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Double-Channel Piezotronic Transistors for Highly Sensitive Pressure Sensing Shuhai Liu, Longfei Wang, Zheng Wang, Yafeng Cai, Xiaolong Feng, Yong Qin, and Zhong Lin Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08447 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018
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Double-Channel Piezotronic Transistors for Highly Sensitive Pressure Sensing Shuhai Liu1†, Longfei Wang2,3†, Zheng Wang4†, Yafeng Cai5, Xiaolong Feng6, Yong Qin1,4* and Zhong Lin Wang2,3,7* 1
School of Advanced Materials and Nanotechnology, Xidian University, Shaanxi 710071, China 2
Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China
3
College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, China 4
Institute of Nanoscience and Nanotechnology, School of Physical Science and Technology, Lanzhou University, Gansu 730000, China
5
Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China
6
Microsystems and Terahertz Research Center, China Academy of Engineering Physics, Chengdu, Sichuan 610200, China 7
School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
†These authors contributed equally to this work. *Corresponding author E-mail:
[email protected],
[email protected].
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ABSTRACT: Piezotronic transistor (PT) that utilize inner crystal potential generated by interface piezoelectric polarization charges as the gate voltage has great potential applications in force/pressure triggered or controlled electronics device, sensors, human-machine communication and microelectromechanical systems. Although the performance of PTs has been partly enhanced by exploring special materials with different geometry or high piezoelectricity, few studies have been focused on the structure design of PT itself to more effectively enhance the performance and structural reliability. Here, an integrated double channel plane piezotronic transistors is invented as a high-performance pressure sensing technology. Owing to the double channel modulation and the plane structure, the PT has the merits of high pressure sensitivity (84.2~104.4 meV/MPa) and high structural reliability,
which
makes
it
has
widely
great
applications,
such
as
human-computer interfacing, biosensor, and health monitoring.
KEYWORDS: piezotronic transistor, ZnO nanoplatelet, pressure sensor, sensitivity, reliability.
Investigating interfacial phenomena and interfacial engineering in semiconductor devices1 is important for their applications in electronics,2,3 optoelectronics4,5 and catalysis.6 Due to the coupling between piezoelectric polarization and semiconductor properties, the piezotronic effect has been widely considered an effective way to engineer band structure at the interfaces in semiconductor devices.7-9 Strain-induced piezoelectric polarization charges in semiconductor device will result in a redistribution of free charge carriers and band structures tuning near the interface, allowing the regulation of electronic transport across the interface under mechanical stimuli.8-12 This kind of devices is known as the piezotronic transistor (PT), and has triggered a wide variety of applications including tactile imaging,13 varistors,14 lasers15 and biomolecule sensors.16 2 ACS Paragon Plus Environment
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In the past decade, almost all the demonstrated piezotronic devices are based on two conventional structures: transversely driven PT (horizontal PT)16-22 and vertically driven PT (vertical PT).13, 23-28 Although horizontal PTs have been applied in flexible electronics and optoelectronics,16-19 vertical PTs are more suitable for direct detection of force/pressure without bending substrates, particularly for high-density integration of PTs for tactile imaging.13,24-26 Vertical PTs are usually composed of three-dimensional
nano/micro-structure
with
single
crystal
piezoelectric
semiconductor sandwiched by one top and one bottom electrodes (Figure 1a left). However, this three-dimensional structure demands for complex microfabrication and leads to difficulty and unreliability in large-scale production, which may limit their further applications.29-32 More importantly, among the two, only one contact works as effective regulating junction, while the other has an opposite effect.7,10,13,20 This single channel (one contact) modulated PT has low structural reliability, because only one channel failure (for example, Schottky contact broken into ohmic contact) will lead to device-malfunction. So, it is highly desired to find schemes for developing the structure and achieving large-scale vertical PT array in a more feasible and reliable way. In addition, sensitivity is one of the most critical properties of sensors. In piezotronics, sensitivity of PT has been partly improved by a variety of means over the past several years, such as doping or surface modification of materials33,35 to reduce the carriers’ shielding effect, and exploration of materials21-23,27,35 with special geometry or high piezoelectricity. However, few studies have been focused on PT’s structure itself to enhance the performance of the piezotronic transistor by increasing the modulation of the strain induced piezoelectric potential on PT’s transport property. In this work, we designed and developed a kind of double-channel piezotronic transistor (DCPT) with merits of high pressure sensitivity and high structural reliability, and further developed a kind of pressure sensor by integrating DCPTs, with a high pressure sensitivity of 84.2~104.4 meV/MPa, which is about 1.1~1.7 times larger than that of a conventional PT.27 The DCPT (Figure 1a right) is regulated by dual Schottky contacts (double channels) formed between two bottom Pd electrodes 3 ACS Paragon Plus Environment
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and the same polarization facet (bottom surface) of ZnO nanoplatelet, and achieves a plane compatible structure by streamlining the three-layer sandwich structure into a two-layer plane structure without top electrodes. Because the two-channels are synchronously modulated in an identical direction (by positive piezoelectric charges under pressure), rather than the opposite modulation in the conventional structures (Figure 1a),13,25-27,35 the DCPT has higher change of the effective Schottky barrier height (∆SBH) (Figure 1b), and thus higher pressure sensitivity. Meanwhile, due to the two-dimensional plane structure and the double channel regulation mechanism, the DCPT presents high structural stability. Even if one channel failure, the DCPT can still work properly. RESULTS AND DISCUSSION DCPTs can be easily integrated by the existing planar micro-fabrication technology (Figure 1c) benefiting from its simple two-layer plane structure. The device fabrication consists of two steps. Firstly, ZnO nanoplatelets synthesized by hydrothermal method36 were uniformly distributed on the substrate in large-scale (at least 4-inch silicon wafer) by the self-assembly method.27 Then, semi-dry PDMS was used to transfer the array of ZnO nanoplatelets to a substrate deposited with interdigital Pd electrodes. After the PDMS is completely dried, the device’s fabrication
is
finished.
This
preparation
not
only
avoids
the
complex
three-dimensional fabrication process, but also can be applied to flexible and transparent electronic/optoelectronic devices, because the nanoplatelets can be transferred in large-scale to any flexible templates. SEM images of the random distribution of ZnO nanoplatelets are presented in Figure 1d. We can find the ZnO nanoplatelets, with the smaller surface (polar (0001) face) up, are randomly distributed on flat surface with a coverage of 43.8% (Figure S1). ZnO nanoplatelets are found to be single crystal with wurtzite structure and have a hexagonal morphology with an average diameter of 4.4 µm and an aspect ratio of 0.4 (Figure 1d and S2-3). Additionally, by piezo-response force microscopy (PFM), the effective piezoelectric coefficient d33 (Figure S4) is measured to be 18.1~20.8 pm/V, which is a relative high value among semiconductors in previous works.37-41 4 ACS Paragon Plus Environment
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Owing to this piezoelectric advantage and the special geometry, nanoplatelet is a suitable piezoelectric component for DCPT. To investigate the piezotronic effect, the energy-band diagram of the PT with and without applied strain is illustrated in Figure 2a. A mirror symmetric energy-band is presented due to the symmetric structure. During compression, positive piezoelectric polarization charges are created at both interfaces of two Schottky contacts within a thickness of 1-2 atomic layers,42 resulting in symmetric reductions of the SBHs (∆Epiezo) and thus symmetrically increasing the current flow through DCPT at both sides (Figure 2b). Therefore, the effective ∆SBH can be derived from the I-V curves, and the applied force can be quantified. To better understand the modulation mechanism, we applied the Schottky theory43 to find a suitable model for DCPT and then derived the piezo-induced ∆SBH (=∆Epiezo) from the I-V curve. Based on the synthesis and measurement conditions, the thermionic-emission-diffusion (TED) and the thermionic-field-emission (TFE) are two suitable models for DCPT.43 ln(I) is approximately proportional to V1/4 in TED and V in TFE. By plotting both curves, we found ln(I)-V1/4 (Figure 2c) is almost linear, indicating the dominant process is the TED model. According to the classic Schottky theory, this linear relationship results from the mirror force, which leads to voltage dependent SBH and makes the barriers not that ‘sharp’.43 By using the classic Schottky formula ∆SBH=-kTln(Istrain/Ifree), the average ∆SBH is found to have a linear relationship with applied pressures in Figure 2d. This is consistent with the piezotronic theory7-10 and shows the effective modulation of piezotronic effect on DCPT. However, the current is also influenced by the change of Schottky contacts. As shown in Figure 2c-d, compared with the uniform change under pressure from 0.25 to 1.00 MPa, the ln(I) and ∆SBH changed dramatically (about 2.26 ln(A) and -58.8 meV) just when an initial pressure (0.25 MPa) was applied. This non-uniform jump may result from a contact potential, indicating non-ideal contacts between bottom electrodes and ZnO nanoplatelets. To solve this problem, further studies are required to optimize the device fabrication. 5 ACS Paragon Plus Environment
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The sensitivity of the device to external stimuli was shown in Figure 2e-f. To avoid the impact of contact potential, we recorded the current while increasing the pressure by a step of 63.5 kPa from 0.75 to 1.00 MPa (Figure 2e). As shown in Figure 2f, a linear relationship is found between the corresponding ln(I) derived from the current in Figure 2e and the applied pressures, which is consistent with the result in Figure 2d. Only considering the piezopotential-induced ∆SBH (Figure 2d-f), we calculated the sensor pressure sensitivity (S=∆SBH/∆Pressure) to be 84.2~104.4 meV/MPa, which is 1.1-1.7 times of that the highest value of the single channel modulated piezotronic transistor in previous report.27 In order to clarify this improvement, we calculate Vrev/(Vrev+Vfor), the reverse biased Schottky voltage drop in the proportion of the total bias, to roughly estimate the voltage-drop distribution along the metal-semiconductor-metal diode in Figure 3. For the sake of simplicity, we assume that the two Schottky barrier heights here are the same at no bias and the voltage drop of ZnO resistance is negligible. Based on the I-V curves of reverse and forward biased Schottky barrier (Figure 3a-b), the Vrev/(Vrev+Vfor) (Figure 3c) as a function of the total bias is derived from experiment data (Figure S5), finite element analysis (Table S1) and the scanning surface potential microscopy data in ref. 44, and found to be voltage-dependent and varied from ~50% to ~75%. This indicates the voltage drop of forwarded Schottky barrier divides at least 25% of the total bias (0~1.2 V) and cannot be ignored in the back-to-back Schottky diode, which is rarely considered in the previous reports.13,20,21,25,26,35 So, if two Schottky contacts can both participate in the regulation and function in an identical direction, here named double channel modulation, the piezotronic device will exhibit high pressure sensitivity as previously shown in Figure 1b. The improvement of DCPT can be attributed to this double channel modulation. This double channel plane structure also endows the DCPT with high structural reliability. Firstly, the simple plane structure of DCPT does not require cumbersome fabrication processes. More importantly, the double channel design allows the device continue to work even if one channel fails (for example, one Schottky contact broken into an ohmic contact). This will undoubtedly help to improve the reliability of PT. To 6 ACS Paragon Plus Environment
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exhibit this, we applied a 3D finite element (FE) model (Young’s modulus E~30 GPa)45 to simulate the piezopotential distribution of DCPT, and its influence on the energy-band and the carrier transport compared with the conventional PT, as schematically illustrated in Figure 4. In the DCPT, the strain-induced positive polarization charges at two contacts play a same effect, both reducing the SBHs (Figure 4a left); while, in the conventional PT, the positive and negative piezopotential respectively introduced at the bottom and the top contacts have opposite effects on the SBHs (Figure 4a right). As a result, if one channel failure, the DCPT will remain working properly (current increased with pressures) owing to the other channel’s modulation (Figure 4b-d), rather than the device-malfunction in the conventional PT (Figure 4e-g). In order to compare the reliability of the two structures, we assume the probability of one channel failure is P (0
