Influence of the thin Si cap layer on SiGe quantum dots morphology

Marcin Żak 1Jean-Yves Laval 2Piotr Dłużewski 1Slawomir Kret 1Daniel Bouchier 3

1. Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland
2. Ecole Supérieure de Physique et de Chimie Industrielles, Laboratoire de Physique du Solide (ESPCI), 10 rue Vauquelin, Paris 75231, France
3. Université Paris-Sud, Institut Électronique Fondamentale (IEF), Bâtiment 220, Orsay 91405, France


We have investigated how a deposition of thin Si layer has influenced on a structure, and composition of SiGe QDs. The samples were grown in ultra high vacuum chemical vapour deposition process (UHVCVD) controlled with an in-situ reflection high energy electron diffraction (RHEED). An initial layer of germanium quantum dots were grown on Si (001) substrates at 700ºC. The resulted Si substrates with SiGe QDs layer were covered by Si caps with nominal thickness between 1.4 – 20.6 nm at temperatures 550ºC and 700ºC.

Cross sectional TEM samples were prepared by grinding, polishing, dimpling, and ion milling, as well as by tripod polishing method. The conventional (TEM) and high-resolution (HRTEM) investigations were performed with a use 2000EX and 2010F Jeol instruments, both operating at 200 keV. The local energy dispersive X-ray (EDX) analysis was carried out with a nanoprobe produced by the field emission gun (FEG) and PGT X-ray analyser. Surface of the samples was examined by an atomic force microscopy (AFM).

We found that Si capping process influences strongly on the shape of the SiGe QDs in both 550ºC and 700ºC deposition temperatures. At the initial stage three types of QDs were existed: pyramid, dome, and asymmetric. The dimension of QD base, taken from 80 images, gave average value 114 nm and was the same for the dome and pyramid shape. The average height was different and gave value 20 nm for dome and 11.5 nm for pyramid respectively. After deposition of 20nm Si cap a shape transition of the QDs has take a place. The both shapes lost facets, QDs were uniformed to flat lens-like shape with height to length ratio diminished by factor of two.

Germanium distribution in SiGe QDs was examined by EDX analysis and a measuring of the local lattice distortion. Both methods gave coherent result 50-60 at% Ge in the case of not covered QDs. For the Si capping layers the Ge content in QDs diminished to around 40-50 at% Ge. It was most pronounced for thicker cap layers.

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Presentation: Poster at E-MRS Fall Meeting 2007, Symposium J, by Marcin Żak
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-10 14:20
Revised:   2009-06-07 00:44
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