J. Hertkorn

Oxygen induced strain field homogenization in AlN nucleation layers and its impact on GaN grown by metal organic vapor phase epitaxy on sapphire: An x-ray diffraction study

This paper presents an x-ray study of GaN, which is grown on nominally undoped and oxygen-doped AlN nucleation layers on sapphire substrates by metal organic vapor phase epitaxy. Without additional oxygen doping a trimodal nucleation distribution of AlN is observed leading to inhomogeneous in-plane strain fields, whereas in oxygen-doped layers a homogeneous distribution of nucleation centers is observed. In both types of nucleation layers extremely sharp correlation peaks occur in transverse ω-scans which are attributed to a high density of edge-type dislocations having an in-plane Burgers vector. The correlation peaks are still visible in the (0002) ω-scans of 500 nm GaN which might mislead an observer to conclude incorrectly that there exists an extremely high structural quality. For the undoped nucleation layers depth-sensitive measurements in grazing incidence geometry reveal a strong thickness dependence of the lattice parameter a, whereas no such dependence is observed for doped samples. For oxygen-...

X-ray diffuse scattering from threading dislocations in epitaxial GaN layers

In this article, we combine diffuse x-ray scattering with a Monte Carlo simulation method for the determination of the dislocation density in thin heteroepitaxial layers. As a model, we consider GaN epitaxial layers containing threading dislocations perpendicular to the surface. The densities of particular types of threading dislocations following from the comparison of measured and simulated distributions of diffusely scattered x-ray intensity are compared with the dislocation densities determined by etching. A good agreement was found.

Time- and locally resolved photoluminescence of semipolar GaInN∕GaN facet light emitting diodes

The authors investigate the carrier lifetime and photoluminescence (PL) intensity of a semipolar GaInN∕GaN sample which was realized by growing five GaInN∕GaN quantum wells on the {11¯01} side facets of selectively grown n-GaN stripes that have a triangular shape running along the ⟨112¯0⟩ direction. Time- and locally resolved PL measurements show drastically reduced lifetimes for the semipolar sample of only 650ps at 4K whereas lifetimes exceeding 50ns were found for a polar reference sample. Furthermore, more than a doubling of the luminescence intensity and a significantly reduced blueshift of the PL peak wavelength with increasing excitation power density provide further evidence for the presence of reduced piezoelectric fields in the semipolar sample.

Highly conductive modulation doped composition graded p-AlGaN/(AlN)/GaN multiheterostructures grown by metalorganic vapor phase epitaxy

In this study, we present theoretical and experimental results regarding highly conductive modulation doped composition graded p-AlGaN/(AlN)/GaN multiheterostructures. Based on simulation results, several multiheterostructures were grown by metalorganic vapor phase epitaxy. Using high resolution x-ray diffraction and x-ray reflectometry, the abruptness of the AlGaN/AlN/GaN interfaces could be determined. Using electron holography, the energetic profile of the valence band could be measured, yielding important information about the vertical carrier transport in such multiheterostructures. The electrical properties of the samples were investigated by measuring the lateral (σL) and vertical (σV) conductivity, respectively. The free hole concentration of a sample optimized in terms of lateral conductivity was measured to be 1.2×1019 cm−3 (295 K) with a mobility of 7 cm2/V s, yielding a record σL of 13.7 (Ω cm)−1. Low temperature Hall measurements (77 K) proved the existence of a two-dimensional hole gas at th...

Optimization of semipolar GaInN/GaN blue/green light emitting diode structures on {1-101} GaN side facets

Bluish-green semipolar GaInN/GaN light emitting diodes (LEDs) were investigated as possible candidates for high-brightness devices even in the long wavelength visible regime. To combine the high material quality known from c-GaN and the advantages of a reduced piezoelectric field, the LED structures were realized on the {1¯101} side facets of selectively grown GaN stripes with triangular cross section. Structural investigations using transmission electron microscopy, scanning electron microscopy, high resolution x-ray diffraction, and atomic force microscopy have been performed and could be related to the luminescence properties in photoluminescence and cathodoluminescence. The defect-related luminescence peaks at 3.3 eV and 3.42 eV typically observed in planar non- and semipolar GaN structures as fingerprints of prismatic and basal plane stacking faults, respectively, could be eliminated in our facet LED structures by optimized growth conditions. Furthermore, an indium incorporation efficiency for these {1¯101} facets is found to be about 50% higher as compared to c-plane growth, what helps significantly to achieve longer wavelength emission in spite of the reduced quantum confined Stark effect in such non- and semipolar materials. Combining these findings, we could realize a bluish-green semipolar light emitting diode on the side facets of our GaN stripes. Continuous wave on-wafer optical output powers as high as 240 µW@20mA could be achieved for about 500nm emission wavelength in electroluminescence measurements. The external efficiency was nearly constant for the investigated current range. Furthermore, the relatively small wavelength shift of about 3 nm for currents between 10mA and 100mA confirmed the reduced piezoelectric field in our LED structures.

Determination of the electronic band structure for a graded modulation-doped AlGaN∕AlN∕GaN superlattice

The electronic band structure of a modulation-doped AlGaN∕AlN∕GaN superlattice structure where the AlGaN layer is compositionally graded has been experimentally determined by electron holography. It is shown that all periods in the superlattice have a similar two-dimensional-electron-gas distribution, indicating no degradation in the quality of the heterostructures during growth. High-resolution potential energy profiles show that the nominally linear grading of the AlGaN barrier layers results in a parabolic profile in Al composition. Knowledge of the nature of energy barriers for electron transfer between channels is important in the optimization of the perpendicular conductivity of AlGaN∕GaN superlattice structures.

Epitaxial growth of coaxial GaInN-GaN hetero-nanotubes

We report about the successful realization of a coaxial hetero structure grown by MOVPE around ZnO nanocolumns. At higher overgrowth temperatures, the ZnO cores completely dissolved leaving GaN nanotube structures with excellent properties. Such tubes could be sheathed by a GaInN-GaN single quantum well structure, as confirmed by photoluminescence and transmission electron microscopy.