Giuseppe Grosso

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.

Tight-binding calculation of optical gain in tensile strained [001]-Ge/SiGe quantum wells

It is known that under a tensile strain of about 2% of the lattice constant, the energy of the bottom conduction state of bulk Ge at the Gamma point falls below the minimum at the L point, leading to a direct gap material. In this paper we investigate how the same condition is realized in tensile strained Ge quantum wells. By means of a tight-binding sp(3)d(5)s(*) model, we study tensile strained Ge/Si(0.2)Ge(0.8) multiple quantum well (MQW) heterostructures grown on a relaxed SiGeSn alloy buffer along the [001] direction. We focus on values of the strain fields at the crossover between the indirect and direct gap regime of the MQWs, and calculate band edge alignments, electronic band structures, and density of states. We also provide a numerical evaluation of the MQW material gain spectra for TE and TM polarization under realistic carrier injection levels, taking into account the leakages related to the occupation of the electronic states at the L point. The analysis of the different orbital contributions to the near-gap states of the complete structure allows us to give a clear interpretation of the numerical results for the strain-dependent TM/TE gain ratio. Our calculations demonstrate the effectiveness of the structures under consideration for light amplification.

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 band intersubband transitions in Ge/SiGe quantum wells

In this letter we present the experimental evidence of 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 terahertz range and are interpreted by means of tight binding calculations which include self-consistent band-bending and depolarization effects. From the comparison of experimental and numerical results a conduction band offset along the L line of about 120 meV has been estimated for the studied heterostructures.

Long intersubband relaxation times in n-type germanium quantum wells

We measured the non-radiative intersubband relaxation time in n-type modulation-doped Ge/SiGe multi-quantum wells of different thickness by means of degenerate pump-probe experiments. The photon energy was tuned to be resonant with the lowest conduction band intersubband transition energy (14-29 meV), as measured by terahertz absorption spectroscopy and in agreement with band structure calculations. Temperature-independent lifetimes in excess of 30 ps were observed.

Electronic conductance of one-dimensional chains with phonon dephasing disorder

In this paper we analyse how electron transport through a one-dimensional chain is modified by the presence of phonon dephasing mechanisms active in a limited strand of the chain. The treatment is based on the nonequilibrium Keldysh Greens function and the self-consistent first Born approximation, with a tight-binding description of the electronic states. A most remarkable feature of the calculated conductance curves is the occurrence of an exponential decrease for small lengths, followed by a slow asymptotic decrease inversely proportional to the strand length. The origin of such a different short-range and long-range behaviour of the conductance, and other observed scaling features, are interpreted with some intuitive understanding of the dephasing mechanisms.

Optical gain in short period Si/Ge superlattices on [001]-SiGe substrates

Results are here presented for the electronic band structure and the material gain of selected Si/Ge superlattices grown on Ge and SiGe buffers along the [001] direction. The chosen superlattices were proposed in the literature as promising direct-gap candidates based on group IV materials. A sp3d5s* tight-binding model has been adopted for the evaluation of the bands and the material gain in the presence of realistic charge injection levels and for different polarizations of the radiation associated with the direct transitions. For the superlattices studied here, we show that in the most favorable case (Si2/Ge14) the peak gain values are only a factor of 6 weaker than the corresponding value obtained for a typical III-V direct gap bulk crystal.

Base and salt 3D forms of Emeraldine II polymers by Car–Parrinello molecular dynamics

Abstract We have studied structural and electronic properties of the three-dimensional crystalline regions of Emeraldine II polymers, in the base (EB-II) and salt (ES-II) forms, by means of first principle Car–Parrinello molecular dynamics. We compare the geometrical structures of the polymer chains in the primitive cells of EB-II and ES-II, pointing out the structural effects due to the protonation with HCl of the iminic nitrogens in the EB-II chains, and the effect of the counterions between neighboring chains. We also analyze the HOMO electron density distribution, band structure and density of states of the resulting bipolaronic structure of ES-II, which is energetically stable and maintains semiconductor character.

Current distribution and conductance quantization in the integer quantum Hall regime

Charge transport of a two-dimensional electron gas in the presence of a magnetic field is studied by means of the Keldysh–Green function formalism and the tight-binding method. We evaluate the spatial distributions of persistent (equilibrium) and transport (nonequilibrium) currents, and give a vivid picture of their profiles. In the quantum Hall regime, we find exact conductance quantization both for persistent currents and for transport currents, even in the presence of impurity scattering centres and moderate disorder.

7th General Conference of the Condensed Matter Division of the European Physical Society

The annual General Conference of the Condensed Matter Division of the European Physical Society has grown into a leading forum for discussion and exchange of the latest scientific developments. The conference series started seven years ago at the initiative of J T Devreese with the Antwerp Conference; it continued with the conferences in Manchester (1982), Lausanne (1983), The Hague (1984), Berlin (1985) and Stockholm (1986). The 7th General Conference was held in Pisa from 7 to 10 April 1987; it was attended by about 900 participants, coming from more than 30 countries of Europe and overseas. The present volume, in two issues T19A and T19B, collects the papers orally presented at the Pisa Conference. The scientific programme contained 90 invited speakers, distributed in plenary lectures, symposia and individual invited talks. The presentation of the papers in this volume follows the pattern of the Conference as closely as possible. The recent explosion of interest in High Temperature Superconductivity motivated, besides the plenary lecture in the programme, the last minute inclusion of an open symposium on this subject. More than forty research groups contributed to this event. The titles of the contributions and the affiliation of the speakers have been included at the end of volume 19B for the benefit of interested readers. Numerous contributions (more than 500), ranging over the entire field of Condensed Matter Physics, were presented in poster sessions. Several poster contributions at the conference (as well as a number of the contributions to the Symposium on High Temperature Superconductivity) will appear as regular articles in Physica Scripta. These articles will later be collected in a Reprint Series of Physica Scripta. We wish to thank the Conference Committees and the individual people who helped in the choice of such stimulating material, and all the contributors to this volume for their cooperation in the timely preparation of manuscripts. A special acknowledgement is also due to the Sponsors, whose generous help has made this manifestation possible.