Jean-Louis Lazzari

Theoretical study of defect impact on two-dimensional MoS2

Our theoretical findings demonstrate for the first time a possibility of band-gap engineering of monolayer MoS2 crystals by oxygen and the presence of vacancies. Oxygen atoms are revealed to substitute sulfur ones, forming stable MoS2−xOx ternary compounds, or adsorb on top of the sulfur atoms. The substituting oxygen provides a decrease of the band gap from 1.86 to 1.64 eV and transforms the material from a direct-gap to an indirect-gap semiconductor. The surface adsorbed oxygen atoms decrease the band gap up to 0.98 eV depending on their location tending to the metallic character of the electron energy bands at a high concentration of the adsorbed atoms. Oxygen plasma processing is proposed as an effective technology for such band-gap modifications.

Modification of band alignment at interface of AlyGa1-ySb/AlxGa1-xAs type-II quantum dots by concentrated sunlight in intermediate-band solar cells with separated absorption and depletion regions

We propose a new intermediate band GaAs solar cell comprising an AlxGa1-xAs absorber with built-in GaSb type-II quantum dots (QDs) [a gradual AlxGa1-xAs absorber with built-in AlyGa1-ySb QDs (0<x=y<0.40) as a variant] separated from the depletion region. We study the modification of the band alignment at type-II interface by two-photon absorption of concentrated sunlight. Our calculation shows that photogenerated carriers produce localized exciton-like electron-hole pairs spatially separated at QDs. Local field of such pairs may essentially modify potential barrier surrounding QDs, increase recombination lifetime of mobile carriers and additional photocurrent generated by two photon absorption. Concentration of about 300-sun pushes by 15% up the conversion efficiency as compared to the efficiency of the reference single junction GaAs solar cell without QDs.

Computer simulation of electronic and magnetic properties of ternary chalcopyrites doped with transition metals

Electronic and magnetic properties of BeSiAs2 and BeGeAs2 ternary compounds with chalcopyrite structure doped with transition metals (Mn, Cr) have been theoretically studied from the first principles. The influence of the substitutional positions of impurity atoms and their type on the appearance of a ferromagnetic (FM) or antiferromagnetic (AFM) state has been analyzed. It was found that magnetic moment of the systems does not depend strongly on the concentration and distance between impurity atoms, while the most important factors observed are the impurity type and substitution sites. Configurations with Mn atoms in the II-group sites are energetically stable in the AFM state, whereas Cr-doped ones seem to be in the FM state. Substitution of IV-group positions by both metals results preferably in the FM state, however these positions are not energetically favorable in comparison with II-group ones. The spin polarization of doped materials is evaluated and their possible application in spintronics is analyzed.

Band gap modifications of two-dimensional defected MoS2

The changes in structural and electronic properties, occurring in one monolayer of MoS2 at different concentrations of oxygen atoms doping and vacancies are investigated by means of ab initio computer simulation. The substitution of sulphur atoms by oxygen ones reduces the band gap for high concentrations only, transforming direct-gap semiconductor into an indirect one, whereas a smaller concentration of oxygen practically does not influence the gap. The presence of sulphur vacancies strongly reduces the band gap, leading to bands overlapping at high concentration and appearance of new bands at the gap region, which are determined by Mo 4d states with the mixture of S 3p states, at low concentrations.