CuInSe2 Heterojunction for

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Self-assembly of the Lateral In2Se3/CuInSe2 Heterojunction for Enhanced Photodetection Zhaoqiang Zheng, Jiandong Yao, and Guowei Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16323 • Publication Date (Web): 09 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017

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ACS Applied Materials & Interfaces

Self-assembly of the Lateral In2Se3/CuInSe2 Heterojunction for Enhanced Photodetection

Zhaoqiang Zheng, Jiandong Yao, Guowei Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China *Corresponding author: [email protected]

Keywords: layered-materials, In2Se3, CuInSe2, lateral junction, photodetectors

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Abstract Layered materials have been found to be the promising candidates for next-generation microelectronic and optoelectronic devices due to their unique electrical and optical properties. The p-n junction is an elementary building block for microelectronics and optoelectronics devices. Herein, using pulsed-laser deposition (PLD) method, we achieve pure In2Se3-based and the lateral p-n heterojunction of In2Se3/CuInSe2-based photodetectors. In comparison to the pure In2Se3-based one, the photodetectors based on the In2Se3/CuInSe2 heterojunction exhibit a tremendous promotion of photodetection performance and obvious rectifying behavior. The photoresponsivity and external quantum efficiency (EQE) of the fabricated heterojunction based device under 532 nm light irradiation are 20.1 A/W and 4698%, respectively. These values are about 7.5 times higher than those of our fabricated pure In2Se3-based devices. We attribute this promotion of photodetection to the suitable band structures of In2Se3 and CuInSe2, which greatly promote the separation of photoexcited electron-hole pairs. This work suggests an effective way to form lateral p-n junction, opening up a new scenario for designing and constructing high performance optoelectronic devices.

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Introduction Over the past decade, layered materials (e.g. In2Se3) have attracted enormous attention due to their exotic electronic and optical attributes such as strong light-matter interaction1, efficient light absorption2, and large surface-to-volume ratio3. These physical properties endued layered materials the potential to trigger the revolution of a wide range of optoelectronic devices, including solar cells, light-emitting devices, and photodetectors4-9. Moreover, the existence of direct bandgap in bulk form renders In2Se3 an attractive candidate in optoelectronic applications2. In optoelectronics, photodetectors are fundamental devices, which are enabled the conversion of optical signal to electronic signals. Photodetectors have been widely applied in various fields including military applications and commercial products in daily life10-11. The performance of photodetectors depends on various factors. These include light absorbency,

photoexcitation,

relaxation,

free

carrier

generation,

charge

trapping/detrapping, recombination, etc12. Under illumination, the active film absorbs photons with energy higher than the bandgap, which result in excited electrons with excess energy. Due to the small bandgaps of layered materials, energy dissipation will inevitably occur during the long relaxation process. The excess energy can be relaxed into thermal sinks via interactions between carriers with intrinsic acoustic or optical phonons12. Thus, effective separation of photoexcited electron-hole pairs and conversion of excess photoexcited energy into electrical current are keys for superior photodetector devices12-14.

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Introducing the p-n junction is a particularly powerful approach to improve the performance of photodetectors. The built-in potential in the depletion region can readily separate and drive the photogenerated electron-hole pairs that migrate toward the electrodes and eventually generate a superior photocurrent to enable efficient energy conversion15-17. Recently, several efforts have been made toward realizing the p-n junction via, for example, the use of local electrostatic gate, appropriate ionic doping procedure, and so on15, 18. However, these methods require either complex device configurations or specialized fabrication procedures. These requirements are not compatible with current semiconductor fabrication technology. Hence, developing a facile and efficient approach to create a functional p-n junction in thin layered semiconductors and achieve superior photodetector is desirable. In this contribution, we conduct a one-step approach to fabricate pure In2Se3-based

and

the

lateral

p-n

heterojunction

of

In2Se3/CuInSe2-based

photodetectors through a simple pulsed-laser deposition (PLD) procedure. Owing to the suitable band structures of In2Se3 and CuInSe2, photoexcited electron-hole pairs can be separated readily and extracted by the electrodes. As a result, the photoresponsivity and EQE of the In2Se3/CuInSe2-based photodetector are about 7.5 times higher than that of our prepared pure In2Se3-based device. Moreover, under illumination, obvious short-circuit current (Isc) and open-circuit voltage (Voc) originated from the formation of p-n junction are also observed from the hetrojunction device. This study suggests a new route for designing and constructing superior

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optoelectronic devices.

Experimental Section Self-assembly preparation of the In2Se3/CuInSe2 photodetector array and pure In2Se3 photodetector. As descripted in Figure 1a-d, PLD was adopted to self-assemble the In2Se3/CuInSe2 photodetector array. In this experiment, the PLD parameters were similar to our previous works19-21. Firstly, Si/SiO2 substrate was washed in acetone, alcohol and deionized water for 10 min to remove the contaminations (Figure 1a). Then seven sets of Au-Cu electrodes were patterned onto the Si/SiO2 substrate by using standard photolithography process, followed by electron beam evaporation (Figure 1b). Subsequently, we covered the electrode pairs with a stainless steel shadow mask for patterning the In2Se3/CuInSe2 film (Figure 1c). Then, they are mounted in the deposition chamber, locating at a distance of 7 cm away from and parallel to the In2Se3 target (99.99%). After evacuating the deposition system to lower than 2*10-4 Pa, highly pure argon background gas with the flowing rate of 50 sccm was guided into the system. Thereupon, rapidly heated the substrate to 365 oC, and maintained the system pressure at 20 Pa. Sequentially, a pulsed KrF excimer laser (λ = 248 nm, pulse duration of 20 ns, repetition rate of 4 Hz) was focused to ablate the target. The operated energy of per pulse was set at 105 mJ, and the total pulse number is 2400. Finally, the system was cooled down to room temperature over several hours. The In2Se3/CuInSe2 photodetector array can be

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observed on the substrate when it was moved out (Figure 1 d). The device structure of a single In2Se3/CuInSe2 photodetector was schematically shown in Figure 1e. PLD was also employed to fabricate pure In2Se3 photodetector. The substrate is Si/SiO2 covered with Au-Au electrodes. The PLD parameters are consistent with those used for the deposition of the In2Se3/CuInSe2 films. Characterization of the In2Se3/CuInSe2 heterojunction film. The structure of the photodetector array was identified using an optical microscope. The surface morphologies of our In2Se3/CuInSe2 film were observed by a scanning electron microscope (SEM, FEI Quanta 400F). Atomic force microscopy (AFM, Bruker Dimension Fastscan) was used to perform the thickness profile measurement and Kelvin probe force microscopy (KPFM) measurements of the heterojunction film. Ultraviolet photoelectron spectroscopy (UPS, Escalab 250) with He I line (21.22 eV) was conducted to estimate the band structure of the samples. X-ray photoelectron spectroscopy (XPS, Escalab 250) with a monochromatic Al Kα source was used to measure the binding energies of the CuInSe2. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were recorded by a transmission electron microscope system (FEI Tecnai G2 F30). X-ray diffraction (XRD, Rigaku D-MAX 2200 VPC) was recorded with Cu Kα radiation (λ = 0.15418 nm) at the speed of 7o min-1. Raman spectra were recorded using a Renishaw InVia Raman spectrometer with a 514 nm laser for excitation. UV-vis-NIR diffuse reflectance spectra (DRS) of the In2Se3 and CuInSe2 were obtained by a spectrophotometer

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(Lambda950, PerkinElmer). Optoelectronic measurements of the heterojunction photodetector array. We carried out the optoelectronic measurements on a Lakeshore probe station equipped with a Keithley 4200 semiconductor characterization system. The photocurrents were measured under light illumination. The illuminations with wavelengths ranging from ultraviolet to near-infrared were generated from monochromatic semiconductor lasers (Viasho). And the intensities of incident lights were measured by an optical power meter.

Results and Discussion Morphology and structure of the In2Se3/CuInSe2 photodetector. An optical microscopy image of a typical pure In2Se3 photodetector is shown in Figure S1a. The greenish color is reflected by the In2Se3 active film. From the SEM image in Figure S1b, we can observe sheet-like morphology. Figure S1c presents the high-resolution TEM image of the In2Se3 active film. Obvious lattice fringes are visible, revealing the high-quality of the In2Se3 film. Inset in Figure S1c shows the SAED pattern of the In2Se3 active film. The sharp diffraction rings indicate the polycrystalline nature of the film. Figure 2a shows the digital photograph of a heterojunction photodetector chip. The chip size is 1×1 cm, and there are seven photodetectors in a chip. Representative optical microscopy image of a single photodetector is presented in Figure 2b. As can be seen, the channel spacing between two parallel electrodes was maintained at 30 µm.

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Moreover, the color of the active film near the copper electrode have transformed into red, which may owing to the reaction with the Cu electrode, layered In2Se3 in this region have transformed into non-layered CuInSe222-23. SEM image in the middle of the active film shows that there is an obviously junction at the interface of the two materials (Figure 2c). High-magnification SEM images in the In2Se3 region (labeled by red box) and CuInSe2 region (labeled by orange box) are presented in Figure 2d and Figure 2e, respectively. Sheet-like and particle-like continuous polycrystalline morphologies are observed. The difference in morphology suggests the creation of In2Se3/CuInSe2 heterojunction. Figure 2f presents the AFM height profile across edge of the heterojunction film. The thickness is calculated to be ca. 14.1 nm. To analyze the detailed chemical composition of the as-synthesized heterojunction film, EDS measure at the interface of the lateral heterojunction is carried out. The EDS spectrum in Figure S2a manifests the coexistence of In, Se, and Cu elements. Their spatial distributions are depicted in the EDS mapping images in Figure 2e-i. Clearly, In and Se elements distribute uniformly throughout the whole heterostructure, while the element Cu locates mainly in one side. It suggests that In2Se3 in the bottom region have converted to CuInSe2 successfully. XPS analysis was performed to verify the elemental composition of CuInSe2. As the wide scanning XPS spectrum in Figure S2b, the binding energies of Cu 2p, In 3d and Se 3d core level were consistent with the literature values24. Figure S2c-e show the high resolution XPS spectra of 2p, In 3d and Se 3d, respectively. The Cu 2P3/2, In 3P5/2, and Se 3d5/2

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binding energy peaks in Figure S2c-e were observed at 932.6, 444.8, and 54.0 eV, respectively, which confirms the existence of Cu+, In3+, and Se2- states25-26. According to the XPS results, the experimental stoichiometric ratio of Cu:In:Se was calculated to be 1:1.06:1.85, which is approximately consistent with the theoretical value (1:1:2). Therefore, we estimate the formation of CuInSe2. In order to further confirm the formation of In2Se3/CuInSe2 heterojunction, XRD and Raman characterizations are performed. As shown the XRD patterns in Figure 3a, the pattern of CuInSe2 contains only one pronounced peak at 26.649o, which can be indexed as (112) plane (JCPDS 87-2265). For the pattern of In2Se3, all diffraction peaks are consistent with data of beta phase In2Se3 (JCPDS 35-1056)27-28. Raman patterns of the In2Se3/CuInSe2 heterojunction film are presented in Figure 3b. As can be seen, Raman spectrum taken from the CuInSe2 region shows a strong peak at 178 cm-1 and two weak peaks at 211 cm-1 and 230 cm-1. They can be regarded as the A1, B2, E mode of CuInSe2, respectively23. While, the positions of the vibration peaks taken from the In2Se3 region are mainly located at 110 cm-1, 151 cm-1, 205 cm-1 and 240 cm-1, which can assign to A1, A1(TO), A1(LO+TO) and A1 symmetry mode of β-In2Se3, respectively29-30. The weak peak at 175 cm-1 is attributed to the In-Se vibrations in the In2Se3/CuInSe2 film30. Figure 3c shows the Raman mapping (CuInSe2 A1 peak of 178 cm-1) at the interface of the In2Se3/CuInSe2 heterojunction film. The CuInSe2 is homogeneously distributed on the bottom part, and the In2Se3/CuInSe2 interface is visible obviously. This Raman mapping has revealed the formation of

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In2Se3/CuInSe2 heterojunction and the boundary between the two materials is clearly visible. Optoelectronic

performance

of

the

In2Se3/CuInSe2

photodetector.

Photodetection performances of the In2Se3/CuInSe2 photodetectors were evaluated upon various light irradiations. We first measured the current versus bias voltage (I-V) characteristics of the In2Se3/CuInSe2 devices in dark and under light illumination (532 nm) with various power intensities ranging from 0.007 to 27 mW/cm2. The corresponding results were logarithmic presented in Figures 4a. Interestingly, these I-V curves exhibit an obvious rectifying behavior. The rectification ratio, defined as the ratio of the forward/reverse current31, is about 10 at Vds = ±2 V under 27 mW/cm2 light irradiation. Considering that the I-V curves of the pure In2Se3 photodetector are symmetrical (Figure S3), the rectifying characteristic of the In2Se3/CuInSe2 heterojunction can thus be attributed to the interaction between In2Se3 and CuInSe2. For further comparison, the inset in Figure 4a and Figure S3 present the enlarged I-V characteristics of the two devices under dark and light illumination. Compared to the pure In2Se3 device, more obvious photovoltaic-like behaviors such as Voc and Isc are exhibited in the In2Se3/CuInSe2 photodetector. The light intensity dependent photocurrent (Iph = Ilight - Idark) is investigated in Figure 4b (blue). We observed that the photocurrent increases continuously with the irradiation power, and the photocurrent of the heterojunction device is much higher than that of the pure In2Se3-based one. Responsivity (R), one of the critical parameters

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for a photodetector, is defined as the generated photocurrent per unit effective incident power. R can be calculated by the equation R = I ph PS , where Iph is the generated photocurrent, P is the light power density, and S is the effective area of the photosensitive region32. We calculated the R values under different incident light intensities in Figure 4b (red). At the light intensity of 7 µW/cm2, our pure In2Se3-based device shows a responsivity of 2.69 A/W. Remarkably, our heterojunction photodetector exhibits a high responsivity of 20.1 A/W, which is ca. 7.5 times higher than that of our prepared pure In2Se3-based device. It is notable that the R decreases with the increasing of irradiation power, which may be explained by the trap states existing at In2Se3/CuInSe2 or the dielectric substrate23, 33. With the measured R here, as shown in Figure 4c, the EQE of our fabricated pure In2Se3-based and In2Se3/CuInSe2-based devices can be calculated to be ∼629% and ∼4698%, respectively, following the relation EQE = hcR/λe, where h is Planck’s constant, c is the light velocity, λ is the excitation wavelength, and e is the electronic charge34. Figure S4 presents the statistic responsivity of seven In2Se3/CuInSe2 devices response to 7 µW/cm2 light illumination. Clearly, the heterojunction detector arrays have high responsivity with a superior uniformity. Reliable and fast responses to light illumination are crucial for high-performance photodetectors. The cycling behaviors of the two photodetectors were checked under the pulse illumination. As depicted in Figure S5, definite photoswitching behaviors were observed and maintain reproducibility for multiple cycles. In Figure 4d,

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temporal responses of the two photodetectors in a complete on/off cycle were recorded with a high temporal resolution. For the In2Se3/CuInSe2 detector, after the illumination was turned on, the photocurrent rapidly rise more than 95% in the initial 8.3 ms. While for the pure In2Se3 device, the photocurrent just rise to 75.5%, followed by a relatively slow tail. The faster photoresponse of the heterostructured In2Se3/CuInSe2 photoconductor could be attributed to the efficient charge separation at the interface. For clear comparison, Table S1 summarizes the relevant performance parameters of other recently developed heterojunction-based photodetectors14, 21, 28-31. In general, our device stands out considering the overall performances, which reveals the superiority of the In2Se3/CuInSe2 heterojunction in the photodetection applications. Then, broadband special responses of the constructed photodetectors were also investigated. Figure 5a presents the temporal photoswitching curves of our In2Se3/CuInSe2 photodetector under 370, 447, 808, and 1064 nm illuminations. Defining the response time as the photocurrent increased from 0 to 80% of the stable photocurrent, the response times of our heterojunction device over the 370-808 nm range are shorter than 8.3 ms, the limited sampling interval of our measurement system. Even for 1064 nm light, the response time is less than 16.6 ms. Figure S6 presents the I-V curves of our In2Se3/CuInSe2 photoconductor measured in dark and under various illumination wavelengths. In all cases, obvious rectifying behavior and significant photocurrents were observed. Figure 5b summarizes the power dependent

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photocurrent upon illumination with different wavelengths at Vds = 2 V. The photocurrents also exhibit positive dependence on the incident light density, which can be attributed to the increase in the number of photogenerated carriers. The corresponding power dependent responsivities are presented in Figure 5c. In all cases, the responsivity also exhibits a negative dependence on the incident light density. Meanwhile, these responsivities are comparable to the of the state-of-the-art commercial Si and Ge photodetectors (R ≈ 0.05 to 0.85 A/W)35. Sequentially, we recorded the photocurrent and corresponding responsivity of the pure In2Se3 photoconductor under various illumination wavelengths in Figure S7. Clearly, our In2Se3/CuInSe2 photoconductor exhibits much better performance than pure In2Se3 device over the UV-vis-NIR range. Remarkably, for the In2Se3 photodetector in 1550 nm (Figure S8a), the photocurrent is still discernible but very weak. It may be arise from the intrinsic defects or native oxides that grow at the surface of the In2Se3 film that could act as efficient energy converters of incident light26. While for the In2Se3/CuInSe2 device, as shown in Figure S8b, we can observe a clear photoresponse in the wavelength of 1550 nm. This property means that the synergistic effect between In2Se3 and CuInSe2 film may broaden the response range of In2Se3 detector. Heterojunction Enhanced Photodetection Mechanism. We can conclude from the above analyses that the In2Se3/CuInSe2 heterojunction can boost the photodetection of pure In2Se3. In this section, we attempt to unveil the plausible photodetection mechanism, especially the synergistic effect between In2Se3 and

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CuInSe2. At first, the band structure of the In2Se3 and CuInSe2 are estimated. UPS is a useful technique to estimate the band structure of samples36. Here, UPS measurements at the CuInSe2 region and In2Se3 region are carried out, respectively. And the corresponding UPS spectra at low kinetic regions are shown in Figure 6a. By linear extrapolation of their leading edges to the base line, the work function of our In2Se3 and CuInSe2 can be measured to be 4.51 eV and 4.9 eV, respectively. Meanwhile, UPS spectra near the off-set part of CuInSe2 and In2Se3 are presented in Figure 6b and Figure 6c, respectively. The difference between the valence band (VB) energy and the Fermi energy can be calculated to be 0.29 eV (21.22 eV-20.93eV) for the CuInSe2 and 1.28 eV (21.22 eV - 19.94 eV) for In2Se3. Thereupon, the bandgaps of the CuInSe2 and In2Se3 are determined to be 0.93 eV and 1.35 eV according to their UV-vis-NIR diffuse reflection spectra (Figure 6d) and their corresponding Tauc plots (Figure 6e, f). Then, we sketch simple bandgap models of CuInSe2 and In2Se3 in Figure 7a. They form a type-II-like junction. In this bandgap model, photoexcited electron-hole pairs (blue arrows in Figure 7b) seem to be separated readily (orange arrows in Figure 7b)37-38. In addition, the photoresponse of our heterojunction detector up to 1550 nm, which is the forbidden optical absorption region for CuInSe2 and In2Se3, arose from the photoinduced electron transfer from the valence band of CuInSe2 to the bottom of the conduction band of In2Se3 (green arrow in Figure 7b). To further verify the photo-induced charges transfer in In2Se3/CuInSe2, Kelvin

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probe force microscopy (KPFM) was utilized to quantitatively analyze the changes in surface potential under illumination39. The synchronous measurements for AFM topography and their corresponding contact potential difference (CPD) distribution at the interface of In2Se3/CuInSe2 heterojunction in the dark and under white light illumination are shown in Figure S9. The CPD between the AFM tip and the local area of In2Se3 or CuInSe2 can be given by CPDIn2Se3 = Φtip - ΦIn2Se3

(1)

CPDCuInSe2 = Φtip - ΦCuInSe2

(2)

where Φti, ΦIn2Se3, andΦCuInSe2 are the work functions of the tip, In2Se3, and CuInSe2, respectively40-41. Therefore, the Fermi level difference (∆Ef) between In2Se3 and CuInSe2 can be obtained by ∆Ef = ΦIn2Se3 - ΦCuInSe2 = CPDCuInSe2 - CPDIn2Se3

(3)

As shown in Figure S8c, the ∆Ef between the In2Se3 and CuInSe2 is determined to be about 386.7 mV in the dark. Under white light illumination, the ∆Ef increases to about 405.4 mV. This significant change of ∆Ef between dark and light illumination can be ascribed to an obvious optoelectric effect occurring in the In2Se3/CuInSe2 heterostructure under light illumination42. Under illumination, a large number of photoexcited electron-hole pairs are generated in the In2Se3 and CuInSe2 film. Subsequently, the type-II band alignment of the In2Se3/CuInSe2 heterostructure will lead to electrons flow into the In2Se3 side, while holes are injected to the CuInSe2 side. As a result, the quasi-Fermi levels of

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these two materials shift in opposite directions, as shown in Figure S10. Therefore, the Fermi level difference between In2Se3 and CuInSe2 widens under illumination, corresponding to increase in ∆Ef. Based on the above discussions, we clearly demonstrate the formation of type-II band alignment in the In2Se3/CuInSe2 heterojunction. The synergistic effects between In2Se3 and CuInSe2 can readily separate photoexcited electron-hole pairs, bringing about the enhanced photodetection.

Conclusion In summary, we have demonstrated the fabrication of the lateral In2Se3/CuInSe2 heterojunction film via a facile PLD technique. The suitable band structures of In2Se3 and CuInSe2 lead to the formation of the type-II heterostructures, which can separate photoexcited electron-hole pairs timely. As a result, the photodetector based on the heterojunction exhibited outstanding photodetection capabilities, including a decent photoresponsivity of 20.1 A/W, faster response time less than 8.3 ms, and broad response range from 370-1550 nm. These parameters were superior to that of the pure In2Se3 device and better than that of other heterojunction-based photodetectors. Moreover, under illumination, obvious photovoltaic-like behaviors were observed from the heterojunction device. Therefore, these results have demonstrated a new approach to design and construct novel optoelectronic devices.

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Supporting Information. Summary of reported photodetectors, morphology, structure and photodetection performances of the pure In2Se3 photodetector, EDS spectrum of the heterojunction, I-V curves and broadband photodetection of the In2Se3/CuInSe2 photoconductor, KPFM measurements at the interface of the In2Se3/CuInSe2 heterojunction.

Acknowledgements This work was supported by National Natural Science Foundation of China (50902097) and State Key Laboratory of Optoelectronic Materials and Technologies.

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Figure Captions Figure 1. (a-d) Schematic diagrams illustrating the fabrication process of the lateral In2Se3-CuInSe2 heterojunction photodetector array. (e) Schematic device structure of a single In2Se3/CuInSe2 photodetector. Figure 2. Morphology and structure of the prepared heterojunction photodetectors. (a) Optical image of a photodetector chip, there are seven detectors in it. (b) High magnification optical image of a single photodetector. (c) SEM image of the interface of the lateral heterojunction film. (d) Zoom-in SEM images of the In2Se3 region indicated by the red box in panel c. (e) Zoom-in SEM images of the CuInSe2 region indicated by the orange box in panel c. (f) AFM height profile of the In2Se3 region of the heterojunction film. The inset shows the AFM image of the scan area. The thickness of the sample is deduced to be 14.1 nm. EDS mapping images of (g) In, (h) Se, and (i) Cu at the interface of the lateral heterojunction. Figure 3.

Further characterizations of the lateral heterojunction film. (a) XRD

patterns of the In2Se3/CuInSe2 heterojunction film. (b) Raman patterns of the In2Se3/CuInSe2 heterojunction film. (c) Raman mapping (CuInSe2 A1 peak of 178 cm-1) at the interface of the In2Se3-CuInSe2 heterojunction film. The wavelength of the laser used for Raman spectroscopy is 514 nm. Figure 4. (a) I-V curves of the In2Se3/CuInSe2 photoconductor measured in dark and under 532 nm light illumination with various power densities. The inset shows enlarged I-V characteristics in dark and under light illumination. (b) Photocurrent

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(blue) and responsivity (R, red) of the two photoconductors at Vds = 2 V. (c) Power intensity dependent external quantum efficiency (EQE) of the two photoconductors at Vds = 2 V. (d) Temporal response of the two photodetectors under 532 nm light illumination. The curves are normalized for clarity, and the time interval is 8.3 ms. Figure 5. Broadband photodetection of the In2Se3/CuInSe2 photoconductor. (a) Temporal photoswitching curves under different illumination wavelengths. The time interval is 8.3 ms. (b) Photocurrent and the corresponding (c) responsivity of the In2Se3/CuInSe2 photoconductor under various illumination wavelengths at Vds = 2 V. Figure 6. (a) UPS spectra near the onset part of CuInSe2 and In2Se3. UPS spectra near the off-set parts of (b) CuInSe2 and (c) In2Se3. Helium I with photon energy of 21.22 eV is used here. (d) UV-vis- NIR absorption spectra of the CuInSe2 and In2Se3. Corresponding Tauc plots of (e) CuInSe2 and (f) In2Se3. Figure 7. (a) Band structures of In2Se3 and CuInSe2 in the heterostructures. (b) Schematic energy band diagram of the heterostructures under light irradiation. Electrons are injected from the CuInSe2 to In2Se3, and holes are injected from the In2Se3 to CuInSe2.

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Figure 1

(a)

SiO2

Electrodes Fabrication

Si

(b)

Cu

Cover the mask

(c)

Au

(e)

CuInSe2

(d)

In2Se3

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In2Se3 Deposition


Figure 2

(b)

(a)

In2Se3 on Au

(c)

2 µm

10 µm CuInSe on Cu 2

(d)

(f)

(e)

14.1 nm

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150 nm

150 nm

(g)

(h)

(i)

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Figure 3

(a)

(b)

(c)

In2Se3

CuInSe2 3 µm

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Figure 4

(a)

(b)

(c)

(d)

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Figure 5

(a)

(b)

(c)

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Figure 6

(a)

(d)

(c)

(b)

(f)

(e)

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Figure 7

(a)

(b)

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TOC

Using pulsed-laser deposition method, we have achieved pure In2Se3-based and the lateral p-n heterojunction of In2Se3/CuInSe2-based photodetectors, which exhibited a tremendous promotion of photodetection performance and obvious rectifying behavior in comparison to the pure In2Se3-based one.

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CuInSe2 Heterojunction for

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