We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.
Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal–semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 Ω−1 cm−1, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.
Transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2, and WSe2, have an interesting two-dimensional (2D) layer structure and semiconducting properties1-3. Scientists have recently discovered that the monolayer structure of MoS2 shows a substantially enhanced light-emitting efficiency because of the quantum confinement effect. The finding of the new direct-bandgap semiconductor material has attracted substantial attention4-7. In addition, the easily stripped layer structure of TMDs is an excellent platform for studying the fundamental properties of 2D materials. Unlike metallic graphene without the bandgap, TMDs have inherent semiconducting characteristics and have a bandgap in the range of 1-2 eV1,3,8. The 2D structures of the ternary compounds of TMDs9 and the possibility of the integration of these compounds with graphene provide an unprecedented opportunity to develop ultrathin and flexible electronic devices.
Unlike graphene, the room temperature electron mobility values of 2D TMDs are at a moderate level (1-200 cm2V−1sec−1 for MoS210-17; approximately 50 cm2V−1sec−1 for MoSe218). The optimal mobility values of graphene have been reported to be higher than 10,000 cm2V−1sec−1.19-21 Nevertheless, semiconducting TMD monolayers exhibit excellent device performance. For instance, the MoS2 and MoSe2 monolayers or multilayer field-effect transistors exhibit extremely high on/off ratios, up to 106-109 10,12,17,18,22. Therefore, it is crucial to understand the fundamental electrical properties of the 2D TMDs and their bulk materials.
However, studies of the electrical properties of the layer materials have been partially hampered because of the difficulty in forming good ohmic contact on the layer crystals. Three approaches, shadow mask deposition (SMD)23, electron beam lithography (EBL)24,25, and focused-ion beam (FIB) deposition,26,27 have been used to form electrical contacts on nanomaterials. Because SMD typically involves the use of a copper grid as the mask, the spacing between two contact electrodes is mostly larger than 10 μm. Unlike EBL and FIB deposition, metal deposition of electrode arrays on a substrate is performed without targeting or selecting nanomaterials of interest in the SMD method. This approach cannot guarantee that the metal patterns are correctly deposited on individual nanomaterials as the electrodes. The result of the SMD method has an element of chance. The EBL and FIB deposition methods are used in the scanning electron microscope (SEM) system; nanomaterials can be directly observed and selected for electrode deposition. In addition, EBL can be used to easily fabricate metal electrodes with a line width and a contact electrode spacing smaller than 100 nm. However, the residual resist on the nanomaterial surface left during lithography inevitably results in the formation of an insulating layer between the metal electrode and the nanomaterial. Thus, EBL leads to high contact resistance.
The main advantage of electrode fabrication through FIB deposition is that it leads to low contact resistance. Because metal deposition is performed by the decomposition of an organometallic precursor by using an ion beam at the defined area, metal deposition and ion bombardment occur simultaneously. This could destroy the metal–semiconductor interface and prevent the formation of Schottky contact. Ion bombardment can also eliminate surface contaminants such as hydrocarbons and native oxides, which decreases contact resistance. Ohmic contact fabrication through FIB deposition has been demonstrated for different nanomaterials27-29. In addition, the entire fabrication procedure in the FIB deposition approach is simpler than that in EBL.
As layer semiconductors typically show highly anisotropic electrical conduction, the conductivity in the layer-to-layer direction is several orders of magnitude lower than that in the in-plane direction30,31. This characteristic increases the difficulty of fabricating ohmic contacts and determining electrical conductivity. Therefore, in this study, FIB deposition was used for studying the electrical properties of layer semiconductor nanostructures.
1. Structural Characterization of MoSe2 Layer Crystals (See Step 1 in Figure 1)
2. Fabrication of MoSe2 Layer Nanocrystal Devices
3. Characterizations of the MoSe2 Layer Nanocrystal Devices
The determined values of the electrical conductance (G) and conductivity (σ) of layer nanomaterials with different thicknesses are highly dependent on the quality of the electrical contacts. The ohmic contacts of the FIB-deposition-fabricated two-terminal MoSe2 devices are characterized by measuring the current–voltage (I–V) curve. The room temperature I–V curves for the two-terminal MoSe2 nanoflake devices with different thicknesses are shown in Figure 2a. The I–V curves follow a linear relationship. This confirms the ohmic contact condition of the MoSe2 devices.
Partial devices with four electrodes were fabricated to further rule out the potential effect of the contact resistance. Figure 2b illustrates the typical I–V curves measured by two-electrode and four-electrode methods at room temperature for the same nanoflake with a thickness at 33 nm. The calculated σ values for the two-probe and four-probe measurements are at 117 and 118 Ω−1cm−1, respectively. Because the σ values calculated using two-probe and four-probe measurements for the same devices were highly similar, the influence of contact resistance on the determined G and σ values in this study was negligible. Figures 2c and 2d illustrate the representative field-emission scanning electron microscope (FESEM) images of the two-terminal and four-terminal MoSe2 devices, respectively. The thicknesses of the MoSe2 nanoflakes on the devices were estimated using atomic force microscopy (AFM) measurements; a sample measurement is shown in Figures 2e and 2f.
The metal–semiconductor interface of the electrical contact in the MoSe2 devices was further examined using high-resolution transmission electron microscopy (HRTEM), selected-area electron diffractometry (SAED), and energy-dispersive X-ray spectroscopy (EDX). Figure 3a shows the cross-sectional transmission electron microscopy (TEM) image of the Pt/MoSe2 interface. The image shows that an alloy layer (25-30 nm) was formed between Pt and MoSe2 because of ion beam bombardment. HRTEM images of the alloy/MoSe2 interface (spot 4, Figure 3b) and the MoSe2 region (spot 3, Figure 3c) show an amorphous alloy formed at the surface of single-crystal MoSe2.
The EDX spectrum and the ring pattern of SAED in Figure 3d show that Pt is the main constituent and that the metal electrode has a polycrystalline structure. Similar measurements, shown in Figure 3e, indicated an ion-bombarded alloy with an amorphous-like structure and containing a mixture of Mo, Se, and Pt at a ratio of 2:4:1. The single-crystal MoSe2 nanoflake was further confirmed by the EDX and SAED measurements, which are shown in Figure 3f.
For the ohmic contact fabricated using the FIB deposition method, in MoSe2 multilayer nanostructures with different thicknesses, the G and σ values can be precisely determined. Figure 4a shows the statistic G values for the MoSe2 nanoflakes with different thicknesses. It can be observed that the G value does not show an observable change or a change in the thickness over two orders of magnitude. This observation is opposite to theoretical predictions, according to which G is linearly dependent on the thickness (t) for a uniform current flow and is written as
Where A is the area for current transport, and l, w, and t are the length, width, and thickness of the conductor, respectively.
The σ value can be obtained using Equation (1). Figure 4b shows the conductivity as a function of the thickness (σ–t). The value of σ increases by over two orders of magnitude, from 4.6 to 1,500 Ω−1cm−1 when t decreases from 2,700 to 6 nm. An inverse power-law σ ∞ t−β is obtained, where the fitted β value is 0.93. The σ values (0.1-1 Ω−1cm−1)36-38 for the thickness in the bulk region (t: 10-100 μm) are also located on the fitted line.
In principle, σ is an intrinsic property without any dimension effects. The strong thickness dependence of σ implies that current conduction mainly occurs at the surface of the MoSe2 layer material. If the surface conduction path is several orders of magnitude higher than the bulk, the G value does not increase and becomes a constant, even if the thickness increases.
Figure 1: Procedure for device fabrication and electrical characterization of MoSe2 nanoflakes. Step 1: Morphological and structural characterizations of MoSe2 bulk layer crystals by XRD and Raman spectroscopy. Step 2: Mechanical exfoliation of bulk layer crystals by dicing tape and observe the morphology of stripped flakes by FESEM. Step 3: Electrode fabrication of nanoflakes by FIB Pt deposition. Step 4: Complete the device by mounting the sample chip on mica substrate and bonding enameled wire on the electrodes of the chip by Ag paste. Step 5: Load sample in the cryogenic probe station. Step 6: Perform I–V measurement and analyze the data.
Figure 2: I–V curve, FESEM, and AFM measurements for two- and four- terminal MoSe2 nanoflake devices. (a) The I–V curves measured by the two-probe method at room temperature for the MoSe2 nanoflakes with different thicknesses at 11, 240, and 1,300 nm. (b) The I–V curves measured by the two-probe and four-probe methods at room temperature for a MoSe2 nanoflake with a thickness at 33 nm. The representative FESEM images of (c) the two-terminal and (d) the four-terminal MoSe2 devices fabricated by FIB approach. (e) A typical AFM image and (f) its cross-sectional height profile along the blue line in (e) for a MoSe2 device with the thickness at ~60 nm. (Reprinted with permission from Ref. 28, Copyright @ The IOP Publishing Ltd.)
Figure 3: HRTEM, SAED, and EDX analyses for the Pt/MoSe2 interface in the device. (a) The cross-sectional TEM image of the Pt metal/MoSe2 semiconductor interface in the MoSe2 nanoflake device (t ~110 nm) fabricated by FIB approach. The numeric labels indicate the different probing regions for HRTEM, SAED, and EDX analyses. 1: Pt metal electrode, 2: ion-bombarded alloy region, 3: MoSe2 multilayer, and 4: alloy/MoSe2 interface. The HRTEM image of (b) the alloy/MoSe2 interface (spot 4) and (c) the MoSe2 region (spot 3). The EDX spectra and the corresponding SAED patterns for (d) the Pt electrode (spot 1), (e) the alloy region (spot 2), and (f) the MoSe2 nanoflake (spot 3), respectively. (Reprinted with permission from Ref. 28, Copyright @ The IOP Publishing Ltd.)
Figure 4: Thickness-dependent conductivity in MoSe2 nanoflakes. (a) The electrical conductance and (b) the log-log plot of the electrical conductivity values for the MoSe2 nanoflake with different thicknesses ranged from 6 to 2,700 nm measured by two-probe (blue solid star) and four-probe (blue open star) methods. The conductivity values of the MoSe2 bulk crystals obtained by our measurements (green open circle) and from the references are also plotted for comparison. The MoSe2 bulks without the thickness information in the Refs. 32, 33, 34 are assumed to be higher than 10 μm and their conductivity levels are represented by green arrows. The red dash line is the fitting line for the conductivity versus thickness data of the MoSe2 nanoflakes. (Reprinted with permission from Ref. 28, Copyright @ The IOP Publishing Ltd.)
The accurate determination of the σ value and its dimension dependence in the layer nanocrystals is highly dependent on the quality of the electrical contacts. The FIB deposition method used for metal electrode deposition played a crucial role throughout the study. According to electrical, structural, and composition analyses, the fabrication of stable and highly reproducible ohmic contacts, using FIB deposition method, in the MoSe2 or MoS2 devices was facilitated by the formation of the amorphous conductive alloy between the Pt metal and MoSe2 layer semiconductor. The defective alloy structure at the MoSe2 surface that shows a high carrier density could effectively minimize the effect of Schottky contact. The surface contaminants of semiconductors, such as native oxides and hydrocarbons, which are generally considered to originate from the insulating layer between the metal and the semiconductor contacts, can be eliminated by ion beam bombardment. The elimination can explain the low contact resistance in the FIB-deposition-fabricated layer crystal devices.
Although the experimental FIB deposition method provides reliable ohmic contacts for the electrode fabrication of layer semiconductor nanostructures, the minimal spacing between the metal electrodes was limited. It was controlled to be above 1 μm in this study. The main reason for the limitation is that the FIB-deposited metal electrode does not have clear edges and sharp side walls because of the Gaussian distribution of the ion beam flux in the radial direction. The lack of clear edges and sharp sidewalls can result in material surface contamination and an electrical short circuit if two electrodes are deposited too close to each other (typically closer than 500 nm).
In addition, material processing in the ion beam environment inevitably damages the material surface, leading to a change in the inherent material properties. To avoid potential damage to the material surface by the ion beam during FIB deposition, we attempted to minimize the ion beam exposure time. Typically, most of the steps of the procedure (including selecting suitable nanocrystals, mapping the position, and recording images) were initially performed in the SEM mode; subsequently, the mode was switched to the FIB mode. Therefore, the sample surface was exposed to the ion beam for a considerably short time (in the snapshot mode), which corresponded to the time taken for operating at FIB mode for identifying the Pt deposited areas. Furthermore, surface protection can be provided by coating an insulating organic material (such as bathocuproine) on the layer nanocrystal prior to FIB deposition (not mentioned in the protocol).
EBL, which is the most widely used method, can provide a much smaller spacing between electrodes (shorter than 100 nm) compared with FIB deposition. Potential damage to the studied materials can be prevented by using EBL. However, EBL requires the use of a resist. Because the complete removal of the resist coated on the material surface is difficult, the residual resist can result in high contact resistance between the contact metal and the studied material. This problem reduces the yield of ohmic contacts considerably and hampers the use of EBL as a microelectrode fabrication method. Therefore, FIB technique can be a good choice for microelectrode fabrication with reliable and reproducible ohmic contact in addition to EBL.
However, in this study, the minimal thickness of MoSe2 layer material only reaches 6 nm (approximately 9-10 monolayers). The electrical contact quality for the ultrathin layer materials with thickness lower than 5 monolayers is still unknown. It is expected that the contact area in the ultrathin layer materials may be totally alloyed because the Pt-Mo-Se alloy thickness (25-30 nm) induced by the ion bombardment is higher than the material thickness. Further work is still required to elaborate the alloy effect on the ohmic contact property using FIB approach.
In reality, the FIB deposition method was developed mainly for material milling or etching on micrometer and nanometer scales. Metal deposition is only an extended use of the method for coating or protecting material surfaces. However, in this report, the FIB deposition method was adopted for ohmic contact fabrication in layer semiconductor nanostructures. The observation of the thickness effect on the transport properties in these 2D nanomaterials was facilitated by the use of the FIB deposition method. Electrode fabrication at the micrometer or submicrometer scale with reliable ohmic contact quality has been a challenge and is crucial for a variety of applications, such as fundamental electrical characterization of nanomaterials, elimination of contact resistance for electronic device processing, and local metallization of material surfaces. The demonstration of microelectrode fabrication on layer nanomaterials using the FIB deposition method can serve as a crucial and useful reference for future researchers and engineers in academia and the industry.
The authors have nothing to disclose.
RSC thanks the support of the National Science Council (NSC) of Taiwan under Project NSC 102-2112-M-011-001-MY3. YSH acknowledges the support of the NSC of Taiwan under Project NSC 100-2112-M-011-001-MY3.
HRTEM&SEAD | FEI (http://www.fei.com/products/tem/tecnai-g2/?ind=MS) | Tecnai™ G2 F-20 | |
SEM&EDS | HITACHI (http://www.hitachi-hitec.com/global/em/sem/sem_index.html) | S-3000H | |
FIB | FEI (http://www.fei.com/products/dualbeam/versa-3d/) | Quanta 3D FEG | |
AFM | BRUKER (http://www.bruker.com/products/surface-analysis/atomic-force-microscopy/dimension-icon/overview.html) | Dimension Icon | |
XRD | Bruker (https://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/x-ray-diffraction/d2-phaser/learn-more.html) | D2 PHASER X-ray Diffractometer | |
Raman | Renishaw (http://www.renishaw.com/en/renishaw-enhancing-efficiency-in-manufacturing-and-healthcare–1030) | inVia Raman microscope system | |
Keithley-4200 | keithley (http://www.keithley.com.tw/products/dcac/currentvoltage/4200scs) | 4200scs | |
ultralow current leakage cryogenic probe station | Lakeshore Cryotronics (http://www.lakeshore.com/) | TTP4 | |
copper foil tape | 3M (http://solutions.3m.com/wps/portal/3M/en_US/Electronics_NA/Electronics/Products/Product_Catalog/~/3M-Copper-Foil-Shielding-Tape-1182?N=4294300025+5153906&&Nr=AND%28hrcy_id%3A8CQ27CX0WMgs_F2LMWMM6M6_N2RL3FHWVK_GPD0K8BC31gv%29&rt=d) | 1182 | |
Ag paste | Well-Being (http://www.gredmann.com/about.htm) | MS-5000 | |
Cu wire | Guv Team (http://www.guvteam.com) | ICUD0D01N | |
dicing tape | Nexteck (http://www.nexteck-corp.com/tw/product-tape.html) | contact vender | |
mica | Centenary Electronic (http://100y.diytrade.com/sdp/307600/4/pl-1175840/0.html) | T0-200 | |
enamel wire | Light-Tech Electronics (http://www.ltc.com.tw/product_info.php/products_id/57631) | S.W.G #38 |