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

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.

In order to determine the reverse-bias leakage current mechanisms in Schottky diodes on Al0.83In0.17N/AlN/GaN heterostructures, the temperature-dependent current-voltage measurements were performed in the temperature range of 250–375 K. In this temperature range, the leakage current was found to be in agreement with the predicted characteristics, which is based on the Frenkel–Poole emission model. The analysis of the reverse current-voltage characteristics dictates that the main process in leakage current flow is the emission of electrons from a trapped state near the metal-semiconductor interface into a continuum of states which associated with each conductive dislocation.

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.

Graphene as transparent electrode for direct observation of hole photoemission from silicon to oxide Appl. Phys. Lett. 102, 123106 (2013) Temperature dependent thermal conductivity of a free-standing graphene nanoribbon Appl. Phys. Lett. 102, 111911 (2013) Directional quantum transport in graphyne p-n junction J. Appl. Phys. 113, 073710 (2013) Charge transport in lightly reduced graphene oxide: A transport energy perspective J. Appl. Phys. 113, 063710 (2013) Effect of chiral property on hot phonon distribution and energy loss rate due to surface polar phonons in a bilayer graphene J. Appl. Phys. 113, 063705 (2013) Additional information on J. Appl. Phys. Shubnikov-de Haas (SdH) and Hall effect measurements performed in a temperature range between 1.8 and 275 K, at an electric field up to 35 kV m À1 and magnetic fields up to 11 T, have been used to investigate the electronic transport properties of monolayer graphene on SiC substrate. The number of layers was determined by the use of the Raman spectroscopy. The carrier density and in-plane effective mass of electrons have been obtained from the periods and temperature dependencies of the amplitude of the SdH oscillations, respectively. The effective mass is in good agreement with the current results in the literature. The two-dimensional (2D) electron energy relaxations in monolayer graphene were also investigated experimentally. The electron temperature (T e ) of hot electrons was obtained from the lattice temperature (T L ) and the applied electric field dependencies of the amplitude of SdH oscillations. The experimental results for the electron temperature dependence of power loss indicate that the energy relaxation of electrons is due to acoustic phonon emission via mixed unscreened piezoelectric interaction and deformation-potential scattering.