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Germanium self-assembled nanoislands and quantum dots are very prospective for CMOS-compatible optoelectronic integrated circuits but their photoluminescence (PL) intensity is still insufficient for many practical applications. Here, it is demonstrated experimentally that the PL of Ge nanoislands in silicon photonic crystal slabs (PCS) with hexagonal lattice can be dramatically enhanced due to the involvement in the emission process of the bounds states in the continuum. These high-Q photonic resonances allow to achieve PL resonant peaks with the quality factor as high as 2200 and with the peak PL enhancement factor of more than two orders of magnitude. The corresponding integrated PL enhancement is demonstrated to be more than one order of magnitude. This effect is studied theoretically by the Fourier modal method in the scattering matrix form. The symmetry of the quasi-normal guided modes in the PCS is described in terms of group theory. This work paves the way toward a new class of optoelectronic components compatible with silicon technology.

Luminescent properties of self-assembled Ge(Si)/SOI nanoislands embedded in twodimensional photonic crystal (PhC) slabs with and without L3 cavities were studied with PhC period a varied between 350 and 600 nm. For small periods (a£450 nm), the nanoisland luminescence, which spans over the wavelength range from 1.2 to 1.6 μm, overlaps with the PhC bandgap resulting in a coupling with the localized modes of an L3 cavity. It is shown that for larger periods (a>450 nm), nanoisland emission couples to the radiative modes above the bandgap located in the vicinity of the Г-point of the photonic crystal Brillouin zone and is characterized by the low group velocity. In this case, a significant (up to 35-fold) increase in the PL intensity was observed in a number of PhCs without a cavity. From a technological point of view, the latter result makes such types of photonic crystal structures particularly promising for the realization of Si-based light emitters operating in the telecommunication wavelength range because, firstly, their manufacture does not require a precise cavity formation and, secondly, they provide a much larger area for the radiating region, as compared with PhC cavities.

The dielectric function of nanocrystalline silicon (nc-Si) with crystallite size in the range of 1 to 3 nm has been determined by spectroscopic ellipsometry in the range of 1.5 to 5.5 eV. ATauc-Lorentz parameterization is used to model the nc-Si optical properties. The nc-Si dielectric function can be used to analyze nondestructively nc-Si thin films where nanocrystallites cannot be detected by x-ray diffraction and Raman spectroscopy. During the last few years, nanocrystalline silicon (nc-Si) films have received great attention in photovoltaics 1 and optoelectronics 2,3. Visible photoluminescence (PL) in the range from 770 to 880 nm, depending on crystallite size, has been reported 4 for nc-Si with a grain size in the range from 1 to 3 nm due to a quantum confinement effect, overcoming the impossibility of crystalline silicon (c-Si) to emit light because of its indirect band gap. Nanocrystals of silicon have been demonstrated recently to act as sensitizer for erbium ions (Er31) incorporated in a silicon-based matrix 5 , and to be the most efficient method for obtaining luminescence at 1.54 m. These applications have in common silicon nanocrystallites with a diameter lower than 3 nm. Optimization of devices requires thin-film nanostructure optimization, understanding of the correlation between the nanostructure and optical/electrical properties and, hence, detection of silicon nanocrystallites volume fraction and size distribution. Conventional structural diagnostics, such as Raman spectroscopy and x-ray diffraction (XRD) cannot detect nanocrystallites with a grain size lower than 50 Å 6. High resolution transmission electron microscopy (HRTEM) allows direct detection of very small nanocrystals, but it is destructive and not applicable as a routine analysis. In this letter, spectroscopic ellipsometry (SE) is demonstrated to be useful for nondestructive detection of silicon nanocrystals with a grain size well below 3 nm and for

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