Alloying and Strain Relaxation in SiGe Islands Grown on Pit-Patterned Si(001) Substrates Probed by Nanotomography

Nanoscale Research Letters, Jun 2009

The three-dimensional composition profiles of individual SiGe/Si(001) islands grown on planar and pit-patterned substrates are determined by atomic force microscopy (AFM)-based nanotomography. The observed differences in lateral and vertical composition gradients are correlated with the island morphology. This approach allowed us to employ AFM to simultaneously gather information on the composition and strain of SiGe islands. Our quantitative analysis demonstrates that for islands with a fixed aspect ratio, a modified geometry of the substrate provides an enhancement of the relaxation, finally leading to a reduced intermixing.

A PDF file should load here. If you do not see its contents the file may be temporarily unavailable at the journal website or you do not have a PDF plug-in installed and enabled in your browser.

Alternatively, you can download the file locally and open with any standalone PDF reader:

http://link.springer.com/content/pdf/10.1007%2Fs11671-009-9360-4.pdf

Alloying and Strain Relaxation in SiGe Islands Grown on Pit-Patterned Si(001) Substrates Probed by Nanotomography

F. Pezzoli 0 1 2 M. Stoffel 0 1 2 T. Merdzhanova 0 1 2 A. Rastelli 0 1 2 O. G. Schmidt 0 1 2 0 Present Address: M. Stoffel Institut fur Halbleitertechnik , Pfaffenwaldring 47, 70569 Stuttgart, Germany 1 F. Pezzoli (&) M. Stoffel A. Rastelli O. G. Schmidt Institute for Integrative Nanosciences , IFW Dresden, Helmholtzstrae 20, 01069 Dresden, Germany 2 T. Merdzhanova Max-Planck-Institut fur Festkorperforschung , Heisenbergstrae 1, 70569 Stuttgart, Germany The three-dimensional composition profiles of individual SiGe/Si(001) islands grown on planar and pit-patterned substrates are determined by atomic force microscopy (AFM)-based nanotomography. The observed differences in lateral and vertical composition gradients are correlated with the island morphology. This approach allowed us to employ AFM to simultaneously gather information on the composition and strain of SiGe islands. Our quantitative analysis demonstrates that for islands with a fixed aspect ratio, a modified geometry of the substrate provides an enhancement of the relaxation, finally leading to a reduced intermixing. - The lattice mismatch between Si and Ge drives the formation of SiGe quantum dots (QD) during strained layer heteroepitaxy [1, 2]. For large-scale integration technologies [3], the position of such islands needs to be accurately controlled on the substrate surface [4]. A viable process relies on the fabrication of lithographically defined pits, which act as a sink for the deposited adatoms, allowing the exact positioning and addressability of individual QDs. In addition, a precise control of the chemical composition of the SiGe islands is required, since the three-dimensional (3D) composition profile ultimately determines their electronic behavior and optical properties. However, little work has been done on this topic, and the different intermixing mechanisms sustaining the growth and evolution of Ge islands in presence of a surface with an extrinsic morphology are still debated [57]. It has been shown that SiGe islands grown on patterned areas have larger volumes than those on the surrounding planar surfaces [8, 9]. These observations are corroborated by a recent comparison of X-ray measurements and finite element calculations, which suggests a different compositional state with a larger intermixing and relaxation on the patterned substrates [7]. However, the compositional differences at the single dot level were not yet considered. In this letter we address the issue of the impact of substrate patterning on shape, composition, and strain relaxation at the single dot level by using atomic force microscopy (AFM)-based nanotomography (NT-AFM). Following Ref. [10], we have recently extended the capabilities of NT-AFM to quantitatively determine the full 3D composition profiles of strained SiGe islands [11]. In this study, we compare lateral and vertical composition gradients of individual SiGe islands grown on pit-pattern and planar Si(001) substrates. Above all, by combining structural data with the average island compositions as obtained by NT-AFM, we are able to determine island strain only by means of an AFM analysis. The experimental ability to map the chemical composition at the nanoscale helps indeed to shed new light on the driving forces governing alloying. Our findings provide direct experimental evidence that a nanostructured surface plays a major role in determining strain relaxation and therefore in defining the compositional profiles of the islands. Experimental Procedure The sample considered here consists of 8.5 monolayer of Ge deposited by molecular beam epitaxy at 700 C on a patterned Si(001) substrate [12]. A 500 9 500 lm2 mesh of pits aligned along the h110i directions was realized by electron beam lithography followed by reactive ion etching. The distance between nearby pits is 450 nm and their depth and width about 25 and 85 nm, respectively. The surface morphology of the sample was analyzed by AFM operating in tapping mode with a super sharp silicon tip (nominal radius of curvature of 2 nm). Fig. 1a shows a 30 9 30 lm2 AFM image of the surface morphology in proximity of a corner of the patterned area. The observed material depletion region is due to a directional diffusion of Ge from the unpatterned, flat surface toward the patterned area, which results in a gradient of the Ge amount available for island formation in the patterned area [9]. Islands close to the pattern edge are therefore larger than those a few microns away from it and some of them exceed the critical size for dislocation introduction [13]. Here we focus on the two areas marked in Fig. 1a, in order to exclude most of the large, dislocated islands at the boundaries of the patterned field. The two island ensembles consist mainly of barnshaped islands [14], as corroborated by a facet analysis (not shown). As reported in Fig. 1b, the mean height of coherent islands is similar, being (49 6) nm and (54 3) nm on the flat and patterned surfaces, (...truncated)


This is a preview of a remote PDF: http://link.springer.com/content/pdf/10.1007%2Fs11671-009-9360-4.pdf

F. Pezzoli, M. Stoffel, T. Merdzhanova, A. Rastelli, O. G. Schmidt. Alloying and Strain Relaxation in SiGe Islands Grown on Pit-Patterned Si(001) Substrates Probed by Nanotomography, Nanoscale Research Letters, 2009, pp. 1073-1077, Volume 4, Issue 9, DOI: 10.1007/s11671-009-9360-4