Ye Weng

Photoluminescence from silicon nitride—no quantum effect

Silicon nitride compounds emit photoluminescence all over the visible range. Recent studies ascribed this luminescence to quantum-size effects within silicon nanocrystals that were either shown or assumed to form inside the silicon nitride matrix; the luminescence of the matrix itself was ignored. In contrast, observing the same luminescence even without the presence of silicon crystallites, our work identifies the silicon nitride matrix itself as responsible for the photoluminescence. All experimental observations are well explained by band tail luminescence from the silicon matrix. In contrast to the silicon nanocrystal approach, our model explains all aspects of the luminescence. As a consequence, we conclude that silicon nitride films are inappropriate if one aims at investigating photoluminescence from silicon nanocrystals within such a matrix.

A Natural Topological Insulator

The earths crust and outer space are rich sources of technologically relevant materials which have found application in a wide range of fields. Well-established examples are diamond, one of the hardest known materials, or graphite as a suitable precursor of graphene. The ongoing drive to discover novel materials useful for (opto)electronic applications has recently drawn strong attention to topological insulators. Here, we report that Kawazulite, a mineral with the approximate composition Bi2(Te,Se)2(Se,S), represents a naturally occurring topological insulator whose electronic properties compete well with those of its synthetic counterparts. Kawazulite flakes with a thickness of a few tens of nanometers were prepared by mechanical exfoliation. They exhibit a low intrinsic bulk doping level and correspondingly a sizable mobility of surface state carriers of more than 1000 cm(2)/(V s) at low temperature. Based on these findings, further minerals which due to their minimized defect densities display even better electronic characteristics may be identified in the future.

Photoluminescence from AlxIn1−xN layers doped with Tb3+ ions

AlxIn1−xN films (0 ≤ x ≤ 1) have been sputter deposited and annealed, both without and with terbium co-doping, to obtain a series of matrices whose band gap energies span the range from around 2 eV to 6 eV. The terbium green luminescence spectra (excitation wavelength 230 nm, i.e. 5.4 eV) are measured at room temperature as a function of the aluminium content (band gap route) and of the terbium concentration (concentration route). The green luminescence assumes a maximum of the integrated intensity at a band gap energy of 4.1 eV (x = 0.7) which can be argued to result from a resonant energy transfer from the host matrix into the Tb luminescence centres. Furthermore, the dependence of this maximum integrated intensity as a function of the Tb concentration, i.e. of the average distance between the Tb centres, suggests the energy transfer from the host material into the Tb luminescent centres to be due to dipole-dipole interaction via an exciton bound to the centre.

Evidence for resonant energy transfer in terbium-doped (Al,In)N films

Measurements of the characteristic luminescence green line family of Tb3+ embedded in magnetron sputtered and annealed AlxIn1−xN films (0 ≤ x ≤ 1) indicate an intensity maximum at around the composition xAl = 0.7 (band gap energy 4.1 eV). The results are described by resonant excitation via excitons bound to the Tb atoms starting from the RESI model (Lozykowski and Jadwisienczak, phys. stat. sol. (b) 244 (2007) 2109).