Side‐Gated In2O3 Nanowire Ferroelectric FETs for High‐Performance Nonvolatile Memory Applications

A new type of ferroelectric FET based on the single nanowire is demonstrated. The design of the side‐gated architecture not only simplifies the manufacturing process but also avoids any postdeposition damage to the organic ferroelectric film. The devices exhibit excellent performances for nonvolatile memory applications, and the memory hysteresis can be effectively modulated by adjusting the side‐gate geometries.

However, since P(VDF-TrFE) is highly soluble in common organic solvents, it is cumbersome to fabricate electronic devices with P(VDF-TrFE) in the top-gated confi guration with short channel lengths (i.e., sub-µm) via conventional ultraviolet or electron-beam lithography (EBL) techniques. [ 5,17 ] In addition, if the P(VDF-TrFE) fi lm is irradiated by the electron-beam, the ferroelectric properties of P(VDF-TrFE) would be degenerated into relaxor (see Figure S1 in the Supporting Information). [ 18 ] As a result, utilizing these organic ferroelectric polymers for advanced electronics gets essentially restricted considering that the corresponding top-gated structure needs complicated manufacturing processes while the simple back-gated device structure is not suitable for high-performance integrated circuits.
At the same time, 1D semiconductor nanowires (NWs) have attracted considerable amount of research attention with the detailed exploration as device channels for many different applications owing to their excellent physical properties. [19][20][21][22] Semiconductor NW material systems with excellent crystallinity are demonstrated with the extraordinary fi eld-effect carrier mobility and superior control in the modulation of their electrical properties. Combining with their nanoscale dimensions, NW devices are highly desirable for the high-density integration of computation chips, logic, and memory cells with the further reduction in power consumption; actually, the fi n fi eld-effect transistor which has been widely used in today's advanced integrated circuits is fabricated based on Si nanowires. [ 23 ] Meanwhile, metaloxide NWs such as In 2 O 3 is a noteworthy NW material system here because of its high mobility (≈1490 cm 2 V −1 s −1 ), inexpensive synthesis, air stability, wide band gap (≈3.6 eV), and good metal-semiconductor contact. [24][25][26] All these unique characteristics have made In 2 O 3 NWs being appropriate for the effi cient deployment in electronics, optoelectronics, sensors, and especially as the transistor channel for high-performance nonvolatile memory operations.
In this work, we have investigated FeFETs using the In 2 O 3 NW and organic P(VDF-TrFE) copolymer with a side-gated architecture fabricated through a relatively simple manufacturing process. The source, drain, and gate patterns were defi ned by only one electron-beam lithography (EBL) step whereas the P(VDF-TrFE) dielectric fi lm was spin-coated onto the sample substrate afterward. This way, there was not any lift-off or etching process required after the deposition of the organic ferroelectric fi lm, not only simplifying the manufacturing process but also avoiding any postdeposition damage to the organic fi lm. In addition, the P(VDF-TrFE) polymer fi lm can act as a protective layer since not any other unit in the fabrication exceeds its capsulation. It is also noted that our In recent years, ferroelectric fi eld-effect transistors (FeFETs) have been widely studied as an important type of memory due to their remarkable characteristics of nonvolatility, low power consumption, fast operation speed, and nondestructive memory operation. [1][2][3][4][5] Until now, among all promising ferroelectric materials explored in FeFETs, organic ferroelectric polymers contain the exceptional advantages of light weight, mechanical fl exibility, and low-temperature solution-base processing, which are ideal for the large area technological utilizations. [6][7][8][9][10] Poly(vinylidene fl uoride-trifl uoroethylene) (P(VDF-TrFE)) copoly mer is one of the typical organic ferroelectric polymers that is frequently employed in various nanoelectronic devices because of its room temperature ferroelectricity, large remnant polarization, and easy forming process with the required annealing temperature below 140 °C. [11][12][13][14] Importantly, the stable remnant polarization of the P(VDF-TrFE) dielectric layer can enable an ultrahigh local electrostatic fi eld in the semiconductor channel, which is much larger than that produced by the gate bias in traditional fi eld-effect transistors; [ 15,16 ] in this case, the transport properties of the channel can be effectively modulated under a relatively low gate bias or even stayed in depletion or accumulation state after the gate bias is removed.
side-gated devices exhibit excellent performances at room temperature and in ambient air. The leakage current, on/off current ratio, and subthreshold slope ( SS ) all present superior values as compared with those of NW-based FeFETs reported previously. [ 5,12,13,17,27 ] Also, the memory hysteresis characteristics can be effectively controlled by adjusting the side-gated geometrical parameters. Moreover, in order to illustrate the versatility of our newly developed NW devices, a memory inverter circuit is as well constructed, which further indicates the technological potency of our ferroelectric side-gated structures for integrated memory circuit applications.
Specifi cally, crystalline In 2 O 3 NWs with the diameters of 40-60 nm and the lengths of tens of micrometers were grown by a chemical vapor deposition (CVD) method, and transferred onto Si substrates with the 270 nm thick thermally overgrown SiO 2 layer. Figure 1 a then displays the device structure of a side-gated FeFET as a nonvolatile memory cell. Scanning electron microscopic (SEM) image of the device is shown in Figure 1 b. The source, drain, and gate patterns were defi ned by using one EBL step, and Cr/Au (10/50 nm) metal stacks were deposited as electrodes followed by a lift-off process. Finally, the P(VDF-TrFE) (70:30 in mol%) fi lm was spin-coated onto the devices with a subsequent thermal treatment on a hot plate for improving the crystallization of P(VDF-TrFE) ferroelectric domains. The thickness of the P(VDF-TrFE) layer was measured to be 100 nm. During the device operation, once a suitable negative gate pulse is applied onto the gate, the polarizations of the ferroelectric dielectric layer will be aligned from the NW to the tip of the gate electrode. When the gate voltage pulse is removed, the remnant polarization of the ferroelectrics provides a local negative fi eld effect, [ 2,16 ] which depletes the electrons in the In 2 O 3 NW channel and results in a low drain current (i.e., the high resistance state as the offstate for erase), as depicted in Figure 1 c. In contrast, as seen in Figure 1 d, when a positive V gs pulse is applied, the NW channel becomes conductive due to the electron accumulation and allows a high drain current (i.e., the low resistance state as the on-state for program).
We fi rst evaluate the transfer characteristics of fabricated devices before coating the P(VDF-TrFE) fi lm. As shown in Figure 2 a, when the device is designed with the side-gate width ( W ) of 600 nm and NW-to-gate distance ( D ) of 400 nm, the electrostatic fi eld produced by gate bias, even up to the V gs sweep range of ±30 V, is not suffi cient to deplete electrons in the NW channel. The inset of Figure 2 a gives a SEM image illustrating the geometrical parameters utilized and varied in this study. After the FeFET fabrication has been completed with the fi nal step of P(VDF-TrFE) deposition, the transfer curve exhibits an obvious memory window for the V gs sweep range raised to ±20 V, with the drain bias of 1 V and device channel length of 3 µm (Figure 2 b). Notably, the hysteresis loop of our devices is traversed in a counterclockwise direction, which is opposite to the typical clockwise hysteresis of In 2 O 3 NW backgated FETs (see Figure S2 in the Supporting Information) while keeping with the polarization hysteresis of P(VDF-TrFE) (see Figure S3 in the Supporting Information). The form of the counterclockwise hysteresis can be understood as follows. The In 2 O 3 nanowire used in this work is a kind of n-type depletion semiconductor. The polarizations of the ferroelectric dielectric is aligned from the NW to the tip of the gate electrode as the negative gate bias increases upon its coercive voltage, and provides a rather high negative fi eld to deplete carriers in the semiconductor channel. As a result, the drain current changes from on-state to off-state at near −20 V. As V gs sweeping back, the ferroelectric remains polarized and the off-state remains stable. Then the polarizations of the ferroelectric dielectric starts to change sign with the positive gate bias increases and the drain current changes from off-state to on-state. Due to the positive fi eld caused by remained ferroelectric polarizations, the on-state can also remain stable until V gs become negative again. [ 28,29 ] The counterclockwise hysteresis demonstrates that the polarization of P(VDF-TrFE) fi lm has a strong effect on the transfer characteristics of our devices. The polarization characteristics of the ferroelectric fi lm are also found to vary significantly depending on the voltage sweep rate. [ 30 ] Therefore, all the devices are measured under the same gate voltage sweep rate of 4 V s −1 in order to establish a consistent platform for the subsequent comparison study among different devices.
Due to the polarization of the ferroelectric fi lm, conductivity of the In 2 O 3 NW device channel can be effectively modulated, yielding an on/off current ratio exceeding 10 6 , along with an excellent SS (110 mV dec −1 ) when the semiconductor channel switches from the on-state to off-state. In addition, the maximum gate leakage current of about 10 −12 A (see Figure S4 in the Supporting Information) is observed repeatedly in our memory devices, being much less than the ones (10 −10 A) of top-gate FeFETs using P(VDF-TrFE) as gate dielectrics, [ 11,16,31 ] in which this smaller leakage can be attributed to the much smaller functional area of the side-gated structure (i.e., only the tip of the gate electrode). Furthermore, Figure 2 c gives the transfer characteristics of the device with different source-drain bias ranging from 10 mV to 1 V. It is clear that the curve does not signifi cantly shift with the increase of source-drain bias for the same V gs sweep range, which suggests a good transistor characteristic for these side-gated devices. The short range V gs sweep after the program and erase pulses also yield two distinct states in our devices, indicating their reliability, even though the corresponding ratio of maximum and minimum drain currents is slightly smaller than that observed in the transfer characteristics of long range V gs scan, as shown in Figure 2 d .
In order to explore the impact of the side-gate geometry on device performances, we fabricated a group of In 2 O 3 NW FeFETs  with different W and D . Fabricated devices with W of 100 nm (narrow gate) and 2 µm (wide gate) are measured, respectively, at different D . Figure 3 a,b presents the transfer characteristics of devices with D of 200 and 800 nm, accordingly. All these devices are measured under the sweep range of ±50 V. It is noted that for the long D , the transistors cannot be cut off under a low negative gate voltage but they do show larger memory windows than the devices with gate tip close to the NW (i.e., short D ). In addition, devices with narrow gate electrode give larger memory windows than the ones of wide gate electrodes for the same D . To further understand and analyze all these observed phenomena with different side-gated geometrical parameters, we perform a simulation to assess the electric fi eld distribution in the channel region by using fi nite element analysis method. Figure 3 c shows the distribution of the modulus of electric fi eld strength in the horizontal plane when W is designed as 100 nm and 2 µm, respectively. Figure 3 d demonstrates the electric fi eld strength distribution along a line segment from the center of the nanowire to the center of the side-gate tip at different W and D . Evidently, the electric fi eld strength decreases with the increasing D under the same gate voltage, bringing about the much larger memory window for devices with long D . Also, both narrow and wide gate devices have dissimilar electric fi eld distribution between the side-gate tip and NW, which concretely shows that the narrow gate electrode generates a spatially decayed fi eld while the wide gate electrode induces a relatively uniform fi eld. The nucleation and growth of switched domains follow different processes under the uniform versus decayed fi eld; in this case, polarization switching occurred in low-fi eld region of decayed fi eld requires much more time consumption. [ 30 ] As a result, devices with narrow gate electrode show larger memory windows than the ones with wide gate electrode.
We then further focused on the devices with uniform W but varied D . Figure 3 e depicts the size of memory windows of these devices divided by their corresponding W . Geometrical parameters are measured by SEM after the electrical characterization and the measured values are all rounded to the nearest 10 nm. It is clear that the changes in memory windows are consistent with the observation discussed above. Moreover, in view of the performance variation accompanied with the nonuniformity of In 2 O 3 NW thickness as well as process deviation in the device fabrication, a statistical study of electrical properties of fabricated devices is necessary in order to obtain a better understanding and to confi rm the geometrical effect here. The turn-off voltage ( V off , defi ned as the gate voltage at which I ds reaches its minimum when V gs sweeps from +50 to −50 V in this study) is chosen to reveal the polarization switching induced by the P(VDF-TrFE) fi lm because the turnon voltage ( V on , defi ned as the gate voltage at which I ds starts to increase when V gs sweeps from −50 to +50 V) is always less stable as compared with V off and the switching from the on-state to the off-state is relatively sharp (see Figure S5 in the Supporting Information). [ 15,32,33 ] Particularly, we compile a statistics regarding the value of E off , where E off refers to the turn-off electric fi eld and it can be simply calculated by V off / D . Figure 3 f shows the E off of devices with different geometrical parameters of W and D , in which all these values are measured by SEM after the electrical characterization and rounded to the nearest 100 nm. More than fi ve devices are measured for each data in the graph. For a fi xed W , the E off of a device does not change dramatically with various D , fi tting well into the thickness dependence model of coercive fi eld of the polyvinylidene fl uoride fi lms reported in previous studies, [ 34,35 ] considering that the thickness of P(VDF-TrFE) between the gate tip and the  NW changes from 200 to 800 nm. In addition, the E off turns smaller with the increasing W , which is in good agreement with the above-discussed simulation results. It is as well worth mentioning that the coercive electric fi eld of P(VDF-TrFE) fi lm appears to be around 50 MV m −1 at the sweep loop frequency of less than 0.1 Hz, [ 30 ] being larger than the turn-off electric fi eld of devices with W of 2 µm. We attribute this to our devices that they are operated under the interaction of ferroelectric polarizations and charge traps existed at the interface between the ferroelectric fi lm and the NW; [ 28,33,36 ] therefore, a lower electric fi eld is required. According to the fi ndings of these geometrical effects, we can then optimize the device design and fabrication for different utilizations with varied operating voltages.
After that, we next test the memory behavior of our In 2 O 3 NW FeFETs. Based on the geometrical effect observed above, devices with D of 400 nm and W of 600 nm are emphasized with the program voltage of 30 V and erase voltage of −30 V in order to assess their device stability and endurance. As shown in Figure 4 a, both on-and off-state are found to be retained stably for at least 50 000 s with an on/off current ratio of about 10 6 . Moreover, as given in Figure 4 b, the stabilized program and erase cycle endurance is also observed beyond 400 cycles. The current dynamic characteristic of the memory cell is shown in Figure 4 c. The device is programmed and erased at V gs of 30 and −30 V with a 0.3 s pulse, respectively, and is read at a V gs of 0 V, as shown in Figure 4 c upper panel; the corresponding I ds in the same time range is given in the lower panel.
Although the gate pulses used in the dynamic characteristic test are much shorter than those used in stability and endurance tests, a dynamic retention ratio of about 10 5 is still observed under V ds of 0.1 V. Here we also studied the current dynamics of devices with the same D and different W (see Figure S6 in the Supporting Information), the result is in good agreement with our discussion about the impact of different gate sizes. All these results demonstrate an excellent nonvolatility memory behavior of our side-gated FeFETs, by reference to ferroelectric memory with top-gated or back-gated structure in previous works. [ 5,13,17,28,36 ] Finally, we fabricated a memory inverter circuit based on a single NW in order to investigate the feasibility of applying our side-gated FeFETs to construct integrated memory circuits. Figure 4 d depicts the SEM image of our memory circuit, which is composed of a side-gated NW FeFET and two NW-based resistors. The equivalent circuit diagram is displayed in Figure 4 e inset. It is noted that the channel length of the NW FeFET is designed to be 800 nm to enlarge the on-state conductance, resulting in the transfer curve shown in Figure 4 e. In brief, two types of memory inverters are tested with their respective resistors R 1 and (R 1 +R 2 ). As shown in the voltage hysteresis characteristics (VHCs) in Figure 4 f, our memory circuit can output a voltage representing the opposite logic level to its input with a hysteresis window approximately equal to the one of the NW-FeFET in the circuit. The inverter with resistors (R 1 +R 2 ) has a smaller low-level output voltage owing to its larger resistance. The short-range V gs sweep after the program and erase pulses can also confi rm the memory effect of our inverter circuit (see    Figure S7 in the Supporting Information). Furthermore, the distinguishability of disparate states in the memory circuit can be estimated by the memory output ratio, [ 5 ] which is defi ned as ( V out,Pro − V out,Er )/ V DD × 100%. With resistors (R 1 +R 2 ), a memory output ratio of ≈85% is achieved in the inverter. As a result, all these have further demonstrated that our side-gated FeFETs are highly compatible with integrated memory circuits in practice as they can be tuned individually and operated independently, acting like top-gated devices.
In summary, FeFETs using the single In 2 O 3 NW and organic P(VDF-TrFE) copolymer dielectric with a side-gated architecture have been demonstrated. There is not any other process required after the deposition of the organic ferroelectric fi lm, not only simplifying the manufacturing process but also avoiding any postdeposition damage to the organic fi lm. The fabricated devices exhibit excellent performances such as the large on/off ratio (>10 6 ), small SS (110 mV dec −1 ) and insignifi cant leakage current (<10 −12 A). The impact of this side-gate geometry on the device performance has also been discussed. The memory hysteresis window and the state switching voltage would get larger when the side gate separates farther away from the channel and the gate electrode becomes thinner. During the typical memory operation with the program and erase pulses of ±30 V, the devices with optimized geometries yield a stable retention for over 5 × 10 4 s, long endurance beyond 400 cycles, indicating their nonvolatility and memory performance. At the end, a memory inverter circuit is fabricated, which confi rms that our side-gated FeFETs are highly compatible with integrated memory circuits. All these results have illustrated the great potency of our ferroelectric sidegated In 2 O 3 NW structures for high-performance nonvolatile memory applications.

Experimental Section
Nanowire Synthesis : The single crystalline In 2 O 3 NWs used in this study were synthesized in a horizontal tube furnace by a CVD method. In 2 O 3 powder and graphite powder were mixed with a weight ratio of 10:1 and then put into a quartz boat. Silicon substrates with 1 nm thick of gold catalyst predeposited on the surface were placed upside-down in the downstream position about 10 cm away from the evaporation source. Then, the entire set-up was inserted into a quartz tube reactor. The source and substrate were heated to 1100 and 900 °C, respectively, and kept heating for 1 h under a constant fl ow of mixed gas (argon/ oxygen = 100:1) at a fl ow rate of 200 sccm. When the system was cooled down to room temperature, a large amount of NWs was formed on the surface of the silicon substrates.
Device Fabrication and Characterization : After the growth, the In 2 O 3 NWs were transferred onto a precleaned p-type silicon substrate with a 270 nm thick thermally overgrown SiO 2 layer. Then the substrates were spin-coated with methyl methacrylate (MMA) and poly(methyl methacrylate) (PMMA), and the EBL (JEOL 6510 with nanometer pattern generation system) was employed to defi ne the source, drain, and gate pattern. The Cr/Au (10/50 nm) electrodes were completed by thermal deposition and lift-off processes. Finally, P(VDF-TrFE) (70:30 mol%) ferroelectric polymer powder was dissolved in the diethyl carbonate with 2.5% wt. and the P(VDF-TrFE) fi lm was spin-coated onto the substrates, followed by the thermal annealing on a hot plate (70 °C for 1 h, and 130 °C for 1 h). Electrical performance of fabricated side-gated NW FETs was assessed using a Lakeshore probe station with Agilent analyzer B1500A and B2912A.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.