Interdiffusion in SiGe alloys studied by x-rays

M. Meduňa¹, J. Novák¹, G. Bauer², V. Holý³, C.V. Falub⁴,

S. Tsujino⁴, D. Grützmacher⁴

¹Institute of Condensed Matter Physics, Masaryk University, Kotlářská 2, 611 37 Brno,

Czech Republic

²Institut für Halbleiterphysik, J Kepler Universität, Altenbergerstrasse 69, A-4040 Linz,

Austria

³Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague,

Czech Republic

⁴Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, CH-5232 Villigen PSI,

Switzerland

mjme@physics.muni.cz

A growing importance of SiGe-based electronic and of optoelectronic devices in recent past is evident due to a significant progress in Si/SiGe band gap and strain engineering [1]. Devices such as MOSFETs are one of these, which attract their attention very intensively due to their application in CMOS circuits. Quantum cascade lasers and light emitters gain their interest for its current threshold and low power consumption. The production of light emitters have been already mastered to a certain extent in III-V materials, but making of optoelectronic devices on the basis of IV-IV materials is more challenging because of their compatibility to a standard Si technology [2].

A typical obstacle that has to be overcome in design, processing and operation of all electronic devices is the thermal load during fabrication and during operation. Due to heat dissipation in the circuits and operation of devices at high temperatures, the diffusion processes of the materials has to be taken into account. Thus a detailed knowledge of the diffusion in SiGe alloys is highly desirable. Unfortunately the interdiffusion process in SiGe is unlinear and strongly depends on Ge concentration [3]. The parameters describing precisely the SiGe intermixing are still under investigation.

The interdiffusion coefficient of mixtures can be described by Arheniuss equation D = D₀ exp(E_A/kT) given by activation energy E_A and diffusion pre-exponential factor D₀. The non-linearity of the diffusion process in SiGe alloys consists in the strong dependence of E_A and D₀ on the Ge concentration X_Ge in Si_1-xGe_x [3,4]. Up to now only activation energies E_A and diffusion prefactors D₀ for X_Ge up to Ge contents of 50% were reported with comparatively large error bars [4]. Recently the authors Aubertine et al. have performed a systematic measurement of interdiffusion in SiGe multilayers with estimation of E_A and D₀ for 0 < X_Ge < 0.20 with relatively small experimental error [5]. To our knowledge the parameters E_A and D₀ for Si_1-xGe_x in the range of X_Ge from 0.5 to 1 are not well reported.

The aim of our investigation was to extend the knowledge about SiGe interdiffusion process also for the range of Ge content from 25 % to 50 % and to determine new values of E_A and D₀. We have annealed simple Si_1-xGe_x multilayers grown by molecular beam epitaxy on relaxed SiGe pseudosubstrate with graded Ge content from pure Si up to constant composition top layer. The multilayer itself consisted nominally from 30 periods of SiGe/Si bilayers sandwiched in between a 20 nm thick step graded layer system and covered by an additional SiGe cap layer due to strain symmetrization. More about the sample structure and about the experiment can be found in our recent publication [6].

In order to study the diffusion properties of SiGe alloys in multilayer structures, we have used in-situ x-ray reflectivity and diffraction techniques performed at ESRF at beamline BM20. The critical temperature, where the interdiffusion started to be observable in our multilayers, was around 700 °C and thus we have performed several isothermal annealing measurements above this temperature in order to obtain activation energy E_A and diffusion pre-exponential factor D₀. Reciprocal space maps were recorded at several stages of annealing and for several temperatures mostly around 800 °C. The diffusion coefficients were obtained from the Ge content profile and from the profile of electron density, which were determined from simulations of specular reflectivity and diffraction data, see Fig. 1.

Figure 1. Evolution of x-ray reflectivity during annealing of Si_0.7Ge_0.3/Si multilayer at 747 °C.

The results were compared with data recently published by Aubertine et al. [5], who report diffusion coefficients showing linear decrease of activation energy and exponential decrease with Ge concentration. Our experiments show good agreement with extrapolation of these data [6] for Ge contents 50 % and 25 %. This statement suggests further generalization whether the dependency of E_A or D₀ is linear or exponential for the whole range of Ge content.

References

1. D. Paul, Physics World, 13, 27 (2000)

2. S. Tsujino, A. Borak, E. Müller, M. Scheinert, C.V. Falub, H. Sigg, D. Grützmacher, M. Giovannini, J. Faist, Appl. Phys. Lett., 86, (2006), 62113.

3. N. R. Zangenberg, J. Lundsgaard Hansen, J. Fage-Pedersen & A. Nylandsted Larsen, Phys. Rev. Lett., 87, (2001), 125901

4. D.B. Aubertine, M.A. Mander, N. Ozguven, A.F. Marshall, P.C. McIntyre, J. O. Chu, P. M. Mooney, J.Appl. Phys., 92, (2002), 5027.

5. D.B. Aubertine & P.C. McIntyre, J.Appl. Phys., 97, (2005), 13531.

6. M. Meduňa, J. Novák, G. Bauer, V. Holý, C.V. Falub, S. Tsujino & D. Grützmacher, Semicond.Sci.Techn., 22, (2007), 447.

Acknowledgements.

This work was supported by SANDiE (NMP4-CT-2004-500101), by the projects MSM0021622410 and MSM0021620834 of the Ministry of Education of Czech Republic and Grant Agency of Czech Republic (project no 202/05/P286).