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GaN x As 1−x samples with x<3% grown by metalorganic vapor phase epitaxy were studied by low-temperature photoluminescence under hydrostatic pressure and photomodulated reflectance spectroscopy. The transformation from N acting as an isoelectronic impurity to N-induced band formation takes place at x≈0.2%. The N level does not shift with respect to the valence band edge of GaNxAs1−x. Concentration as well as hydrostatic-pressure dependence of the GaNxAs1−x bands can be described by a three band kp description of the conduction band state E− and E+ and the valence band at k=0. The model parameters for T<20 and T=300 K were determined by fitting the model to the experimental data. Modeling the linewidth of the E− transition by combining the kp model and ion statistics leads to the conclusion that the electron-hole pairs are strongly localized.

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We report a cross-correlated investigation, performed by means of Raman scattering and infrared spectroscopy, of coupled LO phonon-plasmon modes in bulk GaN. Using different samples with different (high) residual concentrations of free carriers, we find that the high-energy Raman mode follows closely the plasma frequency resolved from the infrared data. On the opposite, the low-frequency modes appears down shifted, with respect to the standard TO phonon frequency, by about 11 cm−1. Both findings agree satisfactorily with predictions of the linear response theory for undamped phonon-plasmon modes and establish Raman scattering as a powerful and nondestructive tool to investigate the residual doping level of GaN up to about 1020 cm−3 .

Phys. Rev. Lett. 87, 292 (1976

We present an investigation of large, isolated, graphene ribbons grown on the C-face of on-axis semi-insulating 6H-SiC wafers. Using a graphite cap to cover the SiC sample, we modify the desorption of the Si species during the Si sublimation process. This results in a better control of the growth kinetics, yielding very long (about 300 µm long, 5 µm wide), homogeneous monolayer graphene ribbons. These ribbons fully occupy unusually large terraces on the step bunched SiC surface, as shown by AFM, optical microscopy and SEM. Raman spectrometry indicates that the thermal stress has been 1

We have investigated the temperature dependence of the fundamental absorption edge of a series ofHg 1 _ x Cd x Te alloys (with composition x ranging from 0.5 to 1). Analyzing our data in the light of the three-dimensional theory of direct-allowed excitons, we find precise .values for the fundamental r -r 6 interband transition energy in a temperature range extendmg from o to 300 K. All experi~ental results, including previous data for HgTe and mercury-rich Hg! _ x Cd x Te alloys, are well accounted for using a simple empirical formula:This expression, which is valid for all compositions O

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