Mat. Res. Bull . , Vol. 24, pp. 1361-1367, 1989. Printed in the USA. 0025-5498/89 $3.00 + .0O Copyright (e) 1989 Pergamon Press plc.

PREPARATION AND CHARACTERIZATION OF GERMANIUM OXIDE THIN FILMS

C. Caperaa a, G. Baud a, J.P. Besse a, P. Bondot b, P. Fessler b and M. Jacquet a aLaboratoire de Physico-Chimie des Mat6riaux, URA CNRS 444,

bLaboratoire de Physique des Milieux Condens6s, URA CNRS 796, Universit6 Blaise Pascal, 63177 Aubi~re Cedex - France

(Received June 29, 1989; Communicated by J .B . Goodenough)

ABSTRACT Thin films of amorphous GeOx (1.4 < x < 2) were obtained by RF cathodic sputtering. The study of local order by EXAFS and IR spectroscopy showed that germanium atoms and tetrahedral units GeO 4 and Ge(O4_yGey) were present in these materials. The refractive index and the dielectric characteristics of these films are given for the phases which are high in oxygen.

MATERIALS INDEX : germanium, oxides.

INTRODUCTION

Numerous studies have been published concerning germanium oxide GeO 2 in its crystalline or in its vitreous form. The optical and electric properties of germanium oxide have been put to advantage in various fields. Since this material can be formed as thin films (1-4), it could find uses in miniaturized devices. Recent studies have shown that it can be used for the manufacture of optical wave guides (5) and of capacitors (6).

We are conducting a study on the preparation of thin films of GeOx by RF cathodic sputtering. In a recent communication (7), we gave our first results concerning the preparation of these oxides. Here, we focus more particularly on the characterization of the structure of these materials using EXAFS and IR spectroscopy. We also give results of refractive index and dielectric constant measurements performed on the thin films we obtained.

EXPERIMENTAL PROCEDURES

Thin films of GeO x having a thickness in the vicinity of 1 ~tm were obtained through RF cathodic sputtering, using an ALCATEL apparatus, model Dion 300, operating at 13.56 MHz. We proceeded by reactive sputtering in an argon-oxygen plasma, using a target of monocrystalline Ge (purity 99.99 %) with a 50 mm diameter. The partial pressure of oxygen in the gas mixture was adjusted with an accuracy in the order of 1 % by means of a DATAMETRICS-DRESSER flowmeter system. In general, the total pressure in the enclosure was 1 Pa, the target-substrate distance 25 mm and the RF power 35 to 50 W. Under these conditions, the rate of deposition is

1361

1362 C. CAPERAA, et al. Vol. 24, No. 11

comprised between 0.3 and 1.5/~.s -I. The thickness and the refractive index of the film were determined by interferometry. Germanium in the deposits was determined by X-ray fluorescence (XRF) using a EG&G-ORTEC spectrometer, model Tefa Monitor, and oxygen was determined by Auger electron spectrometry (AES). The IR absorption spectra of films deposited on a window of monocrystalline germanium were recorded in the range from 400 to 4000 cm -1 with a Fourier transform spectrometer NICOLET, model 20 SX. The EXAFS spectra were obtained at the LURE of Orsay (France) by means of radiation from the DCI storage ring. The measurements were made in the energy range from 11 to 12.1 keV (germanium K-edge : 11.104 keV) by 2.5 eV steps, using a silicon crystal monochromator (311). The dielectric measurements were carried out using a GENERAL RADIO type 1615 capacitance bridge ; the electric contacts were electrodes of platinum deposited on the faces of the samples.

RESULTS AND DISCUSSION

F i l m c o l n n o s i t i o n s Figure 1 illustrates the change in the composition of the GeO x deposits as a function of the

ratio y = PO2/(PAr + PO2) (Po2 and PAr being, respectively, the partial pressure of oxygen and of argon in the plasma).

Ge02

c 0 GeO~

GeO E 0

0 GeO~s i / /

Po./(PAr + Po# (v.)

FIG. I GeO x composition v s PO2/(PAr + PO2)

One can note : - when 0 < y < 1.5 %, the oxygen content increases very rapidly and the GeO 2 composition is reached at y = 1.5 % ; - when 1.5 < y < 5 %, the composition exhibits a plateau, which corresponds to the oxide GeO 2; - when y > 5 %, a slight excess of oxygen is found over the stoichiometric composition. It is believed, that this excess oxygen is incorporated into the film during the sputtering, either as molecular oxygen or in the form of hydroxyl groups.

In this work, the GeO2 films were all prepared using a gas mixture containing 2 % oxygen. Deposits of such a composition can also be achieved from a vitreous GeO 2 target in an argon plasma containing 1% oxygen.

Concerning the structure studies, we mainly focussed our attention on oxides having compositions ranging from GeOl.4 to GeO2, which are all amorphous materials. Table 1 gives the results of composition analysis performed on the films studied by IR and EXAFS.

Vol. 24, No. 11 GERMANIUM OXIDES 1363

Table 1

Determination Determination Sample Po2(%) of germanium of oxygen Composition

Ge % comp. O % comp.

1 0.5 76.0 GeO:.43 23.2 GeOl.37 GeOl.40 2 1 70.9 GeOI.s6 28.6 GeO:.82 GeOl.s4 3 2 70.0 GeO:.94 31.2 GeO2.06 GeO 2

Local structure of the films

EXAFS study at the germanium K-edge In the case of deposits of stoichiometric GeO 2 and of GeO 1.84, the Fourier transform of

the function kx(k) (Fig. 2) shows two peaks at the following distances : Ge - O = 1.74 ]k

and Ge - Ge = 3.13 A,, corresponding exactly to those found with crystalline hexagonal GeO 2, which has a structure characterized by a tridimensional succession of GeO 4 tetrahedrons.

The peak associated with the second neighbours (Ge - Ge) is considerably weakened : this arises from the greater dispersion of the Ge-O-Ge angles in amorphous oxides, as compared with crystalline oxides. On the other hand, in the case of the first neighbours (Ge-O peak), a coordination number higher than 4 is found for amorphous oxides, amounting to 4.4 and 4.2 for, repectively, GeO 2 and GEO1.84.

5

L.

LJ.

J 0 2,0

r

4,0 6,0 r(A) FIG.2

Fourier transforms of EXAFS spectra : crystalline hexagonal GeO 2 ( . . . . ) ; amorphous GeO 2 ( ..... ) ; amorphous GeOl.84 ( - - -)

1364 C. CAPERAA, et al. Vol. 24, No. II

The Fourier transform of the k3g(k) function does not bring any modification to this situation. This result could be due to the presence of some Ge(O4.yGey ) tetrahedrons (or of octahedral GeO 6 groups), in which case an increase of the Ge-O distance should occur in amorphous oxides.

In the case of GeO 1.4, three radial distribution peaks are obtained : - the two previous ones (Ge-O at 1.74 A and Ge-Ge at 3.13 A), both considerably weakened, and further - an additional peak at 2.45 A, which is typical of the Ge-Ge distance in metallic germanium.

The product is therefore a mixture of GeO 4 [or Ge(O4_yGey)] tetrahedral entities and of aggregates of germanium atoms.

Infrared spectroscopic study The infrared absorption spectrum of amorphous GeO 2 films is quite close to those

obtained with crystalline hexagonal GeO 2 and with vitreous GeO 2 (Fig. 3). However, one can note :

- a slight displacement of the streching band Ge-O towards the lower frequencies (870 cm -], as compared to 876 cm -1 for crystalline hexagonal GeO 2 and for vitreous GeO2), - an important weakening of the deformation band, - a dispersion of Ge-O distances lesser than in the vitreous phase.

uJ 0 Z < m n-

I11 <

}

1300 ~100 900 ~0 s0o WAVE NUMBER (cm-~

FIG. 3 Infrared spectra of stoichiometric GeO 2 : a - crystalline GeO 2 ; b = sputtered GeO 2 ;

c - vitreous GeO2

In the case of GeO x (1.4 < x < 2), a broadening and a displacement towards lower frequencies of the two vibration bands occur, which both axe enhanced as x decreases (Fig. 4).

These results would indicate, as the analysis of the EXAFS spectra had indicated, the presence of tetrahedral Ge(O4.yGey) entities in our deposits.

The oxides GeO x appear therefore to be much more complex than we had initially thought. It seems that, as in the case of SiOx (8), we are dealing with compositions intermediate between two limit models :

Vol. 24, No. 11 GERMANIUM OXIDES 1365

- the "RMM" model, corresponding to a statistical distribution of germanium atoms and of GeO4 tetrahedrons in the material ; - the "RBM" model corresponding to a statistical distribution of oxygen and germanium atoms in the tetrahedron Ge(O4.yGey ).

Thus, the oxides GeOx appear to be a mixture containing variable proportions of germanium atoms, of GeO4 tetrahedrons (some of which could be lacunary), of Ge(Oa_yGey) tetrahedrons and of oxygen atoms in the form of 02 molecules or of OH radicals.

ill

m <

~'~cm-t~ ~ cm-I

WAVE ~ R (cm-b

FIG. 4 Infrared spectra of sputtered GeO x : a - GeO 2 ; b - GeO1.84 ; c - GeO 1.40

Phvsical characl~eristics 9f the films

Density The density of the films was found to be p = 3.6 + 0.1 g.cm -3 in the case of stoichiometric

GeO 2 ; this value is in good agreement with that obtained for the vitreous phase (9). When x decreases, p is found to increase, but the accuracy of the measurements is not sufficient to correlate density and composition for small deviations from stoichiometry.

Refractive index Table 2 gives the values of the refractive index n D (~.D = 0.5893 lim), of the dispersion

nF-nc (~-F = 0.4861 ttm, kC = 0.6563 Ixm) and of the Abbe number VD = (riD - 1)/(nF - nc) for oxides GeOl.g 4 and GeO 2. In the case of the stoichiometric oxide, our values are close to those reported by Zhong-Yi Yin and Garside (5) ; they differ only little from those of the vitreous material (9,10).

Table 2

Composition n D nF-n C v D

GEO1.84 1.629 + 0.008 0.014 45 GeO 2 1.608 + 0.008 0.016 38

1366 C. CAPERAA, et al . Vol . 24, No. 11

Dielectric properties Table 3 gives the dielectric characteristics at 20°C obtained with two capacitors annealed at

70°C during 12 hours in ambient air. Table 3

f = 1 kHz f = 5 kHz f = 10 kHz Composition er tan 8xl03 er tan 8xl03 8r tan 8xl03

Ge01.s4 6.02 5.6 5.99 4.4 5.98 4.4 GeO 2 5.95 5.0 5.91 3.0 5.89 2.9

We checked that the thermal treatment does not alter the composition of the deposits. However, a slight reduction of the dielectric constant er and of the loss factor tan 8 was noted. For a same frequency f, the difference in the dielectric constant between the two samples is of the same order of magnitude as the uncertainty resulting from the determination of the geometric capacitance of the capacitors ; tan 8 increases slightly when the oxide becomes non-stoichiometric. The deposited materials have a normal dielectric behaviour : er and tan 8 decrease slightly when the frequency is increased. Our results are in good agreement with those of Umezawa and Sasaki (6).

~QNCLUSION

The study of the local order in thin films of amorphous oxides has shown that these materials have a complex structure. As in the case of vitreous or of crystalline oxide GeO 2 (type a-quartz), the structure of amorphous GeO 2 is based on a tridimensional succession of tetrahedrons GeO4. However, in a small number of these tetrahedrons one (or several) oxygen atoms are replaced by germanium atoms, which gives rise to entities of the Ge(O4_yGey) type. In the case of non-stoichiometric oxides GeO x, the number of these entities Ge(O4_yGey) increases significantly when x decreases. At the same time, metallic germanium appears within the material.

In the case of stoichiometric GeO 2, the density and the refractive index of the deposits are close to those of the vitreous material. A slight deviation from stoichiometry in the material does not alter significantly the dielectric characteristics of the germanium oxide films. Presently, we are broadening the scope of the study to mixed Ge-Pb-O oxides, which exhibit superior dielectric characteristics.

A~YKNOWLEDGMENTS

The authors would like to thank Dr. B. Gruzza for providing the facility for AES measurements.

REFERENCES

(1) K. Suzuki, M. Misawa and Y. Kobayashi, J. Physique C8, 12, 617 (1985).

(2) Y. Takano, Y. Tandoh, H. Ozaki and N. Mori, Phys. Stat. Sol. (b) 130, 431 (1985).

(3) A.L. Shabalov and M.S. Feldman, Phys.Stat. Sol. (a) 83, K11 (1984).

(4) D.A. Jishiashvili and E.R. Kutelia, Phys. Stat. Sol. (b) 14~, K147 (1987).

Vol. 24, No. 11 GERMANIUM OXIDES 1367

(5) Zhong-Yi Yin and B.K. Garside, Appl. Opt., 21, 4324 (1982).

(6) T. Umezawa and K. Sasaki, J. Vac. Sci. Technol., AS, 1783 (1987).

(7) K. Awitor, G. Baud, J.P. Besse, C. Caperaa and M. Jacquet, Proceeding of the First

International Conference on Plasma Surface Engineering (PSE'88 Garmisch- Parten-

kirchen).

(8) Y.N. Sun, A. Feldman and E.N. Farabaugh, Thin Solid Films, 157, 351 (1988).

(9) K. Murthy and J. Ip, J. Amer. Ceram. Soc., 47, 328 (1964).

(10) R.H. Magruder, S. Morgan, D.L. Kinser and R.A. Weeks J. Non-Cryst. Solids, 94, 57

(1987).