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 anisotropic effective electron masses in wurtzite GaN are determined by generalized infrared spectroscopic ellipsometry. Nonpolar (112¯0) oriented thin films allow accessing both effective masses, m⊥* and m∥*, by determining the screened plasma frequencies. A n-type doping range up to 1.7 × 1020 cm−3 is investigated. The effective mass ratio m⊥*/m∥* is obtained with highest accuracy and is found to be 1.11 independent on electron concentration up to 1.2 × 1020 cm−3. For higher electron concentrations, the conduction band non-parabolicity is mirrored in changes. Absolute values for effective electron masses depend on additional input of carrier concentrations determined by Hall effect measurements. We obtain m⊥*=(0.239±0.004)m0 and m∥*=(0.216±0.003)m0 for the parabolic range of the GaN conduction band. Our data are indication of a parabolic GaN conduction band up to an energy of approximately 400 meV above the conduction band minimum.
The dielectric function (DF) of hexagonal AlN on Si (111) is determined in the range between 1 and 9.8 eV by spectroscopic ellipsometry (SE). Due to its large negative crytal-field splitting wurtzite AlN features large dichroism. Showing that SE is sensitive to both components of the DF around the absorption edge, a uniaxial model is applied which yields transition energies for the free excitonic state. The in-plane tensile stress leads to a red-shift of these transitions and to an enlarged splitting. The experimental data are compared to the results of band-structure calculations demonstrating excellent overall agreement. In addition, two high-energy critical points in the ordinary DF were determined at energies of about 7.75 and 8.85 eV.
We report on the implementation of InGaN/GaN-based light-emitting diodes (LEDs) structures on Si(1 1 0) oriented substrates grown by metal–organic vapour phase epitaxy. The total thickness of the completely crack-free device structures on Si(1 1 0) substrates amounts to 2.5 µm exhibiting a comparable or even better layer quality than identical LED structures grown on standard Si(1 1 1) substrates. This result can be explained by a more suited epitaxial relation between the c-plane of the high-temperature AlN seed layer and the Si(1 1 0) surface. The crystallographic structure of the LEDs was analysed by x-ray diffraction and the optical properties were investigated by photo- and electroluminescence. The improved crystallographic quality on Si(1 1 0) goes in line with a higher peak intensity in the photoluminescence measurements. Furthermore, a bright bluish light emission at 490 nm is obtained by an electrical excitation at room temperature.
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