We report the growth of GaN films on the Si(111) substrate by metalorganic chemical vapour phase deposition (MOCVD). Different buffer layers were used to investigate their effects on the structural and optical properties of GaN layers. A series of GaN layers were grown on Si(111) with different buffer layers and buffer thicknesses and were characterized by Nomarski microscopy, atomic force microscopy, high-resolution x-ray diffraction (XRD) and photoluminescence (PL) measurements. We first discuss the optimization of the LT-AlN/HT-AlN/Si(111) templates and then the optimization of the graded AlGaN intermediate layers. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.6µm. The XRD and PL measurements results confirmed that a wurtzite GaN was successfully grown. The resulting GaN film surfaces were flat, mirror-like and crack-free. The mosaic structure in the GaN layers was investigated. With a combination of Williamson-Hall measurements and the fitting of twist angles, it was found that the buffer thickness determines the lateral coherence length, vertical coherence length, as well as the tilt and twist of the mosaic blocks in GaN films. The PL spectra at 8K show that a strong band edge photoluminescence of GaN on Si (111) emits light at an energy of 3.449eV with a full width at half maximum (FWHM) of approximately 16meV. At room temperature, the peak position and FWHM of this emission become 3.390eV and 58meV, respectively. The origin of this peak was attributed to the neutral donor bound exciton. It was found that the optimized total thickness of the AlN and graded AlGaN layers played a very important role in the improvement of quality and in turn reduced the cracks during the growth of GaN/Si(111) epitaxial layers. (Some figures in this article are in colour only in the electronic version) J. Phys. D: Appl. Phys. 41 (2008) 155317 E Arslan et al 2

This study investigated the effects of three bleaching agents (Whiteness Perfect, Whiteness Super, and Whiteness HP) on the color change and refractive index of three dental composites (Admira, Durafill VS, and Gradia Direct). Twenty diskshaped specimens (10×2 mm) of each composite were prepared and divided into four subgroups (n=5). An unbleached group was used as a control, while the remaining specimens in the three subgroups were bleached with one of the bleaching agents respectively. Color change was assessed according to CIELAB color system and refractive indices were determined by phase modulated spectroscopic ellipsometry. Color differences between bleaching and baseline value (ΔE) were less than 3.3 for all groups. However, bleaching with Whiteness HP led to noticeable color changes for Admira and Durafill VS. While this agent had no effect on the refractive indices of these composites, the other two agents containing carbamide peroxide increased their refractive indices. Therefore, results suggested that replacement of such composite restorations may be required after bleaching.

The current-transport mechanisms in ͑Ni/ Au͒ -Al 0,22 Ga 0,78 N / AlN/ GaN heterostructures were studied by using temperature dependent forward-bias current-voltage ͑I-V͒ characteristics in the temperature range of 80-410 K. In order to determine the current mechanisms for ͑Ni/ Au͒ -Al 0,22 Ga 0,78 N / AlN/ GaN heterostructures, we fitted the experimental I-V data to the analytical expressions given for the current-transport mechanisms in a wide range of applied biases and at different temperatures. The contributions of thermionic-emission, generation-recombination, tunneling, leakage currents that are caused by inhomogeneities, and defects at the metal-semiconductor interface current mechanisms were all taken into account. The best fitting results were obtained for the tunneling current mechanism. On the other hand, we did not observe sufficient agreement between the experimental data and the other current mechanisms. The temperature dependencies of the tunneling saturation current ͑I t ͒ and tunneling parameters ͑E 0 ͒ were obtained from fitting results. We observed a weak temperature dependence of the saturation current and the absence of the temperature dependence of the tunneling parameters in this temperature range. The results indicate that in the temperature range of 80-410 K, the mechanism of charge transport in the ͑Ni/ Au͒ −Al 0.22 Ga 0.78 N / AlN/ GaN heterostructure is performed by tunneling among those dislocations intersecting the space charge region. The dislocation density ͑D͒ that was calculated from the I-V characteristics, according to a model of tunneling along the dislocation line, gives the value of 0.24ϫ 10 7 cm −2 . This value is close in magnitude to the dislocation density that was obtained from the x-ray diffraction measurements.

We conducted the present study to design and manufacture a semi-transparent organic solar cell (ST-OSC). First, we formed a transparent top contact as MoO3/Ag/MoO3 in a dielectric/metal/dielectric (DMD) structure. We performed the production of an FTO/ZnO/P3HT:PCBM/MoO3/Ag/MoO3 ST-OSC by integrating MoO3/Ag/MoO3 (10/$$d_{m}$$ d m /$$d_{{od}}$$ d od nm) instead of an Ag electrode in an opaque FTO/ZnO/P3HT:PCBM/MoO3/Ag (–/40/130/10/100 nm) OSC, after theoretically achieving optimal values of optical and electrical parameters depending on Ag layer thickness. The transparency decreased with the increase of $$d_{m}$$ d m values for current DMD. Meanwhile, maximum transmittance and average visible transmittance (AVT) indicated the maximum values of over 92% for $$d_{m} ~$$ d m  = 4 and 8 nm, respectively. For ST-OSCs, the absorption and reflectance increased in the visible region by a wavelength of longer than 560 nm and in the whole near-infrared region by increasing $$d_{m}$$ d m up to 16 nm. Moreover, in the CIE chromaticity diagram, we reported a shift towards the D65 Planckian locus for colour coordinates of current ST-OSCs. Electrical analysis indicated the photogenerated current density and AVT values for $$d_{m} = 6$$ d m = 6  nm as 63.30 mA/cm2 and 38.52%, respectively. Thus, the theoretical and experimental comparison of optical and electrical characteristics confirmed that the manufactured structure is potentially conducive for a high-performance ST-OSC.

The forward current-voltage-temperature characteristics of (Ni/Au)-Al0.83In0.17N/AlN/GaN heterostructures were studied in a temperature range of 80-375 K. The temperature dependences of the tunneling saturation current (It) and tunneling parameters (E0) were obtained. Weak temperature dependence of the saturation current and the absence of temperature dependence of the tunneling parameters were observed in this temperature range. The results indicate that in the temperature range of 80-375 K, the mechanism of charge transport in the (Ni/Au)-Al0.83In0.17N/AlN/GaN heterostructure is performed by tunneling among dislocations intersecting the space-charge region. A model is used for nonuniform tunneling along these dislocations that intersect the space-charge region. The dislocation density that was calculated from the current-voltage characteristics, according to a model of tunneling along the dislocation line, gives the value 7.4 × 10 8 cm −2. This value is close in magnitude to the dislocation density that was obtained from the x-ray diffraction measurements value of 5.9 × 10 8 cm −2. These data show that the current flows manifest a tunneling character, even at room temperature.