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Abstract
Self-driven photodetectors, which can detect optical signals without external voltage bias, are highly attractive in the field of low-power wearable electronics and internet of things. However, currently reported self-driven photodetectors based on van der Waals heterojunctions (vdWHs) are generally limited by low responsivity due to poor light absorption and insufficient photogain. Here, we report p-Te/n-CdSe vdWHs utilizing non-layered CdSe nanobelts as efficient light absorption layer and high mobility Te as ultrafast hole transporting layer. Benefiting from strong interlayer coupling, the Te/CdSe vdWHs exhibit stable and excellent self-powered characteristics, including ultrahigh responsivity of 0.94 A W-1, remarkable detectivity of 8.36 × 1012 Jones at optical power density of 1.18 mW cm-2 under illumination of 405 nm laser, fast response speed of 24 µs, large light on/off ratio exceeding 105, as well as broadband photoresponse (405-1064 nm), which surpass most of the reported vdWHs photodetectors. In addition, the devices display superior photovoltaic characteristics under 532 nm illumination, such as large Voc of 0.55 V, and ultrahigh Isc of 2.73 µA. These results demonstrate the construction of 2D/non-layered semiconductor vdWHs with strong interlayer coupling is a promising strategy for high-performance and low-power consumption devices.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Van der Waals heterojunctions (vdWHs) created by stacking different 2D materials together have attracted significant attention due to their intriguing electronic and physical properties, such as tunneling transistors, non-volatile memory, [1] artificial synapse, [2] light-emitting devices, [3] and photodetectors [4,5]. However, due to the poor light absorption, [6] small built-in potential, [7] and low carrier collection efficiency, [8] obtaining high-performance self-powered photodetectors based on vdWHs is still a challenge. For example, the photoresponsivity (limited to dozens of mA W−1) and quantum efficiencies (most below 55%) of vdW p-n junction devices are still low [8,9]. More recently, the 2D/non-layered semiconductor integration has given more possibilities for high-performance photodetection, such as graphene/Si, [10] WS2/Ge, [9] MoS2/Si, [11] WSe2/ZnO, [12] and BP/MoS2/Si [13] have exhibited fast response speed in a wide spectrum, which benefit from the outstanding advantages of 2D/non-layered semiconductor integration, including enhancement of light absorption and increase of the responsivity, [9,14] the degree of freedom in materials selection, [15] and strong interfacial coupling [16]. As one of the most widely used light absorption materials, nonlayered cadmium selenium (CdSe) has stimulated broad research interest in the past decades for its exceptional optoelectronic properties, [17–19] such as appropriate direct bandgap (1.74 eV), strong optical absorption in the whole visible light range, high electron mobility (720 cm2 V-1 s-1), [18] large signal-to-noise ratio, and fast response times (2 µs), [20] which make it become an ideal building block for optoelectronic devices. In addition, tellurene (Te), a new elemental quasi-2D semiconductor, has attracted enormous attention due to its fascinating electronic, thermal and optoelectronic properties [21–23]. It is a p-type semiconductor with a thickness-dependent bandgap ranging from 0.35 to 1.04 eV, [24] a high hole mobility (700 cm2 V-1 s-1), [25] excessive drain current (550 mA mm-1), [21,26] excellent environmental stability, and broadband absorption spectrum, [27,28] exhibiting great potential for fabricating fast electronic devices. Moreover, Te has also been synthesized with a low-temperature solution method, [24] which renders the easy fabrication of large-scale integrated devices possible.
Herein, we designed and fabricated p-Te/n-CdSe vdWHs for self-powered photodetectors, exhibiting impressive performance in terms of fast response speed of 24 µs, together with the maximum responsivity of 0.94 A W-1, detectivity of 8.36 × 1012 Jones at zero bias, which benefits from the strong light absorption of CdSe nanobelts, effective hole collection of Te flakes and strong interlayer coupling. Furthermore, a pronounced photovoltaic behavior was achieved under 532 nm illumination, with a high open-circuit voltage of 0.55 V, short-circuit current as high as 2.73 µA. These excellent results demonstrate the great potential of Te/CdSe vdWHs in high-performance and low power consumption photodetectors.
2. Material preparation and device fabrication
2.1 Preparation of Te nanoflakes and CdSe nanobelts
The Te nanoflakes were synthesized with hydrothermal method based on Ref [25]. Briefly, 1.5002 g of polyvinylpyrrolidone (PVP, average MW 58 000, Aladdin) and 46 mg of Na2TeO3 were dissolved in 25 mL of distilled water to form a clear solution. Afterwards, 1.660 mL ammonia solution and 0.840 mL hydrazine monohydrate (>98.0%, Aladdin) were added into the mixed solution. The resulting solution was then transferred into a 25 mL Teflon-lined autoclave and heated at 180 °C for 14 h. The obtained silver-grey product was purified by washing with distilled water by centrifugation, and finally the Te flakes were redispersed in distilled water.
The CdSe nanobelts were prepared with the low-pressure physical vapor deposition method in a tube furnace. CdSe powder placed at the center of the furnace was used as the source. Some pieces of SiO2(300 nm)/Si chips coated with 10 nm-thick Au catalysts were used as the collecting substrates and placed at the downstream of the furnace. Prior to the growth process, the quartz tube was pumped to a vacuum of 10−3 Torr. After that, the high-quality Ar was introduced into the tube and the flow was set as 100 sccm. Then, the temperature of the furnace was increased to 850 °C and kept for 1 h. After the reaction, black woollike products can be observed on the SiO2/Si substrate.
2.2 Device fabrication
The p-Te/n-CdSe heterojunction were fabricated with the Poly vinyl alcohol (PVA)-assisted transfer method, [29,30] and the detailed process is schematically illustrated in Fig. S1. Firstly, multilayer Te nanoflakes dispersed in deionized water were spin-coated onto a cleaned SiO2 (300 nm)/Si wafer of 0.8*0.8 cm in size (Shanghai OnWay Technology Co., Ltd). Next, the synthesized CdSe nanobelts were transferred onto a new SiO2/Si substrate using a contact printing method, and the target CdSe nanobelt was picked up with the prepared polydimethylsiloxane (PDMS)/PVA stamp and then precisely transferred on the top of the selected Te nanoflake on a transfer platform equipped with an optical microscope (Shanghai OnWay Technology Co., Ltd). As a result, a Te/CdSe p-n heterojunction was formed. Afterward, the asymmetric electrodes were patterned via an Ultraviolet Maskless Lithography machine (TuoTuo Technology (Suzhou) Co., Ltd.). 40 nm-thick Au (ZhongNuo Advanced Material(Beijing)Technology Co., Ltd) was then thermally evaporated on the Te layer as the drain electrode, followed by a lift-off process in acetone. Another markless lithography process was repeated to define the source electrode pattern on the CdSe side. Subsequently, In/Au (15 nm/25 nm) was evaporated as source contacts followed by a lift-off process.
2.3 Materials and device characterization
Raman and photoluminescence (PL) spectroscopy were measured (Horiba, Evolution HR-R) with an excitation wavelength of 532 nm to analyze the optical properties of the Te/CdSe heterojunction. The thickness of samples was measured by atomic force microscope (AFM) (MultiMode 8, Bruker). The morphology of Te nanoflakes and CdSe nanobelts were characterized by a scanning electron microscope (SEM) integrated with an energy dispersive spectrometer. Finally, all the electrical and photoresponse measurements of the Te/CdSe device were carried out in ambient condition on a probe station equipped with a microscope and a Keysight B1500A semiconductor parameter analyzer. The lasers diode with 405, 450, 670, 808, and 1064 nm was illuminated on the device without focusing and the diameter of the laser spot is about 0.4 cm. In order to investigate the potential of the Te/CdSe heterojunction in the photovoltaic applications, the I-V curves were studied using a 532 nm laser that focused on the device through an optical-fiber and a 10× objective yielding a spot of diameter ∼20 µm. The laser power was varied from 1.6 to 102.3 µW that calibrated with a commercial optical power meter (VEGA, OPHIR). For the temporal-current measurement, a train of laser pulses were generated with an optical beam shutter.
3. Result and discussion
The Te nanoflakes and CdSe nanobelts were synthesized with hydrothermal and chemical vapor deposition (CVD) method, respectively, and their OM, SEM, high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) images are shown in Fig. S2. The continuous lattice fringes in the HRTEM image and bright diffraction spots indicated high-quality Te and CdSe single crystals were successfully synthesized [31,32]. After the successful preparation of individual Te nanoflakes and CdSe nanobelts, the Te/CdSe vdWHs-based devices were then constructed on a SiO2/Si substrate by the PVA-assisted transfer method [29,30]. The prepared CdSe nanobelts were firstly printed onto another SiO2/Si substrate, while the Te flakes were also spin-coated onto a cleaned SiO2/Si substrate. Then the target CdSe nanobelt was precisely picked up and transferred on top of the selected Te nanoflake on a transfer platform. Finally, metal electrodes were deposited onto the Te and CdSe layer with thermal evaporation method. In order to reduce the contact resistance, the high work function (Au) and low work function (In/Au) contacts are used for p-Te and n-CdSe, respectively. Figure 1(a) displays the schematic diagram of the constructed Te/CdSe vdWHs device on a SiO2/Si substrate, and the corresponding OM of the fabricated device is shown in Fig. 1(b). The boundary of Te and CdSe flake was marked by a cyan and green dotted line, respectively, and the effective area of Te/CdSe vdWHs is calculated to be about 180 µm2. To measure the surface morphology and thickness Te and CdSe flake, the device was characterized with AFM as shown in Fig. 1(c). The thickness of the Te and CdSe flakes along the line profile is 35 nm and 130 nm (Fig. 1(d)), respectively. The thick CdSe nanobelt could enhance light absorption and thus increase the photocurrent, which is beneficial for the fabrication of high-performance optoelectronic device. Figure 1(e) exhibits the Raman spectra of individual Te and CdSe flake, as well as the overlap region of the Te/CdSe structure. Specifically, three Raman scattering peaks were observed at 92.7, 121.3, and 143 cm-1 for isolated Te nanoflakes, corresponding to E1 (in-plane), A1 (out-of-plane), and E2 vibration modes, respectively. In the Raman spectrum of CdSe, the first Raman peak at 205 cm-1 belongs to the longitudinal optical (LO) mode and the second peak at 412 cm-1 corresponds to the second-order LO phonon mode (2LO). These Raman peaks are in consistent with previously reported data [17,24]. All the above characteristic Raman peaks corresponding to both materials appeared in the overlapped heterojunction region and showed no noticeable shift, confirming the formation of high quality vdW heterojunctions after the PVA-assisted transfer process. In order to investigate the interface coupling between Te and CdSe, PL measurement was carried out. As exhibited in Fig. 1(f), individual CdSe nanobelt exhibits a pronounced PL peak at 709 nm (1.75 eV) due to the bandgap emission. As a contrast, the Te/CdSe vdWs heterostructure demonstrates a rather weak PL emission of CdSe and an evident PL quenching, which indicates efficient separation and transition of the photogenerated carrier take place at the interface of the heterojunction.
Fig. 1. (a) Schematic and (b) optical microscope (OM) image of the Te/CdSe vdWHs photodetector fabricated on a Si/SiO2 substrate. (c) AFM image of the Te/CdSe heterojunction, the pink and purple lines correspond to the selected regions of Te and CdSe, respectively. (d) The height profiles showing a thickness of∼35 nm and∼130 nm for Te and CdSe nanobelt, respectively. (e) Raman spectra of individual Te, CdSe, and the Te/CdSe heterojunction with 532 nm excitation. (f) PL spectra of the single CdSe and Te/CdSe heterojunction region.
Since the carrier transport behavior at the interface of the heterojunction is essentially determined by the interfacial energy band alignment, [33] it is particularly important to analyze the energy band structure of the Te/CdSe vdWHs. Therefore, we performed kelvin probe force microscopy (KPFM) measurements to quantitatively analyze the surface potential differences between Te and CdSe, and the AFM image was also obtained simultaneously, as presented in Fig. 2(a). The corresponding thicknesses of Te and CdSe flake are measured to be 35 nm and 130 nm based on the white line scan profile in Fig. 2(a), which are nearly the same as that of the device used for optoelectronic characterization.
Fig. 2. (a) AFM image of the Te/CdSe vdWHs. The height profiles of the Te and CdSe are shown in the inset. (b) KPFM image of the Te/CdSe vdWHs. (c) The corresponding potential profile acquired along the red line in panel b. (d) The absorption spectrum of CdSe. (e) Band alignment at the interface of Te and CdSe before contact. (f) Energy band diagrams of the vdWHs under laser illumination at zero bias.
Figure 2(b) presents the KPFM mapping image of the Te/CdSe vdWHs measured under dark condition. Herein, the contact potential difference (${V_{CPD}}$) between the probe tip and the sample can be calculated by
where e is the elementary charge, ${W_{tip}}$ and ${W_{sample}}$ are the work function of the probe tip (Pt/Ir-coated Si tip) and the sample, respectively [34]. Therefore, the distinction between the Fermi levels ($\Delta {E_F}$) for the Te and CdSe flake could be calculated from their difference of CPD values according to the following Eq. (2),The optical bandgap of CdSe can be verified from the UV-vis absorption spectrum shown in Fig. 2(d). A weak absorption edge is observed at about 709 nm for CdSe, which corresponding to an optical bandgap of 1.75 eV according to the Tauc plot formula. Therefore, the energy band profiles of Te and CdSe prior to contact are illustrated in Fig. 2(e), where a type-I band alignment is formed at the interface of Te/CdSe interface. The offsets of the conduction band (ΔEc) and valence band (ΔEv) are 0.91 and 0.49 eV, respectively. When the two materials are stacked together, the higher Fermi level of CdSe would induce electrons move into Te from CdSe, while the large valance band offset makes it difficult for holes to transfer from Te to CdSe, thus a built-in electric field directed from CdSe to Te was formed. Therefore, the PL and KPFM results demonstrates strong interlayer coupling between Te and CdSe layers. Given the carrier concentration of the as-prepared Te four orders magnitude larger than that of CdSe, which can be demonstrated in the following I-V curves, the depletion region mainly exists at the CdSe layer. As shown in Fig. 2(f), driven by the built-in electric field at the Te/CdSe interface, the photogenerated electron–hole pairs were separated and transferred into CdSe and Te, respectively, resulting in the PL quenching of CdSe as shown in Fig. 1(f) and achieving self-powered photodetection at zero bias.
Prior to investigating the electrical properties of the Te/CdSe vdWHs Field effect transistors (FETs), the electrical transport performance of individual Te FET and CdSe FET were firstly characterized. Figure 3(a) and 3(b) present the I−V curves of individual Te and CdSe FET, respectively. They exhibited nearly symmetrical linear curve, suggesting the Schottky barriers between Te-Au and CdSe-In/Au can be ignored. Figure 3(c) shows the transfer curves of back-gated individual Te and CdSe FET at room temperature, which exhibit typical p-type and n-type transport behavior for Te nanoflake and CdSe nanobelt respectively. These results are consistent with the earlier reports of Te and CdSe [25,40].
Fig. 3. Electrical properties of the Te, CdSe, and Te/CdSe heterojuction device: (a), (b) Output curves of individual Te nanoflake and CdSe nanobelt. (c) Transfer curves of individual Te nanoflake CdSe nanobelt under a bias of 0.5 V. (d) Transfer curve of Te/CdSe heterojunction at distinct bias of 0.5 V.
Then, the electrical properties of the Te/CdSe vdWHs were characterized at room temperature. Herein, the metal electrodes on Te and CdSe side were defined as drain and source electrode, respectively. Figure 3(d) presents the transfer curve of the Te/CdSe vdWHs, revealing its ambipolar behavior with on/off ratio of 102. With the gate voltage sweeping from -60 to 60 V, the Ids decreases firstly and then increases with the lowest conductivity point at about -35 V. As the gate voltage is swept from -35 to 60 V, the isolated CdSe and Te/CdSe vdWHs begin to be turned on, leading to continuous increase of channel current. When the gate voltage decreases to less than -35 V, the Fermi level of CdSe shifts toward the middle of the forbidden band. Therefore, the relative distance of Fermi level between Te and CdSe becomes smaller, so does the band bending at the interface. The application of a forward bias to Te would greatly reduce the band offset of VBM, contributing to the transfer process of hole from Te to CdSe. Therefore, the device exhibits p-type conductivity behavior.
The optoelectronic properties of the Te/CdSe device were then systematically investigated to examine its potential as a photodetector. In this study, all the photoresponse properties are measured at Vg = 0 V. Figure 4(a) plots the Ids–Vds curves under dark and the illumination of different wavelengths ranging from 405 to 1064 nm, with the corresponding logarithmic scale curves depicted in the inset. An obvious photovoltaic effect can be observed when the device was exposed to the different wavelength illumination due to the existence of the built-in field, indicating the Te/CdSe vdWHs has a wide spectral response spectrum. Under the global illumination at zero bias, the photoexcited electron-hole pairs in CdSe were separated by internal field and driven to different sides. The electrons and holes accumulated in CdSe and Te layers broke the thermal equilibrium of dark state and created forward open circuit voltage. Meanwhile, the Te/CdSe vdWHs exhibits ultralow dark current (∼0.1 pA) at zero and reverse bias, thus contributing to the low noise current and high sensitivity.
Fig. 4. (a) I−V curves of the Te/CdSe vdWHs device measured under various laser illuminations and the corresponding logarithmic scale curves is shown in the inset. (b) Time-dependent photoresponse of the device irradiated by various light at Vds = 0 V. (c) I−V curves of the Te/CdSe vdWHs device irradiated by 405 nm light with various incident powers. (d) The Vds–dependent output electrical power (Poutput) under different 405 nm light densities. (e) The photocurrent response of the Te/CdSe vdWHs with zero bias under 405 nm. (f) Photo-response of Te/CdSe vdWHs under 405 nm laser irradiation before and after five months. (g) Power density-dependent photocurrent Iph measured with 405 nm at Vds = 0 V. (h) Photo/dark current ratio under 405 nm with various incident powers at Vds = 0 V. (i) Calculated R and D* values for irradiation by 405 nm light at Vds = 0 V.
When the Te/CdSe device is biased at Vds = 0 V, it can work in photovoltaic mode. Figure 4(b) exhibits the temporal photoresponse of the device as a self-powered photodetector under different wavelength excitation. It shows rapid and stable photoresponse from 405 to 1064 nm, further confirming the self-powered photodetection ability of the device from Vis to NIR. Because the wavelength of 808 and 1064 nm is beyond the absorption edge of CdSe, and Te flake is a narrow bandgap (0.32 eV) semiconductor as reported in previous report, [21,25,27] it is expected that the NIR photoresponse is caused by the photoexcitation of Te flakes. However, it was found that the pristine Te-based photodetector has no response to 808 and 1064 nm, [41] probably caused by the high dark current. Therefore, the observed NIR photoresponse can be attributed to the interlayer transition of charge carriers between the VBM of Te to CBM of CdSe as illustrated in Fig. 2(f), indicating the Te/CdSe vdwHs could enhance light–matter interaction.
Next, the effect of laser power on optoelectronic properties at various wavelengths was investigated. Figure 4(c) presents the Ids−Vds curves under dark and 405 nm illumination with different intensities. As the light intensity increases, both the open-circuit voltage (Voc) and short-circuit current (Isc) increase successively owning to the enhancement of photogenerated carriers, and the maximum Voc of 0.38 V and Isc of 13 nA was generated under 0.022 W cm-2 illumination. Similar photovoltaic response can also be observed for 532, 670, 808 and 1064 nm laser (Fig. S3 and S4 in Supplement 1), further demonstrating the device has a self-powered detection feature from Vis to NIR. To determine the photoresponse cut-off wavelength of the Te/CdSe heterojunction, the absorption spectrum and wavelength-dependent photocurrent of the device are measured and shown in Fig. S5. It is evident that the Te/CdSe heterojunction has a broad absorption spectrum from 300 to 2000nm, almost the same as that of Te nanoflake, but the absorption intensity in the visible range was greatly enhanced due to the introduction of CdSe nanoflakes (Fig. S5(b)). The wavelength-dependent photocurrent of the photodetector shown Fig. S5(a) exhibits obvious photocurrent in the visible range, along with a falling edge at ∼715 nm, corresponding to the intrinsic absorption of the CdSe nanobelt. In comparison with the broad absorption spectrum of the Te/CdSe heterojunction, the photodetector shows a narrow-band photoresponse, which originates from the Type-I band structure of the Te/CdSe heterojunction. As exhibited in Fig. 2(f), the Te/CdSe heterojunction has a type-I band alignment. Therefore, the conduction-band offset (CBO) and valance-band offset (VBO) leads to the barrier that inhibits the drifting of photo-generated carriers from p-Te to n-CdSe layer. Only the photogenerated carriers in CdSe layer can cross the heterointerface, and thus the spectral response of the Te/CdSe heterostructure photodetectors should be mainly determined by the CdSe layer. The output electrical power Pel, defined as Pel = Ids×Vds, that was extracted from the Ids−Vds curves in Fig. 4(c) and was plotted in Fig. 4(d). The maximum electrical power could reach 1.613 nW at a bias voltage of 0.23 V when light intensity is 0.022 W cm-2, corresponding to a power conversion efficiency of 4.0%, which is comparable to or even higher than that of most multilayer vdWH-based photovoltaic devices [42,43].
To investigate the photoswitching behavior of the Te/CdSe vdWHs device under light illumination, Fig. 4(e) and Fig. S6 plots the multicycle I-T curves of the Te/CdSe device under the illuminations of different wavelengths with various intensities at Vds = 0 V. It is evident that the device could be switched between on and off states with excellent reproducibility and stability. In particular, the drain current increases rapidly from 10−13 A at dark condition to 10−8 A under 405 nm irradiation, thereby achieving an ultrahigh on/off current ratio of ∼ 105 under a light intensity of 0.028 W cm-2, and this value is higher than that of most previously reported 2D device, which signifies outstanding photoresponse of the device to the visible light. In addition, the device also exhibits excellent long-term stability by storing it in an airtight container. The photocurrent exhibits no noticeable deviation and the on/off ratio even increase slightly after about 5 months (Fig. 4(f)). Furthermore, Fig. S7 presents the typical photoresponse of Te/CdSe device to pulsed light signals with frequencies of 100 Hz, 1 kHz, and 20 kHz, respectively. The device shows a fast, stable, and reversible photoresponse to various optical signals but demonstrates an obvious degradation under a higher frequency (e.g., 20 kHz).
Figure 4(g) presents the device photocurrent at different optical power densities with 405 nm illumination at Vds = 0 V. A steady increase in photocurrent with increasing light density can be observed, and the tendency can be well fitted by the power law equation (Iph$ \propto $Plaserα), [44] where α is ∼0.82, this sublinear correlation indicates the presence of recombination process or trap states in the conduction channel [45,46]. As shown in Fig. 4(h), the photosensitivity (Iph/Idark) of the device was plotted as a function of the light power, which increases from 3.35 × 104 at 1.17 × 10−3 W cm-2 to 2.17 × 105 at 0.022 W cm-2, and this result demonstrates the high sensitivity of the device to visible illumination.
In order to better evaluate the photoresponse performance of the vdWH device as a self-powered photodetector, three important parameters of the photodetector including photocurrent (Iph), responsivity (R) and detectivity (D*) were calculated as function of incident power. The R represents the optical-to-electrical conversion efficiency of a photodetector and is usually expressed by the following Eq. (3),
where Iphoto, Idark, Pin and S are the photocurrent, dark current, incident power intensity and effective area of the device, respectively [47].The D* describes the ability of a photodetector to detect weak signal and can be evaluated according to the following Eq. (4),
where e represents the electron charge (1.6 × 10−19) [48].Based on the Eq. (3) and (4), the calculated R and D* of the Te/CdSe device at different incident power densities of 405 nm laser at zero bias are shown in Fig. 4(i). Simultaneously, both R and D* slightly decrease with increasing incident power, and the maximum values of R and D* could reach up to 0.94 A W-1 and 8.36 × 1012 Jones at Vds = 0 V with an optical power density of 1.18 mW cm-2, respectively, which are not only comparable to that of commercial Si and InGaAs photodetectors (∼1 A W-1, 1012 Jones) [49]. The high R can be attributed to the synergistic effects of the high absorption coefficient of CdSe, large internal electric field, and efficient hole collection of Te, which could contribute to the high photocurrent and thus high responsivity, while the excellent D* benefits from the high R and ultralow dark current (10−13 A) of CdSe nanobelt that arising from the high crystal quality, and therefore the weak light detection ability was greatly enhanced. In order to get an insight into spectral response of the Te/CdSe photodetector, the wavelength dependence of photocurrent, Iph/Idark, R and D* with 532, 670 and 808 nm, were also characterized (Vds = 0 V), respectively (Fig. S8), and the trend is in accord with that measured with λ = 405 nm. It is worth noting that the Te/CdSe photodetector exhibits outstanding photoresponse to near-infrared light with a responsivity of 50 mA W−1, specific detectivity up to 4.9 × 1011 Jones at 808 nm. This is mainly attributed to the enhanced light absorption via interlayer excitations in relevant layers.
In order to further investigate the potential of the Te/CdSe vdWHs for photovoltaic application, the Ids–Vds curves of the device under intensive illumination of a 532 nm focused laser was measured and plotted in Fig. 5(a). During the measurement process, the Te/CdSe device is fixed on a probe station, and the 532 nm laser coupled with an optical-fiber is focused on the junction area of the device through a microscope mounted on top of the probe station. The spot diameter of the laser is about ∼20 µm, and the full power of the laser illuminated on the device is 102 µW, which gives a maximum optical density of 320 mW mm-2. As shown in Fig. 5(a), the device exhibits clear photovoltaic effect under light illumination, and the maximum value of Isc and Voc could reach up to 2.73 µA and 0.55 V at Plaser = 102.3 µW, respectively. The maximum Isc is much higher than most of the self-powered vdWHs photodetectors achieved without gate voltage (see more details in Fig. 5(b)) The excellent values of Voc and Isc indicate the Te/CdSe vdWHs has a promising prospect in the further application of solar cells. The superior photovoltaic response can be understood by the energy band structure of the Te/CdSe vdWHs depicted in Fig. 2(f). When the device is illuminated with 532 nm focused laser, most of the electron-hole pairs are excited in the top CdSe layer because of the relatively thick CdSe nanobelts. With the help of large built-in electric field existed in the CdSe depletion region, the holes will transfer to the Te layer and are effectively collected by the drain electrode, while the electrons move to the CdSe side and are collected by the source electrode. Therefore, a positive Voc and negative Isc were obtained. It is noteworthy that only electrons and holes generated in the CdSe layer could cross the heterojunction and was collected due to the type-I band alignment, which greatly suppressed the recombination of electron and holes and significantly improved the charge collection efficiency, so the Voc and Isc are promoted. The switching durability of the self-powered Te/CdSe photodetector was also characterized under continuously switched 532 nm irradiation, which gives an ultrahigh Iph/Idark ratio of 106 based on the ultralow dark current ∼10−13 A. And the photocurrent shows negligible degradation after 225 temporal response cycles, suggesting good stability of the device (Fig. 5(c)).
Fig. 5. (a) I−V curves of the vdWHs device measured under 532 nm focused laser illumination with different power densities. (b) Comparison of Voc and Isc among reported p-n heterojunctions photodetectors at room temperature. [41,50–56] (c) Photoresponse of Te/CdSe heterojunction with 225 continuous cycles under 532 nm laser irradiation. (d) Rise and decay time of the Te/CdSe vdWH device that measured at Vds = 0 V under the illumination of 520 nm. (e) Optical microscope image of the heterojunction, component materials are outlined in different colors. (f) Scanning photocurrent microscopy images at zero bias with an illumination wavelength of 520 nm.
In addition, response time is also an important parameter for photodetector. The photoresponse speed of the device to 532 nm laser at zero bias voltage was recorded with an oscilloscope and shown in Fig. 5(d). The rise (decay) time τ, defined as the time taken for photocurrent increasing (decreasing) from 10/90% (90/10%) of the maximum value, was calculated to be 24 (25) µs.
Further, we studied the photocurrent generation mechanism, we carried out the scanning photocurrent measurement with a confocal optical microscope to identify photoactive area of the Te/CdSe heterojunction. Figure 5(e) and 5(f) show the optical micrograph image of the scanned area in the device and corresponding scanning photocurrent mapping with the excitation of 520 nm at zero bias, respectively. To distinguish more clearly the different regions of the heterojunction, the edges of Te and CdSe are outlined with pink and yellow dashed line, respectively. A strong photocurrent can be observed at the overlapping region of the Te/CdSe heterojunction, indicating the device can operate under self-powered mode, whereas there is almost no noticeable photocurrent is generated near the metal contacts, which unambiguously demonstrates the photovoltaic response is induced by the heterojunction rather than the Schottky junction. To compare the overall performance of our Te/CdSe device with previously literature, some critical parameters including the R, D*, and τ of this study, together with that in previously reports, are shown in Table 1 (Supplement 1). It can be seen that the performance of the present Te/CdSe photodetector is higher or comparable with that of the best vdW p-n heterojunction photodetectors. The attractive performance of the present devices is attributed to the strong light absorption of CdSe, strong built-in electric field, and the good ohmic contacts at semiconductor/metal interface.
Considering the crystal structure anisotropy of Te nanoflakes, [57] the polarized Raman spectrum of Te nanoflakes and the photocurrent of Te/CdSe vdWHs device were also measured. The intensities of the Raman peaks at different rotational angles were measured and ploted in polar coordinates. Fig. S9 demonstrates the intensities of E1 and A1 modes both fit well the sinusoidal function with a period of 180°. Based on the excellent polarization properties of Te nanoflakes, the polarization photocurrent of this Te/CdSe heterojunction device was measured. Fig. S10 shows the tendency of polarization angle-dependent photocurrent. However, the ratio of anisotropic conductance (Imax/Imin) is smaller compared to other reports, [58,59] possibly due to the relatively thick CdSe nanobelt coated on top of the Te, which has a strong absorption in the visible-light region and absorbs a lot of incoming light. So, we speculate that this heterojunction device may have a better polarization response in the NIR range, which needs to be further investigated in the future.
4. Conclusions
In summary, we have successfully fabricated a novel p-Te/n-CdSe vdWHs with strong interlayer coupling. Owing to the efficient light absorption of CdSe, strong built-in potential and the high mobility of Te, the device exhibits excellent self-powered behavior in a broad response spectrum up to short-wave infrared. Under laser illumination at zero bias, a high light on/off ratio above 105, an ultrahigh responsibility and detectivity of 0.94 A W-1 and 8.36 × 1012 Jones with an optical power density of 1.18 mW cm-2, respectively, and a fast response speed of 24/25 µs were obtained. Moreover, the p-Te/n-CdSe vdWHs present a pronounced photovoltaic with a high Isc and Voc of 2.73 µA and 0.55 V, and a maximum power conversion efficiency of 4.0%. Therefore, the high-performance 2D/non-layered semiconductor vdWHs may find promising applications in future electronic and optoelectronic devices.
Funding
National Natural Science Foundation of China (61804048); Science and Technology Development Program of Henan Province (212102210478, 202102310530).
Acknowledgements
The authors gratefully acknowledge financial support from Natural Science Foundation of China (NSFC Grant Nos. 61804048). This work has been financially supported by Science and Technology Development Program of Henan Province (No. 212102210478 and 202102310530).
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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