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Vanadium dioxide (VO2) manifests an abrupt metal–insulator transition (MIT) from monoclinic to rutile phases, with potential use for tunable electronic and optical properties and spiking neuromorphic devices. Understanding pathways to modulate electronic transport in VO2, as well as its response to irradiation (e.g., for space applications), is critical to better enable these applications. In this work, we investigate the selective modulation of electronic transport in VO2 films subject to different 10 keV helium ion (He+) fluences. Under these conditions, the resistivity in the individual monoclinic and rutile phases varied by 50%–200%, while the MIT transformation temperature remains constant within 4 °C independent of irradiation fluence. Importantly, different trends in the resistivity of the monoclinic and rutile phases were observed both as a function of total He fluence as well as in films grown on different substrates (amorphous SiO2/Si vs single crystal Al2O3). Through a combination of measurements including majority carrier sign via Seebeck, low frequency noise, and TEM, our investigation supports the presence of different kinds of point defects (V in; O in), which may arise due to grain boundary defect interactions. Our work suggests the utility of He irradiation for the selective modulation of VO2 transport properties for neuromorphic, in contrast to other established but non-selective methods, like doping. [ABSTRACT FROM AUTHOR]

Conventional high-energy ion implant processes lack implant depth precision and minimally damaging properties needed to dope atomically thin two-dimensional (2D) semiconductors by ion modification without undesirable side effects. To overcome this limitation, controllable, reproducible, and robust doping methods must be developed for atomically thin semiconductors to enable commercially viable wafer-scale 2D material-based logic, memory, and optical devices. Ultralow energy ion implantation and plasma exposure are among the most promising approaches to realize high carrier concentrations in 2D semiconductors. Here, we develop two different plasma processes using commercially available semiconductor processing tools to achieve controllable electron and hole doping in 2H-MoS2. Doping concentrations are calculated from the measured Fermi level shift within the MoS2 electronic bandgap using x-ray photoelectron spectroscopy. We achieve electron doping up to 1.5 × 1019 cm−3 using a remote argon/hydrogen (H2) plasma process, which controllably generates sulfur vacancies. Hole doping up to 4.2 × 1017 cm−3 is realized using an inductively coupled helium/SF6 plasma, which substitutes fluorine into the MoS2 lattice at sulfur sites. The high doping concentrations reported here highlight the potential of scalable plasma processes for MoS2, which is crucial for enabling complementary circuits based on 2D semiconductors. [ABSTRACT FROM AUTHOR]

We have previously shown that one-dimensional edge contact to two-dimensional heterostructure field effect transistors with a graphene channel, and hexagonal boron nitride, as both the substrate material and top encapsulated layer, show ultrahigh carrier mobility and ultralow carrier noise (low frequency noise, LFN). Although the noise mechanism of the transistor devices in the ON state was modeled based on microscopic scattering mechanisms caused by charge carriers and phonons, the model was based on normalized noise spectral density following Hooge's noise model in the ON-state. In this article, we show that the normalized noise current densities within the charge residual region of two similarly fabricated edge-contacted and encapsulated graphene heterostructure field effect transistors show different trends with overdrive voltage that resemble previously reported results in the literature. Here we explain the poorly understood and conflicting LFN trends in the charge residual region (low charge carrier regime) by correlating the spatial charge inhomogeneity near the Dirac point in heterostructure devices to electron–hole puddles. By systematically studying the temperature-dependent transport, LFN and by modeling the channel characteristics in such devices within the charge residual region, we show that the carriers inside electron–hole puddles get pinned at lower temperatures and unpinned at higher temperatures, as consistently shown in their normalized flat-band spectral variation as a function of overdrive voltage. More generally, near the charge neutrality region of single-layer graphene devices, the LFN is described by a correlated carrier number and mobility fluctuations of the charge carriers. Understanding the carrier physics in encapsulated high-performance and edge-contacted heterostructure two-dimensional devices will aid us in not only engineering low-noise graphene-based FETs for future digital and analog electronics but also understanding the noise associated with Dirac-type materials. [ABSTRACT FROM AUTHOR]

Gold–germanium (Au x Ge 1 − x ) solid solutions have been demonstrated as highly sensitive thin film thermometers for cryogenic applications. However, little is known regarding the performance of the films for thicknesses less than 100 nm. In response, we report on the resistivity and temperature coefficient of resistance (TCR) for sputtered films with thicknesses ranging from 10 to 100 nm and annealed at temperatures from 22 to 200 ° C. The analysis is focused upon composition x = 0.17 , which demonstrates a strong temperature sensitivity over a broad range. The thinnest films are found to provide an enhancement in TCR, which approaches 20% K − 1 at 10 K. Furthermore, reduced anneal temperatures are required to crystallize the Ge matrix and achieve a maximum TCR for films of reduced thickness. These features favor the application of ultra-thin films as high-sensitivity, on-device thermometers in micro- and nanolectromechanical systems. [ABSTRACT FROM AUTHOR]

We examine the DC and radio frequency (RF) response of superconducting transmission line resonators comprised of very thin NbTiN films, < 12 nm in thickness, in the high-temperature limit, where the photon energy is less than the thermal energy. The resonant frequencies of these superconducting resonators show a significant nonlinear response as a function of RF input power, which can approach a frequency shift of Δ f = − 0.15 % in a − 20 dB span in the thinnest film. The strong nonlinear response allows these very thin film resonators to serve as high kinetic inductance parametric amplifiers. [ABSTRACT FROM AUTHOR]

The nuclear spins of low-density implanted Ga atoms in Ge are interesting candidates for solid state-based qubits. To date, activation studies of implanted Ga in Ge have focused on high densities. Here, we extend activation studies into the low-density regime. We use spreading resistance profiling and secondary ion mass spectrometry to derive electrical activation of Ga ions implanted into Ge as a function of the rapid thermal anneal temperature and implant density. We show that for our implant conditions, the activation is best for anneal temperatures between 400 and 650 °C with a maximum activation of 69% at the highest fluence. Below 400 °C, remaining implant damage results in defects that act as superfluous carriers, and above 650 °C, surface roughening and loss of Ga ions are observed. The activation increased monotonically from 10% to 69% as the implant fluence increased from 6 × 10 10 to 6 × 10 12 cm−2. The results provide thermal anneal conditions to be used for initial studies of using low-density Ga atoms in Ge as nuclear spin qubits. [ABSTRACT FROM AUTHOR]

Palladium (Pd) electrodes have enabled superior performance in Te‐based devices. Theory predicts strong Fermi level (EF) pinning near the Te valence band, which likely explains the predominant p‐type conduction in Te transistors. The effects of the native TeOx on the contact polarity has not been explored. In this work, X‐ray and ultraviolet photoelectron spectra (XPS and UPS, respectively) reveal the native surface oxide de‐pins the EF between Pd and trigonal Te (t‐Te) and can reduce contact resistance of hole and electron contacts to Te for back end of line‐compatible complementary logic circuits. Atomic hydrogen reduces the native t‐Te oxide below the XPS detection limit, after which, the EF resides near the t‐Te conduction band edge. Therefore, an n‐type band alignment is formed between Pd and t‐Te via an atomic hydrogen treatment. Furthermore, UPS shows the EF is pinned near the t‐Te conduction band edge. XPS indicates the formation of a PdTex intermetallic at the Pdt‐Te interface also affects the electrostatics of the interface. The concentration of PdTex is 40% higher when TeOx is removed from the t‐Te surface before Pd metallization, likely the root cause of the low (pinned) work function when the native oxide is absent. [ABSTRACT FROM AUTHOR]

Atomically thin tungsten diselenide (WSe2) is a promising 2D semiconductor for nanoelectronics and optoelectronics. Using UV ozone and low‐power O2 plasma treatments, it is demonstrated that the formation of WSe2(1−x)O2x (WSeyOx) leads to hysteretic behavior in vertical transport measurements and also enables to an improvement in the p‐type transfer characteristics in lateral transport measurements. The amount of oxidation correlates well with the resistive switch behavior in oxidized WSe2/graphene, and WSeyOx formation under the electrical contact of the horizontal devices leads to increased p‐branch on/off by 100×. In addition to its effect for residue removal, oxidation on field effect transistor channel also helps mitigate n‐type dominated transfer characteristics of WSe2 commonly seen on sapphire. It is demonstrated that light oxidation of WSe2 is a multifunctional post‐growth treatment that enables vertical resistive switch junctions, contact improvement, and continuous tuning of transistor transport properties. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide, a two-dimensional transition metal dichalcogenide, was analyzed using in situ x-ray photoelectron spectroscopy (XPS). The XPS spectra obtained from a fresh surface which was exfoliated and annealed in ultrahigh vacuum include a survey scan, high resolution spectra of O 1s, C 1s, Mo 3d, S 2s, S 2p, Mo 3p, Mo 4p, S 3s, and the valence band. Quantitative analysis indicates a sulfur deficient surface composition of MoS1.8, and impurities were below the XPS detection limit. [ABSTRACT FROM AUTHOR]

Bismuth selenide (Bi2Se3), a two-dimensional topological insulator material purchased from Alfa Aesar, was analyzed using in situ x-ray photoelectron spectroscopy (XPS). The XPS spectra obtained from a fresh surface exfoliated in ultrahigh vacuum include a survey scan, high resolution spectra of O 1s, Bi 5d, Se 3d, Bi 4f, Se 3p, Se LMM, C 1s, and the valence band. Quantitative analysis indicates a Se deficient surface composition of Bi2Se2.8, which is consistent with the Fermi level position in the conduction band detected in this work. [ABSTRACT FROM AUTHOR]

Bismuth telluride (Bi2Te3) was analyzed using x-ray photoelectron spectroscopy. A freshly exfoliated, oxygen-free flake was analyzed. Spectral regions for O 1s, Te 3d, C 1s, Bi 4f, Bi 5d, and Te 4d were acquired. Bulk quantitative analyses by x-ray fluorescence, inductively coupled plasma-mass spectrometry, and x-ray diffraction indicated that the material was stoichiometric, contained low concentrations of impurities, and was phase pure, respectively. [ABSTRACT FROM AUTHOR]

Covalent p-type doping of WSe2 thin films grown by molecular beam epitaxy and WSe2 exfoliated from bulk crystals is achieved via remote nitrogen plasma exposure. X-ray photoelectron and Raman spectroscopies indicate covalently bonded nitrogen in the WSe2 lattice as well as tunable nitrogen concentration with N2 plasma exposure time. Furthermore, nitrogen incorporation induces compressive strain on the WSe2 lattice after N2 plasma exposure. Finally, atomic force microscopy and scanning tunneling microscopy reveal that N2 plasma treatment needs to be carefully tuned to avoid any unwanted strain or surface damage. [ABSTRACT FROM AUTHOR]

Inconsistent interface control in devices based on two-dimensional materials (2DMs) has limited technological maturation. Astounding variability of 2D/three-dimensional (2D/3D) interface properties has been reported, which has been exacerbated by the lack of direct investigations of buried interfaces commonly found in devices. Herein, we demonstrate a new process that enables the assembly and isolation of device-relevant heterostructures for buried interface characterization. This is achieved by implementing a water-soluble substrate (GeO 2 ), which enables deposition of many materials onto the 2DM and subsequent heterostructure release by dissolving the GeO 2 substrate. Here, we utilize this novel approach to compare how the chemistry, doping, and strain in monolayer MoS 2 heterostructures fabricated by direct deposition vary from those fabricated by transfer techniques to show how interface properties differ with the heterostructure fabrication method. Direct deposition of thick Ni and Ti films is found to react with the monolayer MoS 2 . These interface reactions convert 50% of MoS 2 into intermetallic species, which greatly exceeds the 10% conversion reported previously and 0% observed in transfer-fabricated heterostructures. We also measure notable differences in MoS 2 carrier concentration depending on the heterostructure fabrication method. Direct deposition of thick Au, Ni, and Al 2 O 3 films onto MoS 2 increases the hole concentration by >10 12 cm -2 compared to heterostructures fabricated by transferring MoS 2 onto these materials. Thus, we demonstrate a universal method to fabricate 2D/3D heterostructures and expose buried interfaces for direct characterization.

Semiconducting MoTe 2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains underexplored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe 2 transistors with the highest performance to date, including high saturation current (>400 μA/μm at 80 K and >200 μA/μm at 300 K) and relatively low contact resistance (1.2 to 2 kΩ·μm from 80 to 300 K), achieved with Ag contacts and AlO x encapsulation. We also investigate other contact metals (Sc, Ti, Cr, Au, Ni, Pt), extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer hexagonal boron nitride between MoTe 2 and Sc. These metal-insulator-semiconductor (MIS) contacts partly depin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.

The unique properties of topological insulators such as Bi 2 Se 3 are intriguing for their potential implementation in novel device architectures for low power and defect-tolerant logic and memory devices. Recent improvements in the synthesis of Bi 2 Se 3 have positioned researchers to fabricate new devices to probe the limits of these materials. The fabrication of such devices, of course, requires etching of the topological insulator, in addition to other materials including gate oxides and contacts which may impact the topologically protected surface states. In this paper, we study the impact of He + sputtering and inductively coupled plasma Cl 2 and SF 6 reactive etch chemistries on the physical, chemical, and electronic properties of Bi 2 Se 3 . Chemical analysis by X-ray photoelectron spectroscopy tracks changes in the surface chemistry and Fermi level, showing preferential removal of Se that results in vacancy-induced n-type doping. Chlorine-based chemistry successfully etches Bi 2 Se 3 but with residual Se-Se bonding and interstitial Cl species remaining after the etch. The Se vacancies and residuals can be removed with postetch anneals in a Se environment, repairing Bi 2 Se 3 nearly to the as-grown condition. Critically, in each of these cases, angle-resolved photoemission spectroscopy (ARPES) reveals that the topologically protected surface states remain even after inducing significant surface disorder and chemical changes, demonstrating that topological insulators are quite promising for defect-tolerant electronics. Changes to the ARPES intensity and momentum broadening of the surface states are discussed. Fluorine-based etching aggressively reacts with the film resulting in a relatively thick insulating film of thermodynamically favored BiF 3 on the surface, prohibiting the use of SF 6 -based etching in Bi 2 Se 3 processing.

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe 2 ) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe 2 /sapphire exhibit ambipolar behavior with excellent on/off ratios (∼10 7 ), high current density (1-10 μA·μm -1 ), and good field-effect transistor mobility (∼30 cm 2 ·V -1 ·s -1 ) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.