Elaboration, characterization and ionic conductivity of mixed alkali-germanium oxide thin films

Materials

Chemistry

and Physics,

36 (1993)

183-186

183

MATERIALS CHEMISTRYAND PHYSICS

Elaboration, characterization and ionic conductivity of mixed alkali-germanium oxide thin films K. Awitor, Laboratoire (France)

(Received

G. Baud,

de Physico-Chimie

August

J.P. Besse des Mat&iaq

19, 1992; accepted

and M. Jacquet URA

CNRS

444,

Universite’ Blaise

Pascal-Clermont-Femand,

63177 Aubitre

Cider

April 2, 1993)

Abstract Thin films of mixed xM,O-(1 -x)GeO* oxides (0.125 ~~~0.273 for M = Li; 0.125
Introduction

The purpose of this work was to obtain thin layer deposits of an amorphous solid electrolyte that could be used in microionic devices. Mixed xM,O-(1 -x)GeO, (M = Li, Na) oxides were selected mainly for two reasons: (1) The structure and properties of these phases in the glassy state are well studied [l-9] and their conductivity is known to be high [3, 9-141; therefore, it appeared quite interesting to compare an amorphous thin layer with a bulk glassy material. (2) In addition, they are particularly well adapted to fundamental research; indeed, the analysis of the EXAFS spectra at the germanium K-edge provides much information and enables one to obtain a fine determination of the local order [5, 6, 15-171. Some results for mixed lithium-germanium oxides have been presented in recent work [18, 191.

Experimental

Films about 1 pm thick were deposited using a radiofrequency (r.f.) sputtering apparatus (Alcatel model Dion 300). The targets (50 mm in diameter)

02%0584/93/$6.00

or with xLi,O-( 1 -x)GeO, were made xNa,O-(1-x)GeO, mixed oxides sintered at 850 “C. The plasma was produced by a 2% oxygen-argon mixture at 1 Pa pressure. Generally, the target substrate distance was 25 mm, the r.f. power density was 1.8 W cm-’ and the substrate reached a temperature of 70 “C. The composition of the resulting deposits was determined by X-ray fluorescence analysis (for germanium), flame spectrometry (for lithium and sodium) and Auger spectrometry (for oxygen). The X-ray diagrams were recorded on a D 500 Siemens diffractometer. The EXAFS spectra were obtained at the LURE laboratory in Orsay (France). The IR absorption spectra of the films deposited on windows of germanium were recorded with a Fourier transform spectrometer (Nicolet model 20 SX). The electrical measurements were carried out in a perpendicular (S-type sample) and a parallel (P-type sample) direction to the layer on two different samples obtained by the same sputtering on the same polished alumina substrate. These samples, with deposited platinum electrodes, were analyzed simultaneously in a cell under controlled atmosphere. The d.c. conductivity was measured with a Keithley type 616 ammeter. The a.c. measurements were made by means of the complex

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184

impedance method with a transfer function analyzer (Solartron model FRA 1174) connected to the measurement cell by an impedance adapter.

Results and discussion Chemical composition and crystallization

The chemical analysis of the deposits made from xM,O-(1 -x)GeO, (x = 0.125, 0.182, 0.200, 0.273 for M=Li and x=0.125, 0.182, 0.200 for M=Na) target materials shown that the composition differs only slightly from that of the target. A loss of about 2% alkali metal is observed. This should be attributed to the small amount (about 5 mg) of material used for the analysis. The results relating to lithium have been given previously [18]. Deposits obtained from targets with an alkalimetal concentration higher than those mentioned above could not be investigated, because they are quickly hydrolyzed in the surrounding atmosphere. The mass density of the deposits is very much like that ofglasses of the same composition. It passes through a maximum atx- 0.200 in the case ofxLi,O-(1 -x)GeO, oxides. Whatever the substrate used (crystallized or glassy), the X-ray diffractograms for the different deposits are all characteristic of amorphous materials. Through an investigation of the thermal stability range of the amorphous phases, it was shown that the deposits start to crystallize at about 500 “C on substrates of alumina, while the process takes place at 300 “C on platinum. As a result, conductivity measurements had to be carried out at a temperature lower than 300 “C. Local structure of the deposit EX4FS analysis at the germanium K-edge

We compared the spectra obtained for the deposits with those obtained from the glassy and crystallized phases of the same composition. The structure of the crystallized compounds Li,Ge,O,,, Li,Ge,O,, Li,Ge,O, [20-221 and Na,Ge,O,, [23] is known. In all these phases, germanium is present in two types of environment: - GeO, tetrahedra, with a Ge-0 distance of 1.735 A; - GeO, octahedra, with a Ge-0 distance of 1.893 A. The radial distribution curves generated by the Fourier transform of function k3x(k) are all very much alike. observed for the spectra Figure 1 shows (x = 0.200). xLi,O-( 1 -x)GeO, In all three cases (deposit, glass and crystal) the characteristic first-nei hbour peak corresponds to a Ge-0 distance (1.74 ! ) that is similar for the three types of samples.

r(A)

Fig. 1. Radial distribution curves forxLi+(l (-) deposit; (- --) glass; (-. -) crystal.

-x)GeOz

(x= 0.200):

By comparing the amplitude of the peak with those of the peaks observed for hexagonal crystallized GeO,, where the germanium occupies only tetrahedral sites, and rutile crystallized GeO,, where the germanium occupies only octahedral sites, it is seen that the ratio of the number of GeO, octahedra to the number of GeO, tetrahedra has a lower value for the thin film than is observed for the glassy and crystallized phases of similar composition. In addition, as far as the deposits are concerned, a gradual increase in this ratio is to be noted as the alkali-metal content increases. Cox and McMillan [5] noticed a similar variation in an EXAFS analysis of xLi,O-(1 -x)GeO, glasses. The second-neighbour (Ge-Ge) peak is very much weakened for the amorphous and vitreous oxides, which can be explained by a greater dispersion of the Ge-0-Ge angles for these phases as compared with the crystallized phase. Infrared absorption spectroscopic analysis

It is shown through a comparative study that, for a given composition, the Ge-0 stretching band appears at a higher frequency for deposits than it does for glasses. The spectra obtained for thexLi,O-(1 -x)GeO, phases with x= 0.200 are provided as an example (Fig. 2). It therefore seems that the films obtained by sputtering contain a lower percentage of germanium in the form of octahedra than the corresponding glassy phases. As far as the deposits are concerned (Fig. 3) it is also to be observed that the Ge-0 stretching band shifts toward lower frequencies as the lithium content increases; it goes from 870 cm-’ for x=0 (amorphous GeO,) to 834 cm- ’ for x= 0.273 in the case of xLi,O-(1-x)GeO, phases. This shift shows that the percentage of germanium in octahedral sites increases with increasing alkali-metal content. An analogous phenomenon has been observed for glassy phases of the same stoichiometry [2, 31.

185

this discrepancy being presumably due to the measuring technique.

mw

Iooo

dwave

800

600

100

number ccm-‘)

lZO0

-wave

Fig. 2. IR absorption spectra for thexLi,O-(1 phase: (-) deposit; (---) glass.

1000

800

800

number

-x)GeO*

Fig. 3. IR absorption spectra for the xLi,O-(1 -x)GeO, (a) x=0; (b) x=0.125; (c) x=0.200; (d) x=0.273.

100

ccm-1,

(x = 0.200) deposits:

Therefore, these results lead to the same conclusions as the EXAFS spectra analysis. Ionic conductivity Methods of investigation The electronic conductivity a,, which was measured from the d.c. current-voltage plot (Hebb-Wagner method), is very low compared with the total conductivity 0: The ratio u,/o is less than 2X 10e3 and 5 X 10-3, xLi,O-( 1 -x)GeO, and the respectively, for xNa,O-(1 -x)GeO, deposits for the temperature and concentration range under investigation. Thus the measured a.c. electrical conductivity properties will be attributed hereafter to diffusion of the alkali metals. The complex impedance (Z=Z’ +jZ”) displayed at different frequencies f in diagrams reveals a circular arc, characteristic of the charge volume relaxation process. The arc can be represented by the equation Z=Rl[l+

Effect of the composition Figure 4 displays the conductivity of thin films of mixed oxides for both sample types in the temperature range available for study. The conductivity of mixed xLi,O-(1 -x)GeO, and xNa,O-(1 -x)GeO, oxides increases as the alkali oxide content increases. Similar behaviour was observed with thin films of the B,O,-L&O system [24, 251 as well as M,O-GeO, (M=Li, Na) glasses [3, lo]. For a given alkali-metal composition, in the temperature range investigated, the conductivity is observed to be about 10 times higher when sodium (ionic radius 0.93 A) is replaced by lithium (ionic radius 0.68 A). In the present study, therefore, the best solid electrolyte is xLi,O-(1 -x)GeO, for which x = 0.273. The nearly linearvariation of log UTversus l/Tover the temperature range enables the activation energy to be calculated from the slopes of the lines. For each alkali metal, the activation energy will steadily decrease when the molar fraction increases, as long as ~~0.200 (Fig. 5). The composition with x = 0.200 corresponds to a minimum of the activation energy for xLi,O-(1 -x)GeO, phases. It should be taken into account that this composition also corresponds to a maximum in the mass density and refractive index [19]. Effect of the structure Table 1 compares the electrical conductivity properties of two thin films samples and those obtained

0’2+))‘-“]

in which R stands for the resistance of the sample and r is the relaxation time. The displacement of the center of the circular arc with respect to the real axis can be generally related to a statistical distribution of relaxation times characterized by the parameter (Y.In this study, the values of (Yvary from 0.10 to 0.15. Owing to the different geometrical characteristics of the two samples, the resistance of a P-type sample is about lo5 times that of the corresponding S-type sample; therefore the temperature range available for investigation will be very much restricted. Moreover, only the impedance diagram of the S-type samples is affected by material-electrode interface properties. To obtain consistent results it is necessary to perform a preliminary reheating of the samples at the maximum investigation temperature. The conductivity calculated for an S-type sample was generally observed to be 4-10% lower than that obtained with a P-type sample,

moo/T!K-‘)

*

Fig. 4. Arrhenius diagram of the conductivity of deposits XL&O-(1 -x)GeO, [(a) x=0.125; (b) x=0.182; (c) x=0.200; (d) x=0.273] and deposits xNa,O-(1-x)GeO, [(e) x=0.125; (f) x=0.182; (g) x=0.200].

X

energy vs. x: (a) XL&O-(1 -x)GeO, deposits.

Fig. 5. Activation (b) xNa,O-(1

-x)Ge02

deposits;

186 TABLE 1. Conductivity at 555 K and activation amorphous, glassy and polycrystalline phases Sample

Deposit Glass Polycrystal

energy

xLi,O-( 1 -x)GeO* x=0.125

xNa*O-( 1 -x)GeO* x=0.182

(T (a-’

E, (eV)

(T (a-’

0.98 0.93 1.18

6.0 x 1O-6 1.3 x 10-4 2.3x lo-*

m-‘)

4.3 x1o-s 1.6x1O-4 1.8 X lo-’

m-l)

for

E, (eV) 0.80 0.89 1.30

with glassy and polycrystalline samples of the same composition. As far as xLi,O-( 1 -x)GeO, (x = 0.125) is concerned, the conductivity of the deposit at 555 K is slightly lower than that of the glass [3] but is more than 1000 times that of the polycrystal (82% of theoretical density). The extremely low conductivity of the polycrystal was also observed by Liebert and Huggins [26]. Likewise, the activation energy of the deposit is found to be close to that of the glass, whereas it is very far from that of the polycrystal. samples of various A comparison of xNa,O-( 1 -x)GeO, (x = 0.182) material allows us to reach similar conclusions for the mixed lithium oxides. Most of the time, a comparative study of the xM,O-(1 -x)GeO, phases (M = Li, Na) will show that the electrical properties of the deposit are close to those of the glasses of similar composition, thus allowing us to assume that the conduction phenomena are similar.

The mixed xM,O-(1 -x)GeO, (M = Li, Na) oxides obtained by cathodic r.f. sputtering are all amorphous materials. Structural analysis by EXAFS and IR spectroscopy shows that all the compositions studied contain a mixture of GeO, tetrahedra and GeO, octahedra. As for the glasses of the same stoichiometry, the percentage of GeO, octahedra has been shown to increase with increasing alkali-metal content. In the form of thin films, these mixed oxides are good solid electrolytes. In the temperature range investigated, their conductivity, which is close to that of glassy phases of the same composition, is about 1000 times higher than that of crystallized phases. Activation energies very similar to those of the glasses enable us to assume that the conduction phenomena are also similar.

Acknowledgements The authors would like to thank the LURE laboratory in Orsay, where the EXAFS experiments were carried out, and Professor P. Bondot for his help in interpreting the EXAFS spectra.

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