials up to 1.2 V vs. SGE the main reaction is uniform anodic dissolution. Above this potential porous InP with a surface oxide is formed. The overpotential and anodizing time influence pore diameter (110-250 nm), wall thickness (16-50 nm) and pore length... 

As discussed previously, anodic currents at n-type electrodes can be increased by light excitation if holes are required for the corresponding electrode process. In many cases such a process is the anodic dissolution of the semiconductor material. The question arises now, what kind of process occurs under illumination, if a redox system is added to the electrolyte ... 

PIZZINI It seems that in the case of Ti02 the oxygen evolution takes place under illumination without any titanium dissolution. Apparently water molecules adsorbed at the surface could be involved in the primary reaction step, and we, in fact, observed basification of the titanium oxide comportment. Could not well be this the case of ZnO, where, due to the basification (local) of the solution, dissolution of the crystal ZnO takes place instead of the anodic dissolution of Zn ... 

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