The use of ultra low energy ion implantation is investigated as a doping method for MoS2. 200 eV Cl and Ar implants at doses between 1 × 1013 and 1 × 1015 cm−2 were introduced into exfoliated MoS2 flakes. XPS results for Cl implants show a decrease in core peak binding energies for Mo3d and S2p with increasing dose, implying a p-doping effect. Implantation of MoS2 device channel regions is shown to reduce the channel's conductivity. However, isolated implantation of the contact region with low doses (1 × 1013 cm−2) of Cl and Ar are shown to improve output characteristics by linearizing the IDS-VDS curves and by increasing current through the device. Cl was shown to be more effective than Ar at increasing the current, implying there is a potential chemical effect as well as damage effect. For higher doses (≥ 1 × 1014 cm−2), the current through the device is reduced with increasing dose for both implant species. This report presents the as-implanted, unannealed results. Post-implantation anneals may be necessary to activate the dopants and fully realize the potential of this doping method.

Molybdenum disulfide (MoS 2) is a promising potential replacement for Si in future microelectronic devices. Integration in electronic devices will likely involve the growth or transfer of large-area MoS 2 films onto substrates and subsequent isolation of devices. In this paper, the effect of ion implantation on the electrical properties of MoS 2 is reported. Large-area $4 layer MoS 2 films were implanted by low energy phosphorus plasma at biases of 100, 200, and 300 V and a dose of 1 Â 10 14 cm À2. Electrical measurements using patterned Ni/Au contacts show that after implantation, independent of bias, there is greater than a 10 4 increase in resistivity. TEM and Raman spectroscopy suggest that the film is crystalline prior to and after ion implantation and annealing and that there is no measurable sputtering following implantation. This suggests that the increase in resistivity is likely the result of radiation damage in the MoS 2. The thermal stability of the increase in electrical resistivity was assessed by a series of 15 min anneals beginning at 325 C in a sulfur overpressure and progressing up to 525 C under an Al 2 O 3 ALD cap. The resistivity increase remained unchanged after annealing. These results suggest that implant isolation could provide a preferable alternative to reactive ion etching or chemical etching for electrical isolation of MoS 2 .

The advent of 2D materials in recent years has brought the potential for new device applications at ever decreasing transistor design nodes.  Transition metal dichalcogenides have received much of that attention, with much of the focus on MoS2 thanks to its desirable bandgap.  Studies have shown the advantages of MoS2 devices in key performance metrics such as a high ION/IOFF ratio and a subthreshold swing that approaches or potentially exceeds the fundamental 60 mV/dec limit.  However, the contact resistance remains prohibitively high, and questions remain regarding the ability to dope these materials.  Numerous routes for doping MoS2 have been reported, including surface charge transfer via molecular adsorption, cation substitution during growth, chalcogenide substitution via immersion in solution, and plasma-assisted doping.  The effectiveness of these doping methods is displayed by reductions in contact resistance, increases in drive current, and in some instances a change from n-type to p-type behavior.  This talk will review these recent reports on MoS2 doping. Recent experiments have begun to explore the possibility of doping MoS2 by ion-implantation, which is a legacy doping technique that is often avoided with 2D materials due to their ultra-thin nature.  These experiments utilize ultra low implant energies of 200 eV  which places the species of interest (Cl+ and Ar+) completely within the 3-5 layers of MoS2 being investigated.  Cl is potentially a donor in MoS2 whereas Ar was used to determine the effect of the implant damage on the electrical properties.  These studies focused on room temperature implants at doses between 1 x 1013/cm2 and 1 x 1015/cm2.  Preliminary structural studies utilizing angle-resolved XPS indicate a Cl peak is around 1 nm deep into the MoS2, in rough agreement with Monte Carlo TRIM calculations.  STM studies have revealed a number of defects present after implantation, including sub-surface defects that may be sulfur vacancies as well as surface sulfur vacancies.  The density of these defects track with the dose. Relative to the pristine surface, STS studies have shown little change in tunneling current associated with the sub-surface defects, but enhanced tunneling current associated with the sulfur vacancies. It was observed that as the dose increases the oxidation of the surface as measured by XPS after exposure to the air increases.  Additionally, we are performing transmission line measurements to observe the electrical effect of the implantation.  Using lithography, transmission line masks were patterned onto individual exfoliated flakes. Photoresist was used as a mask to protect the channel and the implantation preceded contact metal (Ni/Au) deposition. Thus, only the contact area was implanted.  Prior to implantation, primarily non-linear IDS-VDS curves are observed for few-layer (<=10 layers) MoS2 flakes with no applied back-gate voltage. After implantation at lower doses, linear IDS-VDS curves are observed implying more ohmic contacts.  This is prior to annealing implying the electrical nature of the implant damage is contributing to better contacts.  However the effect is greater for Cl+ implants than Ar+ implants implying that the effect is not strictly a damage-related phenomenon. At higher doses the contacts went back to being non-ohmic and this may be associated with the aforementioned oxidation after higher dose implantation.  The dose dependence of the role of implant doping on contact resistivity will be discussed as well as the results from post implant annealing to facilitate dopant activation.

Device isolation is a critical but overlooked challenge in the effort to assess MoS2 as a possible material for post-Si technology nodes. Although reactive ion etching or chemical etching can be used to create isolated channels in large-area films, these techniques are problematic due to the high density of dangling bonds that they create. In this study, radiation damage is presented as an alternative to physical isolation of MoS2. Large-area ~4 layer MoS2 films were implanted by a low energy phosphine plasma at a bias of 200 V and a dose of 1 x 10 14 cm -2 . The results of this experiment indicate that there is greater than a 10 4 increase in sheet resistance of the MoS2 without measurable etching of the films. Annealing studies show that the resistance increase is stable up to at least 525°C, suggesting that radiation damage is an alternative to physical isolation for MoS2 devices.