Multi-channel ferroelectric-gate field effect transistors with excellent performance based on ZnO nanofibers

Abstract

Bottom-gate multi-channel ferroelectric-gate field effect transistors (FETs) based on ZnO nanofibers were fabricated, using Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) thin film as the gate insulator and ZnO nanofibers as the semiconductor multi-channel. The electrical characteristics of the multi-channel ZnO nanofiber ferroelectric-gate FETs showed typical p-channel transistor properties with a low threshold voltage of −0.5 V, a high effective field effect mobility, and a large on/off current ratio of 10⁴. These excellent performances are attributed to the multi-channel ZnO nanofiber and BNT ferroelectric-gate insulator.

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1. Introduction

In recent years, ferroelectric-gate field effect transistors (FETs), using ferroelectric materials as a dielectric layer for a metal oxide semiconductor field effect transistor (MOSFET), have attracted a great deal of attention because they have several advantages such as nonvolatility, fast and nondestructive readable operation, capability of high-density integration, and low power consumption.^1–4 As the promising nonvolatile memory components, ferroelectric-gate FETs are widely used in portable electronic devices such as mobile phones, digital cameras, MP3 players, and smart debit cards.¹ Most of previous reports about ferroelectric-gate FETs used thin-film as channel, and these ferroelectric-gate FETs exhibited good electrical properties with large “on” current, high on/off current ratio, low “off” current and large memory window.^5–7 However, these ferroelectric-gate FETs with the thin-film channel also have the drawbacks of low field effect mobility.^5–7 To reduce the field effect mobility, one way is to tune and control the channel conductance. One-dimensional (1D) semiconductor nanostructures as the channel⁸ of ferroelectric-gate FETs can tune and control the channel conductance due to their large surface to volume ratio and high mobility electronic transport properties. In particular, FETs based on nanowire (NW) channel allowed low operation voltage below 1 V as Yao et al.⁹ reported.

Among 1D semiconductor materials, ZnO with large bandgap (3.6 eV) and large exciton binding energy (60 meV) at room temperature, is a well-known semiconducting, piezoelectric, and photoconducting material, which are widely used in FETs.^8–15 Most of previous reports about 1D ZnO nanostructures FETs such as ZnO NW FETs have focused on single NW,^8–10 however, there is lack of a simple and sufficient process to precisely assemble single NWs.¹⁶ Therefore, large-scale fabrication of single NW based devices is still a great challenge. Moreover, in single NW FETs, the small NW cross section limits the “on” state current.¹⁷ ZnO NWs, nanofibers and NW thin film have shown a promising future in large-scale fabrication of devices with a relatively simple and low cost process.¹⁶ What's more, FETs based on multi-nanowire or multi-nanofiber or NW thin film can raise the “on” state current by adjusting the number of NW channels.¹⁷ While in NW thin film FET, the gate was not able to surround the nanowires or even contact directly to the nanowires, resulting in limited gate control.¹⁷ The gate-surrounding structure that can be formed in multi-nanowire FETs allows excellent electrostatic gate control over the NW channel. Thus, multi-channel nanofibers FET can utilize the gate-surrounding structure for optimal gate control, and it can have large “on” state currents.

Therefore, combining ZnO nanofibers as the semiconducting multi-channel with ferroelectric film as the gate dielectric, the FETs will have large “on” state current, high on/off current ratio and high field effect mobility. On the other hand, BNT is a Pb-free ferroelectric thin film with large polarization value. Moreover, the relative dielectric constant of the BNT is very large. Thus, the FET using BNT as the gate dielectric would have smaller threshold voltage, large “on” state currents, and so on. In this work, bottom-gate multi-channel ferroelectric-gate FETs using ZnO nanofibers as the multi-channel and ferroelectric Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) thin film as the gate insulator were fabricated and the electrical characteristics of these FETs have been studied.

2. Experimental

Fig. 1 shows the schematic illustration of the multi-channel ZnO nanofiber/BNT FET. The BNT thin film was prepared via chemical solution deposition on the highly doped n-type silicon (N⁺⁺-Si) which was used as the bottom gate electrode. The details for preparing the BNT thin film were described in our previous work.¹⁸ And then, ZnO nanofibers were fabricated onto the BNT thin film as the semiconducting multi-channel, which were prepared by electrospinning as follows. 0.2667 g zinc acetate dehydrate (Zn(CH₃COO)₂·2H₂O) was dissolved in 2 ml ethanol and stirred for 4 h. 0.36 g poly (vinyl pyrrolidone) (PVP, M_w = 1300 000) was stirred at 1 ml deionized water (H₂O) and 1 ml ethanol for 4 h, then mixed the zinc acetate solution with the PVP solution by stirring for 6 h at room temperature. After 24 h the solution was electrospun and the zinc acetate/PVP composite nanofibers were collected onto the BNT/N⁺⁺-Si substrate, with the electric field for electrospinning set around 19 kV and the distance between the nozzle and the substrate was 15 cm. For aligning the ZnO nano wires, an applied field paralleling to substrate was added during the electrospun. When the mixed solution is electrospun, the nanowires were charged to some extent by the high electrospin electric field. Thus, the charged nanowires can be affected by the applied field paralleling to substrate when it approach to the substrate. The feeding rate of the solution was adjusted to 8 μl min⁻¹ and the collecting time was half an hour. The composite fibers were dried at 80 °C for 12 h, and then thermal annealing at 500 °C for 4 h at the rate of 1 °C min⁻¹. After then, source–drain electrodes were deposited on the ZnO fibers by ion sputtering method using the mask plates. The areas of source and drain were both 3.24 mm². The channel length (L) and width (W) of FETs were 100 and 1050 μm, respectively.

Fig. 1 Schematic illustration of the ZnO nanofiber/BNT FET.

The top-view of the multi-channel ZnO nanofiber/BNT FET, BNT thin film and single ZnO nanofiber located in the channel area of the FET were characterized by scanning electron microscopy (SEM, Hitachi S-4800). The thickness of the BNT gate insulator was measured by the thin-film analyzer (Filmetrics F50). The relative dielectric constant of the BNT thin film and the electrical properties of the multi-channel ZnO nanofiber/BNT FET were measured by the semiconductor parameter analyzer (Agilent B1500A) with a probe system in the dark at room temperature.

3. Results and discussion

The top-view SEM image of the multi-channel ZnO nanofiber/BNT FET is shown in Fig. 2(a). As observed, the distance between the source–drain electrodes is less than 100 μm, which is attributed to the diffraction of the plasma during depositing the electrode.¹⁹ The multi-channel ZnO nanofibers between source–drain electrodes are observed clearly. Fig. 2(b) presents the BNT thin film of the gate insulator below the ZnO nanofibers, and Fig. 2(c) exhibited a single ZnO nanofiber, as observed, the diameter of it is about 125 nm and the surface is smooth.

Fig. 2 The SEM image of (a) the ZnO nanofiber/BNT FET, (b) BNT located in the channel area of the FET, and (c) a single ZnO nanofiber across the source and drain electrodes of the FET.

The thickness image of the BNT thin film is shown in Fig. 3(a). The thickness of a 1 cm × 1 cm BNT thin film is measured by evenly distributed 41 points, which is shown in Fig. 3(a). As observed, the thickness of this BNT thin film is quite uniform and the mean thickness of the thin film is 263 nm. The inset in Fig. 3(a) shows the mean thickness of six batches of BNT thin films, each of them are measured by evenly distributed 41 points, and then their average thickness are obtained. Fig. 3(b) represents the relative dielectric constant of the BNT gate insulator with the frequency varying from 1 kHz to 1 MHz. As observed from Fig. 3(b), the relative dielectric constant of the BNT gate insulator changes relatively small with frequency rages, therefore, the device can work at a wide frequency. Moreover, the value of the relative dielectric constant of the BNT is 352 at 1 MHz, which is much larger than SiO₂ (∼3.9) and HfO₂ (∼20) reported by Miyasako et al.²⁰ and Al₂O₃ (∼9.0) reported by Kim et al.¹²

Fig. 3 (a) The thickness of the BNT gate insulator of the multi-channel ZnO nanofiber/BNT FET. Inset: the mean thickness of six batches of BNT thin films. (b) The relative dielectric constant BNT gate insulator of the multi-channel ZnO nanofiber/BNT FET.

The electrical characteristics of the multi-channel ZnO nanofiber/BNT FET were measured in air ambient. Fig. 4(a) shows the output characteristics (source–drain current versus source–drain bias, I_DS–V_DS), and the V_DS of the FET is swept from 0 to 10 V with the gate voltage (V_GS) varying from 0 to −12 V in −3 V steps. The device exhibits a p-type characteristic for the I_DS increases with increasing V_DS and becomes larger when larger negative V_GS is applied.^9,21 Generally, intrinsic p-type of ZnO is attributed to the zinc vacancy, the unintentional carbon doping and/or surface acceptor levels created by the adsorbed oxygen.^22–25 According to the preparation process of ZnO nanofibers and the results of SEM (Fig. 2(c)) and the high effective field-effect mobility calculated later, we deduce that the adsorbed oxygen is the main reason for the p-type nature of ZnO. The ZnO nanofibers is composed of many small grains with the size of 10–20 nm and left lots of grain boundaries as shown in Fig. 2(c). Meanwhile, the ZnO nanofibers have large specific surface area to contact air. The grain boundaries would adsorb oxygen, and the adsorbed oxygen can capture excess electrons.²² Due to the existence of negative charged states in grain boundaries, the space charge region of the mobile electrons were depleted, the negative grain boundaries charges have to be compensated by positive space charges on both sides of the grain boundaries, this phenomenon could lead to holes accumulating and an inversion of the p-type conductivity beside the grain boundaries. What is more, the FET is operated in enhanced mode with a clear saturation region with saturated drain currents at low drain voltage, when V_GS = −12 V and V_DS = 4 V, the “on” current of the FET reaches to 7.5 × 10⁻⁵ A, which is larger than that of the single-channel ZnO nanofiber FETs on SiO₂ gate insulator reported by Park et al.¹⁰ and Wu et al.²⁶ This is because ZnO nanofibers as the multi-channel can add the total nanofibers cross section between the channel and the ferroelectric-gate insulator can induce much larger charge than that of the normal gate insulator. The transfer characteristics (source–drain current versus gate bias, I_DS–V_GS and square root of I_DS versus V_GS, I_DS^1/2–V_GS) of the multi-channel ZnO NF/BNT FET are shown in Fig. 4(b). The V_GS is swept from −20 V to 2 V in 0.22 V steps at V_DS = 0.1 V. The threshold voltage is determined to be −0.5 V by a linear fit of the I_DS^1/2–V_GS curve of the transistor operating in the saturation region, which is smaller than that of thin-film ferroelectric-gate FETs^5–7 due to high relative dielectric constant of the BNT thin film.

Fig. 4 (a) The output characteristics (source–drain current versus source–drain bias, I_DS–V_DS) of the multi-channel ZnO nanofiber/BNT FET. (b) The transfer characteristics (source–drain current versus gate bias, I_DS–V_GS and square root of I_DS versus V_GS, I_DS^1/2–V_GS) of the multi-channel ZnO nanofiber/BNT FET.

Fig. 5 shows the I_DS–V_GS transfer curves in a semilog graph of the multi-channel ZnO nanofiber/BNT FET at V_DS = 0.1 V with the gate voltage sweeps from 4 to −20 V in a double sweep mode. A counterclockwise hysteresis loop as indicated by arrows is obtained owing to the ferroelectric polarization reversal of the BNT thin films from the I_DS–V_GS characteristics curve, which shows the fabricated device has nonvolatile memory function. The obtained memory window (MW) and on/off current ratio are about 2.4 V and 10⁴, respectively. Moreover, the subthreshold swing (SS) is as small as 0.5 V per decade, which is calculated by

SS = dV_GS/d(log I_DS). (1)

Fig. 5 The transfer characteristics (I_DS–V_GS) shown in a semilog graph of the multi-channel ZnO nanofiber/BNT FET at V_DS = 0.1 V with the gate voltage sweeps from 4 to −20 V in a double sweep mode.

The mobility cannot be directly extracted because it is impossible to directly measure the effective channel width/length (W/L) due to the actual coverage with ZnO NWs is difficult to calculate accurately.²⁷ Assuming 100% coverage, the field-effect mobility of the ZnO NWs FET has been recalculated in the revised manuscript using the following relation,

μ = (dI_DS^1/2/dV_GS)² × 2L/(C × W)

where L means the channel length between source and drain (100 μm), W is the channel width (1050 μm), and C is the equivalent capacitance per unit area of the gate insulator obtained from a parallel plate model,

C = εε₀/h

Where ε is the relative dielectric constant of the BNT gate insulator (352), ε₀ is the dielectric constant of vacuum, and h is the thickness of the BNT gate insulator (263 nm). The field-effect mobility is calculated about 7.45 cm² V⁻¹ s⁻¹, which is much larger than that of the ZnO thin film ferroelectric gate FETs in our previous work,⁵ due to the large surface to volume ratio and high mobility electronic transport properties of ZnO nanofibers. In fact, we can deduce that the effective ZnO NWs FET field-effect mobility is much higher than 7.45 cm² V⁻¹ s⁻¹ when considering the actual coverage of ZnO NWs. From the above results, the multi-channel ZnO nanofiber/BNT FETs exhibit excellent performances, which will be helpful for developing novel nonvolatile memories.

4. Conclusion

In summary, we have fabricated and characterized bottom-gate multi-channel ZnO nanofiber/BNT FETs using ZnO nanofibers as multi-channel and BNT ferroelectric thin film as gate insulator. Typical p-channel enhanced mode transistor operation and drain-current saturation have been observed. The threshold voltage and channel mobility of ZnO nanofiber/BNT FETs are calculated to be −0.5 V and 7.45 cm² V⁻¹ s⁻¹. In particular, the “on” state current is larger than 7.5 × 10⁻⁵ A, the on/off current ratio is approximately 10⁴. These excellent performances are attributed to ZnO nanofibers as the multi-channel to add the total cross section and BNT ferroelectric-gate insulators to induce large charge density due to the spontaneous polarization.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 11272274, 11372266 and 11032010), the Hunan Provincial Natural Science Foundation of China (no. 12JJ1007), the Specialized Research Fund for the Doctoral Program of Higher Education (no. 20114301110004), the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (201143).

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