arXiv subjects

Hartwin Peelaers

Publications and source records attributed to Hartwin Peelaers.

10 recordsLinked to original sources

arXiv2026

Origin of donor compensation in monoclinic (Al$_x$Ga$_{1{\rm -}x})_2$O$_3$ alloys

(Al$_x$Ga$_{1{\rm -}x})_2$O$_3$ alloys are frequently used in heterostructures with monoclinic Ga$_2$O$_3$, resulting in a large conduction-band offset, which leads to charge carrier confinement, a property that is desirable for device applications. However, when (Al$_x$Ga$_{1{\rm -}x})_2$O$_3$ alloys are $n$-type doped with Si, the most efficient shallow donor, there is a significant reduction in the number of charge carriers when the Al content of the alloys is greater than 26%, rendering intentional doping ineffective. Here we show that this compensation is due to cation vacancies forming in response to donor doping. We use density functional theory with the HSE06 hybrid functional to study cation vacancies in monoclinic AlGaO$_3$ and monoclinic Al$_2$O$_3$. We find that vacancies prefer to occupy split-vacancy configurations, similar to vacancies in Ga$_2$O$_3$. Furthermore, by comparing the formation energy of the vacancy with the formation energy of Si donors, we show that vacancies are lower in energy than Si donors, independent of the Fermi level, as soon as the alloys contain more than 16% Al. Therefore, cation vacancies will compensate the donor doping, explaining experimental observations.

cond-mat.mtrl-sci↗

arXiv2026

Electronic and structural properties of V$_2$O$_5$ layered polymorphs

V$_2$O$_5$ is a promising battery electrode material that can intercalate not only Li, but also more abundant alkaline metals such as Na and K, and even multivalent ions such as Al, Ca, Cu, Mg, and Zn. V$_2$O$_5$ exhibits several different polymorphs, and phase transitions between the polymorphs can occur depending on intercalant or external conditions. At least 8 different layered polymorphs have been observed. However, detailed information about the energetics and structural properties of each polymorph is still lacking. To obtain a reliable computational reference, we use hybrid density functional theory calculations to investigate the properties of layered V$_2$O$_5$ polymorphs. We benchmarked several methods to include van der Waals interactions in combination with hybrid functionals, and found that the Grimme D3 method is most accurate. We obtain detailed information on the electronic properties and structures of the various unintercalated polymorphs and show that the main electronic effect of intercalants is a filling of the lowest conduction bands, as the intercalant contributions are well above the conduction-band minimum. Despite the structural differences between the unintercalated polymorphs, we find that they have very similar band gaps and band structures, with the exception of the high temperature and pressure phase $β$.

cond-mat.mtrl-sci↗

arXiv2026

Interface-induced band bending and charge separation in all-organic ZnPc/F$_x$ZnPc heterostructures

Organic semiconductors are attractive building blocks for electronic devices due to their low cost and flexibility. Furthermore, heterostructures with type-II band alignments can efficiently separate photogenerated charges via a charge transfer and separation process. Here, we use density functional theory (DFT) to investigate model interfaces formed by zinc phthalocyanine (ZnPc) and its fluorinated derivatives (F$_8$ZnPc and F$_{16}$ZnPc). We demonstrate that these interfaces not only exhibit a type-II band offset, but also band bending. The band bending causes both the LUMO and HOMO states to localize away from the interface. Therefore, the band bending creates a strong driving force for charge separation. We used ultraviolet photoemission spectroscopy (UPS) to experimentally confirm this predicted band bending. The wavefunction envelopes of vertically-stacked molecules resemble particle-in-a-box states, but this shape is lost when the molecules are staggered. These results elucidate how interface-induced band bending facilitates charge separation in all-organic heterostructures and suggest a design pathway toward improved performance in organic photovoltaic devices.

cond-mat.mtrl-sci↗

arXiv2024

Computational design of optimal heterostructures for $β$-Ga$_2$O$_3$

Ga$_2$O$_3$ is a wide-bandgap material of interest for a wide variety of devices, many of these requiring heterostructures, for instance to achieve carrier confinement. A common method to create such heterostructures is to alloy with In$_2$O$_3$ or Al$_2$O$_3$. However, the lattice constants of these materials are significantly different from those of Ga$_2$O$_3$, leading to large amounts of strain in the resulting heterostructure. If the thickness of the heterostructure is increased, this can lead to cracking. By considering alloys of In$_2$O$_3$ and Al$_2$O$_3$, the lattice constants can be tailored to those of Ga$_2$O$_3$, while still keeping a sizable conduction-band offset. We use density functional theory with hybrid functionals to investigate the structural and electronic properties of In$_2$O$_3$ and Al$_2$O$_3$ alloys in the bixbyite, corundum, and monoclinic structures. We find that the lattice constants increase with In incorporation. Bandgaps decrease nonlinearly with increasing In concentration. We find the (In$_{\rm 0.25}$Al$_{\rm 0.75}$)$_{\rm 2}$O$_{\rm 3}$ monoclinic structure to be of particular interest, as it closely matches the Ga$_2$O$_3$ lattice constants while providing an indirect/direct bandgap of 5.94/5.70 eV and a conduction-band offset of 1 eV compared to Ga$_2$O$_3$.

cond-mat.mtrl-sci↗

arXiv2022

Deformable Molecular Crystal on 2D Crystal: A New Way to Build Nanoscale Periodic Trapping Sites for Interlayer Excitons

The nanoscale moiré pattern formed at 2D transition metal dichalcogenide crystal (TMDC) heterostructures provides periodic trapping sites for excitons, which is essential for realizing various exotic phases such as artificial exciton lattices, Bose-Einstein condensates, and exciton insulators. At organic molecule/TMDC heterostructures, similar periodic potentials can be formed via other degrees of freedom. We utilize the structure deformability of a 2D molecular crystal as a degree of freedom to create a periodic nanoscale potential that can trap interlayer excitons (IXs). Specifically, two semiconducting molecules, PTCDI and PTCDA, which possess similar bandgaps and ionization potentials but form different lattice structures on MoS2, are investigated.The PTCDI lattice on MoS2 is distorted geometrically, which lifts the degeneracy of the two molecules within the crystal's unit cell. The degeneracy lifting results in a spatial variation of the molecular orbital energy, with an amplitude and periodicity of ~ 0.2 eV and ~ 2 nm, respectively. On the other hand, no such energy variation is observed in PTCDA/MoS2, where the PTCDA lattice is much less distorted. The periodic variation in molecular orbital energies provides effective trapping sites for IXs. For IXs formed at PTCDI/MoS2, rapid spatial localization of the electron in the organic layer towards the interface is observed, which demonstrate the effectiveness of these interfacial IX's traps.

cond-mat.mes-hall↗

arXiv2021

Ultrafast Dynamics of Gallium Vacancy Charge States in $β$-Ga$_2$O$_3$

Point defects in crystalline materials often occur in multiple charge states. Although many experimental methods to study and explore point defects are available, techniques to explore the non-equilibrium dynamics of the charge states of these defects at ultrafast (sub-nanosecond) time scales have not been discussed before. We present results from ultrafast optical-pump supercontinuum-probe spectroscopy measurements on $β$-Ga$_2$O$_3$. The study of point defects in $β$-Ga$_2$O$_3$ is essential for its establishment as a material platform for high-power electronics and deep-UV optoelectronics. Use of a supercontinuum probe allows us to obtain the time-resolved absorption spectra of material defects under non-equilibrium conditions with picosecond time resolution. The probe absorption spectra shows defect absorption peaks at two energies, $\sim$2.2 eV and $\sim$1.63 eV, within the 1.3-2.5 eV probe energy bandwidth. The strength of the absorption associated with each peak is time-dependent and the spectral weight shifts from the lower energy peak to the higher energy peak with pump-probe delay. Further, maximum defect absorption is seen for probe polarized along the crystal c-axis. The time-dependent probe absorption spectra and the observed dynamics for all probe wavelengths at all pump-probe delays can be fit with a set of rate equations for a single multi-level defect. Based on first-principles calculations within hybrid density functional theory we attribute the observed absorption features to optical transitions from the valence band to different charge states of Gallium vacancies. Our results demonstrate that broadband ultrafast supercontinuum spectroscopy can be a useful tool to explore charge states of defects and defect dynamics in semiconductors.

cond-mat.mtrl-sci↗

arXiv2020

Intra- and Inter-Conduction Band Optical Absorption Processes in $β$-Ga$_2$O$_3$

$β$-Ga$_2$O$_3$ is an ultra-wide bandgap semiconductor and is thus expected to be optically transparent to light of sub-bandgap wavelengths well into the ultraviolet. Contrary to this expectation, it is found here that free electrons in n-doped $β$-Ga$_2$O$_3$ absorb light from the IR to the UV wavelength range via intra- and inter-conduction band optical transitions. Intra-conduction band absorption occurs via an indirect optical phonon mediated process with a $1/ω^{3}$ dependence in the visible to near-IR wavelength range. This frequency dependence markedly differs from the $1/ω^{2}$ dependence predicted by the Drude model of free-carrier absorption. The inter-conduction band absorption between the lowest conduction band and a higher conduction band occurs via a direct optical process at $λ\sim 349$ nm (3.55 eV). Steady state and ultrafast optical spectroscopy measurements unambiguously identify both these absorption processes and enable quantitative measurements of the inter-conduction band energy, and the frequency dependence of absorption. Whereas the intra-conduction band absorption does not depend on light polarization, inter-conduction band absorption is found to be strongly polarization dependent. The experimental observations, in excellent agreement with recent theoretical predictions for $β$-Ga$_2$O$_3$, provide important limits of sub-bandgap transparency for optoelectronics in the deep-UV to visible wavelength range, and are also of importance for high electric field transport effects in this emerging semiconductor.

physics.app-ph↗

arXiv2015

Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films

We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5-7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5-7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.

cond-mat.mtrl-sci↗

arXiv2014

First-principles study of the mobility of SrTiO$_3$

We investigate the electronic and vibrational spectra of SrTiO$_3$, as well as the coupling between them, using first-principles calculations. We compute electron-phonon scattering rates for the three lowest-energy conduction bands and use Boltzmann transport theory to calculate the room-temperature mobility of SrTiO$_3$. The results agree with experiment and highlight the strong impact of longitudinal optical phonon scattering. Our analysis provides important insights into the key factors that determine room temperature mobility, such as the number of conduction bands and the nature and frequencies of longitudinal phonons. Such insights provide routes to engineering materials with enhanced mobilities.

cond-mat.mtrl-sci↗

arXiv2013

High-Voltage Field Effect Transistors with Wide-Bandgap β-Ga2O3 Nanomembranes

Nanoscale semiconductor materials have been extensively investigated as the channel materials of transistors for energy-efficient low-power logic switches to enable scaling to smaller dimensions. On the opposite end of transistor applications is power electronics for which transistors capable of switching very high voltages are necessary. Miniaturization of energy-efficient power switches can enable the integration with various electronic systems and lead to substantial boosts in energy efficiency. Nanotechnology is yet to have an impact in this arena. In this work, it is demonstrated that nanomembranes of the wide-bandgap semiconductor gallium oxide can be used as channels of transistors capable of switching high voltages, and at the same time can be integrated on any platform. The findings mark a step towards using lessons learnt in nanomaterials and nanotechnology to address a challenge that yet remains untouched by the field.

cond-mat.mes-hall↗