Min Sup Choi

Controlled charge trapping by molybdenum disulphide and graphene in ultrathin heterostructured memory devices

Atomically thin two-dimensional materials have emerged as promising candidates for flexible and transparent electronic applications. Here we show non-volatile memory devices, based on field-effect transistors with large hysteresis, consisting entirely of stacked two-dimensional materials. Graphene and molybdenum disulphide were employed as both channel and charge-trapping layers, whereas hexagonal boron nitride was used as a tunnel barrier. In these ultrathin heterostructured memory devices, the atomically thin molybdenum disulphide or graphene-trapping layer stores charge tunnelled through hexagonal boron nitride, serving as a floating gate to control the charge transport in the graphene or molybdenum disulphide channel. By varying the thicknesses of two-dimensional materials and modifying the stacking order, the hysteresis and conductance polarity of the field-effect transistor can be controlled. These devices show high mobility, high on/off current ratio, large memory window and stable retention, providing a promising route towards flexible and transparent memory devices utilizing atomically thin two-dimensional materials.

Lateral MoS2 p-n junction formed by chemical doping for use in high-performance optoelectronics.

This paper demonstrates a technique to form a lateral homogeneous 2D MoS2 p-n junction by partially stacking 2D h-BN as a mask to p-dope MoS2. The fabricated lateral MoS2 p-n junction with asymmetric electrodes of Pd and Cr/Au displayed a highly efficient photoresponse (maximum external quantum efficiency of ∼7000%, specific detectivity of ∼5 × 10(10) Jones, and light switching ratio of ∼10(3)) and ideal rectifying behavior. The enhanced photoresponse and generation of open-circuit voltage (VOC) and short-circuit current (ISC) were understood to originate from the formation of a p-n junction after chemical doping. Due to the high photoresponse at low VD and VG attributed to its built-in potential, our MoS2 p-n diode made progress toward the realization of low-power operating photodevices. Thus, this study suggests an effective way to form a lateral p-n junction by the h-BN hard masking technique and to improve the photoresponse of MoS2 by the chemical doping process.

Metal-Semiconductor Barrier Modulation for High Photoresponse in Transition Metal Dichalcogenide Field Effect Transistors

A gate-controlled metal-semiconductor barrier modulation and its effect on carrier transport were investigated in two-dimensional (2D) transition metal dichalcogenide (TMDC) field effect transistors (FETs). A strong photoresponse was observed in both unipolar MoS2 and ambipolar WSe2 FETs (i) at the high drain voltage due to a high electric field along the channel for separating photo-excited charge carriers and (ii) at the certain gate voltage due to the optimized barriers for the collection of photo-excited charge carriers at metal contacts. The effective barrier height between Ti/Au and TMDCs was estimated by a low temperature measurement. An ohmic contact behavior and drain-induced barrier lowering (DIBL) were clearly observed in MoS2 FET. In contrast, a Schottky-to-ohmic contact transition was observed in WSe2 FET as the gate voltage increases, due to the change of majority carrier transport from holes to electrons. The gate-dependent barrier modulation effectively controls the carrier transport, demonstrating its great potential in 2D TMDCs for electronic and optoelectronic applications.

Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides

Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (Rc). Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS2 and MoTe2 by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and -0.07 for monolayer MoS2 and MoTe2, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk MoS2. Our results further imply that metal work functions have very little influence on contact properties of 2D-material-based devices. Moreover, we found that Rc is exponentially proportional to SBH, and these processing parameters can be controlled sensitively upon chemical doping into the 2D materials. These findings provide a practical guideline for depinning Fermi level at the 2D interfaces so that polarity control of TMDC-based semiconductors can be achieved efficiently.

Plasma treatments to improve metal contacts in graphene field effect transistor

Graphene formed via chemical vapor deposition was exposed to various plasmas (Ar, O2, N2, and H2) in order to examine its effects on the bonding properties of graphene to metal. After exposing patterned graphene to Ar plasma, the subsequently deposited metal electrodes remained intact, enabling the successful fabrication of field effect transistor arrays. The effects of the enhanced adhesion between graphene and metals were more evident from the O2 plasma than the Ar, N2, and H2 plasmas, suggesting that a chemical reaction of O radicals imparts hydrophilic properties to graphene more effectively than the chemical reaction of H and N radicals or the physical bombardment of Ar ions. The electrical measurements (drain current versus gate voltage) of the field effect transistors before and after Ar plasma exposure confirmed that the plasma treatment is quite effective in controlling the graphene to metal bonding accurately without the need for buffer layers.

High performance vertical tunneling diodes using graphene/hexagonal boron nitride/graphene hetero-structure

A tunneling rectifier prepared from vertically stacked two-dimensional (2D) materials composed of chemically doped graphene electrodes and hexagonal boron nitride (h-BN) tunneling barrier was demonstrated. The asymmetric chemical doping to graphene with linear dispersion property induces rectifying behavior effectively, by facilitating Fowler-Nordheim tunneling at high forward biases. It results in excellent diode performances of a hetero-structured graphene/h-BN/graphene tunneling diode, with an asymmetric factor exceeding 1000, a nonlinearity of ∼40, and a peak sensitivity of ∼12 V−1, which are superior to contending metal-insulator-metal diodes, showing great potential for future flexible and transparent electronic devices.

Effects of plasma treatment on surface properties of ultrathin layered MoS2

This work investigates the use of oxygen plasma (O2) treatment, applied as an inductively coupled plasma, to control the thickness and work function of a MoS2 layer. Plasma-etched MoS2 exhibited a surface roughness similar to that of the pristine MoS2. The MoS2 field effect transistors fabricated using the plasma-etched MoS2 displayed a higher n-type doping concentration than that of pristine MoS2. The x-ray photoelectron spectroscopy was performed to analyze chemical composition to demonstrate the minimum level of chemical reactions occurred upon plasma treatment. Moreover, Kelvin probe force microscopy measurements were conducted to probe the changes in the work function that could be attributed to the changes in the surface potential. The measured work functions suggest the modification of a band structure and n-doping effect after plasma treatments that depended on the number of MoS2 layers. This study suggests that the O2 plasma can control the layer number of the MoS2 as well as the electronic properties of a MoS2 film.

Gate-dependent photoconductivity of single layer graphene grafted with metalloporphyrin molecules

We present the gate-dependent photoconductivity measurements of single layer graphene ribbons grafted with zinc porphyrin molecules Zn(OEP). The Zn(OEP)-graphene showed a maximum 610% increase in its photo-sensitivity compared to the bare graphene samples. Furthermore, the measured photocurrent exhibited strong dependence on the gate bias, light power, and light wavelength. These dependences showed clear evidence of the excitation of the carriers in Zn(OEP) and their energy transfer to graphene.

Effects of Plasma Treatment on Contact Resistance and Sheet Resistance of Graphene FET

We investigated the effect of capacitively coupled Ar plasma treatment on contact resistance (R c ) and channel sheet resistance (R sh ) of graphene field effect transistors (FETs), by varying their channel length in the wide range from 200 nm to 50 μm which formed the transfer length method (TLM) patterns. When the Ar plasma treatment was performed on the long channel (10 ~ 50 μm) graphene FETs for 20 s, R c decreased from 2.4 to 1.15 kΩ·μm. It is understood that this improvement in R c is attributed to the formation of sp³ bonds and dangling bonds by the plasma. However, when the channel length of the FETs decreased down to 200nm, the drain current (I d ) decreased upon the plasma treatment because of the significant increase of channel R sh which was attributed to the atomic structural disorder induced by the plasma across the transfer length at the edge of the channel region. This study suggests a practical guideline to reduce R c using various plasma treatments for the R c sensitive graphene and other 2D material devices, where R c is traded off with Rsh.

Homogeneous molybdenum disulfide tunnel diode formed via chemical doping

We report on a simple, controllable chemical doping method to fabricate a lateral homogeneous MoS2 tunnel diode. MoS2 was doped to degenerate n- (1.6 × 1013 cm−2) and p-type (1.1 × 1013 cm−2) by benzyl viologen and AuCl3, respectively. The n- and p-doping can be patterned on the same MoS2 flake, and the high doping concentration can be maintained by Al2O3 masking together with vacuum annealing. A forward rectifying p-n diode and a band-to-band tunneling induced backward rectifying diode were realized by modulating the doping concentration of both the n- and p-sides. Our approach is a universal stratagem to fabricate diverse 2D homogeneous diodes with various functions.We report on a simple, controllable chemical doping method to fabricate a lateral homogeneous MoS2 tunnel diode. MoS2 was doped to degenerate n- (1.6 × 1013 cm−2) and p-type (1.1 × 1013 cm−2) by benzyl viologen and AuCl3, respectively. The n- and p-doping can be patterned on the same MoS2 flake, and the high doping concentration can be maintained by Al2O3 masking together with vacuum annealing. A forward rectifying p-n diode and a band-to-band tunneling induced backward rectifying diode were realized by modulating the doping concentration of both the n- and p-sides. Our approach is a universal stratagem to fabricate diverse 2D homogeneous diodes with various functions.