- Email: [email protected]
Taking advantage of liquid ammonia to control the surface modification of silicon electrodes
101
J. Electroanal. Chem., 216 (1987) 101-114
Blsevier Sequoia !%A., Lausanne - Printed in The Netherlands
TAKING ADVANTAGE OF LIQUID AMMONIA TO CONTROL SURFACE MODIFICATION OF SILICON ELECTRODES
THE
D. GUYOMARD, M. HBRLBM, C. MATHIEU and J.L. SCULFORT Laboratoire (France)
d’Electrochimie
Interfaciale
du C.N.RS.,
1, Place Aristide
Briand, 92195 Me&m-Bellevue
(Received 6th June 1986; in revised form 14th July 1986)
ABSTRACT
This work shows the advantages of using a non-aqueous solvent such as ammonia, which can be used at cryogenic temperatures and can be prepared anhydrously, to control the state of a silicon electrode surface in contact with the electrolyte. Depending on the in-situ experimental conditions, it was possible to change the oxidized material surface (native oxide) to an oxide-free surface and conversely. An original technique for pretreatment of the electrodes in a solvated electron solution in ammonia was used to corroborate the results obtained with the usual treatment, I-IF under an inert atmosphere. The influence of a superficial oxide layer and the growth of this layer under various conditions, was studied carefully by following the variations of two experimental parameters: the flatband potential and the photocurrent onset potential. By changing the pH, in liquid ammonia, the growth of the superficial oxide layer could be completely controlled; moreover, the oxide could be obtained either by a chemical reaction with traces of water or by a photoelectrochemical route.
(I) INTRODUCTION
Etching semiconductor electrodes before electrochemical studies is necessary. Indeed, etching takes off the superficial layer (e.g. the native oxide), giving a clean and reproducible surface and allowing work with a given electrolyte/semiconductor (ES) interface instead of an uncontrolled electrolyte/superficial layer/semiconductor (EOS) interface. If these ideas are well-known, they still remain difficult to apply experimentally because modification of the interface cannot usually be effected or carefully controlled in situ. The aim of this work was to study carefully the modification of the semiconductor surface in contact with the electrolyte. This was possible with an appropriate choice of the semiconductor material and the electrolytic solvent. Silicon is a very well-known material in physics and in electronics [l], but its electrochemistry is not easy. As a matter of fact, a superficial oxide layer can form
102
in an aqueous electrolyte [2,3], in the air [4,5], and even in non-aqueous electrolytes containing water moisture [6-81. The presence of this uncontrolled superficial layer makes it difficult to reconcile the different results presented in the literature. Silicon electrodes were investigated in water with a suitable etching agent [8] or with an agent stabilizing the oxide thickness [lo]. Chazalviel [11,12] showed the importance of surface defects on the oxide formation kinetics by using defect-free surfaces prepared by mechanochemical etching. The interest in using a non-aqueous solvent with a low water content is therefore evident for fundamental investigations. We chose liquid ammonia, firstly because this solvent can be used at cryogenic temperatures (T < 239 K with P = 1 atm) of which we may expect a slower superficial oxidation and secondly, because the amount of water is completely controlled by the experimental conditions. As a matter of fact, the basic solvent is anhydrous (see text) while acidic or neutral solvents have residual water moisture, the amount of which can be increased deliberately. Ammonia is the only solvent available with these particular properties for the study of the in-situ modifications of silicon electrode surfaces [13]. We present here a report summarizing our results concerning the characterization of oxide-free silicon electrodes and the influence of a deliberately prepared oxide layer on the photoelectrochemical behavior of the electrodes in supporting electrolyte. We investigated the influence of numerous parameters such as n- or p-type silicon, the pH of the ammonia, the technique of etching the electrodes, the immersion time of the etched electrodes in the electrolyte and the amount of moisture in the solvent. Part of this work, dealing with some preliminary results [14] and the behavior of silicon electrodes in basic ammonia [15,16], has already been published. (II) EXPERIMENTAL.
N- and p-type Si single crystals ((100) orientation) were purchased from LAAS (C.N.R.S., Toulouse, France). They were provided with ohmic contacts, polished to a high mirror finish and mounted as described previously [14]. The usual doping concentrations were N* (p-type) = 1016 crne3 and N,, (n-type) = 2 x 1015 cm-3. The usual pretreatment for each electrode involved etching in concentrated HF (40%) during 20 s, drying with blotting paper, and mounting in the cell under an argon or nitrogen stream (to avoid any contact with air). This way of preparing the surface resulted in a reproducible electrode behavior in liquid ammonia. Etching tests in HF + HNO, + CH,COOH (1-2-1) [17] did not give any different results. Ammonia was purified by double distillation from potassium amide. 0.1 mol drne3 KBr was used as the supporting electrolyte after drying under vacuum at 350 K. Before any distillation, the cell was pumped for 24 h. The acidic NH, solutions (NH, + NH,Br) were prepared by condensation of NH, on dry NH,Br. The basic NH, solutions (NH, + KNH,) were prepared in situ by the addition of potassium to the cell in the presence of a platinized platinum wire. These solutions were anhydrous, water being precipitated as KOH (see Section 111.3.iii).
103 TABLE 1 Amount of residual water in purified ammonia at 233 K versus pH [H,O]/mol
PH
dm -3
2x10-3 5x10-4 not detectable
-2 =16 = 30
The NH,Br or KNH, concentrations were 2 X lo-* mol drnm3 in order to stabilize the pH (respectively pH = 2 or pH = 30). The amount of residual water present in carefully purified solvent is reported versus pH in Table 1. The two-compartment cell was described previously [14]. It was thermostatted at 233 K by using a cryostat with an ethanol cooling bath. Potentials were reported versus the Ag/Ag+ (5 X 10m3 mol dmm3) reference electrode (silver reference electrode, denoted SRE); I’,,, = 0 V vs. SHE in water *. The counter electrode was a coiled Au or Pt wire with a large immersed area. The interface was investigated by using classical electrochemical techniques: current-voltage characteristics (cyclic voltammetry), capacitance-voltage characteristics (impedance measurements) and photocurrent-voltage characteristics under white-light illumination. Current-voltage curves were obtained with a PRT 20-2Xtype Tacussel potentiostat without positive feedback resistance compensation. Impedance measurements were performed with a solartron-type Schlumberger system (FRA 1174 and EC1 1186) controlled by an Apple II microcomputer [13,16].
(III) RESULTS AND DISCUSSION
(III. I) Electrochemical
behavior of carefully etched silicon electrodes
(IIZ. 1. i) Voltammograms in the dark Voltammograms obtained on well-etched n-Si electrodes, already published in neutral [14], basic and acidic media [15], are presented in Fig. 1. The redox reactions which limit the electroactivity range of NH, charged with KBr, at a platinum electrode, are the following: In basic or neutral media: n NH, + e- -+ e;(NH,),
(Vs
-2.3
V vs. SRE)
(I)
4NH3+$N,+3NH:+3e-
(IQ
+0.3 v vs. SRE)
(2)
* This potential shift between the reference electrode in NH3 (SRE) and the reference electrode in water (SHE) is calculated and discussed in a short communication that will be presented soon in this Journal.
104 w
J/,A c!o
Clli2 b
! I
a
J/yAeni2
I -1
I 6
I
c
Ijig. 1. Cyclic voltammograms in the dark in supporting liquid ammonia on well-etched n-Si electrodes (1) and psi electrodes (2). u,, = 0.1 V s-l. (a) Basic medium: pH = 30; @) UnbufFeredneutral medium: pH %16; (c) acidic medium: pH = 2.
105
In acidic medium: NH:+e-+NH,+fH,
(Vs
-0.7 V vs. SRE)
reaction (2)
(IQ
+0.5 v vs. SRE)
(3)
(III. I.ii) Voltammograms under white-light illumination The J-V curves obtained on well-etched electrodes under white-light illumination
are summarized in Fig. 2. The photocurrents On n-Si: reaction (2) and Si+x
correspond to the following reactions:
H,0+2xh++SiOX+2xH+
(4)
(residual water)
100
54
a00
-Ioc
a00
-5oQ
Fig. 2. Cyck voltammograms on well-etched electrodes under white-light illumination (about 0.1 W cm-*) in basic (l), neutral (2) and acidic (3) liquid aomonia. o, = 0.1 V s-l. V* are the photocurrent onset potentials. (a) n-S electrodes. ( +, + ) 1” and 2d potential sweeps; ,(-m) 3’d and following sweeps. See Section 111.3.k (b) p-S electrodes.
106 TABLE 2 Relationship of photocurrent onset potentials of n- or p-type silicon samples, etched in HF or unetched, to the pH value of the electrolyte P/V
PH
vs. SRE
n-Si
2 :I6 = 30
p-Si
etched
not etched
etched
not etched
-0.6kO.l -0.6fO.l - 1.0*0.1
+0.7*0.2 +0.7*0.2 +0.4*0.1
-0.35*0.1 -0.6 f0.2 -1.95ItO.l
+ 0.6 & 0.2 +0.5~0.2 - 1.8iO.l
On p-Si: In basic medium : reaction (1) Inneutralmedium:H,O+2e-+H,+2OH(residual water) In acidic medium : reaction (3) The photocurrent onset potentials V* were determined under chopped illtination. They are collected in Table 2. (III.l.iii) Flatband potentials versus pH The flatband potentials were estimated from Mott-Schottky plots deduced from impedance measurements in the dark. The C2-V curves, linear over a wide potential range, were given elsewhere [16,18]. The variation of the flatband potentials of n- and p-type electrodes (etched in HF) with pH is reported in Table 3. Mott-Schottky curves obtained with p-Si electrodes in acidic or neutral media were not perfectly reproducible from one sample to another; the flatband potential range is then given in Table 3. It is important to check that Vfi is consistent with V*. When varying the polarisation applied to a n-type semiconductor, a photocurrent can be obtained only at potentials more positive than V, [19-211. As a matter of fact, the band bending, and consequently the electric field in the space charge layer, then drives the minority photocarriers to the material surface. Comparing the results
TABLE 3 Variation of flatband potentials of n- and p-type electrodes (etched in HF) with pH PH =2 116 = 30
V&/V vs. SRE n-Si
p-Si
-1.1*0.1 - 1.4+0.25 -1.8kO.l
-0.5 to +0.1 -0.8 to -0.2 -0.9kO.2
107
from Table 2 and Table 3, we note that V* and V, are in good agreement because the following relations are verified: V*(n) > V,(n) and V*(p) x V,(p). (III.2) Influence of etching on electrode behavior The quality and the efficiency of the etching of the electrodes were very sensitive to the flatband potentials and the photocurrent onset potentials. (III.2.i) Etching in HF under an Ar or N2 stream
When the n- and p-Si electrodes were not etched, the C-*-V curves were shifted along the potential line, towards more positive potentials, and their linear range was shorter; see Fig. 3 for n-Si in basic medium, for example. The linearity was not very good on p-Si electrodes, but the C-*-V curves were undoubtedly shifted towards more positive potentials. The values of V, (unetched), obtained on n- and p-type electrodes, are not given because of the poor accuracy of the extrapolation potentials at various frequencies. So the amplitude of the V, variation (V, (unetched) V, (etched) was not precisely evaluable. When the n- and p-Si electrodes were not well etched (e.g. too short an etching time), V, was between V, (etched) and V, (unetched). The fact that good Mott-Schottky curves can be obtained only on a carefully etched electrode, is obviously well-known. What is important to us is the shift of V,
1
-2
-1
0
1
Fig. 3. Mott-Schottky plots of n-Si electrodes in basic liquid ammonia. F(2) unetched samples.
v/v
91 kHz. (1) Etched samples;
108
observed on unetched electrodes, which is always positive on n-Si and p-Si electrodes at different pH. When the n-Si and p-Si electrodes were not etched, the J-V curves under white-light illumination were also shifted towards more positive potentials at different pH. The shifts could be evaluated with the new position of the photocurrent onset potential. These onset potentials, measured as described previously, are given in Table 2. These results concerning V, and V* shifts in the presence of a native oxide layer give a good method for detecting the presence of an oxide layer on the Si surface. However, an important point is knowing whether there is any surface oxide on carefully etched electrodes. Is it possible to obtain a more negative I$, value than Vi,, (etched in HF) with a different etching method? To answer this fundamental question we used the pretreatment described below. (III.2.ii) Etching in situ with e,-(NH,), at 233 K A solvated electron solution in ammonia is very easy to obtain: weighted amounts of potassium are dissolved in a basic solvent (if the solvent is not basic, the solvated electron is less stable with time). A solution of [e;(NHs),] = 5 X 10e3 mol dme3 is stable only for about 10 h. Indeed the e;(NH,), species slowly reduces the solvent, yielding hydrogen and amide ions:
e;(NH,>“----NH; (dark blue color)
+ i H, + (PJ- 1) NH, (colorless solution)
(6)
This reaction is thermodynamically spontaneous but its kinetics are very slow [22]. After the decomposition of the solvated electron solution, the pH of the ammonia did not change because the initial amount of NH; was sufficiently high. The Si electrodes were treated as follows: (1) Immersion in basic NH,. (2) Addition of K to the solution to obtain [e;(NH,),] = 5 X 1O-3 mol dmP3 in situ. (3) Waiting for the decomposition of e;(NH,), (solution becomes colorless) according to reaction (6). The impedance measurements were performed in the dark and Mott-Schottky curves were drawn. This treatment led to C2- V curves and extrapolated V,, which were the same as those obtained on electrodes directly immersed in basic medium after etching in HF. The accuracy was excellent with n-Si and only approximate with p-Si (see Table 4). If the electrodes were etched in HF before the treatment with e;(NH3),, no difference was obtained. (111.2.iii) Discussion Vn, was very dependent on the amount of native oxide on the Si surface. These results lead to two important conclusions: on the one hand, the e;(NH,), species
109 TABLE 4 Relationship of the flatband potentials to the etching conditions used for n- and p-type silicon electrodes in basic liquid NH3 Etching conditions
V&/V vs. SRE
Not etched “HF” at 300 K under an AI stream “e; (NH,)” at 233 K 9, “HF + e; (NH3)”
n-si
p-Si
- 1.2 f 0.2 -1.8kO.l -1.8kO.l -1.8kO.l
-0.9+0.1 -0.9kO.2 -0.9kO.2
acts as an etchant of the native oxide and, on the other hand, the two pretreatments (e;(NH,), and HF) leave the same amount of remaining superficial oxide on the Si surface. Because we never obtained a more negative Vi,_,value than the one observed after an HF and/or e;(NH,), treatment, we shall henceforth call these etched Si surfaces oxide-free surfaces. The etching with the e;(NH,), species was done in situ. So it is important to note that we were able to modify the semiconductor surface in contact with the electrolyte from an oxidized surface to an oxide-free one. Now it is possible to give the band-edge energy position of oxide-free silicon electrodes in liquid NH, versus pH. Knowing the doping levels ND and NA of nand p-type Si samples and V, values from Table 3, we deduce V,(p), V,(n), V,(p) and V,(n) from the relationships [19]: V,(n) - V,(n) = (W/e) ln(Nc/N,,) and V,(p) - V,(p) = (kT/e) ln( NV/N,). NC and NV are, respectively, the effective densities of states in the conduction and valence bands. Taking into account the experimental uncertainty of the I$, determination and the bandgap energy, we obtain the relationships: V,(n) = V,(p) and V,(n) = V,(p), which is usually the case in supporting electrolyte [23,24]. These V, and V, values are reported versus pH in Table 5. The energy diagram in Fig. 4 gives a better representation of the band-edge position of oxide-free Si versus pH. The different possible redox reactions determined on a polished platinum electrode [25], are also reported. Our results are compared to those of Bard and co-workers [26] and Krohn and Thomson [27]. Bards results were obtained in an unbuffered neutral medium with KI as electrolyte instead of KBr in our case. Thomson’s results were not experimental; they were TABLE 5 Variation of the band-edge potentials of oxide-free Si electrodes versus pH PH =2 -16 = 30
V”/V vs. SRE
v,/V
- 1.3 fO.l -1.6k0.25 -2.0+0.1
-0.2rtO.l - 0.5 f 0.25 -0.9IkO.l
vs. SE
110
-3-
-3-
.
-2-
*i
-2-
.
0
* 16
32
pH
Fig. 4. Energetic diagrams of oxide-free Si electrodes in supporting liquid ammonia versus pH. Bard’s results (1) and Thomson’s results (2) (see text).
deduced from the observation of the photoinjection of e;(NH,), species in solution from a p-Si photocathode and the knowledge of the e;(NH,),/NH3 redox system potential. The mean variation of the band-edge energy versus pH is 23 mV/unit of pH, that is 0.5 x 2.3 RT/F at 233 K. This variation was already obtained by Morrison and co-workers [28] in aqueous medium. This means there is an analogy between the phenomena in aqueous medium and in NH,: the dependence versus pH is not controlled exclusively by the adsorption of the solvent ions. (III.3) Influence of an oxide layer We reported (see Section 111.2-i) the drastic influence of the native superficial oxide, grown on electrodes exposed to air and not etched before the experiment, on the V, and V* values. We used some parameters to influence the oxide formation in situ on the Si surface, such as the immersion time in the electrolyte of previously etched electrodes and the amount of water in the solvent. Photoelectrochemical oxidation can also be demonstrated.
111
(III.3.i) Spontaneow oxidation of Si samples Well etched Si electrodes were immersed in NH, at different pH. Mott-Schottky curves were obtained at intervals in the dark to follow the changes in V, with time. A second series of experiments was carried out to obtain voltammograms under white-light illumination after a given immersion time. The results depended fundamentally on the pH of the solvent: In neutral (pH = 16) or acidic (pH = 2) purified solutions, Mott-Schottky curves and V, values were shifted towards more positive potentials with increasing immersion times. The V, shift was significant after about 1 h of immersion. A similar variation was observed in V* and was detectable after the same time of immersion. This result shows that a superficial oxide layer is growing slowly with the immersion time of the electrodes in NH,. With added amounts of water in the solvent, the detection time of a superficial oxide decreased. The results were more accurate on n-Si electrodes. It was difficult to evaluate quantitatively the minimum immersion time necessary to form a superficial oxide layer, because of the experimental uncertainty of I’, and V*. We could note only that the oxide formation was not instantaneous at 233 K, even in liquid ammonia containing H,O at a concentration equal to 1 mol dms3. In basic solutions (pH = 30), the Mott-Schottky curves and V, values were not influenced by the immersion time. (III. 3. ii) Electrochemical oxidation of Si samples If a n-Si electrode is biased at a potential more positive than V, under illumination, the photoholes created in the space charge layer are driven to the semiconductor surface [19-211. These photoholes have strong oxidizing properties: they can oxidize the material itself (reaction 4) or a reducing redox species (see Section III.l.ii). The evolution of the J-V curves under illumination on a well-etched n-Si electrode was studied with time (i.e. the number of potential sweeps). The results depended on pH: In neutral or acidic medium, the photocurrent decreased drastically with time and the onset potential shifted towards more positive potentials (see Fig. 2a). At the first positive potential sweep, V* = -0.6 V; at the third sweep, V* was shifted to +0.8 V and the photocurrent was very low. This behaviour is typical of the growth of a passive layer on the Si surface. At basic pH, the J-Y curves under illumination did not depend on the number of potential sweeps, i.e. on time. No passive layer was formed on the Si surface. (III.3.iii) Discussion When the solvent was prepared with the best precautions to mmimize water traces (see Section II), residual water was still present in the neutral and acidic media (see Table l), whereas the amide solutions in ammonia were anhydrous. As a matter of fact, H,O is a weak acid in NH, (pK= 16) [29] and is totally consumed by the strongest base in the solvent, NH;: NH, + H,O + OH- + NH, pK= -16 OH- -I-K+ + KOH
(slightly soluble)
112
The fundamental difference between neutral or acidic and basic media was therefore the residual water. So the above results show that a superficial oxide layer, detectable by a variation of V, or Y*, could grow only if water traces were present in solution. In the same way the passive layer obtained on n-Si under illumination was due to a reaction between the Si surface and residual water. By using acidic or neutral solutions, we were able to modify the semiconductor surface in contact with the electrolyte from an oxide-free surface to an oxidized one. The surface modification, due to spontaneous chemical reaction with water, was faster in the presence of large amounts of water in the solvent. An analogous surface modification was obtained by the electrochemical reaction with photoholes at illuminated n-type silicon. Using a basic solution, we were able to prevent the surface modification of Si electrodes, which needs traces of water in the solvent: since NH; ions were in excess, H,O molecules were completely consumed. Thus, control of the Si surface oxidation was possible in liquid ammonia by changing the experimental conditions. It is interesting to compare the time needed for spontaneous oxidation of the Si surface in our case (about 1 h) with Chazalviel’s results [12]. Chazalviel could measure a faster evolution (about 10 min) of the behavior of the n-Si/acetonitrile + redox system, due to the appearance of surface states in relation to a surface reaction. The very low oxidation rate in our case was certainly due to the low temperature, 233 K. When photoholes oxidized the Si surface, the oxide formed faster (some minutes) if the light intensity was high (0.1 W cmF2). The superficial oxide layer acts as a passive layer, the presence of which leads to the same positive I$, and V* variation on n-Si and p-Si electrodes. The variation of the flatband potential towards more positive potentials can be attributed to an extra potential drop into the oxide layer due to the trapped positive charges Q, 1301, according to the following relationship [19-211:
where C,, is the equivalent capacity of the oxide layer. (IV) CONCLUSION
The use of liquid ammonia containing low concentrations of water or being anhydrous when the pH is basic, allowed the reliable and reproducible characterization of silicon electrodes and the reversible surface modification in situ from an oxide-free surface to an oxidized one and conversely. Two etching techniques for the sample pretreatments, HF under an inert atmosphere at 300 K and solvated electrons in liquid ammonia at 233 K, were studied. They gave similar results for the V, values in basic medium, showing an efficient removal of the native oxide. We could therefore determine the band edge energy position of oxide-free silicon material in contact with liquid ammonia at different pH. The mean dependence of the band edges on pH is approximately 0.5 X 2.3 RT/F.
113
The presence of a superficial oxide layer can be related to a positive shift of the flatband potential and the photocurrent onset potential. They give a reliable tool for detecting the presence and therefore the formation of an oxide. A superficial oxide layer can grow only if water traces are present in the solvent. So controlling the anhydrous character of the solvent by changing the pH, allowed us to control the oxide formation on the Si electrode surface, i.e. the state of the surface in contact with the electrolyte. If the pH was not basic, an oxide layer was obtained in two different ways: it could be formed spontaneously by chemical reaction between the Si surface and residual water or photoelectrochemically using photoholes on a n-Si sample. ACKNOWLEDGEMENT
The authors gratefully acknowledge the referee for helpful critical reading of the manuscript. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
S.M. Sze in Physics of Semiconductor Devices, Wiley Interscience, New York, 1981. F.P. Schmidt and W. Michel, J. EIectrochem. Sot., 104 (1957) 230. A.B. Bocarsly, E.G. Walton and M.S. Wrighton, J. Am. Chem. Sot., 102 (1980) 3390. R.J. Archer, J. EIectrochem. Sot., 104 (1957) 619. S.I. Raider, R. Flits& and M.J. Palmer, J. Electrochem. Sot., 122 (1975) 413. D. Laser and A.J. Bard, J. Phys. Chem., 80 (1976) 459. K.D. Legg, A.B. Ellis, J.M. Bolts and MS. Wrighton, Proc. Natl. Acad. Sci., U.S.A., 74 (1977) 4116. J.N. ChazaIviel, J. EIectrochem. Sot., 129 (1982) 963. M. Matsumara and S.R. Morrison, J. Electroanal. Chem., 147 (1983) 157. B.H. Loo, K.W. Frese and S.R. Morisson, Surf. Sci., 109 (1981) 75. J.N. Chazalviel and T.B. Truong, J. Electroanal Chem., 114 (1980) 299. J.N. Chazalviel and T.B. Truong, J. Am. Chem. Sot., 103 (1981) 7447. D. Guyomard, These de Doctorat d’Etat, Paris VI, 1985. G. Van Amerongen, D. Guyomard, R. Heindl, M. Herlem and J.L. ScuIfort, J. EIectrochem. Sot., 129 (1982) 1998. D. Guyomard, M. Herlem, C. Mathieu, C. Miossec and J.L. Schulfort, J. Electroanal. Chem., 138 (1982) 435. M. Herlem, D. Guyomard, C. Mathieu, J. Belloni and J.L. Sculfort, J. Phys. Chem., 88 (1984) 3826. A. Heller, H.J. Lewerenz and B. Miller, J. Am. Chem. Sot., 103 (1981) 200. D. Guyomard, H. Herlem, C. Mathieu and J.-L. Sculfort, CR. Acad. Sci. Paris, Ser. B, 2% (1983) 1233. W.P. Gomes and F. Cardon, Progr. Surf. Sci., 12 (1982) 155. H. Gerischer in M. Schiavello (Ed.), Photoelectrochemistry, Photocatalysis and Photoreactors, Fundamentals and Developments, Proc. NATO ASI, Vol. 146, Reidel, Dordrecht, 1985, p. 39. D. Guyomard, J. Chim. Phys., 83 (1986) 355. R.R. Dewald, Ber. Bunsenges. Phys. Chem., 1974 (1971) 668. W.H. LafIere, F. Cardon and W.P. Gomes, Surf. Sci., 44 (1974) 541. A.B. Bocarsly, D.G. Bookbinder, R.N. Dominey, N.S. Lewis and MS. Wrighton, J. Am. Chem. Sot., 102 (1980) 3683. M. Herlem, A. Thiebault and J.-J. Minet, J. Electroanal. Chem., 26 (1970) 343. R.E. Malpas, K. Itaya and A.J. Bard, J. Am. Chem. Sot., 103 (1981) 1622.
114
27 28 29 30
C.E. Krohn and J.C. Thomson, Chem. Phys. Lett., 65 (1979) 132. M.J. Madou, B.H. Loo, K.W. Fresc and S.R Morrison, Surf. Sci., 108 (1981) 135. M. Herlem, Thbe de Doctorat d’Etat, Paris, 1966. E.H. Nicollian and J.R. Brews in MOS Physics and Technology, Wiley-Interscience, New York, 1982, p. 156.
J. Electroanal. Chem., 216 (1987) 101-114
Blsevier Sequoia !%A., Lausanne - Printed in The Netherlands
TAKING ADVANTAGE OF LIQUID AMMONIA TO CONTROL SURFACE MODIFICATION OF SILICON ELECTRODES
THE
D. GUYOMARD, M. HBRLBM, C. MATHIEU and J.L. SCULFORT Laboratoire (France)
d’Electrochimie
Interfaciale
du C.N.RS.,
1, Place Aristide
Briand, 92195 Me&m-Bellevue
(Received 6th June 1986; in revised form 14th July 1986)
ABSTRACT
This work shows the advantages of using a non-aqueous solvent such as ammonia, which can be used at cryogenic temperatures and can be prepared anhydrously, to control the state of a silicon electrode surface in contact with the electrolyte. Depending on the in-situ experimental conditions, it was possible to change the oxidized material surface (native oxide) to an oxide-free surface and conversely. An original technique for pretreatment of the electrodes in a solvated electron solution in ammonia was used to corroborate the results obtained with the usual treatment, I-IF under an inert atmosphere. The influence of a superficial oxide layer and the growth of this layer under various conditions, was studied carefully by following the variations of two experimental parameters: the flatband potential and the photocurrent onset potential. By changing the pH, in liquid ammonia, the growth of the superficial oxide layer could be completely controlled; moreover, the oxide could be obtained either by a chemical reaction with traces of water or by a photoelectrochemical route.
(I) INTRODUCTION
Etching semiconductor electrodes before electrochemical studies is necessary. Indeed, etching takes off the superficial layer (e.g. the native oxide), giving a clean and reproducible surface and allowing work with a given electrolyte/semiconductor (ES) interface instead of an uncontrolled electrolyte/superficial layer/semiconductor (EOS) interface. If these ideas are well-known, they still remain difficult to apply experimentally because modification of the interface cannot usually be effected or carefully controlled in situ. The aim of this work was to study carefully the modification of the semiconductor surface in contact with the electrolyte. This was possible with an appropriate choice of the semiconductor material and the electrolytic solvent. Silicon is a very well-known material in physics and in electronics [l], but its electrochemistry is not easy. As a matter of fact, a superficial oxide layer can form
102
in an aqueous electrolyte [2,3], in the air [4,5], and even in non-aqueous electrolytes containing water moisture [6-81. The presence of this uncontrolled superficial layer makes it difficult to reconcile the different results presented in the literature. Silicon electrodes were investigated in water with a suitable etching agent [8] or with an agent stabilizing the oxide thickness [lo]. Chazalviel [11,12] showed the importance of surface defects on the oxide formation kinetics by using defect-free surfaces prepared by mechanochemical etching. The interest in using a non-aqueous solvent with a low water content is therefore evident for fundamental investigations. We chose liquid ammonia, firstly because this solvent can be used at cryogenic temperatures (T < 239 K with P = 1 atm) of which we may expect a slower superficial oxidation and secondly, because the amount of water is completely controlled by the experimental conditions. As a matter of fact, the basic solvent is anhydrous (see text) while acidic or neutral solvents have residual water moisture, the amount of which can be increased deliberately. Ammonia is the only solvent available with these particular properties for the study of the in-situ modifications of silicon electrode surfaces [13]. We present here a report summarizing our results concerning the characterization of oxide-free silicon electrodes and the influence of a deliberately prepared oxide layer on the photoelectrochemical behavior of the electrodes in supporting electrolyte. We investigated the influence of numerous parameters such as n- or p-type silicon, the pH of the ammonia, the technique of etching the electrodes, the immersion time of the etched electrodes in the electrolyte and the amount of moisture in the solvent. Part of this work, dealing with some preliminary results [14] and the behavior of silicon electrodes in basic ammonia [15,16], has already been published. (II) EXPERIMENTAL.
N- and p-type Si single crystals ((100) orientation) were purchased from LAAS (C.N.R.S., Toulouse, France). They were provided with ohmic contacts, polished to a high mirror finish and mounted as described previously [14]. The usual doping concentrations were N* (p-type) = 1016 crne3 and N,, (n-type) = 2 x 1015 cm-3. The usual pretreatment for each electrode involved etching in concentrated HF (40%) during 20 s, drying with blotting paper, and mounting in the cell under an argon or nitrogen stream (to avoid any contact with air). This way of preparing the surface resulted in a reproducible electrode behavior in liquid ammonia. Etching tests in HF + HNO, + CH,COOH (1-2-1) [17] did not give any different results. Ammonia was purified by double distillation from potassium amide. 0.1 mol drne3 KBr was used as the supporting electrolyte after drying under vacuum at 350 K. Before any distillation, the cell was pumped for 24 h. The acidic NH, solutions (NH, + NH,Br) were prepared by condensation of NH, on dry NH,Br. The basic NH, solutions (NH, + KNH,) were prepared in situ by the addition of potassium to the cell in the presence of a platinized platinum wire. These solutions were anhydrous, water being precipitated as KOH (see Section 111.3.iii).
103 TABLE 1 Amount of residual water in purified ammonia at 233 K versus pH [H,O]/mol
PH
dm -3
2x10-3 5x10-4 not detectable
-2 =16 = 30
The NH,Br or KNH, concentrations were 2 X lo-* mol drnm3 in order to stabilize the pH (respectively pH = 2 or pH = 30). The amount of residual water present in carefully purified solvent is reported versus pH in Table 1. The two-compartment cell was described previously [14]. It was thermostatted at 233 K by using a cryostat with an ethanol cooling bath. Potentials were reported versus the Ag/Ag+ (5 X 10m3 mol dmm3) reference electrode (silver reference electrode, denoted SRE); I’,,, = 0 V vs. SHE in water *. The counter electrode was a coiled Au or Pt wire with a large immersed area. The interface was investigated by using classical electrochemical techniques: current-voltage characteristics (cyclic voltammetry), capacitance-voltage characteristics (impedance measurements) and photocurrent-voltage characteristics under white-light illumination. Current-voltage curves were obtained with a PRT 20-2Xtype Tacussel potentiostat without positive feedback resistance compensation. Impedance measurements were performed with a solartron-type Schlumberger system (FRA 1174 and EC1 1186) controlled by an Apple II microcomputer [13,16].
(III) RESULTS AND DISCUSSION
(III. I) Electrochemical
behavior of carefully etched silicon electrodes
(IIZ. 1. i) Voltammograms in the dark Voltammograms obtained on well-etched n-Si electrodes, already published in neutral [14], basic and acidic media [15], are presented in Fig. 1. The redox reactions which limit the electroactivity range of NH, charged with KBr, at a platinum electrode, are the following: In basic or neutral media: n NH, + e- -+ e;(NH,),
(Vs
-2.3
V vs. SRE)
(I)
4NH3+$N,+3NH:+3e-
(IQ
+0.3 v vs. SRE)
(2)
* This potential shift between the reference electrode in NH3 (SRE) and the reference electrode in water (SHE) is calculated and discussed in a short communication that will be presented soon in this Journal.
104 w
J/,A c!o
Clli2 b
! I
a
J/yAeni2
I -1
I 6
I
c
Ijig. 1. Cyclic voltammograms in the dark in supporting liquid ammonia on well-etched n-Si electrodes (1) and psi electrodes (2). u,, = 0.1 V s-l. (a) Basic medium: pH = 30; @) UnbufFeredneutral medium: pH %16; (c) acidic medium: pH = 2.
105
In acidic medium: NH:+e-+NH,+fH,
(Vs
-0.7 V vs. SRE)
reaction (2)
(IQ
+0.5 v vs. SRE)
(3)
(III. I.ii) Voltammograms under white-light illumination The J-V curves obtained on well-etched electrodes under white-light illumination
are summarized in Fig. 2. The photocurrents On n-Si: reaction (2) and Si+x
correspond to the following reactions:
H,0+2xh++SiOX+2xH+
(4)
(residual water)
100
54
a00
-Ioc
a00
-5oQ
Fig. 2. Cyck voltammograms on well-etched electrodes under white-light illumination (about 0.1 W cm-*) in basic (l), neutral (2) and acidic (3) liquid aomonia. o, = 0.1 V s-l. V* are the photocurrent onset potentials. (a) n-S electrodes. ( +, + ) 1” and 2d potential sweeps; ,(-m) 3’d and following sweeps. See Section 111.3.k (b) p-S electrodes.
106 TABLE 2 Relationship of photocurrent onset potentials of n- or p-type silicon samples, etched in HF or unetched, to the pH value of the electrolyte P/V
PH
vs. SRE
n-Si
2 :I6 = 30
p-Si
etched
not etched
etched
not etched
-0.6kO.l -0.6fO.l - 1.0*0.1
+0.7*0.2 +0.7*0.2 +0.4*0.1
-0.35*0.1 -0.6 f0.2 -1.95ItO.l
+ 0.6 & 0.2 +0.5~0.2 - 1.8iO.l
On p-Si: In basic medium : reaction (1) Inneutralmedium:H,O+2e-+H,+2OH(residual water) In acidic medium : reaction (3) The photocurrent onset potentials V* were determined under chopped illtination. They are collected in Table 2. (III.l.iii) Flatband potentials versus pH The flatband potentials were estimated from Mott-Schottky plots deduced from impedance measurements in the dark. The C2-V curves, linear over a wide potential range, were given elsewhere [16,18]. The variation of the flatband potentials of n- and p-type electrodes (etched in HF) with pH is reported in Table 3. Mott-Schottky curves obtained with p-Si electrodes in acidic or neutral media were not perfectly reproducible from one sample to another; the flatband potential range is then given in Table 3. It is important to check that Vfi is consistent with V*. When varying the polarisation applied to a n-type semiconductor, a photocurrent can be obtained only at potentials more positive than V, [19-211. As a matter of fact, the band bending, and consequently the electric field in the space charge layer, then drives the minority photocarriers to the material surface. Comparing the results
TABLE 3 Variation of flatband potentials of n- and p-type electrodes (etched in HF) with pH PH =2 116 = 30
V&/V vs. SRE n-Si
p-Si
-1.1*0.1 - 1.4+0.25 -1.8kO.l
-0.5 to +0.1 -0.8 to -0.2 -0.9kO.2
107
from Table 2 and Table 3, we note that V* and V, are in good agreement because the following relations are verified: V*(n) > V,(n) and V*(p) x V,(p). (III.2) Influence of etching on electrode behavior The quality and the efficiency of the etching of the electrodes were very sensitive to the flatband potentials and the photocurrent onset potentials. (III.2.i) Etching in HF under an Ar or N2 stream
When the n- and p-Si electrodes were not etched, the C-*-V curves were shifted along the potential line, towards more positive potentials, and their linear range was shorter; see Fig. 3 for n-Si in basic medium, for example. The linearity was not very good on p-Si electrodes, but the C-*-V curves were undoubtedly shifted towards more positive potentials. The values of V, (unetched), obtained on n- and p-type electrodes, are not given because of the poor accuracy of the extrapolation potentials at various frequencies. So the amplitude of the V, variation (V, (unetched) V, (etched) was not precisely evaluable. When the n- and p-Si electrodes were not well etched (e.g. too short an etching time), V, was between V, (etched) and V, (unetched). The fact that good Mott-Schottky curves can be obtained only on a carefully etched electrode, is obviously well-known. What is important to us is the shift of V,
1
-2
-1
0
1
Fig. 3. Mott-Schottky plots of n-Si electrodes in basic liquid ammonia. F(2) unetched samples.
v/v
91 kHz. (1) Etched samples;
108
observed on unetched electrodes, which is always positive on n-Si and p-Si electrodes at different pH. When the n-Si and p-Si electrodes were not etched, the J-V curves under white-light illumination were also shifted towards more positive potentials at different pH. The shifts could be evaluated with the new position of the photocurrent onset potential. These onset potentials, measured as described previously, are given in Table 2. These results concerning V, and V* shifts in the presence of a native oxide layer give a good method for detecting the presence of an oxide layer on the Si surface. However, an important point is knowing whether there is any surface oxide on carefully etched electrodes. Is it possible to obtain a more negative I$, value than Vi,, (etched in HF) with a different etching method? To answer this fundamental question we used the pretreatment described below. (III.2.ii) Etching in situ with e,-(NH,), at 233 K A solvated electron solution in ammonia is very easy to obtain: weighted amounts of potassium are dissolved in a basic solvent (if the solvent is not basic, the solvated electron is less stable with time). A solution of [e;(NHs),] = 5 X 10e3 mol dme3 is stable only for about 10 h. Indeed the e;(NH,), species slowly reduces the solvent, yielding hydrogen and amide ions:
e;(NH,>“----NH; (dark blue color)
+ i H, + (PJ- 1) NH, (colorless solution)
(6)
This reaction is thermodynamically spontaneous but its kinetics are very slow [22]. After the decomposition of the solvated electron solution, the pH of the ammonia did not change because the initial amount of NH; was sufficiently high. The Si electrodes were treated as follows: (1) Immersion in basic NH,. (2) Addition of K to the solution to obtain [e;(NH,),] = 5 X 1O-3 mol dmP3 in situ. (3) Waiting for the decomposition of e;(NH,), (solution becomes colorless) according to reaction (6). The impedance measurements were performed in the dark and Mott-Schottky curves were drawn. This treatment led to C2- V curves and extrapolated V,, which were the same as those obtained on electrodes directly immersed in basic medium after etching in HF. The accuracy was excellent with n-Si and only approximate with p-Si (see Table 4). If the electrodes were etched in HF before the treatment with e;(NH3),, no difference was obtained. (111.2.iii) Discussion Vn, was very dependent on the amount of native oxide on the Si surface. These results lead to two important conclusions: on the one hand, the e;(NH,), species
109 TABLE 4 Relationship of the flatband potentials to the etching conditions used for n- and p-type silicon electrodes in basic liquid NH3 Etching conditions
V&/V vs. SRE
Not etched “HF” at 300 K under an AI stream “e; (NH,)” at 233 K 9, “HF + e; (NH3)”
n-si
p-Si
- 1.2 f 0.2 -1.8kO.l -1.8kO.l -1.8kO.l
-0.9+0.1 -0.9kO.2 -0.9kO.2
acts as an etchant of the native oxide and, on the other hand, the two pretreatments (e;(NH,), and HF) leave the same amount of remaining superficial oxide on the Si surface. Because we never obtained a more negative Vi,_,value than the one observed after an HF and/or e;(NH,), treatment, we shall henceforth call these etched Si surfaces oxide-free surfaces. The etching with the e;(NH,), species was done in situ. So it is important to note that we were able to modify the semiconductor surface in contact with the electrolyte from an oxidized surface to an oxide-free one. Now it is possible to give the band-edge energy position of oxide-free silicon electrodes in liquid NH, versus pH. Knowing the doping levels ND and NA of nand p-type Si samples and V, values from Table 3, we deduce V,(p), V,(n), V,(p) and V,(n) from the relationships [19]: V,(n) - V,(n) = (W/e) ln(Nc/N,,) and V,(p) - V,(p) = (kT/e) ln( NV/N,). NC and NV are, respectively, the effective densities of states in the conduction and valence bands. Taking into account the experimental uncertainty of the I$, determination and the bandgap energy, we obtain the relationships: V,(n) = V,(p) and V,(n) = V,(p), which is usually the case in supporting electrolyte [23,24]. These V, and V, values are reported versus pH in Table 5. The energy diagram in Fig. 4 gives a better representation of the band-edge position of oxide-free Si versus pH. The different possible redox reactions determined on a polished platinum electrode [25], are also reported. Our results are compared to those of Bard and co-workers [26] and Krohn and Thomson [27]. Bards results were obtained in an unbuffered neutral medium with KI as electrolyte instead of KBr in our case. Thomson’s results were not experimental; they were TABLE 5 Variation of the band-edge potentials of oxide-free Si electrodes versus pH PH =2 -16 = 30
V”/V vs. SRE
v,/V
- 1.3 fO.l -1.6k0.25 -2.0+0.1
-0.2rtO.l - 0.5 f 0.25 -0.9IkO.l
vs. SE
110
-3-
-3-
.
-2-
*i
-2-
.
0
* 16
32
pH
Fig. 4. Energetic diagrams of oxide-free Si electrodes in supporting liquid ammonia versus pH. Bard’s results (1) and Thomson’s results (2) (see text).
deduced from the observation of the photoinjection of e;(NH,), species in solution from a p-Si photocathode and the knowledge of the e;(NH,),/NH3 redox system potential. The mean variation of the band-edge energy versus pH is 23 mV/unit of pH, that is 0.5 x 2.3 RT/F at 233 K. This variation was already obtained by Morrison and co-workers [28] in aqueous medium. This means there is an analogy between the phenomena in aqueous medium and in NH,: the dependence versus pH is not controlled exclusively by the adsorption of the solvent ions. (III.3) Influence of an oxide layer We reported (see Section 111.2-i) the drastic influence of the native superficial oxide, grown on electrodes exposed to air and not etched before the experiment, on the V, and V* values. We used some parameters to influence the oxide formation in situ on the Si surface, such as the immersion time in the electrolyte of previously etched electrodes and the amount of water in the solvent. Photoelectrochemical oxidation can also be demonstrated.
111
(III.3.i) Spontaneow oxidation of Si samples Well etched Si electrodes were immersed in NH, at different pH. Mott-Schottky curves were obtained at intervals in the dark to follow the changes in V, with time. A second series of experiments was carried out to obtain voltammograms under white-light illumination after a given immersion time. The results depended fundamentally on the pH of the solvent: In neutral (pH = 16) or acidic (pH = 2) purified solutions, Mott-Schottky curves and V, values were shifted towards more positive potentials with increasing immersion times. The V, shift was significant after about 1 h of immersion. A similar variation was observed in V* and was detectable after the same time of immersion. This result shows that a superficial oxide layer is growing slowly with the immersion time of the electrodes in NH,. With added amounts of water in the solvent, the detection time of a superficial oxide decreased. The results were more accurate on n-Si electrodes. It was difficult to evaluate quantitatively the minimum immersion time necessary to form a superficial oxide layer, because of the experimental uncertainty of I’, and V*. We could note only that the oxide formation was not instantaneous at 233 K, even in liquid ammonia containing H,O at a concentration equal to 1 mol dms3. In basic solutions (pH = 30), the Mott-Schottky curves and V, values were not influenced by the immersion time. (III. 3. ii) Electrochemical oxidation of Si samples If a n-Si electrode is biased at a potential more positive than V, under illumination, the photoholes created in the space charge layer are driven to the semiconductor surface [19-211. These photoholes have strong oxidizing properties: they can oxidize the material itself (reaction 4) or a reducing redox species (see Section III.l.ii). The evolution of the J-V curves under illumination on a well-etched n-Si electrode was studied with time (i.e. the number of potential sweeps). The results depended on pH: In neutral or acidic medium, the photocurrent decreased drastically with time and the onset potential shifted towards more positive potentials (see Fig. 2a). At the first positive potential sweep, V* = -0.6 V; at the third sweep, V* was shifted to +0.8 V and the photocurrent was very low. This behaviour is typical of the growth of a passive layer on the Si surface. At basic pH, the J-Y curves under illumination did not depend on the number of potential sweeps, i.e. on time. No passive layer was formed on the Si surface. (III.3.iii) Discussion When the solvent was prepared with the best precautions to mmimize water traces (see Section II), residual water was still present in the neutral and acidic media (see Table l), whereas the amide solutions in ammonia were anhydrous. As a matter of fact, H,O is a weak acid in NH, (pK= 16) [29] and is totally consumed by the strongest base in the solvent, NH;: NH, + H,O + OH- + NH, pK= -16 OH- -I-K+ + KOH
(slightly soluble)
112
The fundamental difference between neutral or acidic and basic media was therefore the residual water. So the above results show that a superficial oxide layer, detectable by a variation of V, or Y*, could grow only if water traces were present in solution. In the same way the passive layer obtained on n-Si under illumination was due to a reaction between the Si surface and residual water. By using acidic or neutral solutions, we were able to modify the semiconductor surface in contact with the electrolyte from an oxide-free surface to an oxidized one. The surface modification, due to spontaneous chemical reaction with water, was faster in the presence of large amounts of water in the solvent. An analogous surface modification was obtained by the electrochemical reaction with photoholes at illuminated n-type silicon. Using a basic solution, we were able to prevent the surface modification of Si electrodes, which needs traces of water in the solvent: since NH; ions were in excess, H,O molecules were completely consumed. Thus, control of the Si surface oxidation was possible in liquid ammonia by changing the experimental conditions. It is interesting to compare the time needed for spontaneous oxidation of the Si surface in our case (about 1 h) with Chazalviel’s results [12]. Chazalviel could measure a faster evolution (about 10 min) of the behavior of the n-Si/acetonitrile + redox system, due to the appearance of surface states in relation to a surface reaction. The very low oxidation rate in our case was certainly due to the low temperature, 233 K. When photoholes oxidized the Si surface, the oxide formed faster (some minutes) if the light intensity was high (0.1 W cmF2). The superficial oxide layer acts as a passive layer, the presence of which leads to the same positive I$, and V* variation on n-Si and p-Si electrodes. The variation of the flatband potential towards more positive potentials can be attributed to an extra potential drop into the oxide layer due to the trapped positive charges Q, 1301, according to the following relationship [19-211:
where C,, is the equivalent capacity of the oxide layer. (IV) CONCLUSION
The use of liquid ammonia containing low concentrations of water or being anhydrous when the pH is basic, allowed the reliable and reproducible characterization of silicon electrodes and the reversible surface modification in situ from an oxide-free surface to an oxidized one and conversely. Two etching techniques for the sample pretreatments, HF under an inert atmosphere at 300 K and solvated electrons in liquid ammonia at 233 K, were studied. They gave similar results for the V, values in basic medium, showing an efficient removal of the native oxide. We could therefore determine the band edge energy position of oxide-free silicon material in contact with liquid ammonia at different pH. The mean dependence of the band edges on pH is approximately 0.5 X 2.3 RT/F.
113
The presence of a superficial oxide layer can be related to a positive shift of the flatband potential and the photocurrent onset potential. They give a reliable tool for detecting the presence and therefore the formation of an oxide. A superficial oxide layer can grow only if water traces are present in the solvent. So controlling the anhydrous character of the solvent by changing the pH, allowed us to control the oxide formation on the Si electrode surface, i.e. the state of the surface in contact with the electrolyte. If the pH was not basic, an oxide layer was obtained in two different ways: it could be formed spontaneously by chemical reaction between the Si surface and residual water or photoelectrochemically using photoholes on a n-Si sample. ACKNOWLEDGEMENT
The authors gratefully acknowledge the referee for helpful critical reading of the manuscript. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
S.M. Sze in Physics of Semiconductor Devices, Wiley Interscience, New York, 1981. F.P. Schmidt and W. Michel, J. EIectrochem. Sot., 104 (1957) 230. A.B. Bocarsly, E.G. Walton and M.S. Wrighton, J. Am. Chem. Sot., 102 (1980) 3390. R.J. Archer, J. EIectrochem. Sot., 104 (1957) 619. S.I. Raider, R. Flits& and M.J. Palmer, J. Electrochem. Sot., 122 (1975) 413. D. Laser and A.J. Bard, J. Phys. Chem., 80 (1976) 459. K.D. Legg, A.B. Ellis, J.M. Bolts and MS. Wrighton, Proc. Natl. Acad. Sci., U.S.A., 74 (1977) 4116. J.N. ChazaIviel, J. EIectrochem. Sot., 129 (1982) 963. M. Matsumara and S.R. Morrison, J. Electroanal. Chem., 147 (1983) 157. B.H. Loo, K.W. Frese and S.R. Morisson, Surf. Sci., 109 (1981) 75. J.N. Chazalviel and T.B. Truong, J. Electroanal Chem., 114 (1980) 299. J.N. Chazalviel and T.B. Truong, J. Am. Chem. Sot., 103 (1981) 7447. D. Guyomard, These de Doctorat d’Etat, Paris VI, 1985. G. Van Amerongen, D. Guyomard, R. Heindl, M. Herlem and J.L. ScuIfort, J. EIectrochem. Sot., 129 (1982) 1998. D. Guyomard, M. Herlem, C. Mathieu, C. Miossec and J.L. Schulfort, J. Electroanal. Chem., 138 (1982) 435. M. Herlem, D. Guyomard, C. Mathieu, J. Belloni and J.L. Sculfort, J. Phys. Chem., 88 (1984) 3826. A. Heller, H.J. Lewerenz and B. Miller, J. Am. Chem. Sot., 103 (1981) 200. D. Guyomard, H. Herlem, C. Mathieu and J.-L. Sculfort, CR. Acad. Sci. Paris, Ser. B, 2% (1983) 1233. W.P. Gomes and F. Cardon, Progr. Surf. Sci., 12 (1982) 155. H. Gerischer in M. Schiavello (Ed.), Photoelectrochemistry, Photocatalysis and Photoreactors, Fundamentals and Developments, Proc. NATO ASI, Vol. 146, Reidel, Dordrecht, 1985, p. 39. D. Guyomard, J. Chim. Phys., 83 (1986) 355. R.R. Dewald, Ber. Bunsenges. Phys. Chem., 1974 (1971) 668. W.H. LafIere, F. Cardon and W.P. Gomes, Surf. Sci., 44 (1974) 541. A.B. Bocarsly, D.G. Bookbinder, R.N. Dominey, N.S. Lewis and MS. Wrighton, J. Am. Chem. Sot., 102 (1980) 3683. M. Herlem, A. Thiebault and J.-J. Minet, J. Electroanal. Chem., 26 (1970) 343. R.E. Malpas, K. Itaya and A.J. Bard, J. Am. Chem. Sot., 103 (1981) 1622.
114
27 28 29 30
C.E. Krohn and J.C. Thomson, Chem. Phys. Lett., 65 (1979) 132. M.J. Madou, B.H. Loo, K.W. Fresc and S.R Morrison, Surf. Sci., 108 (1981) 135. M. Herlem, Thbe de Doctorat d’Etat, Paris, 1966. E.H. Nicollian and J.R. Brews in MOS Physics and Technology, Wiley-Interscience, New York, 1982, p. 156.