Contact, Interface, and Strain Engineering of Two-Dimensional Transition Metal Dichalcogenide Field Effect Transistors
Open Access
- Author:
- Schulman, Daniel Scott
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 06, 2019
- Committee Members:
- Saptarshi Das, Dissertation Advisor/Co-Advisor
Saptarshi Das, Committee Chair/Co-Chair
Susan E Trolier-Mckinstry, Committee Member
Joshua Alexander Robinson, Committee Member
Sukwon Choi, Outside Member - Keywords:
- Electro-Ablation
Field Effect Transistors
Transition Metal Dichalcogenides
Steep Slope Transistor
Strain
Contact Resistance - Abstract:
- For nearly half a century, the advancements in microelectronics have been driven by the predictable miniaturization of semiconductor devices, the heart of which lies the complementary metal oxide semiconductor (CMOS) field effect transistor (FET). The year after year exponential increase in computation power per dollar, an observation colloquially known as Moore’s Law, is no longer sustainable as fundamental limits to materials and device physics are being reached. As the layers in these semiconductor devices are reaching nanometer level thicknesses, two-dimensional (2D) materials such as the transition metal dichalcogenides (TMDCs) are a natural choice to be used in these highly scaled devices. Likewise, the diversity in the different types of 2D materials, their inherent flexibility, high surface to volume ratio, large carrier mobilities, etc. make these materials appealing for “More than Moore” devices which aim to add additional functionality, i.e. low-cost devices, sensors, etc., to existing CMOS technologies. The work in this dissertation focused on four topics related to TMDC semiconductor devices. First, a process to fabricate high performance TMDC FETs using a high κ Al2O3 back-gate on a Pt/TiN/Si substrate with an equivalent oxide thickness (EOT) of 22 nm and channel lengths as small as 100 nm is presented. TMDC FET structures prior to this work are still relatively rudimentary with channel lengths >1 μm and EOT’s >100 nm. Many of the advantages of a 2D semiconductor are only realized when the device electrostatics, i.e. the EOT and channel length, are also scaled. In this work, MoS2, WS2, MoSe2, and WSe2 FETs have been demonstrated with steep subthreshold slope (SS) values as low as ~100 mV/dec and MoS2 contact resistances of ~1kΩ -μm. The fundamental operating physics of these TMDC FETs is still poorly understood, particularly how charge is injected from the metal contacts into the FET channel. This work elucidates how the transistor characteristics in devices with a back-gated geometry, in many regimes of operation, are entirely determined by the gate voltage dependent contact resistance. A dual gate, i.e. a back-gate and top-gate, structure was used to develop and validate a virtual source based model which includes these gated contact effects. When these effects are not recognized, the field effect mobility can be severely either underestimated or overestimated, preventing these materials from being properly characterized. Next, an original sub-60mV/dec subthreshold slope device called the 2D Strain Field Effect Transistor (2D-SFET) which uses the band gap strain response of TMDC materials coupled with a piezoelectric material was explored theoretically and experimentally. A physics-based model was developed to understand how the piezoelectric is able to modulate the bandgap in the 2D-SFET. Landauer ballistic transport simulations show that a sub-60mV/dec SS is achievable. The improvement, ~50 mV/dec, is modest and comes with a large parasitic capacitance arising from the thin, high dielectric constant piezoelectric. Experimentally, the strain in a MoS2/PZT structure was measured in-situ using Raman spectroscopy. While in-plane MoS2 strain was measured and was consistent with finite element analysis (FEA) simulations, no out-of-plane stress was measured, indicating poor strain transfer in the device. TMDC materials in their single layer form, ~0.65 nm thick, are of particular interest for their unique ultra-thin and optoelectronic properties. Electro-ablation (EA) is a thinning process which converts multilayer TMDC materials into a single monolayer via electrochemical oxidation. This work studied the chemistry of the EA process, particularly the effect of pH, solvent, TMDC material, substrate, electrical potential, etc., and characterized them using atomic force microscopy, Raman spectroscopy, photoluminescence, etc. Monolayer MoS2, WS2 and MoSe2 FETs derived from EA materials were then fabricated. The field effect mobility and FET characteristics were on par with the state of the art monolayers grown via chemical vapor deposition (CVD).