Search

We combine density functional theory and many body perturbation theory to investigate the electronic properties of Si(100) and Ge(100) surfaces terminated with halogen atoms (-I, -Br, -Cl, -F) and other chemical functionalizations (-H, -OH, -CH3) addressing the absolute values of their work function, electronic affinity and ionization potential. Our results point out that electronic properties of functionalized surfaces strongly depend on the chemisorbed species and much less on the surface crystal orientation. The presence of halogens at the surface always leads to an increment of the work function, ionization potential and electronic affinity with respect to fully hydrogenated surfaces. On the contrary, the presence of polar -OH and -CH3 groups at the surface leads to a reduction of the aforementioned quantities with respect to the H-terminated system. Starting from the work functions calculated for the Si and Ge passivated surfaces, we apply a simple model to estimate the properties of functionalized SiGe surfaces. The possibility of modulating the work function by changing the chemisorbed species and composition is predicted. The effects induced by different terminations on the band energy line-up profile of SiGe surfaces are then analyzed. Interestingly, our calculations predict a type-II band offset for the H-terminated systems and a type-I band offset for the other cases.

An important challenge in the field of renewable energy is the development of novel nanostructured solar cell devices which implement low-dimensional materials to overcome the limits of traditional photovoltaic systems. For optimal energy conversion in photovoltaic devices, one important requirement is that the full energy of the solar spectrum is effectively used. In this context, the possibility of exploiting features and functionalities induced by the reduced dimensionality of the nanocrystalline phase, in particular by the quantum confinement of the electronic density, can lead to a better use of the carrier excess energy and thus to an increment of the thermodynamic conversion efficiency of the system. Carrier multiplication, i.e. the generation of multiple electron-hole pairs after absorption of one single high-energy photon (with energy at least twice the energy gap of the system), can be exploited to maximize cell performance, promoting a net reduction of loss mechanisms. Over the past fifteen years, carrier multiplication has been recorded in a large variety of semiconductor nanocrystals and other nanostructures. Owing to the role of silicon in solar cell applications, the mission of this review is to summarize the progress in this fascinating research field considering carrier multiplication in Si-based low-dimensional systems, in particular Si nanocrystals, both from the experimental and theoretical point of view, with special attention given to the results obtained by ab initio calculations.

Silicon nanocrystals and nanowires have been extensively studied because of their novel properties and their applications in electronic, optoelectronic, photovoltaic, thermoelectric and biological devices. Here we discuss results from ab initio calculations for undoped and doped Si nanocrystals and nanowires, showing how theory can aid and improve comprehension of the structural, electronic and optical properties of these systems., S. O. acknowledges support/funding from University of Modena and Reggio Emilia under project “FAR2017INTERDISC”. S. O. and I. M. thank the Super-Computing Interuniversity Consortium CINECA for support and high-performance computing resources under the Italian Super-Computing Resource Allocation (ISCRA) initiative, PRACE for awarding us access to the resource MARCONI HPC cluster based in Italy at CINECA. I. M. acknowledges support/funding from European Union H2020-EINFRA-2015-1 and H2020-INFRAEDI-2018-1 programs under grant agreement No. 676598 and No. 824143, project MaX-MAterials at the eXascale. M. A. greatly acknowledges the Transnational Access Programme of the HPC-EUROPA3 (project HPC17PB9IZ). Some of the high-performance computing (HPC) resources for this project were granted by the Institut du development et des ressources en informatique scientifique (IDRIS) under the allocation A0040910089 via GENCI (Grand Equipment National de Calcul Intensif). This work was supported by the ANR HEXSIGE project (ANR-17-CE030-0014-01) of the French Agence Nationale de la Recherche. M. P. acknowledges INFN for financial support through the National project Nemesys. We also acknowledge financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) and MICIU (Ministerio de Ciencia y Universidades) under Grants FEDER-MAT2017-90024-P, FIS2015-64886-C5-4-P and PGC2018-096955-B-C44-P, the Severo Ochoa Centres of Excellence Program under Grant SEV-2015-0496 and the Generalitat de Catalunya under Grant 2017 SGR 1506.

The formation energies of n- and p-type dopants in III–V arsenide and phosphide semiconductors (GaAs, GaP, and InP) are calculated within a first-principles total energy approach. Our findings indicate that—for all the considered systems—both the solubility and the shallowness of the dopant level depend on the crystal phase of the host material (wurtzite or zincblende) and are the result of a complex equilibrium between local structural distortion and electronic charge reorganization. In particular, in the case of acceptors, we demonstrate that impurities are always more stable in the wurtzite lattice with an associated transition energy smaller with respect to the zincblende case. Roughly speaking, this means that it is easier to p-type dope a wurtzite crystal and the charge carrier concentration at a given temperature and doping dose is larger in the wurtzite as well. As for donors, we show that neutral chalcogen impurities have no clear preference for a specific crystal phase, while charged chalcogen impurities favor substitution in the zincblende structure with a transition energy that is smaller when compared to the wurtzite case (thus, charge carriers are more easily thermally excited to the conduction band in the zincblende phase)., We acknowledge financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) under grants FEDER-MAT2017-90024-P and PGC2018-096955-B-C44, the Severo Ochoa Centres of Excellence Program under grant SEV-2015-0496, and the Generalitat de Catalunya under grants no. 2017 SGR 1506. We thank the Centro de Supercomputación de Galicia (CESGA) for the use of their computational resources. M.A. acknowledges the ANR HEXSIGE project (ANR-17-CE030-0014-01) of the French Agence Nationale de la Recherche. Part of the high-performance computing resources for this project were granted by the Institut du développement et des ressources en informatique scientifique (IDRIS) under the allocation A0040910089 via GENCI (Grand Equipement National de Calcul Intensif). S.O. acknowledges support/funding from the University of Modena and Reggio Emilia under project “FAR2017INTERDISC” G.G. acknowledges PRACE for awarding the access to the Marconi system based in Italy at CINECA and the Italian ISCRA program.We acknowledge financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) under grants FEDER-MAT2017-90024-P and PGC2018-096955-B-C44, the Severo Ochoa Centres of Excellence Program under grant SEV-2015-0496, and the Generalitat de Catalunya under grants no. 2017 SGR 1506. We thank the Centro de Supercomputación de Galicia (CESGA) for the use of their computational resources. M.A. acknowledges the ANR HEXSIGE project (ANR-17-CE030-0014-01) of the French Agence Nationale de la Recherche. Part of the high-performance computing resources for this project were granted by the Institut du développement et des ressources en informatique scientifique (IDRIS) under the allocation A0040910089 via GENCI (Grand Equipement National de Calcul Intensif). S.O. acknowledges support/funding from the University of Modena and Reggio Emilia under project “FAR2017INTERDISC” G.G. acknowledges PRACE for awarding the access to the Marconi system based in Italy at CINECA and the Italian ISCRA program.

In recent years,considerable efforts have been done to study silicon, germanium, and silicon/germanium slabs, nanocrystals, and nanowires for optoelectronics and photovoltaic solar energy applications. These zero- one- and two-dimensional systems, with sizes ranging from few to some tenths of nanometers, show unique electronic, optical, and transport properties that are intrinsically associated with their low dimensionality and to the quantum confinement effect. The possibility of understanding the microscopic properties of these systems and modulating their characteristics by doping and passivation can open new perspectives in the development of new, advanced, photovoltaics and optoelectronics devices. In this talk, we will discuss ab-initio theoretical results obtained by our group in the study of electronic, optical, and transport properties of silicon, germanium, and silicon/germanium low dimensional systems. The role played by size, passivation, and doping will be discussed. Moreover, we will show how the interaction between different nanostructures is a promising route to foster the establishment of third-generation photovoltaics.

We studied the physics of common p- and ntype dopants in hexagonal-diamond Si, a Si polymorph that can be synthesized in nanowire geometry without the need of extreme pressure conditions, by means of first-principles electronic structure calculations and compared our results with those for the well-known case of cubic-diamond nanowires. We showed that (i) as observed in recent experiments, at larger diameters (beyond the quantum confinement regime) p-type dopants prefer the hexagonaldiamond phase with respect to the cubic one as a consequence of the stronger degree of three-fold coordination of the former, while n-type dopants are at a first approximation indifferent to the polytype of the host lattice; (ii) in ultrathin nanowires, because of the lower symmetry with respect to bulk systems and the greater freedom of structural relaxation, the order is reversed and both types of dopant slightly favor substitution at cubic lattice sites; (iii) the difference in formation energies leads, particularly in thicker nanowires, to larger concentration differences in different polytypes, which can be relevant for cubic-hexagonal homojunctions; (iv) ultrasmall diameters exhibit, regardless of the crystal phase, a pronounced surface segregation tendency for p-type dopants. Overall these findings shed light on the role of crystal phase in the doping mechanism at the nanoscale and could have a great potential in view of the recent experimental works on group IV nanowires polytypes., M.A. greatly acknowledges the Transnational Access Programme of the HPC-EUROPA3 (project HPC17PB9IZ). Part of the high-performance computing (HPC) resources for this project were granted by the Institut du developpement et des ressources en informatique scientifique (IDRIS) under the allocation A0040910089 via GENCI (Grand Equipement National de Calcul Intensif). This work was supported by the ANR HEXSIGE project (ANR-17-CE030-0014-01) of the French Agence Nationale de la Recherche. We also acknowledge financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) under Grants FEDER-MAT2017-90024-P and FIS2015-64886-C5-4-P, the Severo Ochoa Centres of Excellence Program under Grant SEV-2015-0496, the Generalitat de Catalunya under Grants 2017 SGR 1506. S.O. acknowledges support/funding from University of Modena and Reggio Emilia under project ”FAR2017INTERDISC”.