Semiconductors, structures and devices of reduced dimensionality have attracted much interest during the last few years; in particular for group IV semiconductors. This is due to the improvement of the electron-hole recombination efficiency by quantum confinement effects, and to the possibility of developing Si-based optoelectronics.

It is known that the growth of Ge on Si is of the Stranski-Kastanow type: a 2-D wetting layer initially forms, followed by the appearance of 3-D islands. The degree and type of strain in these islands is currently under investigation although a clear picture of the mechanisms of strain relaxation is not yet available. In particular, there have been some recent indications that intermixing between substrate and epilayer may be significant, but no clear evidence has been provided. In Figure 50 we show AFM images [1] of individual Ge dots contained in our samples: in (a) the morphology is typical of a relaxed structure (dome) while in (b) a single dislocation can be observed attached to a pyramid.

X-ray absorption spectroscopy (XAFS) in the fluorescence mode is able to provide a local structural characterisation of semiconductor nanostructures. Ge K-edge XAFS obtained in our experiment clearly demonstrate that substantial intermixing takes place for Ge dots deposited on Si substrates. In Figure 51 we show selected XAFS spectra along with standards of pure bulk Ge and of a Ge impurity in Si; the spectra of all samples deviate significantly from those of Ge, especially at low k, where a positive oscillation typical of a Si environment can be observed. An essential tool to obtain this new result has been the advanced instrumentation available at BM8 (the GILDA beamline), in particular the multi-element solid state detector, fast digital electronics and specially designed LNT sample holder.

For Ge/Si(001) deposited by CVD at 600°C, with equivalent thicknesses ranging between 5 and 40 nm, we find that Ge is always surrounded by 1 Si atom and 3 Ge atoms. In this thickness range, AFM shows a transition from pyramidal to dome-shaped dots. For Ge/Si(111) deposited by MBE at 500°C, we probe thicknesses of 1 and 1.7 nm; the thinner sample consists essentially of the 2-D wetting layer. For all Ge/Si(111) samples we find increased intermixing, with equal Ge-Si and Ge-Ge coordination numbers of 2. The common conclusion which can be drawn from the data is that considerable diffusion of Si into the Ge dots takes place ­ even at the relatively low growth temperatures employed.

The basic physics underlying these findings is that alloying can reduce the strain energy. In particular, a linear increase in the Si concentration in the Ge dots is expected to reduce quadratically the strain energy. Strain-enhanced diffusion of Si is thus thermodynamically favoured. Our study suggests that interdiffusion must be considered as one of the energy-reduction mechanisms for quantum dots, along with, for example, the formation of dislocations.

Reference
[1] G. Capellini, L. DiGaspare, F. Evangelisti, and E. Palange, Appl. Phys. Lett., 70, 493 (1997).

Principal Publication and Authors
F. Boscherini (a), G. Capellini (b), L. DiGaspare (b), N. Motta (c), F. Rosei (c), S. Mobilio (a), Appl. Phys. Lett., 76, 682 (2000).

(a) Frascati Nat. Lab. and University of Bologna (Italy)
(b) University of Roma Tre (Italy)
(c) University of Roma Tor Vergata (Italy)