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 diffractometer.</li>
		<li>Close the doors of diffractometer.</li>
		<li>Calibrate beam line accordingly to manufacturer&#8217;s instructions.</li>
		<li>Input measurement parameters such as the 2 scan range (10-80&#176;), the increment (0.004&#176;), and the dwell time (0.1 sec).</li>
		<li>Start the DIFFRAC.Measurement Center program on the computer attached to the diffractometer and then save data and name data file according to manufacturer&#8217;s protocol.</li>
		<li>Analyze the XRD pattern by identifying the positions of the diffraction peaks using the software and then compare with the standard data from JCPDS card database to confirm the single out-of-plane orientation and single-crystalline quality of the MoSe2 layer crystals32,33.</li>
	</ol>
	</li>
	<li>Micro-Raman Measurement Procedure
	<ol>
		<li>Perform the Raman equipment calibration using a silicon wafer as the standard sample. The measurement of the silicon wafer is the same as the procedure described below for the interested MoSe2 layer crystal.</li>
		<li>Mount a MoSe2 layer crystal on the slide glass.</li>
		<li>Load the slide glass on the holder of optical microscope and focus the sample surface with a white light source.</li>
		<li>Switch the light source from a white light to a laser beam (wavelength at 514 nm).</li>
		<li>Input measurement parameters such as the wavenumber scan range (150-500 cm-1), the integration time (10 sec), and the number of scan times (10-30 times).</li>
		<li>Start the program on computer attached to the Raman spectrometer and then save data and name data file according to manufacturer&#8217;s protocol.</li>
		<li>Analyze the Raman spectrum by identifying their peak widths and positions using the software and then compare with standard data from references to confirm the crystalline structure type and quality of the MoSe2 layer crystals34,35.</li>
	</ol>
	</li>
</ol>
2. Fabrication of MoSe2 Layer Nanocrystal Devices
<ol>
	<li>Mechanical Exfoliation of Layer Crystals
	<ol>
		<li>Clean tweezers with acetone and alcohol.</li>
		<li>Pick the MoSe2 layer crystals (4 to 8 pieces) with a shiny surface (i.e. mirror-like crystal face) and an area size larger than 0.5 x 0.5 mm2 with the tweezers and put them on the dicing tape with an area size of 20 x 60 mm2.</li>
		<li>Fold the tape in half to exfoliate the layer crystal and repeat the action approximately twenty times. Usually layer crystals can be stripped into many micrometer-sized crystals in width (see Step 2 in Figure 1).</li>
		<li>Load the dicing tape with the layer nanocrystal powder into the SEM chamber to observe the sizes and morphologies of these stripped MoSe2 layer microcrystal. If the width distributions of the layer nanocrystal are at 1-20 &#956;m, the nanocrystal powder can meet the criteria for the device fabrication.</li>
	</ol>
	</li>
	<li>Dispersion of the Layer Nanocrystals on the Device Template
	<ol>
		<li>Place the dicing tape with the layer nanocrystal powder upside-down on the device template. The template is SiO2 (300 nm)-coated silicon substrate with sixteen pre-patterned Ti (30 nm)/Au (90 nm) electrodes on the SiO2 surface (see Step 4 in Figure 1). The area size of the template is 5 x 5 mm2.</li>
		<li>Tap the dicing tape lightly to make some nanocrystals (roughly 10 to 100 pieces) fall on the template.</li>
		<li>Check the number density and dispersion condition of the nanocrystal on the template by optical microscope or sometimes by SEM if the dispersed nanocrystals can be not observed by optical microscope. Usually 2 to 5 pieces of nanocrystals (area size larger than 2 x 2&#160;&#956;m2) dispersed on the center square (with area of 80 x 80 &#956;m2) of the template without overlapping to each other are the better condition for the next FIB processing.</li>
	</ol>
	</li>
	<li>Electrode Fabrication by FIB
	<ol>
		<li>Mount templates on the FIB holder using conducting copper foil tape. Typically, the area of conducting tape of 3 x 2.4 cm2 was required for mounting 6-8 templates.</li>
		<li>Load the holder into the FIB chamber.</li>
		<li>Evacuate the chamber to the vacuum degree down to 10-5 mbar by clicking the button &#8220;Pump&#8221;.</li>
		<li>Set the electron beam current (41 pA) and acceleration voltage (10 kV) for the SEM mode.</li>
		<li>Set the ion beam current (0.1 nA) and acceleration voltage (30 kV) for the FIB mode.</li>
		<li>Warm up the ion beam system and gas-injection-system (GIS) by clicking the button &#8220;beam on&#8221; and the button &#8220;Cold&#8221; in the &#8220;Gas Injection&#8221; block, respectively.</li>
		<li>Turn on the electron-beam by clicking the button &#8220;Beam On&#8221; and focus the image at a low magnification of 100X.</li>
		<li>Set the z-axial working distance (WD) at 10 mm for SEM mode.</li>
		<li>Set the magnification at 5,000X and focus.</li>
		<li>Set the tilt angle of the holder to 52 degrees by clicking the button &#8220;Navigation&#8221; and input the tilt angle &#8220;52&#8221;.</li>
		<li>Select a MoSe2 layer nanocrystal with a certain thickness (ranging from 5 to 3,000 nm) and a rectangular and square shape for the electrode fabrication.</li>
		<li>Take the SEM images at different magnifications (from 1,000X to 10,000X) of the targeted pristine material before electrode fabrication by clicking the button &#8220;Snapshot&#8221;.</li>
		<li>Switch to FIB mode and take an FIB image by the snapshot mode to reduce the exposure time of the targeted material under ion beam bombardment.</li>
		<li>Define the electrode deposition area, select the &#8220;Pt deposition&#8221; mode, and input the thickness (0.2-1.0 &#956;m) value of the deposited Pt electrode.</li>
		<li>Introduce the capillary of GIS into the chamber by clicking the box &#8220;Pt dep&#8221; in the &#8220;Gas Injection&#8221; block.</li>
		<li>Take an image by the snapshot mode again and modify the position of the electrodes if the originally defined pattern shifts slightly.</li>
		<li>Turn on the FIB deposition by clicking the button &#8220;Start Patterning&#8221;.</li>
		<li>After deposition, draw the capillary of GIS back by unclicking the box &#8220;Pt dep&#8221; in the &#8220;Gas Injection&#8221; block.</li>
		<li>Switch to SEM mode and check the result of the deposited Pt electrodes on the layer nanocrystal.</li>
		<li>Take the SEM images at different magnifications of the completed devices with two or four electrodes (see Step 3 in Figure 1).</li>
		<li>Set the tilt angle of the holder return to 0 degrees by clicking the button &#8220;Navigation&#8221; and input the tilt angle &#8220;0&#8221;.</li>
		<li>Take the top-viewed SEM images at different magnifications for the estimations of the material width and electrode inter-distance by clicking the button &#8220;Snapshot&#8221;.</li>
		<li>Turn off the electron-beam and ion beam systems and cool down GIS system by clicking the button &#8220;Beam Off&#8221; and the button &#8220;Warm&#8221; in the &#8220;Gas Injection&#8221; block, respectively.</li>
		<li>Vent the chamber by introducing nitrogen gas by clicking the buttons &#8220;Vent&#8221; and then take the holder out of the chamber. It typically takes 5 to 10 min to finish the venting process.</li>
		<li>Close the chamber door and evacuate the chamber.</li>
	</ol>
	</li>
</ol>
3. Characterizations of the MoSe2 Layer Nanocrystal Devices
<ol>
	<li>Thickness Measurement of the Layer Nanocrystals by AFM
	<ol>
		<li>Install the AFM cantilever to the probe holder.</li>
		<li>Turn on AFM program and select &#8220;ScanAsyst&#8221; mode.</li>
		<li>Load the probe holder and connect it with the laser diode head of AFM station.</li>
		<li>Perform the calibration to align the incident laser beam position and the cantilever according to manufacturer&#8217;s protocol.</li>
		<li>Mount the sample (the template chip with FIB-fabricated layer nanocrystal devices) on the sample holder by Cu foil tape.</li>
		<li>Load the sample holder to the AFM station.</li>
		<li>Move the sample holder to the position approximately underneath the laser beam or AFM cantilever.</li>
		<li>Lower down AFM cantilever to the focus position by focusing the optical microscope image of the layer nanocrystal.</li>
		<li>Input scan parameters such as the scan area (6 x 6-30 x 30 &#956;m2), the frequency (0.5-1.5 Hz), and the resolution (256-512 lines).</li>
		<li>Start the program and save data according to the manufacturer&#8217;s protocol.</li>
		<li>Raise the AFM cantilever and take the sample holder out.</li>
		<li>Load the second sample and repeat the measurement procedure described above if need.</li>
		<li>Estimate the thickness of layer nanocrystals by analyzing the AFM image and height profile using the software &#8220;NanoScope Analysis&#8221;. Select a lateral height profile from the AFM image and determine the average thickness value by the flatten area of the profile. (See Figure 2d and 2e)</li>
	</ol>
	</li>
	<li>Current versus voltage (I-V) measurement of the layer nanocrystals
	<ol>
		<li>Mount the sample (the template chip with FIB-fabricated layer nanocrystal devices) on the mica substrate by Cu foil tape.</li>
		<li>Bond the enameled wires or Cu wires on the electrodes of the chip by Ag paste. (See Step 4 in Figure 1.)</li>
		<li>Load the completed sample in the probe station chamber and fix it on the sample holder by Cu foil tape. The cryogenic probe station was located in the dark environment. (See Step 5 in Figure 1.)</li>
		<li>Solder the electrical wires of the sample and the metal electrodes of the probes one by one.</li>
		<li>Cap the chamber top and evacuate the chamber down to 10-4 mbar. Cool down sample to the 77 K by introducing liquid nitrogen into the probe station. Set the temperature range (usually from 80 to 320 K), interval, and dwell time for the temperature control. (Necessary only for temperature-dependent measurement).</li>
		<li>Set the applied voltage sweeping range (typically from -1 to 1 V), voltage interval (0.01 V), and the limited maximal current (10 or 100&#160;&#956;A) in an ultrahigh-impedance multifunctional electrometer for the two-terminal I-V measurement. For four-terminal measurement, set the applied current sweeping range (typically from -100 to 100 &#956;A) and current interval (1 &#956;A).</li>
		<li>Start the program and save the I-V data at room temperature or at different temperatures.</li>
		<li>Open the chamber cap if necessary and take the sample out of the chamber.</li>
		<li>Load the second sample if need and repeat the procedure described above.</li>
		<li>Analyze the I-V curve by plotting the measured current versus applied voltage data using the software. Fit the I-V curve by selecting the Linear Fitting function. Check the linearity of the I-V curve and obtain the slope value (i.e. conductance value). (See Step 6 in Figure 1.)</li>
		<li>Repeat the Step 3.2.10 for the I-V curves measured at different temperatures if need.</li>
		<li>Calculate the conductivity (&#963;) value according to the equation &#963;= G(t/tw) by adopting the parameters obtained by I-V, SEM, and AFM measurements including conductance (G), thickness (t), width (w) and length (l) of the layer nanocrystal.</li>
		<li>Plot the curves of the conductance and conductivity values versus thickness of layer nanocrystals.</li>
	</ol>
	</li>
</ol>

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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 authors have nothing to disclose.

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 ...

ohmic-contact-fabrication-using-focused-ion-beam-technique-electrical

Research

JoVE Journal

Engineering

12.2K 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. 

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

Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on photoconduction, which has been one of the most commonly used techniques for terahertz generation 1-8. Terahertz generation in a photoconductive emitter is achieved by pumping an ultrafast photoconductor with a pulsed or heterodyned laser illumination. The induced photocurrent, which follows the envelope of the pump laser, is routed to a terahertz radiating antenna connected to the photoconductor contact electrodes to generate terahertz radiation. Although the quantum efficiency of a photoconductive emitter can theoretically reach 100%, the relatively long transport path lengths of photo-generated carriers to the contact electrodes of conventional photoconductors have severely limited their quantum efficiency. Additionally, the carrier screening effect and thermal breakdown strictly limit the maximum output power of conventional photoconductive terahertz sources. To address the quantum efficiency limitations of conventional photoconductive terahertz emitters, we have developed a new photoconductive emitter concept which incorporates a plasmonic contact electrode configuration to offer high quantum-efficiency and ultrafast operation simultaneously. By using nano-scale plasmonic contact electrodes, we significantly reduce the average photo-generated carrier transport path to photoconductor contact electrodes compared to conventional photoconductors 9. Our method also allows increasing photoconductor active area without a considerable increase in the capacitive loading to the antenna, boosting the maximum terahertz radiation power by preventing the carrier screening effect and thermal breakdown at high optical pump powers. By incorporating plasmonic contact electrodes, we demonstrate enhancing the optical-to-terahertz power conversion efficiency of a conventional photoconductive terahertz emitter by a factor of 50 10.

We describe methods for the design, fabrication, and experimental characterization of plasmonic photoconductive emitters, which offer two orders of magnitude higher terahertz power levels compared to conventional photoconductive emitters.

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on ...

Fabrication and Characterization of Disordered Polymer Optical Fibers for Transverse Anderson Localization of Light

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. The developed polymer optical fiber is composed of 80,000 strands of poly (methyl methacrylate) (PMMA) and polystyrene (PS) that are randomly mixed and drawn into a square cross section optical fiber with a side width of 250 &mu;m. Initially, each strand is 200 &mu;m in diameter and 8-inches long. During the mixing process of the original fiber strands, the fibers cross over each other; however, a large draw ratio guarantees that the refractive index profile is invariant along the length of the fiber for several tens of centimeters. The large refractive index difference of 0.1 between the disordered sites results in a small localized beam radius that is comparable to the beam radius of conventional optical fibers. The input light is launched from a standard single mode optical fiber using the butt-coupling method and the near-field output beam from the disordered fiber is imaged using a 40X objective and a CCD camera. The output beam diameter agrees well with the expected results from the numerical simulations. The disordered optical fiber presented in this work is the first device-level implementation of 2D Anderson localization, and can potentially be used for image transport and short-haul optical communication systems.

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. This microstructured fiber can transport a small localized beam with a radius that is comparable to the beam radius of conventional optical fibers.

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. The developed ...

Writing and Low-Temperature Characterization of Oxide Nanostructures

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions. Oxide interfaces exhibit a wide range of properties that can be controlled include conduction, piezoelectric behavior, ferromagnetism, superconductivity and nonlinear optical properties. Recently, methods for controlling these properties at extreme nanoscale dimensions have been discovered and developed. Here are described explicit step-by-step procedures for creating LaAlO3/SrTiO3 nanostructures using a reversible conductive atomic force microscopy technique. The processing steps for creating electrical contacts to the LaAlO3/SrTiO3 interface are first described. Conductive nanostructures are created by applying voltages to a conductive atomic force microscope tip and locally switching the LaAlO3/SrTiO3 interface to a conductive state. A versatile nanolithography toolkit has been developed expressly for the purpose of controlling the atomic force microscope (AFM) tip path and voltage. Then, these nanostructures are placed in a cryostat and transport measurements are performed. The procedures described here should be useful to others wishing to conduct research in oxide nanoelectronics.

Oxide nanostructures provide new opportunities for science and technology. The interfacial conductivity between LaAlO3 and SrTiO3 can be controlled with near-atomic precision using a conductive atomic force microscopy technique. The protocol for creating and measuring conductive nanostructures at LaAlO3/SrTiO3 interfaces is demonstrated.

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions.  Oxide ...

Patterning via Optical Saturable Transitions - Fabrication and Characterization

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical Saturable Transitions (POST). In this technique the chemical properties of organic photochromic molecules that undergo single-photon reactions are exploited, enabling rapid top-down nanopatterning over large areas at low light intensities, thereby, allowing for the circumvention of the far-field diffraction barrier.4 Simple, cost-effective, high throughput and resolution alternatives to nanopatterning are being explored, such as, two-photon polymerization5,6, beam pen lithography (BPL)7, scanning electron beam lithography (SEBL), and focused ion beam (FIB) patterning. However, multi-photon approaches require high light intensities, which limit their potential for high throughput and offer low image contrast. Although, electron and ion beam lithographic processes offer increased resolution, the serial nature of the process is limited to slow writing speeds, which also prevents patterning of features over large areas. Beam-pen lithography is an approach towards parallel near-field optical lithography. However, the gap between the source of the beam and the surface of the photoresist needs to be controlled extremely precisely for good pattern uniformity and this is very challenging to accomplish for large arrays of beams. Patterning via Optical Saturable Transitions (POST) is an alternative optical nanopatterning technique for patterning sub-wavelength features1-3. Since this technique uses single photons instead of electrons, it is extremely fast and does not require high light intensities1-3, opening the door to massive parallelization.

We report that the diffraction limit of conventional optical lithography can be overcome by exploiting the transitions of organic photochromic derivatives induced by their photoisomerization at low light intensities.1-3 This paper outlines our fabrication technique and two locking mechanisms, namely: dissolution of one photoisomer and electrochemical oxidation.

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical ...

Fabrication and Operation of a Nano-Optical Conveyor Belt

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological and physical sciences over the past few decades. The progress made in this field invites further study of even smaller systems and at a larger scale, with tools that could be distributed more easily and made more widely available. Unfortunately, the fundamental laws of diffraction limit the minimum size of the focal spot of a laser beam, which makes particles smaller than a half-wavelength in diameter hard to trap and generally prevents an operator from discriminating between particles which are closer together than one half-wavelength. This precludes the optical manipulation of many closely-spaced nanoparticles and&#160;limits the&#160;resolution of optical-mechanical systems. Furthermore, manipulation using focused beams requires beam-forming or steering optics, which can be very bulky and expensive. To address these limitations in the system scalability of conventional optical trapping our lab has devised an alternative technique which utilizes near-field optics to move particles across a chip. Instead of focusing laser beams in the far-field, the optical near field of plasmonic resonators produces the necessary local optical intensity enhancement to overcome the restrictions of diffraction and manipulate particles at higher resolution. Closely-spaced resonators produce strong optical traps which can be addressed to mediate the hand-off of particles from one to the next in a conveyor-belt-like fashion. Here, we describe how to design and produce a conveyor belt using a gold surface patterned with plasmonic C-shaped resonators and how to operate it with polarized laser light to achieve super-resolution nanoparticle manipulation and transport. The nano-optical conveyor belt chip can be produced using lithography techniques and easily packaged and distributed.

The scalability and resolution of conventional optical manipulation techniques are limited by diffraction. We circumvent the diffraction limit and describe a method of optically transporting nanoparticles across a chip using a gold surface patterned with a path of closely spaced C-shaped plasmonic resonators.

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological ...

Silicon Metal-oxide-semiconductor Quantum Dots for Single-electron Pumping

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by quantum mechanical effects such as tunneling through single dopants, scattering via interface defects, and discrete trap charge states. However, progress in silicon technology has shown that these phenomena can be harnessed and exploited for a new class of quantum-based electronics. Among others, multi-layer-gated silicon metal-oxide-semiconductor (MOS) technology can be used to control single charge or spin confined in electrostatically-defined quantum dots (QD). These QD-based devices are an excellent platform for quantum computing applications and, recently, it has been demonstrated that they can also be used as single-electron pumps, which are accurate sources of quantized current for metrological purposes. Here, we discuss in detail the fabrication protocol for silicon MOS QDs which is relevant to both quantum computing and quantum metrology applications. Moreover, we describe characterization methods to test the integrity of the devices after fabrication. Finally, we give a brief description of the measurement set-up used for charge pumping experiments and show representative results of electric current quantization.

The fabrication process and experimental characterization techniques relevant to single-electron pumps based on silicon metal-oxide-semiconductor quantum dots are discussed.

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by ...

Phase Diagram Characterization Using Magnetic Beads as Liquid Carriers

Magnetic beads with ~1.9 &#181;m average diameter were used to transport microliter volumes of liquids between contiguous liquid segments with a tube for the purpose of investigating phase change of those liquid segments. The magnetic beads were externally controlled using a magnet, allowing for the beads to bridge the air valve between the adjacent liquid segments. A hydrophobic coating was applied to the inner surface of the tube to enhance the separation between two liquid segments. The applied magnetic field formed an aggregate cluster of magnetic beads, capturing a certain liquid amount within the cluster that is referred to as carry-over volume. A fluorescent dye was added to one liquid segment, followed by a series of liquid transfers, which then changed the fluorescence intensity in the neighboring liquid segment. Based on the numerical analysis of the measured fluorescence intensity change, the carry-over volume per mass of magnetic beads has been found to be ~2 to 3 &#181;l/mg. This small amount of liquid allowed for the use of comparatively small liquid segments of a couple hundred microliters, enhancing the feasibility of the device for a lab-in-tube approach. This technique of applying small compositional variation in a liquid volume was applied to analyzing the binary phase diagram between water and the surfactant C12E5 (pentaethylene glycol monododecyl ether), leading to quicker analysis with smaller sample volumes than conventional methods.

Here, we present a protocol to investigate multi-component phase diagrams using externally controlled magnetic beads as liquid carriers in a lab-in-tube approach. This approach can aid in applications that seek to gather further information on phase change in complex liquid systems.

Magnetic beads with ~1.9 &#181;m average diameter were used to transport microliter volumes of liquids between contiguous liquid segments with a tube for ...

Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications of semiconductor materials. However, it is still under debate how the defect structure determines the band structure, and therefore, the recombination behavior of electron-hole pairs responsible for the optical and electrical properties of the extended defects. The present paper is a survey of procedures for the spatially resolved investigation of structural and of physical properties of extended defects in semiconductor materials with a scanning electron microscope (SEM). Representative examples are given for crystalline silicon. The luminescence behavior of extended defects can be investigated by cathodoluminescence (CL) measurements. They are particularly valuable because spectrally and spatially resolved information can be obtained simultaneously. For silicon, with an indirect electronic band structure, CL measurements should be carried out at low temperatures down to 5 K due to the low fraction of radiative recombination processes in comparison to non-radiative transitions at room temperature. For the study of the electrical properties of extended defects, the electron beam induced current (EBIC) technique can be applied. The EBIC image reflects the local distribution of defects due to the increased charge-carrier recombination in their vicinity. The procedure for EBIC investigations is described for measurements at room temperature and at low temperatures. Internal strain fields arising from extended defects can be determined quantitatively by cross-correlation electron backscatter diffraction (ccEBSD). This method is challenging because of the necessary preparation of the sample surface and because of the quality of the diffraction patterns which are recorded during the mapping of the sample. The spatial resolution of the three experimental techniques is compared.

The optical, electrical, and structural properties of dislocations and of grain boundaries in semiconductor materials can be determined by experiments performed in a scanning electron microscope. Electron microscopy has been used to investigate cathodoluminescence, electron beam induced current, and diffraction of backscattered electrons.

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications ...

Focused Ion Beam Fabrication of LiPON-based Solid-state Lithium-ion Nanobatteries for In Situ Testing

Solid-state electrolytes are a promising replacement for current organic liquid electrolytes, enabling higher energy densities and improved safety of lithium-ion (Li-ion) batteries. However, a number of setbacks prevent their integration into commercial devices. The main limiting factor is due to nanoscale phenomena occurring at the electrode/electrolyte interfaces, ultimately leading to degradation of battery operation. These key problems are highly challenging to observe and characterize as these batteries contain multiple buried interfaces. One approach for direct observation of interfacial phenomena in thin film batteries is through the fabrication of electrochemically active nanobatteries by a focused ion beam (FIB). As such, a reliable technique to fabricate nanobatteries was developed and demonstrated in recent work. Herein, a detailed protocol with a step-by-step process is presented to enable the reproduction of this nanobattery fabrication process. In particular, this technique was applied to a thin film battery consisting of LiCoO2/LiPON/a-Si, and has further been previously demonstrated by in situ cycling within a transmission electron microscope.

Focused Ion Beam Fabrication of LiPON-based Solid-state Lithium-ion Nanobatteries for In Situ Testing

A protocol for the fabrication of electrochemically active LiPON-based solid-state lithium-ion nanobatteries using a focused ion beam is presented.

Solid-state electrolytes are a promising replacement for current organic liquid electrolytes, enabling higher energy densities and improved safety of ...

Atmospheric Pressure Fabrication of Large-Sized Single-Layer Rectangular SnSe Flakes

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for applications in two-dimensional nanoelectronics devices. Although many methods to synthesize SnSe nanocrystals have been developed, a simple way to fabricate large-sized single-layer SnSe flakes remains a great challenge. Herein, we show the experimental method to directly grow large-sized single-layer rectangular SnSe flakes on commonly used SiO2/Si insulating substrates using a straightforward two-step fabrication method in an atmospheric pressure quartz tube furnace system. The single-layer rectangular SnSe flakes with an average thickness of ~6.8 &#197; and lateral dimensions of about 30 &#181;m &#215; 50 &#181;m were fabricated by a combination of vapor transport deposition technique and nitrogen etching route. We characterized the morphology, microstructure, and electrical properties of the rectangular SnSe flakes and obtained excellent crystallinity and good electronic properties. This article about the two-step fabrication method can help researchers grow other similar two-dimensional, large-sized, single-layer materials using an atmospheric pressure system.

A protocol is presented demonstrating a two-step fabrication technique to grow large-sized single-layer rectangular shaped SnSe flakes on low-cost SiO2/Si dielectrics wafers in an atmospheric pressure quartz tube furnace system.

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for ...