High-Responsivity Multilayer MoSe₂ Phototransistors with Fast Response Time

Abstract

There is a great interest in phototransistors based on transition metal dichalcogenides because of their interesting optoelectronic properties. However, most emphasis has been put on MoS₂ and little attention has been given to MoSe₂, which has higher optical absorbance. Here, we present a compelling case for multilayer MoSe₂ phototransistors fabricated in a bottom-gate thin-film transistor configuration on SiO₂/Si substrates. Under 650-nm-laser, our MoSe₂ phototransistor exhibited the best performance among MoSe₂ phototransistors in literature, including the highest responsivity (1.4 × 10⁵ AW⁻¹), the highest specific detectivity (5.5 × 10¹³ jones), and the fastest response time (1.7 ms). We also present a qualitative model to describe the device operation based on the combination of photoconductive and photogating effects. These results demonstrate the feasibility of achieving high performance in multilayer MoSe₂ phototransistors, suggesting the possibility of further enhancement in the performance of MoSe₂ phototransistors with proper device engineering.

Introduction

There is a great interest in transition metal dichalcogenides (TMDs), which are composed of vertically stacked layers held together by van der Waals interactions, because of their interesting electronic, optical, and chemical properties^1,2. Unlike graphene, the existence of bandgaps in TMDs^3,4 such as MoS₂ or MoSe₂ offers an attractive possibility of using these layered materials in various device applications. Field-effect transistors (FETs) based on single or multilayer MoS₂ exhibit outstanding performance metrics, including high on/off-current ratio (~10⁷), high mobility (~100 cm²V⁻¹s⁻¹) and low subthreshold swing (~70 mV decade⁻¹)^5,6. As the band structure of TMDs depends on their physical thickness^3,4, FETs based on TMDs are especially promising for optoelectronic devices such as phototransistors. As the optoelectronic properties of early MoS₂ phototransistors improved^7–10, high responsivity (~10⁵ AW⁻¹) and fast response time (~1 ms) were obtained in MoS₂ phototransistors with device engineering such as HfO₂ encapsulation or ferroelectric gate dielectrics^11–13.

While MoS₂ has been the most extensively investigated TMD for device applications, the higher optical absorbance of MoSe₂¹⁴ suggests that MoSe₂ could be more suitable than MoS₂ for the application of phototransistors. However, little attention has been given to the optoelectronic properties of MoSe₂ phototransistors, which has been less impressive than those of MoS₂ phototransistors (responsivity: 0.01–238 AW⁻¹, response time: 5–400 ms)^13,15–19. Therefore, in this study, we explore the optoelectronic properties of MoSe₂ phototransistors fabricated with mechanically-exfoliated multilayer flakes on SiO₂/Si substrates. Our best-performance MoSe₂ phototransistor in a simple bottom-gate thin-film transistor configuration exhibits high responsivity (~1.4 × 10⁵ AW⁻¹) and fast response time (~1.7 ms) under 650-nm-laser surpassing previously reported MoSe₂ phototransistors. We also investigate the dependence of photocurrent on gate voltage and optical power density to describe the device operation based on photoconductive and photogating effects. These results demonstrate the feasibility of achieving high performance in MoSe₂ phototransistors without complicated device structures, suggesting that the performance of MoSe₂ phototransistors could be further enhanced by the combination of optimized device architecture and processing.

Results and Discussion

Before fabricating MoSe₂ transistors, we first measure the optical absorbance of MoSe₂ crystals and mechanically exfoliated flakes on sapphire substrates across visible and near-infrared spectral ranges (Fig. 1(a)). The MoSe₂ crystal is thicker than 100 μm and the thickness of exfoliated MoSe₂ flakes are in the range of 20–80 nm. Both samples show two excitonic absorbance peaks A and B at 1.55 eV and 1.82 eV, respectively, which is consistent with literature²⁰. Next, multilayer MoSe₂ transistors are fabricated on SiO₂/Si substrates. Figure 1(b) shows the optical microscopy image of a completed MoSe₂ transistor along with its schematic cross-section. The measured transfer curve of an MoSe₂ transistor in Fig. 1(c) shows asymmetric ambipolar behavior with strong n-type characteristic (MoSe₂ thickness (t) = 25 nm). For electron transport without light, the MoSe₂ transistor exhibits on/off-current ratio (I_on/I_off) of 10⁵ and field-effect mobility (μ_FE) of 50.6 cm²V⁻¹s⁻¹ extracted from μ_FE = L(dI_d/dV_g)/(WC_oxV_d), where L is the channel length (5 μm), I_d is drain current, V_g is gate voltage, W is the channel width (27 μm), C_ox is the oxide capacitance, and V_d is the drain voltage (1 V). For hole transport without light, I_on/I_off of 10⁴ and μ_FE of 2.8 cm²V⁻¹s⁻¹ are obtained. The n-type-dominant ambipolar behavior of MoSe₂ transistors with Ti/Au electrodes was also observed in literature^21,22. The output curves in Fig. 1(d) show linear region at low V_d suggesting decent contact properties. The transfer and output characteristics of an MoSe₂ transistor in Fig. 1(c,d) show the increase of I_d with the power density of incident light.

Figure 1.

(a) Absorbance spectra of MoSe₂ crystals and mechanically exfoliated flakes on sapphire with two excitonic peaks A and B, (b) optical microscopy image and schematic cross-section of an MoSe₂ phototransistor along the red line, (c) I_d − V_g and (d) I_d − V_d characteristics of an MoSe₂ phototransistor with different optical power of incident light.

The photocurrent (I_ph) of phototransistors based on transition metal dichalcogenides such as MoSe₂ is known to be dominated by photoconductive effect and photogating effect²³. In photoconductive effect, photogenerated excess carriers increase conductivity resulting in increased current. The photocurrent component flowing between two electrodes by photoconductive effect is given by²⁴ $I_{ph}=\left(\mathrm{\Delta }\sigma \right)EWD=\left(\mathrm{\Delta }n\right)q\mu EWD=q\left(\eta P_{in}/h\nu \right)\left(\mu \tau E/L\right)$, where Δσ, E, D, Δn, q, μ, η, P_in, h, ν, and τ are change in conductivity, electric field, depth of absorption region, change in carrier concentration, unit charge, carrier mobility, quantum efficiency, incident optical power, Planck constant, frequency of incident light, and carrier lifetime, respectively. The photoconductive component of I_ph is proportional to areal power density of incident light P_in and weakly depends on V_g^23,24. In photogating effect, one type of photogenerated carriers (electrons or holes) is trapped in localized states and the other type of carriers flows in the channel unrecombined. As this is equivalent to doping by the other type of carriers, photogating effect accompanies a shift of threshold voltage (V_th)²⁵. As V_th shifts, the drain current changes from I_d to I_d + ΔI_d and it follows that²⁶ $I_{ph}=I_d\left(V_g-V_{th}+\mathrm{\Delta }{V}_{th}\right)-I_d\left(V_g-V_{th}\right)\approx g_m\mathrm{\Delta }{V}_{th}=g_m\left(kT/q\right)\ln \left(1+\eta q\lambda P_{in}/I_{dark}hc\right)$, where g_m, k, T, λ, I_dark, and c are transconductance, Boltzmann constant, temperature, wavelength of incident light, dark current, speed of light, respectively. Thus, the photogating component of I_ph shows logarithmic dependence on P_in and is roughly proportional to transconductance (g_m)^23,26.

Figure 2(a) shows I_ph and g_m as a function of V_g for the same device in Fig. 1(c). The calculation of I_ph (I_ph = I_light – I_dark, where I_light is I_d in a detector with light), and g_m (g_m = dI_d/dV_g) is based on the data in Fig. 1(c) at P_in = 18 mWcm⁻². The similarity between I_ph and g_m suggests that photogating effect dominates the photoresponse of MoSe₂ transistors. In the inset of Fig. 2(a), the change in V_th (ΔV_th) for electrons and holes is shown as a function of P_in. The increasing change of V_th with increasing P_in also suggests the domiant role of photogating effect in our MoSe₂ phototransistors. However, the dependence of I_ph on V_g in Fig. 2(b) through (d) suggests that each effect dominates I_ph at different range of V_g. Figure 2(b) through (d) show I_ph of our MoSe₂ phototransistor in an on-state for electrons (at V_g = 40 V), an off-state (at V_g = 0 V), and an on-state for holes (at V_g = −40 V) as a function of P_in in sequence. I_ph is calculated based on the data in Fig. 1(c). The I_ph in an on-state for electrons (at V_g = 40 V) and for holes (at V_g = −40 V) shows logarithmic dependence on P_in suggesting the dominant role of photogating effect. However, the linear dependence of I_ph in an off-state suggests the dominant role of photoconductive effect. Such a distinct dependence of I_ph on V_g regime was also observed in phototransistors based on MoS₂¹⁰, MoTe₂²⁷, compound semiconductors²⁶, and organic semiconductors²⁸.

Figure 2.

(a) I_ph and g_m as a function of V_g; inset shows ΔV_th for electrons and holes as a function of P_in (solid lines: logarithmic fit); I_ph as a function of P_in (b) at V_g = 40 V, (c) at V_g = 0 V, and (d) at V_g = −40 V.

The observed dependence of I_ph on V_g can be understood by the simplified energy band diagrams of an MoSe₂ phototransistor under a bias (V_d) at different V_g in Fig. 3. For mechanically exfoliated MoS₂ flakes and chemical vapor deposited MoS₂ films, the existence of trap state was reported in literature^13,29,30 as a result of structural defects at the surface and inside MoS₂. Similarly, we assume that electron traps and hole traps exist in the energy bandgap of MoSe₂ by structural defects at the surface and inside MoSe₂. In Fig. 3(a) (at V_g = 40 V), the Fermi level (E_F) is located close to the conduction band edge and the majority of electron traps are filled. Without light, I_dark flows by the thermionic emission or tunneling of electrons. With light, the photogenerated holes fill hole traps and additional current I_ph flows by the unrecombined photogenerated electrons. In Fig. 3(b) (at V_g = 0 V), E_F moves toward midgap and the majority of electron traps and hole traps become unfilled. Without light, I_dark is negligible as the high barrier height at the contact allows negligible injection of electrons and holes. With light, I_ph is less than that in Fig. 3(a) as the photogenerated electrons and holes recombine or fill the trap states. In Fig. 3(c) (at V_g = −40 V), E_F is close to the valence band edge and the majority of hole traps are filled. Without light, I_dark flows by the thermionic emission or tunneling of holes. With light, the photogenerated electrons fill electron traps and additional current I_ph flows by the unrecombined photogenerated holes.

Figure 3.

Schematic energy band diagrams of MoSe₂ phototransistors with and without light (a) at V_g = 40 V, (b) at V_g = 0 V, and (c) at V_g = −40 V under an applied bias (V_d).

The performance of an MoSe₂ transistor as a photodetector can be evaluated by responsivity (a measure of the electrical response to light) and specific detectivity (a measure of detector sensitivity)³¹. Responsivity (R) is given by R = (I_light − I_dark)/(P_inA), where A is the area of the detector. Under the assumption that shot noise from I_dark is the major contributor to the total noise, specific detectivity (D*) is given by³² D* = RA^1/2/(2qI_dark)^1/2. Figure 4(a,b) show the calculated R and D* of the MoSe₂ phototransistor at different P_in and V_g. Maximum R of 1.4 × 10⁵ AW⁻¹ and D* of 5.5 × 10¹³ jones are obtained at P_in = 27 μWcm⁻² and V_g = 40 V. These are the highest values of R and D* among MoSe₂ phototransistors reported in literature so far (R = 0.01–238 AW⁻¹ and D* = 1.0 × 10¹¹–7.6 × 10¹¹ jones at P_in = 10–100 mWcm⁻²)^13,15–19. As R and D* increase with decreasing P_in, the enhancement of R and D* in this work may be due to the low P_in compared to that in literature. However, even at comparable P_in in the range of 18–54 mWcm⁻², the maximum R and D* in this work (R = 519 AW⁻¹ and D* = 1.3 × 10¹² jones) are about twice as high as those in literature. In Fig. 4(a,b), the overall dependence of R and D* on P_in and V_g is consistent with literature¹³. R increases as P_in decreases or V_g increases, while D* increases as P_in or V_g decreases. As P_in increases, more holes fill shallow trap states where lifetime is short. This results in faster recombination hence R decreases. When P_in increases, D* also decreases as R decreases and I_dark remians unchanged. When V_g increases, electrical doping at higher V_g reduces contact resistance resulting in higher photocurrent and R. However, as V_g increases, I_dark also increases, which degrades D*.

Figure 4.

(a) R and (b) D* as a function of P_in at different V_g; (c) I_d as a function of time and (d) zoomed-in region in (c).

It needs to be mentioned that our MoSe₂ transistors show wide device-to-device variation of μ_FE, R, and D* (Table S1 in Supplementary Information). Such wide device-to-device variation is commonly observed in the transistors based on transition metal dichalcogenides such as MoSe₂ presumably because of the variation of intrinsic defects in crystals³³. While it is very difficult to pinpoint the origin of high performance in the best device, the correlation between R and μ_FE in this work (Fig. S1 in Supplementary Information) suggests that the enhanced optoelectronic properties may be related to the enhanced electrical performance of our MoSe₂ device. It is also supported by the fact that our MoSe₂ device shows the highest mobility among MoSe₂ transistors in Table 1.

Table 1.

Performance of MoSe₂ phototransistors measured at P_in = 10–100 mWcm⁻².

Type of MoSe2	μFΕ (cm2v−1s−1)	R (AW−1)	D* (jones)	Response time (ms)	Reference
Multilayer flake (Exfoliation)	50.6	519.2	1.3 × 1012	1.7 (rise) 2.2 (fall)	This work
Few layer flake (Exfoliation)	19.7	97.1	—	15 (rise) 30 (fall)	15
Single layer film (CVD)i	—	0.013	—	60 (rise) 60 (fall)	16
Multilayer flake (CVD)i	10.1	93.7	—	400 (rise) 200 (fall)	17
Multilayer flake (Exfoliation)	5.9 or 16ii	0.1 or 16ii	1.0 × 1011ii	5 (fall)ii	13
Few layer flake (Exfoliation)	1.8	0.026	—	20 (rise) 20 (fall)	18
Few layer flake (Exfoliation)	5.1	238	7.6 × 1011	—	19

ⁱChemical vapor deposition.

ⁱⁱWith HfO₂ encapsulation.

We also note the negligible correlation between the optoelectronic properties of MoSe₂ devices and MoSe₂ thickness. This may seem counterintuitive because the width of energy bandgap changes for thin MoS₂ crystals (< ~4 nm in thickness)⁴ and light absorption depends on MoSe₂ thickness. Yet, because the thickness of our MoSe₂ flakes ranges from 20 nm to 80 nm, we expect negligible differences in energy bandgap in our MoSe₂ devices. On the other hand, we expect higher responsivity for devices with thicker MoSe₂ as more light is absorbed in thicker MoSe₂. However, the responsivity shows negligible correlation with thickness of MoSe₂ flakes in this investigation (Fig. S2 in Supplementary Information). This may be due to the variation of intrinsic materials quality overshadowing the effect of thickness. The mobility and detectivity in Fig. S2 also show negligible correlation with the thickness of MoSe₂ flakes, supporting this argument.

To explore the response time of our MoSe₂ phototransistors, we measure the time-resolved photoresponse of our MoSe₂ phototransistors for multiple illumination cycles. Figure 4(c) shows the result for the same device in Fig. 1(c). The incident laser with a power density of 54 mWcm⁻² is modulated with a square wave at 100 Hz at V_g = 15 V and V_d = 10 V. The nearly identical response for multiple cycles suggests the overall rebustness and reproducibility of our MoSe₂ phototransistors. From a zoomed-in region in Fig. 4(d), we obtain rise time of 1.7 ms and fall time of 2.2 ms. (Rise time is calculated as the time taken by current to increase from 10% to 90% of the maximum current. Fall time is calculated as the time taken by current to decrease from 90% to 10% of the maximum current.) This is the fastest response time of MoSe₂ phototransistors ever reported in literature, which ranges from 5 ms to 400 ms^13,15–19. It is intriguing that our MoSe₂ phototransistors exhibit high responsivity and fast response time. Because the long lifetime of carriers in photogating effect suggests slow response to light, the fast response time in our MoSe₂ device may be related to the characteristics of trap states. One possibility is that trap states in our MoSe₂ device may have shorter lifetime and higher density than those in literature. Then, while the shorter lifetime of trap states could provide fast response, the higher density of trap states could provide higher doping enhancing responsivity. However, we may only speculate at this stage and further investigation is needed on the characteristics of trap states including the distribution of trap energy, trap density, trap lifetime, and carrier capture probability.

Table 1 compares μ_FE, R, D*, and response time of MoSe₂ phototransistors in literature. Because the measurement conditions, such as V_g, V_d, P_in, and excitation energy, can influence the device performance, comparable measurement conditions with those in literature are used in this work. Our MoSe₂ phototransistors exhibit the best performance in terms of μ_FE, R, D*, and response time, demonstrating the feasibility of achieving high responsivity and fast response time in multilayer MoSe₂ phototransistors. Future work combining controlled growth of materials with optimized device architecture and processing will further enhance the performance of MoSe₂ phototransistors.

Conclusions

We report high-responsivity multilayer MoSe₂ phototransistors with fast response time fabricated with mechanically-exfoliated MoSe₂ flakes on SiO₂/Si substrates. Our MoSe₂ phototransistors exhibit asymmetric ambipolar behavior with strong n-type characteristic. Without light, high on/off-current ratio of 10⁵ and field-effect mobility of 50.6 cm²V⁻¹s⁻¹ are obtained for electrons. Under 650-nm-laser, our MoSe₂ phototransistor exhibits the best performance among MoSe₂ phototransistors in literature including high responsivity (1.4 × 10⁵ AW⁻¹), high specific detectivity (5.5 × 10¹³ jones), fast rise time (1.7 ms) and fast fall time (2.2 ms). The dependence of photocurrent on gate voltage and optical power density suggest that photocurrent is dominated by photogating effect in on-state and by photoconductive effect in off-state. These results demonstrate the feasibility of achieving high-performance multilayer MoSe₂ phototransistors, providing potentially important implications on using MoSe₂ phototransistors for a variety of applications including touch sensor panels, image sensors, solar cells, and communication devices.

Methods

Device fabrication

Multilayer MoSe₂ flakes were obtained by gold-mediated mechanical exfoliation³⁴ from bulk MoSe₂ crystals (2D Semiconductors) and transferred to highly doped p-type Si wafer with thermally grown SiO₂ (300 nm). The thickness of MoSe₂ flakes measured by atomic force microscope (AFM, Park Systems XE-100) existed between 20 nm and 80 nm. To form source and drain electrodes (100 μm × 100 μm) on top of MoSe₂ flakes, Ti (20 nm) and Au (50 nm) deposited by electron-beam evaporation were patterned using photolithography and etching. The device was then annealed at 200 °C in a vacuum tube furnace for 2 hours (100 sccm Ar and 10 sccm H₂) to remove resist residue and to decrease contact resistance.

Device characterization

Optical absorbance of MoSe₂ was measured by UV-visible spectroscopy (Perkin-Elmer Lambda 35). Electrical characterizations were carried out with current-voltage (I-V) measurements (Agilent 4155 C Semiconductor Parameter Analyzer) at room temperature. The photoresponse of MoSe₂ phototransistors was measured with a 650-nm-laser (beam size of 3 mm) at different power densities (0.027, 18 and 54 mW cm⁻²). Dynamic on/off switching was conducted using a function generator (Tekronix AFG310).

Author Contributions

H.L. and W.C. designed the experiments. H.L. fabricated the devices. H.L., J.A., S.I. and J.K. characterized the devices. H.L. and W.C. wrote the manuscript. All authors reviewed the manuscript.

Competing Interests

The authors declare no competing interests.