CHEN Hong-yu, SHANG Hui-ming, DAI Ming-jin, WANG Yue-fei, LI Bing-sheng, HU Ping-an*

(1. Department of Physics, Harbin Institude of Technology, Harbin 150080, China; 2. School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150080, China; 3. Key Laboratory of Micro-systems and Micro-structures Manufacturing, Ministry of Education, Harbin Institude of Technology, Harbin 150080, China)*Corresponding Authors, E-mail: chenhy@hit.edu.cn; hupa@hit.edu.cn

Abstract: To realize photodetection ranging from UV to visible region with high responsivity and low cost, a novel broadband photodetector based on mixed-dimensional van der Waals (vdW) heterojunction comprising a two dimensional (2D) n-type InSe nanosheet and a p-type Se microwire is proposed. Benefiting from the high crystal micrometer-sized Se microwire and two dimensional InSe nanosheet, the device exhibits a high responsivity ranging from UV to visible region with a sharp cutoff at 700 nm. It is worth pointing out that the responsivity of the device could reach up to 108 mA/W at 460 nm at -5 V. The responsivity is 800% larger than that of pristine Se device. These investigations will broaden our fundamental knowledge of vdW heterostructures, which would open additional opportunities for fabricating low dimensional photodetectors with high performance.

Key words: semiconductors; van der Waals heterojunction; photodetector; multilayer structure

1 Introduction

Inspired by nanoscience, integration of photodetectors with low dimensional semiconductor exhibited tremendous application potentials in air-pollution/waste-water monitoring, forest fire alarm, space secure communication, and medical care,etc[1-5]. Up to now, photodetectors constructed by thin-film or bulk materials, photodetectors including low dimensional building blocks (e.g. quantum dots, micro/nanowires core/shell micro/nanostructures, and nanostructure arrays,etc.) have been explored extensively[6-8]. Compared with traditional photodetectors, devices based on nanostructured semiconductors usually have an advantage of higher responsivity owing to their high surface-area-to-volume ratios and the reduced dimension of effective conductive channel[9]. More importantly, the newfound two dimensional semiconductors with high carrier mobility, ideal bandgap, and chemical stability provided us additional opportunity to fabricate photodetectors with high performance or multifunction[10-12]. And numerous photodetectors based on van der Waals hetero-structures with varying functionalities have been constructed, which overcomes the bottleneck in conventional three dimensional semiconductors[13-15]. Firstly, as an alternative optoelectronics platform, the mixed-dimensional heterostructure usually obtained a larger light-harvesting cross section than that of the individual one. Secondly, owing to the ultrathin 2D materials could be transferred to any other substrate without consideration of crystal lattice mismatch, the preparation process of device would be simpler. Thirdly, due to the interlayer van der Waals bonding without any dangling bonds, trap states plagued traditional semiconductors are often absent in 2D layered crystals[16-18]. Therefore, it would be greatly advantageous to explore novel mix-dimensional heterojunctionsviaa simple and low-cost strategy and high performance.

In this paper, a novel broadband photodetector with a high performance was realized by a facile combination of semicrowire and InSe nanosheet based on our previous workes[19-20]. As an excellent p-type semiconductor, Se exhibits many unique properties, such as low melting point(220 ℃), high conductivity(8×104S·cm-1), and high intrinsic carrier concentration(9.35×1016cm-3). Moreover, the band gap of Se is about 1.67 eV, indicating that Se is an appropriate candidate for fabricating photodetectors with a broadband responsivity ranging from UV to visible region[19,21].At the same time, as an excellent rising star of 2D layered semiconductor, n-type InSe with a direct band gap(Eg=1.26 eV) exhibits great potential to fabricate photodetectors. As an example, electron effective mass of InSe(m*=0.143m0) is smaller than that of MoS2(m*=0.45m0), and the electronic devices based on InSe few-layers exhibit high mobility(103cm2·V-1·s-1) at room temperature[20,22]. Therefore, in conjunction the optical and electronic properties of aforementioned materials, a high performance broadband photodetectors based on InSe/Se mix-dimensional van der Waals heterojunction was successfully constructed. The device could realize highly efficient with high on-off switching ratios, excellent wavelength dependence, and good stability. Simply, the responsivity of the device could reach up to 108 mA·W-1at 460 nm with a fast speed at -5 V. In addition, the device also displays an extremely low dark current of -0.2 pA at -5 V. Thus, these investigations would provide additional pathway to fabricate photodetectors by employing novel mixed-dimensional van der Waals heterostructures.

2 Experiments

Device fabrication: the synthesis of Se microwire was carried out in a horizontal tube furnaceviavapor transport and deposition process. High-purity Se (>99.95%) powers were used as precursors. High-purity(6N) nitrogen with a constant flow of 300 mL·min-1was used as the carrier gas. Keep the temperature of 300 ℃ for 720 min, InSe crystals were prepared by a Bridgeman method, and then the InSe nanosheets were obtained by mechanical exfoliation as described in our previous papers. The selenium powder and indium particles were put into the quartz boat with mole ratio of 1∶1.1, keep the temperature of 660 ℃ for 60 min with 13/7 mL·min-1Ar/H2mixed gas. At the same time, SiO2/Si substrate was cleaned with isopropanol, acetone, ethanol and deionized water. Subsequently, after transferring Se microwire and InSe nanosheet onto the suface of SiO2/Si substrate, In electrodes were deposited by thermal evaporation characterization. Morphologies of the sample were characterized using SEM(Hitachi S-4200). EDX was used to determine the composition of InSe nanosheets. The structure of the samples was studied by a TEM(Tacnai-G2 F30), XRD(DIFFRACTOMETER-6000), and Raman spectroscopy(LabRAM XploRA, laser wavelength is 638 nm). The electrical and optoelectronic performances were analyzed with a xenon lamp, monochromator(Zolix, Omni-λ300i), and a semiconductor characterization system(Keithley, 4200).

3 Results and Discussion

Fig.1 (a) Optical image of Se microwire. (b) SEM image of Se microwire and EDX mapping of Se elements. (c) XRD spectrum of Se microwire.

Fig.2 (a) SEM image of InSe nanosheets and EDX mapping of In and Se elements. (b) XRD spectrum of InSe nanosheets. (c) HRTEM image for InSe nanosheets. Inset is SAED pattern with an orientation along the [001] zone axis. (d) Raman spectrum of the InSe nanosheets. (e) Side and top views of the InSe crystal structure.

After transferring the t-Se microwire and multilayer β-InSe nanosheets to a SiO2/Si substrate by using the methods described previously[24], electrodes were fabricated onto the t-Se microwire and multilayer β-InSe nanosheets, and then a facile p-n junction diode was fabricated. Fig.3(a) shows the schematic diagram of mix-dimensional heterojunction photodetector based on 2D layered InSe and 1D Se microwire. To explore the electrical properties of the device,I-Vcurves were characterized. As displayed in Fig.3(b) and (c), compared to theI-Vcurves of p-Se device, the current of InSe/Se p-n junction exhibits a significant rectification characteristic. The typical photodiode behavior demonstrated that the rectifying behavior comes from the p-n junction instead of the metal-semiconductor contacts. It is worth noting that the hybrid InSe/Se p-n junction photodetector possessed a very low dark current of -0.13 pA even under a bias of -1 V, which is comparable to that of previous best results of low dimensional photodetectors.

The responsivity(Rλ) is an important parameter to evaluate the photoelectric conversion capacity of a photodetector. It can be given by the following equations:

Fig.3 (a) Schematic diagram of InSe/Se mix-dimensional heterojunction photodetector. (b), (c)I-Vcharacteristics of single Se microwire and InSe/Se heterojunction photodetector.

Rλ=Iph/P,

(1)

Iph=Il-Id,

(2)

whereIphis the photocurrent,Idis the dark current,Ilis the light current,Pis the light power,λis the exciting wavelength. To further explore its working wavelength range, the spectra responsivity ranging from 300 to 800 nm was carried out at -5 V bias.

Fig.4 (a) Spectra responsivity of InSe/Se and pure Se devices with irradiance wavelength ranging from 300 to 800 nm under -5 V. (b) Responsivity selectively enhancement of InSe/Se with illumination wavelength ranging from 300 to 800 nm.

As shown in Fig.4(a), responsivities of the mix-dimensional Se/InSe p-n junction and pure Se photodetector are obtained in a wide wavelength range from UV to visible region. The cutoff wavelength of both devices are estimated to be around 700 nm, which agrees quite well with previous Se and InSe photodetectors[19,20,25]. It is worth pointing out that the responsivity of InSe/Se p-n could arrive up to 108 mA·W-1at 460 nm. This value is 8 times larger than that of pristine Se photodetector (Fig.4(b)). The detectivity(D*) is another important figure-of-merits for a photodetector, which reects the ability of the device to detect weak signals from the noise environment. By assuming the shot noise from the dark current is the major contributor, it can be calculated as:

(3)

whereRis the responsivity,qis the electronic charge,Sis the active area. And the calculated results(D*) is as high as 1.5×1011Jones at the wavelength of 460 nm, which is 8 times than that of pure Se photodetector. In addition, value is within an this order of magnitude of theD*for commercial Ge photodetectors[26].

Fig.5 (a) Time-dependent photocurrent response at different light intensities of device at -5 V. (b) Photocurrent of Se/InSe p-n junction device as a function of light intensity at -5 V, the value of the current is in its absolute value form. (c) Schematic illustration of energy levels of InSe/Se heterojunction and charge-transfer process under light illumination.

In addition, the illumination intensity-dependence of the photocurrent was measured under a visible light (400 nm) with the irradiances ranging from 0.03 to 2.76 mW·cm-2. As shown in Fig.5(a), the photocurrent is steadily increasing with the increased light intensities, giving photocurrent values of 2.8×10-2nA at 0.03 mW·cm-2, 8.6×10-2nA at 0.14 mW·cm-2, 1.7×10-1nA at 0.28 mW·cm-2, 6.5×10-1nA at 1.38 mW·cm-2, and 1.1 nA at 2.78 mW·cm-2. This observation is consistent with the fact that the photoelectric conversion efficiency is associated with the absorbed photonux. According to previous reports, the photocurrent can be expressed by a simple power law：

I=APθ,

(4)

whereAis a constant for a certain wavelength, and the exponent (0.5<θ<1) determines the response of the photocurrent to light intensity[27]. By fitting the curves in Fig.5(b) with this equation, the values ofθare calculated to be 0.78 at the wavelengths of 400 nm with an applied voltage of -5 V. This fractional power dependence is likely to be related to the complex processes of electron-hole generation, recombination, and trapping within the p-n junction device[28-29].

To demonstrate the operation mechanism of the mix-dimensional p-n heterojunction structured photodetector, the energy band diagram of Se and InSe is proposed and schematically presented in Fig.5(c). The electron affinities Se and InSe are 3.2 and 4.02 eV, and the corresponding band gaps are 1.7 and 1.26 eV, respectively. After transferring the InSe nanosheet on the surface of Se microwire, the built-in electric field is formed at the interface as a type-Ⅱ heterojunction. Under light illumination, photogenerated electrons and holes could be quickly separated by the built-in field. And the holes would transfer to the valence band of Se, and the electrons in the conduction band of InSe would be collected by electrode, leading to the formation of photocurrent for broadband photodetection.

4 Conclusion

In summary, a novel mix-dimensional heterojunction based on 2D n-type InSe and 1D p-type Se is facilely constructed into a high performance broadband photodetector. Owing to the excellent type-Ⅱ p-n heterojunction formed at the interface of InSe/Se with the conduction band offset (ΔEC) of 0.82 eV and the valence band offset (ΔEV) of 0.38 eV under thermal equilibrium condition, the device exhibited excellent photovoltaic properties in the UV-visible region. The responsivity of the device could reach up to 108 mA·W-1at the wavelength of 460 nm at -5 V, which is 800% larger than that of pristine Se devices. In addition, the device exhibits a very low dark current of -0.2 pA at -5 V. The device possesses a large detectivity of 1011Jones in a broadband region. These results demonstrate that a new approach is established to fabricate controllable, high-performance, and cost efficiency photodetector based on the mix-dimensional semiconductors.