Michele Virgilio

Type-I alignment and direct fundamental gap in SiGe based heterostructures

The electronic properties of strained Si1−xGex alloys epitaxially grown on (001) Si1−yGey relaxed substrates for any x and y Ge concentrations are presented here. Our calculations are based on an sp3d5s* nearest-neighbour tight-binding Hamiltonian and exploit appropriate scaling laws of the Hamiltonian interactions to account for strain effects. Spin–orbit interaction is also included in the Hamiltonian. We first provide the valence and conduction band offsets at the heterointerfaces between Si1−xGex and Si1−yGey, as well as the fundamental energy gap for Si1−xGex strained alloys. We are thus able to distinguish the region in the (x,y) plane where robust type-I alignment is achieved. Then this information on band alignment is exploited to propose a heterostructure which is both type I in -space and direct in -space. With this aim we adopt the decimation–renormalization method for the determination of the electronic properties of the multilayer structure; from the Greens function the energy spectrum and the partial and the total densities of states projected on each layer of the system are obtained. Our conclusion is that by suitable control of alloying, stress, band offsets and folding, truly direct (both in - and in -space) semiconducting heterostructures based on silicon and germanium can be realized. As an example, the case of pure Ge sandwiched between Si0.25Ge0.75 alloys, grown on a Si0.2Ge0.8 substrate, is fully discussed.

Photoluminescence, recombination rate, and gain spectra in optically excited n-type and tensile strained germanium layers

We theoretically investigate the optical properties of photo-excited biaxially strained intrinsic and n-type doped Ge semi-infinite layers using a multi-valley effective mass model. Spatial inhomogeneity of the excess carrier density generated near the sample surface is considered. Strain effects on the band edges, on the band dispersions, and on the orbital compositions of the near gap states involved in radiative recombinations are fully taken into account. We obtain, as a function of the distance from the sample surface, the energy resolved absorption/gain spectra resulting from the contribution of the radiative direct and phonon-assisted band-to-band transitions and from the intra-band free carrier absorption. Photoluminescence spectra are calculated from the spatially dependent spontaneous radiative recombination rate, taking into account energy-dependent self-absorption effects. For suitable combinations of doping density, strain magnitude, pump power, and emitted photon polarization, we find gain v...

Conduction intersubband transitions at normal incidence in Si1?xGex quantum well devices

We show theoretically that it is possible to design SiGe-based quantum well structures in which conduction intersubband transitions are induced by normal incidence infrared radiation. A sp(3)d(5)s(*) tight binding model has been adopted to evaluate the electronic states and optical transitions between lowest conduction confined states of a superlattice composed of one pure Ge quantum well separated by SiGe alloys, grown along the [001] direction. We find that significant optical coupling between confined states in the Ge wells is achieved at normal incidence radiation by the off-diagonal elements of the mass tensor. The minimum energy Ge conduction valleys are, in fact, tilted with respect to the [001] growth axis. For comparison we show that no such coupling can be realized for the conduction states confined in a similar structure composed by Si quantum wells because the ellipsoids of the lowest conduction valleys are oriented along the growth direction.

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.

Valence and conduction intersubband transitions in SiGe, Ge-rich, quantum wells on [001] Si0.5Ge0.5 substrates: A tight-binding approach

Electronic and optical properties of germanium-rich Si∕SiGe quantum wells grown on Si0.5Ge0.5 substrates are investigated by a nearest neighbor tight-binding Hamiltonian. The basis set includes spds* orbitals with both spin states. Appropriate scaling laws account for strain effects. We present full electronic band structure calculations both for valence and conduction bands. Confinement effects on the electronic states are considered in detail. Optical spectra related to hole and electron intersubband transitions are derived. Our results for optical absorption due to valence intersubband transitions show excellent agreement with experimental spectra and previous k∙p calculations. For the same quantum well samples, spectra due to conduction intersubband absorption are provided here.

Strain-modulated Ge superlattices

We present a numerical study of the electronic and optical properties of a model single-element superlattice made of a periodic sequence of relaxed and strained regions of a germanium crystal, realized by means of an externally applied strain. We adopt the tight-binding model to evaluate the strain-driven modifications of the band structure and the optical properties. Superlattice band gaps, spatial confinement of near-gap valence and conduction states, and analysis of their symmetry character, have been obtained for different superlattice periodicities and strain intensities. Our results indicate that, for suitable choices of spatial periodicity and strain values, type-I and direct-gap superlattices, with strong dipole matrix elements, can be realized. Conceptually, we demonstrate that Ge single-element strained superlattices could be active materials for novel Si-compatible optical devices.

Terahertz intersubband transitions in the conduction band of Ge/SiGe multi quantum wells

We present intersubband absorption in the conduction band of compressively strained germanium quantum wells bounded by Ge-rich SiGe barriers. The measured absorption energies are in the 5–12 THz range depending on the well width. These results may be relevant for the design of SiGe-based THz Quantum Cascade emitters.