Correlation between Defect Type, Doping Level and Schottky Barrier in Hexagonal WS₂

One of the representative two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have been widely studied for next generation device application. To realize the high-performance device application of 2D TMDs, it is necessary to understand not only the metal/channel interface related to SBH and contact resistance (R_C) but also properties of channel material. Previous studies approach channel material doping and defect engineering at interface. However, it has not been reported how the defect types and doping concentration of TMDs control the device characteristics on same material.<br/> We investigated the various characteristic analyses depending on two different defects region in a single flake of hexagonal WS₂ FETs. Through photoluminescence (PL) intensity mapping and Raman frequency mapping, it was confirmed that the sulfur vacancy (SV) and the tungsten vacancy (WV) had distinct regions in a single hexagonal WS₂.<br/> Various electrical characteristics on the SV FET and the WV FET at 300 K were compared. In the I_d-V_g characteristics, the SV FET had lower threshold voltage (V_th) and approximately 6 times larger I_d at V_g =60 V than those of the WV FET. On/off ratio of the SV FET had 5 times larger than that of the WV FET. Difference in current was originated from conductivity of each defect region. Using each channel conductivity to extract doping concentration (n) of the SV and WV FETs, they had values of 1.29*10¹⁹ cm^-3, and 1.44*10¹⁸ cm^-3, respectively. Difference in the doping concentration may affect the effective SBH between the SV and WV FETs.<br/> Through the low frequency noise characteristics, the magnitude of the power spectral density (PSD) of the SV FET was approximately 5 times higher than that of WV FET. Normalized PSD of the SV and WV FETs was well fitted by carrier number fluctuation with a correlated carrier mobility fluctuation (CNF+CMF) model. The trap density (N_t) and Coulomb scattering coefficient (α) of the SV and WV FETs were extracted by CNF+CMF model. The N_t of the SV and WV FET was similar with the values of 3.2*10²⁰ cm^-3eV^-1 and 4.0*10²⁰ cm^-3eV^-1, respectively. However, the α related to Coulomb scattering impurity densities of the SV FET (7.7*10⁷ VsC^-1) had 4 times higher than that of WV FET (2.0*10⁷VsC^-1). Because of the reduced channel dimensionality, α for the SV and WV FETs was determined by the carrier density n. Therefore, the SV FET with higher doping concentration had larger α than that of WV FET.<br/> Temperature dependent activation energy (E_a) and SB characteristics for the SV and WV FETs were analyzed according to temperature (300 K – 420 K). In the Arrhenius plot for on current, the trend of E_a for SV and WV FETs was different from 360 K. In high temperature (360 K – 420 K), the E_a of the SV and WV FET was similar as 53 meV and 57 meV, respectively. Before 360 K, the E_a of WV FET had 115 meV, twice that of high temperature region, while the SV FET had a similar 71 meV. In other words, the SV FET was operated without a change in the carrier transport mechanism at the metal-channel interface. However, in the WV FET, the main carrier transport mechanism was changed at 360 K. The SV FET with a higher doping concentration became sharper and narrower SBH than the WV FET. The current of the SV FET flowed well through tunneling at all temperatures. In contrast, most of current was dominant on the external energy because of relatively thicker and higher SBH in the WV FET.<br/> In this study, we compared the various characteristics on SV FET and WV FET in a hexagonal WS₂ FET. Since the SV and the WV have different doping concentration within single material, it is highly look forward to studying the various electronic device application using hexagonal WS₂.