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

Summary

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

Abstract

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.

Introduction

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 '0' and '1'. It is non-volatile, and the write/read process can be completed within nanoseconds. NVME based on organic^1,2,3,4,5,6 and inorganic^{7,8,9,10,11,12,13,14,15} ferroelectric materials have been attempted on flexible substrates. However, such integration is limited by not only the substrates' 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 oxides^{16,17,18,19,20,21,22,23} 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.

Protocol

1. Fabricating Flexible PZT Thin Films

1. Cut a 1 cm x 1 cm mica substrate from a mica sheet with scissors.
2. Fix this 1 cm x 1 cm mica substrate on a desk using double-sided tape.
3. Use tweezers to peel-off the mica layer-by-layer until the desired thickness (50 µm), measured with a micrometer.
4. Paste this freshly cleaved mica substrate on a 5'' substrate holder using a thin layer of silver paint and cure it at 120 °C on a hot plate for 10 minutes to affix mica on substrate firmly.
5. Put the PLD (Pulsed Laser Deposition) substrate holder into the PLD chamber.
6. Select the repetition rate (e.g., 10 Hz) and laser energy (e.g., 300 mJ).
7. Move the focusing lens to the set position.
8. Open the shutter and deposit a 5 nm CoFe₂O₄ (CFO) [Laser energy: 300 mJ, Oxygen pressure: 50 mTorr, Sample temperature: 590 °C, Deposition time: 5 min] thin film as a buffer layer by triggering the laser (Figure 1).
9. Deposit a 20-80 nm SrRuO₃ (SRO) [Laser energy: 300 mJ, Oxygen pressure: 100 mTorr, Sample temperature: 680 °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).
10. Deposit a 150 nm PZT [Laser energy: 300 mJ, Oxygen pressure: 100 mTorr, Sample temperature: 650 °C, Deposition time: 60 min] thin film on the top of SRO bottom electrode by triggering the laser (Figure 1).
11. Vent the chamber using N₂ and remove the PZT/mica sample (Figure 2) when the temperature reaches room temperature.
12. Put the sample on a piece of glass.
13. Put a predesigned mesh with 200 µm diameter on top of the sample. Fix the mesh well and put mesh-sample into the sputtering chamber.
14. Use DC sputtering (10 mA, 8 mbar, 6 min) to deposit Pt top electrodes on the film. Remove the sample after the sputtering.
15. 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.
16. 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.

2. Ferroelectric Characterization

1. Bending Test
   1. 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.
   2. Place the PZT/mica on the test board of the ferroelectric test system and semiconductor device analyzer.
   3. 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.
      1. 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.
   4. 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.
   5. Mount it on the test board of the ferroelectric test system and semiconductor device analyzer.
   6. 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).
   7. Measure the P-E hysteresis loops and C-E curves under various tensile and compressive bending radii (Figure 4).
      1. 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.
   8. Remove the flexible PZT sample when the P-E and C-E measurements are completed.
2. Thermal Stability
   1. Put the PZT/mica on the test board of the ferroelectric test system and semiconductor device analyzer.
   2. Put one measurement probe on the Pt top electrode and put the other measurement probe on the silver-SRO layer.
   3. Open the temperature control system to heat the sample.
   4. Conduct the P-E and C-E measurements at different temperatures (25 °C, 50 °C, 75 °C, 100 °C, 125 °C, 150 °C, 175 °C).
   5. Turn off the heater assembly after the measurements are done.
3. Bending cyclability tests
   1. Mount the flexible PZT/mica into the two grooves of this setup.
   2. Fix one end of the sample while it is bent from the other end with the aid of a motor.
   3. 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).
   4. 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.
   5. Set number of bending cycles (1000) in the computer (Figure 6).
   6. Click the Start button (Figure 6) to initiate the back and forth motor motion.
   7. Remove the sample and measure the P-E to check whether the ferroelectric properties are retained.

Results

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 ag...

Discussion

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...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Equipment
hot plate	Polish	P-20
PLD system	PVD products	PLD 5000
Ferroelectric test system	Radiant Technologies Precisions workstations	RT66A
Semiconductor device analyzer	Agilent	B1500A
Bending molds	home-made		Machined teflon material
Bending stage	home-built		Labview interfaced setup which provides a prescise displacemnt as small as 1 micrometer
Sputtering system	Beijing Elaborate	ETD-3000
Materials
mica(001) sheets	Nilaco corporation	990066
conductive silver paint	Ted Pella, INC	No.16033
CoFe2O4 target	Kurt J.Lesker
SrRuO3 target	Kurt J.Lesker
PbZr0.2Ti0.8O3 target	Kurt J.Lesker
Pt target	Hefei Ke jing