E. Bustarret

OPTICAL CONSTANTS OF EPITAXIAL ALGAN FILMS AND THEIR TEMPERATURE DEPENDENCE

We have studied the dependence of the absorption edge and the refractive index of wurtzite AlxGa1−xN films on temperature and composition using transmission and photothermal deflection spectroscopy. The Al molar fraction of the AlxGa1−xN films grown by plasma induced molecular beam epitaxy was varied through the entire range of composition (0⩽x⩽1). We determined the absorption edges of AlxGa1−xN films and a bowing parameter of 1.3±0.2 eV. The refractive index in the photon energy range between 1 and 5.5 eV and temperatures between 7 and 295 K was deduced from the interference fringes. The static refractive index n(0) changed from 2.29 for GaN to 1.96 for AlN at room temperature. A variation of temperature from 295 to 7 K resulted in a decrease of refractive index (at photon energies close to the band gap) by 0.05±0.01 and in an energy shift of the absorption edge of about 64±5 meV independent of the Al content of the films. Using the Kramers–Kronig dispersion relation and an approximation for the dispersion coefficient for photon energies near the band gap, the refractive index could be described as a function of photon energy, Al content, and temperature.

Experimental determination of the nanocrystalline volume fraction in silicon thin films from Raman spectroscopy

The effect of crystallite sizes L smaller than 100 nm on the integrated Raman cross section Σc of the transverse optical (TO) mode of fcc silicon was studied experimentally in fully nanocrystallized thin films. The Σc/Σa (amorphous) ratio of this mode is shown to be 1 up to L=30 A, and to decay exponentially down to 0.1 at larger L. A systematic procedure taking into account both this effect and the experimental optical absorption coefficient αexp at the excitation wavelength is then proposed for the determination of the crystalline volume fraction in mixed phase (amorphous/nanocrystalline) silicon systems by Raman measurements.

Superconductivity in doped cubic silicon

Although the local resistivity of semiconducting silicon in its standard crystalline form can be changed by many orders of magnitude by doping with elements, superconductivity has so far never been achieved. Hybrid devices combining silicon’s semiconducting properties and superconductivity have therefore remained largely underdeveloped. Here we report that superconductivity can be induced when boron is locally introduced into silicon at concentrations above its equilibrium solubility. For sufficiently high boron doping (typically 100 p.p.m.) silicon becomes metallic. We find that at a higher boron concentration of several per cent, achieved by gas immersion laser doping, silicon becomes superconducting. Electrical resistivity and magnetic susceptibility measurements show that boron-doped silicon (Si:B) made in this way is a superconductor below a transition temperature Tc ≈ 0.35 K, with a critical field of about 0.4 T. Ab initio calculations, corroborated by Raman measurements, strongly suggest that doping is substitutional. The calculated electron–phonon coupling strength is found to be consistent with a conventional phonon-mediated coupling mechanism. Our findings will facilitate the fabrication of new silicon-based superconducting nanostructures and mesoscopic devices with high-quality interfaces.

Dependence of the superconducting transition temperature on the doping level in single crystalline diamond films.

Homoepitaxial diamond layers doped with boron in the 10(20)-10(21) cm(-3) range are shown to be type II superconductors with sharp transitions (approximately 0.2 K) at temperatures increasing from 0 to 2.1 K with boron contents. The critical concentration for the onset of superconductivity in those 001-oriented single-crystalline films is about 5-7 10(20) cm(-3). The H-T phase diagram has been obtained from transport and ac-susceptibility measurements down to 300 mK.

Superconducting group-IV semiconductors.

Despite the amount of experimental and theoretical work on doping-induced superconductivity in covalent semiconductors based on group IV elements over the past four years, many open questions and puzzling results remain to be clarified. The nature of the coupling (whether mediated by electronic correlation, phonons or both), the relationship between the doping concentration and the critical temperature (T(c)), which affects the prospects for higher transition temperatures, and the influence of disorder and dopant homogeneity are debated issues that will determine the future of the field. Here, we present recent achievements and predictions, with a focus on boron-doped diamond and silicon. We also suggest that innovative superconducting devices, combining specific properties of diamond or silicon with the maturity of semiconductor-based technologies, will soon be developed.

Visible light emission at room temperature from partially oxidized amorphous silicon

Abstract In situ boron-doped amorphous hydrogenated silicon films deposited on conductive substrates have been anodized in hydrofluoric acid solutions and subsequently electrochemically oxidized in an aqueous electrolyte. The resulting layers yield a visible luminescence at room temperature similar to that of p-type crystalline porous silicon. Moreover , as-deposited silicon-rich amorphous oxinitrides are also shown to yield a strong visible room-temperature photoluminescence. The incidence of these experimental results on the current debate about the microscopic origin of visible light emission from silicon at room-temperature is discussed.

How to grow cubic GaN with low hexagonal phase content on (001) SiC by molecular beam epitaxy

Molecular beam epitaxy (MBE) of cubic GaN on SiC films deposited by chemical vapor deposition on Si has been investigated by reflection high-energy electron diffraction, x-ray diffraction, photoluminescence, and micro-Raman spectroscopy. The wurtzite/zinc-blende ratio, indicative of the material quality, has been found to depend on both the initial substrate roughness and the N/metal ratio impinging on the surface. The results were consistently analyzed by assuming that the MBE growth of cubic GaN is mainly governed by the impinging active N flux, which directly determines the mean-free path of Ga adatoms.Molecular beam epitaxy (MBE) of cubic GaN on SiC films deposited by chemical vapor deposition on Si has been investigated by reflection high-energy electron diffraction, x-ray diffraction, photoluminescence, and micro-Raman spectroscopy. The wurtzite/zinc-blende ratio, indicative of the material quality, has been found to depend on both the initial substrate roughness and the N/metal ratio impinging on the surface. The results were consistently analyzed by assuming that the MBE growth of cubic GaN is mainly governed by the impinging active N flux, which directly determines the mean-free path of Ga adatoms.

Hydrogen-acceptor interactions in diamond

Abstract Hydrogen-acceptor interactions are investigated in boron-doped diamond through deuterium diffusion experiments followed by SIMS measurements and through infrared absorption spectroscopy. From deuterium diffusion, we show that BD interactions can be properly demonstrated in low compensation B-doped homoepitaxial layers. However, the presence of defects in such layers strongly affects this interaction. The degree of passivation of boron acceptors by deuterium depends on the diffusion temperature. At 550°C or below, the B and D concentrations exactly match giving rise to a complete disappearance of the absorption bands related to the electronic transitions of neutral boron acceptors. Under thermal annealing above 500°C, (B,D) pairs dissociate and neutral boron acceptors recover. At deuterium diffusion temperatures of 700°C, the B passivation is absent.

Hall electron mobility in diamond

The low field Hall mobility of electron in diamond was investigated from room temperature to 873K, both experimentally and theoretically. The acoustic deformation potential for electron scattering is determined by fitting of theoretical calculations to experimental data for high quality {111} homoepitaxial phosphorus-doped diamond films. A good agreement is obtained over a large doping range. A discrepancy for the electron mobilities measured under optical excitation is discussed. The maximum Hall mobility achievable at thermodynamical equilibrium in low compensated n-type diamond is shown to be close to 1000cm2∕Vs at room temperature.

Tunneling spectroscopy and vortex imaging in boron-doped diamond.

We present the first scanning tunneling spectroscopy study of single-crystalline boron-doped diamond. The measurements were performed below 100 mK with a low temperature scanning tunneling microscope. The tunneling density of states displays a clear superconducting gap. The temperature evolution of the order parameter follows the weak-coupling BCS law with Delta(0)/kBTc approximately 1.74. Vortex imaging at low magnetic field also reveals localized states inside the vortex core that are unexpected for such a dirty superconductor.