SiO2 heterostructures

Solid State Communications, Vol. 84, No. 10, pp. 987-989, 1992. Printed in Great Britain.

0038-1098/92 $5.00 + .00 Pergamon Press Ltd

ELASTIC PROPERTIES OF a-Ge : H/Si AND a-Ge : H/SiO2 HETEROSTRUCTURES Hua Xia, J.G. Jiang, Wei Zhang, K.J. Chen and X.K. Zhang National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210008, P.R. China and G. Carlotti, D. Fioretto and G. Socino Laboratorio di Scattering Brillouin, Dipartimento di Fisica, Universit~ di Perugia, 1-06100 Perugia, Italy

(Received 14 July 1992 by V.M. Agranovich) Brillouin light scattering from Rayleigh and Sezawa surface acoustic modes in a-Ge: H films deposited on both c-Si(l 00) and fused silica substrates has been exploited for studying the elastic properties of these heterostructures. Their effective elastic constants have been determined from a fit of the calculated dispersion curves to the measured sound velocities. We have found that the effective shear modulus Can is much lower than that of crystalline Ge. The decrease observed is of about ,-~ 38% when the film is supported by c-Si and of about ~ 44% when supported by fused silica. No appreciable modification of the effective elastic constant C1~, with respect to crystalline Ge, has been observed.

FILMS of hydrogenated amorphous germanium and elastic constants of the supported a-Ge:H films, silicon are of increasing technological importance, within the elastic continuum approximation [5, 6]. owing to their novel and interesting optical and Measurements were carried out in air at room electrical properties. Knowledge of the elastic and temperature by use of a Sandercock-type highvibrational behavior of these heterostructures can be, contrast (3+3)-pass tandem Fabry-P~rot intertherefore, of great interest in view of many potential ferometer [7]. A single-mode of the 5145/k Ar ion device applications in microelectronics and optics. In laser line was used with an incident power of spite of fact that much effort has been made for the 20-50mW and with sampling time of typically I h studies of elastic properties of a-Si:H [1, 2], little is per spectrum. The incident light was polarized in the known about those of a-Ge:H. To this respect, it scattering plane and no analyzer was placed in the would be useful to study the modification of the scattered beam; the aperture of the collecting lens was physical properties of a-Ge:H films with respect to 1:1.8. Measurements have been taken in the backthose of crystalline germanium (c-Ge). scattering interaction geometry. In this report we present experimental results of The hydrogenated amorphous germanium heteroBriliouin light scattering from hydrogenated amor- structures were deposited by a plasma-enhanced phous germanium (a-Ge: H) films grown on Si(00 1) chemical vapor deposition (PECVD) technique and fused silica substrates. The Brillouin scattering onto Si(00 1) and fused silica slabs. The a-Ge:H technique has proved to be a powerful tool for films, 1950/k thick, were grown from GeH 4 (one part gaining a substantial insight into the physics of per volume) and diluted in H2 (40 parts per volume). layered materials since the advent of the high- The pressure in the chamber was 3 × 10-3 Torr, and contrast tandem Fabry-P6rot interferometer. the substrate temperature 250°C; the power density Measurement of the frequency of Rayleigh and was 20 W cm -2. The mass density of the amorphous Sezawa surface modes confined in a supported germanium films is ,-~ 95% of that of the c-Ge, i.e. layer, gives information about its effective elastic 5.05 g cm -3, and with the hydrogen content of about constants and therefore on its mechanical properties 15%. These samples were first studied using Raman [3, 4]. We have observed the scattering from Rayleigh scattering and showed only two main peaks at 80 and and Sezawa modes for different values of the acoustic 275 cm -t, due to maxima in the density of vibrational wavevector; this enabled us to determine the effective states (DOS) corresponding to the TA and TO 987

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Vol. 84, No. 10

a-Ge : H/Si A N D a-Ge : H/SiO2 H E T E R O S T R U C T U R E S

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Fig. 1. Measured Brillouin spectrum of the a-Ge : H/ Si(0 0 1) heterostructure with film thickness h = 1950/k, for an angle of incidence of light 0 = 76 °. The Rayleigh and Sezawa acoustic modes labeled by RW, SWi (i--- 1,2, 3), respectively. The phonon wavevector is along the [1 0 0] direction of the Si substrate.

Fig. 2. Calculated (solid line) and measured (circles) phase velocities of the Rayleigh and Sezawa acoustic modes as a function of Qh for a-Ge: H/Si(00 1). The velocity of bulk transverse wave in the substrate and of the Rayleigh surface wave in the supported film are denoted by v: and v f , respectively. The dashed curves correspond to the velocity dispersions of surface acoustic waves from c-Ge/Si(0 0 1) heterostructure.

phonon modes in the crystalline Ge. This means that the structure of the supported films has short-range order, with the amorphous structure most likely being that of the continuous random network. Figure 1 shows a typical Brillouin spectrum from the a-Ge : H/Si(0 0 1) heterostructure, corresponding to an angle of incidence 0 = 76 °. The most intense inelastic peak, located at about 10GHz, is due to the Rayleigh surface wave (RW). At higher frequencies, the so-called higher-order Sezawa surface modes (SWi) are also seen. Their phase velocities vi are confined to the range v f < vi < v:, where v f and v~ are the bulk transverse sound velocities of the film ( f ) and the substrate (s), respectively. The phase velocity of these acoustic surface waves are directly obtained from the ratio between the measured frequency shift and the wavevector Q fixed by the interaction geometry. In the backscattering geometry used, the wave vector Q of surface phonons coming into the scattering process is: Q = ~ sin 0, with A the optical wavelength and 0 the incidence angle of light. The measured velocity dispersion vi(Qh), is shown in Fig. 2. The experimental points are obtained taking measurements at different angles of incidences in the range between 34 ° and 76 ° . The solid lines in Fig. 2 are the fitted dispersion curves obtained within the elastic continuum approximation, according to the approach described in [5]. They are fitted to the experimental data, assuming the two independent elastic constants of the amorphous film Cll and C44 as free parameters. For the sake of comparison, we have also shown the dispersion curves of c-Ge/Si(0 0 1) system.

As for the c-Si substrate parameters, we have used the following values Cll = 166 GPa, C44 = 79.6 GPa, Ct2 = 63.9GPa, and p = 2.33gcm -3. The film analyzed has a thickness h which is about one half the phonon wavelength (Qh >_ 4) so that the velocity of the Rayleigh surface wave is almost independent of the acoustic wavevector. In this condition its phase velocity depends mainly on C44, while the velocity of Sezawa modes depends on both Cll and C44. These two constants have therefore been determined; the values obtained are: Cit = ( 1 2 9 + 3 ) GPa and Can = (41 + 1)GPa. For a comparison these data with well-known values of crystalline Ge, one can find that the shear constant of the a-Ge : H film to be about ~ 38% lower than that of c-Ge, while no appreciable difference is found for Ci], whose value is reasonably consistent with that deduced from the phase velocity of longitudinal acoustic waves given in [8]. A further measurement has been taken on a a-Ge : H/SiO: heterostructure, grown under the same conditions as the a-Ge: H/Si(00 1) system. A typical Brillouin spectrum obtained for 0 = 76 ° is shown in Fig. 3. This spectrum differs from that of Fig. 1 in two main features: first, the phase velocity of the Rayleigh surface wave is about 5.5% lower than its counterpart in the a - G e : H / S i ( 0 0 1) system; second, the peaks relative to Sezawa modes are much broader since they are located at frequencies higher than the cut-off frequency of guided modes and have therefore the character of leaky modes. The evident decrease of the Rayleigh wave phase velocity in this heterostructure with respect to the

Vol. 84, No. 10

a-Ge : H/Si AND a-Ge : H/SiO2 HETEROSTRUCTURES effective elastic constant C l l c-Ge, has been observed.

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Acknowledgements - This work was supported by the National Natural Science Foundation of China, National Ministry of Education Science Foundation of China, Center for Material Analysis of Nanjing University, and by National Research Council (C.N.R.) of Italy, under the "Progetto Finalizzato" on Electrooptical Technologies.

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1. 2.

3. case of a-Ge : H/Si, reflects in a decrease of the shear modulus C44 of about 10%. A similar dependence of C44 on the substrate has already been found by some of the authors in a recent investigation of the elastic properties of ZnO films deposited on both glass and crystalline silicon [9-12]. This effect can be accounted for assuming that large strains are present at the interface when the film is deposited on crystalline Si, while no appreciable interface effects are present when the substrate is fused silica. In summary, we have investigated the elastic properties of hydrogenated amorphous germanium films deposited on both c-Si and fused silica substrates. We have found that the effective shear modulus C44 of these films is strongly softened with respect to that of c-Ge and depends on the supporting material. Specifically, we have observed a decrease of about 38% in a-Ge: H/c-Si(00 1) and of about 44% in a-Ge:H/SiO2. This softening can be attributed mainly to the structure differences between crystalline and amorphous hydrogenated germanium, and partially to the structural mismatch between film and substrate. No appreciable modification of the

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