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Current oscillations of Cd0.2Hg0.8Te in a Na2SCsOH solution
Electrochimica Acta, Vol. 36, No. S/6, pp, 869415, 1991 Printedin Great Britain.
CURRENT
0013-4686/91$3.00 + 0.00 0 1991. Pergamon Press pk.
OSCILLATIONS OF Cd,, Hh.,Te Na, S-CsOH SOLUTION
IN A
V. MARCU* and H.-H. STREHBLOW~ Institut fur Physikalische Chemie und Elektrochemie, Heinrich-Heine Universitiit, D-4060 Dilsseldorf 1, Germany (Received
19 December
1989; in revised form
18 June 1990)
Abstract-Current oscillations have been obtained in the anodical region of sulphidization of various (CMT) compounds in basic solutions containing Cs+ and S*- ions. According to X-ray photoelectron spectroscopy studies, it is possible to correlate these oscillations to the changes of the Cs surface concentration. After anodization, the samples showed an increase in the Cd/Hg and Hg/Te ratios (changed from Cd,,,Hg,,Te to Cd,,,HgTe). The surface concentrations of OH and Cs change
Cd&g,_,Te
synchronously with the oscillations. These observations are explained by the same model which was previously suggested for the photocurrent oscillations on CdTe (mainly a formation/dissolution of a submonolayer of Te). The anodic reaction corresponds to the decomposition of the CdTe fraction of the alloy with the formation of soluble Te compounds, a thin remaining Te and a CdS-deposit on the surface. Key words:
cadmium mercury telluride, current oscillations, XPS, cesium, surface layers, anodization.
INTRODUCTION
Current oscillations have been known in metal electrochemistry for a long time[ 1,2]. For semiconductor electrodes these effects have been found and studied much later. A large number of important semiconductor materials have already shown such current oscillations: binary compounds-CdTe[3], CdS[4,5] and GaAs[6], as well as some ternary materials, CuXSe, or CuXS,, where X = Fe or In[l. As is the case with their well-known counterparts in homogeneous systems, eg the Belousov-Jabotinsky reaction[8], most of these processes are far from being completely understood. The electrochemistry of CMT has been thoroughly studied, due to its important practical application as a material for ir detectors. The behaviour of the surface has been studied during etching[9-111, anodic oxide formation[l2-191 and, more recently, during sulphidization in organic solutions[20,21]. Various methods were applied in these studies to supplement the pure electrochemical information: X-ray photoelectron spectroscopy (XPS), Rutherford back scattering (RBS), ellipsometry and Raman spectroscopy. We have reported previously on photocurrent oscillations of n-CdTe single crystals in various solutions[3]. The oscillations were attributed to changes in the Helmholtz layer due to the formation/ dissolution of a Te layer and selective Cs adsorption. Our optical measurements have shown that the changes in the Te coverage should be smaller than one monolayer. Single crystal ‘electrodes that do not contain Te were found to give no oscillations and the presence of Cs and sulphide ions was necessary to * Present address: Israel Electrical Corporation. t To whom all correspondence should be addressed.
obtain sustained oscillations[3]. To support the above mentioned model, in the present study we have checked whether oscillations are present on other semiconductor single crystals containing Te and we have tried to’ correlate the oscillations with the surface composition as determined by XPS.
EXPERIMENTAL
Cd,Hg, _,Te single crystals (x = 0.2, 0.7, 0.9, 1.0) have been tested. The following solutions were prepared from analytical grade (pa.) reagents and deionized water (D.I.) (Millipore, MilliQ purification system): Cs,S: CsOH: S (1 M each) (1) and Na,S:CsOH (0.3 M: 1 M) (2). All the experiments were performed potentiostatically, at 25°C without stirring and at constant light intensity, when light was necessary (see Discussion below). Hg/HgO/O.l M KOH (MOE) (E = 0.14 V) or a saturated calomel electrode (see) were used as reference electrodes. A large Pt foil served as counter electrode in a standard three-electrode electrochemical cell. Homemade potentiostats and potential ramp generators were used to control the potential. All potentials are given in reference to the standard hydrogen electrode (she) and are corrected for liquid junction potentials. The electrodes were polished with diamond spray (Struers) to 1 pm and sometimes (see Discussion) etched in Br,:methanol (2% vol.) or K,CrrO, (4 g) : cont. HNOr (10 ml) : H, 0 (20 ml) and reduced at -0.9 V @he) in acetate buffer[l2]. Before placing in the analyser chamber of the ESCALAB 5 (VG Instruments), the samples were flushed with N, or Ar, after sometimes being rinsed with D.I. water and/or absolute ethanol (see Discussion). After a number of oscillations, the sample was extracted from the cell 869
V. MARCUand H.-H.
870 Pt in 0.3 M Na2S + 1 M CsOH dE -= 50 mV s-l dt
lo 8-
-0.4-0.2
0
RESULTS
0.2 0.4 0.6 E(she)N
I
I
0.8
1.0
I
Fig. 1. Potentiodynamic polarization curves of: (a) Pt in 0.3 M Na,S: 1 M CsOH. dE/dt = 50 mV s-‘, A = 2 cm’. (b) Cd,,,H&.,Te in the same electrolyte. 1 and 2 represent
successive scans. and introduced to the vacuum for analysis by XPS. Samples were taken after a few oscillations and after some tens of oscillations. The extraction was performed either when the oscillation was at the maximum or at the minimum current value. For the semi-quantitative XPS measurements the following peaks were used: Te 3d5 (binding energy 573 eV), Cd 3d5 (B.E. 405 eV), Hg 4f7 (B.E. 101 eV), Cs 3d5 (B.E. 725eV) and 0 1s (B.E. 532eV) and the same sensitivity factors as in Ref. [13]. For some experiments a rotating ring disk electrode with a cylindrical single crystal C&.,Hg,,Te disk and Pt ring was used.
0.8 -
a)
Pt
0.6 -
0.4 -
0.1 M KOH
0.2 -
-0.8-0.6-0.4-0.2
0
0.2
STREHBLOW
,
,
,
0.4
0.6
0.8
E(she)N
Fig. 2. Potentiodynamic polarization curves of: (a) same as Fig. la with an electrolyte which has been already used for a few hours in oscillations experiments prior to the scan. (b) Te, HgTe (Br,-CH,OH etched) and Cd,,,Hg,,Te (cleaved in UHV) in 0.1 M KOH (from Refs[lZ, 161).
Since the oscillation might originate from the solution electrochemistry, particularly when potential oscillations have been mentioned during the oxidation of sulphide and other soluble sulphur species[22], one has to examine the behaviour of a Pt electrode in this special electrolyte. Figure la presents the potentiodynamic polarization curve of a Pt electrode in the electrolyte used for oscillations. The numbers 1 and 2 on the curves indicate two successive scans, both from cathodic to anodic potential direction. The first peak (at ca 0.2 V) is due to S*- to So oxidation, while the next rise is due to the formation of soluble sulphur products. No oscillations are seen in these experiments in the region -0.2-0.6 V. In Figure lb the seemingly similar behaviour of the C$,,H&,,Te electrode in the same solution can be observed. Here 1 and 2 represent two successive scans in opposite directions. Following scans are identical and are not represented. The current densities are in this case 10 times larger than those of Fig. la. This is an indication that a different electrode process, due to reactions of the CMT, occurs, as will be shown below by comparison with a few other systems. Figure 2a shows the currentvoltage characteristics of a Pt electrode in a solution of the same composition, but after a several hours use as an electrolyte for oscillations studies. One notices that on top of the peak observed for the fresh solution, an additional one (at ca -0.5 V), absent in the fresh solution, appears when the potential scan starts at more negative values. This peak is due to the oxidation of Te*-. This ion is formed at the Pt counter electrode, during the oscillations, by reduction of the products of simultaneous oxidation of the CMT electrode. Inspection of the anodic polarization curves of Te, HgTe etched in Br,-CH,OH (which enriches the surface in Te) and a vacuum-cleaved CMT sample in KOH (Fig. 2b) indicates that the oxidation of the CdTe fraction of the CMT and of Te (present either as bulk Te or as a thin layer on HgTe) occurs almost at the same potential (ca 0.1-0.2 V). The ratio of the two processes depends on the surface preparation. For an illuminated semiconductor that shows no oscillations, under reverse bias, the i-u curve is determined simply by the total photon flux. One such curve is shown for CdS in Fig. 3b (chopped light). Almost no change of photocurrent with the applied potential is seen between -0.4 and 0.8 V. The situation is different in the case of CdTe: under the same conditions, the photocurrent peaks at ca 0.3 V, due to the electrochemical kinetics. Different photocurrent intensities for opposite scan directions, and phototransients with a maximum and minimum (Fig. 3a) were noticed. This observation is closely related to the oscillations: if the potential scan is stopped in this potential range, the photocurrent starts to oscillate (although the illumination is kept constant)[3]. One should notice the coincidence of the positions of the photocurrent peaks with those of CMT (Fig. lb). A decrease in photocurrent with decreasing anodic potential is expected in a region approaching the flat-band potential, since CdTe is reverse biased. Instead, after the jump due to the beginning of illumination and a normal decrease
Current oscillations of C&Hgs,,Te
O**-
871
a) CdTe CszS + CsOH (1 M
: 1 M)
0.8 0.4 -
-1.2
-0.8
0
-0.4
0.4
0.8
E(shePV
Fig. 3. Potentiodynamic polarization curves of: (a) n-CdTe (continuous curve represents the cathodic scan, while the previous anodic scan is, for simplicity, only partly represented in the upper right comer) and (b) n-CdS in Cs,S :CsOH (1 M each) (the slight difference in the cathodic dark current between the anodic and cathodic scans is not relevant for the discussion). Chopped light: alternating dark (lower) and light (higher) currents.
in the photocurrent, due to the reduction of the potential barrier with the cathodic scan, an increase follows at more negative potentials (eg around 0.1 V) (Fig. 3a). CMT in 0.3 M Na2S + 1 M CsOH 45” c
48 mV
27 mV
0
If23 mV
OmV
l time/s
Fig. 4. Current oscillations of Cd,,,H&,,Te in dependence of the potential.
The CMT electrodes were cleaned, as described above, introduced in the cell under potential control, at a potential slightly negative of that at which the current rises sharply (ie 0 V in Fig. 1b). Subsequently, the potential was slowly increased until the current started to drop. At this potential the scan was stopped and most of the electrodes started to oscillate with a frequency in the range of 0.1-0.5 Hz (slightly depending on various parameters such as potential, temperature and surface preparation) and an amplitude close to 60% of the maximal value of the anodic current. Figure 4 depicts these oscillations for different electrode potentials (the potential at which the oscillations started on this electrode was lower than that of the sharp drop in Fig. lb). The potential region in which the oscillations take place is much narrower than in the case of CdTe[3]. The potential of 48 mV in Fig. 4 represents the maximum potential at which coherent oscillations in the region of interest (-0.2-0.4 V) occur. On some of the electrodes, oscillations were noticed for potentials larger than 0.4 V, which probably are due to similar electrochemical behaviour during the oxidation of HgTe (or HgS) part of the compound. The structure of the oscillations is shown in more detail in Fig. 5. The shape of the wave corresponds to Fig. If of Ref. [23], where the oscillations were attributed to a chemical reaction of the anode with one of the oxidation products of the electrolysis of the solution. The shape of the wave could give hints for excluding certain mechanisms involving sudden changes in the state of the electrode (eg gas bubbles[4] or a breakdown of a passive layer). It also shows that the mathematical treatment we used before, in order to explain the features of the CdTe oscillations[3], is only approximately true, since it would involve a perfect sine shape.
V. MARCUand H.-H. STREHBLOW CMT in 0,3 M Na2S + 1 M CsOH
60000 -
al CMTimi”
50000 IlOmV
40000 30000
la
Te
:I::!!+
r
-
10s
time/s
Fig.
5. Typical fine structure of the oscillations Cd,,H&,,Te (same conditions as in Fig. 1b).
To obtain additional information on the surface reactions, X-ray photoelectron spectra were taken from the Cd,,,Hg,,,Te specimen. Figure 6 presents a general survey of the XPS spectrum of a reference sample (polished only). In Fig. 7, surveys of two samples immersed at the top and the bottom of the current oscillation, respectively, are compared. The pronounced change in the relative intensity of the Cs ion signal with respect to Cd (or C) is quite obvious. Table 1 compares qualitatively the oscillations for the different parameters such as the composition of the electrode and of the electrolyte. One concludes that electrodes which are more stable to photooxidation, like CdS and CdSe[24], show no oscillations, whatever the composition of the solution. The presence of Te in the electrode and sulphide in the solution are prerequisites for oscillations, since no current instabilities were observed in solutions containing only hydroxide[ 12-141. A thick Te layer forms in basic solutions[24]. The above mentioned conditions are still not enough for the generation of sustained oscillations: Cs+ ions play an important role. The situation described by weak oscillations in Table 1 actually refers to very low amplitudes (slightly above noise level), irregular and with much longer periods (tens of seconds). The change of the surface composition during the oscillations, as deduced from XPS results can be seen
5-
800
of
1000
1200
EbleV
Fig. 7. XPS survey spectrum of Cd,,Hg,,Te samples after oscillations. The samples were immersed under potentiostatic control when the current was: (a) at minimum; (b) at maximum. (N.B. the a.u. here are 10,000 times smaller than in Fig. 6.)
in Table 2. Table 3 compares the characteristics of the oscillations of CdTe and CMT. Experiments performed with the rotating ring disk electrode showed only a weak dependence of the ring reduction currents on the disk oscillations. Changes of only < 1% were observed in the ring current during the oscillations, which is negligibly small compared to 25% expected theoretically. This indicates that the formation of surface layers and incorporation of corrosion products therein is responsible for the oscillations.
DISCUSSION The best method for preparing the electrodes prior to anodization, in view of their subseqent analysis by XPS, was mechanical polishing. This pretreatment is reproducible and prevents any changes in the surface stoichiometry. We checked that the electrochemical behaviour and the occurrence of the oscillations is not altered by the different surface treatments mentioned above. Oscillations also occur on electrodes etched in Br,-CH,OH and reduced in acetate buffer (a standard pretreatment favoured by electrochemists)[ 121. This etching procedure increases the amount of Te on the surface. The subsequent reduction of Te to Te2- in the acetate buffer slightly Table 1. Crystal
Solution
0
1 200
I 400
1 600
800
I 1000
EbIeV
Fig. 6. XPS survey spectrum of a reference Cd,,Hg,,Te sample (mechanically polished only).
Oscillations
OH- + S*-
CdTe CdS, CdSe
Weak No
OH- + S2- + Cs+
Cd,Hg,., Te x = 0.2, 0.7, 0.9, 1
Strong
CdS CdSe
No No
Current oscillations of Cc&,H&,,Te Table 2. Initial
873
MODEL
Cd:Hg:Te
=0.2:0.8:1
After: 1.4:0.9: 1 1.2:1.0:1
depletes the CMT surface in Cd ions in addition. This paradoxical behaviour, which was observed in the case of the cathodic decomposition of clean CdSe as well[25] is explained by the competing CdTe or HgTe reduction. Cd is formed, by the later reaction, which in contact with the air oxidizes to CdO. The oxide in turn dissolves in D.I. water during rinsing, while part of the telluride can be oxidized to insoluble Te. Thus, the expected ratio of atomic composition is reversed. The alternative etching in K,Cr, O,-containing solutions removes the oxides and damaged sites from the surface, but leaves a Cr contamination which interferes with the Te determination by XPS. In order to keep the surface concentrations as close to those present during the oscillations, most of the samples were transferred to the XPS analysis chamber without further rinsing. No artifacts are introduced by this manipulation method, since, qualitatively, the same results are obtained when the samples are rinsed with water or ethanol. The transfer of the model used to explain the photocurrent oscillations of CdTe[3] to the case of CMT involves the following steps (Fig. 8): 1. The CdTe part of the CMT is oxidized to Cd*+, Te (unstable at these potentials) and TeS:-, 2. The Cd2+ ions diffuse through the Cd depleted layer of the electrode (which is closer in stoichiometry to HgTe) to the electrolyte interface, where they react with S*- and precipitate on the surface as CdS. 3. A thin Te layer, produced in step 1, successively forms and dissolves as TeSz-, 4. Cs+ is adsorbed on the Te layer, but not on the clean CMT surface. The experimental evidence presented above cannot distinguish between the independent Cs+ and OH- adsorption, because of ion pairing[26,27]. Their presence facilitates the dissolution of Te (see last paragraph of Discussion). Steps 3 and 4 generate the oscillating behaviour. The following discussion will show how the present results support the model. It is obvious from Table 1, that the oscillations require the presence of Te. Comparison of Fig. lb and the curves in Figs 2b and 3a, which represent the shape of the photocurrent-potential curve of CdTe and replotted literature data for Te, CMT and HgTe oxidation, respectively, suggests that the region of the oscillation is the same as that of CdTe and Te oxidation. Much larger currents are observed in the presence of sulphide ions (compare Figs 2b and lb). This observation is due to the formation of the more
Cd pool layer (- HgTe) Fig. 8. Model explaining the oscillations. CdTe denotes the part of CMT which is oxidized.
stable CdS and the better dissolution of Te in sulphide solutions. The larger reaction rate might be a reason for the oscillating behaviour (however, in the case of CdTe, where the reaction rate can be controlled by the light intensity, oscillations were observed for light fluxes differing by three orders of magnitude). The first peak in Fig. 2a is due to the oxidation of Tez- to Tea. Te*- was formed at the cathodic potentials of the Pt counter electrode, during the oscillations, from the soluble TeS:- ions, generated by step 1 at the CMT electrode. The second peak will be shown to correspond to the Te(0) to Te( +4) oxidation and not to sulphide oxidation as in Fig. la. Indeed, the oxidation charge of the second peak in Fig. 2a (0.534 mC) represents, within a 10% accuracy, the charge necessary for the oxidation of the Te produced during the first peak of the scan. The charge of the first peak (0.237 mC), is about half of that of the more positive one. This refers to the explanation that the number of electrons involved in the Te(0) oxidation to Te(4+) is 4, the double of that of the Te(2-) to Te(0) oxidation. We therefore conclude that the main process occurring during the oscillations is not S formation (as is the case during the oxidation of the electrolyte on Pt-Fig. la), but Te oxidation. As was mentioned in the Introduction, we attributed the oscillations to the change in the amount of Te at the surface. The valence state of the Te at the CMT surface is of decisive importance for the interpretation. However, it is almost impossible to identify Te(0) in the presence of a large amount of Te(2-)[12-15, 18, 191, witQ only 0.2 eV chemical shift between the peaks of Te(0) and Te(2 -). Instead, we focused our attention on another aspect, derived from the semi-quantitative analysis of the spectra. Table 2 indicates a clear increase in the amount of Cd on the surface compared to the bulk composition.
Table 3. Crystal Parameter Light Potential Temperature
CdTe
CMT
Necessary (non-linear effect) Osc. occur in large range (1 V) Change in induction time frequency
Not necessary Narrow (20 mV) Small effects
V. MARCUand H.-H.
874
This can be explained by the formation of CdS as a consequence of steps 1 and 2. As was shown previously[3], CdS does not generate current oscillations in the electrolyte solutions used in the present study. CMT electrodes, similar to CdTe[3], produce sustained oscillations for as long as 2 h. Therefore one concludes that the CdS deposits cover the surface over a long time compared to the oscillation period. A second observation from the data in Table 2 is that, in time, the ratio of Hg: Te gets closer to 1: 1. This is a result of Cd loss in step 1. A layer depleted in Cd is formed under the CdS on the surface. Its composition tends to that of the unoxidized HgTe. The second component of the oxidizing CdTe is Te. From thermodynamic considerations it is expected that it will be transformed into soluble TeS:- (or TeO:-) ions as described in step 1. However, assuming the adsorption characteristics described in step 4, the experimentally measured changes of Cs+ concentration on the surface (Fig. 7) indicate that a Te layer forms on the surface according to step 3. Its appearance and dissolution is responsible for the oscillations. The fact that the oxidation of Te to soluble sulphotelurites (step 1) is hindered can be explained either by kinetical or thermodynamical reasons. A kinetic mechanism would involve two consecutive steps: formation of Te and its oxidation. If a Te layer is unstable at the steady state, oscillations occur. The thermodynamic mechanism would involve a change of the local conditions (eg pH or potential drop in the Helmholtz layer) which would make alternatively Te or its soluble oxidation products stable. In the case of CdTe, photocurrent oscillations occur over a large potential range (Table 3). Thus, a kinetic mechanism seems more likely. Cs+ ions are known to adsorb on the surface of II-VI semiconductor compounds having a wiirtzite structure. Such electrodes are stable and do not show oscillations (eg CdS and CdSe). On II-VI semiconductor compounds having a zincblende structure (like CdTe and CMT) Cs+ penetrates in the bulk material[28]. Consequently, it does not considerably accumulate on the latter’s surfaces. We assume that Cs+ adsorbes on the Te surface as well (step 4). Its presence is accompanied by a change in the OHconcentration. Thus Cs+ surface concentration is a sensitive gauge of the state of the surface which correlates with the oscillatory behaviour. The POSSible role of Cs+ in amplifying the oscillations was discussed in Ref. [3]: it was supposed to modify the Helmholtz layer structure, to change the surface electronic properties or to speed-up the Te layer dissolution. We performed an experiment that allows a qualitative estimate of the time necessary for dissolving a given amount of Te powder in sulphide with or without Cs in the solution. In the former case the time was reduced by one half.
STREHBLOW
a Te layer and the related change in the surface concentration of Cs+ and OH-. Several differences are pointed out: in the case of CdTe light is necessary for the oscillations, while on Cd,,H&,,Te they take place in the dark as well; the potential range in which the oscillations take place is much narrower for Cd,,,Hg,,,Te, which is also relatively insensitive to changes in the electrolyte temperature. Apart from their importance as systems with peculiar electrochemical features, the present oscillations can be important because of a recently published application of electrochemical sulphidization of the surface of CMT with the purpose of its stabilization. Acknowledgements-V.M.
acknowledges a fellowship from the Alexander von Humboldt Foundation and wishes to thank Professor R. Tenne and Dr D. Laser for drawing his attention to the CMT materials. We are grateful to Dr R. Triboulet for donating some of the samples used in this study and to Mr H.-W. Hoppe for help with the XPS measurements and related discussions.
REFERENCES 1. I. Epelboin, C. Gabrielli and M. Keddam, Comprehensive Treatise of Elecirochemistry, Vol. 9 (Edited by E. Yeager, J. GM. Bockris, B. E. Conway and S. Sarangapani), Chap. 6, p. 148. Plenum Press, New York (1984). 2. J. Wojtowicz, Modern Aspects of Electrochemistry, Vol. 8, pp. 47-120. Plenum Press, New York (1972). 3. V. Marcu and R. Tenne, J. phys. Chem. 92,7089 (1988). 4. B. Vainas, J. electroanai. Chem. 184, 215 (1985). 5. P. Josseaux, J. C. Micheau and A. Kirsch-De Mesmaeker, Elecfrochim. Acta 30, 1093 (1985). 6. R. L. Van Meirhaeghe, F. Cardon and W. P. Gomes, Electrochim. Acta 24, 1047 (1979). 7. S. Cattarin and H. Tributsch, Chem. Phys. Left. 148,
221 (1988). 8. R. J. Field and M. Burger (Eds), Oscillafion and Travelling Waves in Chemical Systems. John Wiley,
New York (1985). 9. S. P. Kowalczyk and J. T. Cheung, J. Vat. Sci. Technol. 18, 944 (1981). 10. D. R. Rhiger and R. E. Kvaas, J. Vat. Sci. Technol. 21, 168 (1982). 11. M. Seelmann-Eggbert and H. J. Richter, J. Vat. Sci. Technol. A6, 2699 (1988).
12. M.
Sakashita,
B. L&he1
and
H.-H.
Strehblow,
J. electroanal. Chem. 140, 75 (1982).
13. M.
Sakashita,
H.-H.
Strehblow
and
M.
Bettini,
and
M.
Bettini,
J. electrochem. Sot. 129, 739 (1982).
14. M. Sakashita,
H.-H.
Strehblow
J. electrochem. Sot. 129, 1710 (1982).
15. M. Seelmann-Eggbert,
G. Brandt and H. J. Richter,
J. Vat. Sci. Technol. AZ, 11 (1984).
16. D. Laser, J. A. Silberman, W. E. Spicer and J. A. Wilson, J. electrochem. Sot. 134, 462 (1987). 17. H. Arwin and D. E. Aspnes, J. Vat. Sci. Technol. AZ, 1316 (1984).
18. R. F. C. Farrow, P. N. J. Dennis, H. E. Bishop, N. R. Smart and J. T. M. Wotherspoon, Thin Solid Films 88, 87 (1982).
CONCLUSIONS Current oscillations were obtained on several CMT materials, showing that the mechanisms of the oscillations on CdTe and CMT are closely related. Both can be explained by the formation and dissolution of
19. A. B. Cristie, I. Sutherland and J. M. Walls, Surf. Sci. 135, 225 (1983).
20. Y. Nemirovsky, L. Burstein and I. Kidron, J. appl. Phys. 58, 366 (1985).
21. R. J. Strong, J. D. Luttmer, D. D. Little, T. H. Teherani and C. R. Helms, J. Vat. Sci. Technol. AS, 3207 (1987). 22 H. Gerischer, Z. Elektrochem. 54, 540 (1950).
Current oscillations of Cd,2H&,,Te 23. U. F. Franck and R. Fitzhugh, Z. Elektrochem. 65,156 (1961). 24. A. B. Ellis, S. W. Kaiser, J. M. Bolts and M. S. Wrighton, J. Am. them. Sot. 99, 2839 (1977). 25. V. Marcu and H.-H. Strehblow, J. electrochem. Sot. (in press).
815
26. S. Haupt, U. Collisi, H. D. Soeckmann and H.-H. Strehblow, J. electroanal. Chem. 194, 179 (1985). 27. S. Licht, R. Tenne, H. Flaischer and J. Manassen, J. electrochem. Sot. 133, 52 (1986). 28. M. Peisah, C. A. Rabe, C. A. Pineda, D. Mahalu and R. Tenne, J. Vat. Sci. Technol. E6, 1506 (1988).
CURRENT
0013-4686/91$3.00 + 0.00 0 1991. Pergamon Press pk.
OSCILLATIONS OF Cd,, Hh.,Te Na, S-CsOH SOLUTION
IN A
V. MARCU* and H.-H. STREHBLOW~ Institut fur Physikalische Chemie und Elektrochemie, Heinrich-Heine Universitiit, D-4060 Dilsseldorf 1, Germany (Received
19 December
1989; in revised form
18 June 1990)
Abstract-Current oscillations have been obtained in the anodical region of sulphidization of various (CMT) compounds in basic solutions containing Cs+ and S*- ions. According to X-ray photoelectron spectroscopy studies, it is possible to correlate these oscillations to the changes of the Cs surface concentration. After anodization, the samples showed an increase in the Cd/Hg and Hg/Te ratios (changed from Cd,,,Hg,,Te to Cd,,,HgTe). The surface concentrations of OH and Cs change
Cd&g,_,Te
synchronously with the oscillations. These observations are explained by the same model which was previously suggested for the photocurrent oscillations on CdTe (mainly a formation/dissolution of a submonolayer of Te). The anodic reaction corresponds to the decomposition of the CdTe fraction of the alloy with the formation of soluble Te compounds, a thin remaining Te and a CdS-deposit on the surface. Key words:
cadmium mercury telluride, current oscillations, XPS, cesium, surface layers, anodization.
INTRODUCTION
Current oscillations have been known in metal electrochemistry for a long time[ 1,2]. For semiconductor electrodes these effects have been found and studied much later. A large number of important semiconductor materials have already shown such current oscillations: binary compounds-CdTe[3], CdS[4,5] and GaAs[6], as well as some ternary materials, CuXSe, or CuXS,, where X = Fe or In[l. As is the case with their well-known counterparts in homogeneous systems, eg the Belousov-Jabotinsky reaction[8], most of these processes are far from being completely understood. The electrochemistry of CMT has been thoroughly studied, due to its important practical application as a material for ir detectors. The behaviour of the surface has been studied during etching[9-111, anodic oxide formation[l2-191 and, more recently, during sulphidization in organic solutions[20,21]. Various methods were applied in these studies to supplement the pure electrochemical information: X-ray photoelectron spectroscopy (XPS), Rutherford back scattering (RBS), ellipsometry and Raman spectroscopy. We have reported previously on photocurrent oscillations of n-CdTe single crystals in various solutions[3]. The oscillations were attributed to changes in the Helmholtz layer due to the formation/ dissolution of a Te layer and selective Cs adsorption. Our optical measurements have shown that the changes in the Te coverage should be smaller than one monolayer. Single crystal ‘electrodes that do not contain Te were found to give no oscillations and the presence of Cs and sulphide ions was necessary to * Present address: Israel Electrical Corporation. t To whom all correspondence should be addressed.
obtain sustained oscillations[3]. To support the above mentioned model, in the present study we have checked whether oscillations are present on other semiconductor single crystals containing Te and we have tried to’ correlate the oscillations with the surface composition as determined by XPS.
EXPERIMENTAL
Cd,Hg, _,Te single crystals (x = 0.2, 0.7, 0.9, 1.0) have been tested. The following solutions were prepared from analytical grade (pa.) reagents and deionized water (D.I.) (Millipore, MilliQ purification system): Cs,S: CsOH: S (1 M each) (1) and Na,S:CsOH (0.3 M: 1 M) (2). All the experiments were performed potentiostatically, at 25°C without stirring and at constant light intensity, when light was necessary (see Discussion below). Hg/HgO/O.l M KOH (MOE) (E = 0.14 V) or a saturated calomel electrode (see) were used as reference electrodes. A large Pt foil served as counter electrode in a standard three-electrode electrochemical cell. Homemade potentiostats and potential ramp generators were used to control the potential. All potentials are given in reference to the standard hydrogen electrode (she) and are corrected for liquid junction potentials. The electrodes were polished with diamond spray (Struers) to 1 pm and sometimes (see Discussion) etched in Br,:methanol (2% vol.) or K,CrrO, (4 g) : cont. HNOr (10 ml) : H, 0 (20 ml) and reduced at -0.9 V @he) in acetate buffer[l2]. Before placing in the analyser chamber of the ESCALAB 5 (VG Instruments), the samples were flushed with N, or Ar, after sometimes being rinsed with D.I. water and/or absolute ethanol (see Discussion). After a number of oscillations, the sample was extracted from the cell 869
V. MARCUand H.-H.
870 Pt in 0.3 M Na2S + 1 M CsOH dE -= 50 mV s-l dt
lo 8-
-0.4-0.2
0
RESULTS
0.2 0.4 0.6 E(she)N
I
I
0.8
1.0
I
Fig. 1. Potentiodynamic polarization curves of: (a) Pt in 0.3 M Na,S: 1 M CsOH. dE/dt = 50 mV s-‘, A = 2 cm’. (b) Cd,,,H&.,Te in the same electrolyte. 1 and 2 represent
successive scans. and introduced to the vacuum for analysis by XPS. Samples were taken after a few oscillations and after some tens of oscillations. The extraction was performed either when the oscillation was at the maximum or at the minimum current value. For the semi-quantitative XPS measurements the following peaks were used: Te 3d5 (binding energy 573 eV), Cd 3d5 (B.E. 405 eV), Hg 4f7 (B.E. 101 eV), Cs 3d5 (B.E. 725eV) and 0 1s (B.E. 532eV) and the same sensitivity factors as in Ref. [13]. For some experiments a rotating ring disk electrode with a cylindrical single crystal C&.,Hg,,Te disk and Pt ring was used.
0.8 -
a)
Pt
0.6 -
0.4 -
0.1 M KOH
0.2 -
-0.8-0.6-0.4-0.2
0
0.2
STREHBLOW
,
,
,
0.4
0.6
0.8
E(she)N
Fig. 2. Potentiodynamic polarization curves of: (a) same as Fig. la with an electrolyte which has been already used for a few hours in oscillations experiments prior to the scan. (b) Te, HgTe (Br,-CH,OH etched) and Cd,,,Hg,,Te (cleaved in UHV) in 0.1 M KOH (from Refs[lZ, 161).
Since the oscillation might originate from the solution electrochemistry, particularly when potential oscillations have been mentioned during the oxidation of sulphide and other soluble sulphur species[22], one has to examine the behaviour of a Pt electrode in this special electrolyte. Figure la presents the potentiodynamic polarization curve of a Pt electrode in the electrolyte used for oscillations. The numbers 1 and 2 on the curves indicate two successive scans, both from cathodic to anodic potential direction. The first peak (at ca 0.2 V) is due to S*- to So oxidation, while the next rise is due to the formation of soluble sulphur products. No oscillations are seen in these experiments in the region -0.2-0.6 V. In Figure lb the seemingly similar behaviour of the C$,,H&,,Te electrode in the same solution can be observed. Here 1 and 2 represent two successive scans in opposite directions. Following scans are identical and are not represented. The current densities are in this case 10 times larger than those of Fig. la. This is an indication that a different electrode process, due to reactions of the CMT, occurs, as will be shown below by comparison with a few other systems. Figure 2a shows the currentvoltage characteristics of a Pt electrode in a solution of the same composition, but after a several hours use as an electrolyte for oscillations studies. One notices that on top of the peak observed for the fresh solution, an additional one (at ca -0.5 V), absent in the fresh solution, appears when the potential scan starts at more negative values. This peak is due to the oxidation of Te*-. This ion is formed at the Pt counter electrode, during the oscillations, by reduction of the products of simultaneous oxidation of the CMT electrode. Inspection of the anodic polarization curves of Te, HgTe etched in Br,-CH,OH (which enriches the surface in Te) and a vacuum-cleaved CMT sample in KOH (Fig. 2b) indicates that the oxidation of the CdTe fraction of the CMT and of Te (present either as bulk Te or as a thin layer on HgTe) occurs almost at the same potential (ca 0.1-0.2 V). The ratio of the two processes depends on the surface preparation. For an illuminated semiconductor that shows no oscillations, under reverse bias, the i-u curve is determined simply by the total photon flux. One such curve is shown for CdS in Fig. 3b (chopped light). Almost no change of photocurrent with the applied potential is seen between -0.4 and 0.8 V. The situation is different in the case of CdTe: under the same conditions, the photocurrent peaks at ca 0.3 V, due to the electrochemical kinetics. Different photocurrent intensities for opposite scan directions, and phototransients with a maximum and minimum (Fig. 3a) were noticed. This observation is closely related to the oscillations: if the potential scan is stopped in this potential range, the photocurrent starts to oscillate (although the illumination is kept constant)[3]. One should notice the coincidence of the positions of the photocurrent peaks with those of CMT (Fig. lb). A decrease in photocurrent with decreasing anodic potential is expected in a region approaching the flat-band potential, since CdTe is reverse biased. Instead, after the jump due to the beginning of illumination and a normal decrease
Current oscillations of C&Hgs,,Te
O**-
871
a) CdTe CszS + CsOH (1 M
: 1 M)
0.8 0.4 -
-1.2
-0.8
0
-0.4
0.4
0.8
E(shePV
Fig. 3. Potentiodynamic polarization curves of: (a) n-CdTe (continuous curve represents the cathodic scan, while the previous anodic scan is, for simplicity, only partly represented in the upper right comer) and (b) n-CdS in Cs,S :CsOH (1 M each) (the slight difference in the cathodic dark current between the anodic and cathodic scans is not relevant for the discussion). Chopped light: alternating dark (lower) and light (higher) currents.
in the photocurrent, due to the reduction of the potential barrier with the cathodic scan, an increase follows at more negative potentials (eg around 0.1 V) (Fig. 3a). CMT in 0.3 M Na2S + 1 M CsOH 45” c
48 mV
27 mV
0
If23 mV
OmV
l time/s
Fig. 4. Current oscillations of Cd,,,H&,,Te in dependence of the potential.
The CMT electrodes were cleaned, as described above, introduced in the cell under potential control, at a potential slightly negative of that at which the current rises sharply (ie 0 V in Fig. 1b). Subsequently, the potential was slowly increased until the current started to drop. At this potential the scan was stopped and most of the electrodes started to oscillate with a frequency in the range of 0.1-0.5 Hz (slightly depending on various parameters such as potential, temperature and surface preparation) and an amplitude close to 60% of the maximal value of the anodic current. Figure 4 depicts these oscillations for different electrode potentials (the potential at which the oscillations started on this electrode was lower than that of the sharp drop in Fig. lb). The potential region in which the oscillations take place is much narrower than in the case of CdTe[3]. The potential of 48 mV in Fig. 4 represents the maximum potential at which coherent oscillations in the region of interest (-0.2-0.4 V) occur. On some of the electrodes, oscillations were noticed for potentials larger than 0.4 V, which probably are due to similar electrochemical behaviour during the oxidation of HgTe (or HgS) part of the compound. The structure of the oscillations is shown in more detail in Fig. 5. The shape of the wave corresponds to Fig. If of Ref. [23], where the oscillations were attributed to a chemical reaction of the anode with one of the oxidation products of the electrolysis of the solution. The shape of the wave could give hints for excluding certain mechanisms involving sudden changes in the state of the electrode (eg gas bubbles[4] or a breakdown of a passive layer). It also shows that the mathematical treatment we used before, in order to explain the features of the CdTe oscillations[3], is only approximately true, since it would involve a perfect sine shape.
V. MARCUand H.-H. STREHBLOW CMT in 0,3 M Na2S + 1 M CsOH
60000 -
al CMTimi”
50000 IlOmV
40000 30000
la
Te
:I::!!+
r
-
10s
time/s
Fig.
5. Typical fine structure of the oscillations Cd,,H&,,Te (same conditions as in Fig. 1b).
To obtain additional information on the surface reactions, X-ray photoelectron spectra were taken from the Cd,,,Hg,,,Te specimen. Figure 6 presents a general survey of the XPS spectrum of a reference sample (polished only). In Fig. 7, surveys of two samples immersed at the top and the bottom of the current oscillation, respectively, are compared. The pronounced change in the relative intensity of the Cs ion signal with respect to Cd (or C) is quite obvious. Table 1 compares qualitatively the oscillations for the different parameters such as the composition of the electrode and of the electrolyte. One concludes that electrodes which are more stable to photooxidation, like CdS and CdSe[24], show no oscillations, whatever the composition of the solution. The presence of Te in the electrode and sulphide in the solution are prerequisites for oscillations, since no current instabilities were observed in solutions containing only hydroxide[ 12-141. A thick Te layer forms in basic solutions[24]. The above mentioned conditions are still not enough for the generation of sustained oscillations: Cs+ ions play an important role. The situation described by weak oscillations in Table 1 actually refers to very low amplitudes (slightly above noise level), irregular and with much longer periods (tens of seconds). The change of the surface composition during the oscillations, as deduced from XPS results can be seen
5-
800
of
1000
1200
EbleV
Fig. 7. XPS survey spectrum of Cd,,Hg,,Te samples after oscillations. The samples were immersed under potentiostatic control when the current was: (a) at minimum; (b) at maximum. (N.B. the a.u. here are 10,000 times smaller than in Fig. 6.)
in Table 2. Table 3 compares the characteristics of the oscillations of CdTe and CMT. Experiments performed with the rotating ring disk electrode showed only a weak dependence of the ring reduction currents on the disk oscillations. Changes of only < 1% were observed in the ring current during the oscillations, which is negligibly small compared to 25% expected theoretically. This indicates that the formation of surface layers and incorporation of corrosion products therein is responsible for the oscillations.
DISCUSSION The best method for preparing the electrodes prior to anodization, in view of their subseqent analysis by XPS, was mechanical polishing. This pretreatment is reproducible and prevents any changes in the surface stoichiometry. We checked that the electrochemical behaviour and the occurrence of the oscillations is not altered by the different surface treatments mentioned above. Oscillations also occur on electrodes etched in Br,-CH,OH and reduced in acetate buffer (a standard pretreatment favoured by electrochemists)[ 121. This etching procedure increases the amount of Te on the surface. The subsequent reduction of Te to Te2- in the acetate buffer slightly Table 1. Crystal
Solution
0
1 200
I 400
1 600
800
I 1000
EbIeV
Fig. 6. XPS survey spectrum of a reference Cd,,Hg,,Te sample (mechanically polished only).
Oscillations
OH- + S*-
CdTe CdS, CdSe
Weak No
OH- + S2- + Cs+
Cd,Hg,., Te x = 0.2, 0.7, 0.9, 1
Strong
CdS CdSe
No No
Current oscillations of Cc&,H&,,Te Table 2. Initial
873
MODEL
Cd:Hg:Te
=0.2:0.8:1
After: 1.4:0.9: 1 1.2:1.0:1
depletes the CMT surface in Cd ions in addition. This paradoxical behaviour, which was observed in the case of the cathodic decomposition of clean CdSe as well[25] is explained by the competing CdTe or HgTe reduction. Cd is formed, by the later reaction, which in contact with the air oxidizes to CdO. The oxide in turn dissolves in D.I. water during rinsing, while part of the telluride can be oxidized to insoluble Te. Thus, the expected ratio of atomic composition is reversed. The alternative etching in K,Cr, O,-containing solutions removes the oxides and damaged sites from the surface, but leaves a Cr contamination which interferes with the Te determination by XPS. In order to keep the surface concentrations as close to those present during the oscillations, most of the samples were transferred to the XPS analysis chamber without further rinsing. No artifacts are introduced by this manipulation method, since, qualitatively, the same results are obtained when the samples are rinsed with water or ethanol. The transfer of the model used to explain the photocurrent oscillations of CdTe[3] to the case of CMT involves the following steps (Fig. 8): 1. The CdTe part of the CMT is oxidized to Cd*+, Te (unstable at these potentials) and TeS:-, 2. The Cd2+ ions diffuse through the Cd depleted layer of the electrode (which is closer in stoichiometry to HgTe) to the electrolyte interface, where they react with S*- and precipitate on the surface as CdS. 3. A thin Te layer, produced in step 1, successively forms and dissolves as TeSz-, 4. Cs+ is adsorbed on the Te layer, but not on the clean CMT surface. The experimental evidence presented above cannot distinguish between the independent Cs+ and OH- adsorption, because of ion pairing[26,27]. Their presence facilitates the dissolution of Te (see last paragraph of Discussion). Steps 3 and 4 generate the oscillating behaviour. The following discussion will show how the present results support the model. It is obvious from Table 1, that the oscillations require the presence of Te. Comparison of Fig. lb and the curves in Figs 2b and 3a, which represent the shape of the photocurrent-potential curve of CdTe and replotted literature data for Te, CMT and HgTe oxidation, respectively, suggests that the region of the oscillation is the same as that of CdTe and Te oxidation. Much larger currents are observed in the presence of sulphide ions (compare Figs 2b and lb). This observation is due to the formation of the more
Cd pool layer (- HgTe) Fig. 8. Model explaining the oscillations. CdTe denotes the part of CMT which is oxidized.
stable CdS and the better dissolution of Te in sulphide solutions. The larger reaction rate might be a reason for the oscillating behaviour (however, in the case of CdTe, where the reaction rate can be controlled by the light intensity, oscillations were observed for light fluxes differing by three orders of magnitude). The first peak in Fig. 2a is due to the oxidation of Tez- to Tea. Te*- was formed at the cathodic potentials of the Pt counter electrode, during the oscillations, from the soluble TeS:- ions, generated by step 1 at the CMT electrode. The second peak will be shown to correspond to the Te(0) to Te( +4) oxidation and not to sulphide oxidation as in Fig. la. Indeed, the oxidation charge of the second peak in Fig. 2a (0.534 mC) represents, within a 10% accuracy, the charge necessary for the oxidation of the Te produced during the first peak of the scan. The charge of the first peak (0.237 mC), is about half of that of the more positive one. This refers to the explanation that the number of electrons involved in the Te(0) oxidation to Te(4+) is 4, the double of that of the Te(2-) to Te(0) oxidation. We therefore conclude that the main process occurring during the oscillations is not S formation (as is the case during the oxidation of the electrolyte on Pt-Fig. la), but Te oxidation. As was mentioned in the Introduction, we attributed the oscillations to the change in the amount of Te at the surface. The valence state of the Te at the CMT surface is of decisive importance for the interpretation. However, it is almost impossible to identify Te(0) in the presence of a large amount of Te(2-)[12-15, 18, 191, witQ only 0.2 eV chemical shift between the peaks of Te(0) and Te(2 -). Instead, we focused our attention on another aspect, derived from the semi-quantitative analysis of the spectra. Table 2 indicates a clear increase in the amount of Cd on the surface compared to the bulk composition.
Table 3. Crystal Parameter Light Potential Temperature
CdTe
CMT
Necessary (non-linear effect) Osc. occur in large range (1 V) Change in induction time frequency
Not necessary Narrow (20 mV) Small effects
V. MARCUand H.-H.
874
This can be explained by the formation of CdS as a consequence of steps 1 and 2. As was shown previously[3], CdS does not generate current oscillations in the electrolyte solutions used in the present study. CMT electrodes, similar to CdTe[3], produce sustained oscillations for as long as 2 h. Therefore one concludes that the CdS deposits cover the surface over a long time compared to the oscillation period. A second observation from the data in Table 2 is that, in time, the ratio of Hg: Te gets closer to 1: 1. This is a result of Cd loss in step 1. A layer depleted in Cd is formed under the CdS on the surface. Its composition tends to that of the unoxidized HgTe. The second component of the oxidizing CdTe is Te. From thermodynamic considerations it is expected that it will be transformed into soluble TeS:- (or TeO:-) ions as described in step 1. However, assuming the adsorption characteristics described in step 4, the experimentally measured changes of Cs+ concentration on the surface (Fig. 7) indicate that a Te layer forms on the surface according to step 3. Its appearance and dissolution is responsible for the oscillations. The fact that the oxidation of Te to soluble sulphotelurites (step 1) is hindered can be explained either by kinetical or thermodynamical reasons. A kinetic mechanism would involve two consecutive steps: formation of Te and its oxidation. If a Te layer is unstable at the steady state, oscillations occur. The thermodynamic mechanism would involve a change of the local conditions (eg pH or potential drop in the Helmholtz layer) which would make alternatively Te or its soluble oxidation products stable. In the case of CdTe, photocurrent oscillations occur over a large potential range (Table 3). Thus, a kinetic mechanism seems more likely. Cs+ ions are known to adsorb on the surface of II-VI semiconductor compounds having a wiirtzite structure. Such electrodes are stable and do not show oscillations (eg CdS and CdSe). On II-VI semiconductor compounds having a zincblende structure (like CdTe and CMT) Cs+ penetrates in the bulk material[28]. Consequently, it does not considerably accumulate on the latter’s surfaces. We assume that Cs+ adsorbes on the Te surface as well (step 4). Its presence is accompanied by a change in the OHconcentration. Thus Cs+ surface concentration is a sensitive gauge of the state of the surface which correlates with the oscillatory behaviour. The POSSible role of Cs+ in amplifying the oscillations was discussed in Ref. [3]: it was supposed to modify the Helmholtz layer structure, to change the surface electronic properties or to speed-up the Te layer dissolution. We performed an experiment that allows a qualitative estimate of the time necessary for dissolving a given amount of Te powder in sulphide with or without Cs in the solution. In the former case the time was reduced by one half.
STREHBLOW
a Te layer and the related change in the surface concentration of Cs+ and OH-. Several differences are pointed out: in the case of CdTe light is necessary for the oscillations, while on Cd,,H&,,Te they take place in the dark as well; the potential range in which the oscillations take place is much narrower for Cd,,,Hg,,,Te, which is also relatively insensitive to changes in the electrolyte temperature. Apart from their importance as systems with peculiar electrochemical features, the present oscillations can be important because of a recently published application of electrochemical sulphidization of the surface of CMT with the purpose of its stabilization. Acknowledgements-V.M.
acknowledges a fellowship from the Alexander von Humboldt Foundation and wishes to thank Professor R. Tenne and Dr D. Laser for drawing his attention to the CMT materials. We are grateful to Dr R. Triboulet for donating some of the samples used in this study and to Mr H.-W. Hoppe for help with the XPS measurements and related discussions.
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Sakashita,
B. L&he1
and
H.-H.
Strehblow,
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13. M.
Sakashita,
H.-H.
Strehblow
and
M.
Bettini,
and
M.
Bettini,
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14. M. Sakashita,
H.-H.
Strehblow
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G. Brandt and H. J. Richter,
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CONCLUSIONS Current oscillations were obtained on several CMT materials, showing that the mechanisms of the oscillations on CdTe and CMT are closely related. Both can be explained by the formation and dissolution of
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Current oscillations of Cd,2H&,,Te 23. U. F. Franck and R. Fitzhugh, Z. Elektrochem. 65,156 (1961). 24. A. B. Ellis, S. W. Kaiser, J. M. Bolts and M. S. Wrighton, J. Am. them. Sot. 99, 2839 (1977). 25. V. Marcu and H.-H. Strehblow, J. electrochem. Sot. (in press).
815
26. S. Haupt, U. Collisi, H. D. Soeckmann and H.-H. Strehblow, J. electroanal. Chem. 194, 179 (1985). 27. S. Licht, R. Tenne, H. Flaischer and J. Manassen, J. electrochem. Sot. 133, 52 (1986). 28. M. Peisah, C. A. Rabe, C. A. Pineda, D. Mahalu and R. Tenne, J. Vat. Sci. Technol. E6, 1506 (1988).