Ingo Tischer

Cathodoluminescence of GaInN quantum wells grown on nonpolar a plane GaN: Intense emission from pit facets

GaxIn1−xN quantum wells grown by metal organic vapor phase epitaxy on a plane GaN grown on r plane sapphire substrate typically show relatively large surface pits. We show by correlation of low temperature photoluminescence, cathodoluminescence, scanning and transmission electron microscopy that the different semipolar side facets of these pits dominate the overall luminescence signal of such layers.

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)%.

Three-dimensional reciprocal space mapping of diffuse scattering for the study of stacking faults in semipolar (\bf 11{\overline 2}2) GaN layers grown from the sidewall of an r-patterned sapphire substrate

Three-dimensional reciprocal space mapping of semipolar (11{\overline 2}2) GaN grown on stripe-patterned r-plane (1{\overline 1}02) sapphire substrates is found to be a powerful and crucial method for the analysis of diffuse scattering originating from stacking faults that are diffracting in a noncoplanar geometry. Additionally, by measuring three-dimensional reciprocal space maps (3D-RSMs) of several reflections, the transmission electron microscopy visibility criteria could be confirmed. Furthermore, similar to cathodoluminescence, the 3D-RSM method could be used in future as a reliable tool to distinguish clearly between the diffuse scattering signals coming from prismatic and from basal plane stacking faults and from partial dislocations in semipolar (11{\overline 2}2) GaN. The fitting of the diffuse scattering intensity profile along the stacking fault streaks with a simulation based on the Monte Carlo approach has delivered an accurate determination of the basal plane stacking fault density. A reduction of the stacking fault density due to the intercalation of an SiN interlayer in the GaN layer deposited on the sidewall of the pre-patterned sapphire substrate has led to an improvement of the optoelectronic properties, influenced by the crystal quality, as has been demonstrated by a locally resolved cathodoluminescence investigation.

Structural and cathodoluminescence properties of ZnO nanorods after Ga-implantation and annealing

Single-crystalline ZnO nanorods were implanted with 30 keV Ga+ ions and fluences between 5×1012 and 1.5×1016 cm−2. Annealing treatments at temperatures up to 700 °C for 1 h were carried out to reduce implantation-induced structural defects. The structural and optical properties of the nanorods were studied by transmission electron microscopy (TEM) and cathodoluminescence (CL) spectroscopy. TEM shows that extended implantation defects vanish completely in nanorods implanted with doses up to 5×1013 cm−2 after annealing at 700 °C. Dislocation loops remain after implantation with higher fluences. The CL intensity of as-grown nanorods and implanted ZnO nanorods is low. Annealing at 700 °C leads to a significant increase in the CL intensity for as-grown nanorods and implanted with a dose of up to 5×1013 cm−2. The strong CL intensity in implanted and annealed nanorods correlates with the complete disappearance of extended structural defects.

Three-dimensional cathodoluminescence characterization of a semipolar GaInN based LED sample

A semipolar GaInN based light-emitting diode (LED) sample is investigated by three-dimensionally resolved cathodoluminescence (CL) mapping. Similar to conventional depth-resolved CL spectroscopy (DRCLS), the spatial resolution perpendicular to the sample surface is obtained by calibration of the CL data with Monte-Carlo-simulations (MCSs) of the primary electron beam scattering. In addition to conventional MCSs, we take into account semiconductor-specific processes like exciton diffusion and the influence of the band gap energy. With this method, the structure of the LED sample under investigation can be analyzed without additional sample preparation, like cleaving of cross sections. The measurement yields the thickness of the p-type GaN layer, the vertical position of the quantum wells, and a defect analysis of the underlying n-type GaN, including the determination of the free charge carrier density. The layer arrangement reconstructed from the DRCLS data is in good agreement with the nominal parameters defined by the growth conditions.A semipolar GaInN based light-emitting diode (LED) sample is investigated by three-dimensionally resolved cathodoluminescence (CL) mapping. Similar to conventional depth-resolved CL spectroscopy (DRCLS), the spatial resolution perpendicular to the sample surface is obtained by calibration of the CL data with Monte-Carlo-simulations (MCSs) of the primary electron beam scattering. In addition to conventional MCSs, we take into account semiconductor-specific processes like exciton diffusion and the influence of the band gap energy. With this method, the structure of the LED sample under investigation can be analyzed without additional sample preparation, like cleaving of cross sections. The measurement yields the thickness of the p-type GaN layer, the vertical position of the quantum wells, and a defect analysis of the underlying n-type GaN, including the determination of the free charge carrier density. The layer arrangement reconstructed from the DRCLS data is in good agreement with the nominal parameters ...

Determination of axial and lateral exciton diffusion length in GaN by electron energy dependent cathodoluminescence

We demonstrate the application of low-temperature cathodoluminescence (CL) with high lateral, depth, and spectral resolution to determine both the lateral (i.e., perpendicular to the incident primary electron beam) and axial (i.e., parallel to the electron beam) diffusion length of excitons in semiconductor materials. The lateral diffusion length in GaN is investigated by the decrease of the GaN-related luminescence signal when approaching an interface to Ga(In)N based quantum well stripes. The axial diffusion length in GaN is evaluated from a comparison of the results of depth-resolved CL spectroscopy (DRCLS) measurements with predictions from Monte Carlo simulations on the size and shape of the excitation volume. The lateral diffusion length was found to be (95 ± 40) nm for nominally undoped GaN, and the axial exciton diffusion length was determined to be (150 ± 25) nm. The application of the DRCLS method is also presented on a semipolar (112¯2) sample, resulting in a value of (70 ± 10) nm in p-type GaN.

Studies on Defect Reduction in AlGaN Heterostructures by Integrating an In-situ SiN Interlayer

We have decreased the dislocation density in AlxGa1-xN epitaxial layers grown on sapphire wafers by introducing an in-situ deposited SiN nano-mask layer. Taking together results obtained by transmission electron microscopy, photoluminescence, cathodoluminescence, and X-ray diffraction, we were able to derive a schematic model about the AlGaN growth on the SiN nanomask: On the open pores of the nano-mask, Ga-rich AlGaN hillocks develop, whereas on the SiN layer Al-rich AlGaN nucleates owing to the reduced selectivity of Al-containing material. Once the hillocks are formed, Ga-rich material is more efficiently incorporated on the inclined side-facets leading to an Al-rich coverage of the central c-plane part of the hillocks. We observed a bending of the dislocations towards the side-facets of the hillocks, which eventually leads to dislocation bundles with increased probability of dislocation annihilation, separated by fairly defect-free regions. Thus, we could achieve a significant reduction of the edge-type dislocation density in these epitaxial layers.

Optical Properties of ZnO/GaN/InGaN Core–Shell Nanorods

Upright ZnO/GaN/InGaN core–shell nanorods arrayed in a well defined pattern are very good candidates for sensing applications. In our approach, we grew single ZnO nanopillars on top of ordered GaN pyramids, which were subsequently overgrown with GaN and a single InGaN quantum well, followed by a final GaN barrier layer. Spatially and spectrally resolved low temperature cathodoluminescence was used to investigate the optical properties of the rods. We found the dominant quantum well luminescence to be well defined and homogeneously distributed, with a maximum intensity at the edges of the pillars. Although the hydrogen atmosphere during the nitride growth together with the elevated growth temperature should lead to complete desorption of the initial ZnO pillar template, we found evidence for ZnO relicts on the pillar surface, and for incorporation of Zn in GaN at the tips of the rods. Furthermore, we were able to distinguish between the luminescence contributions from the quantum well, Zn-doped GaN, and possible structural defects.

Stacking fault emission in GaN: Influence of n-type doping

We present spatially and spectrally resolved cathodoluminescence investigations on the cross section of semipolar (112¯2) gallium nitride epitaxial layers with high background doping level. The locally varying high carrier concentration leads in emission to a free electron recombination band (FERB) governed on the high energy side by conduction band filling. For the basal plane stacking fault (BSF) of type I1, typically emitting at ≈3.41 eV in low doped GaN, we find a blue shift in emission correlated to the FERB high energy tail. This shift can be perfectly modeled and understood in a quantum well model for the BSF, taking also into account the varying doping level in the barrier region. Thus, the carrier concentration can be finally calculated either from the actual position of the I1 BSF or alternatively from the FERB-related near band edge emission.

Evidence of terbium and oxygen co-segregation in annealed AlN:Tb

Analytical scanning transmission electron microscopy has been applied to study aluminium nitride (AlN) doped with terbium (Tb) and annealed at 800 °C. The correlation of the maps of Tb and oxygen (O) from electron energy-loss spectrum (EELS) imaging proves that these two elements co-segregate, replacing aluminium (Al) and nitrogen (N) atoms, respectively. This agrees well with modelling which predicted the existence of Tb–O complexes needed to fit all lines in the rather complicated cathodoluminescence emission spectrum of the sample.