We present a systematic study on the influence of epitaxial strain and hole concentration on the magnetic anisotropy in ͑Ga,Mn͒As at 4.2 K. The strain was gradually varied over a wide range from tensile to compressive by growing a series of ͑Ga,Mn͒As layers with 5% Mn on relaxed graded ͑In,Ga͒As/GaAs templates with different In concentration. The hole density, the Curie temperature, and the relaxed lattice constant of the as-grown and annealed ͑Ga,Mn͒As layers turned out to be essentially unaffected by the strain. Angledependent magnetotransport measurements performed at different magnetic-field strengths were used to probe the magnetic anisotropy. The measurements reveal a pronounced linear dependence of the uniaxial out-of-plane anisotropy on both strain and hole density. Whereas the uniaxial and cubic in-plane anisotropies are nearly constant, the cubic out-of-plane anisotropy changes sign when the magnetic easy axis flips from in-plane to out-of-plane. The experimental results for the magnetic anisotropy are quantitatively compared with calculations of the free energy based on a mean-field Zener model. Almost perfect agreement between experiment and theory is found for the uniaxial out-of-plane and cubic in-plane anisotropy parameters of the as-grown samples. In addition, magnetostriction constants are derived from the anisotropy data.

Selective-area epitaxy is used to form three-dimensional (3D) GaN structures providing semipolar crystal facets. On full 2-in. sapphire wafers we demonstrate the realization of excellent semipolar material quality by introducing inverse GaN pyramids. When depositing InGaN quantum wells on such a surface, the specific geometry influences thickness and composition of the films and can be nicely modeled by gas phase diffusion processes. Various investigation methods are used to confirm the drastically reduced piezoelectric polarization on the semipolar planes. Complete electrically driven light-emitting diode test structures emitting in the blue and blue/green spectral regions show reasonable output powers in the milliwatt regime. Finally, first results of the integration of the 3D structures into a conventional laser design are presented

We propose and realize a novel concept of a self-organized three-dimensional metamaterial with a plasma frequency in the visible regime. We utilize the concept of self-rolling strained layers to roll up InGaAs/GaAs/Ag multilayers with multiple rotations. The walls of the resulting tubes represent a radial superlattice with a tunable layer thickness ratio and lattice constant. We show that the plasma frequency of the radial superlattice can be tuned over a broad range in the visible and near infrared by changing the layer thickness ratio in good agreement with an effective metamaterial description. Finite difference time domain simulations reveal that the rolled-up radial superlattices can be used as hyperlenses in the visible.

Semipolar ð1122Þ oriented GaN has been grown on a prestructured r-plane sapphire substrate. By using silicon doped marker layers (MLs) we have been able to monitor the growth evolution of the stripes until coalescence. With that technique we correlated the growth type (direction) with the results of cathodoluminescence (CL) and transmission electron microscopy. Both characterization methods show only a few defects for the major part of the structure and a relatively high defect density for material grown in a-direction at one side of the stripes. It is shown that during coalescence these defects are mainly terminated resulting in a flat, planar ð1122Þ GaN layer with strongly reduced defect density. Additionally, X-ray diffraction (XRD) measurements show the high quality of these layers.

The longitudinal and transverse resistivities of differently strained ͑Ga,Mn͒As layers are theoretically and experimentally studied as a function of the magnetization orientation. The strain in the series of ͑Ga,Mn͒As layers is gradually varied from compressive to tensile using ͑In,Ga͒As templates with different In concentrations. Analytical expressions for the resistivities are derived from a series expansion of the resistivity tensor with respect to the direction cosines of the magnetization. In order to quantitatively model the experimental data, terms up to the fourth order have to be included. The expressions derived are generally valid for any single-crystalline cubic and tetragonal ferromagnet and apply to arbitrary surface orientations and current directions. The model phenomenologically incorporates the longitudinal and transverse anisotropic magnetoresistance as well as the anomalous Hall effect. The resistivity parameters obtained from a comparison between experiment and theory are found to systematically vary with the strain in the layer.