Jochen Bruckbauer

Silicon, being the fundamental material for modern semiconductor devices, has seen continuous advancements to enhance its electrical and mechanical properties. Strain engineering is a well-established technique for improving the performance of silicon-based devices. In this paper, we propose a simple method for inducing and permanently maintaining strain in silicon through pure physical bending. By subjecting the silicon substrate to a controlled bending process, we demonstrate the generation of strain levels that persist even after the removal of external stress, with a maximum strain value of 0.4%. We present a comprehensive study of the mechanics behind this phenomenon, a full finite element mechanical model, and experimental verification of the bending-induced strain in Si membranes using electron backscatter diffraction measurements. Our findings show the potential of this approach for strain engineering in high-performance silicon-based technologies without resorting to complex and expensive fabrication techniques.

Owing to its unique properties, N-polar GaN offers several advantages over Ga-polar GaN, particularly for applications in high power electronics. However, the growth of high-quality N-polar material is challenging. One dominant issue is the increased surface roughness, due to the occurrence of hexagonal-shaped hillocks, referred to as hexagons, on the material’s surface. Although there are different methods to reduce the density of these hillocks, such as the use of vicinal substrates or optimum growth conditions, the properties of such hillocks are not extensively studied. Here, we investigate the crystallographic and luminescence properties of these hexagonal features using the techniques of electron backscatter diffraction (EBSD) and cathodoluminescence (CL) hyperspectral imaging in the scanning electron microscope combined with micro-Raman mapping. CL revealed increased light emission from the top of the hexagons compared with the surrounding material. Additionally, dark spots in intensity images, associated with non-radiative recombination at threading dislocations, could be resolved on top of the hexagons, but not in the surrounding area, implying improved material quality of the hexagons. Extensive strain analysis using EBSD revealed that the hexagons are composed of equivalent triangular segments with tensile strain along symmetrically equivalent ⟨112¯0⟩ directions. As the hexagons become larger, this strain increases with the distance from the center. This was confirmed by mapping the Raman E2 (high) mode. Overall, this provides crucial insight into the strain state of these hexagonal features.

The impact of underlying electron transport layer (SnO ₂ , TiO ₂ and ZnO) on the optoelectronic and structural properties of the CsPbI ₂ Br perovskite overlayer is explored using Cathodoluminescence (CL) and Electron Back Scatter Diffraction (EBSD).

We present the application of electron backscatter diffraction (EBSD) as a technique for characterizing wurtzite (wz) and zincblende (zb) polytypes of GaN grown upon micropatterned Si (001) substrates. The Si substrate is etched to create parallel V-shaped grooves with opposing {111} facets before the deposition of GaN. EBSD revealed that wz-GaN growth fronts initially form on the {111} Si facets before undergoing a transition from a wurtzite to zincblende structure as the two growth fronts meet. Orientation analysis of the GaN structures revealed that the wz-GaN growth fronts had different growth orientations but shared the same crystallographic relationship with the zb-GaN such that ⊥{303¯8}wz∥⟨110⟩zb, ⟨112¯0⟩wz∥⟨110⟩zb, and ⊥{303¯4}wz∥⟨001⟩zb. Furthermore, the crystallographic relationship, {0001}wz-GaN∥{111}zb-GaN∥{111}Si, and alignment of the wz- and zb-GaN with respect to the Si substrate was investigated. The two wz-GaN ⟨0001⟩ growth directions were expected to coalesce at an angle of 109.5°; however, measurements revealed an angle of 108°. The resultant misalignment of 1.5° induces misorientation in the zb-GaN crystal lattice. While the degree of misorientation within the zb-GaN lattice is low, <1°, the zb-GaN lattice is deformed and bends toward the wz-GaN interfaces about the specimen direction parallel to the length of the V-groove. Further EBSD measurements over larger areas of the sample revealed that these results were consistent across the sample. However, it was also revealed that additional factors induce changes in the orientation of the zb-GaN lattice, which may relate to the initial growth conditions of the zb-GaN.

Cl⁻ anions remain in the CsPbI₂Br film after annealing directing the preferential crystal orientation of the films along [100].

Extended defects, like threading dislocations, are detrimental to the performance of optoelectronic devices. In the scanning electron microscope, dislocations are traditionally imaged using diodes to monitor changes in backscattered electron intensity as the electron beam is scanned over the sample, with the sample positioned so the electron beam is at, or close to the Bragg angle for a crystal plane/planes. Here, we use a pixelated detector instead of single diodes, specifically an electron backscatter diffraction (EBSD) detector. We present postprocessing techniques to extract images of dislocations and surface steps, for a nitride thin film, from measurements of backscattered electron intensities and intensity distributions in unprocessed EBSD patterns. In virtual diode (VD) imaging, the backscattered electron intensity is monitored for a selected segment of the unprocessed EBSD patterns. In center of mass (COM) imaging, the position of the center of the backscattered electron intensity distribution is monitored. Additionally, both methods can be combined (VDCOM). Using both VD and VDCOM, images of only threading dislocations, or dislocations and surface steps can be produced, with VDCOM images exhibiting better signal-to-noise. The applicability of VDCOM imaging is demonstrated across a range of nitride semiconductor thin films, with varying surface step and dislocation densities.

We demonstrate the use of a low-cost liquid-crystal-based wavelength-tunable filter and CMOS video camera to add hyperspectral imaging capabilities to a probe station equipped with a simple optical microscope. The resultant setup is used to rapidly resolve the spectral and spatial variations in electroluminescence typically observed for InxGa1−xN/GaN light-emitting diodes. Applying standard statistical analyses of variation within the multivariate datasets, such as moments and principal components, we observe inhomogeneities on a spectral scale significantly smaller than the bandwidth of the tunable filter. The resultant tool offers an alternative to scanning beam luminescence techniques for high-throughput hyperspectral analysis of optoelectronic devices.

The use of organic materials and the replacement of rare-earth elements in the making of light-emitting devices has been increasingly popular over the last decades. Herein, the synthesis and characterisation of a novel organic green-emitting material (GreenCin), based on a fluorene-benzothiadiazole-fluorene (Flu-BT-Flu) core structure, and its performance as a down-converting layer in tandem with commercial blue light-emitting diodes (LEDs) for white light emission are reported. This material has been functionalised with cinnamate-groups to enable the emissive material to react with the cross-linker tetra(cinnamoyloxymethyl)methane (TCM), to produce stable films with high performance in hybrid LEDs. The hybrid devices can generate white light with a good colour rendering index (CRI) of 69. The hybrid devices also have ×2.6 increased luminous efficacy (107 lm W⁻¹) and ×2.4 increased radiant flux (24 mW) when compared with hybrid devices using non-cross-linked analogues of GreenCin. Additionally, the hybrid devices containing GreenCin have a high blue-to-white efficacy value (defined by dividing the luminous flux of a hybrid device by the radiant flux of the underlying blue LED), of 213 lm W⁻¹, for which inorganic phosphors have values in the range of 200–300 lm W⁻¹.

Polyvinylpyrrolidone (PVP) can be used to produce upconversion nanoparticles (UCNPs) in an advantageous manner, i.e. at modest temperatures in open-to-air conditions with simple hotplate and flask apparatus. However, the influence of PVP parameters on the formation of UCNPs has not been previously investigated. In this exploratory study, we establish that PVP molecular weight and relative amount of PVP can greatly influence the morphology and diameter of NaYF₄:Yb,Er UCNPs produced via the PVP-assisted route. At nominal amounts of PVP, varying the molecular weight of PVP in synthesis between 10,000 g mol⁻¹ (PVP10), 40,000 g mol⁻¹ (PVP40), and 55,000 g mol⁻¹ (PVP55), had minimal effect on UCNP morphology, whereas reducing the quantity of PVP10 and PVP40 in the reaction to 10% of the nominal amount resulted in two notable effects: (1) the generation of a greater range of UCNP diameters and (2) the production of an unexpected sub-population of rhombus-shaped UCNPs. Bulk and individual nanoparticle analysis indicates that all UCNP morphologies were cubic (α-phase) crystal structure and consisted of NaYF₄:Yb,Er. Optical emission properties exhibited only modest green and red luminescence emission ratio when PVP parameters were varied. However, separately produced PVP40 NaYF₄:Yb,Tm UCNPs exhibited a much more intense and dual-band blue/red emission. This exploratory work demonstrates that tailoring PVP content in synthesis of UCNPs can greatly alter morphology of UCNPs produced and should be carefully considered in experimental design. However, the underlying mechanisms of action of the role PVP plays in this synthesis remain unclear. Ultimately, significant further work is still required to fully elucidate the relevant chemistry to achieve full control of PVP-UCNP synthesis.