Current Rectification in a Structure: ReSe2/Au Contacts on Both Sides of ReSe2
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Schottky effect of two-dimensional materials is important for nanoscale electrics. A ReSe2 flake is transferred to be suspended between an Au sink and an Au nanofilm. This device is initially designed to measure the transport properties of the ReSe2 flake. However, a rectification behavior is observed in the experiment from 273 to 340 K. The rectification coefficient is about 10. The microstructure and elements composition are systematically analyzed. The ReSe2 flake and the Au film are found to be in contact with the Si substrate from the scanning electron microscope image in slant view of 45°. The ReSe2/Si and Si/Au contacts are p-n heterojunction and Schottky contacts. Asymmetry of both contacts results in the rectification behavior. The prediction based on the thermionic emission theory agrees well with experimental data.
KeywordsReSe2 Rectification Two-dimensional materials
Atomic force microscope
Energy dispersive spectroscopy
Field effect transistor
Scanning electron microscope
Transition metal dichalcogenides
Rectification behaviors of metal-semiconductor contacts, where the current varies with the direction of the applied voltage, are widely used in Schottky barrier diode, field effect transistor (FET), and metal-oxide-semiconductor FET. Schottky explained the behavior by depletion layers on the semiconductor side of such interfaces . Differences of electron work function between metal and semiconductor lead to the rectification behavior named Schottky effect . The contact between metal and two-dimensional (2D) semiconductor materials is a Schottky contact when the metal has a higher electron work function than an n-type 2D semiconductor materials or lower electron work function than a p-type 2D semiconductor. The Schottky effect of metal/2D materials has great applications in micro-photo detectors, micro-FETs, gas sensors, and phototransistors . Among 2D materials, transition metal dichalcogenides (TMDs) have attracted much attention because they have a sizable bandgap  and the bandgap transits from indirect to direct as the thickness is reduced to monolayer . The bandgap ensures that TMDs can be used for many applications, i.e., FETs and solar cells . TMDs can be also used in thermoelectric field , which has drawn wide attention [6, 7, 8, 9]. Many experiments have been done to explore properties and applications of TMDs such as MoS2, MoSe2, WSe2, and WS2. Lopez-Sanchez et al.  made ultrasensitive monolayer phototransistors with MoS2. Britnell et al.  made a WS2/graphene heterostructure and demonstrated its application in photovoltaic device. WSe2, as an ambipolar semiconductor, was controlled with double electrostatic gates to fabricate a light-emitting diode [12, 13]. Among TMDs, ReSe2 is different from other group VI TMDs because ReSe2 belongs to group VII TMDs with an extra electron in d orbitals, which leads to strong in-plane anisotropy . A few studies have explored the electrical properties of ReSe2 due to its special band structure. Current rectification is explored with a ReSe2/WS2 p-n heterojunction  and ReSe2/MoS2 p-n heterojunction . FET is made to investigate the electrical properties of metal/semiconductor contacts like ReSe2/metal or ReS2/metal [17, 18, 19].
In this letter, a ReSe2 flake is suspended across an Au sink and an Au nanoribbon electrode. The device is originally designed to measure the thermal and electrical conductivities of the ReSe2 flake. Measurements were performed at 340 K, 310 K, 280 K, and 273 K.
Firstly, the Si substrate with Au electrodes was fabricated. The 400-μm-thick undoped Si substrate was oxidized to form a 180-nm-thick SiO2 layer after initial cleaning, and a 320-nm-thick electron beam resist was deposited on the SiO2 surface by means of spin coating. Au was deposited by physical vapor deposition to fabricate the Au nano-electrodes and the Au nanofilm in the pattern which was prepared by electron beam lithography. By putting the sample into the photoresist developer, the electron beam resist was etched and the Au electrode and film were left. At last, the SiO2 layer is etched by buffered hydrofluoric acid and the Si layer under the Au nanofilm is etched by CF4 plasma to fabricate a suspended nanofilm which is about 6 μm above the Si substrate.
The direction along A-B-C is defined as positive, or vice versa, and a direct current was applied. The voltage, V, across the Au-ReSe2-Au junctions was measured by a high accuracy digital multimeter (Keitheley 2002, 8.5 digits), while the current, I, was determined through measuring the voltage across a reference resistor in series. The I-V curves of the ReSe2/Au junctions for forward and inverse voltage were measured at different temperatures in a physical property measurement system (quantum design).
Results and Discussion
Calculated ideality factor for ReSe2-Si and Si-Au contacts
Equation (5) shows that the ideality factor is inversely proportional to the temperature. The ideality factor significantly decreases with temperature only at low temperature and changes slowly when the temperature is over 300 K [28, 29]. However, as shown in Table 1, the reverse saturation current increases significantly with the temperature which is different from the ideality factor. It can be explained by Eq. (2). According to Eq. (2), the reverse saturation current increases with temperature because T2 and exp (− qΦB/kT) increase with temperature. Due to the exponential relationship between exp (− qΦB/kT) and − qΦB/kT, exp (− qΦB/kT) increases significantly with temperature. Based on the research by Zhu et al , qΦB of the Au/Si contact in the experiment at 273 K and 295 K are 0.77 eV and 0.79 eV, respectively. The calculated results show that the reverse saturation current at 295 K is six times as much as the reverse saturation current at 273 K, explaining why the reverse saturation current increases significantly with temperature.
In conclusion, a rectification behavior is observed in the contacts where a ReSe2 flake suspended across Au substrate and Au nanofilm at different temperature. The SEM image of the suspended ReSe2 flake in slant view of 45° shows that the ReSe2 flake and the Au nanofilm are in contact with the Si substrate and the EDS map illustrated the elements composition, ReSe1.67. The contact between the ReSe2 flake and the Si substrate is responsible for the rectification behavior. The ReSe2-Si and Si-Au contacts are both rectification contacts forming another circuit, and asymmetry of both contacts results in the apparent rectification behavior. The calculated results based on Schottky current equation considered the Si-Au Schottky contact, and the ReSe2-Si p-n heterojunction agrees well with experiments results.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 51776224, 51576105, 51636002, 51827807, and 51336009), the Science Fund for Creative Research Groups (Grant No. 51621062), Beijing Municipal Science & Technology Commission (No Z161100002116030), and the Tsinghua University Initiative Scientific Research Program.
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