The etching of InP in HCl solutions : a chemical mechanism

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The etching of InP in HCl solutions : a chemical mechanism

Citation for published version (APA):

Notten, P. H. L. (1984). The etching of InP in HCl solutions : a chemical mechanism. Journal of the

Electrochemical Society, 131(11), 2641-2644. https://doi.org/10.1149/1.2115375

DOI:

10.1149/1.2115375

Document status and date:

Published: 01/01/1984

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Vol. 1 3 1 , N o . 11

E V A P O R A T E D WO~ F I L M 2641 20. R. J. Cotton and J. W. Rabalais,

Inorg. Chem.,

15, 236

(1976).

21. K. T. Ng and D. M. Hercules,

J. Phys. Chem.,

80, 2094 (1976).

22. D. R. Renn,

J. Electron. Spectro. Relat. Phenom.,

9, 26 (1976).

23. J-P. Gabano, " L i t h i u m Balleries," p. 27, A c a d e m i c Press, New York (1983).

The Etching of InP in HCI Solutions: A Chemical Mechanism

P. H. L. N o t t e n

Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands

A B S T R A C T

The etch rate of I n P in solutions of high HC] concentration was s h o w n to be i n d e p e n d e n t of the applied potential in a w i d e potential range negative with respect to the flatband value. Dissolution of the solid led to the formation of PHi. The etch rate, w h i c h was not mass-transport controlled, was first order in molecular HC1 concentration. The results lead us to c o n c l u d e that, in HC1 etchants, I n P is dissolved by a purely chemical mechanism. The influence of chemical etching on the anodic behavior of I n P in these electrolytes is described.

A q u e o u s etching of III-V materials is often an important step in device t e c h n o l o g y (1, 2). While anodic etching is s o m e t i m e s used (3), the necessity of m a k i n g electrical contact to slices of small dimensions is invariably a disad- vantage. Methods not involving an external current or voltage source are therefore favored (1, 2). S u c h m e t h o d s can be divided into two classes: "electroless" and "chemical" dissolution. Electroless dissolution involves two sep- arate p o t e n t i a l - d e p e n d e n t electrochemical reactions: the oxidation of the solid and the reduction of an oxidizing agent. The principle is illustrated in Fig. 1 for the etching of a p-type III-V s e m i c o n d u c t o r (4). F o r these materials, holes in the valence band are required at the surface for b o n d breaking. As a result, the anodic dissolution current increases at potentials close to the flatband value (VrB) as s h o w n in curve (a). For electroless dissolution, holes must be supplied by an oxidizing agent in solution. If the distri- bution function of the oxidizing agent overlaps with the valence band of the semiconductor, t h e n r e d u c t i o n via hole injection into the valence band is e x p e c t e d [curve (b)]. Here, we have assumed that the reduction reaction is diffusion controlled,

i.e.,

the cathodic partial current is in- d e p e n d e n t of potential. Curve (c) represents the total m e a s u r e d current-potential curve in the electrolyte containing the oxidizing agent. At the rest potential, the anodic and cathodic partial currents are equal. If the potential is changed from this value using an external source, then the etch rate of the s e m i c o n d u c t o r will, o f course, also change.

Gerischer and co-workers (5, 6) have shown that semiconductors can be dissolved by a purely chemical mechanism, w h i c h is characterized by the absence of any influence of the surface carrier concentration on the etch rate. S u c h behavior is observed with bifunctional agents, such as halogen or H~O.., molecules, w h i c h are capable of f o r m i n g two n e w bonds with the s e m i c o n d u c t o r surface more or less simultaneously. For GaAs dissolution in bro- m i n e solution, for example, they suggest a coordinated reaction s e q u e n c e i n v o l v i n g t h e breaking of Ga-As and Br-Br bonds and the simultaneous formation of Ga-Br and As-Br bonds.

Etchants based on HC1 are widely used for I n P semi- c o n d u c t o r devices (7, 8). The 'presence of other acids in the HC1 solution has a significant influence on the etch rate. However, I n P does not dissolve in conventional etchants involving s i m p l e oxidizing agents. In order to re- solve the question of the dissolution mechanism, we studied both the etching and electrochemistry of p - I n P in various HC1 solutions.

E x p e r i m e n t a l

The p-type I n P slices used in this work w e r e m a d e from liquid-encapsulated Czochralski material with a carrier

density in the range 1-2 • 10 is cm -3. The (100) face was e x p o s e d to the solution. The diameter of the electrodes was 3 ram, with the e x c e p t i o n of the I n P rotating disk, w h i c h had a diameter of 4 mm.

The current-potential m e a s u r e m e n t s were carried out u n d e r potentiostatic control in a c o n v e n t i o n a l cell using a Pt counterelectrode and a saturated calomel electrode (SCE) as reference. All potentials are given with respect to this SCE. A Solartron 1172 F r e q u e n c y R e s p o n s e Ana- lyser was e m p l o y e d for d e t e r m i n i n g the flatband potentials. All i m p e d a n c e m e a s u r e m e n t s were carried out at a f r e q u e n c y of 10 kHz.

The total dissolution rate of I n P at various potentials was d e t e r m i n e d analytically by m e a s u r i n g the i n d i u m concentration in the etching solution by induced-

coupled-plasma (ICP) emission spectrometry. The

etehant was passed over the I n P electrode, w h i c h was m o u n t e d in a glass m i c r o e l e c t r o c h e m i c a l flow cell as described by Haroutiounian

et al.

(9). I n d i u m concentrations as low as 0.5 p p m could be d e t e r m i n e d w i t h a rela- tive a c c u r a c y of about 5%. An L K B Varioperpex peristaltic p u m p was used to p u m p the solution t h r o u g h the flow cell. The flow rate of the solution, mainly deter-

u - o o t - o _u "(3 o JE

8

0 (a)

l

I

/ (c)

///I

V(SCE)

(b)

Fig. 1. Curve (a) represents the theoretical partial anodic dissolution current of a p-lnP electrode in the dark, and curve (b) the diffusion- limited reduction current of an oxidizing agent as a function of the po- tential. Curve (c) gives the total current-potential curve.

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2 6 4 2 J . E l e c t r o c h e m . Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y N o v e m b e r 1984

m i n e d by the lower limit of the indium detection, was in the range from 0.2 to 1.0 m]/min.

The gas was analyzed qualitatively using the color- detector tube m e t h o d ("Dr~ger" tube), w h i c h in our case gave a color reaction specific for phosphine. A gas bu- rette was used for the quantitative gas analysis.

Results

Flow-cell experiments.--In Fig. 2, results are given for the potentiostatic etching of p-InP in the dark in HC1 solutions. Figure 2a shows the current-potential curves of this electrode in 3M and 6M HC1. At negative potentials the current is very low in both cases. In the vicinity of the flatband potential (VFB = 0.73V vs. SCE) the anodic current increases, as expected. It should be noted that the anodic curve for the 6M HC1 solution is shifted slightly in the cathodic direction with respect to that for the 3M solution.

The total dissolution rate, according to the ICP analysis, is shown in Fig. 2b as a function of the potential. For the 3M HC1 solution, the dissolution rate follows the current closely. At negative potentials the etch rate (rt) is v e r y low and increases considerably near the flatband potential. If we assume that in this case etching is due solely to anodic dissolution, then it can be easily shown that six holes are r e q u i r e d to dissolve one I n P entity. The total e t c h rate potential curve for the 6M HC1 case differs m a r k e d l y from that of the c o r r e s p o n d i n g current- potential curve. This difference is most obvious at negative potentials, at w h i c h the I n P dissolves at a rate essen- tially i n d e p e n d e n t of applied potential. Since no cathodic reaction occurs in this range, we m u s t c o n c l u d e that I n P is chemically dissolved by HC1. If we assume that anodic dissolution in 6M HC1 here also requires six holes per InP, t h e n the chemical etch rate can be calculated from the total etch rate and the m e a s u r e d anodic current. Fig- ure 2c shows that the chemical etch rate (rchem) remains

15

O

--- 10

E

5

0

0.50

r- E

E

0.25

im.4,-, X

C =

~

0'2 f x

j

OI

9 3M HCL

x 6M HCI

_ e J )q ){ x ~ 9 A ~--'C_

(a)

(b)

(c)

constant, e v e n at potentials at which the electrode dissolves anodically.

A further e x a m p l e is s h o w n in Fig. 3 for 1.5M HC1 in concentrated acetic acid solution. Etchants based on HC1 and acetic acid are often used in I n P technology (7). Fig- ure 3a shows the current-potential curve, and Fig. 3b the calculated chemical etch rate as a function of the elec- t r o d e potential. The results in the cathodic region are similar to those found w i t h 6M HC1 solution; the current is low and a high chemical etch rate is found. The slow increase of the chemical etch rate with potential is probably due to a r o u g h e n i n g of the electrode surface during the experiment. The increase in the anodic current at potentials near VFB is considerably lower than that found in a HC1 solution of the same concentration but without acetic acid. A dramatic decrease of the chemical dissolution rate is observed w h e n the anodic dissolution starts. In this potential range, we observed the formation of an orange- colored film at the electrode surface. P h o s p h o r u s was found in this film by E D A X analysis. Obviously, a passivating film is formed during anodic dissolution. This film inhibits both the anodic and the chemical dissolution reactions.

It should be noted that the chemical dissolution rate de- pends markedly on the surface condition of the I n P electrode. Since induction effects are observed, care m u s t be t a k e n to ensure a constant dissolution rate before mea- s u r e m e n t s are made. This can be done by pre-etching the electrode in the same etching solution.

The results shown so far suggest that the chemical etch rate of I n P is strongly d e p e n d e n t on HC1 concentration. This d e p e n d e n c e was studied by varying the HC1 concentration over a wide range for two different systems. An I n P crystal was chemically etched in these solutions in the flow cell (the m e a s u r e d current was zero), and the chemical dissolution rate was again analytically deter- m i n e d by I C P emission spectrometry. The results are shown in Fig. 4. For curve (a), the concentration was varied by diluting concentrated HC1 with water. At 9M HC1, the chemical dissolution rate is high and decreases rapidly as the HC1 concentration is lowered. For a 5M HC1

%

E 15 10 0

(a)

& 4, m m ; 0.6 E 0.~ 0,2 0

(b)

I I I

-0.5

0

0.5 V(SCE)

Fig. 2. Potentiostaticnlly measured

current-potential

curves (a), total etch rate as a function of the electrode potential (b), and chemical etch rate as a function of the electrode potential (c) for a p-lnP electrode in the dark in a 3M HCI ( 0 ) and 6M HCI (x) solution in water.

I i i

-0.5 0 0.5

V(SCE)

Fig. 3. Potentiostatically measured current-potential curve (a) and chemical etch rate as a function of the electrode potential (b) in the dark for o p-lnP electrode in a solution of 1.SM HCI in concentrated acetic acid.

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Vol. 131, N o . 11

E T C H I N G O F I n P I N H C 1 S O L U T I O N S 2 6 4 3 r- E c .

/

"l

(b)

/~

(a)

/

./

J

/

I I

O0

6

8

10 [HCL]

(Moi/t)

Fig. 4. Chemical etch rate as a function of the HCI concentration for a p-lnP crystal in the dark. The HCI concentration was varied by diluting with water [curve (a)] and concentrated acetic acid [curve

(b)].

c o n c e n t r a t i o n , t h e e t c h rate is less t h a n 40 ~,/min a n d e v e n l o w e r for m o r e d i l u t e solutions. F o r c u r v e (b), c o n c e n t r a - t e d HC1 was d i l u t e d w i t h c o n c e n t r a t e d acetic acid. It is s t r i k i n g t h a t h e r e a l i n e a r r e l a t i o n s h i p b e t w e e n e t c h rate a n d HC1 c o n c e n t r a t i o n is o b s e r v e d and t h a t t h e e t c h rate is significantly h i g h e r t h a n in t h e c o r r e s p o n d i n g HC1-H20 solutions. To d e c i d e w h e t h e r t h e d i s s o l u t i o n is a d i f f u s i o n or a k i n e t i c a l l y c o n t r o l l e d process, t h e e t c h rate w a s m e a s u r e d as a f u n c t i o n of t h e r o t a t i o n rate u s i n g a p - I n P r o t a t i n g d i s k e l e c t r o d e at o p e n circuit. F i g u r e 5 s h o w s t h a t t h e c h e m i c a l e t c h rate m e a s u r e d in a 3M HC1 in c o n c e n t r a t e d acetic a c i d s o l u t i o n is e s s e n t i a l l y i n d e p e n d e n t of t h e rota- t i o n rate (N). T h i s m e a n s t h a t t h e rate o f t h e d i s s o l u t i o n r e a c t i o n is k i n e t i c a l l y c o n t r o l l e d .

Gas analysis.--When

an I n P crystal w a s d i s s o l v e d at t h e rest p o t e n t i a l in a c o n c e n t r a t e d HC1 solution, gas evo- l u t i o n w a s o b s e r v e d at t h e solid surface. U s i n g t h e color- d e t e c t o r m e t h o d , w e s h o w e d t h a t this gas w a s p h o s p h i n e . A c l e a r color c h a n g e w a s i n d e e d o b s e r v e d e v e n w i t h t h e m o s t i n s e n s i t i v e tubes. We d e t e r m i n e d t h e gas

1.5

1.0 "E

E

~ 0.5

0

Fig. 5. Chemical etch rate in the dark as a function of the rotation speed of a p-lnP electrode in a solution of 3M HCI in concentrated acetic acid.

q u a n t i t a t i v e l y w i t h a gas b u r e t t e a n d s h o w e d t h a t t h e p h o s p h o r u s is c o n v e r t e d for 100% (• 1%) as PHi.

Electrochemical measurements.--The

c u r r e n t - p o t e n t i a l c u r v e s of p - I n P in 1 N H2SO4 a n d in d i f f e r e n t HC1 s o l u t i o n s in t h e d a r k are s h o w n in Fig. 6. In all cases, t h e b l o c k i n g c u r r e n t in t h e c a t h o d i c r e g i o n was v e r y l o w (< 0.02 mA/cm2). T h e o n s e t of t h e a n o d i c c u r r e n t in t h e case o f 1N H~SO~ [curve (a)] o c c u r s n e a r t h e f l a t b a n d p o t e n t i a l (VFB = 0.73V), as e x p e c t e d . T h e a n o d i e c u r r e n t for 1M HC1 [ c u r v e (b)] is shifted s o m e 150 m V in t h e n e g a t i v e p o t e n - tial d i r e c t i o n w i t h r e s p e c t to t h e H~SO~ case. With an in- c r e a s e in t h e HC1 c o n c e n t r a t i o n , this e f f e c t b e c o m e s m o r e p r o n o u n c e d . F o r t h e 9M HC1 solution, t h e shift a m o u n t s to a b o u t 350 mV. W h e n t h e c u r v e w a s m e a s u r e d again in 1N H2SO4 after t h e HC1 e x p e r i m e n t s , e x a c t l y t h e s a m e re- s u l t w a s o b t a i n e d as in t h e first m e a s u r e m e n t [ c u r v e (a)_].

We also m e a s u r e d t h e M o t t - S c h o t t k y plots for e a c h of t h e s o l u t i o n s u s e d in Fig. 6. T h e f l a t b a n d p o t e n t i a l d i d n o t d e p e n d on t h e HC1 c o n c e n t r a t i o n : VrB = 0.725 -+ 0.050V in all cases. T h e s l o p e of t h e M o t t - S c h o t t k y plots d e c r e a s e d s o m e w h a t as t h e HC1 c o n c e n t r a t i o n was increased. T h i s is p r o b a b l y d u e to an i n c r e a s e in t h e surface a r e a of t h e e l e c t r o d e d u e to r o u g h e n i n g as a r e s u l t of etching. A similar e f f e c t c a n also be s e e n in Fig. 3b.

Discussion

F r o m Fig. 2 a n d 4, it is o b v i o u s t h a t c h e m i c a l dissolu- t i o n o c c u r s in a q u e o u s s o l u t i o n w h e n t h e HC1 c o n c e n t r a - t i o n e x c e e d s a c e r t a i n critical value. T h e r a t e is s t r o n g l y d e p e n d e n t on t h e HC1 c o n c e n t r a t i o n a n d b e c o m e s v e r y l o w at v a l u e s l o w e r t h a n 5M [see Fig. 4, c u r v e (a)]. T h i s s u g g e s t s t h a t t h e e t c h rate d e p e n d s on t h e d e g r e e of dis- s o c i a t i o n of HC1 m o l e c u l e s . A l t h o u g h it is clear t h a t at l o w c o n c e n t r a t i o n s t h e d i s s o l u t i o n of HC1 is c o m p l e t e , t h e r e is a c o n s i d e r a b l e d i s c r e p a n c y in t h e l i t e r a t u r e w i t h r e s p e c t to h i g h e r c o n c e n t r a t i o n s (10). C a l c u l a t i o n s b a s e d o n v a p o r p r e s s u r e m e a s u r e m e n t s a n d on H a m m e t t func- tions s h o w t h a t t h e c o n c e n t r a t i o n of u n d i s s o c i a t e d HC1 b e g i n s to i n c r e a s e s i g n i f i c a n t l y a b o v e 5 mol/1 (11, 12). I n o r d e r to a v o i d t h e u n c e r t a i n t y i n v o l v e d w h e n HC1 is di- l u t e d w i t h water, w e s t u d i e d t h e d i s s o l u t i o n rate in HC1- acetic a c i d solutions. T h e d i s s s o c i a t i o n c o n s t a n t of HC1 in acetic a c i d (Ka = 10 -s~5) is m u c h l o w e r t h a n in w a t e r (Ka = 10 +3) (13). C o n s e q u e n t l y , t h e d e g r e e of d i s s o c i a t i o n of HC1 in acetic acid is n e g l i g i b l e , e v e n at l o w HC1 c o n c e n - trations. T h e c h e m i c a l e t c h rate c o u l d t h e r e f o r e be studied as a f u n c t i o n of t h e m o l e c u l a r H C t c o n c e n t r a t i o n . T h e l i n e a r d e p e n d e n c e of t h e c h e m i c a l d i s s o l u t i o n r a t e on t h e HC1 c o n c e n t r a t i o n in c o n c e n t r a t e d acetic acid [Fig. 4, c u r v e (b)] i n d e e d c o n f i r m s t h a t c h e m i c a l d i s s o l u t i o n is d e t e r m i n e d by t h e m o l e c u l a r HC1 c o n c e n t r a t i o n . T h e I

I

1.0-

(e){d)(c)(b) (a)

9 v

0.5

I L I

2000

z.()O0

0 - 0 . 2

0

0.2

0.4

0.6 N (r.p.m.)

V (SEE}

Fig. 6. Current-potential curves for a p-lnP electrode in the dark in IN H2SO4 (a), 1M HCI (b), 5M HCI (c), 7M HCI.(d), and 9M HCI (e) in water. Scan rate: 10 mV/s.

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2644 J. E l e c t r o c h e m . Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y N o v e m b e r 1984

high etch rate of HC1 solutions diluted with concentrated acetic acid compared to that for aqueous solutions, also found by Adachi (7), can be understood in this way. Fig- ure 5 shows that the etch rate is i n d e p e n d e n t of the rotation rate of an I n P electrode meaning that the chemical etch rate is kinetically determined by these HC1 molecules.

The m e c h a n i s m presented by Gerischer and Wallem- Mattes for the chemical dissolution of semiconductors involves symmetrical bifunctional etching agents such as H~O~ and halogen molecules (5, 6). Although we are, in the present work, dealing w i t h an asymmetrical HC1 mole- cule, we propose a reaction scheme similar to that for symmetrical agents. The first step involves a synchro- nous exchange of bonds: In-C1 and P-H bonds replace the original H-C1 and In-P bonds. This is very likely the rate- d e t e r m i n i n g step

CI - - H CI H

z

>

I

- - I n - - P - - - - I n P - -

/

\

/

\

Since the indium and phosphorus atoms at the surface are triply b o n d e d to n e i g h b o r i n g atoms, two further bonds m u s t be b r o k e n in an analogous m a n n e r to r e m o v e each atom from the lattice

CI H

I

I n P - + 2 H C I ~ I n C i 3 + P H 3 ~

/

\

I n d i u m is therefore dissolved as hydrolyzed InCl~ and PH:~ is evolved as a gas. S u c h a m e c h a n i s m can account for the etching results observed.

The chemical etch rate of I n P in HC1 solution is inde- p e n d e n t of potential. At potentials near the flatband value (Fig. 2 and 3) the p-InP electrode dissolves anodieally. The rate of the anodie etching increases as the surface hole concentration is increased, i.e., as the potential is m a d e more positive. In aqueous HC1 solutions, we have shown that six holes are required to dissolve one In-P entity. This means that both In and P are oxidized to the trivalent state, as is c o m m o n for I I L V materials (3-6). With a considerable concentration of acetic acid in t h e HC1 solution, however, a film is formed on the electrode w h i c h inhibits both the chemical and anodic dissolution (Fig. 3). E D A X m e a s u r e m e n t s have shown that this layer contains phosphorus but no indium or chlorine. We sus- peet that, in this case, p h o s p h o r u s is not oxidized directly during anodic dissolution

I n P + 3h + --> In nI 4- P

A similar three-hole m e c h a n i s m has b e e n shown for GaP (14).

It is obvious from Fig. 6 that the chemical dissolution rate strongly influences the electrochemical behavior of InP. Anodic dissolution starts at a more negative potential as the HC1 concentration is increased, although the flatband potential does not change. This m u s t m e a n that the activation energy for electrochemical dissolution is lowered by chemical etching. A similar effect has been d e m o n s t r a t e d by Gerischer and Wallem-Mattes (6) for the dissolution of GaAs in b r o m i n e solution. As in the GaAs case, this result can be explained if we assume that rup-

ture of the first In-P surface bond is rate d e t e r m i n i n g for anodic dissolution

• •

\

/

\ /

/

I n : P + 2 h + + 2 X - ~ I n P

/

\

/

/ \

I f this bond is b r o k e n during a chemical attack by HC1, t h e n the r e m a i n i n g bonds are more easily attacked anodically and the onset potential for anodic dissolution is c o n s e q u e n t l y lowered, indicating that it is, indeed, likely that the first step is the rate-determining dissolution step.

Conclusions

The dissolution of I n P in concentrated HC1 solutions follows a chemical m e c h a n i s m in which undissociated HC1 molecules play a decisive role. It seems likely that other etchants for InP, such as H B r and Br2 (7, 8), are based on a similar mechanism. The reason w h y I n P does not undergo electroless dissolution in conventional etchants containing oxidizing agents is probably related to the presence of a thin, highly resistant oxide layer on the semiconductor (15). S u c h a layer, w h i c h can inhibit ei- ther the dissolution of the solid or hole injection from the oxidizing agent, is unlikely to be present at the high HC1 concentrations used here.

Acknowledgment

The author wishes to t h a n k Dr. J. J. Kelly, Mr. J. E. A. M. v a n den Meerakker, and Dr. R. Mernming (Philips Forschungslabor, Hamburg, Germany) for helpful discus- sions, and the Analytical D e p a r t m e n t of Ir. P. J. R o m m e r s for the indium analyses.

Manuscript submitted March 19, 1984; revised manuscript received J u n e 11, 1984.

Philips Research Laboratories assisted in meeting the publication costs of this article.

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