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.

1. Structural Characterization of MoSe2 Layer Crystals (See Step 1 in Figure 1)
<ol>
	<li>XRD Measurement Procedure
	<ol>
		<li>Mount a MoSe2 layer crystal (with the size range of 5 x 5 x 0.1-10 x 10 x 0.5 mm3) or crystal powder (which was mixed with quartz powder and binder and was smeared on the slide glass) on the holder.</li>
		<li>Press the holder by a slide glass to ensure layer crystal surface parallel to the holder surface.</li>
		<li>Load the sample holder into the diffractomet...

<ol>
	<li>Wilson, J. A., Yoffe, A. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+transition+metal+dichalcogenides+discussion+and+interpretation+of+the+observed+optical,+electrical+and+structural+properties.">The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties.</a> Adv. Phys. 18 (73), 193-335 (1969).</li><li>Ataca, C., Sahin, H., Ciraci, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-layer+MX2+transition-metal+oxides+and+dichalcogenides+in+a+honeycomb-like+structure.">Single-layer MX2 transition-metal oxides and dichalcogenides in a honeycomb-like structure.</a> J. Phys. Chem. C. 116 (16), 8983-8999 (2012).</li><li>Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N., Strano, M. 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T., Ye, P. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Channel+length+scaling+of+MoS2+MOSFETs.">Channel length scaling of MoS2 MOSFETs.</a> ACS Nano. 6 (10), 8563-8569 (2012).</li><li>Ghatak, S., Pal, A. N., Ghosh, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nature+of+electronic+states+in+atomically+thin+MoS2+field-effect+transistors.">Nature of electronic states in atomically thin MoS2 field-effect transistors.</a> ACS Nano. 5 (10), 7707-7712 (2011).</li><li>Ong, Z. Y., Fischetti, M. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mobility+enhancement+and+temperature+dependence+in+top-gated+single-layer+MoS2.">Mobility enhancement and temperature dependence in top-gated single-layer MoS2.</a> Phys. Rev. B. 88 (16), (2013).</li><li>Hwang, W. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparative+study+of+chemically+synthesized+and+exfoliated+multilayer+MoS2+field-effect+transistors.">Comparative study of chemically synthesized and exfoliated multilayer MoS2 field-effect transistors.</a> Appl. Phys. Lett. 102 (4), 165316 (2013).</li><li>Park, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Oxygen+environmental+and+passivation+effects+on+molybdenum+disulfide+field+effect+transistors.">Oxygen environmental and passivation effects on molybdenum disulfide field effect transistors.</a> Nanotechnology. 24 (9), 095202 (2013).</li><li>Wu, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High+mobility+and+high+on/off+ratio+field-effect+transistors+based+on+chemical+vapor+deposited+single-crystal+MoS2+grains.">High mobility and high on/off ratio field-effect transistors based on chemical vapor deposited single-crystal MoS2 grains.</a> Appl. Phys. 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S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two-dimensional+gas+of+massless+Dirac+fermions+in+graphene.">Two-dimensional gas of massless Dirac fermions in graphene.</a> Nature. 438 (7065), 197-200 (2005).</li><li>Zhang, Y., Tan, Y. W., Stormer, H. L., Kim, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+observation+of+the+quantum+Hall+effect+and+Berry's+phase+in+graphene.">Experimental observation of the quantum Hall effect and Berry's phase in graphene.</a> Nature. 438 (7065), 201-204 (2005).</li><li>Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V., Kis, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-layer+MoS2+transistors.">Single-layer MoS2 transistors.</a> Nature Nanotech. 6 (3), 147-150 (2011).</li><li>Chang, C. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrical+transport+properties+of+single+GaN+and+InN+nanowires.">Electrical transport properties of single GaN and InN nanowires.</a> J. Electro. 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E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Disorder+effects+in+focused-ion-beam-deposited+Pt+contacts+on+GaN+nanowires.">Disorder effects in focused-ion-beam-deposited Pt contacts on GaN nanowires.</a> Nano Lett. 5 (10), 2029-2033 (2005).</li><li>Chen, R. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anomalous+quantum+efficiency+for+photoconduction+and+its+power+dependence+in+metal+oxide+semiconductor+nanowires.">Anomalous quantum efficiency for photoconduction and its power dependence in metal oxide semiconductor nanowires.</a> Nanoscale. 5 (15), 6867-6873 (2013).</li><li>Chen, R. S., Tang, C. C., Shen, W. C., Huang, Y. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thickness-dependent+electrical+conductivities+and+ohmic+contacts+in+transition+metal+dichalcogenides+multilayers.">Thickness-dependent electrical conductivities and ohmic contacts in transition metal dichalcogenides multilayers.</a> Nanotechnology. 25 (41), (2014).</li><li>Huang, Y. H., Peng, C. C., Chen, R. S., Huang, Y. S., Ho, C. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transport+properties+in+semiconducting+NbS2+nanoflakes.">Transport properties in semiconducting NbS2 nanoflakes.</a> Appl. Phys. Lett. 105 (9), (2014).</li><li>Hu, S. Y., Liang, C. H., Tiong, K. K., Lee, Y. C., Huang, Y. 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H., Merlin, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Raman Scattering in Materials Science. , (2000).</li><li>Tonndorf, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photoluminescence+emission+and+Raman+response+of+monolayer+MoS2,+MoSe2,+and+WSe2.">Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2.</a> Opt. Express. 21 (4), 4908-4916 (2013).</li><li>Hu, S. Y., Liang, C. H., Tiong, K. K., Lee, Y. C., Huang, Y. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+and+characterization+of+large+niobium-doped+MoSe2+single+crystals.">Preparation and characterization of large niobium-doped MoSe2 single crystals.</a> J. Crystal Growth. 285 (3), 408-414 (2005).</li><li>Hu, S. Y., Liang, C. H., Tiong, K. K., Huang, Y. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+Re+dopant+on+the+electrical+and+optical+properties+of+MoSe2+single+crystals.">Effect of Re dopant on the electrical and optical properties of MoSe2 single crystals.</a> J. Alloys Compounds. 442 (1-2), 1-2 (2007).</li><li>Bougouma, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth+and+characterization+of+large,+high+quality+MoSe2+single+crystals.">Growth and characterization of large, high quality MoSe2 single crystals.</a> J. Crystal Growth. 363, 122-127 (2013).</li></ol>

The accurate determination of the &#963; 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 t...

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&#8722;1sec&#8722;1 for MoS210-17; approximately 50 cm2V&#8722;1sec&#8722;1 for MoSe218). The optimal mobility values of graphene have been reported to be higher than 10,000 cm2V&#8722;1sec&#8722;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 &#956;m. 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&#8211;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.

<table>
	<tbody>
		<tr>
			<td>HRTEM&#38;SEAD</td>
			<td>FEI (http://www.fei.com/products/tem/tecnai-g2/?ind=MS)</td>
			<td>Tecnai&#8482; G2 F-20</td>
			<td></td>
		</tr>
		<tr>
			<td>SEM&#38;EDS</td>
			<td>HITACHI (http://www.hitachi-hitec.com/global/em/sem/sem_index.html)</td>
			<td>S-3000H</td>
			<td></td>
		</tr>
		<tr>
			<td>FIB</td>
			<td>FEI (http://www.fei.com/products/dualbeam/versa-3d/)</td>
			<td>Quanta 3D FEG</td>
			<td></td>
		</tr>
		<tr>
			<td>AFM</td>
			<td>BRUKER (http://www.bruker.com/products/surface-analysis/atomic-force-microscopy/dimension-icon/overview.html)</td>
			<td>Dimension Icon</td>
			<td></td>
		</tr>
		<tr>
			<td>XRD</td>
			<td>Bruker (https://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/x-ray-diffraction/d2-phaser/learn-more.html)</td>
			<td>D2 PHASER X-ray Diffractometer</td>
			<td></td>
		</tr>
		<tr>
			<td>Raman</td>
			<td>Renishaw (http://www.renishaw.com/en/renishaw-enhancing-efficiency-in-manufacturing-and-healthcare--1030)</td>
			<td>inVia Raman microscope system</td>
			<td></td>
		</tr>
		<tr>
			<td>Keithley-4200</td>
			<td>keithley (http://www.keithley.com.tw/products/dcac/currentvoltage/4200scs)</td>
			<td>4200scs</td>
			<td></td>
		</tr>
		<tr>
			<td>ultralow current leakage cryogenic probe station</td>
			<td>Lakeshore Cryotronics (http://www.lakeshore.com/)</td>
			<td>TTP4</td>
			<td></td>
		</tr>
		<tr>
			<td>copper foil tape</td>
			<td>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&#38;&#38;Nr=AND%28hrcy_id%3A8CQ27CX0WMgs_F2LMWMM6M6_N2RL3FHWVK_GPD0K8BC31gv%29&#38;rt=d)</td>
			<td>1182</td>
			<td></td>
		</tr>
		<tr>
			<td>Ag paste</td>
			<td>Well-Being (http://www.gredmann.com/about.htm)</td>
			<td>MS-5000</td>
			<td></td>
		</tr>
		<tr>
			<td>Cu wire</td>
			<td>Guv Team (http://www.guvteam.com)</td>
			<td>ICUD0D01N</td>
			<td></td>
		</tr>
		<tr>
			<td>dicing tape</td>
			<td>Nexteck (http://www.nexteck-corp.com/tw/product-tape.html)</td>
			<td>contact vender</td>
			<td></td>
		</tr>
		<tr>
			<td>mica</td>
			<td>Centenary Electronic (http://100y.diytrade.com/sdp/307600/4/pl-1175840/0.html)</td>
			<td>T0-200</td>
			<td></td>
		</tr>
		<tr>
			<td>enamel wire</td>
			<td>Light-Tech Electronics (http://www.ltc.com.tw/product_info.php/products_id/57631)</td>
			<td>S.W.G #38</td>
			<td></td>
		</tr>
	</tbody>
</table>

ohmic contact fabrication using a focused-ion beam technique and electrical characterization for layer semiconductor nanostructures

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&#8211;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 &#937;&#8722;1 cm&#8722;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.

The determined values of the electrical conductance (G) and conductivity (&#963;) 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&#8211;voltage (I&#8211;V) curve. The room temperature I&#8211;V curves for the two-terminal MoSe2 nanoflake devices with different t...

Watch this Scientific Journal Video about Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures at JoVE.com

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

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 ...

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12.1K vistas.  National Taiwan University of Science and Technology. 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. 