Timo Aschenbrenner

Highly ordered catalyst-free and mask-free GaN nanorods on r-plane sapphire.

Self-organized and highly ordered GaN nanorods were grown without catalyst on r-plane sapphire using a combination of molecular beam epitaxy and metal-organic vapor-phase epitaxy. AlN nucleation centers for the nanorods were prepared by nitridation of the sapphire in a metal-organic vapor-phase epitaxy reactor, while the nanorods were grown by molecular beam epitaxy. A coalesced two-dimensional GaN layer was observed between the nanorods. The nanorods are inclined by 62 degrees towards the [Formula: see text]-directions of the a-plane GaN layer. The high degree of ordering and the structural perfection were confirmed by micro-photoluminescence measurements.

Optical and structural characterization of AlInN layers for optoelectronic applications

Al1−xInxN layers with an indium content between x=10.5% and x=24% were grown by metal-organic vapor-phase epitaxy and characterized concerning their optical, structural and morphological properties with regard to the realization of optoelectronic devices. The indium content and the strain of these layers were measured by high resolution x-ray diffraction. Ellipsometric measurements were used to determine the optical constants [refractive index n(λ) and extinction coefficient κ(λ)] in dependence of wavelength and indium content. The values determined for the electronic bandgaps are in good agreement with theoretical predictions and previous publications on this topic but are more focused on AlInN layers which are pseudomorphically grown on GaN. A bowing parameter of b=10.3±0.1 was determined for fully strained layers with an indium content between 13% and 24%. In order to investigate the suitability of these layers for use in distributed Bragg reflectors, the surface morphology is characterized with respec...

Light-emitting diode based on mask- and catalyst-free grown N-polar GaN nanorods.

We report on the fabrication of a light-emitting diode based on GaN nanorods containing InGaN quantum wells. The unique system consists of tilted N-polar nanorods of high crystalline quality. Photoluminescence, electroluminescence, and spatially resolved cathodoluminescence investigations consistently show quantum well emission around 2.6 eV. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy measurements reveal a truncated shape of the quantum wells with In contents of (15 ± 5)%.

Polytypism in GaAs nanowires: determination of the interplanar spacing of wurtzite GaAs by X-ray diffraction

In GaAs nanowires grown along the cubic [111]c direction, zinc blende and wurtzite arrangements have been observed in their stacking sequence, since the energetic barriers for nucleation are typically of similar order of magnitude. It is known that the interplanar spacing of the (111)c Ga (or As) planes in the zinc blende polytype varies slightly from the wurtzite polytype. However, different values have been reported in the literature. Here, the ratio of the interplanar spacing of these polytypes is extracted based on X-ray diffraction measurements for thin GaAs nanowires with a mean diameter of 18-25 nm. The measurements are performed with a nano-focused beam which facilitates the separation of the scattering of nanowires and of parasitic growth. The interplanar spacing of the (111)c Ga (or As) planes in the wurtzite arrangement in GaAs nanowires is observed to be 0.66% ± 0.02% larger than in the zinc blende arrangement.

Influence of static atomic displacements on composition quantification of AlGaN/GaN heterostructures from HAADF-STEM images.

In an earlier publication Rosenauer et al. introduced a method for determination of composition in AlGaN/GaN heterostructures from high-angle annular dark field (HAADF) images. Static atomic displacements (SADs) were neglected during simulation of reference data because of the similar covalent radii of Al and Ga. However, SADs have been shown (Grillo et al.) to influence the intensity in HAADF images and therefore could be the reason for an observed slight discrepancy between measured and nominal concentrations. In the present study parameters of the Stillinger-Weber potential were varied in order to fit computed elastic constants, lattice parameters and bonding energies to experimental ones. A reference data set of HAADF images was simulated, in which the new parameterization was used to account for SADs. Two reference samples containing AlGaN layers with different Al concentrations were investigated and Al concentrations in the layers determined based on the new data set. We found that these concentrations were in good agreement with nominal concentrations as well as concentrations determined using alternative techniques such as strain state analysis and energy dispersive X-ray spectroscopy.

Optical properties of InGaN quantum dots in monolithic pillar microcavities

The integration of InGaN quantum dots into GaN-based monolithic microcavities grown by metal-organic vapor-phase epitaxy is demonstrated. Microphotoluminescence spectra reveal distinct spectrally sharp emission lines around 2.73 eV, which can be attributed to the emission of single InGaN quantum dots. The samples are structured into airpost pillar microcavities. The longitudinal and transversal mode spectra of these cavities are in good agreement with theoretical calculations based on a vectorial transfer-matrix method. Quality factors up to Q=280 have been achieved.

Blue monolithic II-VI-based vertical-cavity surface-emitting laser

We report on laser operation of optically pumped, fully epitaxial blue vertical-cavity surface-emitting lasers. The ZnSe-based structures posses a bottom and top distributed Bragg reflector consisting of ZnMgSSe as high-index material and a short-period superlattice of MgS/ZnCdSe as low-index material. The cavity has an optical thickness of λ and contains three ZnSe quantum wells surrounded by ZnMgSSe barriers. To illustrate the specific issues related to this demanding material combination, we compare two epitaxial structures of different crystalline quality. A minimum threshold of 5 pJ is observed for laser emission at 443 nm comparable to values reported for nitride based vertical-cavity surface-emitting lasers.

A structural investigation of highly ordered catalyst-?and mask-free GaN nanorods

GaN nanorods were grown on r-plane sapphire substrates by a two-step approach. Nucleation sites for the nanorods were provided by the formation of AlN islands during nitridation in a metal organic vapor phase system. These islands are a-plane oriented as expected for nitride growth on r-plane sapphire. The nanorods themselves were grown by plasma assisted molecular beam epitaxy. The nanorods show an inclination towards the surface normal of 28.3° and are highly ordered. Studies with high resolution x-ray diffraction polar plots reveal the epitaxial relationship between the substrate and nanorods as a c-direction growth on inclined m-plane facets of the nitridated islands. The determined lattice constants show nanorods which are strain free. The growth direction of the nanorods has been confirmed in a transmission electron microscope by convergent beam electron diffraction patterns to be in the N-polar [Formula: see text] direction.

TEM characterization of catalyst- and mask-free grown GaN nanorods

Catalyst- and mask-free grown GaN nanorods have been investigated using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy filtered transmission electron microscopy (EFTEM). The nanorods were grown on nitridated r-plane sapphire substrates in a molecular beam epitaxy reactor. We investigated samples directly after the nitridation and after the overgrowth of the structure with GaN. High resolution transmission electron microscopy (HRTEM) and EFTEM revealed that AlN islands have formed due to nitridation. After overgrowth, the AlN islands could not be observed any more, neither by EFTEM nor by Z-contrast imaging. Instead, a smooth layer consisting of AlGaN was found. The investigation of the overgrown sample revealed that an a-plane GaN layer and GaN nanorods on top of the a-plane GaN have formed. The nanorods reduced from top of the a-plane GaN towards the a-plane GaN/sapphire interface suggesting that the nanorods originate at the AlN islands found after nitridation. However, this could not be shown unambiguously. The number of threading dislocations in the nanorods was very low. The analysis of the epitaxial relationship to the a-plane GaN showed that the nanorods grew along the [000-1] direction, and the [1-100] direction of the rods was parallel to the [0001] direction of the a-plane GaN.

Trap levels in the atomic layer deposition-ZnO/GaN heterojunction—Thermal admittance spectroscopy studies

In this work, a n-ZnO/p-GaN heterojunction is analyzed using admittance spectroscopy techniques. Capacitance transient measurements performed at 10 kHz reveal four majority-carrier deep levels, the most important one located at approximately 0.57 eV below the ZnO conduction band (CB) edge with a density about two orders of magnitude below the doping level (NT = 4 × 1015 cm−3). The others, located at 0.20 eV, 0.65 eV, and 0.73 eV, are about three orders of magnitude below the doping level (NT = 4–9 × 1014 cm−3).