1,721,063 research outputs found
Tight-binding approach to electronic and optical properties of strained SiGe quantum wells
No full textValley splitting and optical intersubband transitions at parallel and normal incidence in [001]-Ge/SiGe quantum wells
No full textWe investigate intervalley splitting in the conduction band of strained [001]-Ge quantum well (QW) systems with finite SiGe alloy barriers by means of a sp3d5s∗ tight-binding Hamiltonian. We find that interaction between germanium bulk L minima splits each confined subband into a doublet. We first characterize this splitting as a function of the well width and of the strength of a uniform electric field superimposed along the growth direction. Varying the well width, an oscillating behavior of the splitting magnitude similar to that predicted for Si QW systems is observed and explained. Then we focus on the optical intersubband transitions occurring between states belonging to the fundamental and the first-excited doublets. Selection rules for intersubband transitions at normal and parallel incidence are discussed exploiting the parity character of the involved doublet states. Numerical results for infrared-absorption spectra, evaluated for both symmetric and biased Ge QWs, support our findings
Type-I alignment and direct fundamental gap in SiGe based heterostructures
No full textThe 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 Green's 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
Conduction intersubband transitions at normal incidence in Si1-xGex quantum well devices
No full textWe 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 sp3d5s* 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
Strain-modulated Ge superlattices
No full textWe 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
Optical transitions between valley split subbands in biased Si quantum wells
No full textBy a tight-binding sp3d5s* model, we study numerically the optical transitions involving the lowest conduction states confined in strained [001] Si quantum wells. These states belong to the fundamental and to the first excited quantum well (QW) subbands, each one split into a doublet by intervalley interaction. Both hard wall and finite SiGe barriers boundary conditions for the QWs are considered. Amplitudes of the doublet splittings as a function of the well width and of a uniform electric field superimposed along the growth direction are first investigated. Then, we study atomic contributions and parity character of the doublet wave functions to derive selection rules for interdoublet optical transitions. Finally, we demonstrate the role of intervalley coupling and the effectiveness of the selection rules here presented, for the interpretation of the absorption spectrum of a n-type Si QW between SiGe barriers, evaluated at different temperatures
Valley splitting and selection rules for inter-doublets optical transitions in strained [001]-Si/SiGe heterostructures
No full textBy an sp3d5s* tight-binding model we investigate temperature and electric field effects on the optical intersubband transitions between valley split conduction states confined in strained Si/SiGe [0 0 1]-quantum wells. By the same model we analyze the symmetry of the confined states and deduce selection rules for the involved optical transitions. The selection rules here provided predict specific signatures in the intersubband absorption spectra which can be tuned by proper control of thermal population of the states and by changes in the intervally coupling induced by perpendicular electric fields
Optical spin orientation in strained Ge/SiGe quantum wells: A tight-binding approach
No full textWe present a theoretical investigation of electron-spin optical orientation in strained Ge/SiGe quantum wells. The atomistic sp3d5s∗ nearest-neighbor tight-binding model adopted, allows us to obtain the spin polarization of the excited electrons, as a function of frequency and direction of the incident radiation, in the presence of external fields, different strain conditions, and taking into account contributions arising from states out of the Brillouin zone center. Orbital projected densities of states further highlight spin mixing of the localized spin-orbitals composing the states involved in the optical transitions. Our results illustrate the potential of the adopted method as a theoretical tool to support experiments on optical manipulation of spins in group IV based heterostructures
Quantum-confined Stark effect in Ge/SiGe quantum wells: A tight-binding description
No full textWe present a tight-binding study of the strong quantum confined Stark effect (QCSE) involving direct transitions in Ge∕SiGe quantum wells. Our aim is to provide a theoretical and numerical description of the experimental results by Kuo et al. Nature 437 1334 (2005) by means of a tight-binding model. In the presence and in the absence of external electric fields, we are able to assign of the states involved in the observed transitions. Oscillator strengths for normal and parallel incident radiation are evaluated. In particular, the genuine direct transitions in the Ge region, at the Γ point, and the direct transitions coming from states along the Δ lines folded at Γ, are discriminated; their energy shift as a function of a superimposed field is evaluated, and their role in the QCSE is evidenced. Excitonic effects below and above the interband threshold are also included in our calculations; they contribute to a close reproduction of the experimental absorption spectra for different superimposed uniform electric fields
Confinement and interwell coupling effects in Ge double quantum wells pseudomorphic to a Si(001) substrate
No full textWe present a theoretical study of the electronic properties of two coupled Ge quantum wells separated by a variable number of Si monolayers, epitaxially grown on a Si (001) substrate. We adopt the real space decimation-renormalization method and a sp3d5s* nearest neighbors tight binding Hamiltonian for the description of the electronic states of bulk Si and Ge crystals. Strain, band offsets and spin-orbit interactions are properly taken into account. From the Green’s function of the multilayer structure considered, the energy spectrum and partial and total densities of states projected on each layer and orbital are obtained. This has allowed us to investigate the nature of the valence and conduction confined states and the effect of interwell coupling on the optical properties. In particular we show that position and red shift with Ge width of the experimental no phonon luminescence line of these structures are well interpreted by the present calculation and that additional luminescence lines are predicted
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