ABSTRACT
Despite growing interest in doping two-dimensional (2D) transition metal dichalcogenides (TMDs) for future layered semiconductor devices, controllability is currently limited to only heavy doping (degenerate regime). This causes 2D materials to act as metallic layers, and an ion implantation technique with precise doping controllability is not available for these materials (e.g., MoS2, MoSe2, WS2, WSe2, graphene). Since adjustment of the electrical and optical properties of 2D materials is possible within a light (nondegenerate) doping regime, a wide-range doping capability including nondegenerate and degenerate regimes is a critical aspect of the design and fabrication of 2D TMD-based electronic and optoelectronic devices. Here, we demonstrate a wide-range controllable n-doping method on a 2D TMD material (exfoliated trilayer and bulk MoS2) with the assistance of a phosphorus silicate glass (PSG) insulating layer, which has the broadest doping range among the results reported to date (between 3.6 × 10(10) and 8.3 × 10(12) cm(-2)) and is also applicable to other 2D semiconductors. This is achieved through (1) a three-step process consisting of, first, dopant out-diffusion between 700 and 900 °C, second, thermal activation at 500 °C, and, third, optical activation above 5 µW steps and (2) weight percentage adjustment of P atoms in PSG (2 and 5 wt %). We anticipate our widely controllable n-doping method to be a starting point for the successful integration of future layered semiconductor devices.
ABSTRACT
Despite heightened interest in 2D transition-metal dichalcogenide (TMD) doping methods for future layered semiconductor devices, most doping research is currently limited to molybdenum disulfide (MoS2), which is generally used for n-channel 2D transistors. In addition, previously reported TMD doping techniques result in only high-level doping concentrations (degenerate) in which TMD materials behave as near-metallic layers. Here, we demonstrate a controllable nondegenerate p-type doping (p-doping) technique on tungsten diselenide (WSe2) for p-channel 2D transistors by adjusting the concentration of octadecyltrichlorosilane (OTS). This p-doping phenomenon originates from the methyl (-CH3) functional groups in OTS, which exhibit a positive pole and consequently reduce the electron carrier density in WSe2. The controlled p-doping levels are between 2.1 × 10(11) and 5.2 × 10(11) cm(-2) in the nondegenerate regime, where the performance parameters of WSe2-based electronic and optoelectronic devices can be properly designed or optimized (threshold voltage↑, on-/off-currents↑, field-effect mobility↑, photoresponsivity↓, and detectivity↓ as the doping level increases). The p-doping effect provided by OTS is sustained in ambient air for a long time showing small changes in the device performance (18-34% loss of ΔVTH initially achieved by OTS doping for 60 h). Furthermore, performance degradation is almost completely recovered by additional thermal annealing at 120 °C. Through Raman spectroscopy and electrical/optical measurements, we have also confirmed that the OTS doping phenomenon is independent of the thickness of the WSe2 films. We expect that our controllable p-doping method will make it possible to successfully integrate future layered semiconductor devices.
ABSTRACT
Deoxyribonucleic acid (DNA) and two-dimensional (2D) transition metal dichalcogenide (TMD) nanotechnology holds great potential for the development of extremely small devices with increasingly complex functionality. However, most current research related to DNA is limited to crystal growth and synthesis. In addition, since controllable doping methods like ion implantation can cause fatal crystal damage to 2D TMD materials, it is very hard to achieve a low-level doping concentration (nondegenerate regime) on TMD in the present state of technology. Here, we report a nondegenerate doping phenomenon for TMD materials (MoS2 and WSe2, which represent n- and p-channel materials, respectively) using DNA and slightly modified DNA by metal ions (Zn(2+), Ni(2+), Co(2+), and Cu(2+)), named as M-DNA. This study is an example of interdisciplinary convergence research between DNA nanotechnology and TMD-based 2D device technology. The phosphate backbone (PO4(-)) in DNA attracts and holds hole carriers in the TMD region, n-doping the TMD films. Conversely, M-DNA nanostructures, which are functionalized by intercalating metal ions, have positive dipole moments and consequently reduce the electron carrier density of TMD materials, resulting in p-doping phenomenon. N-doping by DNA occurs at â¼6.4 × 10(10) cm(-2) on MoS2 and â¼7.3 × 10(9) cm(-2) on WSe2, which is uniform across the TMD area. p-Doping which is uniformly achieved by M-DNA occurs between 2.3 × 10(10) and 5.5 × 10(10) cm(-2) on MoS2 and between 2.4 × 10(10) and 5.0 × 10(10) cm(-2) on WSe2. These doping levels are in the nondegenerate regime, allowing for the proper design of performance parameters of TMD-based electronic and optoelectronic devices (VTH, on-/off-currents, field-effect mobility, photoresponsivity, and detectivity). In addition, by controlling the metal ions used, the p-doping level of TMD materials, which also influences their performance parameters, can be controlled. This interdisciplinary convergence research will allow for the successful integration of future layered semiconductor devices requiring extremely small and very complicated structures.
