Full-chip synthesis and transport characterization of monolayer semiconducting transition metal dichalcogenides
- In recent years, monolayer (1L) semiconducting transition metal dichalcogenides (TMDs) have attracted significant attention for use in a plethora of electronic applications, including logic and memory. However, few studies have examined important metrics for scaling device numbers up and dimensions down for eventual system-level applications. Among the many problems that have been waiting to be solved are demonstration of large area synthesis, description of device-to-device variation, and characterization of velocity saturation. In this work, I will explore these issues principally for 1L MoS2. First, I highlight the recent advancements in solid-source chemical vapor deposition of 1L two-dimensional (2D) semiconductors. I begin by reviewing the theory and conditions necessary for van der Waals epitaxy of large, highly crystalline films of 1L materials. Using this understanding, I establish growth of MoS2, WS2, MoSe2, and WSe2 crystals on SiO2, and perform extensive spectroscopic characterization verifying the quality and integrity of the materials. For MoS2, I also demonstrate an optimized process that yields films large enough (~cm^2) to fabricate thousands of devices simultaneously by optical lithography. Next, I discuss the low-field electrical transport of devices fabricated from these large MoS2 films. In additional to careful measurements revealing the highest mobility (30 to 50 cm^2/V/s) and lowest contact resistance (~1 kΩ∙μm) in any undoped 1L semiconductor, I employ statistical data for devices fabricated on a single, die-sized film to show exceptionally low values for hysteresis (< 5 mV per nm of equivalent SiO2 thickness), and low variation in intrinsic carrier density (~10^11 cm^-2) and drift mobility (~3 cm^2/V/s). I also demonstrate remarkable reliability through bias temperature instability measurements, which constitute an improvement by two orders of magnitude compared to other 2D devices. Finally, I focus on measuring the high-field saturation velocity of electrons in 1L MoS2. A detailed thermal model is used to account for device self-heating, and reveals that devices in high-field operation self-heat by ~200 °C, severely limiting drift velocity. Temperature-dependent measurements subsequently reveal a velocity of 4.3×10^6 cm/s at 295 K, high enough to allow for saturation currents to easily exceed 1.5 mA/μm for digital electronics. This work represents the first evaluation of its kind carried out for 2D electronics, and addresses critical issues to consider for scaling 1L semiconductors for larger applications.
|Type of resource
|electronic; electronic resource; remote
|1 online resource.
|Smithe, Kirby Kurtis Hayes
|Stanford University, Department of Electrical Engineering
|Wong, Hon-Sum Philip, 1959-
|Wong, Hon-Sum Philip, 1959-
|Statement of responsibility
|Kirby Kurtis Hayes Smithe.
|Submitted to the Department of Electrical Engineering.
|Thesis (Ph.D.)--Stanford University, 2018.
- © 2018 by Kirby Smithe
- This work is licensed under a Creative Commons Attribution 3.0 Unported license (CC BY).
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