Hassel and Marianne Ledbetter MatSE Colloquium - "Understanding and Mitigating Defects in Two-Dimensional Semiconductors"

Current semiconductor technology largely relies on three-dimensional materials, where dangling bonds are unavoidable and the resulting disorder at dissimilar interfaces are unavoidable. 2D semiconductors, which lack dangling bonds, are one of the leading candidates for next-generation transistor materials. Of particular focus for the semiconductor community is a class of 2D semiconductors known as transition metal dichalcogenides (TMDs). However, compared to industrial semiconductor materials (e.g. silicon), defects in 2D semiconductor TMDs are substantially high. Despite several different approaches to modifying growth parameters, the majority of monolayer TMDs grown are limited to carrier mobilities of just a few thousand cm2/Vs at cryogenic temperatures. These low carrier mobilities in TMDs are primarily attributed to scattering by charged defects (typically around 0.1% to 1% of all atomic sites) and isovalent defects (~1% to 5% of all atomic sites). In recent years, a promising synthesis route has been identified for producing high-quality transition metal selenides and tellurides using excess chalcogen as a flux; reducing defect densities to 0.001% and 0.1% for charged and isovalent defects, respectively. This reduction in defects shows considerable improvement in carrier mobility for monolayer WSe2, with measured low-temperature hole mobilities of nearly 70,000 cm2/Vs. Though the chalcogen self-flux method has improved TMDs considerably, the residual defect densities are still relatively high compared to state-of-the-art semiconductors. In addition, while the origin of isovalent defects have been identified as primarily oxygen substitutions at the chalcogen site, the origin of the much more impactful charged defects has remained relatively uninvestigated. Without knowing the identity of the prominent charged defect types present in TMDs, it’s nearly impossible to develop further strategies for mitigating these defect types. In this talk I will discuss some of the targeted strategies we have implemented for mitigating oxygen substitutional impurities in TMDs and the limits of these strategies. In addition, I will discuss the efforts we have made in identifying the potential candidates for charged defects in TMDs and possible strategies to further mitigate these charged defects in transition metal sulfides, selenides, and tellurides.