Proximity Effects and the Generalized Ginzberg-Landau Equation

A computer program has been developed which finds solutions of the generalized Ginzburg-Landau equation subject to de Gennes\u27s boundary conditions. The resulting information on the spatially varying order parameter is then interpreted vis-a-vis experimental data on both superconductor-insulator-normal-metal —superconductor Josephson tunneling systems, and ultrasonic-surface-wave attenuation

Direct-Gap Photoluminescence from a Si-Ge Multilayer Super Unit Cell Grown on Si0.4Ge0.6

Both Si and Ge possess indirect band gaps, which makes them very inefficient light emitters. One way to overcome this limitation is through band gap engineering. In this regard, M. d’Avezac et al. [Phys. Rev. Lett., 108, 027401 (2012)] predicted that a strained SiGe2Si2Ge2SiGen super unit cell on Si0.4Ge0.6 would have a direct and dipole-allowed gap of 0.863 eV, which is ideally suited for optical fiber applications. Here we report on the epitaxial growth of such a structure and its optical properties, for which purpose two similar samples were prepared by molecular beam epitaxy and solid phase epitaxy. Photoluminescence (PL) spectra were obtained at low temperatures (6–25 K) with excitation at wavelengths of 405 and 458 nm, selected to emphasize the light emission from the sample superstructure. A strong low-energy PL quadruplet is seen, with peaks near 727, 758, 792 and 822 meV at 6 K, together with a much weaker peak at 871 MeV. The ratio of intensities of the strong and weak peaks is the same in both samples. The weak peak at 871 meV is assigned to the dipole-allowed direct-gap transition associated with the super unit cell. The four strong peaks are attributed to dislocation related emission lines of the thick relaxed Si0.4Ge0.6 transition layer on Si

Bright photoluminescence from ordered arrays of SiGe nanowires grown on Si(111)

We report on the optical properties of SiGe nanowires (NWs) grown by molecular beam epitaxy (MBE) in ordered arrays on SiO2/ Si(111) substrates. The production method employs Au catalysts with self-limited sizes deposited in SiO2-free sites opened-up in the substrate by focused ion beam patterning for the preferential nucleation and growth of these well-organized NWs. The NWs thus produced have a diameter of 200 nm, a length of 200 nm, and a Ge concentration x = 0.15. Their photoluminescence (PL) spectra were measured at low temperatures (from 6 to 25 K) with excitation at 405 and 458 nm. There are four major features in the energy range of interest (980\u20131120 meV) at energies of 1040.7, 1082.8, 1092.5, and 1098.5 meV, which are assigned to the NW-transverse optic (TO) Si\u2013Si mode, NW-transverse acoustic (TA), Si\u2013substrate\u2013TO and NW-no-phonon (NP) lines, respectively. From these results the NW TA and TO phonon energies are found to be 15.7 and 57.8 meV, respectively, which agree very well with the values expected for bulk Si1 12xGex with x = 0.15, while the measured NW NP energy of 1099 meV would indicate a bulklike Ge concentration of x = 0.14. Both of these concentrations values, as determined from PL, are in agreement with the target value. The NWs are too large in diameter for a quantum confinement induced energy shift in the band gap. Nevertheless, NW PL is readily observed, indicating that efficient carrier recombination is occurring within the NWs.Peer reviewed: YesNRC publication: Ye