A hybrid TiO 2 -carbon nanofiber hierarchical nanostructure has been fabricated by the metal-organic chemical vapor deposition of a TiO 2 layer onto the vertically aligned carbon nanofiber array. As the deposition time increases from 10 to 60 min, the TiO 2 coating changes from the particulated conformal ultrathin film to a nanoneedle-like texture along the sidewall of the carbon nanofibers. X-ray diffraction indicates that the TiO 2 form anatase crystals with the coherent length over 50 nm, in good agreement with transmission electron microscopy images. Photoluminescence spectra are taken at both room temperature and 10 K. The absence of photoluminescence emission indicates that the electron-hole recombination in TiO 2 is completely quenched in such core-shell hybrid structure. The charge separation may be much more effective at the TiO 2 -carbon nanofiber heterojunction. The nanoneedle-like texture drastically increases the effective surface area of the TiO 2 while maintaining a much more effective electrical wiring through the highly conductive carbon nanofiber core. Such hierarchical architecture may be used as a novel anode material for dye-sensitized solar cells.

Photoluminescence spectroscopy was employed to probe the nature of optical transitions involving Al vacancy (VAl) and vacancy-oxygen complex (VAl-ON) in AlN. An emission line near 2 eV due to the recombination between the 2− charge state of (VAl-ON)2−/1−, and the valence band was directly observed under a below bandgap excitation scheme. This photoluminescence (PL) band was further resolved into two emission lines at 1.9 and 2.1 eV, due to the anisotropic binding energies of VAl-ON complex caused by two different bonding configurations–the substitutional ON sits along c-axis or sits on one of the three equivalent tetrahedral positions. Moreover, under an above bandgap excitation scheme, a donor-acceptor pair like transition involving shallow donors and (VAl-ON)2−/1− deep acceptors, which is the “yellow-luminescence” band counterpart in AlN, was also seen to split into two emission lines at 3.884 and 4.026 eV for the same physical reason. Together with previous results, a more complete picture for the optical transitions involving cation vacancy related deep centers in AlGaN alloy system has been constructed.

Photoluminescence (PL) emission spectroscopy was employed to probe the nature of deep center emissions in GaN. The room temperature PL spectrum measured in the infrared (IR) region revealed an emission band centered around 1.23 eV. Based on detailed analysis of both the IR and visible emission spectra, we suggest that this emission band is a band-to-impurity transition involving a deep level complex consisting of a gallium vacancy and an oxygen atom sitting on one of the neighboring nitrogen sites; the (VGa–ON)2− charge state of (VGa–ON)2−/1−. Two electronic structures, which arise due to two different configurations of (VGa–ON)2−/1−, with ON either along the c-axis (axial configuration) or in one of the three equivalent tetrahedral positions (basal configuration), were observed. Our result also provides explicit evidence that both the yellow luminescence band and the 1.23 eV emission line in GaN are related to a common deep center, which is believed to be (VGa–ON)2−/1−.

Mg-doped InxGa1−xN alloys were grown by metal organic chemical vapor deposition on semi-insulating c-GaN/sapphire templates. Hall effect measurements showed that Mg-doped InxGa1−xN epilayers are p-type for x up to 0.35. Mg-acceptor levels (EA) as a function of x, (x up to 0.35), were experimentally evaluated from the temperature dependent hole concentration. The observed EA in Mg-doped In0.35Ga0.65N alloys was about 43 meV, which is roughly four times smaller than that in Mg doped GaN. A room temperature resistivity as low as 0.4 Ω cm (with a hole concentration ∼5×1018 cm−3 and hole mobility ∼3 cm2/V s) was obtained in Mg-doped In0.22Ga0.78N. It was observed that the photoluminescence (PL) intensity associated with the Mg related emission line decreases exponentially with x. The Mg energy levels in InGaN alloys obtained from PL measurements are consistent with those obtained from Hall-effect measurements.

The yellow color of bulk AlN crystals was found to be caused by the optical absorption of light with wavelengths shorter than that of yellow. This yellow impurity limits UV transparency and hence restricts the applications of AlN substrates for deep UV optoelectronic devices. Here, the optical properties of AlN epilayers, polycrystalline AlN, and bulk AlN single crystals have been investigated using photoluminescence ͑PL͒ spectroscopy to address the origin of this yellow appearance. An emission band with a linewidth of ϳ0.3 eV ͑at 10 K͒ was observed at ϳ2.78 eV. We propose that the origin of the yellow color in bulk AlN is due to a band-to-impurity absorption involving the excitation of electrons from the valence band to the doubly negative charged state, ͑V Al 2− ͒, of isolated aluminum vacancies, ͑V Al ͒ 3−/2− described by V Al 2− +h =V Al 3− +h + . In such a context, the reverse process is responsible for the 2.78 eV PL emission.