G. Capellini

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.

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...

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.

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...

Freezing shape and composition of Ge/Si(001) self-assembled islands during silicon capping

We use atomic force microscopy, x-ray photoemission spectroscopy, and x-ray absorption spectroscopy to study the effect of the deposition of a Si cap layer by chemical vapor deposition on the morphology and composition of a Ge island layer grown at 600°C. We found that the capping of self-assembled Ge islands under a silicon layer results in high-quality, atomically flat layer only at deposition temperature above 700°C. On the other hand at this temperature Ge–Si intermixing and island coarsening are greatly enhanced, resulting in an increased average island volume. Here we show that the predeposition at low temperature of a thin cap layer preserves island shape, size, and composition when the capped islands undergo a subsequent process at higher temperature up to 750°C. It is shown, therefore, that with a two-step capping process it is possible to combine the benefit of a low temperature capping, which reduces island alloying and coarsening, with that of a high temperature capping which is needed to reco...

Interface and nanostructure evolution of cobalt germanides on Ge(001)

Cobalt germanide (CoxGey) is a candidate system for low resistance contact modules in future Ge devices in Si-based micro and nanoelectronics. In this paper, we present a detailed structural, morphological, and compositional study on CoxGey formation on Ge(001) at room temperature metal deposition and subsequent annealing. Scanning tunneling microscopy and low energy electron diffraction clearly demonstrate that room temperature deposition of approximately four monolayers of Co on Ge(001) results in the Volmer Weber growth mode, while subsequent thermal annealing leads to the formation of a Co-germanide continuous wetting layer which evolves gradually towards the growth of elongated CoxGey nanostructures. Two types of CoxGey nanostructures, namely, flattop- and ridge-type, were observed and a systematic study on their evolution as a function of temperature is presented. Additional transmission electron microscopy and x-ray photoemission spectroscopy measurements allowed us to monitor the reaction between ...

Radiative and non-radiative recombinations in tensile strained Ge microstrips: Photoluminescence experiments and modeling

Tensile germanium microstrips are candidate as gain material in Si-based light emitting devices due to the beneficial effect of the strain field on the radiative recombination rate. In this work, we thoroughly investigate their radiative recombination spectra by means of micro-photoluminescence experiments at different temperatures and excitation powers carried out on samples featuring different tensile strain values. For sake of comparison, bulk Ge(001) photoluminescence is also discussed. The experimental findings are interpreted in light of a numerical modeling based on a multi-valley effective mass approach, taking in to account the depth dependence of the photo-induced carrier density and of the self-absorption effect. The theoretical modeling allowed us to quantitatively describe the observed increase of the photoluminescence intensity for increasing values of strain, excitation power, and temperature. The temperature dependence of the non-radiative recombination time in this material has been inferred thanks to the model calibration procedure.

Growth and evolution of nickel germanide nanostructures on Ge(001)

Nickel germanide is deemed an excellent material system for low resistance contact formation for future Ge device modules integrated into mainstream, Si-based integrated circuit technologies. In this study, we present a multi-technique experimental study on the formation processes of nickel germanides on Ge(001). We demonstrate that room temperature deposition of ∼1 nm of Ni on Ge(001) is realized in the Volmer-Weber growth mode. Subsequent thermal annealing results first in the formation of a continuous NixGey wetting layer featuring well-defined terrace morphology. Upon increasing the annealing temperature to 300 °C, we observed the onset of a de-wetting process, characterized by the appearance of voids on the NixGey terraces. Annealing above 300 °C enhances this de-wetting process and the surface evolves gradually towards the formation of well-ordered, rectangular NixGey 3D nanostructures. Annealing up to 500 °C induces an Ostwald ripening phenomenon, with smaller nanoislands disappearing and larger ones increasing their size. Subsequent annealing to higher temperatures drives the Ni-germanide diffusion into the bulk and the consequent formation of highly ordered, {111} faceted Ni-Ge nanocrystals featuring an epitaxial relationship with the substrate Ni-Ge (101); (010) || Ge(001); (110).

Narrow intersubband transitions in n-type Ge/SiGe multi-quantum wells: Control of the terahertz absorption energy trough the temperature dependent depolarization shift

In this paper we present a detailed study of the intersubband absorption occurring between electron states confined in strained Ge multi-quantum wells as a function of the temperature. The high structural quality of the samples is reflected by the very narrow absorption line-shape constant with temperature. We observe a temperature driven charge transfer occurring between the ground and the first excited subband which, in turn, induces a change in the depolarization shift and consequently in the energy of the absorbance peak. The experimental observations are well accounted for by a multi-valley k·p model.