B. L O V R E ~ EAND K J. O’M. BOCKRIS
1308
formed in N,N-dimethylformamide were completely soluble in the solvent. This may indicate that poly-
Vol. 63
mers formed in the presence of water have a higher molecular weight and are more highly crosslinked.
THE POTENTIAL O F THE SEMICONDUCTOR-SOLUTION INTERFACE I N THE ABSENCE OF N E T CURRENT FLOW: GERMANIUM BY B.LOVRE~EK’ AND J. O’M. BOCKRIS John Harrison Laboratory of Chemistry, University of Pennsylvania, Philadelphia
4, Pennsylvania
Received December 80, 1868
The otential a t the interface between n- and p-type Ge electrodes of crystal orientations 110 and 111 has been measured in high6 ure aqueous solutions of GeOz, of p H 0-14. The potential was affected by stirring and by impurities in the low pH range. 8ver certain ranges of pH, belapH 2 2.303RTIF. GeOz and H2 concentrations have no definite effect. O2 tends t o increase the range in which be/bpH = 2.303RTlF. No essential difference was observed between n- and p-type samples, or those of different crystal orientation. Calculation of the mixed potential indicates only a small deviation from a thermodynamically reversible potential. Calculation of potential-pH relations for various reactions involving Ge suggests a si njficant error in the previously accepted values. The observed results correspond to those of the reaction Ge HzO e 8 e O 2H+ 2eo-, the germanous oxide appearing in two forms, depending on pH and age of the electrode. The recorded phenomena are consistent with this mechanism.
+
+
Introduction The properties of the semiconductor-solution interface (sc.-s.i.1 merits an increasing interest. Thus, it has numerous technological implications: e.g., in etching, electropolishing, electrodeposition of semiconductors, and the preparation of p-n junctions by deposition of metals such as In onto the semiconductor. The fundamental electrochemical processes which underly these technological possibilities are as yet almost completely unexplored.2-s Fundamental investigations of the sc.-s.i. are needed, not only because of their usefulness to technology, but also because the electrode kinetics of processes at the sc.-s.i. involve aspects novel to this field, e.g., the presence of two types of electron flow. Moreover, electrochemical measurements offer certain advantages in elucidation of the structure of the surface region in semiconductors compared with measurements made for this purpose with the semiconductor in contact with a gas (g). Thus: (i) The surface of sc.’s cannot be cleaned by methods successfully used for the preparation of the surfaces of metals; but they may be electrodissolved into highly decontaminated solutions. I n general, “preparation” of a “clean” surface for a sc. is more easily achieved in solution than in contact with the gas phase. (ii) A larger range of potentials (and their free energy levels) can be achieved for the sc.-s.i. than for the sc.-g. interface. Hence, the possibility of determining the energy of surface states is increased. (iii) By controlled specific adsorption of ions from solution, a greater variation in the potential distribution a t the sc.-si. is possible than st the sc.-g. interface. The study of the kinetics and mechanism of processes a t the sc.-s.i. interface has as its object the elucidation of the path and rate-determining step of the electrode reaction. However, before a de-
+
tailed study of this may be made, it is necessary to ascertain what is the over-all reaction at the sc.-s.i., in a given system, Le., the process given by the thermodynamics of the electrode-solution interface a t equilibrium. A study of this subject for the interface Ge(n- and p-type, single crystals)-aqueous solution is herewith recorded. No previous experimental study of the thermodynamics of this type of system has been published (but cf. Nichols and Cooper“). Experimental
(i) General.-Preliminary work carried out with solutions of Analytical Purity showed a considerable instability of potential. Decontamination of the solution from parasitic oxidation-reduction systems showed not only an increase in stability, but also a substantial change in values of de/bpH in acid solutions. Consequently, precautions were taken throughout the work t o achieve relatively clean conditions of solution and electrode surface. The cell and connecting glass parts were made of As-free Pyrex. Only glass was used in the cell and purification trains except for a very short connection of gas cylinder t o the train by means of Tygon. Purified helium could be introduced into all arts of the apparatus and was used to eliminate oxygen theregom. It also was used to impel solutions from one part of the system t o another. (ii) Cell and Temperature Conditions.-The cell contained arrangements whereby four electrodes and a glass reference electrode could be introduced into the solution. The cell, together with certain parts of the purification trains, were maintained in a darkened air thermostat maintained at 2 5 f 1 ’ . (iii) Electrodes.-The electrodes were single crystals supplied by the Philco Corporation in the form of discs, diameter c. 1 cm. and thickness 0.5 mm. They were defined by the designation “n” or “p”; the specific resistivity; and the crystal face. Because it was desired to carry out the measurements on a specific crystal face, the electrode could not be heated into the form ot a sphere.? Sealing into glass is also difficult because of the danger of deformation and introduction of unintentional impurities. The preparation of the electrodes was carried out by: (i) washing in cellosolve; (ii) washing in distilled water; (iii) treatment for one minute in C . P . - ~ ; (iv) washing in dis(1) On leave of absence from the Chemistry Department of the tilled water; ( v ) soldering electrical connections of tinned Technical Faculty,University of Zagreb, Zagreb, Yugoslavia. copper wire onto back of electrode; (vi) repetition of wash(2) W.H. Brattain and C. G. B . Garrett, Phys. Rev., 94, 750 (1954); ing in cellosolve and distilled water; (vii) covering solder and
Teehn. J . , 94, 129 (1955). (0) M. L. Nichols and S. R. Cooper, Ind. Eng. Chem., Anal. Ed., 7 , (3) D.R. Turner, J . Electrochem. Soc., 103,252 (1950). (4) H. Gerisoher and F. Beck, 2.phyaik. Chem. N . F . , 13,389 (1957). 350 (1935). (7) J. O’M. Bookris, B. E. Conway and W. RIehl, J . Sci. Instr., 33, (5) M .Green, Chapter V in “Modetn Aspeots of Electrochemistry,’’ 400 (1950). IJ, J. Q’M. Bvckris, Ed., Butterwvrtb’s, London, 3959. Bel? &at.
Sept., 1959
~ ' O T W N T I A LOF
SEMICONUUCYOR-SOLUTION INTERFACW
surrounding area with polystyrene coating; (viii) electrodes were then pulled against the ground end of a gIass tube by means of the connecting wire. The lower part of this tube then was filled with molten paraffin.* (iv). Purification of Materials.-H2SOd (A. R.) was distilled in a low pressure of purified He. KzSO, (A. R.) was twice recrystallized in an atmosphere of purified He. KOH (A. R.) was dissolved in conductance water and subject to electrolysis, using pure Hg as a cathode. The K amalgam formed was transferred directly and without contact with air to a decomposition vessel. Herein, it was allowed to dissolve to form pure KOH. GeOz was made up by dissolving the spectroscopically pure material (Vieille Montagne) in conductance water. Helium was purified by passage through Hopcalite, soda lime, active Cu powder at 200°,9 SiOzgel (2 t.ubes), three traps maintained a t liquid Nz temperatures, two of which contained activated charcoal; and finally through a fritted disc in conductance water. (v) Electrical.-A conventional tube potentiometer was used for the potential measurements and a second for the pH measurements. A saturated calomel electrode was used as a reference (potentials recorded in this paper refer to the standard Hz scale). (vi) Procedure.-After cleaning in chromic-sulfuric acid and washing with distilled water, the cell was placed in position and connected by means of glass bridges to the other parts of the apparatus. The glass electrode was introduced into one of the electrode holders in the main compartment of the cell; others were closed by means of ground glass plugs. The cell was left in contact with distilled water overnight. The pre-electrolysis compartment (Fig. 1) was filled with 0.1 N I
