Strain compensation by heavy boron doping in Si 1x Ge x layers grown by solid phase epitaxy
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Strain compensation by heavy boron doping in Si12x Gex layers grown by solid phase epitaxy A. Rodr´ıguez, T. Rodr´ıguez, and A. Sanz-Herv´as Departamento de Tecnolog´ıa Electr´onica, ETSI de Telecomunicaci´on, UPM, Ciudad Universitaria s/n, 28040 Madrid, Spain
A. Kling and J. C. Soares Centro de F´ısica Nuclear, Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1699 Lisboa Codex, Portugal
M. F. da Silva Departamento de F´ısica, Instituto Technol´ogico e Nuclear, Estrada Nacional 10, 2685 Sacav´em, Portugal
C. Ballesteros Departamento de F´ısica, Escuela Polit´ecnica Superior, Universidad Carlos III, C./Butarque 15, 28911 Legan´es, Madrid, Spain
R. M. Gwilliam Department of Electronic and Electrical Engineering, University of Surrey, Guildford, Surrey, GU2 5XH, United Kingdom (Received 18 October 1996; accepted 20 February 1997)
The strain compensation produced by heavy boron doping in Si12x Gex layers with x 0.21, 0.26, and 0.34 grown by solid phase epitaxy on (001) Si wafers has been analyzed using high resolution electron microscopy, high resolution x-ray diffractometry, and ion channeling. The structure of the epilayers consists of a defect-free region located next to the layer-substrate interface and a top region which contains strain-relieving defects. In the undoped samples the defect-free layers are partially relaxed and their relaxation increases as the Ge fraction increases. Substitutional boron incorporated to the SiGe lattice to levels of 2.8 6 0.3 3 1020 cm23 during the growth process reduces the lattice mismatch between the epilayers and the substrate. The boron-doped, defect-free layers are thicker than the corresponding undoped layers of the same Ge content and their strain relaxation is lower. It has been shown that it is possible to grow at least 27 nm thick defect-free and fully strained heavily boron-doped layers with x 0.21 by solid phase epitaxy.
I. INTRODUCTION
p 1 -doped fully strained epitaxial Si12x Gex layers with 0.20 < x < 0.35 and thicknesses of around 20 nm are of interest for the fabrication of long wavelength infrared detectors with cutoff wavelengths up to 15 mm.1,2 Solid phase epitaxy (SPE) is a suitable method to grow high quality epitaxial layers at low temperatures starting from amorphous material in contact with a crystalline substrate. This technique also allows one to obtain very high electrically active dopant concentrations in SiGe, well above the impurity equilibrium solubility.3 Using this growth method, boron doping of SiGe layers with carrier concentrations of 3 3 1020 cm23 has been reported recently.4 The growth of undoped SiGe layers on Si by SPE has been studied, in most cases using samples with x , 0.17. It has been shown that the layers grow without defects up to a critical thickness above which strain-relieving defects nucleate and extend throughout the rest of the layer.5–8 The theoretically predicted critical thicknesses for compositions in the 1698
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No.
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