Tunable optoelectronic properties of a two-dimensional graphene/α-In₂Se₃/graphene-based ferroelectric semiconductor field-effect transistor

Abstract

Two-dimensional (2D) materials are promising for future electronic and optoelectronic devices. In particular, 2D material-based photodetectors have been widely studied because of their excellent photodetection performance. Owing to its excellent electrical and optical characteristics, 2D indium selenide (α-In₂Se₃) is a good candidate for photodetection applications. In addition, α-In₂Se₃ samples, including atom-thick α-In₂Se₃ layers, present ferroelectric properties. Herein, we report the fabrication and electrical and optoelectronic properties of multilayered graphene (Gr)/α-In₂Se₃/Gr-based ferroelectric semiconductor field-effect transistors (FeS-FETs). Furthermore, we discuss the physical mechanisms affecting electronic and optoelectronic transport in the Gr/α-In₂Se₃/Gr heterostructure. Large hysteresis was observed in the transfer characteristic curves and it was attributed to the ferroelectric polarization of MTL α-In₂Se₃ and carrier trapping–detrapping effects. The optoelectronic performance of the fabricated FeS-FETs depended on the ferroelectric properties of α-In₂Se₃ and can be easily tuned to achieve the maximum photoresponsivity and specific detectivity of 10 AW⁻¹ and 4.4 × 10¹² cmHz^1/2 W⁻¹, respectively.

1 Introduction

Indium selenide (α-In₂Se₃) is a III₂–VI₃ binary chalcogenide material with a bandgap in the range of 1.39–1.45 eV [1]. The excellent electrical and optical properties of α-In₂Se₃ render it a good candidate for photodetection applications [2,3,4,5]. Moreover, α-In₂Se₃ samples, including atom-thick α-In₂Se₃ layers, exhibit ferroelectric properties at room temperature. Ding et al. [6] reported that α-In₂Se₃ exhibited a spontaneous polarization that can be reoriented using an external electric field; therefore, α-In₂Se₃ is suitable for memory devices and applications, such as solar cells and photovoltaics [1], wide-range photodetectors [7], neuromorphic computing systems [8], and photocatalytic water splitting devices [9]. Recently, Si et al. [10] have fabricated a ferroelectric semiconductor field-effect transistor (FeS-FET)-based memory device using two-dimensional (2D) α-In₂Se₃ and have utilized it to address the shortcomings of non-destructive ferroelectric field-effect transistors featuring ferroelectric materials as gate insulators, such as short retention times, charge trapping, and current leaking through the ferroelectric insulator.

In this study, we fabricated a new FeS-FET device based on multilayered (MTL) graphene (Gr) as the drain and source contacts and 2D MTL α-In₂Se₃ as the channel (Gr/α-In₂Se₃/Gr) and analyzed its electrical properties and the physical mechanisms affecting the electronic and optoelectronic transport in the device. The effects of the ferroelectric nature of α-In₂Se₃ and trapping–detrapping on the optoelectronic performance of the photodetector are discussed herein. The maximum photoresponsivity and specific detectivity of the Gr/α-In₂Se₃/Gr-based FeS-FET-based photodetector were 10 AW⁻¹ and 4.4 × 10¹² cmHz^1/2 W⁻¹, respectively, which indicated the high performance of the device for photodetection applications.

2 Experimental section

2.1 Experimental details

The Gr/α-In₂Se₃/Gr-based FeS-FET-based photodetector was fabricated using a mechanical exfoliation technique. Gr and α-In₂Se₃, were purchased from HQ Graphene and were first mechanically exfoliated onto polydimethylsiloxane (PDMS) (Dow Corning, Toray Co., Ltd). Then, MTL Gr contacts were deposited using PDMS on 50 nm titanium electrodes that were deposited by electron beam evaporation. Thereafter, the MTL 2D α-In₂Se₃ nanosheet was transferred from PDMS by mechanical exfoliation technique on the MTL Gr contacts. A p-type Si substrate with a 300 nm thick silicon dioxide (SiO₂) layer was used as the dielectric gate insulator.

2.2 Characterization techniques

Park NX20 atomic force microscope (AFM) and JEOLJSM-7500F + EDS(Oxford) were used for morphological and topographic analysis characterizations. The electrical and optoelectronic properties of the device were evaluated using a Keithley 4200A-SCS parameter analyzer and mobiken laser power meter LP1 was used for power light calibration prior the experiments. The laser power(P) ranged between 0.016 and 1.55 μW, and the wavelength and laser spot diameter were 532 nm and 20 mm, respectively.

3 Results and discussion

3.1 Morphological analysis

The schematic diagram and corresponding scanning electron micrograph of the fabricated device are presented in Fig. 1a and b, respectively. The length of the channel between the MTL Gr contacts was estimated to be 12 μm. The atomic force microscopy images of the MTL materials are illustrated in Fig. 1c. Topographic analysis revealed that the thicknesses of the drain MTL Gr contacts, MTL In₂Se₃, and source MTL Gr contacts were 125, 160, and 60 nm, respectively (Fig. 1d).

Fig. 1

a Schematic diagram and b scanning electron micrograph of the of the fabricated multilayered (MTL) graphene Gr/α-In₂Se₃/Gr device. c Atomic force microscopy images of MTL Gr and α-In₂Se₃. d Thicknesses of the two-dimensional drain MTL Gr contacts (red), MTL In₂Se₃ (green), and source MTL Gr (blue) contacts determined using topographic analysis. Here, I_DS, V_DS, and V_GS denote drain current, drain-source voltage, and gate-source voltage, respectively (Color figure online)

3.2 Electrical characterization

The drain current (I_DS) dependence on the drain-source voltage (V_DS) at different gate-source voltages (V_GS) of the Gr/α-In₂Se₃/Gr-based FeS-FET heterostructure wherein the MTL Gr contacts present a Schottky barrier is illustrated in Fig. 2a. Under a negative back-gate voltage (V_BG), the electronic transport was dominated by minority carriers, resulting in a low I_DS. However, when V_BG exceeded the flat-band voltage, both types of carriers (holes in the valence band and electrons in the conduction band) were involved in electronic transport, resulting in high I_DS values. I_DS increased as V_BG increased from − 10 to 40 V (Fig. 2a, inset), confirming the n-type and normally off behavior of the device. The hysteresis observed in the inset of Fig. 2a is explained in the last section of this paper. The I_DS–V_DS characteristics of the Gr/α-In₂Se₃/Gr device at different P values are presented in Fig. 2b. I_DS increased with increasing P owing to the increase in number of photogenerated carriers in MTL α-In₂Se₃.

Fig. 2

a Drain current (I_DS)–drain-source voltage (V_DS) diagrams of the Gr/α-In₂Se₃/Gr device at different gate-source voltages (V_GS). The transfer characteristic curves of the Gr/α-In₂Se₃/Gr device at V_DS of 3, 4, and 5 V are included in the inset. b I_DS–V_DS characteristics at different laser power densities. Here V_BG denotes back-gate voltage

3.3 Optoelectronic characterization

The dependence of the optoelectronic properties of the Gr/α-In₂Se₃/Gr device on the V_BG applied along the vertical direction of the Cu/p-Si/SiO₂/α-In₂Se₃ structure are illustrated in Fig. 3a–d. The transfer characteristic curves of the Gr/α-In₂Se₃/Gr device at different P values are presented in Fig. 3a.

Fig. 3

a Drain current (I_DS)–back-gate voltage (V_BG) transfer characteristic curves of the of graphene (Gr)/α-In₂Se₃/Gr-based ferroelectric semiconductor field-effect transistor in the dark and at different laser power densities and b photocurrent (I_PH), c photoresponsivity (R_λ), and d specific detectivity (D^*) dependence on V_BG at different laser power densities, at V_DS of 1 V

The photocurrent (I_PH) was measured by the difference between the I_DS values under light irradiation and in the dark. I_PH increased with increasing P owing to the increase in density of the photoexcited carriers (Fig. 3b); moreover, I_PH did not reach saturation. The dependence of photoresponsivity (R_λ), which was calculated as follows: R_λ = I_PH/P, on V_BG is illustrated in Fig. 3c. R_λ increased with increasing V_BG because I_PH increased, and decreased with increasing P. The specific detectivity (D^*) represents the capability of FeS-FETs to detect weak signals, and can be calculated as follows: D^* = R_λ × A^1/2/(2 × e × I_dark)^1/2, where I_dark, the current in the dark, is the main contributor to shot noise, A is the effective irradiation area, and e is the electron charge. D^* increased to a maximum of 1.77 × 10¹² cmHz^1/2 W⁻¹ when V_DS and V_BG were 1 and 5 V, respectively and decreased when V_BG exceeded 5 V. A detailed explanation of the changes in R_λ and D^* with the forward and reverse I_DS is included in the last section of the paper (Fig. 5).

The I_DS–V_BG transfer characteristic curves in the forward and reverse directions, when V_BG increased from − 10 to 40 V and then decreased back to − 10 V are presented in Fig. 4a. I_DS presented a clockwise hysteresis loops in the dark and under laser irradiation. Other 2D material-based FETs, such as MoS₂ [11] and WSe₂ [12], exhibited electrical hysteresis; however, α-In₂Se₃ presented a wider memory window than MoS₂ and WSe₂ (Fig. 4a). Si et al. [10] attributed the electrical hysteresis of α-In₂Se₃ to the ferroelectric polarization of the α-In₂Se₃ layer. Furthermore, Li et al. [13] ascribed the large memory window of α-In₂Se₃ to its high charge capturing capability. To better understand the physical mechanism affecting the electronic and optoelectronic transports in forward and reverse directions, we drew the band diagrams of the Gr/α-In₂Se₃/Gr device along the vertical direction of the Cu/p-Si/SiO₂/α-In₂Se₃ system (Fig. 4c). The bandgap energy, work function, and electron affinity of α-In₂Se₃ were 1.4 eV [14], 4.35 eV [15], and 3.6 eV [16], respectively (Fig. 4c-i). At equilibrium, a depletion region formed at the SiO₂–α-In₂Se₃ interface (Fig. 4c-ii), that is, the Gr/α-In₂Se₃/Gr device was normally off in the depletion regime. When a very low negative V_BG was applied to the Gr/α-In₂Se₃/Gr device (Fig. 4c-iii), electrons were injected from the electron traps at the SiO₂–α-In₂Se₃ interface into the conduction channel. In addition, the electrical dipole in the α-In₂Se₃ layer was polarized in the downward direction, which pushed electrons from the surface of the α-In₂Se₃ layer into its bulk (Fig. 4c-iii). In general, traps in α-In₂Se₃ are ascribed to the natural oxide layer at the α-In₂Se₃–SiO₂ interface [17] and the adsorption of water and oxygen molecules at the α-In₂Se₃ surface [13, 18]. Therefore, the threshold V_BG shifted to negative values in the forward direction owing to an increase in electron density.

Fig. 4

Dependence of a drain current (I_DS) and b photocurrent (I_PH) on the back-gate voltage (V_BG) and laser power density in the forward and reverse directions, at V_DS of 5 V. c Band diagrams of the Gr/α-In₂Se₃/Gr device in the vertical direction at different operating conditions illustrating the effect of the electrical dipole polarization on electronic transport. Here Gr and MTL denote graphene and multilayered, respectively, and V_FB and E_f denote flat-band voltage, and Fermi energy level, respectively

Nevertheless, by increasing V_BG to positive values (Fig. 4c-iv), electrons, which were the majority charge carriers accumulated at the SiO₂–α-In₂Se₃ interface and I_DS increased. In this case, the electrical dipole in the α-In₂Se₃ layer was polarized in the upward direction. Therefore, electrons were pushed toward the surface far from the conduction layer; moreover, because V_BG was positive, electrons started to fill the electron traps at the SiO₂–α-In₂Se₃ interface. Consequently, in reverse mode (Fig. 4c-v), fewer electrons were available for electronic transport, and the threshold voltage shifted to positive values.

Under light illumination, the concentration of charge carriers in α-In₂Se₃ increased owing to the generation of electron–hole pairs, which increased I_PH and shifted the threshold voltage in the negative direction (Fig. 4b). Conversely, under light excitation, electrons were removed from the trap states at the SiO₂–α-In₂Se₃ interface and the α-In₂Se₃ surface and were injected into the conduction band of α-In₂Se₃. Because electron detrapping occurred in reverse mode, the threshold voltage shifted more toward the negative values of V_BG and increased with increasing P, which explained hysteresis (or memory window) narrowing.

The hysteresis loops of α-In₂Se₃ strongly affect its optoelectronic properties. R_λ presented clockwise hysteresis with a maximum value of 10 AW⁻¹ when V_BG exceeded 30 V (Fig. 5a). R_λ decreased with increasing P from 0.016 to 0.93 μW owing to trapping–detrapping effects. Similarly, D^* presented a pinched hysteresis loop (Fig. 5b). D^* increased to 4.4 × 10¹² cmHz^1/2 W⁻¹ when V_DS and V_BG were 5 and 35 V, respectively. The pinched loop hysteresis of D^* was attributed to the decrease in D^* at high V_BG values, which was caused by the exponential increase in I_dark (insets of Figs. 2a and 4a).

Fig. 5

Dependence of a photoresponsivity (R_λ) and b specific detectivity (D^*) on the back-gate voltage (V_bg) and laser power density in the forward and reverse directions, at V_DS of 5 V

4 Conclusion

In summary, we fabricated a new MTL Gr/α-In₂Se₃/Gr-based FeS-FET, analyzed its electrical and optoelectronic properties, and investigated the physical mechanisms responsible for tuning its optoelectronic performance. The large hysteresis observed in the transfer characteristic curves of the Gr/α-In₂Se₃/Gr device was attribute to the ferroelectric polarization of MTL α-In₂Se₃ and carrier trapping–detrapping effects. The maximum R_λ and D^* values of the Gr/α-In₂Se₃/Gr device were 10 AW⁻¹ and 4.4 × 10¹² cmHz^1/2 W⁻¹, respectively, confirming that the Gr/α-In₂Se₃/Gr-based FeS-FET photodetector can be a promising device for photodetection applications.