Enhancing Carrier Mobility with Process Induced Strain in Monolayer MoS₂ Transistors

Two-dimensional materials are a promising candidate for future non-silicon electronics based in the "More Moore" development path, owing to their ability to circumvent many fundamental challenges hindering the continued scaling of current silicon-based technologies. Using atomically-thin channels strongly suppresses undesirable short channel effects such as drain-induced barrier-lowering and body punchthrough. However, there are important knowledge gaps in how thin film deposition materials and processes impact 2D materials properties, leading to enormous variability in reported device performance metrics. Understanding these interactions has practical implications on improving not just the reliabililty but also enhancing the performance of 2D transistors. Analogously, one of the many commercially implemented performance enhancement techniques in CMOS is the deposition of high stress capping layers, which increase the carrier mobilities of NMOS and PMOS transistors. The key question is how thin film deposition will impact doping and strain in 2D materials, and how to unravel their relative effects. Here, we systematically quantify the effects of thin film processed induced strain on the carrier mobility of monolayer MoS₂ transistors. To achieve this, we encapsulate the monolayer MoS₂ transistors under a HfO₂ barrier layer via atomic layer deposition before using E-beam evaporation to sequentially deposit MgO films on the same set of devices, while measuring the Raman spectra and transport between each deposition. The evaporated MgO film has a built-in internal compressive stress, which applies a tensile strain to the underlying transistor channel. This approach both maximizes data consistency by removing sample-to-sample variation.<br/><br/>We use vector decomposition on the Raman peak shifts in the channel to unravel the relative strain and doping induced by the film deposition. We found a systematic shift in the strain as a function of MgO thickness reaching a value of 0.45% at 150 nm. The initial deposition of MgO created a doping shift of 3×10¹²/cm². The addition of the HfO₂ barrier suppressed further doping fluctuations to under 1×10¹²/cm². Raman mapping confirmed the uniformity of channel strain for up to 150 nm of MgO, at which point we observe slip induced nonuniformities in the strain distribution. We measured the transfer and output characteristics of the MoS₂ transistor as a function of film thickness. Under 0.45% tensile strain, the MoS₂ transistors exhibit a electron mobility enhancement of 40%, a saturation current enhancement of 45%, and a -15 V shift in the threshold voltage. This yields the rate of mobility enhancement of n-type MoS₂ to be 90% per percent of biaxial strain.<br/><br/>We are now extending this experiment to p-type transition metal dichalcogenide transistors as well as compressive strains, to fully realize strain-enhanced two-dimensional CMOS logic. We are applying the Transfer Length Method to systematically isolate the relative impact of contact resistances versus channel conductivity on transistor performance. These results form a critical foundation for the design and characterization of commercialized high-speed two-dimensional integrated electronics, using existing thin-film deposition technologies in industry.