Florestano Evangelisti

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.

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.

Single-electron transistor based on modulation-doped SiGe heterostructures

We report the characterization of a single-electron transistor based on bended wires fabricated on modulation-doped SiGe two-dimensional electron gas. Electrical measurements show a diamond-shaped stability plot and a nonperiodic sequence of conductance peaks. The device behavior suggests the presence of disorder-induced multiple islands along the wire. Conductance oscillations remain well pronounced above liquid helium temperature.

Spectroscopic ellipsometric study of the size evolution of Ge islands grown on Si (100)

In this article we discuss the use of spectroscopic ellipsometry for an in situ and real time probe of three-dimensional self-organized Ge island growth on Si (100) surfaces. We will show that atomic force microscopy and x-ray photoemission spectroscopy can be combined with spectroscopic ellipsometry to give information on the size and shape evolution of the Ge islands as well as on the amount of Ge deposited on the Si surface.

Detection of terahertz radiation by AlGaN/GaN field-effect transistors

High electron mobility transistors can work as room-temperature direct detectors of radiation at frequency much higher than their cutoff frequency. One open issue is how the radiation couples to the sub-wavelength transistor channel. Here, we studied the coupling of radiation to an AlGaN/GaN transistor with cut-off frequency of 30 GHz. Local irradiation with a Free Electron Laser source at 0.15 THz allowed us to selectively couple the signal to the channel through one transistor terminal at a time. Far-field experiments at 0.15–0.94 THz were also performed in order to study the nonlinear properties of the transistor channel.

Investigation of Ge on Si(100) quantum wells by photoelectron spectroscopies

In the present work we report on the evolution of the electronic states as a function of Ge overlayer thickness ranging from a fraction of a monolayer to few tens of monolayers. The Ge states above the Si valence band top could be clearly detected at every overlayer thickness d. For d ∼6u2009A, i.e., at the critical thickness for the transition from strained to unstrained overlayers. For these larger thicknesses, the Ge edge becomes broad and featureless. The energy separation between the Ge onset and the Si valence top increases up to 0.70 eV at larger overlayer thicknesses.

Terahertz electrodynamics in high electron-mobility transistors

Plasmon current, charge oscillations, and associated electromagnetic fields in the THz range of a two-dimensional electron gas forming the channel of a high electron mobility transistor have been analysed by solving the full set of Euler, continuity, and Maxwell equations. Upon imposing boundary conditions, a finite-length channel behaves as a plasmonic cavity exhibiting strong resonance effects when the wavelength is commensurate with the channel length. It is shown that, if the relaxation time is not exceedingly short, the resonance effects should survive and be detectable. The presence of a metallic gate has a strong effect on the longitudinal waves, by changing both dispersion relation and propagation velocity for modes of wavelength larger than the gate-channel separation. Furthermore, gate and channel behave as a planar transmission line that readily transfers the gate-source signal to the gate-drain load impedance. Finally, it is suggested that the non-linear nature of the Euler equation can explai...

X-ray scanning microscope study of strain instabilities in low mismatched SiGe alloys grown on Si(001) substrates

An X-ray scanning microscope study of self-organized morphological defects arising during the growth of thick SiGe virtual substrates grown on Si(001) surface is reported. The 200 nm microscope resolution permitted to probe the local alloy composition. The measured variations of the Ge content in the alloy demonstrate that defect formation and evolution represent a strain relief mechanism. The observed large stress concentration inside the defect and the absence of the cross-hatch pattern on its internal facets are findings in close analogy with the lateral strain relaxation of self-organized islands in heterostructures.

Photoluminescence of strained and relaxed multilayered Ge islands on Si(001)

In this letter, we report on the optical properties of multistacked Ge quantum dots grown on Si(001). We demonstrate that the two emission bands observed in the photoluminescence spectra are related to the presence of strained and relaxed islands. Within both types of islands, the experimental findings support the hypothesis of the presence of quantum confining regions whose dimensions are governed by the strain relaxation process.

Defects in SiGe virtual substrates for high mobility electron gas

We have investigated the morphology defects, in particular, pits present in compositionally graded SiGe thick films heteroepitaxially grown on Si(001) surfaces. Besides the pits clearly related to impurities and interface defects, a class of pits intrinsic to the strained-layer growth has been found. Two stable and one metastable pit shapes have been identified that can be related to the alloy film thickness and pit dimension. The influence of the pits on the mobility of the two dimensional electron gas grown on these kinds of substrates has been also investigated.