High-sensitive two-dimensional PbI₂ photodetector with ultrashort channel

Abstract

Photodetectors based on two-dimensional (2D) semiconductors have attracted many research interests owing to their excellent optoelectronic characteristics and application potential for highly integrated applications. However, the unique morphology of 2D materials also restricts the further improvement of the device performance, as the carrier transport is very susceptible to intrinsic and extrinsic environment of the materials. Here, we report the highest responsivity (172 A/W) achieved so far for a PbI₂-based photodetector at room temperature, which is an order of magnitude higher than previously reported. Thermal scanning probe lithography (t-SPL) was used to pattern electrodes to realize the ultrashort channel (~60 nm) in the devices. The shortening of the channel length greatly reduces the probability of the photo-generated carriers being scattered during the transport process, which increases the photocurrent density and thus the responsivity. Our work shows that the combination of emerging processing technologies and 2D materials is an effective route to shrink device size and improve device performance.

Graphical abstract

Keywords

two-dimensional photodetectors / carrier scattering / ultrashort channel / thermal scanning probe lithography / PbI₂ nanosheets

1 Introduction

Photodetectors have been widely applied in many fields including digital display, biomedical imaging, environment monitoring and so forth [1, 2]. In general, the operation of most photodetectors involves three main processes: light absorption [3, 4], electron−hole pair separation [5, 6], and carrier transport. Although less attention has been paid to carrier transport than the first two in previous studies, it is crucial to the performance of photodetectors. In recent years, two-dimensional (2D) semiconductors have shown great application potential in the photodetectors owing to their unique structures and superior optoelectronic properties [7-10]. Due to the atomic thickness and strong surface/interface effect, the performance of 2D photodetectors is more strongly related to carrier scattering than conventional devices [11]. Carrier scattering, usually caused by lattice vibrations [12-14], ionized impurities [15, 16], and other material disorders [17], is one of the most important factors affecting carrier transport.

Previous studies on 2D electronic devices have shown that when the channel length was shortened to below 100 nm, the carrier scattering would be obviously suppressed and manifested as a significant improvement in the current density. For example, the current density of the MoS₂ field-effect transistors (FETs) was increased by two orders of magnitude by shortening the channel length down to sub-10 nm [18, 19]. Similar phenomenon was also found in WSe₂ transistors. The highest current density ever reported in 2D transition metal dichalcogenides (TMD) transistors to date was realized by fabricating bilayer WSe₂ transistors with sub-100 nm channel-length [20]. Considering 2D semiconductors are immune to short-channel effects, shortening the channel length is also one of the effective means to reduce device size and improve integration [21-23]. Unfortunately, the fabrication of short-channel devices is extremely challenging. At present, the main preparation methods of short-channel devices are either require specific electrode materials [18, 20] or device structures [21, 24-26], or cause unavoidable damage to the fragile 2D materials [27]. Thermal scanning probe lithography (t-SPL) is an emerging direct-write nanolithography method, using thermal energy from a heated nanometer-scaled tip to induce local material modification [28-32]. It enables in situ simultaneous patterning and imaging of 2D materials with sub-10 nm resolution without damaging the materials or contaminating the surface, which is very important for fabricating ultrashort channel devices [33-36]. However, it has not been used in the preparation of 2D photodetectors with ultrashort channel.

In this work, we introduced the t-SPL method to the fabrication of 2D photodetectors for the first time. Multilayered-PbI₂ is a kind of direct-bandgap semiconductors with wide bandgap (~2.5 eV), large absorption coefficient, high resistivity, and large atomic number, which is promising for 2D ultraviolet photodetectors, room-temperature X-ray/γ-ray detectors, and spin-photonics applications [37-44]. However, the performance of PbI₂-based photodetectors has been unsatisfactory due to the suboptimal charge transport. As a good demonstration, the sensitivity of the photodetector based on PbI₂ nanosheet was significantly improved by shortening the channel length to 60 nm. The sharp increase in sensitivity mainly comes from the greatly suppressed carrier scattering brought about by the shortened channel length. In this way, an ultra-high responsivity up to 172 A/W at room temperature was realized, which is one order of magnitude higher than the highest responsivity of previously reported PbI₂-based photodetectors. Our work fully demonstrates that the introduction of advanced processing methods can facilitate the fabrication of ultrashort channel devices and significantly improve their optoelectronic performance. It will also greatly promote the application of 2D semiconductors in high-performance and high-integration optoelectronic devices.

2 Experimental section

Firstly, PbI₂ nanosheets were prepared on PDMS (polydimethylsiloxane) substrate via solution method according to previous report [45, 46] [Fig.1(a)]. A piece of PbI₂ nanosheet with regular shape, smooth surface, and lateral dimension of approximately 42 μm was selected for the construction of photodetector [Fig.1(b)].

Fig.1 The fabrication process of PbI2-based photodetector with ultrashort channel. (a) Schematic illustration of synthesizing PbI2 nanosheets via solution-processed method. (b) The optical image of the as-grown PbI2 nanosheet with regular hexagonal shape and flat surface. (c) Schematic illustration of the t-SPL patterning process for the fabrication of metal electrodes with ultrashort spacing. (d) SEM image of electrodes with a spacing of 60 nm. (e) Schematic illustration of transfer process. (f) Optical image of a PbI2-based photodetector with ultrashort channel.

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The patterning of electrodes was processed with the t-SPL method [Fig.1(c)]. To be specific, the PMGI (polymethylglutarimide) film and heat-sensitive PPA (polyphthalaldehyde) film were spin-coated onto Si/SiO₂ substrate successively to form a dual-polymer stacking as a resist. By precisely controlling the movement of a heated nano-probe (200 °C), etching of the PPA film with ultrahigh resolution can be achieved. The pattern was further etched into the underlying PMGI with TMAH (tetramethylammonium hydroxide) solution. Then a stack of Cr/Au [3 nm/25 nm, provided by ZhongNuo Advanced Material (Beijing) technology Co., Ltd] metals was deposited using electron-beam evaporation. After that, the specimens were soaked in Remover PG for metal lift-off. As can be seen in Fig.1(d), a metal electrode array with a minimum spacing of 60 nm was realized.

At last, a piece of PbI₂ nanosheet was transferred onto the patterned electrode array followed by a brief annealing in argon at 150 °C for 2 hours to improve the quality of the van der Waals contact between the metal and the semiconductor [Fig.1(e) and (f)].

3 Results and discussion

With the help of t-SPL method, a 2D PbI₂-based photodetector with a lateral metal−semiconductor−metal configuration and ultrashort channel was fabricated. The schematic diagram of the photodetector is shown in the inset of Fig.2(a). PbI₂ nanosheets with smooth surfaces were selected for the construction of photodetectors, which can ensure high-quality contact between the nanosheets and electrodes, so that the transport of photogenerated carriers at the interface would not be restricted.

Fig.2 Photodetection performance of PbI2-based ultrashort channel photodetector at room temperature. The wavelength of the incident laser is 405 nm. (a) I−V characteristics in the dark and under different incident power densities. Inset: A schematic drawing of the configuration of photodetector. (b) Photocurrent and responsivity versus incident power density with Vds = 3 V. (c) I−V characteristics of photodetector in the dark and under different incident power densities with stepped positive Vg. (d) Responsivity as a function of the positive Vg. Inset: Schematic diagram of device structure with a top-gate.

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The output characteristic curves of the photodetector were measured in the dark and under laser irradiation with different power densities. As shown in Fig.2(a), source-drain current (I_ds) maintains a linear and symmetrical relationship with bias voltage (V_ds), proving that a good ohmic contact was formed between the PbI₂ nanosheet and the electrodes. At the same time, with the increase of the incident light power density, I_ds corresponding to the same V_ds shows a gradual increase trend.

The dependence of the net photocurrent ($I_{\text{ph}}$) on the incident light power (P) can be described by the power law: $I_{\text{ph}}\propto P^{\beta }$. The value of the exponent β reflects the utilization rate of the photodetector to the incident light. Fig.2(b) plots the relationship between $I_{\text{ph}}$ and P of our photodetector. The fitted exponent β is 0.80, suggesting that there is a photogating effect in our device [38]. The existence of the photogating effect can be further confirmed by the shift of the transfer curve with the change of the gate voltage (V_g) [47] [Fig. S1 of the Electronic Supplementary Materials (ESM)]. The derived responsivity (R) values under different incident power densities were also plotted in Fig.2(b). The responsivity was derived from $R=\frac{I_{\mathrm{p}\mathrm{h}}}{PS}$ [38], where S is the effective area of the photodetector. The responsivity as high as 172 A/W was achieved when V_ds was 3 V and P was 0.47 mW/cm². To the best of our knowledge, it is the highest responsivity ever reported in PbI₂-based photodetectors, which is an order of magnitude higher than the highest responsivity of previously reported devices. Moreover, the responsivity decreases with increasing illumination power, which can be attributed to the higher probability of carrier recombination during transport under high carrier concentration, resulting in a decrease in photocurrent.

It is well-known that the electrical properties of 2D semiconductors can be effectively tuned by an external electric field which is usually generated by applying a vertical bias [48]. Since the devices prepared with t-SPL method usually have a structure in which the electrodes are below the channel, a top gate can be easily added to the fabricated photodetector without any damage to the original device structure [Fig.2(d) inset]. Here, a photodetector with a top gate was fabricated to study the gate-controlled photodetection performance of photodetectors with ultrashort channel. h-BN was used as the gate dielectric. Considering PbI₂ nanosheet is a p-type semiconductor [4, 49], positive V_g was applied to the photodetector to reduce the concentration of holes in the channel. Comparing the output curves under different V_g [Fig.2(c)], it can be found that I_ds gradually decreases as the positive V_g increases, no matter in the dark or with light illumination (405 nm, 0.36 mW/cm²).

At the same time, the increase of the positive V_g has a more obvious effect on I_ds under light illumination than in the dark, resulting in the responsivity decreases [Fig.2(d)]. It can be attributed to the above-mentioned photogating effect being suppressed by the applied positive V_g. The photogating effect is a way of modulating the channel conductance with light-induced or electrically-driven gate field. Higher photocurrent can be achieved with the promoted gain, as the lifetime of photo-generated minority carriers is prolonged in low-dimensional channel, where impurities and defects may act as trap centers rather than recombination centers [11, 50]. In our case, when the external positive V_g was applied, these trap centers were quickly filled by the continuously injected electrons from gate. Therefore, the concentration of free photo-generated holes decreased with the increase of the positive V_g, resulting in a drop in the responsivity of the photodetector. In addition, the derived responsivity of photodetectors with top gate were slightly lower than that of non-top-gated devices under the same test conditions. This is because the huge non-transparent metal top gate electrode blocks part of the incident light, so that the actual power density irradiated on the channel is much less than the measured incident power density used in the calculation of responsivity.

In order to investigate the effect of channel length on the responsivity of photodetectors, we summarized and plotted the responsivity as a function of the corresponding channel length of previously reported PbI₂-based photodetectors [37-40, 49, 51-56] (Fig. S2 of the ESM) as shown in Fig.3(a). It can be observed that the responsivity shows an exponential upward trend with the shortening of channel length, which can be attribute to the significant suppression of carrier scattering in short-channel devices.

Fig.3 (a) The responsivity of our ultrashort channel photodetector as compared to previous PbI2-based photodetectors with different channel lengths. (b) Schematic image of the photoconductive effect under light illumination in the photodetectors. (c) Diffusive transport of carries when the channel length is longer than the λMFP. The insets show three main scattering mechanisms. (d) Ballistic transport when the channel length is shorter than the λMFP.

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As a photoconductive detector, the generation of photocurrent in our device mainly comes from the photo-generated electron−holes pairs [Fig.3(b)]. The electron−hole pairs are separated by the applied V_ds, resulting in the increase of the free carrier concentration in the semiconductor. The efficient transport of free carriers forms the photocurrent. It should be noted that multilayered PbI₂, as a 2D material, has a large specific surface area and strong surface/interface effects, so the carrier transport in photodetectors based on PbI₂ nanosheets is very susceptible to the internal conditions of the material and the external environment, such as lattice vibration, atomic defects, strain, adsorbates, surface roughness, and charged impurities [17]. In the long channels whose length is much longer than the mean free path (λ_MFP), which is defined as the average distance an electron travels before its momentum is changed by elastic scattering from a static scattering center, the transport of photogenerated carriers would be easily scattered by ionized impurities or remote optical phonons, or collide with electrically neutral impurities to change the transport direction [57] [Fig.3(c)]. At this time, diffusive transport accounts for a high proportion of charge transport, which seriously reduces the carrier transport efficiency.

When the channel length is shortened to less than the λ_MFP, the distance that the photogenerated carriers have to travel before reaching the electrodes was shortened greatly [25]. As a result, the carriers would be hardly scattered by impurities or defects during transport, and the proportion of ballistic transport in carrier transport will be increased significantly [58, 59] [Fig.3(d)]. As far as photodetectors are concerned, the improvement of carrier transport efficiency will increase not only the density of the current under illumination, but also dark current. Fortunately, since the generation of photocurrent comes from the surge in the number of free carriers, high effective carrier transport would increase the current under illumination much more significantly than the dark current. Therefore, the net photocurrent of short-channel photodetectors will be significantly improved compared with long-channel devices, resulting in a sharp promote in responsivity.

To further study the transport behavior of photo-generated carriers in the channel, temperature-dependent optoelectronic performance characterization was performed. All of the photocurrent was obtained at the V_ds of 3 V. The dependence of the photocurrent on the incident power density at different temperatures was demonstrated in Fig.4(a). It can be observed that the photocurrent always increases gradually with the increase of power density regardless of the varying temperature. The promotion effect of the power increase on the I_ph becomes more significant as the temperature decreases, indicating that the transport efficiency of photo-generated carriers is enhanced with the decrease of temperature. The relationships between the derived responsivity and incident power density at different temperatures were shown in Fig.4(b), which were all consistent with the trend that the responsivity decreases with the increase of power density observed at room temperature.

Fig.4 Temperature-dependent photoelectric characteristics of PbI2 ultrashort channel photodetector. All data are obtained under 3 V bias. (a, b) Photocurrent and responsivity as a function of incident power density at different temperatures. (c) Responsivity to incident light with a power density of 0.47 mW/cm2 at different temperatures. (d) Detectivity (black line) and EQE (blue line) as a function of incident power density at 80 K.

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It is worth mentioning that although shortening the channel length can significantly reduce the possibility of carriers being scattered, the ballistic proportion in carrier transport is difficult to reach 100% [60]. The unavoidable interaction between carriers and various intrinsic lattice phonons is the main cause of scattering during transport at room temperature. The probability of phonon scattering of carriers during transport is proportional to $T^{3/2}$, and can be derived from $P_{\text{s}}=\frac{{\epsilon }_{\text{c}}^2{k}_{0}T{\left(m^{\ast }\right)}^2}{\text{π}\rho {\hslash }^4{u}^2}v$, where k₀ is the Boltzmann constant, m^* is the effective mass of the carrier, ρ is the lattice density, u is the velocity of the longitudinal elastic wave, ${\epsilon }_{\text{c}}$ is the deformation potential constant and v is the velocity of the electron [61]. Hence, the probability of carriers being scattered will further decrease after the temperature drops. We extracted the highest responsivity that the photodetector can achieve at different temperatures, and plotted the relationship between responsivity and temperature [Fig.4(c)]. As the temperature drops from 290 K to 80 K, the responsivity increases exponentially, reaching the highest responsivity of 627 A/W at 80 K owing to suppressed phonon scattering and more effective carrier transport at low temperature [17, 59].

Detectivity (D^*) and external quantum efficiency (EQE) are two other key parameters to evaluate the photo-sensitivity of a photodetector, and can be given by equations: $D^{\ast }=\frac{R{S}^{1/2}}{(2e{\text{I}}_{\text{dark}})1/2}$ and $\text{EQE}=\frac{Rhc}{e\lambda }$ [25] respectively, where I_dark is the dark current, e is the electron charge, h is the Plank constant, c is the velocity of light, and λ is the wavelength of incident laser (λ = 405 nm). Fig.4(d) showed the dependence of D^* and EQE on illumination power density with V_ds fixed at 3 V. As can be seen, both of D^* and EQE increase with decreasing of power density. At a power density of 0.47 mW/cm², D^* and EQE are 1.36 × 10¹³ Jones and 1.92 × 10⁵ % respectively, indicating the excellent performance of our PbI₂-based photodetector with ultrashort channel (Tab.1).

Tab.1 Comparison of various figures of merit of different representative PbI2-based photodetectors.

Morphology	Channel width (μm)	Bias (V)	Incident light (nm)	Responsivity (A·W−1)	D* (Jones)	Ref.
Nanosheet	0.06	3	405	172	4.06 × 1012	This work
Nanosheet	20	5	375	0.51	4.0 × l010	[37]
Nanosheet	5	5	450	0.56	−	[38]
Nanosheet	10	5	450	0.04	3.31 × l010	[40]
Nanosheet	80	1.9	450	0.0001	−	[51]
Nanosheet	2	5	470	0.72	1.04 × 1010	[52]
Nanosheet	17	10	405	0.0013	−	[53]
Nanosheet	10	5	405	0.41	3.1 × 1011	[55]
Nanobelt	20	5	445	0.013	−	[39]
Single crystal	20	15	440	11.3	−	[49]
Single crystal	60	10	450	0.18	3.23 × 1011	[54]

4 Conclusion

In a word, the shortening of channel length plays an important role in improving the performance of 2D photodetectors. Based on the electrode array with a spacing as short as 60 nm, PbI₂-based photodetector with ultrashort channel was fabricated. It was found that the photodetector exhibits an ultrahigh responsivity of 172 A/W at room temperature and 627 A/W at 80 K, which is an order of magnitude higher than the highest responsivity of the ever-reported PbI₂-based photodetectors. The improvement of responsivity is mainly due to the ultrashort channel length. The shortening of the channel length significantly reduces the probability of scattering during carrier transport, resulting in improved carrier transfer. Our work heralds new possibilities created by the combination of novel processing techniques and 2D semiconductors, which could guide the development of highly integrated optoelectronic devices in the future.

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Declarations

The authors declare that they have no competing interests and there are no conflicts.

Electronic supplementary materials

The online version contains supplementary material available at https://doi.org/10.1007/s11467-023-1323-1 and https://journal.hep.com.cn/fop/EN/10.1007/s11467-023-1323-1.

Acknowledgements

The work was financially supported by the National Key R&D Program of China (Grant Nos. 2020YFA0308900 and 2022YFB3602801) and the National Natural Science Foundation References of China (Grant No. 92064010).

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