A Stable UV Photodetector Based on n-ZnS/p-CuSCN Nanofilm with High On/Off Ratio

WEI Yao-qi，QUAN Jia-le，ZHAO Qing-qiang，ZHOU Ming-chen，HAN San-can

Abstract：Herein，We fabricated a CuSCN nanofilm ultraviolet（UV）photodetector（PD）using an in situ growth method.When the bias is -1 V and the incident light is 350 nm，the on/off ratio of the CuSCN PD is～94，and the rise/decay time is～1.41 s/1.44 s.However，such a device still cannot be called a high-performance photodetector.To improve the optoelectronic properties of CuSCN nanofilm further，we fabricated a UV photodetector based on n-ZnS/p-CuSCN composite nanofilm and analyzed its morphology，composition，and properties.The photocurrent and dark current of the ZnS/CuSCN UV photodetectors are 1.22×10-5 A and 4.8×10-9 A，respectively（at -1 V，350 nm）.The ZnS/CuSCN nanofilms' on/off ratio of～2 542 and rise/decay time is 0.47 s/0.48 s.Besides，the n-ZnS/p-CuSCN nanofilm UV PDs have the best responsivity and detectivity at 350 nm with 5.17 mA/W and 1.32 × 1011 Jones，respectively.In addition，the n-ZnS/p-CuSCN composite film is stable at room temperature，which indicates its great potential as a high-performance UV photodetector.

Key words：photodetector；p-n junction；ZnS/CuSCN；on/off ratio

1 Introduction

Photodetectors are electronic devices that convert light signals into electrical signals and play an essential role in human life[1-2].In particular,ultraviolet（UV）photodetectors（PDs）are widely used in optical communication,biosensing,photoelectric detection,and environmental monitoring[3-5].It is commonly accepted that an excellent photodetector should have high sensitivity,high signal-to-noise ratio,high spectral selectivity,high speed,and high stability[6].

Several general wide bandgap semiconductors have been used for UV PDs in the past decade[7-8],such as GaN[9],ZnO[10],In2O3[11],CuSCN[12],etc.Especially,CuSCN is a broadband transparent p-type semiconductor with a bandgap of about 3.7 eV with excellent UV selectivity[13].Many CuSCN nanofilm photodetectors have been reported to have high UV sensitivity and stability.Yanget al.[14]and Kaet al.[15]assembled a CuSCN-based UV PD with response/recovery time of 150 μs/30 μs and 6.7 μs/0.95 ms,respectively.However,the on/off ratio is not as good as expected,at 200 and 512.In recent years,Yanet al.[16]constructed a Cs2AgBiBr6/CuSCN photodetector with a high detectivity（1.03×1013Jones）and high photocurrent（7×10-7A）,but the on/off ratio is still not ideal.

This paper has obtained CuSCN nanofilm by anin situgrowth method.However,the rise/decay time（1.41 s/1.44 s）and the on/off ratio（～94）of the PD based on CuSCN are not ideal due to surface defects and crystallinity.Researchers have proposed several approaches to solve the above problems,including the introduction of Scotty junctions,heterojunctions[17-18],surface modifications[19],and size optimization[20].Composites often exhibit superior performance over single materials due to heterojunctions and synergistic effects between materials[21-23].Therefore,to obtain a high-performance CuSCN-based UV PD,we prefer an n-type semiconductor with the highest absorbance in the UV region as the composite.

Zinc sulfide（ZnS）,a highly crystalline n-type semiconductor material,has good electrical and optical properties and is widely used in optoelectronic devices[24-27].The bandgap of ZnS is about 3.6 eV[28],indicating its excellent UV selectivity and potential material for improving CuSCN UV PD's performance.Many ZnS PDs with different nanostructures have been demonstrated to exhibit excellent properties,such as thin films[29],nanowires[30],nanoarrays[31],quantum dots[32],etc.Reports on ZnS photodetectors show that responsibility of 0.1 mA/W and a rise/decay time of 0.35 s/0.07 s under UV irradiation[33].Therefore,ZnS is a promising n-type material to build p-n junctions with CuSCN.In particular,the built-in electric field in the vicinity of the PN junction effectively promotes the separation of the current carrier[34-35].The composites are expected to have faster responses and higher photocurrents,enabling high-performance photodetectors.

This work obtainedin situgrowth CuSCN nanofilm PDs（on/off ratio～94,rise/decay time～1.41 s/1.44 s）.Subsequently,we fabricated a ZnS/CuSCN nanofilm photodetector with a high on/off ratio and stability based on the CuSCN nanofilm.Ultraviolet optoelectronic properties of PDs based on composite films have been improved.The on/off ratio of the ZnS/CuSCN PD is～2 542,while the rise/decay time is 0.47 s/0.48 s（at -1 V,350 nm）.In addition,the responsivity and detection of the device are 5.17 mA/cm2and 1.32 × 1011Jones,respectively.Therefore,ZnS/CuSCN device is expected to be a candidate for high-performance UV photodetectors.

2 Experiment

2.1 Synthesis of CuSCN Films

A copper foil（99.99% purity,1 cm×1 cm）was cleaned in 0.1 mol/L HCl and then rinsed by ethanol and DIW,followed by air blow-drying.Then it was immersed in an alkaline solution for a while.Next,the sample was annealed at 500 ℃for 1 h and kept for 1 h to obtain Cu2O nanofilm.Used a constant flow of N2（99.99%）with 80 mL/min as protecting and carrier gas through the entire process.Finally,the Cu2O was added to the KSCN solution（5 mmol/L）for 72 h at room temperature.

2.2 Synthesis of ZnS

（NH4）2S aqueous solution（3.5 mL,8%）was stirred at 60 ℃.Then 5 mmol Zn（NO3）2·6H2O was dissolved in（NH4）2S for 2 h,excess ethanol was added to the obtained solution,the product was collected by centrifugation at 10 000 r/min for 20 min,dried the ZnS powder in a high temperature drying oven for 2 h.

2.3 Fabrication of ZnS/CuSCN Films

The ZnS powder was dissolved in N,N-dimethylformamide（0.01 mol/L）and applied drops onto the surface of CuSCN film.

2.4 Characterization

Morphological electron microscopy（SEM,FEI Quanta 450FEG）of the samples was analyzed by scanning method.Transmission electron microscopy（TEM）,high-resolution transmission electron microscopy（HRTEM）,and high angle annular darkfield（HAADF）were also performed for transmission electron microscopy（Tecnai G2 F30）.The phase structures were investigated by using X-ray diffraction（XRD,D8-Advance/Germany）.In addition,EDS spectra of scanning transmission electron microscopy（STEM）mode were also used to study the distribution of elements.XPS was performed by a spectrometer（Thermo Scientific K-Alpha）.Raman spectroscopy was tested on a LabRAM HR Evolution spectrometer with a 532 nm laser.UV-Vis was characterized by a UV-Vis diffuse reflectance spectrometer（UV-3600）.The optoelectronic properties of the devices are described by a semiconductor characterization system（Keithley 4200-SCS）.

3 Results and Discussion

The XRD patterns of ZnS and ZnS/CuSCN composites are shown in Fig.1（a）.Diffraction peaks appearing at 16°,27.1°,32.5°,34.4°,50.1°,58.5°are caused by（006）,（012）,（0012）,（018）,（116）,（027）,match the standard peak of hexagonal CuSCN（JCPDS 73-1855）.The diffraction peaks with 2θvalues of 28.6°,33.1°,47.5°,56.4° and 78.8° belong to the（002）,（100）,（112）and（210）crystal orientations of the ZnS cubic crystal structure,respectively.All peaks are perfectly matched,confirming the successful preparation of the uniform crystal structure.The ZnS was analyzed according to the reference card（JCPDS＃99-0097）,as seen in Fig.S1.The XRD pattern of the ZnS/CuSCN nanofilm possesses peaks of both ZnS and CuSCN,which confirms the successful preparation of the ZnS/CuSCN nanofilm.Fig.1（b）shows Raman spectra of ZnS/CuSCN film,which shows three different Raman peaks.The ZnS/CuSCN nanofilm offers three Raman peaks at 260,350,2 175 cm-1.The peak of 2 175 cm-1is attributed to CuSCN.Fig.S2 is the Raman spectrum of a single CuSCN,showing that there is only one Raman peak of CuSCN located at 2 174 cm-1,which is almost consistent with the Raman peak of CuSCN in ZnS/CuSCN composites.Fig.S3 shows the Raman spectra of ZnS,further conducted to confirm ZnS formation in the ZnS/CuSCN nanocomposite.There are two apparent peaks at 260 cm-1and 350 cm-1,ascribed to the E1/A1（TO）and E1/A1（LO）vibrational modes of the single ZnS phase[36-37].The absence of the A1（TO）phonon peak in the Raman spectrum proves that the synthesized ZnS is a pure ZB（zinc-blende structure）phase.The broad features at 350-450 cm-1are related to the secondorder Raman scattering,including the combination and overtone of several ZnS phonon modes.The peak position of the ZnS bulk phonon modes is close to the reported bulk values,indicating the excellent crystal quality with strain-free growth of ZnS[38].

Fig.1 （a）The XRD patterns of ZnS/CuSCN sample.（b）Raman spectra of ZnS/CuSCN.

Fig.2（a）-（d）show the SEM images of CuSCN and ZnS/CuSCN.The surface of pure CuSCN exhibits large-area bulk stacks,a high-density structure,and is very uniform.In addition,Fig.2（c）-（d）indicate that the ZnS film is deposited on the surface of the CuSCN,which is more dense and uniform than pure CuSCN.This structure is beneficial to electron transfer and has certain structural advantages compared with CuSCN.

Fig.2 （a）-（b）SEM images of CuSCN nanofilm.（c）-（d）SEM images of ZnS/CuSCN composite.

The HRTEM images in Fig.3（a）show different lattice spacing of 0.33 nm and 0.295 nm on the CuSCN surface,corresponding to the（012）and（018）planes of CuSCN,respectively,which are consistent with the XRD results.The EDX analysis in Fig.3（b）shows almost the same elemental distribution of Cu,S,N,and C in CuSCN,which further confirms the presence of CuSCN substrates.

Fig.3 （a）HRTEM images of CuSCN.（b）HAADF-STEM images of CuSCN，and the corresponding EDX mapping of C，Cu，N，and S elements.

The UV-Vis diffuse reflectance spectra（DRS）of CuSCN film,ZnS,and ZnS/CuSCN film are shown in Fig.4（a）-（c）.They showed strong absorption in the ultraviolet region.Fig.4（a）-（c）inset images show（αhν）2versusphoton energyhνplots of CuSCN,ZnS,and ZnS/CuSCN,respectively.According to the UV-Vis absorption spectrum analysis and the formula calculation,theEgof CuSCN,ZnS,and ZnS/CuSCN is 3.61,3.62,3.71 eV,respectively.The calculatedEgof ZnS/CuSCN is more significant than ZnS.The above analysis proves that ZnS/CuSCN has excellent application potential for UV detection.XPS measurement was employed to analyze the elementary composition of ZnS/CuSCN.Fig.5（a）confirmed the existence of Cu,S,C,N,and Zn.The binding energies of the C1s spectrum of ZnS/CuSCN are appeared at 285.2 eV and 285.8 eV,respectively,in Fig.5（b）,which were ascribed to the presence of C in CuSCN.Besides,the peak at 284.6 eV is attributed to extraneous carbon-based pollutants.Fig.5（c）showed the N 1s spectrum of ZnS/CuSCN.There are two peaks distributed at 398.2 eV and 399.8 eV,respectively.Fig.5（d）showed the S 2p XPS spectra for ZnS/CuSCN,the S 2p1/2and S 2P3/2occurred at 164.5 eV and 163.2 eV[39].And the error is within ± 0.2 eV between them,attributed to the S in CuSCN.Fig.5（c）shows the core energy levels in the N 1s spectra of ZnS/CuSCN are located at 398.2 eV and 399.8 eV,respectively,and Fig.5（e）depicts the high-resolution spectrum of Cu 2p.The fitted peaks at 952.45 eV（Cu 2p1/2）and at 932.6 eV（Cu 2p3/2）match well with that of Cu+[40-42].Fig.5（f）shows Zn 2p XPS spectrum of ZnS/CuSCN,splits into Zn 2p3/2（1 022.1 eV）and Zn 2p1/2（1 044.9 eV）[42-43],which is consistent with that of Zn2+.In summary,XPS analysis confirms the successful recombination of ZnS/CuSCN,providing a basis for the possibility of forming high-quality p-n junctions of ZnS/CuSCN.

Fig.4 （a）-（c）The UV-Vis DRS and the corresponding（αhν）2 versus photon energy plots of the activated CuSCN，ZnS and ZnS/CuSCN.

Fig.5 XPS spectra.（a）Survey spectrum of ZnS/CuSCN.（b）ZnS/CuSCN C1 spectra.（c）ZnS/CuSCN N 1s spectra.（d）ZnS/CuSCN S 2p spectra.（e）ZnS/CuSCN Cu 2p spectra.（f）ZnS/CuSCN Zn 2p spectra.

The photoelectric properties were investigated using a two-probe method at room temperature regarding ZnS/CuSCN composite PDs by constructing silver paste with a small area as the electrodes（～1 cm2）.Fig.6（a）shows typical current-voltage（I-V）curves of ZnS/CuSCN and CuSCN devices under -1-1 V bias,irradiated with 350 nm UV light（～2.36 mW/cm2）.TheI-Vcurves indicate that CuSCN composite with ZnS improves the optoelectronic performance compared with CuSCN.Notably,theIVcurve of the ZnS/CuSCN device exhibits asymmetry under forwarding bias,which confirms that the ZnS/CuSCN devices form a high-quality p-n junction[44].The photocurrent of ZnS/CuSCN is 1.22×10-5A,and the dark current is 4.8×10-9A.The dark current of CuSCN is 3.01×10-8A.Compared with CuSCN PDs,the decrease in the dark current of ZnS/CuSCN PDs due to the formation of a denser film on the surface after adding ZnS,which reduces the surface defects.Thus,it reduces the ZnS/CuSCN device's surface recombination current and surface leakage current[45],confirms that the ZnS/CuSCN nanofilm UV photodetector is excellent.Fig.6（c）shows the on/off ratio（Ilight/Idark）of the ZnS/CuSCN composite films is 2 542,which is greatly improved compared with the CuSCN nanofilm（～94）,is due to the decrease of dark current and the increase of photocurrent after CuSCN composite ZnS.The ZnS/CuSCN PDs show an excellent on/off ratio,about one order of magnitude higher than CuSCN PDs.The ZnS/CuSCN PDs offer an excellent on/off ratio,approximately one order of magnitude higher than that of CuSCN PDs.The responsivity is defined asR=Iph/Pin,whereIphisIlight-Ilight,andPinis the average optical power irradiating to the sample,which indicates how much photocurrent can be produced by unit incident power on the photodetector.The ZnS/CuSCN responds well to UV light in the UVA band,demonstrating the best responsivity of 5.17 mA/W upon 350 nm UV irradiation.The detectivity is one of the most critical parameters for photodetectors,defined by the equation,D*=RA1/2/（2eIdark）1/2,whereAis the effective photosensitive area of the device.The ZnS/CuSCN shows a high detectivity ofD*=1.32×1011Jones towards 350 nm.Moreover,these ZnS/CuSCN composite films maintain excellent stability at room temperature and last for more than 30 d without significant decay,as shown in Fig.S4.Five response recovery cycles of the ZnS/CuSCN PD are shown in Fig.6（b）,which also confirms the stability and repeatability of the device.But it is shown in Fig.6（b）that the dark current of ZnS/CuSCN PDs does not offer a horizontal straight line.Possible reasons for this problem might be the instability of the contact junction region between the ZnS and CuSCN.Fig.6（d）shows that the rise time and decay time of the ZnS/CuSCN PDs are 0.47 s and 0.48 s,respectively.However,the rise time and decay time of CuSCN PDs are 1.41 s and 1.44 s,respectively,which suggests that ZnS/CuSCN nanofilm respond faster.

Fig.6 （a）The I-V curves of ZnS/CuSCN PD and pure CuSCN PD under dark and light illumination of 350 nm.（b）I-t curves of ZnS/CuSCN PD and CuSCN PD with periodical on/off switching upon 350 nm UV light illumination under -1 V bias.（c）A single period of the I-t curve on a semilogarithmic plot of ZnS/CuSCN PD and CuSCN PD.（d）Response speed curves of CuSCN/ZnS PD and CuSCN PD.

The responsivity is an essential parameter to the sensitivity of the photodetector.According to Fig.7（a）,the ZnS/CuSCN PD exhibits outstanding UV photodetector performance.The results in Fig.7（b）indicate that the responsivity and detectivity of the ZnS/CuSCN device increased gradually from 300 nm to 350 nm.The ZnS/CuSCN device has the best responsivity of 5.17 mA/W andD*of 1.32×1011Jones when using 350 nm light illumination.Fig.7（c）shows the（current-time）I-tcurve of the ZnS/CuSCN photodetectors at-1 V,the incident wavelength is 350 nm,under different optical densities（0.45,0.95,1.43,1.89,2.36 mW/cm2）,the corresponding photocurrent values are 2.26×10-6,4.25×10-6,6.79×10-6,1.01×10-5,1.22×10-5A,respectively,indicating that the photocurrent is proportional to the light intensity density.The above results show that the ZnS/CuSCN UV PDs are still highly stable even when operated at different powers.Fig.7（d）shows that the responsivity of the ZnS/CuSCN PD is proportional to the optical power,calculated by exponential fitting.The value ofθis 0.99.Compared with recent reports on p-type PDs,ZnS/CuSCN device's on/off ratio is outstanding（Tab.1）,and the all-around performance is better than other devices.Fig.7（e）-（f）shows schematic diagrams and corresponding theoretical band alignments of the ZnS/CuSCN devices.In the UV light test,UV light was first irradiated on the ZnS layer on the ZnS/CuSCN nanofilm and then reached the underlying CuSCN.ZnS has efficient electron mobility and can serve as an outer layer to provide an efficient transport path for photogenerated electrons.Meanwhile,the ZnS/CuSCN nanofilm belongs to the polycrystalline structure with more defects and interface states.Carriers are easily recombined through defects in transport and interfacial states.More importantly,the gradient energy level formed between ZnS and CuSCN facilitates an effective charge separation process and suppresses carrier recombination,desirable for a high-performance UV PD.

Fig.7 Photoelectric properties of ZnS/CuSCN.（a）I-V curves under dark and light illumination of 300，350，400，450，500 nm.（b）Spectral responsivity and D* of ZnS/CuSCN.（c）I-t curves at -1 V bias upon 350 nm light illumination at different light intensities.（d）Corresponding fitting curves for the relationship between the photocurrent and the light intensity.（e）Schematic representations of the device.（f）The energy band alignment of the various layers in the ZnS/CuSCN.

Tab.1 The characteristic parameters of p-type PDs in the literature

4 Conclusion

In conclusion,we have developed a high-performance UV photodetector based on a ZnS/CuSCN p-n heterojunction.The on/off ratio of ZnS/CuSCN UV PD compared to CuSCN UV PD has been dramatically increased from the previous 94 times on/off ratio to 2 542 times.The ZnS/CuSCN UV PD decreases the dark current to 4.8×10-9A and improves its photocurrent to 1.22×10-5A.Moreover,the ZnS/CuSCN PD has the best responsivity and detectivity of 5.17 mA/W and 1.32×1011Jones（at-1 V,350 nm）.In addition,the rise/decay time of ZnS/CuSCN PD is 0.47 s/0.48 s.ZnS/CuSCN PD also shows good cycling stability at different power levels.The results reported here provide a facile route to fabricate CuSCN-based UV photodetectors with high sensitivity,speed,and stability and lay a solid foundation for subsequent applications.

Supplementary Information and Response Letter are available for this paper at：http://cjl.lightpublishing.cn/thesisDetails#10.37188/CJL.20220069.