Martien Ilse Den Hertog

Top-down fabrication of GaN-based nanowires offers a promising alternative to bottom-up approaches, providing enhanced control over geometry and doping profiles. In this work, single nanowire photodetectors were realized from planar GaN p−n junction structures, including a single p−n junction (SPN) and a structure incorporating additionally a tunnel junction (SPNTJ). Nanowires with diameters in the 150−300 nm range were fabricated using nanosphere lithography followed by dry and crystallographic wet etching. Electrical and optoelectronic characterization of individual nanowires contacted on Si3N4 membranes reveal rectifying behavior and ultraviolet photoresponse with a sharp cut-off at ∼364 nm. The devices exhibit responsivities in the range of 8−20 mA/W under reverse bias, increasing by 1−3 orders of magnitude under forward bias. The photocurrent follows a power-law dependence on optical irradiance, Iph∝Poptβ, with the exponent β strongly influenced by nanowire diameter and bias conditions. Linear response is achieved under reverse bias for thinner nanowires (150 nm), while larger diameter nanowires (300 nm) display pronounced sublinear behavior. The device operation is interpreted using a model combining junction-driven photocurrent generation and surface-state-controlled series resistance, accounting for the role of nanowire depletion and surface effects in determining linearity and dynamics. Results demonstrate that the performance of top-down fabricated nanowire photodetectors is comparable to that of their bottom-up counterparts in terms of responsivity, linearity, and temporal response. These results demonstrate the viability of top-down approaches for the fabrication of high-performance nanowire-based optoelectronic devices with improved design flexibility.

InN nanowires were grown on Si⟨111⟩ and Si⟨100⟩ substrates by plasma-assisted molecular beam epitaxy using a thin AlN buffer layer at temperatures compatible with the thermal budget limitation imposed by back-end-of-line processing. Reflection high-energy electron diffraction reveals differences in the AlN nucleation on the two substrate orientations, with higher structural disorder in the case of Si⟨100⟩. However, vertically aligned nanowires with a hexagonal cross section and N polarity are obtained on both substrates. A statistical analysis of nanowire morphology as a function of growth temperature indicates similar trends (increasing diameter and length, and decreasing density with increasing temperature) on Si⟨111⟩ and Si⟨100⟩, which are explained by adatom kinetics during growth. Nanowires on Si⟨100⟩ exhibit improved uniformity and reduced tapering, attributed to the different nanowire nucleation due to microstructural properties of the AlN buffer layer. The results demonstrate the feasibility of growing high-quality InN nanowires on Si⟨100⟩, supporting their potential for monolithic integration of nanowire-based photodetectors on silicon.

We have examined a focused ion beam prepared silicon p–n junction lamella with different thicknesses by Lorentz 4D scanning transmission electron microscopy using center-of-mass (CoM) analysis. Using convergence semi-angles of 992, 207, and 83 μrad, we demonstrate diffraction limited spatial resolution combined with deflection detection limits on the sample as good as 0.49 μrad. We assess electron beam deflection detection limits away from the p–n junction, as well as the variation along the junction, and compare these with the variation in a vacuum region. With the larger semi-convergence angle of 992 μrad, a rigid shift of the transmitted disk was observed in the junction, while using smaller angles, a redistribution of intensity in the transmitted beam occurs, leading to an underestimation of the electric field related to the larger electron probe size. It is shown that electric field detection limits as good as 0.033, 0.0072, and 0.0051 MV cm−1 could be obtained for decreasing semi-convergence angles (at 376 nm sample thickness), equivalent to angular detection limits of 3.2, 0.68, and 0.49 μrad, respectively. For an optimal detection limit, we show that about 50 counts per pixel within the diffracted disk are needed for all settings, allowing rapid acquisition of diffraction maps, mitigating sample drift and beam damage issues. Moreover, we demonstrate that the CoM shift increases linearly with electrically active sample thickness, as must be the case for a quantitative signal.

There has been extensive activity exploring the doping of semiconducting two-dimensional (2D) transition metal dichalcogenides in order to tune their electronic and magnetic properties. The outcome of doping depends on various factors, including the intrinsic properties of the host material, the nature of the dopants used, their spatial distribution, as well as their interactions with other types of defects. A thorough atomic-level analysis is essential to fully understand these mechanisms. In this work, the vanadium-doped WSe₂ monolayer grown by molecular beam epitaxy is investigated using four-dimensional scanning transmission electron microscopy (4D-STEM). Through center-of-mass-based reconstruction, atomic-scale maps are produced, allowing the visualization of both the electric field and the electrostatic potential around individual V atoms. To provide quantitative insights, these results are successfully compared to multislice image simulations based on ab initio calculations, accounting for lens aberrations. Finally, a negative charge around the V dopants is detected as a drop in the electrostatic potential, unambiguously demonstrating that 4D-STEM can be used to detect and to accurately analyze single-dopant charge states in semiconducting 2D materials.

Understanding the electronic transport of metal-semiconductor heterojunctions is of utmost importance for a wide range of emerging nanoelectronic devices like adaptive transistors, biosensors, and quantum devices. Here, we provide a comparison and in-depth discussion of the investigated Schottky heterojunction devices based on Si and Ge nanowires contacted with pure single-crystal Al. Key for the fabrication of these devices is the selective solid-state metal-semiconductor exchange of Si and Ge nanowires into Al, delivering void-free, single-crystal Al contacts with flat Schottky junctions, distinct from the bulk counterparts. Thereof, a systematic comparison of the temperature-dependent charge carrier injection and transport in Si and Ge by means of current-bias spectroscopy is visualized by 2D colormaps. Thus, it reveals important insights into the operation mechanisms and regimes that cannot be exploited by conventional single-sweep output and transfer characteristics. Importantly, it was found that the Al-Si system shows symmetric effective Schottky barrier (SB) heights for holes and electrons, whereas the Al-Ge system reveals a highly transparent contact for holes due to Fermi level pinning close to the valence band with charge carrier injection saturation due to a thinned effective SB. Moreover, thermionic field emission limits the overall electron conduction, indicating a distinct SB for electrons.

In this work, bottom‐up Al–Si–Al nanowire (NW) heterostructures are presented, which act as a prototype vehicle toward top‐down fabricated nanosheet (NS) and multi‐wire (MW) reconfigurable field‐effect transistors (RFETs). Evaluating the key parameters of these transistors regarding the on‐ and off‐currents as well as threshold voltages for n‐ and p‐type operation exhibit a high degree of symmetry. Most notably also a low device‐to‐device variability is achieved. In this respect, the investigated Al–Si material system reveals its relevance for reconfigurable logic cells obtained from Si NSs. To show the versatility of the proposed devices, this work reports on a combinational wired‐AND gate obtained from a multi‐gate RFET. Additionally, up‐scaling the current is achieved by realizing a MW RFET without compromising reconfigurability. The Al–Si–Al platform has substantial potential to enable complex adaptive and self‐learning combinational and sequential circuits with energy efficient and small footprint computing paradigms as well as for native components for hardware security circuits.

Epitaxial growth has become a promising route to achieve highly crystalline continuous two-dimensional layers. However, high-quality layer production with expected electrical properties is still challenging due to the defects induced by the coalescence between imperfectly aligned domains. In order to control their intrinsic properties at the device scale, the synthesized materials should be described as a patchwork of coalesced domains. Here, we report multi-scale and multi-structural analysis on highly oriented epitaxial WS₂ and WSe₂ monolayers using scanning transmission electron microscopy (STEM) techniques. Characteristic domain junctions are first identified and classified based on the detailed atomic structure analysis using aberration corrected STEM imaging. Mapping orientation, polar direction and phase at the micrometer scale using four-dimensional STEM enabled to access the density and the distribution of the specific domain junctions. Our results validate a readily applicable process for the study of highly oriented epitaxial transition metal dichalcogenides, providing an overview of synthesized materials from large scale down to atomic scale with multiple structural information.