MAPbBr3 single crystal based metal-semiconductor-metal photodetector enhanced by localized surface plasmon

Hybrid organic-inorganic lead halide perovskites (HOIPs) have appealed to researchers on account of excellent optoelectronic properties. Compared with films which possess grain boundaries, HOIPs single crystals with fewer defects behave excellent transport and recombination performances. In the family of HOIPs, single crystals of MAPbX3 (MA = CH3NH3 +, X = Cl, Br or I) are recognized as the most competitive candidates for optoelectronic applications. However, the photodetectors based on MAPbX3 have difficulties in detecting weak signals for lacking of gains without structure optimizations and extra energy transfer channels. In this study, taking advantage of MAPbBr3 single crystal (100) facets, planar metal-semiconductor-metal (MSM) photodetectors were fabricated with Au zigzag electrodes and modified Au nanoparticles (NPs) to realize localized Au surface plasmons (SPs). Compared to device without Au NPs, 2 times enhancement of photocurrent and responsivity have been achieved under 630 nm photon irradiation and 5 V bias. Furthermore, the surface metal structures can inhibit ionic migration to a certain extent. Potential mechanisms of the enhancements and suppressions are discussed in details to reveal the applications of this technique.


Introduction
The hybrid organic-inorganic lead halide perovskites (HOIPs), which chemical formula is ABX 3 , (the A-site is occupied by a small organic cation, (e.g., CH 3 NH 3 + , CH 3 (NH 2 ) + and MA + , FA + for short.), while B-site is a divalent metal (e.g., Ge 2+ , Sn 2+ , or Pb 2+ ) and the X-site is a halogen (e.g., Cl − , Br − , I − ) show extraordinary application potentials in light-emitting diodes (LEDs) [1], lasers [2], photodetectors [3] and solar cells [4,5] on account of their excellent photoelectric characteristics such as direct bandgap [6], long carrier lifetime [7,8], high photon absorption efficiency [9] and outstanding carrier diffusion ability [10]. MAPbBr 3 of the MAPbX 3 family exhibits great optical absorption ability in the visible spectra band [11,12], which makes it highly anticipated in photodetectors [13] and light-emitting devices [14]. Furthermore, in contrast with MAPbI 3 , MAPbBr 3 shows more advantages on account of chemical and thermal stability as preferred choice for optoelectronic applications in visible band. Single crystals have significant advantages in optoelectronic device fabrication due to their lower density of inherent defects, such as surface states, different lattice orientations, and grain boundaries [15][16][17]. With the single crystal showing its wide potential superiorities, such as longer carrier diffusion length and lifetime, several types of single crystal photodetectors have been demonstrated, such as metal-semiconductor-metal (MSM)-type [18], Schottky-type [19], p-n/p-i-n-type [20] photodetectors and plentiful results have been obtained. However, the photodetectors above have difficulties in detecting weak signals for lacking of structure and external optimizations. Although avalanche photodiodes (APD) detectors can realize single photon detection, high requirements of material quality, fabrication arts, high operating voltage and low operating temperature Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
limit their wide applications [21]. In recent years, many approaches have been developed to improve the performances of the devices [22], but high responsivity and external quantum efficiency (EQE) devices are still highly expected in this area. In this situation, SPs has been explored to obtain high-sensitivity photodetectors.
Recently, the technique of SPs has been employed in many areas. For example, biosensing [23], lasers [24], nanophotonics [25], integrated optics [26] and so on. As for photodetectors, plenty of results based on III-V group materials , Silicon , and two-dimensional materials have been reported. For III-V group materials, Tong et al proposed an InAsSb-based hetero-n-i-p photodiode integrated with an Au-based two-dimensional squarelattice hole array structure, which responsivity and detectivity increased about 3 times at 150 mV bias [27]. Qiu et al designed a n-InAsSb/n-GaSb heterodetector integrated with a two-dimensional metallic square-hole array, which photocurrent increased about 2 times at 1.80 μm [28]. For Silicon-based device, Sobhani et al reported an n-doped silicon Schottky photodetector with an Au grating layer. The grating geometry enables around three times larger photocurrent responsivity than pristine device [29]. As a member of two-dimensional materials, graphene was applied to photoconductive type detector. Yao et al showed an end-to-end plasmonic antenna structure fabricated on a graphene sheet, which demonstrated more than 200 times enhancement of responsivity at mid-infrared band compared to devices without antennas [30]. Besides, Molybdenum disulfide (MoS 2 ) was a novel two-dimensional material for optoelectronic devices. Miao et al deposited 4 nm thick Au nanoparticles sparsely onto few-layer MoS 2 phototransistors, and observed two times increase in the photocurrent response [31]. In addition, the SPs enhanced photodetector based HOIPs thin films performances have also been reported [26,[32][33][34][35][36]. while single crystal related research is rarely introduced especially MSMtype photodetector based on it. By deliberate design of device morphology or configuration, photodetectors based on HOIPs single crystal can achieve preferable performance.
In this research, a MAPbBr 3 single crystal based MSM-type photodetector had been successfully fabricated and systematic studies of optoelectronic performances had been conducted, which included photocurrents, responsivities, EQEs and detectivities in different conditions. Our results indicate that the SPs introduce a significant increasement of photocurrents and responsivities under different wavelength irradiation lights, and finally a suppression of ionic migration has also been observed.

Materials and methods
2.1. Synthesis of CH 3 NH 3 Br CH 3 NH 3 Br was synthesized by reacting HBr (40%) with slight excess CH 3 NH 2 solution (40%) in an ice bath in ambient atmosphere. Waiting for reaction about 6 h and then sealing the mixed solutions tightly with temperature of 60°C for 24 h. The crystallization of CH 3 NH 3 Br was achieved by evaporation of water at 60°C, and white CH 3 NH 3 Br powder was gradually precipitated out.

Single crystal growth
MAPbBr 3 single crystals were grown by unchanged temperature method [37,38]. The unchanged temperature method was as follows: dissolving CH 3 NH 3 Br and PbBr 2 (98%) with 1:1 molar ratio in 10 ml DMF solution (99.5%) at room temperature by stirring continually. Then a 0.6 mol L-1 transparent solution was obtained. Next step was adding a small seed crystal into the transparent solution and the solution was kept at 85°C. Large size MAPbBr 3 single crystals were obtained after 1-2 days. Figure 1(a) depicts the schematic diagram of the MAPbBr 3 single crystal growth process. By filtering the depositions from the mixed solution which was placed in constant temperature water bath at 85°C and then evaporating the saturated clarified liquid, spontaneous nucleation of grains was observed. After a few steps of disposing, large crystals of high quality can be obtained. Figure 1(b) shows the photo of a single crystal of MAPbBr 3 with 0.6 cm×0.6 cm size. The corresponding XRD patterns of the MAPbBr 3 powder and single crystal are shown in figure 1(c).

Device fabrications
Planar photodetectors were made on (100) lattice plane of MAPbBr 3 single crystal. The single crystal facet was carefully polished by abrasive paper to reduce surface polycrystallinity, trap-related defects and decomposition which lead to performance reduction of optoelectronic device. The device was manufactured as soon as possible after polishing. During the Au sputtering process, the hollow aluminum contact pattern mask was put on the facet and then Au zigzag electrodes can be obtained under the blank area of the hollow aluminum mask after Au sputtering. The effective light absorption size can be calculated by the 2000 μm side length of device and the 100 μm width and separation distances of the interdigited area.

Characterizations and measurements
X-ray diffraction patterns of MAPbBr 3 single crystal powder were measured by a D/Max2500PC X-ray diffractometer with Cu K α1 irradation, and the scan range was 10°-70°. Surface morphologies were measured using Atomic Force Microscope (AFM) in tapping mode by HORIBA AFM tester. Photoluminesence (PL) spectrum were obtained using IdeaOptics spectrometer with 375 nm continuous laser as excited light source. Time-resolved PL was acquired with a time correlated single photon counting detector (PicoQuant) with 375 nm pulse laser. The optoelectronic properties of photodetector were investigated at ambient temperature and measured by Keithley 4200 A instrument. In order to compare the influence of SPs on optoelectronic properties, the measurements were conducted in varied LD power (at fixed wavelength) and wavelength (at fixed power) respectively to test the pristine or hybrid photodetectors sequentially. The NPs were sputtered overall 2 s in the condition of 8×10 −2 mbar vacuum degree.

Results and discussion
The figures 2(a), (b) show the surface topography of pristine single crystal and corresponding 3D images. The figures 2(d), (e) show the surface topography with randomly dispersed Au NPs of hybrid single crystal and corresponding 3D images, Which the average size is on the order of 100 nanometers. The device morphology and measurements configurations are shown in figures 2(c), (f). Plane MSM-type photodetector was manufactured on (100) facet of MAPbBr 3 single crystal and image of surface appearance is demonstrated in figure 2(c). Au zigzag electrodes can be obtained under the blank area of the hollow aluminum mask after sputtering process. Hence, the hollow aluminum contact pattern mask area was the corresponding light absorbing area of the photodetector. Figure 2(f) illustrates the configurations of photoelectric measurements employing an electrical properties analyzer combined with a probe station. The dark current measurement was investigated in the condition of a shielding box and the photocurrents were obtained under illuminations of two different semiconductor laser diodes (LDs, 532 nm, 630 nm) as excited light sources. All the measurements were performed without and with Au NPs sequentially.
Dark current, photocurrent, responsivity, EQE and detectivity are the key parameters to evaluate the performances of photodetectors. Figure 3(a) shows the dark current and photocurrents obtained by using a 630 nm LD with different irradiation powers respectively. The dark current is as low as nanoamperometer order of magnitude benefitting from the high crystal quality. Under laser illumination, currents exhibit a huge increase in contrast to the dark condition and the photocurrent grow up to 2.5×10 −7 A under 5 V bias voltage with 0.5 mW irradiation power. Photocurrents shown in figure 3(a) increase with the growth of incident light power densities as more photons can excite more electron-hole pairs contributing to the photocurrents. The photocurrent under 5 mW irradiation power to dark current ratio both in 5 V bias is 29.7. Moreover, the increasement trends of photocurrents can be divided by the 0.4 V inflection point. The photocurrents increase sharply when the bias voltage is lower than 0.4 V, which can be ascribed to the exists of Au-MAPbBr 3 Schottky junctions [39]. Nearly-linear curve is obtained when the bias voltage exceeds 0.4 V Which can be conclude to the ohmic region [40]. Additionally, the ion migration is one of the considerable factors to suppress the increase of currents. With regard to halide perovskites, vacancies can get over lower activation energies to migrate in a hopping mode between neighboring positions when the bias increases gradually [41,42] which introduce the cations and anions accumulate near the cathode and anode to generate a built-in electric field impeding the photocurrents increase dramatically. Figure 3(b) depicts the dark current and photocurrents versus bias voltage in different irradiation powers at fixed 630 nm laser with Au NPs. The dark current increases for nearly an order of magnitude in contrast with the forementioned sample and the photocurrents improve simultaneously. For instance, under 5 V voltage bias and 0.5 mW irradiation, the photocurrent reaches 5×10 −7 A and two times enhancement has been obtained compared with pristine sample. Figure 3(b) still illustrates proportional relationship between photocurrents and irradiation power and the photocurrent under 5 mW irradiation power to dark current ratio both in 5 V bias is 6.9. Whereas, the shape of curve is far different from before. Except for conventional ohmic region can be observed at lower voltage region, the I-V characteristics of the single crystal change at V TFL and the currents show an abrupt increase implying the transition into the trap-filled limit (TFL), where the trap states are filled or trapped by charged carriers [42]. The value of inflection point V TFL can be calculated by the following formula: where e is the elementary charge, N traps is the trap density of the crystal, L is the length between the two electrodes, ε and ε 0 stand for the dielectric constant of MAPbBr 3 and the vacuum permittivity respectively. The value of V TFL decreases with the increase of irradiation power can be easily observed from figure 3(b). Increased irradiation power can excite more carriers to fill the traps so that the trap density of the crystal decreases. Moreover, by comparing the growth trend of photocurrents between figures 3(a) and (b) we can find that Au NPs not only enhance the photocurrents response, but also prevents the effect of ion migration on photocurrents. The Au NPs structures located on the surface of crystal suppress ion migration which mainly occurs on the surface of the single crystal. The responsivity R, EQE, and detectivity (D) can be calculated by the following formula: where I ph and I dark are photocurrents and dark current. P irr is irradiation light power density, and S eff is the effective area of the detector, h represents the Planck constant and c is the speed of light. λ indicates the wavelength of incident light. Responsivity refers to the ratio of the output signal of a photodetector to the light power, which is the main parameters of devices performances to evaluate and depict the photoelectric conversion ability of photodetector. EQE presents the ability of the device to convert the number of photons from the incident light to the number of carriers flowing into the external circuit. D represents the detection level of the detector. As the incident laser wavelength is fixed at 630 nm and the irradiation power is varied from 0.1 mW to 0.5 mW, the responsivities and EQEs of single crystal photodetector without or with Au NPs are shown in figure 3(c), d respectively. We can obviously infer that the responsivities and EQEs rely on the bias voltages and illumination powers which are positively correlated with the bias. The shapes of these two illustrations are very similar to the trend of former I-V characteristics. For the detailed numerical results, the highest value of responsivity and EQE in figure 3(c) are 1.7×10 −3 A W −1 and 0.22% respectively under 5 V bias and 0.1 mW. In addition, the highest value of responsivity and EQE with Au NPs in figure 3(d) are 2.4×10 −3 A W −1 and 0.37% respectively, at the same condition. Compared to the maximum responsivity and EQE of MSM photodetector without Au NPs, an enhancement of 41% and 68% has been obtained respectively. By further increasing the irradiation power, both responsivity and EQE start to decrease. Figure 3(e) shows that the D start to decrease when the voltage exceeds 0.5 V which indicates the instability of the device when facing high voltage applied. However, the detector modified with Au NPs shows outstanding stability even when the voltage exceeds 2.5 V exhibiting in figure 3(f). Which also proves implicitly that surface Au structures can improve the detector's stability by suppressing the ionic migration. The relevant data comparison of pristine and hybrid photodetectors are shown in table 1.
In order to study the response of photodetector with Au NPs to different light wavelengths, we used 630 nm and 532 nm laser (all the lasers above were fixed at same output power) to illuminate (100) lattice plane of the device. Figure 4(a) depicts the photocurrents of pristine device under two different wavelength illuminations. The photocurrent response of the device to 630 nm LD shows slightly inferior, which the highest value is about 2.3×10 −7 A at 5 V bias. The orange MAPbBr 3 single crystal inclines to reflect rather than absorbing 630 nm wavelength photons is the reason why the photocurrent under red light irradiation shows insensitivity in contrast with other two wavelengths. The photocurrent illuminated by 532 nm laser is around 2.2×10 −5 A under 5 V bias. Besides, the device under 532 nm laser irradiation performs higher current response than other laser is due to the resonance absorption as the photon energy consistent with the value of E g of MAPbBr 3 single crystal reported before [38]. Furthermore, we can see the increase rates of the two curves are still limited by the ion migration. Figure 4(b) reveals the photocurrents response of the device with Au NPs under two different wavelengths. Compared with figure 4(a), the photocurrents show distinct increasement tendency with   The photocurrents enhancement in device are primarily ascribed to the electrical effects of Au SPs play a pivotal part in promoting the charge extraction and transport. The pristine MAPbBr 3 single crystal exhibited a PL peak at around 532 nm with the excitation wavelength of 375 nm. With the introduction of Au NPs, drastic PL quenching was observed as presented in figure 5(a), implying that the excited electrons in MAPbBr 3 probably transfer to Au NPs by reducing the charge recombination [43]. The decrease in PL lifetime for the hybrid single crystal also validates the effective electron transfer process between perovskite and Au NPs as proved by timeresolved PL measurements ( figure 5(b)). Notably, when Au NPs are directly attached to the Au electrodes and contacted with the perovskite, the photon-generated carriers in the perovskite can be extracted to the Au electrodes through the Au NPs under illumination, causing the carrier collection efficiency increased [44].

Conclusions
In summary, we have investigated the enhancement of performances on planar MSM photodetectors based on MAPbBr 3 single crystal with sputtered Au NPs by measuring and calculating photocurrent, responsivity, EQE and detectivity. By comparing with the device without Au NPs using different incident light powers or wavelengths, photocurrents, responsivities, and EQEs all displayed a certain of improvements. Additionally, the Au SPs not only enhanced the performances parameters, but also suppress the ion migration of the MAPbBr 3 crystals by reducing the trap states. This phenomenon is particularly evident for device with NPs under 630 nm wavelength light sources. The study on the perovskite with SPs establish a scope for developing further fundamental knowledge and practical applications of perovskite-based especially single crystal optoelectronic and electronic devices.