Name: a fabrication and measurement method for a flexible ferroelectric element based on van der waals heteroepitaxy
Uploaded: 2018-04-08T14:00:00.000+00:00
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Description: Watch this Scientific Journal Video about A Fabrication and Measurement Method for a Flexible Ferroelectric Element Based on Van Der Waals Heteroepitaxy at JoVE.com

This work was supported by National Natural Science Foundation of China (Grant Nos. 11402221 and 11502224), the State Key Laboratory of Intense Pulsed Radiation simulation and effect (SKLIPR1513) and the Hunan Provincial Key Research and Development Plan (No. 2016WK2014).

1. Fabricating Flexible PZT Thin Films
<ol>
	<li>Cut a 1 cm x 1 cm mica substrate from a mica sheet with scissors.</li>
	<li>Fix this 1 cm x 1 cm mica substrate on a desk using double-sided tape.</li>
	<li>Use tweezers to peel-off the mica layer-by-layer until the desired thickness (50 &#181;m), measured with a micrometer.</li>
	<li>Paste this freshly cleaved mica substrate on a 5&#39;&#39; substrate holder using a thin layer of silver paint and cure it at 120 &#176;C on a hot plate for 10 minutes to affix mica on substrate firmly.</li>
	<li>Put the PLD (Pulsed Laser Deposition) substrate holder into the PLD chamber.</li>
	<li>Select the repetition rate (e.g., 10 Hz) and laser energy (e.g., 300 mJ).</li>
	<li>Move the focusing lens to the set position.</li>
	<li>Open the shutter and deposit a 5 nm CoFe2O4 (CFO) [Laser energy: 300 mJ, Oxygen pressure: 50 mTorr, Sample temperature: 590 &#176;C, Deposition time: 5 min] thin film as a buffer layer by triggering the laser (Figure 1).</li>
	<li>Deposit a 20-80 nm SrRuO3 (SRO) [Laser energy: 300 mJ, Oxygen pressure: 100 mTorr, Sample temperature: 680 &#176;C, Deposition time: 10-30 min] on the CFO buffer layer as the bottom electrode for subsequent electrical performance tests by triggering the laser (Figure 1).</li>
	<li>Deposit a 150 nm PZT [Laser energy: 300 mJ, Oxygen pressure: 100 mTorr, Sample temperature: 650 &#176;C, Deposition time: 60 min] thin film on the top of SRO bottom electrode by triggering the laser (Figure 1).</li>
	<li>Vent the chamber using N2 and remove the PZT/mica sample (Figure 2) when the temperature reaches room temperature.</li>
	<li>Put the sample on a piece of glass.</li>
	<li>Put a predesigned mesh with 200 &#181;m diameter on top of the sample. Fix the mesh well and put mesh-sample into the sputtering chamber.</li>
	<li>Use DC sputtering (10 mA, 8 mbar, 6 min) to deposit Pt top electrodes on the film. Remove the sample after the sputtering.</li>
	<li>Use a knife or 20% HF acid to remove a 1 mm x 1 mm PZT section. This is to uncover the bottom SRO electrode and form many small flexible ferroelectric capacitors. 
	NOTE: Grow SRO as the bottom electrode, and then deposit Pt on top of the electrodes on the films by DC sputtering to form many small capacitors to measure the electronic properties of the PZT thin film, which is shown in Figure 3.</li>
	<li>Paint a coat of conductive silver on the exposed SRO to increase the electrical conductivity of the bottom SRO electrode. Ensure that the conductive silver can contact the exposed SRO.</li>
</ol>
2. Ferroelectric Characterization
<ol>
	<li>Bending Test
	<ol>
		<li>On the backside of the flexible sample, glue a piece of paper with the same size as the sample for easy transfer of the sample from one stage to another.</li>
		<li>Place the PZT/mica on the test board of the ferroelectric test system and semiconductor device analyzer.</li>
		<li>Put one measurement probe of the ferroelectric test system and semiconductor device analyzer on the Pt top electrode and put the other measurement probe on the silver-SRO layer to get the polarization-electric field (P-E) hysteresis loops and capacitance-electric field (C-E) curves while the sample is unbent.
		<ol>
			<li>Measure the P-E hysteresis loops with the two probes at a 2 kHz frequency and at 4 V. Measure the C-E curves with the two probes at a 1 MHz frequency and at 4 V. Remove the unbent sample.</li>
		</ol>
		</li>
		<li>Secure the flexible PZT/mica thin film on the desired mold using double-sided tape. Take care to avoid the slipping/gliding of mica during the measurement.</li>
		<li>Mount it on the test board of the ferroelectric test system and semiconductor device analyzer.</li>
		<li>Put one probe on the Pt top electrode while the other probe touches the bottom SRO electrode through the silver coating similar to the configuration used earlier (step 2.1.3).</li>
		<li>Measure the P-E hysteresis loops and C-E curves under various tensile and compressive bending radii (Figure 4).
		<ol>
			<li>Measure the P-E hysteresis loops with the two probes at a 2 kHz frequency and at 4 V. Measure the C-E curves with the two probes at a 1 MHz frequency and at 4 V.</li>
		</ol>
		</li>
		<li>Remove the flexible PZT sample when the P-E and C-E measurements are completed.</li>
	</ol>
	</li>
	<li>Thermal Stability
	<ol>
		<li>Put the PZT/mica on the test board of the ferroelectric test system and semiconductor device analyzer.</li>
		<li>Put one measurement probe on the Pt top electrode and put the other measurement probe on the silver-SRO layer.</li>
		<li>Open the temperature control system to heat the sample.</li>
		<li>Conduct the P-E and C-E measurements at different temperatures (25 &#176;C, 50 &#176;C, 75 &#176;C, 100 &#176;C, 125 &#176;C, 150 &#176;C, 175 &#176;C).</li>
		<li>Turn off the heater assembly after the measurements are done.</li>
	</ol>
	</li>
	<li>Bending cyclability tests
	<ol>
		<li>Mount the flexible PZT/mica into the two grooves of this setup.</li>
		<li>Fix one end of the sample while it is bent from the other end with the aid of a motor.</li>
		<li>Use a ruler to measure the PZT/mica length along with the movement (bending) direction of the motor prior to the 8 mm bending process (Figure 5).</li>
		<li>Calculate the movement length C to bend the sample 5 mm according to the formula: C=L-2Rsin(L/2R), where L is the length of PZT/mica in unbent state, R is the bending radii, and C is the movement length of the motor.</li>
		<li>Set number of bending cycles (1000) in the computer (Figure 6).</li>
		<li>Click the Start button (Figure 6) to initiate the back and forth motor motion.</li>
		<li>Remove the sample and measure the P-E to check whether the ferroelectric properties are retained.</li>
	</ol>
	</li>
</ol>

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The authors have no competing financial interests to disclose.

The key step in the fabrication of ferroelectric elements lies in the use of a clean and even/flat substrate surface. Though freshly cleaved mica surface is atomically smooth, it is necessary to pay attention to preventing surfaces from suffering visible splintering, split layers, cracks, inclusions, etc. After deposition of the PZT layer, the sample was cooled under a high oxygen pressure (200-500 Torr) to reduce the oxygen vacancies. Ex situ top platinum electrodes were deposited via a predefined mesh to form many Pt/PZT/SRO capacitor elements. To carry out bending tests, the sample was attached to a piece of paper of similar dimensions to enable an easy transfer of the sample between different molds. The molds used to mechanically strain the sample under compressive or tensile states were printed by a 3D printer. During the cycling tests, both ends of the sample were held firmly so as to avoid slipping of mica layers.
However, the inherent small area of uniformity of the PLD technique restricts its applicability in large scale production. The process to choose a good piece of mica without cracks is also time-consuming. Mica cannot be stretched and compressed, and accordingly the devices grown on mica cannot be stretched or compressed, too. A lot of materials grown on mica need a buffer layer to get a good-quality film, which increases the complexity for the production process. These inherent issues restrict the development of flexible devices. Thus, it is necessary to understand in detail the mechanisms governing the nucleation and growth during van der Waals epitaxy and electronic coupling across van der Waals-heterointerfaces in order to circumvent this these issues.
Currently employed strategies for realizing flexible NVME include use of a polymer substrate, thinning of substrate or epitaxial-transfer technique. Though polymer substrates exhibit excellent mechanical compliance, their low temperature stability affects the device performance in a negative way. Also, thinning substrate8 or epitaxial growth and subsequent transfer on flexible polymer substrate15 involves a multistep tedious process. The van der Waals epitaxy involving mica22,23 not only lessens the lattice and thermal matching conditions but also relieves the substrate clamping effect, beneficial for realizing epitaxial systems with performance metrics comparable to single crystalline bulk counterparts as reflected in PZT/mica. Furthermore, 2D layered mica substrate gives the advantage of realizing free-standing-like memory elements that maintain robust ferroelectric behavior against mechanical and thermal constraints. The PZT/mica system possesses the best performance among all the flexible memory elements to date20, which circumvents the issues of various approaches stated above.
The transparency of mica can be exploited to achieve transparent NVME. Due to the nature of (quasi) van der Waals epitaxy, the material database can be expanded beyond the limited material combinations inherent to conventional epitaxy. It is anticipated that van der Waals epitaxy on mica will trigger substantial research interest in the design and development of next generation of flexible electronic devices.

The successful fabrication of flexible nonvolatile memory elements (NVME) plays a key role in exploiting the full potential of flexible electronics. NVME shall feature light weight, low cost, low-power consumption, fast speed and high storage density capabilities besides data storage, information processing and communication. Perovskite Pb (Zr,Ti)O3 (PZT) acts as a popular system for such applications considering its large polarization, fast polarization switching, high Curie temperature, low coercive field and high piezoelectric coefficient. In ferroelectric nonvolatile memories, an external voltage pulse can switch the two remnant polarizations between two stable directions, represented by &#39;0&#39; and &#39;1&#39;. It is non-volatile, and the write/read process can be completed within nanoseconds. NVME based on organic1,2,3,4,5,6 and inorganic7,8,9,10,11,12,13,14,15 ferroelectric materials have been attempted on flexible substrates. However, such integration is limited by not only the substrates&#39; inability of high-temperature growth but also the degraded device performance, current leakage and electrical shorting due to their rougher surfaces. Despite promising results, alternate strategies like the thinning of substrate8 and the epitaxial layer transfer on a flexible substrate15 suffer restricted viability in view of the sophisticated multistep process, the unpredictability of transfer, and the limited applicability.
For the aforementioned reasons, it is critical to explore an appropriate substrate that is able to overcome limited thermal and operational stabilities of soft substrates to further advance flexible electronics. A natural muscovite mica (KAl2(AlSi3O10)(OH)2) substrate with unique features like atomically smooth surfaces, high thermal stability, chemical inertness, high transparency, mechanical flexibility, and compatibility with current fabrication methods can be used to effectively deal with these issues. More so, the two-dimensional layered structure of monoclinic mica supports van der Waals epitaxy, which mitigates lattice and thermal matching conditions, thereby significantly suppressing the substrate clamping effect. These advantages have been exploited in the direct growth of functional oxides16,17,18,19,20,21,22,23 on muscovite recently, in view of flexible device applications.
Herein, we describe a protocol to directly grow epitaxial yet flexible lead zirconium titanate (PZT) thin films on muscovite mica. This is achieved through a pulsed laser deposition process relying on the versatile properties of mica, resulting in van der Waals heteroepitaxy. Such fabricated structures retain all the superior properties of epitaxial PZT on rigid single crystalline substrates and exhibits excellent thermal and mechanical stabilities. This simple and reliable approach provides a technological advantage over multistep-transfer and substrate thinning strategies and facilitates the development of much-awaited flexible yet single-crystalline non-volatile memory elements prerequisite for next-generation smart devices with high performance.

<table><tbody><tr><td>&lt;strong&gt;Equipment&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>hot plate</td><td>Polish</td><td>P-20</td><td></td></tr><tr><td>PLD system</td><td>PVD products</td><td>PLD 5000</td><td></td></tr><tr><td>Ferroelectric test system&amp;nbsp;</td><td>Radiant Technologies Precisions workstations&amp;nbsp;</td><td>RT66A</td><td></td></tr><tr><td>Semiconductor device analyzer&amp;nbsp;</td><td>Agilent&amp;nbsp;</td><td>B1500A</td><td></td></tr><tr><td>Bending molds</td><td>home-made</td><td></td><td>Machined teflon material</td></tr><tr><td>Bending stage</td><td>home-built</td><td></td><td>Labview interfaced setup which provides a prescise displacemnt as small as 1 micrometer</td></tr><tr><td>Sputtering system</td><td>Beijing&amp;nbsp;Elaborate</td><td>ETD-3000</td><td></td></tr><tr><td>&lt;strong&gt;Materials&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>mica(001) sheets</td><td>Nilaco corporation&amp;nbsp;</td><td>990066</td><td></td></tr><tr><td>conductive silver paint</td><td>Ted Pella, INC</td><td>No.16033</td><td></td></tr><tr><td>CoFe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; target</td><td>Kurt J.Lesker</td><td></td><td></td></tr><tr><td>SrRuO&lt;sub&gt;3&lt;/sub&gt; target</td><td>Kurt J.Lesker</td><td></td><td></td></tr><tr><td>PbZr&lt;sub&gt;0.2&lt;/sub&gt;Ti&lt;sub&gt;0.8&lt;/sub&gt;O&lt;sub&gt;3 &lt;/sub&gt;target</td><td>Kurt J.Lesker</td><td></td><td></td></tr><tr><td>Pt target</td><td>Hefei Ke jing</td><td></td><td></td></tr></tbody></table>

a fabrication and measurement method for a flexible ferroelectric element based on van der waals heteroepitaxy

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on high-density data storage and low-power consumption capabilities. However, the high-quality oxide based nonvolatile memory on flexible substrates is often constrained by the material characteristics and the inevitable high-temperature fabrication process. In this paper, a protocol is proposed to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica. The versatile deposition technique and measurement method enable the fabrication of flexible yet single-crystalline non-volatile memory elements necessary for the next generation of smart devices.

The epitaxial PZT/SRO/CFO/mica thin films were deposited with the pulsed laser deposition technique as outlined in Step 1. Figure 1 shows the growth scheme and Figure 2 shows an actual flexible NVM element based on the PZT.

Mechanical stability is a crucial aspect of flexible device application. The macroscopic ferroelectric performance of the heterostructure against mechanical flexing was evaluated under both tensile and compressive bending. Figure 7a and 7b show the P-E and C-E hysteresis loops of the PZT capacitors under various compressive and tensile bending radii (R). Figure 7c shows constant Psat, Pr, Ec and capacitance values within experimental errors under different bending radius. The corresponding nominal strain values estimated by <img alt="Equation 1" src="/files/ftp_upload/57221/57221eq1.jpg" /> where &#951; = tf /tS, &#967; = Yf /YS, Yf is Young&#39;s modulus of the PZT layer and YS is Young&#39;s modulus of the mica are also marked. These results suggest that the PZT thin &#64257;lm capacitor maintains stable electrical properties under mechanical constraints required for the flexible electronics device applications, which was also checked by Raman spectroscopy20.
The well-saturated and symmetric polarization-electric field (P-E) hysteresis loops and the capacitance-electric field (C-E) with the &#34;butterfly&#34; curves of the heterostructure measured at 1 MHz and temperatures ranging from 25-175 &#176;C for a new device are shown in Figure 8a and 8b, respectively. This ferroelectric capacitor exhibits constant saturation polarization (Psat), a remnant polarization (Pr), a coercive field (Ec) and capacitance in a wide temperature range as shown in Figure 8c. The heterostructure also maintains high retention and endurance at room temperature as well as at 100 &#176;C20. These results imply that the PZT/mica heterostructure can have potential applications in high-temperature electronic devices.
A series of cyclability tests were carried out to validate PZT/mica heterostructures for practical applications. Figure 9 shows P-E loops before and after 1000 bending cycles in both tensile and compressive strain states. The P-E loops at different bending modes are displaced vertically for the sake of convenience. It is noteworthy that the heterostructure retains its ferroelectric behavior even after 1000 bending cycles at a bending radius of 5 mm irrespective of the nature bending strain.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57221/57221fig1.jpg" /> 
Figure 1. The growth scheme of a flexible memory element on mica. Evacuate the chamber to a base pressure (~10-6 Torr) and raise the sample temperature to 590 &#176;C. Adjust the oxygen pressure to 50 mTorr to grow the CFO. Increase the temperature to 680 &#176;C and adjust the oxygen pressure to 100 mTorr to grow the SRO. Decrease the temperature to 650 &#176;C and adjust the oxygen pressure to 100 mTorr to grow the PZT. <a href="/files/ftp_upload/57221/57221fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57221/57221fig2.jpg" /> 
Figure 2. The photograph of a flexible memory element on mica. The flexible memory element can be bent easily. <a href="/files/ftp_upload/57221/57221fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57221/57221fig3.jpg" /> 
Figure 3.The schematic configuration for measuring P-E hysteresis loops and C-E curve. Contact the SRO bottom electrode using one probe, while the other probe contacts the Pt top electrodes on the films to measure the electronic properties of the PZT thin film. <a href="/files/ftp_upload/57221/57221fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57221/57221fig4.jpg" /> 
Figure 4. Molds with different fixed bending radii (R). The bending mold designs were made/drawn using autoCAD and printed using a 3D printer. These molds of fixed bending radii (R) induce the reported compressive and tensile bending strains (R = &#177;12.5 mm, &#177;10.0 mm, &#177;7.5 mm, &#177;5.0 mm, &#177;2.5 mm, the positive (negative) sign corresponds to tensile (compressive) strain that the heterostructures undergo when mounted on them). <a href="/files/ftp_upload/57221/57221fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/57221/57221fig5.jpg" /> 
Figure 5. Bending stage to perform bending cycles test. The heterostructure length (C) is measured by a rule in unbent state. For the bending cycle measurement, use a computer-aided home built bending setup. The bending stage consists of two arms with grooves to hold thin sheets. One arm is fixed while the other arm can be moved to bend a thin sheet with a stepper motor interfaced with the computer. <a href="/files/ftp_upload/57221/57221fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/57221/57221fig6.jpg" /> 
Figure 6. Program protocol to perform bending tests. Use a computer-aided home built bending setup to control the movement of the motor. The setup allows the length of the sample to be bent by providing the displacement as small as 1 &#181;m on the bending stage. One can set the bending radii (see 2.3.4) as well as perform bending cycles. <a href="/files/ftp_upload/57221/57221fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/57221/57221fig7.jpg" /> 
Figure 7. Ferroelectric properties under different bending radii. Electric field dependence of (a) polarization and (b) capacitance under various tensile and compressive bending radii. (c) Saturation polarization (Psat), remnant polarization (Pr), coercive field (Ec) and capacitance as a function of bending radius. Corresponding strain values are also indicated (see text). This Figure has been modified with permission20. <a href="/files/ftp_upload/57221/57221fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/57221/57221fig8.jpg" /> 
Figure 8. Ferroelectric properties under high temperature. Electric field dependence of (a) polarization and (b) capacitance at different temperatures. (c) Thermal evolution of Psat, Pr, Ec and capacitance. This Figure has been modified with permission20. <a href="/files/ftp_upload/57221/57221fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 9" class="xfigimg" src="/files/ftp_upload/57221/57221fig9.jpg" /> 
Figure 9. Ferroelectric properties after bending cycles. P-E hysteresis loops under tensile and compressive bending radius of 5 mm before and after 10 to 1000 bending cycles. This Figure has been modified with permission20. <a href="/files/ftp_upload/57221/57221fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about A Fabrication and Measurement Method for a Flexible Ferroelectric Element Based on Van Der Waals Heteroepitaxy at JoVE.com

A Fabrication and Measurement Method for a Flexible Ferroelectric Element Based on Van Der Waals Heteroepitaxy

In this paper, we present a protocol to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica.

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on ...

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8.2K Views.  Xiangtan University. In this paper, we present a protocol to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica.

Video: A Fabrication and Measurement Method for a Flexible Ferroelectric Element Based on Van Der Waals Heteroepitaxy

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices 1-3. Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

We describe the use of pulsed laser deposition (PLD), photolithography and wire-bonding techniques to create micrometer scale complex oxides devices. The PLD is utilized to grow epitaxial thin films. Photolithography and wire-bonding techniques are introduced to create practical devices for measurement purposes.

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from ...

Sputter Growth and Characterization of Metamagnetic B2-ordered FeRh Epilayers

Chemically ordered alloys are useful in a variety of magnetic nanotechnologies. They are most conveniently prepared at an industrial scale using sputtering techniques. Here we describe a method for preparing epitaxial thin films of B2-ordered FeRh by sputter deposition onto single crystal MgO substrates. Deposition at a slow rate onto a heated substrate allows time for the adatoms to both settle into a lattice with a well-defined epitaxial relationship with the substrate and also to find their proper places in the Fe and Rh sublattices of the B2 structure. The structure is conveniently characterized with X-ray reflectometry and diffraction and can be visualised directly using transmission electron micrograph cross-sections. B2-ordered FeRh exhibits an unusual metamagnetic phase transition: the ground state is antiferromagnetic but the alloy transforms into a ferromagnet on heating with a typical transition temperature of about 380 K. This is accompanied by a 1% volume expansion of the unit cell: isotropic in bulk, but laterally clamped in an epilayer. The presence of the antiferromagnetic ground state and the associated first order phase transition is very sensitive to the correct equiatomic stoichiometry and proper B2 ordering, and so is a convenient means to demonstrate the quality of the layers that can be deposited with this approach. We also give some examples of the various techniques by which the change in phase can be detected.

A method to prepare epitaxial layers of ordered alloys by sputtering is described. The B2-ordered FeRh compound is used as an example, as it displays a metamagnetic transition that depends sensitively on the degree of chemical order and the exact composition of the alloy.

Chemically ordered alloys are useful in a variety of magnetic nanotechnologies. They are most conveniently prepared at an industrial scale using ...

Chemical Vapor Deposition of an Organic Magnet, Vanadium Tetracyanoethylene

Recent progress in the field of organic materials has yielded devices such as organic light emitting diodes (OLEDs) which have advantages not found in traditional materials, including low cost and mechanical flexibility. In a similar vein, it would be advantageous to expand the use of organics into high frequency electronics and spin-based electronics. This work presents a synthetic process for the growth of thin films of the room temperature organic ferrimagnet, vanadium tetracyanoethylene (V[TCNE]x, x~2) by low temperature chemical vapor deposition (CVD). The thin film is grown at &#60;60 &#176;C, and can accommodate a wide variety of substrates including, but not limited to, silicon, glass, Teflon and flexible substrates. The conformal deposition is conducive to pre-patterned and three-dimensional structures as well. Additionally this technique can yield films with thicknesses ranging from 30 nm to several microns. Recent progress in optimization of film growth creates a film whose qualities, such as higher Curie temperature (600 K), improved magnetic homogeneity, and narrow ferromagnetic resonance line-width (1.5 G) show promise for a variety of applications in spintronics and microwave electronics.

We present the synthesis of the organic-based ferrimagnet vanadium tetracyanoethylene (V[TCNE]x, x~2) via low temperature chemical vapor deposition (CVD). This optimized recipe yields an increase in Curie temperature from 400 K to over 600 K and a dramatic improvement in magnetic resonance properties.

Recent progress in the field of organic materials has yielded devices such as organic light emitting diodes (OLEDs) which have advantages not found in ...

Chemistry

Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film synthesis) for the purpose of exploratory materials research. Certain materials pose a challenge wherein the traditional bulk Hall bar device fabrication method is insufficient to produce a measureable device for sample transport measurement, principally because the single crystal size is too small to attach wire leads to the sample in a Hall bar configuration. This can be, for example, because the first batch of a new material synthesized yields very small single crystals or because flakes of samples of one to very few monolayers are desired. In order to enable rapid characterization of materials that may be carried out in parallel with improvements to their growth methodology, a method of device fabrication for very small samples has been devised to permit the characterization of novel materials as soon as a preliminary batch has been produced. A slight variation of this methodology is applicable to producing devices using exfoliated samples of two-dimensional materials such as graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDs), as well as multilayer heterostructures of such materials. Here we present detailed protocols for the experimental device fabrication of fragments and flakes of novel materials with micron-sized dimensions onto substrate and subsequent measurement in a commercial superconducting magnet, dry helium close-cycle cryostat magnetotransport system at temperatures down to 0.300 K and magnetic fields up to 12 T.

We describe the methodology of mechanical exfoliation and deposition of flakes of novel materials with micron-sized dimensions onto substrate, fabrication of experimental device structures for transport experimentation, and the magnetotransport measurement in a dry helium close-cycle cryostat at temperatures down to 0.300 K and magnetic fields up to 12 T.

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film ...

Epitaxial Growth of Perovskite Strontium Titanate on Germanium via Atomic Layer Deposition

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and conformality by taking advantage of self-limiting reactions between vapor-phase precursors and the growing film. Perovskite oxides present potential for next-generation electronic materials, but to-date have mostly been deposited by physical methods. This work outlines a method for depositing SrTiO3 (STO) on germanium using ALD. Germanium has higher carrier mobilities than silicon and therefore offers an alternative semiconductor material with faster device operation. This method takes advantage of the instability of germanium's native oxide by using thermal deoxidation to clean and reconstruct the Ge (001) surface to the 2&#215;1 structure. 2-nm thick, amorphous STO is then deposited by ALD. The STO film is annealed under ultra-high vacuum and crystallizes on the reconstructed Ge surface. Reflection high-energy electron diffraction (RHEED) is used during this annealing step to monitor the STO crystallization. The thin, crystalline layer of STO acts as a template for subsequent growth of STO that is crystalline as-grown, as confirmed by RHEED. In situ X-ray photoelectron spectroscopy is used to verify film stoichiometry before and after the annealing step, as well as after subsequent STO growth. This procedure provides framework for additional perovskite oxides to be deposited on semiconductors via chemical methods in addition to the integration of more sophisticated heterostructures already achievable by physical methods.

This work details the procedures for the growth and characterization of crystalline SrTiO3 directly on germanium substrates by atomic layer deposition. The procedure illustrates the ability of an all-chemical growth method to integrate oxides monolithically onto semiconductors for metal-oxide semiconductor devices.

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and ...

Fabrication of Flexible Image Sensor Based on Lateral NIPIN Phototransistors

Flexible photodetectors have been intensely studied for the use of curved image sensors, which are a crucial component in bio-inspired imaging systems, but several challenging points remain, such as a low absorption efficiency due to a thin active layer and low flexibility. We present an advanced method to fabricate a flexible phototransistor array with an improved electrical performance. The outstanding electrical performance is driven by a low dark current owing to deep impurity doping. Stretchable and flexible metal interconnectors simultaneously offer electrical and mechanical stabilities in a highly deformed state. The protocol explicitly describes the fabrication process of the phototransistor using a thin silicon membrane. By measuring I-V characteristics of the completed device in deformed states, we demonstrate that this approach improves the mechanical and electrical stabilities of the phototransistor array. We expect that this approach to a flexible phototransistor can be widely used for the applications of not only next-generation imaging systems/optoelectronics but also wearable devices such as tactile/pressure/temperature sensors and health monitors.

We present a detailed method to fabricate a deformable lateral NIPIN phototransistor array for curved image sensors. The phototransistor array with an open mesh form, which is composed of thin silicon islands and stretchable metal interconnectors, provides flexibility and stretchability. The parameter analyzer characterizes the electrical property of the fabricated phototransistor.

Flexible photodetectors have been intensely studied for the use of curved image sensors, which are a crucial component in bio-inspired imaging systems, ...

Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

The synthesis of high quality bulk and thin film (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O entropy-stabilized oxides is presented.

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and ...

A Standard and Reliable Method to Fabricate Two-Dimensional Nanoelectronics

Two-dimensional (2D) materials have attracted huge attention due to their unique properties and potential applications. Since wafer scale synthesis of 2D materials is still in nascent stages, scientists cannot fully rely on traditional semiconductor techniques for related research. Delicate processes from locating the materials to electrode definition need to be well controlled. In this article, a universal fabrication protocol required in manufacturing nanoscale electronics, such as 2D quasi-heterojunction bipolar transistors (Q-HBT), and 2D back-gated transistors are demonstrated. This protocol includes the determination of material position, electron beam lithography (EBL), metal electrode definition, et al. A step by step narrative of the fabrication procedures for these devices are also presented. Furthermore, results show that each of the fabricated devices has achieved high performance with high repeatability. This work reveals a comprehensive description of process flow for preparing 2D nano-electronics, enables the research groups to access this information, and pave the way toward future electronics.

The article aims to introduce a standard and reliable fabrication procedure for the development of future low dimensional nanoelectronics.

Two-dimensional (2D) materials have attracted huge attention due to their unique properties and potential applications. Since wafer scale synthesis of 2D ...

Measuring Magnetically-Tuned Ferroelectric Polarization in Liquid Crystals

Materials showing coupling phenomena between magnetism and (ferro)electricity, i.e., magnetoelectric effects, have attracted a great deal of attention due to their potential applications for future device technologies such as sensors and storage. However, conventional approaches, which usually utilize materials containing magnetic metal ions (or radicals), have a major problem: only a few materials have been found to show the coupling phenomena at room temperature. Recently, we proposed a new approach to achieve room-temperature magnetoelectrics. In contrast to conventional approaches, our alternative proposal focuses on a completely different material, a &#34;liquid crystal&#34;, free from magnetic metal ions. In such liquid crystals, a magnetic field can be utilized to control the orientational state of constituent molecules and the corresponding electric polarization through magnetic anisotropy of the molecules; it is an unprecedented mechanism of the magnetoelectric effect. In this context, this paper provides a protocol to measure ferroelectric properties induced by a magnetic field, that is, the direct magnetoelectric effect, in a liquid crystal. With the method detailed here, we successfully detected magnetically-tuned electric polarization in the chiral smectic C phase of a liquid crystal at room temperature. Taken together with the flexibility of constituent molecules, which directly affects the magnetoelectric responses, the introduced method will serve to allow liquid crystal cells to acquire more functions as room-temperature magnetoelectrics and associated optical materials.

In this report, we present a protocol to examine direct magnetoelectric effects, i.e., induction of ferroelectric polarization by applying magnetic fields, in liquid crystals. This protocol provides a unique approach, supported by the softness of liquid crystals, to achieve room-temperature magnetoelectrics.

Materials showing coupling phenomena between magnetism and (ferro)electricity, i.e., magnetoelectric effects, have attracted a great deal of attention due ...

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We demonstrate not only lateral control but, importantly, also control over the angular alignment of adjacent layers. The core of the technique is represented by a home-built transfer setup which allows the user to control the position of the individual crystals involved in the transfer. This is achieved with sub-micrometer (translational) and sub-degree (angular) precision. Prior to stacking them together, the isolated crystals are individually manipulated by custom-designed moving stages that are controlled by a programmed software interface. Moreover, since the entire transfer setup is computer controlled, the user can remotely create precise heterostructures without coming into direct contact with the transfer setup, labeling this technique as &#8220;hands-free&#8221;. In addition to presenting the transfer set-up, we also describe two techniques for preparing the crystals that are subsequently stacked.

In this work we describe a technique that is used to create new crystals (van der Waals heterostructures) by stacking ultrathin layered 2D materials with precise control over position and relative orientation.

In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We ...