Excitonic giant Zeeman effect in GaN:Mn^3+

We describe a direct observation of the excitonic giant Zeeman splitting in (Ga,Mn)N, a wide-gap III-V diluted magnetic semiconductor. Reflectivity and absorption spectra measured at low temperatures display the A and B excitons, with a shift under magnetic field due to s,p-d exchange interactions. Using an excitonic model, we determine the difference of exchange integrals between Mn^3+ and free carriers in GaN, N_0(alpha-beta)=-1.2 +/- 0.2 eV. Assuming a reasonable value of alpha, this implies a positive sign of beta which corresponds to a rarely observed ferromagnetic interaction between the magnetic ions and the holes.Comment: 4 pages, 4 figure

Growth and optical properties of GaN/AlN quantum wells

We demonstrate the growth of GaN/AlN quantum well structures by plasma-assisted molecular-beam epitaxy by taking advantage of the surfactant effect of Ga. The GaN/AlN quantum wells show photoluminescence emission with photon energies in the range between 4.2 and 2.3 eV for well widths between 0.7 and 2.6 nm, respectively. An internal electric field strength of $9.2\pm 1.0$ MV/cm is deduced from the dependence of the emission energy on the well width.Comment: Submitted to AP

Local boron doping quantification in homoepitaxial diamond structures

The capability of transmission electronmicroscopy (TEM) using the high angle annular dark fieldmode (HAADF,also labelled Z-contrast) to quantify boron concentration, in the high doping range between 1019cm−3 and 1021cm−3, is demonstrated. Thanks to the large relative variation of atomic number Z between carbon and boron, doping concentration maps and profiles are obtained with a nanometer-scale resolution. A novel numerical simulation procedure allows the boron concentration quantification and demonstrates the high sensitivity and spatial resolution of the technique.4 page

Paramagnetic gold in a highly disordered Au-Ni-O alloy

Magnetic materials are usually classified into a distinct category such as diamagnets, paramagnets or ferromagnets. The enormous progress in materials science allows one nowadays, however, to change the magnetic nature of an element in a material. Gold, in bulk form, is traditionally a diamagnet. But in a ferromagnetic environment, it can adopt an induced ferromagnetic moment. Moreover, the growth of gold under certain conditions may lead to a spontaneous ferromagnetic or paramagnetic response. Here, we report on paramagnetic gold in a highly disordered Au–Ni–O alloy and focus on the unusual magnetic response. Such materials are mainly considered for plasmonic applications. Thin films containing Au, Ni and NiO are fabricated by co-deposition of Ni and Au in a medium vacuum of 2 × 10−2 mbar. As a result, Au is in a fully disordered state forming in some cases isolated nanocrystallites of up to 4 nm in diameter as revealed by high resolution transmission electron microscopy. The disorder and the environment, which is rich in oxygen, lead to remarkable magnetic properties of Au: an induced ferromagnetic and a paramagnetic state. This can be proven by measuring the x-ray magnetic circular dichroism. Our experiments show a way to establish and monitor Au paramagnetism in alloys

Waveguide saturable absorbers at 1.55 μm based on intraband transitions in GaN/AlN QDs

We report on the design, fabrication and optical characterization of GaN/AlN quantum-dot-based waveguides for all-optical switching via intraband absorption saturation at 1.55 μm. The transmittance of the TMpolarized light increases with the incident optical power due to the saturation of the s-p z intraband absorption in the QDs. Single-mode waveguides with a ridge width of 2 μm and a length of 1.5 mm display 10 dB absorption saturation of the TM-polarized light for an input pulse energy of 8 p J and 150 fs. © 2013 Optical Society of America.This work was supported by Spanish Government Project TEC2012-37958-C02-01, Comunidad de Madrid Project S2009/ESP-178, and the EU Marie Curie IEF ‘SolarIn’ (#331745), and EU ERC-StG ‘TeraGaN’ (#278428) projects.Peer Reviewe

Strain Distribution in GaN/AlN Quantum-Dot Superlattices

The two-dimensional strain distribution in a GaN/AlN quantum-dot (QD) superlattice is measured from high-resolution transmission electron microscopy images using the geometrical phase analysis. The results are compared to elastic theoretical calculations using a combination of Fourier transform and Green's function techniques. The GaN/AlN system appears to be a model system for a comparison between theory and experiments as interdiffusion between GaN and AlN is negligible. We verify that for the case of a three-dimensional system, such as a QD, the biaxial strain approximation is not valid. Furthermore, we demonstrate that the presence of QDs induces a modulation in the strain state of the AlN barriers which is the driving force for the vertical alignment of the GaN QDs in the AlN matrix.</p

Spontaneous Transformation of Thin Films into ZnO Nanowire Arrays with High Structural and Optical Quality

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