Opto-Electronic Properties of Isotopically Purified Monolayer MoS₂

Two-dimensional (2D) semiconductors have emerged as candidates for nanoscale transistors and other opto-electronics. Gate-all-around (GAA) nanosheets, the next-generation transistor architecture, are projected to have exceptional electrostatic control especially with 2D semiconductor channels [1]. Molybdenum disulfide, MoS₂, is presently one of the most promising materials for n-type 2D transistors, owing to advancements made regarding its industrially compatible large-area growth quality [2], variety of doping techniques [3], and contact schemes [4].

Interestingly, Mo has a wide range of natural mass isotopes, the most common ones between 92 to 100 mass, which have been shown to affect phonon dynamics [5] and optical properties in various 2D heterostructures [6]. Such isotopically pure MoS₂ monolayers have displayed up to 50% higher thermal conductivity than naturally-occurring MoS₂, at room temperature [5]. However, how isotope purification impacts the growth quality of 2D semiconductors and their subsequent electrical performance remains entirely unexplored.

Here, we investigate the impact that isotope purification has on the growth of monolayer MoS₂, by characterizing its optical properties and field-effect transistor performance. We prepare monolayer MoS₂ by chemical vapor deposition (CVD), growing three types of samples using 92, 98, and natural isotope abundance MoO₃ precursors (denoted as 92-MoS₂, 98-MoS₂, and Nat-MoS₂). We then conduct extensive room-temperature Raman spectroscopic mapping to elucidate the effects that isotope engineering has on the phonon modes in monolayer MoS₂. We observe that the isotopically pure samples have sharper E' peaks of ~2.35 cm^-1 in linewidth (at 298 K) compared to Nat-MoS₂ with ~2.75 cm^-1, owing to decreased mass disorder. We confirm that Raman peak position varies with atomic weight, where we observe steep slopes of -23.3 cm^-1 amu^-1/2 and -36.3 cm^-1 amu^-1/2 for the E' and 2LA(M) modes respectively. We also conduct low-temperature photoluminescence (PL), where we observe enhanced contribution in the A-exciton peak and suppressed defect-mediated emission in the 92-MoS₂ at 10 K.

We also fabricate transistors of varying channel lengths (100 nm to 1 µm), where the MoS₂ films are grown directly onto 100 nm SiO₂ on p++ Si global back-gates. Our initial investigations suggest that the threshold voltage may be slightly tuned with atomic mass. Moreover, the isotopically pure 92-MoS₂ devices display roughly twice the transconductance and on-state current in long-channel structures. Overall, the 98-MoS₂ and Nat-MoS₂ devices performed similarly to each other, which matches the observations of low-temperature PL. Isotope purity is expected to yield only small modifications to the MoS₂ band structure [6], which cannot explain the improved electrical performance. Thus, we suggest that using a 92-MoO₃ precursor enables films with fewer defects and/or fewer charged impurities, ostensibly due to more uniform Mo diffusion during the growth process.

In summary, we have employed both Raman mapping and low-temperature PL to study the effects of isotope engineering on the properties of monolayer MoS₂. We find that using a 92-MoO₃ precursor yields higher quality films with double the electron mobility and improved PL emission. Further work entails visualizing the films more directly to quantify defects and impurities. These results suggest that isotope purification may be used as a platform to enhance 2D device performance. T.P. acknowledges support from the NSF MPS-Ascend Postdoctoral fellowship.

References:
[1] K.P. O’Brien, et al., Nat. Commun., 14, 6400 (2023).
[2] J. Zhu, et al., Nat. Nanotechnol., 18, 456-463 (2023).
[3] C.J. McClellan, et al., ACS Nano, 15, 1587-1596 (2021).
[4] Z. Sun, et al., ACS Nano, 18, 22444-22453 (2024).
[5] X. Li, et al., ACS Nano, 13, 2481-2489 (2019).
[6] Y. Yu, et al., Sci. Adv., 10, eadj0758 (2024).