Broadband Detection Based on 2D Bi₂Se₃/ZnO Nanowire Heterojunction

Abstract

The investigation of photodetectors with broadband response and high responsivity is essential. Zinc Oxide (ZnO) nanowire has the potential of application in photodetectors, owing to the great optoelectrical property and good stability in the atmosphere. However, due to a large number of nonradiative centers at interface and the capture of surface state electrons, the photocurrent of ZnO based photodetectors is still low. In this work, 2D Bi₂Se₃/ZnO NWAs heterojunction with type-I band alignment is established. This heterojunction device shows not only an enhanced photoresponsivity of 0.15 A/W at 377 nm three times of the bare ZnO nanowire (0.046 A/W), but also a broadband photoresponse from UV to near infrared region has been achieved. These results indicate that the Bi₂Se₃/ZnO NWAs type-I heterojunction is an ideal photodetector in broadband detection.

1. Introduction

Over the past decades, photodetectors have been extensively used in both military and civil fields, such as living cell inspection [1], night vision [2], optical communications [3], atmospheric [4], etc. [5,6,7,8]. Photodetectors have become indispensable in daily life. Among these semiconductor materials for making photodetectors, ZnO has the potential of application in UV photodetectors. Owing to the great optoelectrical property and good stability in the atmosphere, ZnO is widely researched with the band gap of 3.37 eV, of which brought by high exciton binding energy (60 meV) at room temperature, noise interference is suppressed. In addition, the merits of being low-cost, non-toxic and easy to prepare make it more attractive to be investigated [9,10,11]. Various morphologies of ZnO have been prepared during years of research, such as nanoparticle, nanowire, nanotube, nanofilm, and nanosheet [12,13,14,15]. The morphology with different dimensions has different characteristics. With the morphology of one-dimensional nanowire arrays (NWAs), not only the space between the nanowire is well backed up for the strengthening of light trapping ability but also the superior transport properties provide the fast electron transport channel when making comparisons with its bulk and thin-film structures [16,17].

However, until now, the photoresponse performance of the ZnO nanowires UV detector is still lower than the theoretical predicted value, due to a large number of nonradiative centers at interface and electron trapped by the surface states [18].

Many methods have been used to optimize the performance of the ZnO nanowires UV photodetector (UVPD). Yang et al. reported photocurrent enhanced through optimizing ZnO seed layer growth condition [19]. Research of Kim et al. shows that NiO/Ni coated ZnO NWs reveal raised D0X transition due to the increasing oxygen deficiency which is responsible for increasing donor density [20]. Zhang et al. reported a two-dimensional graphene/ZnO nanowire mixed-dimensional van der Waals heterostructure for high-performance photosensing [21]. Zang et al. reported enhancing photoresponse based on ZnO nanoparticles decorated CsPbBr₃ films [22]. Sumesh et al. reported broadband and highly sensitive photodetector based on ZnO/WS₂ heterojunction [23]. So far, more and more two-dimensional materials/ZnO nanowires mixed dimensional heterojunctions have been reported [24,25,26,27].

Up to now, as the discovery of the unique characteristic of topological insulators (TI), further attention is paid to TI to fabricate photodetectors [28,29,30,31]. Taking advantage of Dirac dispersion and spin-momentum locking property brought by 2D surface electrons and time-reversal symmetry, back scattering in the Dirac fermions caused by nonmagnetic impurities is prevented, which enable the outstanding transport characteristics, thus reducing the dark current to obtain higher performance [32]. With direct band gap of 0.3 eV and weak Van der Waals’ force between each two layers, Bi₂Se₃ has very infusive photoelectric properties, such as tunable surface bandgap, polarization-sensitive photocurrent, and thickness dependent optical absorption [33,34]. These special properties make Bi₂Se₃ promise for binding with ZnO to build high performance photodetector in the UV and visible region. In contrast, there are scarcely any reports about 2D Bi₂Se₃/ZnO nanowire mixed-dimensional detectors.

In this work, hydrothermal synthesized ZnO NWAs is composite with 2D Bi₂Se₃. The broadband photodetector based on Bi₂Se₃/ZnO NWAs heterojunction photodetector is fabricated. Enhanced UV to visible responsivity is realized in the Bi₂Se₃/ZnO NWAs heterojunction photodetector. The mechanism of enhanced response in Bi₂Se₃/ZnO NWAs heterojunction is dealt with in detail through thorough inspections of photoelectric response characteristic combined with Raman scattering measurements, optical properties, and band gap structure.

2. Materials and Methods

2.1. Syntheses of ZnO NWAs

ZnO NWAs were synthesized via a simple hydrothermal method with buffer layer sputtered on fluorine-doped tin oxide (FTO) glass. A thin buffer layer was deposited with RF magnetron sputtering, during which process, the O₂:Ar flow ratio was controlled at 18:42 in 1 Pa under room temperature for 5 min, after which, it was annealed at 400 °C for 1 h. On the basis of buffer layer, an ordinary hydrothermal method was applied for the synthesis of ZnO nanowire. Zn(AC)₂·6H₂O and C₆H₁₂N₄ (HMT)with an equal amount of 0.0009 mol were added to 30 mL deionized water and stirred for 5 min, respectively. Subsequently, the two aqueous solutions were mixed and stirred for 5 min. Then, the obtained solution was sealed in autocave at 90 °C for 4 h. Afterwards, synthesized sample was washed with deionized water several times and dried in the air. In this way, ZnO NWAs was prepared.

2.2. Syntheses of 2D Bi₂Se₃

2D Bi₂Se₃ is provided in SixCarbon Technology Shenzhen.

2D Bi₂Se₃ was synthesized via a standard chemical vaporous deposition method. The reactions were conducted in a tube furnace with dual heating zone. 0.1 g 99.995% Bi₂O₃ (Bi₂O₃, 6CARBON, Shenzhen, Guangzhou, China) powder was placed in the high temperature zone to heat up to 700 degrees. 0.5 g 99.999% purity Se (Se, 6CARBON, Shenzhen, Guangzhou, China) particles were placed in the low temperature zone to heat up to 300 degrees. Under mixed carrier gas of argon and hydrogen at flow rates of 200 sccm and 15 sccm, respectively, the clean sapphire film placed 5 cm below Bi₂O₃ was heated to 500 degrees for 15 min.

2.3. The Transfer of 2D Bi₂Se₃

Methyl methacrylate (PMMA) was coated on obtained 2D Bi₂Se₃. After curing, PMMA was placed in pure water and heated to 90 degrees for 1 h. Then, it was quickly plunged into ice water to separate Bi₂Se₃ which attached to PMMA from sapphire base. Then, the film was placed on ZnO NWAs and finally PMMA was removed with acetone to obtain the transferred Bi₂Se₃ film.

2.4. Characterization

The morphologies of ZnO NWAs and Bi₂Se₃ film were conducted on a field-emission scanning electron microscopy (FE-SEM, ZEISS Merlin Compact, Oberkochen, Germany). Surface morphologies of Bi₂Se₃ film were also characterized by atomic force microscopy (AFM, Dimension Fastscan, Bruker, Billerica, MA, USA). Raman were carried out on (LabRAM HR Evolution, Horiba, Paris, France) with an excitation wavelength of 532 nm. Photoluminescence (PL) spectrums were recorded in CCD using the same instrument system with Raman by He–Cd laser line of 325 nm with fixed excitation intensity at room temperature. Both measurement of PL and Raman use the same equipment. The responsivity of samples was investigated by Zolix responsivity measurement system (DSR600, Zolix, Beijing, China), which calibrated via standard silicon cells. The spectral responsivity was measured in terms of the current signal within the range of 300–1000 nm at 4 V bias under room temperature.

3. Results

Figure 1a shows the dramatic structure of the photodetector. Bi₂Se₃ is transferred onto the ZnO nanowires and then onto a steam plate with platinum electrodes. The morphologies of ZnO NWAs and 2D Bi₂Se₃ were further confirmed through SEM and AFM exhibited in Figures S1 and S2 (Supplementary Materials). Due to stress and other reasons, the Bi₂Se₃ split into several pieces after the transfer. The photodetector is made by the standard semiconductor fabrication techniques. The interdigital metal electrodes, which are defined on a 300 nm Pt layer by the conventional UV photolithography and lift-off procedure, are 0.5 mm long and 300 μm wide, with a 200 μm gap. There were 10 fingers in our interdigital structure, 5 up and 5 down [35], and Figure 1b is a physical image of the detector.

As a widely used non-damaged measurement, Raman spectroscopy could be used to investigate intralayer vibration modes, interlayer vibration modes, and the layer coupling in 2D materials effectively [36,37]. Figure 2 showed Raman curves of ZnO NWs, 2D Bi₂Se₃ and Bi₂Se₃/ZnO NWAs. Ascribed to the perpendicular laser incident direction relative to the c-axis of sample surface, there were only two peaks located in 100 cm⁻¹ and 438 cm⁻¹ corresponding to $E_2^{low}$ mode and $E_2^{high}$ mode of ZnO NWAs in curve (a), respectively, in which the strong $E_2^{high}$ phonon mode peak was observed, indicating the good crystalline quality [38,39,40]. Located at 72.8 cm⁻¹, 133.3 cm⁻¹ and 174.6 cm⁻¹, three peaks observed in curve (b) could be assigned to $A_{1g}^1$, $E_g^2$ and $A_{1g}^2$ vibrational mode in turn for Bi₂Se₃ [41]. A1g modes were out of plane vibrations and Eg modes were vibrations in-plane [42]. There were two peaks of ZnO NWAs and one peak of Bi₂Se₃ making their appearance in curve (c), exhibiting the consistent material nature after compositing.

Furthermore, it can be seen in Figure 2, after being transferred onto the ZnO nanowires, the Bi₂Se₃ mode slightly shifts from 133.3 cm⁻¹ to the lower frequency 132 cm⁻¹ corresponding to the 2D Bi₂Se₃. This can be ascribed to the effect of residual stress [43]. The magnitude of the stress can be given by the following formula ε = Δω/χ, where χ is the shift rates of Raman vibrational modes. Based on the previous literature reports, the shift rates of $E_g^2$ mode under biaxial strain is ∼5.2 cm⁻¹ per % strain [43,44]. Therefore, the stress in the heterojunction should be 0.25%.

Since the photoluminescence characteristic could give an index to the degree of electron and hole recombination of different materials, PL test was carried out on ZnO NWAs and Bi₂Se₃/ZnO NWAs to find out the change after the heterojunction forming in Figure 3. The sharp emission peak of ZnO NWAs with high intensity at 377 nm derived from near band-edge emissions [45]. In contrast, caused by oxygen vacancies [46] and chemisorbed O₂ in the air [47,48], the peak emerging at the visible region is relatively weak, suggesting the high crystal quality. The PL spectrum of Bi₂Se₃/ZnO NWAs presents an obvious decrease in near band-edge emissions peak in ZnO NWAs, indicating the weakened electron hole recombination. The anomalous decreased PL emission mainly originated from the more efficient separation of photogenerated carriers at the strain-tailored heterointerfaces [43]. In the test of photoluminescence spectrum, we found that ZnO interband luminescence peaks also moved significantly, from 377 nm of simple ZnO nanowires to 378 nm of heterojunctions, which was caused by the band changes brought by stress, according to the relevant literature reports [43]. In the visible light part, the luminous emission peak shifts from 550 nm to 510 nm. It is believed that the radiant peak at 510 nm could be ascribed to the emission of Bi₂Se₃ [49]. Furthermore, it can be found that after composition with Bi₂Se₃, the luminescence in the visible region is significantly enhanced, combining with the analysis of the Bi₂Se₃/ZnO NWAs heterojunction energy band structure below. It can be deduced that Bi₂Se₃ and ZnO form a type-I band structure, leading to the transfer of photogenerated electrons from ZnO to Bi₂Se₃. This electron transport process results in the reduction of ZnO emission and the enhancement of Bi₂Se₃ emission. This will be discussed in detail in the energy band section.

To assess the photoresponse performance of Bi₂Se₃/ZnO NWAs heterojunction, Pt interdigital electrode was sputtered with a mask to fabricate photodetector, as depicted in Figure 1. Responsivity performance is measured as shown in Figure 4. The spectral responsivity was measured in terms of the current signal in the range of 300–1000 nm at 4 V bias. All the samples revealed UV photoresponse with a cutoff wavelength of 365 nm. This can be attributed to the response of ZnO nanowires. After being transferred with the Bi₂Se₃, the response of ZnO nanowires enhanced from 0.046 AW⁻¹ to 0.15 AW⁻¹, three times higher than the bare ZnO nanowires. Furthermore, the photocurrent of Bi₂Se₃/ZnO NWAs at visible and near-infrared regions is also increased compared with bare ZnO NWAs. The spectral responses in the visible and near infrared regions are from Bi₂Se₃, corresponding to the band gap and PL spectrum of Bi₂Se₃ [50]. The performance of ordinary photodetectors constructed by nanowire and transferred 2D material could be restricted to a large extent. It is difficult to obtain effective responsivity promotion for them as the result of high interface impedance brought by impurity introduced during transfer and the limited contact area between materials of one and two dimensions. Therefore, the performance could be restricted to a large extent. In contrast, the threefold increase responsiveness of the Bi₂Se₃/ZnO NWAs photodetector shows it is superior to similar detectors owing to the repressed back scattering caused by the intrinsic characteristic of TI [32,51]. In addition, the spectral test shows that the heterojunction not only significantly improves the photoelectric response in the ultraviolet region but also has good spectral response in the visible to near-infrared region, indicating that the heterojunction device has the ability to prepare wide-spectrum detectors.

For ZnO based optoelectronic devices, oxygen molecules adsorbed on the surface of the ZnO nanowires capture the free electrons, due to the surface adsorption and desorption. Therefore, a depletion layer with low conductivity is created near the surface of the film, and results in a reduced photocurrent [52,53].

In order to study the enhance response mechanism of Bi₂Se₃/ZnO NWAs heterojunctions, the energy band structure of heterojunctions is analyzed. For the heterojunction, band alignment also plays a crucial role in the spectral response characteristics [54].

The band structure of Bi₂Se₃/ZnO can be deduced from the Anderson model shown in Figure 5 [55,56]. Based on previously reported energy band data of Bi₂Se₃ and ZnO [33,34,43,49,50,52,57,58], the conduction band offset (CBO) of the Bi₂Se₃/ZnO heterojunction is worked out to be 1.67 ± 0.15 eV. With this band alignment, electrons can easily drift from ZnO to the Bi₂Se₃. As a result, the recombination of electrons and holes may mainly take place on the Bi₂Se₃ side rather than in ZnO nanowires. This will mitigate the impact of the ZnO surface adsorption and desorption. Hence, at the same time of leading to an increase in the UV current of the Bi₂Se₃/ZnO NWAs heterojunction, the drifting of electrons also results in a decrease of ZnO emission in the UV region. In this way, the realization of Bi₂Se₃/ZnO NWAs heterojunction photodetectors makes responsivity of Bi₂Se₃/ZnO NWAs improve noticeably in the UV region, however, the absorption range broadened effectively.

4. Conclusions

In summary, this work presents a novel broadband heterojunction photodetector based on 2D Bi₂Se₃/ZnO NWAs. Combining the merit of 2D Bi₂Se₃ and ZnO, the responsivity spectrum of the photodetector exhibited the improved responsivity covering UV-Visible-Near Infrared range. The properties of a responsivity of 0.15 A/W, nearly three times to the bare ZnO, benefit from unique advantages of type-I band alignments. More importantly, the special heterojunction photodetector with type-I energy band structure injects a large number of electrons into the ultrathin 2D material to enhance responsivity of Visible-Near Infrared wavelength while exporting electrons to the electrode rapidly to avoid electron and hole recombination, making it a promising and yet-to-be-extended way for novel exploration for more applications.