Amplification of pyroelectric device with WSe₂ field effect transistor and ferroelectric gating

Since the discovery of infrared (IR) about two centuries ago,¹ IR detectors have found applications in medical imaging, military equipment, environmental sensing, among others.² Pyroelectric devices are employed in IR detectors and are finding increased applications in IR sensing.^2,3 Pyroelectricity is the property of certain crystals to spontaneously polarize in response to a change in temperature.⁴ Thin-film pyroelectric IR detectors have been produced from lead zirconium titanate (PZT) and the pyroelectric current measured as a result of changes in temperature. The current so measured is dependent on the device area and is typically in the order of tens of pico-amps.⁵ Graphene and two-dimensional (2D) transition metal dichalcogenides (TMDs) have been integrated with a pyroelectric device to improve the output signal of the IR detector² and performance of the 2D field effect transistor (FET),⁶ respectively.

TMDs, such as tungsten diselenide (WSe₂) and molybdenum disulfide (MoS₂), have found increased applications in electronic devices by virtue of their promising electronic band structure.⁷ TMDs consist of one atom of a transition metal and two atoms of a chalcogen element covalently bonded together (X–M–X). The atoms form a hexagonal arrangement, and adjacent planes are held together by a weak van der Waals interaction.⁸ As a result of this weak bonding, layers of TMDs can be exfoliated and deposited onto a substrate for further processing. PZT is increasingly employed for ferroelectric field effect transistor devices owing to its high capacitance, low coercive field, chemical stability, and compatibility with fabrication processes.

In this work, a standalone pyroelectric device (SPD) made from PZT with a top and bottom electrode has been fabricated and the pyroelectric current measured with respect to temperature. Also, 45 nm thick WSe₂ has been exfoliated and deposited onto the thin-film PZT. WSe₂ FET has been integrated with PZT [integrated pyroelectric device (IPD)] with a view to amplify the device output. The characteristics of the SPD and IPD as a function of temperature have been analyzed. In addition, the PZT based WSe₂ FET has been gated, and the transfer characteristics of the device have been studied.

The devices have been fabricated using microfabrication processes. The PZT has been sputtered from an individual target of the constituent elements at about 600 °C forming thin-film PZT. Both the platinum and PZT have been deposited via sputtering onto the SiO₂ substrate. Using scotch-tape, the WSe₂ flake has been exfoliated and deposited onto the PZT substrate. By mask-less lithography, the photoresist has been patterned and the metal electrodes (titanium/aluminum) deposited as the source and drain. Figure 1(a) shows the 45 nm thick WSe₂ flake on PZT with the metal electrodes. Atomic force microscopy has been used to confirm the thickness of the WSe₂ flake. Figure 1(b) shows the device schematic entailing the material stack and electrical connections. The platinum serves as the bottom electrode for gating the device.

The devices have been tested using a Keithley parameter analyzer fitted with a temperature chuck. The temperature chuck is connected to the ATT-systems module responsible for controlling and measuring the temperature. Temperature values of between 21 and 32 °C have been applied to the measurements. Additionally, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) have been performed on the PZT to study its crystal quality. The SEM and EBSD experiments have been carried out using a Carl Zeiss SIGMA HD VP field emission SEM with an Oxford Instruments Aztec Synergy EBSD system. For SEM, an accelerating voltage of 3 kV and a working distance of approximately 5.5 mm have been applied. The EBSD condition entails a 70° tilt of the sample, a 10 kV accelerating voltage, and a working distance of ∼12.5 mm.

The images in Fig. 2 are standard secondary electron (SE) images of the surface of the PZT. The SEM image in Fig. 2(a) has been taken directly from the top of the sample, and the grain size is about 100 nm. However, the image in Fig. 2(b) has been taken from a 70° tilt, and it can be seen that many of the crystals grow to a pyramid shape. Some of the crystals are being pointed at the top and some are narrowing, but not quite forming a point.

Figure 3(a) shows the diffraction pattern formed by the electron beam interacting with a single point on the 70° tilted PZT sample grown on the platinum (Pt) bottom electrode. With the aid of the Aztec system database, a solution for the diffraction pattern has been indexed (Tetragonal, Space Group 99).⁹ The successful detection of the electron diffraction pattern and the consequent indexing [Fig. 3(a)] indicate that the PZT exists in tetragonal crystals. In addition, an area of single crystal grains from the PZT has been mapped and plotted in the pole figure [Fig. 3(b)] with the assistance of the MTEX program. It can be seen that for the PZT, the data points of the (111) crystal facet are gathered in the center and side of the pole figures, and the data points of the (100) crystal facet are gathered on the side, indicating a tetragonal shape. The pole figures indicate that PZT grown on Pt seed layers shows a good correlation for an orientation along the [111] direction normal to the sample surface in agreement with Ref. 10.

To characterize the quality of the WSe₂, Raman spectroscopy has been performed on the 45 nm thick WSe₂ shown in Fig. 4(a). An in-plane E¹_2g mode from the out-of-phase vibration and an A_1g mode from the out-of-plane vibrations of WSe₂ both appear as a single peak (E¹_2g + A_1g) at 252 cm⁻¹. The Raman shift of about 252 cm⁻¹ shows that WSe₂ is of good quality. The sample has been excited with a 450 nm laser under room temperature and atmospheric pressure. The B¹_2g indicates that the sample is of several layers thick. The B¹_2g peak intensity diminishes as the number of layers increases in response to vibrational dampening coming from a lower interlayer interaction.¹¹ 

Figure 4(b) shows the output characteristics of the IPD gated from 0 to −20 V with a step gate voltage of 5 V and a sweeping drain-source voltage (V_ds) from 0 to 5 V. The plots begin linearly, and as V_ds increases, the drain-source current (I_ds) begins to pinch-off as it nears saturation, indicating generic MOSFET behavior.

The schematic in the inset of Fig. 5(a) shows the PZT sandwiched with a top and bottom metal electrode. The device dimensions allow for the detection of infrared radiation.¹² Due to the thickness of the PZT, infrared wavelength can be converted into heat resulting in the release of charge from the pyroelectric effect. Pyroelectric materials do not rely on the Seebeck effect¹³ and can be deployed in a spatially uniform temperature environment. The device has been tested against changes in temperature both during heating and cooling. From Fig. 5(a), the pyroelectric current measured during heating (21–32 °C) of the device stands around 124–145 pA, while about 46–55 pA was measured during cooling (32–21 °C). The higher pyroelectric current magnitude observed during heating is attributed to a higher degree of polarization in the PZT during heating. The degree of polarization in this instance is a factor of the rate of change of temperature of the heating element, which heats up faster than it cools. Additionally, the piezoelectric effect could arise from material expansion, and this is believed to be responsible for the unstable pyroelectric current measured during heating. The change in polarity of the measured pyroelectric current is a result of the reversal in polarization of the pyroelectric device and consequently, its dipole moment.¹⁴ The area of the SPD top electrode is 0.64 mm², and the pyroelectric coefficient of the PZT has been calculated to be 1.755 × 10⁻⁴ C/m²K using Eq. (1),

where p is the pyroelectric coefficient, i is the pyroelectric current, A is the area, and $dT/dt$ is the rate of change in temperature.

A similar PZT based pyroelectric device has been demonstrated by Ref. 5 and has been tested across a temperature range of 30–60 °C, producing a current output of about 100–120 pA. Yang et al. reported on a PZT nanogenerator that is 175 mm thick and 5 mm in electrode length, which produces a pyroelectric current of about 430 nA.¹⁵ 

The effect of temperature change on the output current of both the IPD and SPD is shown in Fig. 5(b). The SPD plots in red and blue depict the pyroelectric current density of 1–1.8 nA/mm² and 0.8–1 nA/mm² for heating and cooling, respectively. On the other hand, the IPD represented by the black and orange plots has been biased with a V_ds of 1 V. The current density measured during heating is about 20–35 nA/mm², while 16–19 nA/mm² has been measured during cooling. The combined area of the WSe₂ and the electrodes is about 0.0526 mm². No gate voltage has been applied, and the IPD shows an amplified current output of over ten orders of magnitude higher than what is obtainable from the SPD. It is believed that a combined bolometric and pyroelectric effect is responsible for the measured output of the IPD. It is possible that absorbed heat could affect the temperature coefficient of resistance (TCR) of the WSe₂, ultimately affecting the current output of the device. Additionally, the temperature change can result in the release of charges due to the pyroelectric effect, which in turn modulates the WSe₂ resistance.² 

To investigate the ferroelectric behavior of the IPD, the device has been back-gated from −16 to 16 V under a varied drain-source voltage (V_ds). In Fig. 6, each loop represents the ferroelectric response of the WSe₂ FET to the polarization of the PZT. The point where the drain-source current (I_ds) is maximum represents the maximum polarization achieved by the PZT. It can be seen that the maximum I_ds is 0.36 μA when the maximum gate voltage (V_gs) is −16 V. A similar current output of 0.5 μA has been realized from a ferroelectric FET based on WSe₂ on CuInP₂S₆.¹⁶ The hysteresis direction of the IPD is shown by the arrows in the graph, which is in the same direction as the polarization vs voltage (P–V) curve.¹⁷ Hence, the IPD shows a good ferroelectric response as it has the expected anti-clockwise hysteresis behavior in sync with the polarization effects of the ferroelectric gate dielectric. Similar anti-clockwise hysteresis graphs have been obtained for MoS₂ FET.¹⁸ The actual origin of hysteresis is still under debate; however, the postulated reasons for current–voltage hysteresis include the ferroelectric effect, ionic motions within the ferroelectric material, and charge trapping and detrapping.^19,20 The ferroelectric effect can be described as spontaneous electric polarization induced in dielectrics by an applied electric field.²¹ Research has shown that ferroelectric crystals can change symmetry, which, in turn, can affect charge carrier extraction.²² Ionic migration occurs in ferroelectrics under a voltage bias and leads to a buildup of internal potential. The speed of the ion migration can have an effect on the current–voltage hysteresis. Furthermore, charge traps formed as a result of defects within the ferroelectric material can further explain the origin of hysteresis.^19,20 These trapped electrons and holes significantly affect the charge transport properties of the material.²³ Additionally, V_M in Fig. 6 represents the memory window and is defined as the difference in V_gs that occurs at the current value corresponding to the midpoint of the maximum and minimum possible current values of the device. As can be seen in the figure, the V_M of each loop is similar in range despite changes in the V_ds from 1.2 to 2 V, indicating the stability of the memory window. The memory window for this device is about 7 ± 1 V for a V_gs range of 32 V (−16 to 16 V). The memory window in this experiment is comparable to the memory window of MoS₂-PZT FET, indicating possibility for memory application.²⁴ 

The data from Figs. 5(b) and 6 show the effects of temperature and electric field on the IPD, respectively. From changes in the IPD temperature (21–32 °C) in Fig 5(b), a corresponding increase in the output current density of about 15 nA/mm² has been observed. In comparison with the effect of gate voltage (0 to −15 V) in Fig. 6, a corresponding current output of about 370 nA (7 μA/mm²) has been observed. It could be concluded that higher polarization in PZT is realized from the gate voltage via the ferroelectric effect than from the temperature change.

The design, fabrication, and operation of a WSe₂-PZT FET (IPD) and a PZT sandwiched between metal electrodes (SPD) has been presented. The current density output of the IPD and SPD is ∼16 nA/mm² and ∼1 nA/mm², respectively. Thus, the integrated device amplifies the current output of the standalone device by over ten orders of magnitude. In addition, it has been observed that the gate voltage induces more polarization in the PZT than the temperature change alone. Anti-clockwise hysteresis behavior measured in the device shows direct control of PZT polarization on the WSe₂ FET. The performance of the PZT based WSe₂ FET would help guide future experiments to develop high performing electronic devices.

The work of S. C. Mbisike has been supported by the Petroleum Technology Development Fund (PTDF) and also by Pyreos Limited. S. C. Mbisike wishes to acknowledge Imo State University.

The authors would like to acknowledge the UK Engineering and Physical Sciences Research Council (EPSRC).

For the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) license to any Author Accepted Manuscript version arising from this submission.

The authors have no conflicts to disclose.

The data that support the findings of this study are available within the article.