NON-AQUEOUS ELECTROLYTES IN THE ELECTROREFLECTANCE STUDIES

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NON-AQUEOUS ELECTROLYTES IN THE

ELECTROREFLECTANCE STUDIES

F. Luke, J. Hele Ic

To cite this version:

F. Luke, J. Hele Ic. NON-AQUEOUS ELECTROLYTES IN THE

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JOURNAL DE PHYSIQUE Colloque C5, supplkment au no 11, Tome 38, Novembre 1977, page C5-201

NON-AQUEOUS ELECTROLYTES

IN

THE

ELECTROREFLECTANCE STUDIES

F.

LUKES

and

J. HELESIC

Dept. of Solid State Physics, Faculty of Science,

J. E. Purkyng University, Brno, Czechoslovakia

RBsumk.

-

On a etudie plusieurs 6lectrolytes non aqueux pour des etudes d'6lectror6flectance. Nous avons prouve expbrimentalement qu'ils donnent de bons spectres dYClectror6flectance. On compare des rksultats obtenus sur des monocristaux de germanium & la fois dans des Blectrolytes aqueux et non aqueux. Les 6lectrolytes dkcrits peuvent 8tre utilises jusqu'h des longueurs d'onde d'au moins 6 Dm. On montre aussi la possibilit6 d'utilisation de ces electrolytes non aqueux pour des klectror6flectances du germanium & basse temperature 210 K

.

Abstract.

-

Several non-aqueous electrolytes have been tested in the electroreflectance studies. We have proved experimentally that they give us good electroreflectance spectra compared with spectra of germanium single-crystals obtained both in aqueous and non-aqueous electrolytes. The described electrolytes may be used in the wavelength range of at least up to 6 wm. There has also been demonstrated the possibility of using non-aqueous electrolytes at low temperatures by studying the electroreflectance spectra of germanium at 210 K.

1 . Introduction.

-

Though the non-aqueous electrolytes were suggested a long time ago by Cardona and his coworkers [I, 21 as a possible tool to obtain the electroreflectance (ER) spectra of semiconductors, they have not been often used in practice.

The most important advantages of non-aqueous electrolytes when compared with aqueous solutions of suitable salts are as follows :

layer between the sample and the window. We applied the square-wave ac voltage ; its frequency was 30 Hz. We used the following electrolytes : 0.1 N solution of K,SO, in water, 0.2 N solution of KNO, in dimethylsulphoxide (DMSO), 0.2 N solution of KNO, in dimethylformamide (DMFA), 0.2 N solution of KNO, in ethylalcohol, saturated solution of KNO, in acetone. The electrolytes were mostly gettered with 1) they enable us to extend the ER measurements crushed Ge.

into the farther infrared region ; We measured the reflectance R of the samples and its change AR produced by applied ac voltage separa- 2, make the ER measurements at lower tempe- _{tely. The curve}_AJZIR_{was then calculate&}

ratures generally possible ; _{In the experiments when the modulating ac voltage} 3) they enable us to study the ER spectra of mate- _{was added to a saw-like slowlv variable dc bias we} rials reacting with water.

We have tested several non-aqueous electrolytes which could serve suitably in the ER studies concerning the above three aspects. Some results of our studies are presented.

2. Experimental.

-

Our experimental arrangement used in the studies of the ER spectra by the electrolyte method was of the standard type. We used a tungsten lamp and the Nernst filament as light sources and a cooled InSb detector (Mullard, ORP 13) in the energy range of 0.25-1 eV and a PbS detector (Phillips, 61 SV) in the energy range of 0.5-1.5 eV. Our Zeiss mono- chromator SPM 1 was equipped with an LiF prism. The platinum electrode and the sample were situated in a plastic vessel with an LiF entrance window. The sample was pressed by the help of a plastic screw against the LiF window to realize sufficiently thin liquid

coupled the Datapulse 410 sweep-function generator with a sine-wave precision generator TESLA BM-269. The measurement at low temperature carried out with the help of solid carbon dioxide was performed in a special cryostat designed by Schmidt and BoE6nek [3]. In the upper part of a copper rod, which had been cooled, a space was drilled where the electrolyte was filled. A part of the copper wall was cut and a silica plate was glued to the copper. The silica plate served as an entrance window for the radiation incident on the sample in the electrolyte. This system can be generally used even at lower temperatures if suitable solvents could be found.

3. Results and discussion. - There are problems with the interpretation of the electrolyte-type ER spectra generally. Earlier experimental studies performed by Gobrecht et al. [4] proved that there appear

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many difficulties even with the interpretation of the ER spectra of Ge obtained in aqueous electrolytes in spite of the fact that the electrochemistry of the processes at the Ge-aqueous electrolyte interface is sufficiently well known, which cannot be said about any other semiconductor-electrolyte system. Nevertheless, the detailed analysis of the ER spectra of Ge in an aqueous electrolyte performed by Aspnes and Frova [5] leads to the conclusion that at least the spectra of Ge obtained in aqueous solutions of suitable salts are good ER spectra which can be interpreted quantitatively. On the other hand, the electroabsorption (EA) spectra of PbS near

E, transition obtained also by the electrolyte method

but with the help of the non-aqueous electrolyte (propylene carbonate with tetrabutyl ammonium per- chlorate as a solute) studied .by Aspnes and Car- dona [6] prove that there appear a broad curve (background) which may be reasonably explained only as an

electro-optic effect due to the modulation of the optical properties of the electrolyte. This has been attributed especially to the properties of the Gouy layer adjacent to the studied PbS film. While the potential drop across the Gouy layer can be disregarded in strong electrolytes, i. e. mostly in aqueous solutions of suitable salts, it may be quite considerable in weak electrolytes to which mostly the non-aqueous electrolytes based on organic solvents belong. Moreover, slow adjustment processes in the Gouy layer can lead to a drift of the point of operation [6]. This means that there appears a phase shift of the ER signal with respect to the modulation voltage. This phase shift may depend on the instantaneous value of the applied ac

+

dc

potentials.

It is evident that the detailed study concerning the use of non-aqueous electrolytes in ER measurements should include the electrochemistry of the semiconductor-electrolyte interface. As a matter of fact, we did not follow this procedure in our work since it is the problem per se. We simply used the fact that the ER spectra of Ge obtained with the help of aqueous electrolytes are good ER spectra. Then we compared these spectra with those obtained with the help of some non- aqueous electrolytes. We also performed some other comparative studies to find whether there was an overall similarity between the properties of Ge- aqueous electrolyte interface and of Ge-non-aqueous electrolyte interface.

In our experiments we tried to fulfil some reasonable requirements concerning the properties of electrolytes, better to say, of solvents, namely :

1) they should have the possibly lowest absorption in infrared ;

2) they must not be poisonous ; 3) they shoud not evaporate rapidly.

Some of the solvents that we tested and which seem to be suitable for the use in the infrared region are reviewed in table I where there are also given several of their physicochemical parameters 171. The useful limit of transmission of light depends substantially on the thickness of the liquid layer between the entrance window and the sample. Almost in all cases it is neces- sary to use fairly thin liquid layers to obtain a reasonable ER signal within the broad wavelength range in infrared. But the same has to be done even with aqueous electrolytes in near infrared [8, 91 and in spite of this fact we cannot use aqueous electrolytes above about 2 ym. Since we used a cooled InSb detector we could measure only up to about 5.5 ym. Most of the solvents given in table I can be used at least up to this limit.

We found that the lineshapes of ER spectra of Ge were essentially the same in all non-aqueous electrolytes we used and that they were identical with those obtained in aqueous electrolytes. We studied the dependence of three main peaks o the ER curves of Ge near E, transition for three Ge samples (n-Ge 1 with

p = 8.6 Qcm, n-Ge 2 with p = 0.63 Qcm and p-Ge 3 with p = 22.4 Qcm). Figure 1 shows the dependence of the magnitude of the AR signal on the dc bias for constant amplitude of ac modulation voltage (V,, = 0.2 V) for the sample p-Ge 3. It is evident that all the curves obtained in aqueous and in non-aqueous electrolytes as well are qualitatively the same. The signals in non-aqueous electrolytes are comparable with those in aqueous ones in magnitude for the same

dc and ac voltages. Also the transition from depletion to the accumulation layer at the semiconductor surface may be realized in non-aqueous electrolytes similarly as in aqueous ones as proves the change of the sign of the AR signal which occurs at the crossing of the flat- band position. The hysteresis curves are also similar in both types of electrolytes. dc bias needed to reach the flat-band position is different in different electro-

Physical parameters of organic solvents

Solvent - Acetone Dimethylformamide Dimethylsulphoxide Ethylalcohol Specific

Poiling Melting conductivity Relative dielectric point O C point Q - l cm-l x lo8 constant

-

56.2

-

95.4 0.05-0.13 20.7

158

-

61 3-975 36

189 18.6 2-3 46.6

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NON-AQUEOUS ELECTROLYTES IN THE ELECT'ROREFLECTANCE STUDIES C5-203

lytes which is connected with different values of the surface potential of Ge in these electrolytes. This is not surprising. There is no sign of any background on our

ARIA = f ( E ) curves which could be connected with the change of the optical properties of the electrolytes, especially inside the Gouy layer.

The absorption is still rather high, at least in certain regions in infrared for most non-aqueous electrolytes given here (there exists a strong absorption band in the region of 0.40-0.44 eV for DMSO, etc.). Thercfore we tried to find still better solvents. Thus, the transmission of CCl, is very high within the range of 1-6.2 pm with the exception of a weak absorption band near 4.4 pm. Also C,Cl, is characterized by high transmission within the region of 1-8.5 pm.

We did not find any ER signal for n-Ge 2 in CCI, saturated with either anhydrous gaseous HCl or with 1 N solution of HCIO,. We tried to use CCl, mixed with another non-aqueous solvent to increase the transmission of the electrolyte. The result for a mixture of CCI, with 0.2 N solution of KNO, in DMSO is given in figure 2. It is evident that the AR signal decreases

0

nonlinearly with decreasing DMSO component. The

FIG. 3. - AR = f ( 1 ) for n-Ge2 in c2C1.1 saturated with gaseous HCI ; the sample was kept 72 hours in the electrolyte before the measurement started (full line) : a) dc

-

- 5 V ,

ac = 45 V ; dotted line - after another 6 hours of applied dc

+

ac voltages ; b) dc =

-

25 V, ac = 25 V .

full line. After another 6 hours of both dc and a c

J

-

{ l o 4 Q 5

-

A / ~ ~ _{voltages applied to the sample the result was such as}

I I - ---

--

I---

.

.,

,;s'

]'*<\

-- - ,

.

* \

change of the lineshape of AR = f ( A ) curve corres-

F ~ G . 2.

-

AR = f ( l ) for n-Ge 2 in a mixture o f 0 . 2 N solution given by the dotted line (Fig. 3a). For 1 N solution of of KNO3 in DMSO : CC1,. HClO, in C,CI, we obtained the ER spectra imme-

J

-

- g -

₄ Q lo 5 \ \ ,

-

\ I 0 -5 -10 - 10 I

- 0 8 _O _{d c / V} -08 '

b

,Jv . o e properties of the electrolytes.

We also tested C2C14 saturated with gaseous HCl.

FIG. 1.

-

AR

= f

(dc) for P-Ge 3 for three peaks on AR=f ( E ) _The _{ER spectrum of the sample n-Ge 2 kept for} curve near Eo transition : - low energy peak, - - - main

peak,

-.--

high energy peak : a ) aqueous electrolyte ; 72 hours in the electrolyte is shown in figures 3a, b,

6) 1 N solution of KNO3 in DMSO.

', -

I

ts, \ - ---+I--, -- -

---

, . .

i

- - , I

ponds to decreasing intensity of the electric field E inside the semiconductor in perfect agreement with the physically similar dependence demonstrated by Aspnes and Frova [5] for ER spectra of Ge in aqueous electro- lyte also near E, transition. The ER spectra of Ge in the mixturc of CCI, with acetone saturated with KNO, show considerably greater decrease than should cor- respond to the ratio of both components though the lineshape of the spectra is similar as well as in previous case. It is evident that the mechanism of the interaction between different solvents is quite complicated and there seem to exist good possibilities to influence the

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(25-204 F. LUKES AND J .

HELESIC

FIG. 4.

-

AR = j (A) for n-Ge 2 in 1 N solution of HC104 in

C2C14:a) d c = - 2 5 V , a c = 2 5 V ; b ) d c = O , a c = 5 0 V .

diately after immersing the sample into the electrolyte. The results are shown in figure 4.

The common feature of the ER spectra of n-Ge 2 in Cl,Cl, saturated either with gaseous HCI or with HCIO, is relatively strong, almost constant background on the AR = f

(A)

curve both below and above the structure characterizing the E, transition. This background is connected probably with the modulation of the optical properties of the Gouy layer of the electrolyte adjacent to Ge sample.

After performing the measurements with Ge we studied ER spectra of n-type InAs in the energy range of 0.3-1 5 eV at room temperature, that means in the range of E, and E,

+ A ,

transitions. These measurements and their physical interpretation will be described in detail in another paper. Here we only intend to present some specific results connected with the use of non-aqueous electrolytes.

We used saturated KNO, solution in DMSO. InAs samples cut from single-crystals were (111) oriented. They were polished and then etched in the mixture H F : HNO, : CH,COOH : Br (15 : 75 : 15 : 0.06). We found that the ER signal obtained in the considered energy range (0.25-1.5 eV) depended on the history of the sample. This is demonstrated on figure 5 where ER signal of n-InAs 6 at E = 0.35 eV (local maximum on the ER curve corresponding according to our analysis

Fro. 5.

-

AR = f(V) for n-InAs 6 : V+max = 2.7 V ,

V-m, = - 2.7 V on sample vs. Pt electrode ; the curves 4 and 5 are given in a half scale.

to the maximum of the Franz-Keldysh type ER effect near the energy E,) is given. The period of slow saw- like wave substituting the dc bias was 200 s

(f = 5 x

lo-,

Hz) and the frequency of the sine-

wave ac signal was 30 Hz. The first three cycles (1-3) were taken continuously and the measurements started

FIG. 6. - AR/R = f (E) for p-Ge in saturated solution of

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NON-AQUEOUS ELECTROLYTES I N THE ELECTROREFLECTANCE STUDIES C5-205

just at point A, at the same moment when both dc and ac fields were switched on. Also curves 4 and 5 were taken continuously but with a 30 minute delay. In the meantime similar curves were taken at different energies continuously. This experiment shows that only after considerably long time we get the curves which are more or less reproducible as is proved by curves 4 and 5 which are identical within a reasonable experimental accuracy. This effect is probably connected with the occupation of slow surface states. We have not yet made more detailed experiments in this respect. Figure 6 shows the ER spectra of p-Ge ( p =2.6 Rcm) taken at T = 292 K and at T = 210 K. Ethylalcohol with saturated solution of KNO, served as electrolyte. It is evident that both spectra characterizing the E, transition are qualitatively the same and are typical of p-types samples with medium impurity concentration. The increase of the specific resistance of the electrolyte by more than one order at T = 210 K compared with the value a t T = 292 K may be paralysed by an increase of the applied dc and ac voltages. The temperature coefficient of Eo, namely

agrees perfectly with the value

dEo/dT = 3.72 x lo-, eV/K [lo].

4. Conclusion. -- Though we cannot yet conclude unambiguously that non-aqueous electrolytes are essentially equivalent to aqueous ones or to an MOS system in the ER studies we believe that we have justified the use of at least several non-aqueous electrolytes presented here to obtain good ER spectra in the infrared region. There is still a possibility that a background appears superimposed on true ER spectrum in some cases but in our opinion we may avoid the complications in the interpretation of experimentally found ER curves especially when we succeed in crossing the flat-band position. That is, of course, possible only for the Franz-Keldysh type ER spectra, not for the band-population effect [ll]. Our results

prove that the electrolytes discussed here give, at least for Ge and InAs, good results. It is remarkable that the Franz-Keldysh type ER spectrum of the sample n-Ge 2 in 1 N solution of HCIO, in C,CI, could be changed by applying suitable dc voltage (see Fig. 4) from the accu- mulation to the depletion-type curve while the sign and the amplitude of the background remained the same. This differs from the curves obtained by Aspnes and Cardona [6] representing the EA curves of PbS films near Eo fundamental gap where both the sign of the EA spectrum and that of the background attributed to the electro-optic effect in the electrolyte, especially in the Gouy layer, change with changing the dc bias. Our results may be explained as a modulation of the optical properties of the electrolyte adjacent to the sample, especially of its refractive index n,. The assumed change An,, which probably does not change with only the relative change of the applied ac voltage by shifting the dc bias (while the magnitude of the ac voltage is constant), may lead to constant change AR,

beyond the region of the Franz- Keldyshtype ER effect. Beyond this region the contribution of the ER effect of Ge is equal to zero. The applied ac and dc voltages as well werevery high, about two orders higher than for aqueous electrolytes, which means that the potential drop must be quite high within the Gouy layer which may conclude in the observed change of reflectance.

We have also proved that non-aqueous electrolytes may be used for study the ER spectra at low temperatures. This is advantageous especially for heavily doped materials where we cannot use the Schottky barrier and the application of the MOS structure does not lead to good results according to our experiences since it is very difficult to influence the surface barrier by changing the

dc bias-mostly the breakdown occurs.

5. Acknowledgments.

-

We would like to acknow- ledge fruitful discussions with Dr. M. Kratochvil and Dr. L. Kigovii as well as their help in the choice of organic solvents.

References [I] CARDONA, M., SHAKLEE, K. L. and POLLACK, F. H., Phys.

Rev. 154 (1967) 696.

[2] CARDONA, M., Modulation Spectroscopy, in Solid State Phys., edited by F . Scitz, D. Turnbull, and H. Ehren- reich (Academic Press, New York) 1969, Suppl. 11. [3] SCHMIDT, E. and B ~ ~ ~ I N E K , L., J. Phys. E 4 (1971) 250. [4] GOBRECHT, H., THULL, R., HEIN, F. and SCHALDACH, M.,

Ber. Bunsenges. phys. Chem. 73 (1969) 68.

[5] ASPNES, D. E. and FROVA, A., Phys. Rev. B 2 (1970) 1037.

[6] ASPNES, D. E. and CAKDONA, M., Phys. Rev. 173 (1968) 714. [7] JANZ, G. J. and TOMKINS, R. P. T., NOII-(IQII~OUS Electro1.vtes

Handbook, vol. 1 (Academic Press, New York) 1972.

[8] CARDONA, M., SHAKLEE, K. L. and POLLACK, F. H., Phys. Lett. 23 (1966) 37.

[9] LUKES, F. and SCHMIDT, E., Phys. Lett. 23 (1966) 413. [lo] SERAPHIN, B. O., in Proc. of 7th Int. Conf. on Physics of,

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C5-206 F. LUKE$ AND J. HELESIC

DISCUSSION

D. LYNCH (a comment.

-

Many of the advantages A. ROUSSEAU.

-

Avez-vous observe l'influence de of the use of non-aqueous electrolytes may by obtained traces d'oxygkne ou d'eau dans les milieux non aqueux with the metal-film Schottky barrier configuration. sur les spectres d'ClectrorCflexion.