Papers by Author: Adam Gali

Abstract: A set of lines in the photoluminescence spectra of 4H-, 6H-, and 15R-SiC in the near-infrared are attributed to Nb-related defects on the ground of doping experiments conducted with 4H-SiC. A model based on a an exciton bound at the Nb-centre in an asymmetric split vacancy configuration at a hexagonal site is proposed, which explains the structure of the luminescence spectrum and the observed Zeeman splitting of the lines.

Abstract: Molecule-sized fluorescent emitters are much sought-after to probe biomolecules in living cells. We demonstrate here by time-dependent density functional calculations that the experimentally achievable 1-2 nm sized silicon carbide nanocrystals can emit light in the nearinfrared region after introducing appropriate color centers in them. These near-infrared luminescent silicon carbide nanocrystals may act as ideal fluorophores for in vivo bioimaging.

Abstract: Isolated point defects possessing high spin ground state and below-band-gap excitation may play a key role in realizing solid state quantum bits in semiconductors which are the basic building blocks of quantum computers. Silicon vacancy in silicon carbide provides these features making it a feasible candidate in this special and emerging field of science. However, it has been not clarified what is the exact nature of the luminescence of silicon vacancy detected in hexagonal polytypes. This is the first crucial step needed to understand this basic defect in silicon carbide. We report density functional theory based calculations on silicon vacancy defect. Based on the obtained results we identify the silicon vacancy related photoluminescence signals with the negatively charged defect.

Abstract: We have investigated the absorption of 0.9, 1.4 nm silicon carbide nanoparticles (SiC NPs) by time-dependent density functional calculations, focusing on the effect of different oxygen adsorbates of the surface. We have found that negatively charged Si-O−, Si-COO− defects dramatically lower the optical gap of SiC NPs. Our findings can help interpret recent controversary experiments on colloidal SiC NPs.

Abstract: The electronic structure and absorption spectrum of hydrogenated silicon carbide nanocrystals (SiC NC) have been determined by first principles calculations. We show that the reconstructed surface can significantly change not just the onset of absorption but the shape of the spectrum at higher energies. We compare our results with two recent experiments on ultrasmall SiC NCs.

Abstract: Large-scale and gap error free calculations of the electronic structure of vacancies in 4H-SiC have been carried out using a hybrid density functional (HSE06) and an accurate charge correction scheme. Based on the results the carbon vacancy is proposed to be responsible for the Z1/2 and EH6/7 DLTS centers.

Abstract: A brief overview about the recent progress in developing the methods to calculate the properties of defects in solids is given and some recent examples on vacancy-related defects in SiC are presented.

Abstract: In this paper we revisit sharp low temperature luminescence lines (LTPL) previously generated by high dose 1018 to 1020 cm-2 electron beams in an electron microscope and now produced by low dose 1015 cm-2 electron, 5x1010 cm-2 proton and helium ion irradiation. New no phonon lines E0, F0, θ0, Φ0, K0, G0, J0, M0 and phonon replicas are found. Phonon replicas up to the fifth harmonic are well accounted for by theory giving convincing new evidence that the di-carbon antisite is responsible for these deep defect lines.

Abstract: We investigate the neutral divacancy in SiC by means of first principles calculations and group theory analysis. We identify the nature of the PL transitions associated with this defect. We show that how the spin state may be manipulated optically in this defect.

Abstract: Defects introduced by electron irradiation at ~80-100 K in 3C-, 4H- and 6H-SiC were studied by electron paramagnetic resonance (EPR). A number of EPR spectra, labelled LE1-10, were detected. Combining EPR and supercell calculations, we will show that the LE1 center in 3C-SiC with C2v symmetry and an electron spin S=3/2 is related to the (VSi-Sii)3+ Frenkel pair between the silicon vacancy and a second neighbour Sii interstitial along the <100> direction. Results on other centers, possibly also related to interstitials, are discussed.