Gunnar Kusch

Influence of substrate miscut angle on surface morphology and luminescence properties of AlGaN

The influence of substrate miscut on Al0.5Ga0.5 N layers was investigated using cathodoluminescence (CL) hyperspectral imaging and secondary electron imaging in an environmental scanning electron microscope. The samples were also characterized using atomic force microscopy and high resolution X-ray diffraction. It was found that small changes in substrate miscut have a strong influence on the morphology and luminescence properties of the AlGaN layers. Two different types are resolved. For low miscut angle, a crack-free morphology consisting of randomly sized domains is observed, between which there are notable shifts in the AlGaN near band edge emission energy. For high miscut angle, a morphology with step bunches and compositional inhomogeneities along the step bunches, evidenced by an additional CL peak along the step bunches, are observed.

Spatial clustering of defect luminescence centers in Si-doped low resistivity Al0.82Ga0.18N

A series of Si-doped AlN-rich AlGaN layers with low resistivities was characterized by a combination of nanoscale imaging techniques. Utilizing the capability of scanning electron microscopy to reliably investigate the same sample area with different techniques, it was possible to determine the effect of doping concentration, defect distribution, and morphology on the luminescence properties of these layers. Cathodoluminescence shows that the dominant defect luminescence depends on the Si-doping concentration. For lower doped samples, the most intense peak was centered between 3.36 eV and 3.39 eV, while an additional, stronger peak appears at 3 eV for the highest doped sample. These peaks were attributed to the (VIII-ON)2− complex and the VIII3− vacancy, respectively. Multimode imaging using cathodoluminescence, secondary electrons, electron channeling contrast, and atomic force microscopy demonstrates that the luminescence intensity of these peaks is not homogeneously distributed but shows a strong dependence on the topography and on the distribution of screw dislocations.

Multi-wavelength emission from a single InGaN/GaN nanorod analyzed by cathodoluminescence hyperspectral imaging

Multiple luminescence peaks emitted by a single InGaN/GaN quantum-well(QW) nanorod, extending from the blue to the red, were analysed by a combination of electron microscope based imaging techniques. Utilizing the capability of cathodoluminescence hyperspectral imaging it was possible to investigate spatial variations in the luminescence properties on a nanoscale. The high optical quality of a single GaN nanorod was demonstrated, evidenced by a narrow band-edge peak and the absence of any luminescence associated with the yellow defect band. Additionally two spatially confined broad luminescence bands were observed, consisting of multiple peaks ranging from 395 nm to 480 nm and 490 nm to 650 nm. The lower energy band originates from broad c-plane QWs located at the apex of the nanorod and the higher energy band from the semipolar QWs on the pyramidal nanorod tip. Comparing the experimentally observed peak positions with peak positions obtained from plane wave modelling and 3D finite difference time domain(FDTD) modelling shows modulation of the nanorod luminescence by cavity modes. By studying the influence of these modes we demonstrate that this can be exploited as an additional parameter in engineering the emission profile of LEDs.

Hybrid top-down/bottom-up fabrication of a highly uniform and organized faceted AlN nanorod scaffold

As a route to the formation of regular arrays of AlN nanorods, in contrast to other III-V materials, the use of selective area growth via metal organic vapor phase epitaxy (MOVPE) has so far not been successful. Therefore, in this work we report the fabrication of a highly uniform and ordered AlN nanorod scaffold using an alternative hybrid top-down etching and bottom-up regrowth approach. The nanorods are created across a full 2-inch AlN template by combining Displacement Talbot Lithography and lift-off to create a Ni nanodot mask, followed by chlorine-based dry etching. Additional KOH-based wet etching is used to tune the morphology and the diameter of the nanorods. The resulting smooth and straight morphology of the nanorods after the two-step dry-wet etching process is used as a template to recover the AlN facets of the nanorods via MOVPE regrowth. The facet recovery is performed for various growth times to investigate the growth mechanism and the change in morphology of the AlN nanorods. Structural characterization highlights, first, an efficient dislocation filtering resulting from the ~130 nm diameter nanorods achieved after the two-step dry-wet etching process, and second, a dislocation bending induced by the AlN facet regrowth. A strong AlN near band edge emission is observed from the nanorods both before and after regrowth. The achievement of a highly uniform and organized faceted AlN nanorod scaffold having smooth and straight non-polar facets and improved structural and optical quality is a major stepping stone toward the fabrication of deep UV core-shell-based AlN or AlxGa1-xN templates.

Deep UV Emission from Highly Ordered AlGaN/AlN Core–Shell Nanorods

Three-dimensional core-shell nanostructures could resolve key problems existing in conventional planar deep UV light-emitting diode (LED) technology due to their high structural quality, high-quality nonpolar growth leading to a reduced quantum-confined Stark effect and their ability to improve light extraction. Currently, a major hurdle to their implementation in UV LEDs is the difficulty of growing such nanostructures from Al xGa1- xN materials with a bottom-up approach. In this paper, we report the successful fabrication of an AlN/Al xGa1- xN/AlN core-shell structure using an original hybrid top-down/bottom-up approach, thus representing a breakthrough in applying core-shell architecture to deep UV emission. Various AlN/Al xGa1- xN/AlN core-shell structures were grown on optimized AlN nanorod arrays. These were created using displacement Talbot lithography (DTL), a two-step dry-wet etching process, and optimized AlN metal organic vapor phase epitaxy regrowth conditions to achieve the facet recovery of straight and smooth AlN nonpolar facets, a necessary requirement for subsequent growth. Cathodoluminescence hyperspectral imaging of the emission characteristics revealed that 229 nm deep UV emission was achieved from the highly uniform array of core-shell AlN/Al xGa1- xN/AlN structures, which represents the shortest wavelength achieved so far with a core-shell architecture. This hybrid top-down/bottom-up approach represents a major advance for the fabrication of deep UV LEDs based on core-shell nanostructures.

Optical properties and resonant cavity modes in axial InGaN/GaN nanotube microcavities

Microcavities based on group-III nitride material offer a notable platform for the investigation of light-matter interactions as well as the development of devices such as high efficiency light emitting diodes (LEDs) and low-threshold nanolasers. Disk or tube geometries in particular are attractive for low-threshold lasing applications due to their ability to support high finesse whispering gallery modes (WGMs) and small modal volumes. In this article we present the fabrication of homogenous and dense arrays of axial InGaN/GaN nanotubes via a combination of displacement Talbot lithography (DTL) for patterning and inductively coupled plasma top-down dry-etching. Optical characterization highlights the homogeneous emission from nanotube structures. Power-dependent continuous excitation reveals a non-uniform light distribution within a single nanotube, with vertical confinement between the bottom and top facets, and radial confinement within the active region. Finite-difference time-domain simulations, taking into account the particular shape of the outer diameter, indicate that the cavity mode of a single nanotube has a mixed WGM-vertical Fabry-Perot mode (FPM) nature. Additional simulations demonstrate that the improvement of the shape symmetry and dimensions primarily influence the Q-factor of the WGMs whereas the position of the active region impacts the coupling efficiency with one or a family of vertical FPMs. These results show that regular arrays of axial InGaN/GaN nanotubes can be achieved via a low-cost, fast and large-scale process based on DTL and top-down etching. These techniques open a new perspective for cost effective fabrication of nano-LED and nano-laser structures along with bio-chemical sensing applications.

Cathodoluminescence hyperspectral imaging of nitride semiconductors : introducing new variables

Cathodoluminescence (CL) hyperspectral imaging—the acquisition of a full optical emission spectrum at each pixel of an image—has become firmly established as a measurement mode in scanning electron microscopy (SEM) [1]. CL is sensitive to the structural, compositional and electrical properties of a sample, and the inherent multimode nature of SEM makes it possible to combine CL with other techniques which are also sensitive to one or more of these properties. For example, combining CL hyperspectral imaging with simultaneous X-ray microanalysis has been used to probe composition variations within semiconductors [2] and minerals [3]. In this work we present recent results combining CL hyperspectral imaging with other SEM modes, and also demonstrate the benefits of introducing additional measurement parameters, such as sample applied bias and microscope chamber pressure. Structural defects are central in determining the performance of electrical and optical devices based on III-nitride semiconductors. To better understand the effect of threading dislocations on carrier recombination in such materials, we have carried out CL imaging together with electron channeling contrast imaging (ECCI), which is sensitive to the lattice distortions associated with such defects. Using this combination of techniques we have shown that both pure edge dislocations and mixed (combination of edge + screw) dislocations act as nonradiative recombination centers in GaN epilayers [4]. Since CL provides information only about radiative recombination, it needs to be coupled with other techniques to provide a fuller picture of the different carrier loss mechanisms which limit device performance, an objective we achieved by measuring CL together with electron beam induced current (EBIC). This signal originates within the junction of an electrically contacted device (measured via an external circuit) and is dependent on the total (i.e. radiative + nonradiative) recombination. By carrying out both techniques simultaneously on InxGa1-xN/GaN light-emitting diodes (LEDs), we have separated out nonradiative recombination from other loss mechanisms, such as light extraction or electrical contacting issues [5]. These CL/EBIC measurements on contacted LEDs have opened up the possibility of introducing an additional variable into CL imaging: applied bias voltage. Figure 1 shows an example of such measurements for an InGaN/GaN LED device. This capability allows the emission to be imaged under conditions closer to those of a device in normal operation. It also allows us—by imaging at several bias values—to differentiate between the drift and diffusion components of the current, and to probe the influence of the quantum-confined Stark effect in the active region of the LED. Furthermore, comparison with electroluminescence (EL) hyperspectral images yields additional information: for example, in some areas of the LED we see correlations between contrast features in EL and EBIC images which are not present in the CL, implying that they originate in the diode junction or contacts rather than in the active layer itself [5]. Finally, we have demonstrated CL hyperspectral imaging in an environmental SEM operating with a variable pressure of water vapor. This has allowed us to measure for the first time such insulating