Electrochemical effects in metall - a-Si:H - metal* structures

Centre de recherche erz microilectronique et Dipartet~ierlt de ginie ilectrique, Utliver:riti de Sherbrooke, 2500, boulevard Universiti, Sherbrooke (Quibec), Canada J l K 2R1

AND

M. R. WERTHEIMER Groupe des couches minces et Dipartement de ghzie physique,

~ c o l e polytechriique de Montrial, C.P. 6079, Succursale A, Morlrrial (Quibec), Carlada H3C 3A7 Received June 27, 1990

In the course of studying microwave plasma-deposited a-Si:H, some M I - a-Si:H - M-, structures (where MI and M2 represent dissimilar metals) exhibited a behavior similar to that of galvanic cells. For many pairs of metallic electrodes we have observed, in the dark, an open-circuit voltage (V,) and a short-circuit current (l,,) that were stable over a period of several months. Such effects have been variously reported for MI-polymer-M2 cells, where no doubt remains as to their electrochemical origin. We describe here the observed phenomena, along with results from chemical profiling (elastic recoil detection analysis and Auger electron spectroscopy) studies, aging characterization by Fourier transform infrared spectroscopy, morphological studies by scanning transmission electron microscopy, low-frequency permittivity measurements. and 1-V behavior in the dark and under illumination, and we interpret these results in terms of electrochemical reactions.

Lors de l 'ttude du a-Si:H dCposC par plasma micro-ondes, certaines structures M I - a-Si:H - M2 (ob M I et M2 representent des mCtaux diffirents) ont montrC un comportement similaire a celui d'une pile galvanique. Plusieurs paires d'tlectrodes differentes donnent lieu, dans l'obscuritt, a une tension en circuit ouvert (V,,,) ainsi qu ' i un courant en court-circuit (I,,) qui sont tous deux stables pour des periodes de plusieurs mois. Pareils effets furent maintes fois signalis pour des cellules MI- polymkre-Mz, oh leur origine ilectrochimique est bien Ctablie. Nous prisentons ici une description des divers phCnomknes observts, ainsi que des rksultats de profilage chimique (dktection du recul elastique et spectroscopie des Clectrons Auger) des cellules, d'ttudes de vieillissement par spectroscopie infrarouge par transformte de Fourier, des ttudes morphologiques par

~ ~

microscopie Clectronique balayage, ainsi que des mesures de permittivitk B basse frkquence et des mesures courant-tension a 1'obscuritC et la lumikre. Ces risultats sont finalement interprCtCs en termes de reactions Clectrochimiques.

[Traduit par la revue] Can. 1. Phys. 68. 1396 (1990)

1. Introduction In previous studies conducted in our laboratory (1, 2) and

elsewhere (3-6), it was shown that for a number of polymeric dielectrics a MI-polymer-M2 cell (where M I and M2 repre- sent dissimilar metals) will display a spontaneous open-circuit voltage (V,,) and a short-circuit current (I,,). Such polymers include polyethylene terephthalate (PET) (I), polyvinyl bu- tyral (PVB) (4), polyamides (5), polyimides ( I ) , regenerated- cellulose (6), and others. Recently, Leclerc and Yelon (7) have shown that the behavior of these cells is similar to what is expected from an electrochemical system where the current is limited by the ohmic drop in the electrolyte. They have also shown that, in the presence of a water-vapor atmosphere, wa- ter affects both the ionic transport in the electrolyte and the electrode reactions.

When some of our M I - a-Si:H - M2 structures were ob- served to behave in a similar fashion to the polymer cells, we naturally attempted to interpret our data in the framework mentioned above. To gain knowledge of both the physics and chemistry of metal - a-Si:H interfaces and of the electrochem- ical effects themselves, we have conducted numerous further

FIG. 1. Top view ( a ) and cross section ( b ) of a MI - a-Si:H - M2 cell: 1, glass; 2, M I electrode; 3, a-Si:H; and 4, M2 electrode.

(iii) Aging studies: submitting a-Si:H samples to Fourier transform infrared (FTIR) spectroscopy analysis at different ageing intervals, to monitor possible chemical modifications.

(iv) Optoelectronic studies: to compare the I-V character- istics in the dark and under illumination.

(v) Ambient studies: to characterize the cell behavior when exposed to different gaseous atmospheres.

(vi) Permittivity studies: to determine the cell behavior as a function of frequency under alternating voltage, especially when exposed to differing degrees of relative humidity. - -

studies, namely: (i) Physico-chemical studies: using chemical-profiling tech- 2. Experimental

niclues (elastic recoil detection analysis (ERDI and Auger elec- The M I - a-Si:H - M2 cells under study comprise films of . ~

tron spectroscopy (AES)), to identify any detectable c;mposi- a-Si:H between two thin-metallic electrodes ( ~ i g . I). The M2 tional changes in the electrodes and (or) a-Si:H. electrode is first evaporated on a clean glass substrate (Corn-

(ii) Morphological studies: using scanning transmission ing Glass 7059, or microscope slide glass) of 2.5 x 2.5 cm. electron microscopy (STEM) to determine whether the amor- This electrode is normally 50 nm thick. Then the substrate phous silicon structure can be related to the electrochemical is transferred to a glow-discharge deposition system where properties. the a-Si:H deposition takes place from silane (SiH4) feed gas.

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ET AL. 1397

The L M P ~ ~ microwave plasma reactor used to produce the present a-Si:H films has been described elsewhere (8, 9). All the samples studied here were intentionally maintained in the glow zone of the plasma. The resulting a-Si:H is highly resis- tive and morphologically different from semiconducting mate- rial grown outside this zone (9). The thickness of these films ranged from 0.5 to 2.0 pm, as determined from interferometric measurements. After reintroduction into the evaporation sys- tem, the M I electrode is evaporated onto the a-Si:H, usually to a thickness of 50 nm. Finally, both electrodes are connected to the electrical measuring system by thin copper wires, attached using silver epoxy paste.

About 40 cells have been studied here, half of which were Au - a-Si:H - Al, a configuration that produces large V,, values. These samples have been used to observe the basic electrochemical phenomena (V, and I,,) and also to conduct the physico-chemical and morphological studies (ERD, AES, and STEM) mentioned above. Special samples of a-Si:H de- posited on crystalline silicon substrates have been used for FTIR analysis.

All voltage and current measurements were performed with either Keithley Model 616 or Model 642 electrometers. Un- less otherwise specified, these measurements were taken with the sample in the dark, at room temperature and in ambient atmosphere.

Complex permittivity measurements (at frequencies between 2 mHz-10 kHz) were conducted using a Solartron Frequency Response Analyser (Model 1170). At frequencies <5 Hz, a Keithley Model 616 electrometer was used to increase the in- put impedance of the Solartron. Low-frequency results were also obtained from the Fourier transform of the transient cur- rent (measured with a Model 616 electrometer) induced by a step voltage (10).

3. Results and interpretation 3.1. Morphological and physico-chemical studies

The first evidence of structural changes with time in these cells was a progressive evolution of the apparent color of the external electrode M I . For example, the gold electrode of a Au - a-Si:H - A1 cell was observed to evolve from its original yel- low metallic luster, successively to dull yellow, indigo, violet, and finally black, within about 60-90 d. The color sequence observed and the rate of change were found to depend upon the nature of the electrode. For example, an indium electrode (In - a-Si:H - Au) evolved from metallic grey to dull grey, indigo, and black in only a few hours. Similar color changes have also been observed by others. For example, Carlson (1 1) reported surface coloration over a palladium contact in a-Si:H Schottky-barrier solar cells.

Such changes are reminiscent of electrochemical corrosion, in which a noble metal like gold would be expected to corrode much more slowly than a reactive metal like indium, as we have indeed observed. In the following text it will be shown that the phenomenon is not simple, at least not in the Au - a-Si:H case.

Electron microscopy has been performed on a-Si:H sam- ples, either deposited directly on a glass substrate, deposited on the M2 electrode, or taken from the region between MI and M2 (see Fig. 1). In all the cases we observed (Fig. 2) a colum- nar structure perpendicular to the plane of the substrate, this structure depends on the deposition parameters, but not on the time elapsed since deposition. The column diameter is quite variable, ranging from 10 to 300 nm; columnar structures in a-

Si:H are well known and have been studied by several authors (12, 13). As the substrate temperature during deposition (Ts) is raised to about 300"C, the mean column diameter becomes smaller, accompanied by a rise in photoconductivity. It can also be seen from Fig. 2 that these samples display significant surface roughness, an observation known to correlate with the presence of the columnar structure (12, 13).

We have observed that for aged Au - a-Si:H - A1 samples over 1 year old, the MI electrode readily acquires an electric charge when exposed to the electron beam in the microscope, suggesting the presence of a dielectric layer on the electrode surface. This is further confirmed by the impossibility of estab- lishing an electrical contact with an electrode whose coloration has begun to change, even if only slightly.

AES and ERD analysis were performed to characterize the chemical composition of aged samples. Figure 3 shows the atomic concentrations of the constituent elements as a func- tion of etching time for a 1-year-old Au - a-Si:H - A1 cell deposited at Ts = 50°C. It shows the presence of a surface layer composed of 10% carbon, 35% oxygen, 30% silicon, and 25% gold. This indicates that this layer is formed mainly of SiO, (14).

At the very surface, the Si Auger peak (92 eV) is shifted by about 10 eV. After the first atomic layers have been etched, a second peak appears, within 1 eV of its normal position. Further etching causes the surface peak to disappear entirely, leaving only the second peak. This observation is related to the earlier mentioned charging effect, brought about by the insulating nature of the surface film.

Figure 3 also shows that the gold electrode contains a sig- nificant concentration of silicon (3%) that has diffused through the entire layer. The aluminum electrode contains a high con- centration of oxygen, as expected for such a reactive metal.

At the Au - a-Si:H interface, interdiffusion appears to have taken place, but this could also be a manifestation of the sur- face roughness of the amorphous silicon film. The presence of gold in the silicon could also be due to a "knocking-in" process induced by the argon ions used to etch the sample. As mentioned above, the a-Si:H layer of the sample in Fig. 3 was deposited at T, = 50°C; samples made at higher Ts dis- played lower hydrogen contents, but were similar in all other important respects.

The results obtained with the present Au - a-Si:H - A1 cells agree with those published by Hiraki et al. (15, 16) for Au - c-Si and by Hara et al. (17) for Au - a-Si:H. Like these authors, we find that silicon diffuses through the gold layer, at the surface of which it accumulates and spontaneously re- acts with the atmosphere to form an oxide film, even at room temperature.

Finally, a few samples of a-Si:H - c-Si were examined by FTIR spectroscopy (18-20). Figure 4a shows a series of ab- sorption bands between 1000-1200 cm-I, which are attributed to the presence of contaminants (C, 0 , and N). Their intensity is seen to rise with time after the samples have been exposed to an ambient atmosphere; also observed are absorption bands assigned to CH, near 3000 cm-', along with the progressive disappearance of the SiH2 band near 2100 cm-I .

Figure 46 illustrates the evolution of certain bands in the 1000-1200 cm-I region. While it reconfirms the presence of carbon and oxygen, this figure also demonstrates that con- tamination occurs progressively after the samples become ex- posed to an ambient atmosphere. These results show that the columnar a-Si:H is by itself quite reactive with atmospheric

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1398 CAN. I. PHYS. VOL. 68, 1990

L G L A S S

a - S i : H ( s u r f a c e )

a - S i : H ( cross section)

* a - S i : H ( s u r f a c e )

+ a - S i : H

(cross section) * A L

\ G L A S S

FIG. 2. Microphotographs of a-Si:H grown in the glow zone of the plasma; (a) a-Si:H on glass and (b) a-Si:H on Al.

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PAQUIN ET AL

FIG 3. AES and ERD atomic concentration profiles for a Au - a-Si:H - Al sample. The substrate temperature was T, = 50°C. The hydrogen profile has been measured by ERD and all other elements by AES.

constituents; without a metallic electrode, the reactions go to enhances both the current and voltage, the single exception completion with a relaxation time of about 20 h. being sample no. 6. Note here that an important part of the

The preceding results and observations clearly indicate that incident light is not transmitted through the thick (50 nm) sur- the cells undergo diffusion, oxidation, and corrosion processes face electrode. Such a photovoltaic effect requires the presence all at a significant rate. of a small band gap electronic conductor, i.e., a semiconduc-

tor. Should this semiconductor be the result of a surface reac- 3.2. Electrochemical, optoelectronic, and dielectric properties tion between a metal electrode and the amorphous silicon, one

As already mentioned, many of the present samples exhib- would expect each and every sample to exhibit a photovoltaic ited spontaneous J,, and V,. Values for a few representative effect. Since it is not the case, we are led to believe that our Au - a-Si:H - A1 cells are reported in Table 1; these values a-Si:H consists of two bulk phases (21, 22); one which is a di- were obtained after the cells had been aged for at least 100 d. electric ionic conductor, the other a semiconducting electronic Both J,, and V,, values for a given cell were stable for periods conductor. Clearly, the electrochemical effects require the for- exceeding 1 year, for all these samples. However, it is obvious mer phase, while the photovoltaic effect requires the latter. that the particular values differ considerably from one case to Since our material is not very reproducible it may happen that another. Such lack of reproducibility has also been reported sample no. 6 lacks the semiconducting phase and is unable to by Leclerc and Yelon (7) for the case of polymer-based cells. exhibit a photocurrent. They attributed it to the varying content of Impurities in their While V , and J,, values presented in Tables 1-3 pertain polymeric electrolytes. As this presumably also applies to a- to aged samples, and were constant for long periods of time, Si:H, where physico-chemical and morphological differences their initial behavior after fabrication of a cell is quite different. occur even among samples produced at nominally the same Both V , and J,, have negligibly small values until the surface T, and other fabrication parameters, the dispersion of values coloration changes as described in Sect. 3.1. comes as no surprise. When a cell, initially in ambient atmosphere, is exposed to

Table 2 reports the maximum V,,, and J,, values obtained high vacuum (2 x lop4 Pa), J,, is found to decrease by two or with different MI-M2 pairs, for T, = 150°C. It is noteworthy three orders of magnitude and V , also becomes very small. that the values obtained for certain MI-M2 combinations are When subsequently returned to the atmosphere, they both re- quite different from those for the corresponding M2-MI pair sume their original values. This is typical of an electrochemical (see samples nos. 5-13, 18, 19), and that nonzero values are system in which a gas participates in an electrodic process. To obtained for certain MI-MI pairs (see samples nos. 1 1 and examine this further, V,,, and J,, were measured under var- 12). We must mention here that M I - a-Si:H - M I are not ious controlled atmospheres. Introducing He, Ar, N2, or 0 2

as symmetric as one would think. It should be kept in mind into the sample chamber did not yield V, or J,, values that that the lower electrode is deposited on glass while the upper differed from those obtained under high vacuum. However, electrode is exposed to air, the a-Si:H is grown on the lower when these varlous gases were saturated with water vapor, re- electrode while the upper electrode is deposited on it, and sults were similar to those in an ambient atmosphere. Figure that the a-Si:H is likely inhomogeneous simply because, as 5 shows the shifts in V , under various conditions and their it grows, its structure becomes less dependent on that of the time evolution. Note particularly the transient polarity reversal initial metallic substrate, as can be seen in Fig. 2b. observed when H2 is injected.

In Table 3 we compare V, and J,, values measured in the To further examine the role of humidity, low-frequency per- dark and under AM-2 illumination (35 mw.crnP2) for sam- mittivity measurements were carried out on M I - a-Si:H - M2 ples that exhibit a photovoltaic effect. Evidently, illumination cells. Figure 6 illustrates a typical result for the loss tangent

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CAN. J. PHYS. VOL. 68, 1990

- 8 ' 4300 36bo 32bo 2800 znbo 2000 1600 1200 sbo nbo

Wf3VE NUMBERS

T ime ( h ) FIG. 4. Evolution of the IR spectra for a-Si:H grown in the glow zone of the plasma. (a) IR spectra taken after exposition times of 6 h (curve

1) and 47 d (curve 2) to ambient atmosphere. (b) Evolution of certain bands in the 1 0 0 ~ 1 2 0 0 cm-' region.

(tan 6 = d l / d ) as a function of frequency, in which curve 1 corresponds to a cell under a dry atmosphere, while curve 2 represents the same cell at 100% relative humidity. One ob- serves relaxation peaks of roughly equal heights, but with the high-relative-humidity peak shifted about four decades higher in frequency.

We have published elsewhere (21) an interpretation of these results using the framework of interfacial polarization, the Maxwell-Wagner-Sillars (MWS) theory for conducting inclu- sions embedded in a dielectric matrix (10, 23, 24), considering that the columnar material (Fig. 2) is a semiconductor while the interjoining material is a dielectric (21, 25). Using the MWS model, we infer that the presence of water molecules in the sample causes an increase of two to four decades in the conductivity of the insulating medium. These observations

strengthen the interpretation given above for the electrochem- ical and photovoltaic effects. However, we cannot determine whether this increase in conductivity must be attributed to an increase in the concentration of ionic species in this medium, or to an increase in their mobility, or to both simultaneously.

Temperature also affects V , ind J,,, but we have not con- ducted a systematic study of this parameter; Leclerc and Yelon (26) have presented a successful semi-empirical model for the water-vapor pressure and temperature dependence of V , and J,,. Some of their conclusions apply equally well to the present samples, namely:

(i) The present a-Si:H exhibits a dominant ionic conductiv- ity, since an electronic conductor cannot exhibit an open-circuit ~oltage.

(ii) At least one electrode is the site of an electrochemical

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PAQUIN ET AL. 1401

TABLE I. Open-circuit voltages V,, and short-circuit current densities J,, as a function of the substrate temperature Ts. All measurements were made in the dark

on Au - a-Si:H - Al cells

Sample

TABLE 2. Open-circuit voltages Voc and short- circuit current densities J,,, for different pairs of electrodes. The substrate temperature was Ts = 150°C throughout. Allmeasurements were

made in the dark -

v, Jsc Sample M I M2 (mV) ( n A ~ m - ~ )

In- a-SI t i-AU

-0.2 I I 1

TIME ( h / d i v

FIG 5. Schematic representation of the open-circuit voltage (V,) behavior of our samples as a function of time under various con- trolled atmospheres, in the dark and at room temperature; ( a ) ambi- ent atmosphere; (b) 100% relative humidity; ( c ) under high vacuum ( a 2 x Pa); (d) He, Ar, or O2 (M 120 Wa); ( e ) Hz (M 120 kPa) and, ( f ) returning to ambient atmosphere from an Hz atmosphere.

Au- 0-5: H- Al

10-2 - -TCFT-4 +------ ~ o l o r t r o n +

I O - ~ I

1 4 ~ lo0 lo2 lo4

Frequency ( H z )

FIG. 6. Example of the variation of tan 6 induced by water vapor in a-Si:H, as measured by transient current Fourier transform (TCFT) and with a solarton frequency response analyser; (1) under high vac- uum and (2) under 100% relative humidity.

TABLE 3. Open-circuit voltages Voc and short-circuit current densities J,, in the dark and under illumination (AM-2 (35 mE cm2)). The

substrate temperature was T, = 150°C throughout

voc (mV) Jsc (nA cm-')

Sample M I M2 Dark Light Dark Light

process in which water is involved, or water acts as a disso- ciator to generate one or more of the ionic species that are involved in an electrochemical process.

4. Discussion and conclusions The present M I - a-Si:H - M2 samples have displayed a

hitherto unreported behavior of a-Si:H, that is, the presence of spontaneous and stable open-circuit voltage and short-circuit current in the dark. We have shown this behavior to be very similar to that observed in MI-polymer-Mz samples, and as in that case we attribute it to an electrochemical phenomenon. One must keep in mind that such an open-circuit voltage rules out the possibility of dominant electronic conduction, since in such a case the electrolyte would act as an internal short- circuit, placing both electrodes at the same potential. Con- versely, in the present samples the electronic conductivity of our material is negligible with respect to ionic conductivity, at least in the dark and at room temperature.

Electron microscopy clearly shows the columnar structure of our a-Si:H. While the dielectric, electrochemical, and pho- tovoltaic results lead us to believe that the columnar material is a semiconducting electronic conductor, the intercolumnar material is an insulating ionic conductor (Pemn (22) has al- ready shown that these two materials are quite different). The intercolumnar material may completely or partially encapsu- late the electronic conductor insulating it from at least one electrode. In both cases the ionic conduction would appear dominant, allowing the existence of a V,. The interfacial po-

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1402 CAN. J . PHYS. VOL. 68, 1990

larization observed at low frequencies would then take place at the interface between the electronic and ionic conductor. For an electrical current to flow through such an interface requires an electrochemical charge-transfer reaction from an ion to an electron (or vise-versa).

It may also well be that there is an electrochemical equi- librium or process between the two phases when no external voltage is applied. This could contribute to an open-circuit voltage if the material near the substrate is significantly differ- ent from the one near the upper electrode. It would however be insufficient to explain why the observed V,, is a function of the nature of the metallic electrode or why identical effects are observed in polymer-based cells. Clearly, at least one of the metal - a-Si:H interfaces must be the site of electrochemical equilibrium or processes, the chemical nature of which is still to be determined.

The transient behavior observed under H2 atmosphere (see Fig. 5) may well be caused by a hydrogen evolution reaction:

(not to be confused with the evolution of hydrogen observed in amorphous silicon (27), which is a nonelectrochemical exodiffusion process). For such a reaction, according to the Nernst law:

[H' 1' EH2 IX In -

pH2

where EH, is the hydrogen half-cell potential, and p ~ , the par- tial pressire of molecular hydrogen. If exposure to hydrogen temporarily changes the ratio [ H + ] ~ / ~ H , the measured voltage will accordingly undergo a temporary change. Figure 5 sug- gests that a hydrogen evolution reaction is taking place at one or at both electrodes.

While the electrochemical nature of the observed e hen om- ena thus seems well established, not much insight has been gained as to the nature of the reactions. Water was shown to affect both the current and voltage, raising the following possibilities:

(i) Water is involved in one or more electrode reactions. These reactions may be in equilibrium or not.

(ii) Water acts as a dissociator, raising the concentration of one or more ionic species, which species are involved both in charge transport mechanisms (thereby affecting J,,) and elec- trodic processes (affecting V,,).

(iii) The electrochemical reactions in the presence of water vapor and under high vacuum are completely different.

The work of Leclerc and Yelon (26, 28) has shown that for M I - polymer - M2 cells, hypotheses (i) and (ii) are most likely to be true, while (iii) seems unlikely. For M I - a-Si:H - M2 cells, our results seem to confirm their conclusions. As in polymers (29, 30) protons are most likely to be the dominant current-carrying species, because of the high concentration of hydrogen in our material and also because; due to their small size, they are expected to have a very high mobility.

The fact that V, and J,, are not reproducible for any given pair of metallic electrodes strongly suggests that t h e - ~ I s ~ : H films obtained by the plasma deposition technique used are neither homogeneous nor very reproducible. This is not sur-

~ -

prising in view of the sizeable number of fabrication variables involved, and of the immense subtlety of this material that is evident from the huge body of world literature on the subject. For exam~le . we have shown that the columnar structure of the . ,

particular material studied here is a sensitive function of T,,

the substrate temperature during deposition. Films deposited at elevated T, become much denser, due to enhanced surface mobility of the precursor species during deposition; indeed, the best semiconducting (device grade) a-Si:H is smooth and struc- turally homogeneous, and in some cases without any discern- able columnar structure. The absence of V,, when T, > 250°C, and the accompanying vast improvement of electronic conduc- tion processes signal the transition to domination of electronic conduction processes.

Acknowledgements The authors wish to thank S. Poulin-Dandurand for ERD

measurements, D. Masson for FTIR spectroscopy, and J . Ma- sounave for AES and STEM. We are also indebted to Profes- sor A. Yelon for helpful discussions. This work was supported by the Natural Sciences and Engineering Research Council of Canada and by the Fonds FCAR (Formation de chercheurs et aide i la recherche) of Quebec.

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