Yan Zhang

In Situ Fabrication of Vertical Multilayered MoS2/Si Homotype Heterojunction for High‐Speed Visible–Near‐Infrared Photodetectors

c2D transition metal dichalcogenides (TMDCs)-based heterostructures have been demonstrated to achieve superior light absorption and photovoltaic effects theoretically and experimentally, making them extremely attractive for realizing optoelectronic devices. In this work, a vertical multilayered n-MoS2/n-silicon homotype heterojunction is fabricated, which takes advantage of multilayered MoS2 grown in situ directly on plane silicon. Electrical characterization reveals that the resultant device exhibits high sensitivity to visible-near-infrared light with responsivity up to 11.9 A W(-1). Notably, the photodetector shows high-speed response time of ≈ 30.5 µs/71.6 µs and capability to work under higher pulsed light irradiation approaching 100 kHz. The high response speed could be attributed to a good quality of the multilayer MoS2 , as well as in situ device fabrication process. These findings suggest that the multilayered MoS2 /Si homotype heterojunction have great potential application in the field of visible-near-infrared detection and might be used as elements for construction of high-speed integrated optoelectronic sensor circuitry.

Tuning the p-type conductivity of ZnSe nanowires via silver doping for rectifying and photovoltaic device applications

Applications of one-dimensional (1D) semiconductor nanostructures in nanoelectronics and nano-optoelectronics rely on the ability to rationally tune their electrical transport properties. Here we report the synthesis of single-crystalline Ag-doped ZnSe nanowires (NWs) by using silver sulfide (Ag2S) as the p-type dopant via a thermal evaporation method. The ZnSe:Ag NWs had the zinc blende structure with [11 ] growth orientation. Significantly, the conductivities of the NWs could be tuned over 9 orders of magnitude by adjusting the Ag doping levels. Field-effect transistors (FETs) constructed from the ZnSe:Ag NWs verified their p-type nature with a hole concentration of up to 2.1 × 1019 cm−3, which is the highest value achieved for p-type ZnSe nanostructures thus far. Schottky barrier diodes (SBDs) based on the ZnSe:Ag NW/ITO junctions exhibited remarkable rectifying behavior, with a rectification ratio of >107 and a small ideality factor of ∼1.29 at 320 K. Moreover, photovoltaic devices were fabricated from the ZnSe NW array/Si p–n heterojunctions by aligning the p-ZnSe NWs in a parallel fashion on a n-Si substrate. The device with a graphene top electrode showed a large fill factor (FF) of 61%, yielding a power conversion efficiency of ∼1.04%. The realization of p-type ZnSe NWs with tunable conductivity opens up opportunities for a host of high-performance nanoelectronic and nano-optoelectronic devices.

High-speed ultraviolet-visible-near infrared photodiodes based on p-ZnS nanoribbon–n-silicon heterojunction

Ag-doped p-type ZnS nanoribbons (NRs) with a high hole concentration of 5.1 × 1018 cm−3 and high carrier mobility of 154.0 cm2 V−2 s−1 were synthesized by using silver sulfide (Ag2S) as the Ag source. Excellent ohmic contact to p-ZnS NR with specific contact resistivity as low as 5.6 × 10−7 Ω cm2 was achieved by using bilayer Cu (4 nm)–Au electrode, which according to the depth profiling X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analysis can help to form a thin Cu2S interfacial layer between the electrode. Based on the high quality ZnS NRs and achievement on ohmic contact, p–n photodiodes have been constructed from the p-ZnS nanoribbon (NR)–n-Si heterojunction with a response speed as high as ∼48 μs (rise time). Furthermore, the device also exhibits stable optoelectrical properties with high sensitivity to UV-visible-NIR light and an enhancement of responsivities of 1.1 × 103 AW−1 for 254 nm under a reverse bias of 0.5 V. These generality of the above results shows that the p-ZnS NR–n-Si heterojunction will have potential applications in future high-performance photodetectors.

Construction of crossed heterojunctions from p-ZnTe and n-CdSe nanoribbons and their photoresponse properties

Sb-doped p-type ZnTe nanoribbons (NRs) and Ga-doped n-type CdSe NRs were synthesized via a co-thermal evaporation method in a horizontal tube furnace, respectively. Crossbar heterojunction diode (HD) devices were constructed from p-ZnTe:Sb NRs and n-CdSe NRs by a convenient route. The p-ZnTe/n-CdSe NR HD device exhibits a significant rectification characteristic with a rectification ratio up to 103 within ±5 V and a low turn-on voltage of 2.6 V. Photoresponse analysis reveals that such HD devices were highly sensitive to light illumination with excellent stability, reproducibility and fast response speeds of 37/118 μs at reverse bias voltage. It is expected that such HD devices will have great potential applications in electronic and optoelectronic devices in the future.

Solution assembly MoS2 nanopetals/GaAs n–n homotype heterojunction with ultrafast and low noise photoresponse using graphene as carrier collector

MoS2, the classical representative of layered structure transition metal dichalcogenides (TMDCs), has been widely used as an ideal n-type semiconductor, offering an interesting opportunity to construct heterostructures with other 2D layered or 3D bulk materials for ultrafast optoelectronic applications. In this work, we report the synthesis of ultrathin MoS2 nanopetals via a solution-processable route, and the solution assembly of a 2D MoS2 nanopetal/GaAs n–n homotype heterojunction using graphene as the carrier collector. The fabricated devices have excellent photoresponse characteristics including a good detectivity of ∼2.28 × 1011 Jones, a noise current approaching 0.015 pA Hz−1/2 at zero bias and notably a very fast response speed, up to ∼1.87/3.53 μs with a broad photoresponse range. More interestingly, the device could respond to fast pulsed illumination up to 1 MHz, far exceeding the performance of many current congeneric 2D nanostructured and solution-processable photodetectors reported. These results suggest that our devices, together with the solution assembly methodology of the device described herein, can be utilized to give large-scale integration of low-cost, high-performance photodetectors, thus opening up new possibilities for 2D layered material-based photovoltaic and optoelectronic applications in the future.

Design and construction of ultra-thin MoSe2 nanosheet-based heterojunction for high-speed and low-noise photodetection

Advances in the photocurrent conversion of two-dimensional (2D) transition metal dichalcogenides have enabled the realization and application of ultrasensitive and broad-spectral photodetectors. The requirements of previous devices constantly drive for complex technological implementation, resulting in limits in scale and complexity. Furthermore, the development of large-area and low-cost photodetectors would be beneficial for applications. Therefore, we demonstrate a novel design of a heterojunction photodetector based on solution-processed ultrathin MoSe2 nanosheets to satisfy the requirements of its application. The photodetector exhibits a high sensitivity to visible–near infrared light, with a linear dynamic range over 124 decibels (dB), a detectivity of ~1.2 × 1012 Jones, and noise current approaching 0.1 pA·Hz–1/2 at zero bias. Significantly, the device shows an ultra-high response speed up to 30 ns with a 3-dB predicted bandwidth over 32 MHz, which is far better than that of most of the 2D nanostructured and solution-processable photodetectors reported thus far and is comparable to that of commercial Si photodetectors. Combining our results with material-preparation methods, together with the methodology of device fabrication presented herein, can provide a pathway for the large-area integration of low-cost, high-speed photodetectors.

Ultralow-voltage and high gain photoconductor based on ZnS:Ga nanoribbons for the detection of low-intensity ultraviolet light

A low-intensity ultraviolet photodetector (PD), with a gain as high as ∼2.4 × 106, has been successfully constructed based on gallium (Ga) doped zinc sulfide (ZnS) nanoribbons (NRs). The device exhibits excellent photoconductive properties upon a bias voltage as low as ∼0.01 V in terms of high sensitivity to UV light with an intensity of 1 μW cm−2 (corresponding to an incident power of 10−14 W), relatively fast response times of ∼3.2 ms, and an extremely high detectivity of ∼1.3 × 1019 cm Hz1/2 W−1. The high gain and fast response time are attributed to the excellent ohmic contact obtained by using a high quality ITO electrode and having a carrier mobility as high as 130 cm2 V−1 s−1, which was confirmed from the back-gate field effect transistors. These results show that the single-crystalline n-type ZnS:Ga NRs will have potential applications in future high-performance low-intensity ultraviolet photodetectors.

Large conductance switching nonvolatile memories based on p-ZnS nanoribbon/n-Si heterojunction

A new structure nonvolatile memory, with large conductance switching (on/off ratio > 106), has been constructed from a p-ZnS nanoribbon (NR)/n-Si heterojunction. The p-type ZnS NRs were obtained using cuprous sulfide (Cu2S) as the Cu dopant. Excellent Ohmic contact to p-type ZnS NRs was achieved by using a Cu/Au bilayer electrode, which contributed to the formation of the thin Cu2S interfacial layer between the electrode and the NR, as confirmed from the combined X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analysis. These devices exhibit a stable and reproducible hysteresis and excellent memory characteristics with a long retention time of 1 × 105 s and good endurance >6 months at room temperature. The electrical switching behavior could be attributed to the charge trapping and detrapping in interface states at the junction. The approach could potentially provide a viable way to create new advanced nonvolatile memory devices with simple structure and to fabricate large conductance switching and large-capacity data storage.

Core–shell CdS:Ga–ZnTe:Sb p–n nano-heterojunctions: fabrication and optoelectronic characteristics

In this study, we reported on the construction of p–n junctions based on crystalline Ga-doped CdS–polycrystalline ZnTe nanostructures (NSs) for optoelectronic device application. The coaxial nano-heterojunction was fabricated by a two-step growth method. It is found that the absorption edge of CdS:Ga–ZnTe:Sb core–shell NSs red shifted to about 580 nm, compared with CdS nanowires (520 nm). The as-fabricated core–shell p–n junction exhibited obvious rectification characteristics with a low turn-on voltage of ∼0.25 V. What is more, it showed stable and repeatable photoresponse to 638 nm light illumination, with a responsivity and a detectivity of 1.55 × 103 A W−1 and 8.7 × 1013 cm Hz1/2 W−1, respectively, much higher than other photodetectors with similar device configurations. The generality of this study suggests that the present coaxial CdS:Ga–ZnTe:Sb core–shell nano-heterojunction will have great potential applications in future nano-optoelectronic devices.

High performance visible-near-infrared PbS-quantum-dots/indium Schottky diodes for photodetectors

Here we fabricate self-powered photodetectors based on PbS-quantum-dots/indium Schottky barrier diodes successfully. These devices exhibit excellent repeatability and stability at a high frequency (up to1 MHz), and show a typical fast rise time/fall time of ∼0.8 μs/3.2 μs. They also show excellent rectification ratios up to 104 with bias from -0.5 V to +0.5 V in the dark and a pronounced photovoltaic performance under light illumination. Moreover, the devices demonstrate high sensitivity in weak light illumination detection (detectivity) approaching 1012 Jones and low noise currents <1 pAHz-1/2. These findings suggest great application potential of PbS-quantum-dots for advanced fast response, low noise current, high detectivity and high stability photodetectors.