Synthesis of 2D Quantum Materials for Nanoelectronic and Nanophotonic Applications

Citation

Lin, Wei-Hsiang (2021) Synthesis of 2D Quantum Materials for Nanoelectronic and Nanophotonic Applications. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/vh7k-4w84. https://resolver.caltech.edu/CaltechTHESIS:04262021-085536699

Abstract

2D materials have attracted tremendous attention for a variety of properties such as ultra-low body thickness, ultra-high mobility, and tunable bandgap. These unique merits of the 2D materials bring in the significant improvements and new perspectives in the digital CMOS scaling, analog performance, as well as the 3D integration of wafer stacking.

In this thesis, we explore van der Waals materials for future CMOS technologies. Chapter 2 introduces a compatible and a single-step method for synthesizing high-mobility monolayer graphene (MLG) in merely a few minutes by means of plasma-enhanced chemical vapor deposition (PECVD) techniques without the need of active heating. This environment enables graphene growth on different surfaces at relatively low temperatures, which paves ways to a CMOS-compatible approach to graphene synthesis. Chapter 3 describes the development of a synthesis method that controls the growth of large-area h-BN films from monolayer to 30 atomic layers, and summarizes the characterizations of the properties of these h-BN films that demonstrate the high-quality of these materials.

New degrees of freedom possess the immense potential and attract huge attentions as the imminent end of "Moore's Law". Compared with the traditional charge degree of freedom, spin and valley are the other two additional internal degree of freedom in solid-state electronics which enable the spintronic and valleytronic devices with high integration density, fast processing speed, low power dissipation, and non-volatility. Monolayer transition-metal dichalcogenides (TMDCs) in the 2H-phase are semiconductors promising for opto-valleytronic and opto-spintronic applications because of their strong spin-valley coupling. In chapter 4, we report detailed studies of opto-valleytronic properties of heterogeneous domains in CVD-grown monolayer WS₂ single crystals. By illuminating WS₂ with off-resonance circularly-polarized light and measuring the resulting spatially resolved circularly-polarized emission (P_circ), we find large circular polarization increases significantly to nearly 90% at 80 K. In Chapter 5, it is reported that valley polarized PL of monolayer WS₂ can be efficiently tailored at room temperature (RT) through the surface plasmon-exciton interaction with plasmonic Archimedes spiral (PAS) nanostructures. The DVP of WS₂ using 2 turns (2T) and 4 turns (4T) of PAS can reach up to 40% and 50% at RT, respectively. Further enhancement and continuous control of excitonic valley polarization in electrostatically doped monolayer WS₂ are demonstrated. Under the circularly polarized light on WS₂-2TPAS heterostructure, 40% valley polarization of exciton without electrostatic doping is icreased to 70% by modulating the carrier doping via a backgate. This enhancement of valley polarization may be attributed to the screening of momentum-dependent long-range electron-hole exchange interactions. The demonstration of electrical tunability in the valley-polarized emission from WS₂-PAS heterostructures provides new strategies to harness valley excitons for application in ultrathin valleytronic devices.

In contrast to future optical switch applications, in Chpater 6, it is reported that Ternary tellurides based on alloying different 2D transition metal dichalcogenides can result in interesting new 2D materials with tunable optical and electrical properties. Additionally, such alloys can provide opportunities for significantly improving the electrical contact properties at the metal-semiconductor interface. In particular, realization of practical devices based on the 2D materials will require overcoming the typical Fermi-level pinning limitations of the electrical contacts at the metal-semiconductor interface and ultimately approaching the ideal Schottky-Mott limit. In this work, we develop a simple method of stacking 3D/2D electrical metal contacts onto dangling-bond-free 2D semiconductors in order to surmount the typical issue of Fermi-level pinning. Specifically, contacts of Au, graphene/Au, and WTe₂/Au are transferred onto WS_1.94Te_0.06 alloy-based devices via a new transfer method. The WS_1.94Te_0.06 field-effect transistors (FETs) with WTe₂/Au contacts reveal a field-effect mobility of 25 cm²V⁻¹s⁻¹, an on/off current ratio of 10⁶, and extremely low contact resistance of 8 kΩ μm. These electrical properties are far more superior to similar devices with either Au or graphene/Au contacts, which may be attributed to the fact that the work function of WTe₂ is close to the band edge of the WS_1.94Te_0.06 alloy so that the resulting metal-semiconductor interface of the FETs are free from Fermi-level pinning. The Schottky barrier heights of the WS_1.94Te_0.06-FETs with WTe₂/Au contacts also follow the general trend of the Schottky-Mott limit, implying high-quality electrical contacts. Finally, in Chapter 7, several promising opportunities were proposed for future CMOS integrated circuits based on monolayer semiconductors.

Thesis Files