The transport characteristics of graphene devices with low n‐ or p‐type carrier density (∼1010–1011 cm‐2), fabricated using a new process that results in minimal organic surface residues, are reported. The p‐type molecular doping responsible for the low carrier densities is initiated by aqua regia. The resulting devices exhibit highly developed ν = 2 quantized Hall resistance plateaus at magnetic field strengths of less than 4 T.

We report a firm evidence of enhanced luminescence from InGaN-like clusters in InxAlyGa1−x−yN quaternary alloys. Photoluminescence (PL) and Raman scattering measurements have been employed to study the optical properties of these alloys. The excellent correlation between the phonon replica structures accompanying luminescence line and the observed InGaN-related phonon modes in Raman spectra provide a powerful evidence showing that the existence of InGaN-like clusters is responsible for the enhanced luminescence in InxAlyGa1−x−yN quaternary alloys. In addition, the dependence of the PL emission energy on temperature in the low-temperature regime and on excitation power density can also be explained consistently with recombination mechanisms involving the localized states attributed to InGaN-like cluster size fluctuations.

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PACS : 78.20.Ci; 78.55.Cr; 78.67.De We have investigated the crystal orientation dependence of optical properties in In x Ga 1--x N/GaN multiple quantum wells. The spectral peak and intensity of the micro-photoluminescence emission for different crystal orientations were found to have sixfold symmetry. Quite interestingly, the refractive index obtained from the interference pattern, also varies with the crystal orientation. The 60 degree periodic anisotropy of electronic transitions as well as optical parameters were interpreted in terms of the formation of hexagonal truncated pyramidal In-rich clusters. Our result provides an alternative solution to improve the designs of photonic and electronic devices based on nitride semiconductors.InGaN compound semiconductor is one of the potential materials for optical devices [1, 2], due to its tunability of the energy gap covering the visible to near ultraviolet light spectra. According to previous calculation [3,4], it has been shown that the properties of wurtzite GaN-based materials strongly depend on the crystal orientation. Therefore, the investigation of the dependence of physical properties on crystal orientation not only can establish the fundamental understanding of GaN-based materials, but it also provides an alternative solution to improve the properties of the associated devices. Such devices include field effect transistors (FETs), optical waveguides, LDs, and photodetectors. For example, the hole effective mass of wurtzite GaN-based materials is much heavier than that of conventional zinc-blende materials such as GaAs. Thus, the threshold current density of wurtzite-GaN quantum-well lasers is intrinsically much higher than that of GaAs QW lasers. It will be a great advantage if one can choose a particular orientation which has a small effective mass and optical gain as predicted theoretically by Niwa et al. [4]. However, there are only a rather limited experimental works devoted to the study of the dependence of GaN-based materials on crystal orientation [5]. The aim of the present work is to develop the simple and nondestructive PL method to detect the crystal anisotropy and to establish the crystal orientation effects on optical properties in In x Ga 1--x N/GaN multiple quantum wells (MQWs).The samples studied in this work were prepared by low-pressure metalorganic chemical vapor deposition. A series of 10 periods of 20 A thick In 0.22 Ga 0.78 N wells and 90 A thick GaN barriers were grown on (0001) sapphire. The multiple-quantum-well layers were sandwiched with 6 mm n-GaN layer and a 0.2 mm p-GaN capping layers, which is phys.