M. De Seta

SiGe intermixing in Ge/Si(100) islands

We have applied atomic force microscopy and x-ray photoemission spectroscopy to the study of SiGe intermixing in Ge/Si(100) self-assembled islands. We have quantified the Ge/Si alloying as a function of the deposition temperature in the 500–850 °C range. The Si content inside the islands varies from 0% at 550 °C up to 72% at 850 °C. As a consequence of the reduction of the effective mismatch due to the observed SiGe intermixing, the critical base width for island nucleation increases from 25 nm for Tdep<600 °C up to 270 nm for Tdep=850 °C.

Ordering self-assembled islands without substrate patterning

The self-patterning of the strain field that arises in the growth of stacked multilayers of heteroepitaxial islands, together with the capability of tuning the island size by acting on the deposition temperature, are here exploited to obtain self-organization, resulting in well-ordered clusters composed of regularly disposed, nanosized islands. Our results show that the island spatial distribution can be tuned from a random one to a well-ordered square lattice of island clusters, and that the number of islands inside each cluster can be selected. Moreover, due to the dipole repulsive interaction between adjacent islands, the islands themselves arrange in an ordered fashion inside a single cluster along the same [010]-[100] crystalline directions of the long-range cluster ordering.

Strain relaxation in high Ge content SiGe layers deposited on Si

We have used Raman spectroscopy, transmission electron microscopy, x-ray diffraction, and x-ray photoemission spectroscopy to investigate strain relaxation mechanism of Si0.22Ge0.78 heteroepitaxial layer deposited on Si substrates in tensile, neutral, and compressive strain conditions. The three regimes have been obtained by interposing between the SiGe layer and the substrate a fully relaxed Ge layer, a partially relaxed Ge layer, or growing directly the alloy on Si. We found that the deposition of a Ge buffer layer prior to the growth of the SiGe is very promising in view of the realization of thin virtual substrates on silicon to be used for the deposition of strain-controlled high Ge content SiGe alloys. We demonstrate that this is mainly due to the strain relaxation mechanism in the Ge layer occurring via insertion of pure edge 90° misfit dislocations (MDs) and to the confinement of threading arms in to the Ge layer due to a second MD network formed at the SiGe/Ge heterointerface.

Intermixing-promoted scaling of Ge/Si(100) island sizes

The shape evolution and the effect of deposition temperature on size and composition of chemical vapor deposition grown Ge/Si(100) islands have been investigated in the deposition temperature range 450–850 °C. It is found that the increase of the growth temperature above 600 °C entails a strong island enlargement due to an increased Si/Ge intermixing. The crystallographic structure of the islands was investigated by transmission electron microscopy. The analysis of the resulting Moire pattern reveals that the island lattice deformation decreases with increasing island size and that the effective mismatch e between the silicon substrate and the epilayer decreases with increasing deposition temperature. The island nucleation size, the mean size of coherent islands and the critical size for the insertion of misfit dislocations have been found to scale as e−2, e−2, and e−1, respectively. The agreement of our experimental scaling results with the predictions of theoretical calculation performed for homogeneous...

High temperature x ray diffraction measurements on Ge/Si(001) heterostructures: A study on the residual tensile strain

Ge/Si(001) heterostructures grown by means of ultrahigh vacuum chemical vapor deposition have been investigated by means of variable temperature high resolution x ray diffraction in order to investigate the origin of the residual tensile strain observed in this system. To this purpose, we have simultaneously measured the in- and out-of-plane lattice parameters of the deposited Ge films and of the underlying Si substrate, thus allowing us to directly measure the Ge strain evolution as the epilayer was annealed up to and over the deposition temperature and cooled back to room temperature. We have observed that the tensile strain, resulting from the different Si and Ge thermal expansion coefficient, is partially compensated by the residual compressive heteroepitaxial strain, due to the hardening limit of Ge. This limited the tensile strain observable in these heterostructures to ∼0.002.

Ge/Si(100) islands: Growth dynamics versus growth rate

The effect of the deposition rate on the size, density, and uniformity of Ge islands grown on Si(100) is investigated. Upon changing the growth rate from 4 to 110 ML/min the island density increases by one order of magnitude and the strained dome base decreases from 84 to 55 nm. A narrowing of the island size distribution was also observed. We discuss these experimental findings by taking into account island–island interaction effects.

Strain-induced ordering of small Ge islands in clusters at the surface of multilayered Si–Ge nanostructures

Classical molecular-dynamics simulations based on the Tersoff potential are used to compute at the atomic level the strain-induced potential well generated at the surface of the capping layer by a buried, three-dimensional Ge island on Si(001). A simple model is outlined in order to predict the configurational arrangement for the nucleation of small Ge islands in such a potential well. The theoretical predictions are compared with atomic force microscope images of multilayered SiGe nanostructures grown by chemical vapor deposition. The cluster configuration is shown to be strongly dependent on the capping layer thickness, and to closely mimic the behavior predicted by the model.

Agglomeration process in thin silicon-, strained silicon-, and silicon germanium-on insulator substrates

In this paper we present a comparative study of the agglomeration process in silicon-on-insulator (SOI), silicon germanium-om-insulator (SGOI), and strained SOI (SSOI) thin layers under thermal annealing in ultrahigh vacuum. In particular, we provide the first evidence and characterization of agglomeration in SGOI and SSOI substrates. A common agglomeration dynamics is observed in all the substrates investigated, with the semiconductor-on-insulator layer thickness being the main parameter governing it. These findings provide a better understanding of the surface-energy-driven dewetting phenomenon in semiconductor layers and allow us to single out the influence of the surface and stress energies on the void formation and evolution, as well as on the size and density of the agglomerated islands.

Influence of the growth parameters on self-assembled Ge islands on Si(100)

The effect of deposition temperature on the growth dynamic, the shape, the size and the composition of Chemical Vapor Deposition (CVD) grown Ge/Si(100) islands have been investigated in the range between 500 and 850 °C. We found that the growth dynamic of the islands changes strongly between 500 and 600 °C, going from a kinetically limited growth regime to nearly equilibrium conditions. At higher temperatures the island growth is instead mainly affected by Ge/Si alloying. We found that the increase of the growth temperature above 600 °C results in an increased Si/Ge alloying, the mean Ge concentration in the islands changing from x = 0.75 at 600 °C to 0.28 at 850 °C. The determined SiGe intermixing and the consequent reduction of the effective mismatch completely accounts for observed island enlargement in the same temperature range.

Evolution of Ge/Si(001) islands during Si capping at high temperature

We discuss the effect of the deposition of a Si cap layer on the composition and morphological properties of Ge(Si)∕Si(001) self-assembled islands deposited by chemical vapor deposition at 750°C. The morphological evolution of the island shape was investigated by means of atomic force microscopy and the actual island composition has been measured by means of x-ray photoemission spectroscopy and x-ray absorption spectroscopy techniques. At an early stage of Si capping, Si atoms are incorporated in the island layer. As a consequence, we observe a reverse Stranski-Krastanov growth dynamics in agreement with the volume-composition stability diagram proposed for domes, pyramids, and prepyramids in the GexSi1−x∕Si(100) system. We find that the island burying begins when the Ge average composition reaches the value x=0.28. Once the islands are buried under a thin silicon layer their composition is unaffected by subsequent silicon deposition. We conclude that strain relief, rather than thermal diffusion, is the m...