arXiv subjects

Johanna Eichhorn

Publications and source records attributed to Johanna Eichhorn.

3 recordsLinked to original sources

arXiv2026

Quantifying Thermal, Photovoltage, and Defect Contributions to Transient Absorption of Ta$_{3}$N$_{5}$ Photoanodes

Ta$_{3}$N$_{5}$ is among the most intensively studied photoanode materials for solar-driven water oxidation, yet its performance often remains limited by short carrier lifetimes and defect mediated recombination. Although transient absorption spectroscopy is widely used to probe carrier dynamics in photoelectrodes, spectral assignments are frequently ambiguous due to overlapping contributions. Here, microsecond-to-second transient absorption of Ta$_{3}$N$_{5}$ thin films is combined with complementary optical spectroscopies to disentangle contributions from lattice heating, electrostatics, and defect states. Photoreflectance reveals three critical points in the Ta$_{3}$N$_{5}$ band structure, including two anisotropic near-edge transitions at 2.14 eV and 2.27 eV and a higher-lying transition near 2.80 eV, all closely aligned with dominant transient absorption features. A previously unreported photo-induced absorption at 2.80 eV is attributed to pump-induced lattice heating, while potential-dependent measurements reveal that near-edge bleach features arise from pump-induced band flattening and subsequent surface photovoltage relaxation. Fitting transient absorption spectra with independently measured thermal and electrostatic components enables quantification of both thermal and photovoltage dynamics, while the sub-bandgap response provides insight into the redistribution of defect charge states. Thus, this approach to quantifying thermal, electrostatic, and defect-mediated contributions to microsecond-to-second transient absorption provides broadly applicable insights into photoexcitation and relaxation mechanisms in functional semiconductor photoelectrodes.

cond-mat.mtrl-sci↗

arXiv2024

Atomically Flat Dielectric Patterns for Band Gap Engineering and Lateral Junction Formation in MoSe$_2$ Monolayers

Combining a precise sputter etching method with subsequent AlO$_x$ growth within an atomic layer deposition chamber enables fabrication of atomically flat lateral patterns of SiO$_2$ and AlO$_x$. The transfer of MoSe$_2$ monolayers onto these dielectrically modulated substrates results in formation of lateral heterojunctions, with the flat substrate topography leading to minimal strain across the junction. Kelvin probe force microscopy (KPFM) measurements show significant variations in the contact potential difference (CPD) across the interface, with AlO$_x$ regions inducing a 230~mV increase in CPD. Spatially resolved photoluminescence spectroscopy reveals shifts in spectral weight of neutral and charged exciton species across the different dielectric regions. On the AlO$_x$ side, the Fermi energy moves closer to the conduction band, leading to a higher trion-to-exciton ratio, indicating a bandgap shift consistent with CPD changes. In addition, transient reflection spectroscopy highlights the influence of the dielectric environment on carrier dynamics, with the SiO$_2$ side exhibiting rapid carrier decay typical of neutral exciton recombination. In contrast, the AlO$_x$ side shows slower, mixed decay behavior consistent with conversion of trions back into excitons. These results demonstrate how dielectric substrate engineering can tune the electronic and optical characteristics of proximal two-dimensional materials, allowing scalable fabrication of advanced junctions for novel (opto)electronics applications.

cond-mat.mtrl-sci↗

arXiv2022

Spatially-Modulated Silicon Interface Energetics via Hydrogen Plasma-Assisted Atomic Layer Deposition of Ultrathin Alumina

Atomic layer deposition (ALD) is a key technique for the continued scaling of semiconductor devices, which increasingly relies on reproducible and scalable processes for interface manipulation of 3D structured surfaces on the atomic scale. While ALD allows the synthesis of conformal films at low temperature with utmost control over the thickness, atomically-defined closed coatings and surface modifications are still extremely difficult to achieve because of three-dimensional growth during nucleation. Here, we present a route towards sub-nanometer thin and continuous aluminum oxide (AlOx) coatings on silicon (Si) substrates for the spatial control of the surface charge density and interface energetics. We use trimethylaluminum (TMA) in combination with remote hydrogen plasma instead of a gas-phase oxidant for the transformation of silicon oxide into alumina (AlOx). During the initial ALD cycles, TMA reacts with the surface oxide (SiO2) on silicon until there is a saturation of bindings sites, after which the oxygen from the underlying surface oxide is consumed, thereby transforming the silicon oxide into Si capped with AlOx. Depending on the number of ALD cycles, the SiO2 can be partially or fully transformed, which we exploit to create sub-nanometer thin and continuous AlOx layers deposited in selected regions defined by lithographic patterning. The resulting patterned surfaces are characterized by lateral AlOx/SiO2 interfaces possessing step heights as small as 0.3 nm and surface potential steps in excess of 0.4 V. In addition, the introduction of fixed negative charges of $9 \times 10^{12}$ cm$^{-2}$ enables modulation of the surface band bending, which is relevant to the field-effect passivation of Si and low-impedance charge transfer across contact interfaces.

cond-mat.mtrl-sci↗