Two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors are potential candidates as channel materials for future CMOS technology. A major roadblock in realizing a technologically relevant MOSFET with 2D materials is the lack of methods of creating fixed, controllable doping, which is a critical element in MOSFET design, especially for realizing low resistance source and drain contacts. This work investigates doping techniques such as spin-on-diffussant (SoD) processing and ion implantation to realize doping in 2D materials such as MoS2 and WSe2.
For both materials (flakes and large area CVD-grown), significant changes in the current-voltage characteristics is observed with commercially available phosphorus-containing SoD. By spin-coating and curing at up to 1000 C, the ON state current is improved to ~100 uA/um while the gate modulation of the channel current is almost completely suppressed. With MoS2 which typically shows n-type behavior, only the OFF-state current is affected. On the other hand, WSe2, which typically shows ambipolar behavior, the I-V properties show a dependence with anneal temperature: at 500 C, the n-branch and the OFF-state currents are significantly suppressed while the p-branch current significantly increases; for annealing at 1000 C, the ON-state current remains the same while the OFF-state current increases by a few orders of magnitude. Raman measurements show that the characteristic peaks for the TMDC are not significantly affected by the SoD processing, indicating preservation of the TMDC crystal structure. This, combined with the gate-field independent conductance indicates a change of the background charge density in TMDC, and is an encouraging result for contact-region doping in TMDC.
With ion-implantation, directly implanting the TMDC with species such as P, N and As with ion energies 5-20 keV is observed to lead to significant crystal damage, indicated by a decrease in the characteristics Raman peak intensity as a function of ion energy. Electrically, both the OFF and the ON state currents are degraded, likely due to the amorphization of the TMDC. Post-implant anneals up to 1000 C are observed to not significantly affect the characteristics. SIMS measurements indicate the presence of the ion species inside the MoS2, suggesting the anneal temperatures are insufficient for dopant activation. Indirect ion implantation approaches such as implanting the substrate prior to MoS2 growth are explored. XPS analysis shows barely detectable levels of the ion species inside the MoS2, consistent with the relatively lower effective ion dose achieved with this method. However, electrical characterization shows shifts in the threshold voltage, suggesting modifications in the background charge in the MoS2, without degradation in the ON or OFF-state currents. A statistical improvement in carrier mobility is also observed with substrate implantation, likely due to modification of the charged impurity distribution.