arXiv subjects

Alex Henning

Publications and source records attributed to Alex Henning.

12 recordsLinked to original sources

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

Using metal-organic frameworks to confine liquid samples for nanoscale NV-NMR

Atomic-scale magnetic field sensors based on nitrogen vacancy (NV) defects in diamonds are an exciting platform for nanoscale nuclear magnetic resonance (NMR) spectroscopy. The detection of NMR signals from a few zeptoliters to single molecules or even single nuclear spins has been demonstrated using NV-centers close to the diamond surface. However, fast molecular diffusion of sample molecules in and out of nanoscale detection volumes impedes their detection and limits current experiments to solid-state or highly viscous samples. Here, we show that restricting diffusion by confinement enables nanoscale NMR spectroscopy of liquid samples. Our approach uses metal-organic frameworks (MOF) with angstrom-sized pores on a diamond chip to trap sample molecules near the NV-centers. This enables the detection of NMR signals from a liquid sample, which would not be detectable without confinement. These results set the route for nanoscale liquid-phase NMR with high spectral resolution.

physics.app-ph↗

arXiv2021

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↗

arXiv2021

Scalable transparent conductive thin films with electronically passive interfaces for direct chemical vapor deposition of 2D materials

We present a novel transparent conductive support structure for two-dimensional (2D) materials that provides an electronically passive 2D/3D interface while also enabling facile interfacial charge transport. This structure, which comprises an evaporated nanocrystalline carbon (nc-C) film beneath an atomic layer deposited alumina (ALD AlOx) layer, is thermally stable and allows direct chemical vapor deposition (CVD) of 2D materials onto the surface. When the nc-C/AlOx is deposited onto a 270 nm SiO2 layer on Si, strong optical contrast for monolayer flakes is retained. Raman spectroscopy reveals good crystal quality for MoS2 and we observe a ten-fold photoluminescence intensity enhancement compared to flakes on conventional Si/SiO2. Tunneling across the ultrathin AlOx enables interfacial charge injection, which we demonstrate by artifact-free scanning electron microscopy and photoemission electron microscopy. Thus, this combination of scalable fabrication and electronic conductivity across a weakly interacting 2D/3D interface opens up new application and characterization opportunities for 2D materials.

physics.app-ph↗

arXiv2021

A Roadmap for Controlled and Efficient n-type Doping of Self-assisted GaAs Nanowires Grown by Molecular Beam Epitaxy

N-type doping of GaAs nanowires has proven to be difficult because the amphoteric character of silicon impurities is enhanced by the nanowire growth mechanism and growth conditions. The controllable growth of n-type GaAs nanowires with carrier density as high as 1020 electron/cm3 by self-assisted molecular beam epitaxy using Te donors is demonstrated here. Carrier density and electron mobility of highly doped nanowires are extracted through a combination of transport measurement and Kelvin probe force microscopy analysis in single-wire field-effect devices. Low-temperature photoluminescence is used to characterize the Te-doped nanowires over several orders of magnitude of the impurity concentration. The combined use of those techniques allows the precise definition of the growth conditions required for effective Te incorporation.

cond-mat.mtrl-sci↗

arXiv2021

Surface NMR using quantum sensors in diamond

Characterization of the molecular properties of surfaces under ambient or chemically reactive conditions is a fundamental scientific challenge. Moreover, many traditional analytical techniques used for probing surfaces often lack dynamic or molecular selectivity, which limits their applicability for mechanistic and kinetic studies under realistic chemical conditions. Nuclear magnetic resonance spectroscopy (NMR) is a widely used technique and would be ideal for probing interfaces due to the molecular information it provides noninvasively. However, it lacks the sensitivity to probe the small number of spins at surfaces. Here, we use nitrogen vacancy (NV) centers in diamond as quantum sensors to optically detect nuclear magnetic resonance signals from chemically modified aluminum oxide surfaces, prepared with atomic layer deposition (ALD). With the surface NV-NMR technique, we are able to monitor in real-time the formation kinetics of a self assembled monolayer (SAM) based on phosphonate anchoring chemistry to the surface. This demonstrates the capability of quantum sensors as a new surface-sensitive tool with sub-monolayer sensitivity for in-situ NMR analysis with the additional advantage of a strongly reduced technical complexity.

physics.app-ph↗

arXiv2021

Aluminum Oxide at the Monolayer Limit via Oxidant-free Plasma-Assisted Atomic Layer Deposition on GaN

Atomic layer deposition (ALD) is an essential tool in semiconductor device fabrication that allows the growth of ultrathin and conformal films to precisely form heterostructures and tune interface properties. The self-limiting nature of the chemical reactions during ALD provides excellent control over the layer thickness. However, in contrast to idealized growth models, it is experimentally challenging to create continuous monolayers by ALD because surface inhomogeneities and precursor steric interactions result in island growth during film nucleation. Thus, the ability to create pin-hole free monolayers by ALD would offer new opportunities for controlling interfacial charge and mass transport in semiconductor devices, as well as for tailoring surface chemistry. Here, we report full encapsulation of c-plane gallium nitride (GaN) with an ultimately thin (~3 {\AA}) aluminum oxide (AlOx) monolayer, which is enabled by the partial conversion of the GaN surface oxide into AlOx using a combination of trimethylaluminum deposition and hydrogen plasma exposure. Introduction of monolayer AlOx significantly modifies the physical and chemical properties of the surface, decreasing the work function and introducing new chemical reactivity to the GaN surface. This tunable interfacial chemistry is highlighted by the reactivity of the modified surface with phosphonic acids under standard conditions, which results in self-assembled monolayers with densities approaching the theoretical limit. More broadly, the presented monolayer AlOx deposition scheme can be extended to other dielectrics and III-V-based semiconductors, with significant relevance for applications in optoelectronics, chemical sensing, and (photo)electrocatalysis.

cond-mat.mtrl-sci↗

arXiv2018

Charge Separation at Mixed-Dimensional Single and Multilayer MoS2/Silicon Nanowire Heterojunctions

Layered two-dimensional (2-D) semiconductors can be combined with other low-dimensional semiconductors to form non-planar mixed-dimensional van der Waals (vdW) heterojunctions whose charge transport behavior is influenced by the heterojunction geometry, providing a new degree of freedom to engineer device functions. Towards that end, we investigated the photoresponse of Si nanowire/MoS2 heterojunction diodes with scanning photocurrent microscopy and time-resolved photocurrent measurements. Comparison of n-Si/MoS2 isotype heterojunctions with p-Si/MoS2 heterojunction diodes under varying biases shows that the depletion region in the p-n heterojunction promotes exciton dissociation and carrier collection. We measure an instrument limited response time of 1 us, which is 10 times faster than previously reported response times for planar Si/MoS2 devices, highlighting the advantages of the 1-D/2-D heterojunction. Finite element simulations of device models provide a detailed understanding of how the electrostatics affect charge transport in nanowire/vdW heterojunctions and inform the design of future vdW heterojunction photodetectors and transistors.

physics.app-ph↗

arXiv2018

Self-Aligned van der Waals Heterojunction Diodes and Transistors

A general self-aligned fabrication scheme is reported here for a diverse class of electronic devices based on van der Waals materials and heterojunctions. In particular, self-alignment enables the fabrication of source-gated transistors in monolayer MoS2 with near-ideal current saturation characteristics and channel lengths down to 135 nm. Furthermore, self-alignment of van der Waals p-n heterojunction diodes achieves complete electrostatic control of both the p-type and n-type constituent semiconductors in a dual-gated geometry, resulting in gate-tunable mean and variance of anti-ambipolar Gaussian characteristics. Through finite-element device simulations, the operating principles of source-gated transistors and dual-gated anti-ambipolar devices are elucidated, thus providing design rules for additional devices that employ self-aligned geometries. For example, the versatility of this scheme is demonstrated via contact-doped MoS2 homojunction diodes and mixed-dimensional heterojunctions based on organic semiconductors. The scalability of this approach is also shown by fabricating self-aligned short-channel transistors with sub-diffraction channel lengths in the range of 150 nm to 800 nm using photolithography on large-area MoS2 films grown by chemical vapor deposition. Overall, this self-aligned fabrication method represents an important step towards the scalable integration of van der Waals heterojunction devices into more sophisticated circuits and systems.

physics.app-ph↗

arXiv2017

Control of interlayer delocalization in 2H transition metal dichalcogenides

It is assessed in detail both experimentally and theoretically how the interlayer coupling of transition metal dichalcogenides controls the electronic properties of the respective devices. Gated transition metal dichalcogenide structures show electrons and holes to either localize in individual monolayers, or delocalize beyond multiple layers - depending on the balance between spin-orbit inter- action and interlayer hopping. This balance depends on layer thickness, momentum space symmetry points and applied gate fields. A good quantitative agreement of predictions and measurements of the quantum confined Stark effect in gated MoS2 systems unveils intralayer excitons as major source for the observed photoluminesence.

cond-mat.mes-hall↗

arXiv2015

Multiple State EFN Transistors

Electrostatically Formed Nanowire (EFN) based transistors have been suggested in the past as gas sensing devices. These transistors are multiple gate transistors in which the source to drain conduction path is determined by the bias applied to the back gate, and two junction gates. If a specific bias is applied to the side gates, the conduction band electrons between them are confined to a well-defined area forming a narrow channel- the Electrostatically Formed Nanowire. Recent work has shown that by applying non-symmetric bias on the side gates, the lateral position of the EFN can be controlled. We propose a novel Multiple State EFN Transistor (MSET) that utilizes this degree of freedom for the implementation of complete multiplexer functionality in a single transistor like device. The multiplexer functionality allows a very simple implementation of binary and multiple valued logic functions.

cs.ET↗

arXiv2014

Why Lead Methylammonium tri-IODIDE perovskite-based solar cells requires a mesoporous electron transporting scaffold (but not necessarily a hole conductor)

CH3NH3PbI3-based solar cells were characterized with electron beam-induced current (EBIC), and compared to CH3NH3PbI3-xClx ones. A spatial map of charge separation efficiency in working cells shows p-i-n structures for both thin film cells. Effective diffusion lengths, LD, (from EBIC profile) show that holes are extracted significantly more efficiently than electrons in CH3NH3PbI3, explaining why CH3NH3PbI3-based cells require mesoporous electron conductors, while CH3NH3PbI3-xClx ones, where LD values are comparable for both charge types, do not.

physics.chem-ph↗