Search

We report magnetoresistance measurements on epitaxial films of the intrinsic ferromagnetic semiconductor GdN electron doped with ∼ 10 20 cm − 3 to ∼ 10 21 cm − 3 . The magnetoresistance across the temperature range of 10–300 K is dominated by a reduction of spin-disorder scattering in the presence of a magnetic field, imposing a resistance reduction of 27% in a field of 8 T. We show that the magnetoresistance closely follows the magnetic disorder as signaled by the departure of the magnetization from its fully saturated value M s of 7 μ B / G d 3 + . [ABSTRACT FROM AUTHOR]

A blue light emitting diode (LED) is grown on top of a (Ga, In)N/GaN multiple quantum well (QW) acting as a light converter from blue to green-yellow wavelength. The blue light is produced by electrical injection, while the green-yellow emitting QWs are optically pumped by the blue photons. It is shown that the final color of the LED is strongly dependent on the blue pumping wavelength, the absorption and the internal quantum efficiency of the light converter. Depending on these parameters, blue to green LEDs or even white LEDs can be obtained. In addition, the injection current dependence of the LED electroluminescence is measured and analyzed. A very low blueshift is observed as a function of the injection current. It is explained by the fact that the carrier density per QW in the light converter stays relatively low compared to the case of classical current-injected green LEDs. [ABSTRACT FROM AUTHOR]

We show that the shape of GaN nanostructures grown by molecular beam epitaxy on AlxGa1-xN (0001) surfaces, for x≥0.4, can be controlled via the ammonia pressure. The nanostructures are obtained from a two dimensional to three dimensional transition of a GaN layer occurring upon a growth interruption. Atomic force microscopy measurements show that depending on the ammonia pressure during the growth interruption, dot or dash-shaped nanostructures can be obtained. Low temperature photoluminescence measurements reveal a large redshift in the emission energy of the quantum dashes, as compared to the quantum dots. By simply adjusting the GaN deposited thickness, it is shown that quantum dashes enable to strongly extend the emission range of GaN/Al0.5Ga0.5N nanostructures from the violet-blue (∼400–470 nm) to the green-orange range (∼500–600 nm). [ABSTRACT FROM AUTHOR]

A method is presented to achieve thick high quality crack-free AlGaN layers on GaN. This method uses jointly plastic relaxation and lateral growth. In a first step, plastic relaxation by cracking and misfit dislocation introduction is realized. Then the cracks are overgrown to obtain a smooth surface. By this reproducible technique, we grew smooth metal-organic chemical vapor deposition Al[sub 0.2]Ga[sub 0.8]N films with a threading dislocation density as low as 5×10[sup 8] cm[sup -2]. This result is the best ever reported for crack-free AlGaN growth over a large area. The control of the complete plastic relaxation opens up perspectives for the realization of high performance devices. In order to explain the mechanisms involved in the full relaxation of the AlGaN/GaN heterostructure, we propose a relaxation scheme and discuss its different steps. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The refractive indices of several Al[sub x]Ga[sub 1-x]N alloys deposited on silicon are determined by ellipsometry and reflectivity experiments at room temperature. The AlGaN layers are grown on (111)Si substrate by molecular-beam epitaxy on top of an AlN/GaN/AlN buffer in order to reduce the strain of the alloy. The Al composition is deduced from energy dispersive x-ray spectroscopy and photoluminescence experiments. The refractive index n and the extinction coefficient k are determined in the 300–600 nm range. For the transparent region of Al[sub x]Ga[sub 1-x]N, the refractive index is given in form of a Sellmeier law. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ammonia pressure is used for controlling the shape of GaN nanostructures grown by molecular beam epitaxy on [Al.sub.x][Ga.sub.1-x]N (0001) surfaces. The studies have shown that quantum dashes enable to strongly extend the emission range of GaN/[Al.sub.0.5][Ga.sub.0.5]N nanostructures from the violet-blue to the green-orange range by adjusting the GaN deposited thickness.

The refractive indices of several AI(sub x)Ga(sub 1)Ga(sub x-1)N alloys deposited on silicon are determined by ellipsometry and reflective experiments at room temperature. It is determined that the refractive indices of device quality, strain-compensated AIGaN layers on Si substrates with high accuracy, should permit the design and growth of vertical cavity structures and distributed Bragg reflectors on silicon substrates.

We report persistent photoconductivity in Mg-doped GdN thin films grown by molecular beam epitaxy. Temperature-dependent measurements were carried out in the time and frequency domains to probe the nature of Mg impurities in GdN. The results reveal an initial fast decay followed by a slow persistent photoconductivity. The magnitude of the photoconductivity as well as the characteristic fast- and slow-decay times was found to decrease systematically with increasing the Mg-doping level. Our experimental results suggest that Mg impurities in epitaxial GdN thin films act as acceptor-like centres. Interestingly they also show that the incorporation of Mg result in a significant decrease in the concentration of nitrogen vacancies, as is demonstrated also to be in agreement with an ab initio calculation. [ABSTRACT FROM AUTHOR]