HIGH TEMPERATURE IN-SITU INVESTIGATION OF SI/SIGE MULTILAYERS AND CASCADE STRUCTURES

M. Meduňa¹ J. Novák^{1, 2}, C.V. Falub³, G. Bauer², 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

Electroluminescence from Si/SiGe structures grown either pseudomorphically on (001) Si substrates [1] or on Si_0.5Ge_0.5 pseudosubstrates [2] has been observed in the wavelength region around 10 mm. The realization of Si/SiGe cascade structures with an emission in the 100 mm wavelength region is a next step in construction of quantum cascade laser (QCL). Nevertheless the design and realization of Si-based QCL is very complicated and requires enormously stable growth conditions.

Since such emitters, with high Ge content in SiGe layers, are usually exposed to high temperatures during device operation, it is very important to determine the temperature stability and diffusion properties of this metastable QCL structures. Due to the high strain present within the individual layers, the QCL structures can suffer strain relaxation through dislocation formation at high temperatures and the interface roughness and morphology can change as well.

In this study, we focus on the structural and temperature stability investigations of SiGe multilayers and quantum cascade samples with a Ge content of 30% and 80%. The structural stability of multilayers is enhanced by strain symmetrization of Si_0.25Ge_0.25 (Si_0.5Ge_0.5) pseudosubstrate and cap layer. A series of multilayers with different layer thicknesses and multilayer periods were measured by x-ray reflectivity under grazing incidence and high angle diffraction during in-situ annealing at ROBL beamline at ESRF in Grenoble.

X-ray reflectivity and diffraction reciprocal space maps (RSM) for all structures have been obtained at room temperature and during several isothermal annealing processes for temperatures ranging from 550°C up to 824°C, using a wavelength l=1.5405 Å. The annealing was performed under high vacuum using a Be dome chamber present at the beamline. The details of the experiment on Si_0.2Ge_0.8 samples were published in Ref. [3].

From the temporal evolution of x-ray reflectivity data at high temperature, we obtained an evolution of Si-SiGe interfaces induced by diffusion, while the temporal evolution of diffraction RSMs provides information about the strain status in the multilayer during annealing. The series of isothermic in-situ annealing measurements show, that a significant change in reflectivity RSMs, corresponding to the inter-diffusion in Si_0.7Ge_0.3/Si layers, depend on the individual layer thickness. In samples with the smaller period, the disappearance of the Bragg peaks starts to be observable much earlier, than in samples with larger layer thicknesses, already at a temperature of 690°C. Moreover the evolution of the envelope curve of Bragg peaks, corresponding to the Si/SiGe interface shift, saturates after certain time and next significant change start to appear after further increase of the sample temperature.

The results of our experiment demonstrate that the critical temperature for Ge inter-diffusion process in Si_0.7Ge_0.3 and Si_0.2Ge_0.8 depends not only on Ge content but also most likely on the thickness of the individual layers and varies within the interval from 700°C to 800°C. This effect was observed as for in-situ annealed samples containing MQWs with periods 5.8 – 13.5 nm as in complicated QCL structure annealed ex-situ.

[1] G. Dehlinger, L. Diehl, U. Gennser, H. Sigg J. Faist, K. Ensslin, D. Grützmacher, and E. Müller, Science, 290 (2001) 2277-2280.

[2] D. Grützmacher, S. Tsujino, C.V. Falub, A. Borak, L. Diehl, E. Müller, H. Sigg, U. Gennser, T. Fromherz, M. Meduňa, G. Bauer, J. Faist, O. Kermarrec, Mater.Sci. in Semi.Proc. 8 (2005) 401-409.

[3] 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-A125