IN-SITU INVESTIGATIONS OF SI AND GE INTERDIFFUSION IN SI CASCADE STRUCTURES

M. Meduňa¹, J. Novák¹, V. Holý¹, C.V. Falub³, G. Bauer², and D. Grützmacher³

¹Institute of Condensed Matter Physics, Masaryk University, Kotlářská 2, Brno, Czech Republic
²Institute of Semiconductor Physics, J. Kepler University, Altenbergerstrasse 69, Linz, Austria

³Laboratory for Micro- and Nanotechnology, PSI, Villigen-PSI, Switzerland

The importance of knowledge of diffusion properties of Si and Ge is crucial for production of SiGe based cascade emitters. Electroluminescence has been observed from specially designed Si/SiGe quantum cascade structures [1] and the implementation of quantum cascade laser in SiGe technology is still in demand. Nevertheless heating due to thermal losses during operation of such devices is a significant factor and the knowledge of the temperature stability of cascade structures and the diffusion processes of Ge and Si in SiGe are very important.

The diffusion properties of Ge in either Si or in Si_1-xGe_x alloys are still not well understood. Their determination is complicated by the strong non-linear dependence of interdiffusion coefficient D=D₀(X_Ge)exp[-E_A(X_Ge)/kT] on Ge content X_Ge in Si_1-xGe_x [2]. In recent publications [3] 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. 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, but they are necessary for the numerical modeling of non-linear diffusion processes of Si and Ge in Si_(1-x)Ge_x cascade structures with Ge contents up to 80%. Since the typical cascade structures are very complicated and the evaluation and analysis of its measured data are quite difficult, the most suitable for x-ray measurements are simple MQW structrures.

In our case we have studied series of strain symmetrized MQW structures grown on Si_0.25Ge_0.25 and Si_0.5Ge_0.5 pseudosubstrates with thicknesses and periods similar to usual cascade structures. The Ge content in these structures was 30% and 80% with multilayer periods in the range from 6 to 13 nm. X-ray reflectivity (XRR) and diffraction (XRD) reciprocal space maps for all structures have been recorded at room temperature and during several in-situ isothermal annealing processes for temperatures ranging from 550°C up to 824°C. The annealing was performed under high vacuum using a Be dome chamber at ROBL beamline at ESRF [4].

The obtained measurements were compared with simulations of diffraction curves for MQW structures using the evolution of Ge content profile according to nonlinear diffusion equation.

The Ge content profile obtained from diffusion equation was used for simulating the XRR or XRD spectra. Since the interdiffusion coefficient depends strongly on Ge content, the interpretation of the data is still quite difficult.

[1] L. Diehl, S. Mentese, E. Müller, D. Grützmacher, H. Sigg, T. Fromherz, J. Faist, U. Gennser, Y. Campidelli, O. Kermarrec, D. Bensahel, Physica E, 16, (2003), 315.

[2] N. R. Zangenberg, J. L. Hansen, J. Fage-Pedersen, and A. N. Larsen, Phys. Rev. Lett., 87, (2001) , 125901.

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

[4] M. Meduňa, J. Novák, C.V. Falub, G. Chen, G. Bauer, S. Tsujino, D. Grützmacher, E. Müller, Y. Campidelli, O. Kermarrec, D. Bensahel, N. Schell, J. Phys. D: Appl. Phys., 38 , (2005), A121.