United States

Abstract Body: Atomically thin (two-dimensional, 2D) semiconductors have shown great potential as the fundamental building blocks for next-generation electronics. However, all the 2D semiconductors that have been experimentally made so far have room-temperature electron mobility lower than that of bulk silicon, which is not understood. Here, by using first-principles calculations and reformulating the transport equations to isolate and quantify contributions of different mobility-determining factors, we show that the universally low mobility of 2D semiconductors originates from the high “density of scatterings,” which is intrinsic to the 2D material with a parabolic electron band. The density of scatterings characterizes the density of phonons that can interact with the electrons and can be fully determined from the electron and phonon band structures without knowledge of electron-phonon coupling strength. Our work reveals the underlying physics limiting the electron mobility of 2D semiconductors and offers a descriptor to quickly assess the mobility. Ref: Long Cheng, Chenmu Zhang, and Yuanyue Liu, Phys. Rev. Lett. 125, 177701 (2020)

EMC 2021

Why Two-Dimensional Semiconductors Generally Have Low Electron Mobility

2d semiconductor; mobility; first-principles calculations

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Abstract Body: The use of p-type superlattices (SL) is valuable for III-nitride semiconductor-based LEDs and pFETs (p-type Field effect transistors) because the polarization effects create a periodic oscillation of the energy bands, enhancing the ionization of the deep acceptors (Ex: Mg) [1-3]. Recently, the existence of acceptor traps 0.8eV above the valence band of GaN, located at the positive polarization interfaces in III-nitride systems, was shown [4]. This acceptor trap level was the main source of holes in systems which had insufficient or no Mg doping in them. The present study experimentally demonstrates the effect of these acceptor traps on the charge and mobility of holes in N-polar p-type GaN/(AlN/AlGaN) superlattices. N-polar modulation doped GaN/(AlN/Al0.24Ga0.75N) 7-period superlattices were grown using metalorganic chemical vapor deposition (MOCVD). Each superlattice period was 20nm thick composed of 5nm GaN:Mg /5nm unintentionally doped (u.i.d) GaN/ x nm u.i.d. AlN/ (5-x) nm u.i.d AlGaN/ 5nm AlGaN:Mg, where the AlN layer thickness, x, was 0 nm ≤ x ≤ 1.6 nm. In addition to the AlN thickness series, the Mg doping was varied for each AlN thickness with 3.75×1018 cm-3 ≤ [Mg]≤ 2.5×1019 cm-3. The Mg doping in the samples was calibrated using Secondary Ion Mass Spectroscopy, and the SL quality and composition was measured using X-Ray Diffraction. Mg activation was carried out at 800○C and Pd/Au contacts were deposited to prepare the samples for room temperature Hall measurements. The experimental results were compared to those calculated using STR-FETIS. When the band-bending of both GaN and AlN was < 0.8eV, the sheet charge concentration of holes (ps) measured using Hall matched with the calculated values. For each of the samples where band-bending of either GaN or AlN was ≥ 0.8eV(without accounting for the impact of trap ionization on band-bending), the measured ps was greater than the amount of Mg dopants put into the sample. This showed that the excess holes came from the acceptor traps at the positive polarization interfaces, here - AlGaN/GaN and AlN/AlGaN interfaces. The acceptor trap ionized when either band-bending was ≥ 0.8eV and the Fermi level was pinned at this trap level leading to the very high measured ps. A low sheet resistance (~3.4kΩ/■) was observed for N-polar modulation doped GaN/AlN/AlGaN SLs, with ps~1.3×1014 cm-2 and µp~14 cm2/Vs. For the samples in this study, the two-dimensional hole gas (2DHG) exists at the GaN/AlN interface or a GaN/AlGaN interface, if no AlN present. It was observed for each of the AlN thickness series (with various Mg doping levels) the µp decreased in samples where the acceptor traps ionized. It was further observed within those set of samples that µp decreased to a great extent (>50% decrease) for those samples where AlN band-bending was ≥ 0.8eV – independent of the GaN band-bending in the same sample. The 2DHG is closer to the AlN/AlGaN positive polarization interface (PPI-1) (0 to 1.6nm) compared to the AlGaN/GaN positive polarization interface (PPI-2)(10nm). Therefore, the more dominant source of scattering (and hence the decreased mobility) are the ionized acceptor traps which exist at the AlN/AlGaN positive polarization interface, the closer interface. While the ionized traps at AlGaN/GaN interface also cause scattering, their effect is not as pronounced because they are far away from the 2DHG. This explains the higher mobility observed in in samples where only the GaN band-bending was ≥ 0.8eV, and the AlN band-bending was < 0.8eV leading to the ionization of traps at only the AlGaN/GaN positive polarization interface (PPI-2). Further details of simulations, experiments and methodology will be presented at the conference. [1] P. Kozodoy, et al., APL(1999) [2] A. Raj, et. al., IEEE-EDL (2020) [3] C.J. Zollner, et. al., OME (2020) [4] A. Krishna, et. al., APL (2020)

Effect of Acceptor Traps at Positive Polarization Interfaces on the Charge and Mobility of Holes in N-Polar P-Type GaN/(AlN/AlGaN) Superlattices

Abstract Body: β -Ga2O3 has attracted considerable technological and scientific interest due to its ultra-wide ~ 4.8 eV band gap and its extremely high projected ~ 8 MV/cm breakdown field, both of which can advance high-power and high-speed electronics. However, to reach the full potential of these applications, an understanding and control of electrically - active native point defects and impurities in β -Ga2O3 is needed. Besides their contribution to shallow dopants, deep levels of such extrinsic states can compensate free carriers and have a major impact on their recombination and transport, and possibly their ultimate breakdown voltage strength. Luminescence studies [1-3] have identified commonly observed emissions at 2.5 eV, 2.8 eV, 3.0 eV, 3.5 eV and 3.8 eV, some of whose nature can be related to native defects. The deep level defect nature of extrinsic impurities such as Ir in β -Ga2O3 is of interest since high quality single crystal growth techniques such as edge-defined film-fed growth (EFG) make use of iridium dies and crucibles and because large EFG crystals can be grown with quality that is suitable for technological applications. Hence, the impact of Ir in β -Ga2O3 is of both scientific and practical concern. We used depth-resolved cathodoluminescence spectroscopy (DRCLS) and surface photovoltage spectroscopy (SPS) to explore the nature of optical transitions at Ir- Tamura Corp. (010) UID EFG interfaces. Low-kV e-beam excitation through 25 nm-thick, e-beam-deposited Ir-Ga2O3 diodes provided DRCL spectra with near-nm depth resolution. Near - surface and bulk DRCLS of adjacent bare Ga2O3 reveals a prominent ~3.2 eV emission similar to a photoluminescence/cathodoluminescence feature reported previously [4,5], while our SPS identifies a similar transition located 3.15 eV± 0.1 eV below the conduction band. These results are consistent with various theoretical and experimental studies that attribute the ~3.2 eV emission to recombination involving self-trapped holes. Near-nm scale DRCLS at the Ir-Ga2O3 interface shows the emergence of 2.1 eV and 2.6 eV features, which is consistent with transitions associated with the complementary IrGa(II) (0/+) and IrGa(II) (+/0) levels predicted theoretically [6]. The ~3.2 eV emission peak energy maximum also shifts downward towards the Ir-Ga2O3 interface, which correlates with the emergence of a sharp ~2.9 eV emission that can possibly be identified with a IrGa(I) charge state transition. These measurements provide a distinction as well as possible interplay between deep level emission due to Ir impurities and possible self-trapped polarons that can introduce deep level recombination that affect free carrier transport, density, and dielectric breakdown. These results also suggest that Ir is readily incorporated into the Ga2O3 lattice and can produce a major luminescence feature that may produce significant free carrier recombination and compensation. Research efforts are currently underway to perform rapid-thermal annealing on the Ir-deposited sample to determine the activation energy and diffusion coefficient for Ir in Ga2O3. DNR, MSH, and LJB gratefully acknowledge AFOSR Grant No. FA9550-18-1-0066 (Ali Sayir). [1] H. Gao et al., Appl. Phys. Lett. 112, 242102 (2018). [2] H. Gao et al., J. Phys. D: Appl. Phys. 53 465102 (2020). [3] Y. Wang et al., Sci. Rep. 8, 18075 (2018). [4] S. Yamaoka et al., Physical Review B 95, 094304 (2017). [5] T. Onuma et al., Appl. Phys. Lett. 103, 041910 (2013). [6] J.R. Ritter et al., Appl. Phys. Lett. 113, 052101 (2018).

Deep Levels and Self-Trapped Excitons at Iridium/Edge-Fed Grown β-Ga2O3 Interfaces

Abstract Body: β-gallium oxide (Ga2O3) is a promising material for applications in power electronics and as a transparent conducting oxide due to its wide bandgap of 4.8 eV and high breakdown electric field of 8 MV/cm. Fundamental properties such as doping, mobility, optical absorption, luminescence, and charge trapping are determined by the device’s ensemble of defects. Thermodynamic simulations model the limiting case where the system has reached equilibrium; however, all real systems are limited by both kinetics and thermodynamics during device processing and operation. Here, we present a coupled reaction-diffusion solver used to study time-dependent, one- and zero-dimensional systems, going beyond thermodynamic predictions for defects. By using a detailed reaction mechanism, we can model carrier-defect interactions, such as trapping and emission, resulting in an in-depth description of time-dependent device behavior and accurate device characterization. The solver uses the finite volume method to approximate spatial derivatives and implicit time stepping for time derivatives. This allows us to model surface defect formation and subsequent diffusion into the bulk, as well as bulk defect formation. Using an implicit time stepping scheme enables us to study an extraordinary range of time scales, from femtoseconds to years, unlocking possibilities to simulate device lifetime reliability in addition to device processing.

Time-Dependant Simulations of Defect Kinetics and Thermodynamics in Gallium Oxide

Abstract Body: Rare-earth chromites (RCrO3) have Néel temperature (TN, above which these become paraelectric) in the range of 100-280 K, which is much higher than the magnetic transitions observed in rare-earth manganites (RMnO3). These family of compounds have been studied for the magnetoelectric-multiferroic (ME-MF) for their potential applications in various devices including magnetic field sensors, energy harvesting devices, and magnetic refrigeration (MR), to name some. In case of ME-MF based devices, it is desirable to have the transitions close to room temperature. In this talk, the structure, dielectric, and magnetic properties of some RMnO3 and RCrO3 based materials will be presented with a focus on their band gap and mutliferroic behavior and the structure-property relations.

Rare-Earth Chromites and Rare-Earth Manganites—Structural-Property Relations

Abstract Body: In recent years, III-V semiconductor nanowires (NWs) have significantly attracted researchers due to one-dimensional architecture, quantum confinement effects, and a higher tolerance for stress-strain mismatch that allow greater freedom in engineering combinations of material systems in a variety of various nanowire architectures to meet the demands of next-generation optoelectronic devices. Dopant incorporation in a well-controlled manner is essential to realize advanced devices in NW configuration successfully. Unfortunately, the knowledge obtained on dopant incorporation and carrier concentration from the thin film studies cannot be directly translated to NWs due to the dopant’s influence on the growth kinetics and different growth mechanisms along with the axial and radial directions. The commonly used measurement techniques in thin films for the determination of carrier concentration namely, Hall effect, field-effect transistor, and capacitance-voltage require highly sophisticated lithography steps. In the last decade, several characterization methods have evolved for the assessment of dopants in NWs namely, off-axis electron holography for which the sample preparation is complex, and it requires additional information on the NW thickness and homogeneity; secondary ion mass spectrometry and atom probe tomography requires a standard of known dopant concentration and is destructive. Therefore, a combination of conductive-atomic force microscopy (C-AFM), scanning Kelvin probe microscopy (SKPM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS), provide a set of excellent characterization techniques for doping assessment, as they do not involve any complex contact fabrications. In this work, we evaluate the carrier concentration and incorporation of Beryllium (Be) dopants in GaAs NWs and Tellurium (Te) dopants in GaAsSb NWs grown using Ga-assisted molecular beam epitaxy (MBE) with variation in Be cell temperatures (TBe) from 750 oC to 950 oC and Te cell temperatures (TGaTe) from 500 oC to 570 oC, respectively. The extensive research interest in GaAs and GaAsSb NWs stems from its narrow bandgap region covering the important optical telecommunication wavelengths of 1.3 µm and 1.55 µm. The current-voltage measurements using C-AFM of the vertical single NW showed a significant enhancement in current for GaAs NWs doped with TBe at 950 oC and GaAsSb NWs doped with TGaTe at 550 oC. The carrier concentration of Be-doped GaAs and Te-doped GaAsSb NWs are found using COMSOL Multiphysics fitting. From XPS and UPS, the atomic percent, work function, and carrier concentration of Be-doped GaAs NWs and Te-doped GaAsSb NWs are determined. A shift in Fermi level towards the valence band with increasing TBe for GaAs NWs, provided evidence towards higher Be incorporation with TBe - 950 oC doped NWs having a work function of 5.6 eV. Whereas the Fermi level is shifted towards the conduction band in Te–doped GaAsSb NWs. The values of electron density from UPS concur very well with the values of electron density determined from C-AFM. The change in surface potential observed in doped NWs in SKPM characterization also provided strong evidence of Be and Te incorporation. Hence, these surface analytical tools without any contact fabrication are found to be powerful characterization techniques for direct measurement of the dopant levels in NWs, which are critical for bandgap engineering design of optoelectronic devices. Acknowledgement This material is based upon research supported by the Air Force Office of Scientific Research (AFOSR) under grant number W911NF1910002.

Doping Assessment of Ga-Assisted MBE Grown Be-Doped GaAs and Te-Doped GaAsSb Nanowires for Infrared Photodetector Application

Abstract Body: Point defects in wide bandgap semiconductors can provide robust, on-demand single photons that can be used in quantum technologies. Despite the appeal of defect-based single-photon emitters (SPEs), sub-diffractional length-scales of defects make traditional optical characterization techniques insensitive to local variations in the material response. Traditionally, photoluminescence (PL) spectroscopy is used to measure the emission spectrum and even map local intensities, however, spatial resolution is typically restricted to sizes on the order of a micron. However, advances in nano-optic probe technologies such as scattering-type scanning near-field optical microscopy (s-SNOM), can provide optical information about a material with nanoscale spatial resolution (< 20nm)1. Here, s-SNOM employs local evanescent fields confined to the apex of a metallized AFM tip following irradiation with a monochromatic laser to probe the material response with this sub-diffractional spatial resolution. While s-SNOM can be performed with a range of frequencies from visible to THz, infrared frequencies are used to exploit the contrast in vibrational resonances between a material and any local defects, thereby creating a two-dimensional spatial map. The tradeoff for high resolution s-SNOM is excessive scan times over large areas (>100μm2) and imaging at only a single incident frequency, therefore another imaging technique must be used to navigate the sample of interest and to identify spectroscopic resonances of interest. Here, PL and nano-FTIR spectroscopies offer a highly complementary toolset with s-SNOM. Using PL intensity mapping in a scanning confocal microscope, ensembles of nano diamonds (d=134nm) with nitrogen-vacancy (NV) defects can be identified by high-intensity regions (Fig. 1a) and verified from the corresponding collected emission spectrum (Fig. 1b). FTIR spectroscopic measurements of similar diamond samples with very high NV densities have been found to correlate with additional vibrational absorption bands in the 1100-1400cm-1 range (Fig. 1c) with two prominent peaks2, indicative of vibrational modes due to the modified crystal bonds making up the NV defect. By mapping spatial regions featuring nano diamond NVs via s-SNOM, we hypothesize that one can map out the local distribution of the defect vibrational states. This is presented through s-SNOM phase maps recorded within a region of high PL intensity using a probing frequency (1177cm-1) within the absorption band of the NVs (Fig. 1d). Regions of high phase contrast correspond to high local absorption and three regions, circled in white, display magnitudes of high contrast at this frequency. However, the phase contrast is diminished when probing the sample with a different frequency (1450cm-1), outside of the NV absorption band (Fig. 1e). Regions of high phase contrast present in both images (1d,e) are attributed to fabrication residue, polymethyl methacrylate, which absorbs at both probed frequencies. The s-SNOM and PL maps suggest that the highlighted features are nano diamonds containing photoluminescent NVs but is still inconclusive. However, nano-FTIR provides direct measurements of IR reflection and absorption spectra from samples without diffraction limitations3 using s-SNOM principles. Employing it to measure the spectra of the high contrast regions will enable a direct comparison to its bulk counterpart, conclusively determining the nature of the additional vibrational modes. Correlating PL with s-SNOM imaging has been demonstrated on extended defects, but not so with point defects4. Diamond NVs are appropriate for developing this technique because of their high brightness at room temperature5. This technique could provide a non-destructive method of investigating defect-based SPEs in materials such as hexagonal boron nitride and gallium nitride, whose defect structures have not been fully characterized but are yet still under intense investigation.

Identifying Defect Origins of Single-Photon Emitters Using Infrared Nano-Optic Probes

Abstract Body: Core-shell semiconductor nanowires (NWs) are an attractive alternative to planar devices for multiple applications, including field-effect transistors, photovoltaic devices, light-emitting diodes, and lasers. The core-shell radial junction structure enables electron- or photo-induced electron-hole pairs (EHPs) to be collected with shorter diffusion lengths due to the small NW diameter. Furthermore, the collection of the EHPs inside the core is free from surface recombination, potentially improving the energy harvesting efficiency. Electron-induced-beam current (EBIC) measurement is a complementary technique in situ in a scanning electron microscope (SEM), collecting excess carriers in a biased diode as a function of beam position and accelerating voltage. In this talk, we will present the analysis of carrier diffusion properties in unprocessed, free standing core-shell GaAs NW diodes. The carrier kinetics in both the n-type core and the p-type shell were determined by analyzing radial EBIC profiles as a function of beam energy. These profiles are highly sensitive to changes in depletion widths and minority carrier diffusion lengths due to geometric effects. Combined with Monte Carlo simulations, they permitted measurement of the n-type core, minority carrier diffusion length, as well as the depletion widths as a function of radial direction. A relatively short minority carrier diffusion length (50 nm) found near {112}B surface facets can be attributed to bulk point defects originating from low-temperature growth (400 °C). Depletion widths were found to be smaller in <112>B directions than in <112>A, likely due to different dopant corporation efficiencies of Te and Zn in the core and the shell, respectively, and to the very thin shell on the {112}B surfaces.

Measurement of Minority Carrier Diffusion Lengths in Core-Shell GaAs NW p-n Junctions

Abstract Body: Printed electronics is an inexpensive way of producing electronic devices for every-day applications such as flexible biosensors, affordable RFID price tags and large-area optoelectronics. Transistors are the most fundamental components of many digital and analog circuits. Printed organic thin-film transistors (OTFT) consist of different functional layers deposited from solution. The conductive electrodes are commonly printed using metal nanoparticles that need to be sintered to become conductive. These electrodes are usually printed on other previously solution-processed layers. However, annealing the electrodes will impose another annealing step on the previously deposited layers. Depending on the chemical structure of these layers, their surface or bulk might undergo changes that might affect the final device performance. In bottom-contact bottom-gate organic transistors, the source and drain electrodes are printed onto the gate dielectric layer, which will undergo the electrode post-annealing. In this work, the effect of changing the annealing temperature of silver electrodes in p-type organic transistors is studied. ITO-coated glass is used as the substrate, where the ITO layer works as the gate electrode. A dielectric layer of Teflon AF 1600 (3% in Fluorinert FC-40) then covers the ITO layer through two spin-coating steps. This dielectric layer is subsequently annealed at 150°C for 30 minutes resulting in 450 nm thickness. Since this dielectric layer has low surface energy, it is not possible to form a line of connected ink droplets through printing, therefore the dielectric layer is plasma treated to increase its surface energy. Afterward, for the source and drain electrodes, silver nanoparticle ink is inkjet-printed with a custom-made printer and a 60 μm diameter nozzle. The transistors have varying channel length (20 - 75 μm) and 1.3 mm width. These electrodes are then sintered at different temperatures for 30 minutes (after 5 minutes heating at 60°C to prevent coffee ring effect). This step also acts as a second post-annealing step for the dielectric layer. Finally, the organic semiconductor PDBPyBT is spin-coated in the last step, followed by annealing at 120°C for 10 minutes. The transistors are p-type and are biased at a drain voltage of -120 V in saturation. Silver annealing temperatures of 80°C, 100°C, 120°C, and 200°C are studied. Devices annealed at 120°C show the best performance with the maximum saturation field-effect mobility of 0.13 cm2/V.s and the maximum saturation Ion/off ratio of 3.41x107. The difference in performance between different electrode sintering temperatures cannot be explained by changes in electrode conductivity because it is much lower than the channel resistance in all studied cases for this ink. The bulk dielectric properties of the Teflon gate dielectric also remain unchanged during the electrode annealing step with a dielectric constant of 2.1. However, the Teflon surface properties are changed. Water contact angle increases monotonically from 85° with no post-annealing to 120° with 200°C post-annealing. With 200°C electrode annealing, this leads to partial dewetting and accumulation of the semiconductor between the more wettable source and drain electrodes. The resulting large thickness of the semiconductor mainly deteriorates device off-state performance reducing the average on-off-ratio from 1.14x107 to 7.25x105. For 80°C, 100°C, and 120°C electrode annealing, the semiconductor thickness remains constant at 100 nm, however, the Teflon surface becomes more hydrophobic. It has been observed before that organic semiconductors perform better on more hydrophobic surfaces, which usually requires a chemical surface modification step. Here, we show that this can be achieved by a simple post-annealing step, which is already part of device fabrication to sinter the source and drain electrodes and should therefore be considered when designing the process flow of the complete device.

Effect of Annealing Printed Electrodes on Gate Dielectric Hydrophobicity and Device Performance in Organic Transistors

Abstract Body: Conjugated polymers (CPs) have found great success as active materials in organic electrochemical transistors (OECTs), which can be integrated into bioelectronic devices for cell monitoring and stimulation.1,2 The success of CPs in this application is dependent on their material design: CPs must exhibit electrochemical activity in aqueous environments and support OECT operation in the energy-efficient, accumulation mode. Current materials such as PEDOT:PSS present several challenges, including depletion mode operation in OECTs, which is less energy-efficient; water solubility, which necessitates crosslinking or post-treatment;3 and high acidity, which can limit their biocompatibility with specific cell lines.4 Consequently, there is a clear need to develop new CPs for aqueous OECTs which overcome these limitations. To this end, we have synthesised and characterised a new CP, poly(ethylenedioxythiophene) functionalised with isopropyl-protected phosphonate groups (PEDOT-Phos). This material is readily processed in organic solvents, can be proton doped with non-oxidising acids, and exhibits reversible electrochemical activity in aqueous electrolytes. Most importantly, we used PEDOT-Phos as the channel material in aqueous, accumulation mode OECTs which boast low threshold voltages and high volumetric capacitances and transconductances, without requiring additives or post-treatments. Overall, PEDOT-Phos is a promising electronic material for applications in energy-efficient, organic bioelectronics. References: (1) Hopkins, J.; Travaglini, L.; Lauto, A.; Cramer, T.; Fraboni, B.; Seidel, J.; Mawad, D. Photoactive Organic Substrates for Cell Stimulation: Progress and Perspectives. Adv. Mater. Technol. 2019, 1800744, 1800744. https://doi.org/10.1002/admt.201800744. (2) Fidanovski, K.; Mawad, D. Conjugated Polymers in Bioelectronics: Addressing the Interface Challenge. Adv. Healthc. Mater. 2019, 8 (10), 1–9. https://doi.org/10.1002/adhm.201900053. (3) Stavrinidou, E.; Leleux, P.; Rajaona, H.; Khodagholy, D.; Rivnay, J.; Lindau, M.; Sanaur, S.; Malliaras, G. G. Direct Measurement of Ion Mobility in a Conducting Polymer. Adv. Mater. 2013, 25 (32), 4488–4493. https://doi.org/10.1002/adma.201301240. (4) Mantione, D.; del Agua, I.; Sanchez-Sanchez, A.; Mecerreyes, D. Poly(3,4-Ethylenedioxythiophene) (PEDOT) Derivatives: Innovative Conductive Polymers for Bioelectronics. Polymers (Basel). 2017, 9 (12), 354. https://doi.org/10.3390/polym9080354.

Synthesis and Characterisation of Phosphonated PEDOT—An Organic Material for Energy-Efficient Bioelectronics

Abstract Body: Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs) have captivated the attention of the scientific community for about a decade now because they have demonstrated a multitude of desirable properties for flexible electronics and optoelectronics. However, a major materials challenge of 2D semiconductors is that they tend to form a substantial Schottky barrier (SB) with most metals used for making electrical contacts, severely limiting both the fundamental research and electronics/optoelectronics applications of 2D semiconductors. Efforts to eliminate/minimize the SB by selecting contact metals with high (low) work function for holes (electrons) have been hampered by the nearly ubiquitous presence of Fermi-level pinning. Van der Waals (vdW) contacts have recently emerged as a promising contact-engineering strategy to minimize the Fermi-level pinning effect at the metal/2D semiconductor interface. Electrical contacts formed by transferring prefabricated metal electrodes onto 2D semiconductors demonstrated SB heights (SBHs) approaching the Schottky-Mott limit, which was attributed to the minimization of structural and chemical defects at the interface by avoiding direct metal deposition on the 2D semiconductors. However, these contacts are still depletion-type with a notable positive SBH even when the work function of metal is significantly larger than the ionization energy of 2D semiconductors, indicating that Fermi-level pinning is still present. To date, accumulation-type ohmic contacts completely free of a SB have not been experimentally demonstrated in 2D semiconductor FET devices to the best of our knowledge. We report the realization of accumulation-type vdW Ohmic contacts between nearly intrinsic WSe2 and degenerately p-doped MoS2 (p+-MoS2). Dual-gated WSe2 field-effect transistors (FETs) with p+-MoS2 bottom-contacts were fabricated as a test structure to demonstrate true Ohmic contacts free of a Schottky barrier without the need of electrostatically gating the contact regions. Despite that the back-gate electric field is screened by the p+-MoS2 bottom-contacts and the nearly intrinsic nature of WSe2, these devices exhibit ohmic behavior and similarly high performance in the back-gate as well as in the top-gate configuration (with the other gate grounded), including linear output characteristics, a high on/off ratio of 108, and two-terminal extrinsic field-effect mobility exceeding 130 cm2V-1s-1 at room temperature, which approaches the intrinsic mobility of few-layer WSe2 (~ 200 cm2V-1s-1) with further reduction of the series resistance of excess p+-MoS2. The observed ohmic behavior persists to low temperatures. Theoretical modeling shows that a p+-MoS2/WSe2 junction effectively acts as an accumulation-type metal/semiconductor ohmic contact, as signified by an extremely thin depletion region of ~ 1nm in the p+-MoS2 side of the junction and that a substantial accumulation layer of free holes in the WSe2 side. The accumulation-type ohmic contacts between p+-MoS2 and WSe2 is further verified by the results of dual-gated measurements. We attribute the accumulation-type ohmic contacts to nearly ideal metal-semiconductor interface of a vdW junction consisting of 2D materials with similar crystalline and electronic structures, where the SBH is dictated by the Schottky-Mott rule. Because the work function of p+-MoS2 (acting as a 2D metal) is slightly larger than the ionization energy of WSe2 as verified by X-ray photoelectron spectroscopy (XPS), a strong hole accumulation region develops in WSe2 at the vdW interface as predicted by the Schottky-Mott rule. The 2D vdW contacts with negligible Fermi-level pinning can also be used as the baseline to study the mechanism of Fermi-level pinning at the metal/2D-semiconductor interface. *This work was partially supported by NSF (DMR-2004445; ECCS-1849578) and the Kaskas Scholarship Funds.

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Effect of Acceptor Traps at Positive Polarization Interfaces on the Charge and Mobility of Holes in N-Polar P-Type GaN/(AlN/AlGaN) Superlattices

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Effect of Acceptor Traps at Positive Polarization Interfaces on the Charge and Mobility of Holes in N-Polar P-Type GaN/(AlN/AlGaN) Superlattices

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