P-Type Doping of Tungsten Diselenide (WSe₂) Thin Films Using Nitric Oxide (NO)

WSe₂ has garnered interest for use in complementary metal–oxide–semiconductor (CMOS) applications because it is one of the few transition metal dichalcogenides (TMDs) with inherent p-type conductivity. However, Fermi level pinning near the mid-gap prevents the full realization of high-performance WSe₂ devices. Additionally, the relatively low intrinsic carrier concentration limits the film's conductivity. P-type doping, however, can address both challenges. Doping the material at the metal contact to the degenerate limit can de-pin the Fermi level, and lightly doping the channel region can improve device on-currents.<br/>Previously it has been reported that p-type doping of WSe₂ can be accomplished by substituting the metal site with acceptors such as V, Rh, and Ta. Although this results in stable incorporation due to the six-fold bonding coordination, such substitution must be done during film growth when reactive metal sites are accessible, and the energetics are favorable. Alternatively, others have shown that atomic substitution at the chalcogen site can be accomplished with the pnictogens (N, As, etc.) after film growth due to the accessibility of the chalcogen sites at the crystal surface and the availability of native vacancies. Nitrogen-based plasmas can be used to provide the energy and generate highly reactive species for this chalcogen substitution, but this approach can lead to excessive film damage. Among all the pnictogens, nitrogen is an ideal candidate because it has the lowest formation energy for chalcogen substitution. Others have shown that thermal treatment of processed WSe₂ field-effect transistors under an atmosphere of NO results in substantial improvements in on-state currents. However, the mechanism of p-type doping of WSe₂ by NO and its impact on contact resistance are not yet fully understood.<br/>In this work, we report on a low temperature post-growth annealing process to substitute NO at Se sites and induce p-type doping in WSe₂. MOCVD was used to grow high quality few-layer epitaxial films of WSe₂ at the 2-inch wafer scale on c-plane sapphire. Using a hot walled tube furnace setup, these films were then exposed to NO at temperatures ranging between 60 - 300 °C for durations from 2 - 120 minutes. Complementary studies were conducted under air and ultra-high purity argon. These results help elucidate the chemical action of NO as a doping agent versus air induced oxidation and deconvolute purely thermally derived effects.<br/>The films were characterized by scanning electron and atomic force microscopy (SEM and AFM) techniques as well as Raman, photoluminescence (PL) and x-ray spectroscopy (XPS). XPS results indicate substitution beginning at temperatures as low as 100 °C and a complete conversion to a suboxide of tungsten oxynitride (WN_xO_1-x) above 250 °C. Further, the doping amount can be incrementally tuned by modulation of the annealing temperature and time. AFM results show a 2D morphology is still maintained after complete conversion from tungsten diselenide (WSe₂) to a suboxide of tungsten oxynitride (WN_xO_1-x).<br/>Treated films were transferred to SiO₂substrates where back gated field effect transistor (FET) arrays were fabricated and tested. FET arrays fabricated from untreated WSe₂ were also annealed under NO across the same range of temperatures and durations. FET characteristics were extracted, and preliminary results show a clear improvement in FET device performance with I_on values improving by at least three orders of magnitude. Additionaly, doping to the degenerate limit was also achieved. These results are of interest because they indicate a viable path towards the realization of high-performance nanoscale WSe₂-based CMOS devices.