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ELECTROCHEMICAL ASPECTSOF GERMANIUM DISSOLUTION
Some Electrochemical Aspects of Germanium Dissolution. Simultaneous Chemical and Electrochemical Oxidation
by Walter E. Reid, Jr. Electrochemistry Section, National Bureau of Standards, Washington, D . C.
(Received December 90, 1964)
The dissolution of germanium by oxidation-reduction agents was investigated for both unpolarized and polarized specimens. The rate of the chemical oxidation process is expressed directly as a corrosion current, io, proportional to the oxidation-reduction potential, E ” , above a certain minimum value, E,, and obeys the relation i, = k(E” - Em). The electrode potential, VE, for this case can be correlated with i, and with E”. A mathematical expression is obtained relating E” with i, and i,, the respective rates of the chemical and electrochemical oxidation rates. This relation E” = 0.21(2i,/ia 1) log i, enables the interrelation of these quantities to be better understood and indicates the reasons for “current multiplication.” The enhancement of the chemical corrosion current by anodic polarization is due to an increase in the number of surface sites and the enhancement of the anodic process by chemical corrosion is due to the partially oxidized state of the surface. The techniques used give a general method for determining fairly accurately the standard electrode potentials of oxidation-reduction couples not easily measured by conventional means. For Ge the values are: E o G e - - G e t l = 0.24 v.; Eo-Gett-Ge+4 = 0.00 v.
+
Certain aspects of the behavior of the germanium anode in the presence of oxidizing reagents have been studied The effects of simultaneous anodic and chemical dissociation for varying current densities and oxidizing agents of different oxidationreduction potentials have not been thoroughly investigated, although some studies of this nature have been made by Beck and Geri~cher.~-a A fairly extensive investigation of the chemical corrosion of Ge and Si has been made by Turner.’ The anodic and chemical dissolution behavior of a p-type Ge anode was examined in the presence of oxidizing agents of varying E” values for a range of current densities to determine the interdependence of these variables in the dissolution process. As a part of this investigation the mechanism of current increase brought about by oxidizing agents, so-called “current multiplication,” was also investigated. I . The Chemical Oxidation Reaction. If we place two dissimilar metals in contact or mix two solutions containing oxidizing materials of different standard electrode potentials ( E ” ) , we observe a transfer of electrons from the higher to the lower energy state.
With a metal in contact with an oxidizing solution the process becomes expressable as a corrosion current or rate which will vary with the concentration of oxidizing material provided the reaction products are soluble in the solution. Charge transfer occurs directly across the metal-solution interface, and for a clean surface is controlled only by the rate of transport of oxidizing species to the metal surface and the activation energy of the process. For a series of similar oxidizing agents of differing E”, the corrosion rate will be a linear function of the E” values a t constant temperature, stirring speed, and concentration, as will be discussed below. (1) D. R. Turner, “The Electrochemistry of Semiconductors,” P. J. Holmes, Ed., Academic Press Inc., New York, N . Y., 1962. (2) J. F. Dewald, ”Semiconductors,” N. B. Hannay, Ed., Reinhold Publishing Corp., New York, N. Y., 1959. (3) E. A. Efimov and I. G. Erusalimchik, “Electrochemistry of Germanium and Silicon,” Sigma Press, Washington, D. C., 1963 (English Translation). (4) F. Beck and H . Gerischer, 2 . Elektrochem., 63, 943 (1959). (5) F. Beck and H. Gerischer, ibid., 63, 500 (1959). (6) H. Gerischer and F. Beck, Z . p h y s i k . Chem. (Frankfurt), 24, 378 (1960). (7) D. R. Turner, J. Electrochem. Soc., 107, 810 (1960).
Volume 69, Number 7
J u l y 1965
WALTERE. REID,JR.
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The intercept of the resulting straight line in the plot of E" 21s. io will be the minimum standard oxidizing potential, E,, required to oxidize the metal. If the metal has only two stable valence states, the intercept will give the E" of the lower valence state in general, since stepwise electron removal must occur,8 and the initial electron removal usually involves the higher energy requirements.
Experimental Procedure A p-type Ge specimen of resistivity 2.3 ohm cm. was used in almost all of the experiments; however, n-type Ge was found to give identical results in several of the corrosion experiments. A definite area (5 cm.z) of the Ge specimen was exposed to the oxidizing solution under examination, and the weight loss after a fixed time was determined. This weight loss was converted to a corrosion current density by means of Faraday's law: i, = (wt. loss/t)(S/equiv. wt.)(l/A). The corrosion current for each oxidizing solution was determined a t least three times. If the experimental work was interrupted for several days, it was found necessary to remove the thick oxide film found on the specimen with CP-4 etchant; otherwise, the corrosion rates were abnormally low. Since the corrosion current varied linearly with the concentration of the oxidizing ion (Figure 1), the experiments 'were performed for short time intervals (-15 min.) so that the concentration would not change drastically. The initial concentration of the oxidizing agent was 0.1 N in all cases. All experiments were performed in a 400-ml. Pyrex tall-form beaker containing 100 ml. of solution. A Fisher magnetic stirrer Model 14-1511-1, was used with a 27 X 6 mm. Teflon-coated stirring bar. The stirrer was operated a t its minimum speed. The stirring rate was such that at these concentrations the maximum possible corrosion rate or limiting current was about 27 ma./cni.2 as determined from the corrosion of a Cu specimen. It was found for Cu that a series of oxidizing agents with E" > -0.60 v. gave an i, value essentially constant a t 27 nia./cm.2 indicating diffusion control. Below this E" value the Cu was not dissolved. This value of 27 nia./cm.* was taken as the maximum possible corrosion rate for the experimental conditions described. The following solutions were used for the corrosion studies of Ge
0.1 N Ce4+in 1 M HzS04; E"
0.1 N K3Fe(CN)Bin 0.1 N HzS04; E"
=
c
.06
2
4
6
8
10
12 14 io(mo/cme )
16
18
20
22
24
Figure 1. Dependence of corrosion current on concentration of oxidation-reduction agent.
0.56 v.
0.1 N VOz+ in 1 d l HzSO,; E o = 0.92 v. (this investigation) The Journal of Physical Chemistry
1.43 v.
The K3Fe(CN)e solution was used immediately after preparation as it decomposed slowly upon standing. The experiments with this solution were kept as brief as possible, as a white precipitate, believed to be a germanium ferrocyanide compound, was slowly formed on the Ge specimen, and thus in the electrochemical oxidation studies a slight increase in potential was occasionally required to maintain a constant current. I n similar experiments involving copper, an analogous compound was formed very readily. It is noted that the VOz+-V02+ couple (E" = 0.92 v.) is reduced to the V02+-VO+ couple (E" = 0.30 v.). The corrosion current of the latter is assumed to be sufficiently small for the conditions used so as not to affect seriously the value obtained for the corrosion current of the former. The short exposure times and the dependence of io upon concentration justify this assumption. All potential measurements were made with the Hgl Hg2S0411 M HzS04 reference electrode ( E = 0.67 v.) and are expressed relative to the normal hydrogen electrode. The junction potential was assumed to be less than the experimental error and was neglected. Measurements made of the potential of Ge in the 1 M H$04 with and without stirring indicated no significant change (
