Holm Kirmse

The present work investigates the use of the refractory metal alloy TiW as a possible candidate for the realization of ohmic contacts to the ultrawide bandgap semiconductor β-Ga₂O₃. Ohmic contact properties were analyzed by transfer length measurements of TiW contacts annealed at temperatures between 400 and 900 °C. Optimum contact properties with a contact resistance down to 1.5 × 10⁻⁵ Ω cm² were achieved after annealing at 700 °C in nitrogen on highly doped β-Ga₂O₃. However, a significant contact resistance increase was observed at annealing temperatures above 700 °C. Cross-sectional analyses of the contacts using scanning transmission electron microscopy revealed the formation of a TiO_x interfacial layer of 3–5 nm between TiW and β-Ga₂O₃. This interlayer features an amorphous structure and most probably possesses a high amount of vacancies and/or Ga impurities supporting charge carrier injection. Upon annealing at temperatures of 900 °C, the interlayer increases in thickness up to 15 nm, featuring crystalline-like properties, suggesting the formation of rutile TiO₂. Although severe morphological changes at higher annealing temperatures were also verified by atomic force microscopy, the root cause for the contact resistance increase is attributed to the structural changes in thickness and crystallinity of the interfacial layer.

Liquid-phase crystallization (LPC) of thin silicon layers on glass substrates is a technique to fabricate solar cells with low energy and material consumption and open-circuit voltages comparable to multicrystalline silicon wafer cells. We studied the impact of different passivation layers deposited with plasma-enhanced chemical vapor deposition (PECVD) on the cell quality. Silicon nitride (SiN$_{\rm {x}}$) and ultrathin silicon oxide (SiO$_{\rm {x}}$) layers with varying thicknesses were used. In addition, we plasma-treated the SiN$_{\rm {x}}$ surface to form a thin silicon oxynitride (SiO$_{\rm {x}}$N$_{\rm {y}}$) passivation layer. Plasma oxidation is an attractive alternative to PECVD, since thicknesses of ultrathin layers can be controlled easier as compared with PECVD. We found that the cell performance is influenced by the passivation layer. Particularly, cells on the 9 nm PECVD SiO$_{\rm {x}}$ passivation layer are worse than cells on the 11 nm SiO$_{\rm {x}}$N$_{\rm {y}}$ layer. We modeled cell parameters employing ASPIN3 to estimate effective diffusion lengths. Using transmission electron microscopy as well as electron energy loss spectroscopy, we studied the effect of the plasma treatment on the morphology and elemental distribution at the interface between passivation layer and LPC-Si absorber. Our best interdigitated back contacted solar cell with an efficiency >14% is based on the SiO$_{\rm {x}}$N$_{\rm {y}}$ layer.

Preparation of hybrid inorganic-organic systems (HIOS) for transmission electron microscopy (TEM) in cross sectional view is the key for understanding the interfacial structure. Strikingly different materials properties like hardness, cleavability and heat sensitivity limit the number of applicable preparation strategies. Successful preparation of a HIOS system combining ZnO and para-sexiphenyl (6P) is realized by ultramicrotomy. It is shown that the alignment of the cutting plane with respect to the (0001) cleavage plane of ZnO plays a decisive role for successful preparation of extended TEM lamellae and the preservation of the HIOS structure. In particular, for (0001) oriented ZnO substrates the optimum cut direction is parallel to the HIOS interface. In cross-sectional high-resolution TEM images (100) lattice planes of 6P are observed proving the appropriate preparation strategy.

Porous nitrogen-doped hollow carbon spheres (NHCSs) with tailored particle size, nitrogen content and wall thickness were prepared by grafting a thin layer of poly(ionic liquid) as carbon precursor onto silica particle surface followed by carbonization and template removal. In the next step, Fe₂O₃ nanoparticles were anchored onto the NHCSs via an impregnation-thermal decomposition route using Fe(NO₃)₃ as iron source. The as-synthesized Fe₂O₃–NHCS composite was utilized as the anode material for a lithium-ion battery, which exhibited a high reversible capacity of 1120 mAh g⁻¹ at a rate of 100 mA g⁻¹, and a coulombic efficiency of ∼98% at a rate of 500 mA g⁻¹ after 65 cycles.