Optical Characterization of Self-Assembled Ge Dots on Silicon
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Optical Characterization Of Self-Assembled Ge Dots On Silicon F.Marabelli1, A.Rastelli1,2, A.Valsesia1, H.von Känel2 1 2
INFM-Phys.Dept.``A.Volta'', University of Pavia, Italy Laboratorium für Festkörperphysik, ETH Zürich, Switzerland
ABSTRACT Self assembled quantum dots of Ge were obtained by magnetron sputter epitaxy of seven monolayers of Ge on a 33nm thick undoped Si buffer grown on top of a p-doped (100) Si substrate. The samples obtained in this manner were then capped with an increasing number of silicon layers in order to study the effect of Si deposition on the strain and the morphology of the dots. They were characterized ``ex situ'' by spectroscopic ellipsometry and Raman spectroscopy. The optical experiments revealed well defined differences between the capped and uncapped samples and among samples with different cap thicknesses. By monitoring the energy and the splitting of the E0’, E1 and E2 interband optical transitions of Ge and the Ge-Si vibrational mode, the optical measurements evidence strain effects as well as the formation of SiGe alloy, in agreement with the ``in situ'' STM measurements.
INTRODUCTION Structural and Morphological features of self assembled island are connected to a series of other different behaviours more directly related to electronic and vibrational properties. In a system like Si/Ge both composition and strain strongly affect the electronic structure of the resulting material. [1,2] Optical spectroscopy, and spectroscopic ellipsometry in particular, is a very sensitive tool of investigation of this kind of problems. [2-5] Actually, many data exist in the literature on SixGe1-x bulk or relaxed samples [3,6]. The situation is more difficult for the strained samples, on which only theoretical data and investigations of only few selected stoichiometries are available [1,2,4,7]. In the present work we present some optical results obtained on a series of samples obtained by self assembling of Ge island on a silicon substrate. The aim is to discriminate between strain and intermixing effects on such a series of samples.
EXPERIMENT AND DISCUSSION The samples were prepared by ultra high vacuum magnetron sputtering epitaxy. 60 nm of silicon buffer were deposited on pre-cleaned Si(001) substrates; on top of the buffer 7 monolayers (ML) of Ge were epitaxially grown, followed in many cases by Si capping at 450°C. Further details on the sample preparation and structural characterization are given in ref.[8]. A series of samples were prepared by capping the Ge island formed after the Ge deposition with 1, 4, 8 and 16 ML of silicon, respectively. One samples was kept uncapped. An epitaxial film of 7
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ML of germanium capped with 16 ML of silicon was prepared at low temperature in order to have as a reference a fully strained, homogeneous Ge layer. Spectroscopic ellipsometry were measured with a Sopra spectrometer mod. ES 4G at an incidence angle of 70° and 75°. The spectral resolution was less than 5 meV over the whole spectral range (1.4-5 eV). A beam condenser used to focus t
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