1. Introduction

Two-dimensional (2D) materials were widely studied as the next generation of flexible and transparent electronic devices, especially, photodetectors [1–6]. N-type 2D materials are more suitable for photodetection application due to the high electrons mobility compared with holes’ mobility in P-type 2D materials. However, the electronic and optoelectronic properties of P-type 2D materials are needed for the realization of more complex structures such as PN, Schottky photodiodes and heterostructures based photodetectors [7–9]. In addition to their flexibility and highly transparency, nano-layered two-dimensional materials based photodetectors show high performance compared with commercial photodetectors [10,11]. Recently, we reported on the fabrication and characterizations of metal–semiconductor–metal (MSM) based photodetectors where multilayered gallium selenide (GaSe), with has an indirect bandgap of around 2.11 eV [12], was used as the active layer, showed excellent optoelectronic properties in the visible light range [13–15]. In this paper, we fabricated back-gated field effect transistor (FET) structure based on nanolayered GaSe as active layer and copper contact as back gate contact. The photoresponsivity and the specific detectivity can be tunable by the gate voltage to 0.9 AW^–1 and 8.08 × 10¹¹ cmHz^0.5W^–1, respectively.

2. Experimental

Figure 1(a) represents the schematic of the back-gated field effect transistor based photodetector, where the channel length and width were 47 and 20 μm, respectively. The nanolayered GaSe (HQgraphene, Netherlands) was deposited using mechanical exfoliation technique. Figure 1(b) shows an optical photo of the nanolayered GaSe deposited on titanium electrodes, where the active surface was estimated as 0.305 × 10⁻⁴ cm². We annealed the device at 400°C under nitrogen gas atmosphere for two hours. The nanolayered GaSe thickness was determined as 45 nm, as shown in Fig. 1(c), by atomic force microscopy (AFM) using Pico plus 5500 AFM (Agilent Technologies, USA). Figure 1(d) represents the transfer characteristics of the back-gated FET at different value of drain source voltage (V_ds). The back-gated FET showed excellent gate control capability, and ${\text{I}}_{ON}/{\text{I}}_{OFF}$ that exceed 10³ at drain source voltage of 10V. The optoelectronic properties of the back-gated FET based on nanolayered GaSe were measured under 532 nm laser.

 

Fig. 1 (a) Schematic illustrating the back-gated phototransistor based on nano-layered GaSe. (b) Optical image of the GaSe flake, (c) AFM image and the thickness of the GaSe flake determined by the topographic analysis, and (d) transfer characteristics at different drain-source voltage values.

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3. Results and discussion

The effective mobility (${\text{μ}}_{\text{eff}}$) was deduced from the drain current – gate voltage transfer function as 0.13 cm²V⁻¹s⁻¹ at drain source voltage of 10V using the equation:

Figure 2(a) represents the energy band diagrams of GaSe along the vertical axis showing the Si/SiO₂/GaSe structure before contact. The energy band gap and the electron affinity are, respectively, 2.11 eV and 2.7 eV [12,16]. The Fermi level in GaSe is 0.18 eV above the valence band [17]. According to the electrical measurement shown in Fig. 1(d) the back-gated FET after contact is in the depletion mode. At this condition and for higher gate voltage, the majority carriers, holes in this case, are pushed to the surface, as shown in Fig. 2(b). However, by decreasing the gate voltage to values lower than the flat-band condition voltage (V_FB), holes are collected at the interface between the nanolayered GaSe and the SiO₂ and electrons are pushed to the surface, at this condition the phototransistor is in the accumulation mode, as seen in Fig. 2(c).

 

Fig. 2 Energy band diagrams of the back-gated FET based on GaSe. (a) Before contact, (b) Depletion (V_G > V_FB) and (c) Accumulation (V_G < V_FB).

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The transfer characteristics at drain source voltage (V_ds) of 5 V and at different values of laser power is shown in Fig. 3(a). Increasing the laser power enhanced creation of the photo-carriers in the nanolayered GaSe, which explain the increase in the drain current. Figure 3(b) is the gate voltage dependence of the photocurrent measured using the equation ${\text{I}}_{\text{ph}}=\text{I}\left(\text{light}\right)-\text{I}(\text{dark})$ at different laser power values. Under light excitation the density of the photogenerated carriers, electrons in the conduction band and holes in the valence band, is enhanced. In addition, by decreasing the gate voltage more holes are collected at the interface and contributed to the conduction, which explains the increase in the photocurrent. The photocurrent showed peak at the gate voltage of −30V, at this value of gate voltage both minority carriers (electrons) in the conduction band and majority carriers (holes) in the valence band can be collected at drain and source, respectively, and contributed to the conduction [18]. The photocurrent can be expressed as function of the laser power ($\text{P}$) using the equation ${\text{I}}_{\text{ph}}\propto {\text{P}}^{\alpha }$, where $\alpha$ is an exponent. The exponent $\alpha$ was deduced as function of gate voltage and represented in Fig. 3(c). As we can see, the exponent $\alpha$ is lower than unit, which is the signature of the photogating effect originated from long-lived states from traps [19]. The exponent $\alpha$ drops at gate voltage of −30V, because both electrons and holes contribute to the conduction and can be trapped at the surface and the interface, respectively. The oxygen and water in the atmosphere are the main origin of trapping effect at the surface of the GaSe flake [20], however, lattice mismatch between SiO₂ and GaSe flake can be the main origin of the trapping effect at the interface.

 

Fig. 3 Laser power dependence of the (a) Transfer characteristics and (b) the photocurrent versus the gate voltage of the nano-layered GaSe based back-gated field effect transistor. (c) Variation of the coefficient $\alpha$ in function of the gate voltage.

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The photoresponsivity was calculated as function of the gate voltage at different values of laser power using the equation ${\text{R}}_{\text{λ}}={\text{I}}_{\text{ph}}/\text{P}$, where P is the laser power. As seen in Fig. 4(a), the photoresponsivity decreased with increasing the laser power due to the saturation of trap states [10]. The photoresponsivity increases with decreasing the gate voltage and showed maximum value of 1.4 AW^–1, at laser power of 0.015 μW. The external quantum efficiency (EQE) was measured as 326% at the same values of gate voltage and laser power using the equation$\text{ EQE}={\text{hcR}}_{\text{λ}}/\text{eλ}$, where $\text{h}$ is the Planck’s constant, $\text{e}$ is the elementary electron charge, $\text{c}$ is the light velocity, and $\text{λ}$ is the excitation wavelength.

 

Fig. 4 Laser power and gate voltage dependence of the (a) photoresponsivity, and (b) specific detectivity of the nano-layered GaSe based back-gated field effect transistor.

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The specific detectivity is deduced from the photoresponsivity using the equation ${\text{D}}^{*}={\text{R}}_{\text{λ}}\times {\left(A\right)}^{0.5}/{\left(2e{I}_{dark}\right)}^{0.5} \left(cm H{z}^{1/2} W^{-1}\right)$ [21], where $A$is the effective area of the nanolayered GaSe in $c{m}^2$ (0.305 × 10⁻⁴ cm²). Figure 4(b) represents the specific detectivity as function of the gate voltage measured at different values of laser power. The specific detectivity showed a maximum value of 8.08 × 10¹¹ cmHz^0.5W^–1 at gate voltage of −18V, corresponds to photoresponsivity and external quantum efficiency of 0.9 AW^–1 and 210%, respectively. The specific detectivity is higher than previously reported detectivity in p-type two-dimensional materials in the visible range [8,22–24].

4. Results and discussion

In conclusion, nanolayered GaSe based field effect transistor showed excellent gate control capability with an ${\text{I}}_{ON}/{\text{I}}_{OFF}$ value exceeding 10³. The back-gated FET showed high specific detectivity making the nanolayered GaSe promising p-type two-dimensional material for photodetection application. Fabrication vertical configuration of PN photodiode based on gallium selenide as p-type 2D materials can enhance both the photoresponsivity and the detectivity, a subject which is interesting for future research.