Very-Thin Dopant Layer on MoS₂ Monolayer to Get Degeneracy/Heavily Doped Situation

Transition metal dichalcogenides (TMDCs) are an emerging class of materials with versatile and unique electrical, optical, physical, chemical, and mechanical properties. They have a general formula of MX₂, where M is a transition metal atom (like Mo, W, etc.), and X is a chalcogen atom (like S, Se, or Te). Examples of TMDCs, are molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂), tungsten disulfide (WS₂), and tungsten diselenide (WSe₂), etc. Because of its robustness like high mobility, excellent gate controllability, high on/off current ratio, ultra-low stand-by current, and good stability, etc., MoS₂ is the widely studied material in the family of TMDCs. MoS₂ monolayer behaves as a semiconductor with a direct electronic bandgap of 2.4 eV. It has potential applications in nanoelectronics, optoelectronics, and flexible electronics, etc.<br/><br/>Doping is a very significant approach to manipulate electronic and photonic characteristics of various 2D materials for advanced applications in neuromorphic hardware, logical circuits, and optoelectronic devices, because by doping, we can improve the device performance by controlling the carrier concentration. Moreover, in electronic devices, the doping technique can improve the on-state current by reducing the effective barrier and contact resistance height at the metal/TMD junction, as contact resistance is a major issue in 2D materials.<br/><br/>In the present work, at first, we fabricated a thin-film transistor (TFT) with MoS₂ ML as channel (with channel length of 5 µm), and Bi as source/drain. This TFT was then doped with an asymmetrical molecule (this molecule has a lone pair of electrons, which helps to dope MoS₂ ML. Doping has been done only in the channel region). It has been found that after doping, degeneracy is obtained. The doping was also confirmed from Raman and PL spectroscopy, which clearly shows red-shift in A_1g and A peaks, respectively.<br/><br/>The doping was then extended from a single device to an array of devices, where all the devices have MoS₂ ML as channel, and Bi as source/drain. This array was fabricated using photolithography, with channel width and length varied from 5 to 50 µm. The array was then again doped with the same asymmetrical molecule (discussed above) to understand the doping mechanism, i.e., how doping is done, when the channel length and width changes. Is the doping same or not, in all the devices. It has been found that for the array, doping is not uniform. In other words, doping depends more on channel width as compared to channel length, i.e., doping is more dominant on the devices that have 50 µm channel width and less dominant on the devices with 5 µm channel width. This may be due to the SF₆ etching, which is used to etch the ML. Apart from SF₆ etching, substrate wettability may also be a reason. This is also confirmed from the AFM studies, that doping on 50 µm channel width is more like a thin-film, while on 5 µm channel width is more like particle type.<br/><br/>Details will be discussed in the meeting.