Session F12: Devices from 2D Materials -- Transport

Electronic properties of correlated two-dimensional materials

Two-dimensional (2D) atomic crystals, best exemplified by graphene, have emerged as a new class of material that may impact future science and technology. So far semiconducting 2D materials attracted most attention. Meanwhile, vast opportunities exists in correlated 2D materials: the reduced dimensionality may lead to novel properties that are vastly different from that in the bulk, and the exposed surface makes these materials highly tunable. In this talk I will discuss the emerging opportunities in correlated 2D materials. In particular, I will talk about two correlated 2D materials that we found particularly interesting – metallic 2D ferromagnet Fe₃GeTe₂ and monolayer Bi₂Sr₂CaCu₂O_8+δ. We explore their electronic properties while the doping and dimensionality of the 2D systems are modulated.   

Studying Ambipolar Tellurene Field Effect Transistors using Microwave Near-Field Microscopy

The successful development of nanoscale semiconducting devices requires precise control over adjoining regions of n- and p-type transport. Key challenges remain the development of new materials with bipolar transport as desired for homojunction devices as well as techniques capable of studying local variations in carrier type and associated conductivity with nanometer spatial resolution. Here we image local electronic variations in ambipolar field effect transistors made from 2D films (tellurene) of the 1D van der Waals material tellurium using near-field scanning microwave microscopy (SMM). We perform SMM imaging together with differential measurements to study spatial variations in both carrier type and the associated conductivity as a function of the applied global backgate voltage. We produce nanometer resolved maps of the local carrier equivalence backgate voltage and show that the apparent device conductivity minimum determined from transport measurements in fact arises from the local coexistence of p- and n-type regions.   

Effects of H2 Interaction with MoS2: Electronic Behavior and Sensing Applications

In this work, we probe the electronic properties of monolayer MoS₂ via interaction with H₂, aiming for sensing application. The MoS₂ FET exhibit a response to H₂ in a wide range of concentration (0.1-90 %) at relatively low temperatures (300-473 K). These H₂ sensor show desirable properties such absence of catalytic metal dopants (Pt or Pd) and full reversibility. Based in three experiments, we demonstrate that the conductivity of MoS₂ increases as a function of the H₂ concentration due to a reversible charge transferring process. Further, we propose that such process originates from the dissociative H₂ adsorption driven by interaction with sulfur vacancies in the MoS₂ surface (V_S). This proposal is in agreement with related DFT studies about H₂ adsorption on MoS₂. Finally, measurements on partially defect-passivated MoS₂ FETs using atomic layer deposited Al₂O₃ consisting of an experimental indication that the V_S plays an important role in the H₂ interaction with the MoS₂. Our findings provide insights for futures applications in catalytic process between monolayer MoS₂ and H₂.   

Anisotropic electron transport behaviors in few-layer ReSe2

The transition metal dichalcogenide of ReSe2 had drawn much attention for decades since it is a layered semiconductor with strong anisotropy on the basal plane. The Re atoms are arranged to form clusters with the clusters aligned in a specified ‘b’-axis. After the improvement of material science, the anisotropy on the basal plane can be studied on single layer ReSe2. In this talk, we will discuss the electrical exploration of the anisotropy on the basal plane of ReSe2. We applied the mechanical exfoliation to make few-layer ReSe2 flakes and the standard electron-beam lithography to make circularly oriented probing electrodes on the basal plane. We checked that the contact resistance is very small as compared with the ReSe2 channel resistance. Using the circularly arranged electrodes, we studied electron transport in the direction parallel to the b-axis and in the direction making an angle with the b-axis. We studied angle dependence of electron mobility, the disorder parameter T0 of the Mott’s variable range hopping model, the metal-insulator transition, and the thermoelectric power. We observe an enhanced conductivity in the b-axis on the basal plane of the ReSe2.   

Low-Frequency Noise Spectroscopy of the Charge-Density-Wave Phase Transitions in Vertical Tantalum Disulfide Devices

Noise spectroscopy has proven itself as an effective tool for investigating the phase transitions in two-dimensional (2D) charge density wave (CDW) materials [1]. Almost all studies of CDW effects in 2D systems focused on transport along the atomic planes. Here we report results of investigation of CDW transitions in vertical 1T-TaS₂ devices. We observed two jumps in electrical resistivity – at the temperature range from 150 K to 180 K, and another at 80 K to 85 K. The low-frequency noise spectral density, revealed strong peaks at these transition points, sometimes changing by as much as three orders-of-magnitude. The higher temperature feature can be associated with the transition between the commensurate and nearly commensurate CDW states. The lower temperature transition can indicate the debated “hidden” CDW phase. The obtained results are important for the proposed applications of vertical van der Waals heterostructures in memory and logic gates.
[1] G. Liu, et. al., "Low-frequency current fluctuations and sliding of the charge density waves in 2D materials," Nano Lett., 18, 3630 (2018).
  

Type Control of MoOx/MoS2 Heterostructure Transistors by Oxidation Treatments

Molybdenum disulfide (MoS₂) has attracted much attention due to great potential applications as electronics devices. By mechanical exfoliation, few-layer MoS₂ can be obtained and made into field-effect transistors (FETs). In particular, we demonstrated that heating device under ozone exposure for several hours can change the device behaviors from natively n-type to either ambipolar or p-type. The treatments resulted in oxidation of MoS₂ surfaces to molybdenum oxide (MoO_x). Due to exhibiting high work function, MoO_x was used as an efficient hole contact. By adjusting the time and temperature of oxidation treatments, the formation of MoO_x and the work function of contact electrodes can be modulated. The MoS₂ FETs after weak and strong oxidation treatments presented ambipolar and p-type feature, respectively. In MoO_x/MoS₂ heterostructure, the devices showed the Schottky contact behaviors. The effective Schottky barrier depended on both the gate voltage and source-drain voltage. Additionally, the electrical transport and transfer characteristics of ambipolar and p-type FETs were separately explored. In this report, the oxidation treatments not only simplify the complex fabrication but also improve the diversity of applications for nanoelectronic devices.   

Reduction of Current-Voltage Hysteresis in Graphene Field Effect Transistor Achieved with Dry Transfer Using Flexible Tape Supporter

In the conventional wet transfer method of Chemical Vapor Deposition (CVD) graphene, it is inevitable to have water molecules trapped between graphene and substrate. The trapped water molecules can cause the hysteretic behavior in current-voltage curves of graphene field effect transistor (GFET). Here, a new dry transfer method adopting the Kapton tape as an additional flexible supporting layer is demonstrated. The N₂ blowing and heating processes are added to vaporize the water molecules adsorbed on graphene layer right before the transfer step. By comparing the I-V characteristics of wet- and dry-transferred GFET, the field effect mobility is found to be larger for the dry-transfer GFET in comparison with the wet-transferred one, possibly due to the more uniform Coulomb potential landscape. Also, the hysteretic behavior is found to be reduced substantially in accordance with the decrease of trapped water molecules. The obtained electron field effect mobilities are ~1118 cm²/Vs and ~415 cm²/Vs for dry- and wet-transferred graphene, respectively. Our dry transfer method can provide a simple and reliable way to transfer the CVD graphene onto an arbitrary substrate with its as-grown electrical properties being preserved, regardless of the substrate size.   

High-Efficiency Thermoelectric 2D Tellurium Devices with Accumulation-type Metal-to-Semiconductor Contacts

The paradigm of a good thermal electrical material is usually a heavily doped narrow bandgap semiconductor with good electrical conductivity and low thermal conductivity. Bulk Te has been theoretically predicted and experimentally demonstrated to be an outstanding thermoelectric material. Recent achievements in growing high quality 2D Te films allow us to explore the thermoelectric performance of 2D Te. Using nano-fabricated heater and thermometer, we were able to measure the room temperature Seebeck coefficient and power factor of 0.41 mV/K and 31.7 μW/cm●K. Using the thermal conductivity of bulk Te, we can estimate the lower bound of ZT value to be ~0.32 at 300 K, whereas the true ZT value should be much higher considering the suppression of thermal conductivity in thin films. The details of film thickness dependence are on-going and will be reported. Thermoelectrical current mapping was performed with a laser heater, and we found high work function metals such as Palladium can form rare accumulation type metal-to-semiconductor contacts to Te, which allows thermoelectrically generated carriers to be collected more efficiently. High-performance thermoelectric Te devices will have broad applications for energy harvesting or as a Peltier cooler in microsystems.   

Contact Engineering of 2D Semiconductors using Ultrathin Transition Metal Dichalcogenides as a Contact Interlayer

Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs) have emerged as a promising candidate for post-silicon electronics. However, a major impediment in the realization of their electronics applications is their tendency to form a substantial Schottky barrier with most commonly used metals to form electrical contacts. Various strategies to reduce this barrier are deficient as they lack thermal or chemical stability, or reduce the overall performance. Here, we present a new method to significantly reduce the barrier height at the semiconductor/metal interface by inserting ultrathin TMD as an interlayer. Particularly, we observed an order of magnitude reduction of contact resistance to 1.6 kΩ●μm and significant improvement of overall device performance in MoS₂ transistors as the Schottky barrier height is lowered from ~ 100 meV to ~ 40 meV by inserting a WSe₂ interlayer between the MoS₂ channel and the metal electrodes. A significant advantage of using 2D semiconductors as a contact interlayer as opposed to previously reported insulator interlayers is that 2D semiconductor interlayers with appropriate band alignments are able to effectively reduce the Schottky barrier height without introducing a substantial tunneling barrier at the contacts.   

Strong Spin-Orbit Interaction in ultra clean graphene/ transition-metal dichalcogenide heterostructures

Van der Waals heterostructures based on stacking two dimensional materials gives rise to new possibilities for designing tunable electronic systems. While combining the merits of individual layers, heterostructures provide a platform for studying interfacial interactions [1]. Among the many phenomena achieved by this proximity effect, inducing spin-orbit coupling (SOC) in graphene has attracted much attention for potential applications in topological physics. We study the enhancement of SOC in graphene induced by transition-metal dichalcogenides (TMDs). High carrier mobility at low temperature suggests that our devices have high quality graphene and very clean graphene/TMD interfaces. We measure magnetoresistance in these devices, which is large and positive, indicating the presence of ultra-strong weak antilocalization (WAL), as a signature of SOC induced in graphene. In contrast to previous works [2-3], our WAL strength is much higher, and we have explored the variation with carrier density more systematically. The WAL feature gradually decreases with increasing temperature and becomes unobservable above 60K.

[1] Geim, Andre K., et al. Nature 499.7459 (2013): 419.
[2] Wang, Zhe, et al. Nature communications 6 (2015): 8339.
[3] Yang, Bowen, et al. 2D Materials 3.3 (2016): 031012.   

Ultimate Limit in Size and Performance of WSe2 Vertical Diodes

Precise doping-profile engineering in van der Waals (vdW) heterostructures is a key element to promote optimal device performance in various electrical and optical applications with two-dimensional vdW materials. Here, we report tungsten diselenide-(WSe₂) based pure vertical diodes with atomically defined p-, i- and n-channel regions. Externally modulated p- and n-doped layers are respectively formed on the bottom and the top facets of WSe₂ single crystals by direct evaporations of high and low work-function metals platinum and gadolinium. Since metal-induced dopings are restricted to the first few layers of WSe₂, atomically sharp p-i-n heterojunctions are naturally formed in the homogeneous WSe₂ layers. As the number of layers increases, charge transport through the vertical WSe₂ p-i-n heterojunctions is characterized by a series of quantum tunneling events, namely direct tunneling, Fowler–Nordheim (FN), and Schottky emission (SE). With optimally selected WSe₂ thickness, where FN and SE tunneling events prevail, our vertical heterojunctions show superb diode characteristics of an unprecedentedly high current density (≥ 2 × 10⁵ A/cm²) and low turn-on voltages while maintaining good current rectification.   

Direct observation of ballistic transport in vertical direction of Black Phosphorus

Ballistic transport is the transport of carriers without scattering. In this regime, carriers travel freely and coherently in semiconductors and therefore the ballistic transport is highly desirable for the development of low power, high speed logic and quantum circuits. Due to its puckered lattice structure, the out-of-plane mobility of BP is comparable to in-plane mobility. Here, we report the direct observation of ballistic transport of BP in out-of-plane direction. The ballistic transport observed in vertical direction of BP is within a 3D framework which is different from 1D ballistic transport. Our results shed light on the development of efficiently facilitating carriers on the nanoscale.   

Charge Transport in Graphene-based multi-molecules junctions

The realization of stable and reliable molecular junctions taking advantage of graphene electrodes faces several issues. Nanoscale gaps with graphene electrodes can indeed exhibit signatures mimicking those of molecules, with gate-dependent resonance features [1,2]. Substrate effects can also play a role: Silicon dioxide has for instance been reported to yield feature-rich charge-transport characteristics in nanoscale graphene gaps, primarily due to switching within the oxide [3].
We report here on the realization of mechanically and electronically robust graphene-based multi-molecule junctions [4]. The mechanical stability is achieved by anchoring molecules directly to the substrate using silanization, rather than to graphene electrodes. The electronic stability is due to a large overlap between the electronic π-systems of neighboring head groups. The nature of the stacking leads to junctions less sensitive to the electrode properties. The junctions are reproducible throughout several devices and operate up to room temperature.

[1] M. El Abbassi et al. Nanoscale 9, 17312 (2017)
[2] V.M. García-Suarez,et al., Nanoscale 10, 18169 (2018)
[3] L. Posa et al., Nano letters 17, 6783 (2017)
[4] M. El Abbassi et al., to be submitted