2470
Jovan Antula and Bernhard F. Becker
Investigation of the Cathodic Reduction of Lithium and Arsenic Ions on Monocrystalline Silicon by Cyclic Voltammetry
Jovan Antula' MPl, 8 Munich 12, P.O. Box 12 0 1 05, West Germany
and Bernhard F. Becker Institute for Inorganic Chemistry, Technical University of Munich, 8 Munchen 2, West Germany (Received June 4, 1975)
Linear sweep cyclic voltammetry has been applied for qualitatively testing silicon-electrolyte systems for their suitability for electrolytically doping silicon. The metals investigated as dopants were lithium and arsenic. Both are shown to penetrate cathodically into silicon. In the case of lithium, the formation of the intermetallic phase occurs partly by direct electrochemical incorporation. The diagnostic criteria used in the evaluation of the cyclic voltammograms are discussed.
Introduction
A semiconductor is electrolytically doped when ions of a dopant material penetrate into it from solution under the influence of an applied electric fie1d.l Linear sweep cyclic voltammetry is employed here as a rapid means for testing semiconductor-electrolyte systems on their suitability for achieving an electrolytical doping by the cathodic penetration of metals. The semiconductor of interest was silicon and the metals investigated as dopants were lithium and arsenic. The particular interest in lithium as a dopant for silicon is related to lithium-drifted radiation detectors and to lithium-doped solar cellsq2Arsenic shows the highest solubility of all the classical dopants in silicona3 The entire complex of electrochemical alloying or the formation of intermetallic compounds has received considerable attention over the last few years, because it is related to the problem of new batteries and fuel cells.* The cathodic penetration of lithium into various metals such as Sn, Pb, Al, Au, Zn, Cd, Ag, Pt,5 Mg,5,6and Ga7 and of alkali metals into graphite8 has recently been investigated. Intermetallic compounds are formed when Li is deposited on As, Sb, or Tl.9 Na and K have been introduced into Si by electrochemical reduction from salt melts.1° Experimental Section (a) Cyclic Voltammetry. Principle and use of the method of cyclic voltammetry in its application to the study of electrode-electrolyte behavior have been adequately described in the l i t e r a t ~ r e . ~ , ~ J ~ , ~ ~ The electrical circuit employed is shown in Figure 1. A potentiostat (Bank, Gottingen, Wenking Type 68 T S 10) coupled with a triangular voltage generator (SMP 69 or VSG 72, Bank, Gottingen) regulated the potential between the working electrode and the reference electrode (saturated calomel K 401, Radiometer, Copenhagen). The plotter was a Hewlett-Packard 7004B X-Y recorder. The measurements were carried out in a glass cell containing 5 ml of test electrolyte. A 0.6-cm2 platinum plate positioned parallel to and at a distance of 0.5 cm from the working electrode served as the counterelectrode. The reference electrode was positioned in its own compartment The Journal of Physical Chemistry, Vol. 79, No. 23, 1975
and separated from the test electrolyte by a porcelain diaphragm. The reference electrode compartment culminated in the main cell as a Luggin capillary placed 1mm from the surface of the working electrode. All measurements were carried out a t room temperature. (b) Cyclic Voltammograms. Typical linear sweep cyclic voltammograms of the systems investigated are shown in Figures 2-9. The voltage scan was started from a potential of zero current flow and proceeded through the cathodic reaction on the left and below the voltage axis (negative current peak) to the anodic reaction above the voltage axis (positive current peak). All potentials are referenced to the saturated calomel electrode. (c) Working Electrodes and Electrolytes. Optically polished monocrystalline silicon, either of p or n type, with a geometrical surface area of about 1.5 cm2 served as the working electrode. T o ensure the same starting conditions before each measurement, the silicon plate was cleaned with benzene, immersed for 3 min in 10%hydrofluoric acid, and subsequently washed with absolute methanol before being dried in an atmosphere of dry nitrogen. In order to compare the behavior of the semiconductor electrode with that of a metal, a chemically pure platinum plate of about 0.6-cm2 surface area was substituted for silicon as the working electrode material. The nonaqueous propylene carbonate-0.1 M lithium perchlorate electrolyte was used for investigating lithium penetration, since hydrogen evolution from water competes with or even prevents the deposition of alkali metals.6 The measurements were carried out under an atmosphere of dry nitrogen. Arsenic is deposited on silicon just before hydrogen evolution starts in aqueous acids. Therefore a 1%solution of arsenic trioxide in concentrated hydrochloric acid could be employed as a test electrolyte for arsenic penetration. Discussion Since the enthalpy of a metal is lessened by alloying, a more positive dissolution potential is displayed by the incorporated form than by the free metal.g Depending on this enthalpy difference, two dissolution peaks or a broadened peak are observed in a cyclic voltammogram if incorporated as well as free metal is simultaneously present. The res-
2471
Reduction of Li and As on Monocrystalline Silicon
VOLTAGE
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a
-
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SATURATED CALOMEL ELECTRODE
- -3
Figure 1. Schematic representation of the system used for cyclic volammetry measurements.
- -4
- -5 I -3
-1.5 u (V) - 2 Figure 4. Cyclic voltammogram of lithium deposition and dissolution on monocrystalline silicon (n-type, specific resistivity 0.06 12 cm): v = 0.1 v min-’
-3.5
-2.5
1-6 -2.5
-3
u ( v ) -2
-1
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-2
Figure 2. Cyclic voltammogram of lithium deposition and dissolution on platinum metal: sweep rate (v) = 0.1 V min-l.
-1
u [VI Figure 5. Cyclic voltammogram of lithium deposition and dissolution on monocrystalline silicon (p-type, specific resistivity 0.01 12 cm): v = 0.3 V min-’.
-
1-
U
E 0-1 -
u
20
-2-
I -3
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u[VI Figure 3. Cyclic voltammogram of lithium deposition and dissolution on monocrystalline silicon (n-type, specific resistivity 0.06 i2 cm): v = 1 V min-l.
olution into two peaks also depends on the relative amounts of material available for electrochemical dissolution. The area enclosed by a peak is proportional to the charge transfered and therefore to the amount of material converted in an electrode process. The cyclic voltammograms recorded with lithium and arsenic electrolytes will be discussed on the basis of these criteria. (a) Lithium. Lithium deposited on platinum gives rise to a single, sharp positive peak a t -2.8 V on reoxidation (Fig-
Flgure 8. Cyclic voltammogram of arsenic deposition and dissolution on platinum metal: v = 3 V mine‘. ure 2). The electrodeposition is reversible and without any sign of incorporation of lithium into platinum over the range of voltage sweep rates from 3 to 0.1 V min-l. It has been established however, that lithium can be electrochemically alloyed with platinum from the solution used.5 The failure to detect this alloying by cyclic voltammetry demonstrates that the extent and/or rate of penetraThe Journal ofPhysical Chemistry, Vol. 79, No. 23. 1975
2472
Jovan Antub and Bernhard F. Becker
I
i -1.5
-1
-0.5
0
0.5
1
1.5
Ubl
Cyclic voitammogram of monocrystalline silicon (n-type, specific resistivity 0.06 R cm) in concentrated hydrochloric acid in the absence of arsenic: w = 0.03 V min-’. Figure 7.
-0.41
-1.5
-1
- 0.5
0
0.5
1
1.5
u [VI Figure 8. Cyclic voltammogram of arsenic deposition and dissolution on monocrystalline silicon (n-type, specific resistivity 0.06 R cm): w = 0.3 V min-’.
-a E
O 0.11
’
*
I
1
A
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Y
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Figure 9. Cyclic voitammogram of arsenic deposition and dissolution on monocrystalline silicon (n-type. specific resistivity 0.06 R cm): w = 0.03 V min-I.
tion of lithium into platinum muat be low. In fact, after being held a t -3 V for 5 min a second, small lithium dissolution peak does appear at -2.8 V. The overlapping of the reduction current curves in Figure 2 can be ascribed to a slight crystallization overpotential at the beginning of the lithium deposition on platinum.!) In contrast to its behavior with platinum, lithium deposited on n- or p-type silicon already shows a pronounced broadening of its positive dissolution peak at voltage sweep rates of 1 and 0.3 V min-’ (Figures 3 and 5), ana even a splitting into two peaks at 0.1 V min-’ (Figure 4). In Figure 4 it becomes particularly obvious that there are a t least two “types” of lithium present after reduction from solution on silicon. The more cathodic dissolution peak is assigned to The Journal of Physical Chemistry, Vol. 79, No. 23, 1975
free lithium merely deposited on the surface of the electrode. Penetration into silicon with the formation of an intermetallic phase accounts for the lithium of lower energy.6*9 The negative deposition-current peak in Figures 3-5 displays a “shoulder” slightly anodic of the potential where the main current begins to flow and a t which free surface lithium is deposited. Alloying by way of a secondary, purely chemical reaction between deposited metal and the substrate electrode does not lead to a deposition of the metal a t more positive potential^.^ The incorporation of lithium into silicon must therefore be a t least partly due to a direct electrochemical penetration reaction. At slower voltage sweep rates the direct electrochemical incorporation of lithium into silicon gains in significance, as is evidenced by the pronounced deposition shoulder a t -2.8 V in Figure 4. Also more lithium is incorporated into the electrode a t slower sweep rates. This is indicated by the relative area under the alloyed and the surface lithium dissolution peak in Figure 4 as compared to Figure 3. Since the amount of lithium redissolved from the alloyed form is greater than the amount incorporated by direct electrochemical alloying (Figures 3-5), alloying by way of a secondary, chemical reaction must also take place. This reaction is more important at fast sweep rates, where most of the metal is primarily deposited on the surface of the electrode (Figure 3). The high reversibility of the lithium penetration into silicon indicates that the incorporated metal ions retain a high mobility within the silicon lattice.6 This is in accordance with the known high diffusion constant of lithium in silicon, corresponding to an interstitial d ~ p a n t . ~ The anodic displacement of the dissolution peak of surface lithium on silicon (Figures 3-5) of up to 0.8 V against that on platinum (Figure 2) implies a higher activation polarization for lithium dissolution from silicon than from platinum. A similar observation has been made with lithium deposited on T1, Sb, and As electrode^.^ (b) Arsenic. Arsenic(II1) reduction to the metal (-0.2 V) and reoxidation (+0.4V) on platinum are shown in Figure 6. The electrode potential in the hydrochloric acid electrolyte used for testing the arsenic deposition is limited cathodically by the evolution of hydrogen and anodically by that of oxygen. The stability range of this medium is dependent on the electrode material (Figures 6 and 7). Because of the large potential difference between both electrode processes a pronounced activation polarization must evidently be overcome before either one or both of them take place. Arsenic deposition on silicon is accompanied by an even greater activation overpotential, the deposition and dissolution peaks being separated by about 1.5 V (Figures 8 and 9). In contrast to the single arsenic dissolution peak observed from platinum (Figure 6), two peaks can be discerned with silicon electrodes (Figures 8 and 9), as was the case with lithium. The more anodic peak a t +0.8 V ascribed to incorporated arsenic again increases relative to the dissolution peak of free surface arsenic a t +0.6 V with a decrease in the voltage sweep rate (Figures 8 and 9). Two distinct arsenic reduction peaks cannot be observed under any conditions. There also does not appear to be a significant anodic shift of the deposition potential with a decrease in sweep rate, Thus, contrary to lithium, a direct electrochemical incorporation of arsenic into silicon is not indicated by cyclic voltammetry.
2473
Kinetics of Two Simultaneous Second-Order Reactions
Conclusion cyclic voltammetry has proven to be a quick, easy method for qualitatively testing electrolytes as to their suitability for electrolytically doping silicon. The incorporation of cathodically lithium or arsenic into the electrode - deposited material is characterized by a broadening of the reoxidation peak or its resolution into two peaks, as compared to the dissolution from an essentially inert electrode material such as platinum. The more anodic dissolution peak which arises from the incorporated metal is enhanced by slower voltage sweep rates. Both lithium and arsenic showed a high reversibility of the primary incorporation which, a t least in the case of lithium, is partly of a direct electrochemical nature. A secondary chemical reaction between deposited surface metal and the electrode material also leads to alloying. Direct electrochemical incorporation of arsenic into silicon could not be detected. From the dissolution current strengths under similar conditions of concentration, electrode area, and voltage sweep rate it can be estimated that the rate of alloy formation between lithium and silicon is about an order of magnitude larger than that of arsenic with silicon. The validity of the method was subsequently tested by electrolytically doping silicon with lithium and arsenic from the solutions examined here. The penetration of the metal ions from solution into the electrode material was verified by means of secondary ions mass spectroscopy1 and electric resistance measurements.13
Cyclic voltammetry cannot however absolutely exclude that alloying will take place. A very low rate or extent of alloying fails to give rise to.the above mentioned characteristic features in a cyclic voltammogram. The deposition of lithium on Platinum demonstrates this.
Acknowledgments. The authors are indebted to Professor Dr. H. P. Fritz of the Institute for Inorganic Chemistry of the Technical University of Munich for the use of laboratory facilities and to Dr. J. 0. Besenhard of the same institute for many useful discussions. Thanks are due to the company Wacker-Chemie, Burghausen, Germany, for providing the silicon material. References and Notes (1) J. Antula and G. Staudenmair, to be submitted for publication. (2) J. C. Larue, Phys. Status SolidiA, 6, 143 (1971). (3) W. Harth, “Haibleitertechnologle”, 8. G. Teubner, Stuttgart, 1972. (4) A. K. Vljh, “Electrochemistry of Metals and Semiconductors”, Marcel Dekker, New York, N.Y., 1973. (5) A. N. Dey, J. Electrochem. Soc., 118, 1547 (1971). (6) M. M. Nicholson, J. Nectrochem. SOC.,121, 734 (1974). (7) B.N. Kababov, Elektrokhimiya, 10, 765 (1974). (8) J. 0. Besenhardand H. P.Fritz, J. Nectroanal. Chem., 53, 329 (1974). (9) J. 0. Besenhard and H. P. Fritz, Nectrochim. Acta, 20, 513 (1975). (IO) L. Svob, Solidstate Nectron., 10, 991 (1967). (11) M.Shaw and A. H. Remanick, US Report No. 66-37 286 (1966). (12) R. N. Adams, “Electrochemistry at Solid Electrodes”, Marcel Dekker, New York, N.Y., 1969. (13) J. Antula, to be submitted for publication.
Kinetics of Two Simultaneous Second-Order Reactions Occurring in Different Zones Malcolm Dole,* Chang S. Hsu, V. M. Patel, and G. N. Patel Department of Chemistry, Bay/or University, Waco, Texas 76703 (Recelved January 30, 1975; Revised Manuscript Received August 20, 1975) Publication costs assisted by Eaylor University
Equations have been derived for the case of free radicals recombining according to the second-order kinetics with or without diffusion control under the conditions that there are two simultaneous spatially separated recombination reactions but that only the overall free-radical concentration can be observed. The properties of these equations are discussed and methods for determining the three independent parameters in the first case and five in the second developed. The resulting equations have been applied to the interpretation of data obtained in studying the decay of allyl chain free radicals in irradiated extended chain crystalline polyethylene.
1. Introduction Some years ago’ Dole and Inokuti drived the conditions for first- or second-order kinetics of reactions in which the reacting species were isolated in multiple reaction zones, the reaction rate constant being the same in each zone. They did not consider the case where the reaction rate qonstant was different in different zones. In this paper we wish to look into the mathematical consequences of having two reaction zones with different reaction rate constants, but
under the condition that only the overall concentration of the reacting species can be measured. Such a situation would exist, for example, in a second-order recombination reaction between allylic type chain free radicals in the crystalline or amorphous phases of an irradiated polymer such as polyethylene. In the case of the decay of alkyl type free radicals in irradiated polyethylene, Johnson, Wen, and Dole2 demonstrated that the data could be quantitatively interpreted in terms of two simultaneous first-order reac-
