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Observing Charge Transfer Interaction in CuI and MoS2 Heterojunction for Photoresponsive Device Application Rakesh D. Mahyavanshi, Pradeep Desai, Ajinkya Ranade, Masaki Tanemura, and Golap Kalita ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00069 • Publication Date (Web): 17 Feb 2019 Downloaded from http://pubs.acs.org on February 17, 2019
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ACS Applied Electronic Materials
Observing Charge Transfer Interaction in CuI and MoS2 Heterojunction for Photoresponsive Device Application Rakesh D. Mahyavanshi[a], *, Pradeep Desai1, Ajinkya Ranade[a], Masaki Tanemura[a], Golap Kalita[a],[b],* [a]Department
of Physical Science and Engineering, Nagoya Institute of Technology, Gokisocho, Showa-ku, Nagoya 466-8555, Japan
[b]Frontier
Research Institute for Material Science, Nagoya Institute of Technology, Gokisocho, Showa-ku, Nagoya 466-8555, Japan
Abstract: Charge transfer interaction at the interface of semiconducting layered materials is of great interest to develop effective heterojunction optoelectronic devices. Here, we demonstrate the charge transfer interaction and formation of an active heterojunction between the molybdenum disulfide (MoS2) layer and p-type copper iodide (CuI) exhibiting excellent photoresponsive properties. The CuI film was fabricated by solid phase iodization of a copper film and direct thermal evaporation processes, which lead to formation of a transparent conducting layer. The thermally evaporated CuI film showed the main diffraction peak for (111) plane, confirming the formation of γ-CuI cubic crystal structure along the (111) plane on the MoS2 layers. Photoluminescence (PL) quenching effect was observed for the γ-CuI/MoS2 heterostructure, which can be attributed to the spontaneous separation of charge carriers at the interface. A photoresponsivity of 0.27 A/W was obtained at a bias voltage of 5V for the γ-CuI/MoS2 heterojunction device with illumination of monochromatic light. Our finding shows that the excellent photoresponsivity in the fabricated heterojunction is due to the formation of an effective interface between the two materials for efficient exciton dissociation and charge separation. Keywords: n-type MoS2; p-type CuI; cubic crystalline structure; interface quality; photoresponsive device 1 ACS Paragon Plus Environment
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1. Introduction Transition metal dichalcogenides (TMDCs) based layered materials have attracted significant attention for developing novel nanoelectronic and optoelectronic devices considering their unique band structure properties.1-4 The transition from indirect to direct band gap for monolayer, light-matter interaction, spin and valley physics are the fascinating aspects of the two-dimensional (2D) TMDCs.2-9 Among the family of TMDCs, molybdenum disulfide (MoS2) with n-type semiconducting nature has been utilized for fabrication of transistors, memory devices, photodetectors, tunneling diodes, light-emitting diodes, and photovoltaic devices.1,
6-8,9-22
Field effect transistor (FET) fabricated with single layer MoS2 showed
reasonable room temperature mobility, and excellent switching characteristics with on/off current ratio of ~1010.23,24 Photoluminescence (PL) efficiency with near unity quantum yield has been achieved for the chemically treated sulfur-based TMDCs layers.25 In addition, TMDCs based layered materials have been used for fabrication of Schottky and p-n junctions showing ultrahigh photosensitivity for photodiodes and solar cell applications.26–32 Integration of the TMDCs layers with three-dimensional (3D) bulk semiconductor (such as silicon, gallium nitride, etc.) has been also explored to create 2D/3D active heterojunction devices.32-38 The MoS2-silicon heterojunction is the most explored system for fabrication of efficient photosensors and solar cells.36,38 The 2D TMDCs can be readily integrated to fabricate a heterostructure without constraints of lattice matching due to the dangling bond free lattice and strong van-der-waals interaction. Another important aspect in such heterostructures is the ultrafast charge transfer interaction process at the interfaces, which enables the development of high photoresponsive and efficient optoelectronic devices.39,40 The charge transfer interaction in two different TMDCs layered materials such as MoS2-WSe2 heterojunction has been investigated.40 The stack of MoS2 and WSe2 monolayer forms a p-n junction, creating a built-in electric field across the interface thereby facilitating efficient 2 ACS Paragon Plus Environment
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ACS Applied Electronic Materials
electron-hole separation and transfer.39,40 The charge transfer interaction in TMDCs layered materials with wide bandgap semiconductors has also significant importance considering the possibility of developing transparent van-der-waals heterojunction devices. The integration of the TMDCs layered materials with gallium nitride and silicon carbide semiconductors has been investigated for optoelectronic device applications.38,41 However, most of the oxide and nitride based wide bandgap semiconductors are normally n-type, whereas the counterpart TMDCs layers are normally p-type. The γ-copper iodide (γ-CuI) with p-type conductivity is one of the notable halide wide bandgap (~3.1 eV) semiconductor with 2D sheet-like structures and can be significant to integrate with n-type TMDCs layered semiconductors.42-45 γ-CuI with excellent hole-transporting property has a great prospect in high efficiency solar cells, photosensors and other device applications.45,46 The charge transfer interaction and formation of effective junction potential in p-type γ-CuI and n-type TMDCs layers are still to be explored for fabrication of efficient optoelectronic device components. In this prospect, we explored the possibility of fabricating γ-CuI/MoS2 based heterojunction and its interface properties for developing a photoresponsive device. Significant quenching of PL peak for the MoS2 layer in γ-CuI/MoS2 heterostructure was observed attributing to the spontaneous separation of charge carriers at the interface. Excellent photoresponsivity was obtained for the γ-CuI/MoS2 heterojunction device with illumination of monochromatic light, indicating the possibility of application in the photodiodes and solar cells. This can be attributed to the effective exciton dissociation and charge separation at the interface of γ-CuI (111) and MoS2 layer heterostructure due to the presence of a strong build-in field. In what follows, we demonstrate the fabrication process of γ-CuI cubic crystal structure along the (111) plane on the MoS2 layer and studied their interface properties to develop a photoresponsive device. 2. Experimental section 2.1 CVD Synthesis of MoS2 layer 3 ACS Paragon Plus Environment
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MoS2 layer was synthesized by chemical vapor deposition (CVD) on SiO2/Si and Al2O3 substrates. A modified assembly of small inner tube was used in conjunction with a regular quartz tube and with two furnace system. The substrates were cleaned using isopropyl alcohol and acetone each for 10 minutes with sonication. Subsequently, the substrate was dried using argon gas flow to avoid atmospheric contamination. Sulfur (1gm) was kept in one end of the inner small tube under low temperature furnace (LTF) at 200 °C and MoO3 powder (10mg) was kept in ceramic boat covered (face-down) with cleaned SiO2/Si substrate, at the other end of the small inner tube under high temperature furnace (HTF) heated to 750 °C. In the optimized condition, HTF was started first to attain the temperature of 600 °C, at this point the LTF heating is started. As the LTF reaches 200 °C, the HTF also reaches evaporation temperature for MoO3. It is observed that the evaporation of sulfur and MoO3 powder need be synchronized to get a uniform layer of MoS2 on the substrate. Growth time was kept at 40 minutes after which, first LTF was cooled down to avoid any secondary sulfur deposition on substrate surface and then HTF was cooled to room temperature. 2.2 CuI deposition on MoS2 layers Two different approaches were explored for the deposition of CuI on MoS2 layers. In one approach, first copper (Cu) film was deposited on MoS2 substrate using thermal evaporator. 0.1gm of Cu (on tungsten boat) was evaporated at 40A current under a chamber pressure of 6x10-4 Pa for three minutes. Subsequently, the Cu film on MoS2 layer was treated with iodine (I) vapor created by iodine beads (approximately 20gm) in atmospheric condition and room temperature (25 °C) in a sealed box for 2 hr. as shown in figure 2a. In another approach, CuI powder appx. 0.05 gm in tungsten boat (Wako Pure Chemical Industries Ltd. Purity-95%) was directly deposited using vacuum thermal evaporator at a chamber pressure of 6x10-4 Pa and at current of 40A for about 20 seconds. The substrate gets slightly heated (
