UC Berkeley

Device architecture and materials innovations have enabled transistor scaling for the last several decades, boosting the performance of electronics, increasing the speed of communication and computational systems, lowering power consumption and reducing costs per operation. Two-dimensional (2D) materials have gained tremendous attention in the last decade, after the discovery of graphene which has exceptional properties like high carrier mobility, ultra-thin van der Waals connected layers (~ 0.3 nm thick), high tensile strength, etc.

Transition metal dichalcogenides (TMDs) are a family of 2D materials similar to graphene, with an important difference, many of them have an electronic bandgap. Some examples like MoS2 and WS2 are also direct-band gap materials at the monolayer limit (~ 0.7 nm thick). The impressive electronic and optical properties along with their ultra-thin nature, have made them potential candidates for use in future electronics and optoelectronic applications, along with other nanomaterials like carbon nanotubes.

Device physics and the electronic and optical properties of two-dimensional semiconductors are investigated in this thesis, with emphasis on TMDs. The first chapter presents an outline of the thesis and introduces 2D materials. Chapter 2 investigates the electronic properties of TMDs with focus on applications in sub-5 nm gate length transistors for low-power applications. MoS¬2 channel transistors with 1-nm long carbon nanotube (CNT) gate electrodes are experimentally demonstrated, showing good On/Off current ratio of ~ 10^6 and good subthreshold swing of ~ 65 mV/decade. The electrostatics of the transistors are investigated using simulations which demonstrate an effective channel length of ~ 1 nm in the On state and ~ 4 nm in the Off state.

Chapter 3 considers the impact of an atomic-scale gate on the performance of nanoscale transistors, for example 2D materials like graphene and metallic TMDs like WTe2, and 1D gates like graphene nanoribbons (GNRs) and CNTs. As the size of gates approaches atomic-limits, the low electronic density of states (DOS) in the gate limits the channel charge and thus the drain current. In addition, the gate DOS can be engineered to achieve a desired shape for the transfer characteristics of a transistor. The effect of gate quantum capacitance on nanoscale transistors is experimentally demonstrated, with the observation of room-temperature quantization features in CNT gated ultra-thin silicon-on-insulator (SOI) transistors (~ 3 nm thick SOI layer), which can be correlated to the Van Hove singularities in the 1D DOS of the CNT gate.

Strain engineering is an important tool used to boost mobility of carriers and enhance the performance of transistors. In chapter 4, the evolution of the electronic band structure of multilayer WSe2 as a function of uniaxial tensile strain is investigated using photoluminescence and Raman spectroscopy. A strain induced indirect to direct bandgap transition is observed in multilayer WSe2 with a ~ 35 x increase in photoluminescence in bilayer WSe2.

Chapter 5 focuses on a materials processing technique to selectively obtain large area monolayer 2D materials preferentially, with high yield. A gold mediated exfoliation technique is developed to selectively transfer monolayer 2D materials onto arbitrary substrates. A gold layer deposited on top of a 2D material crystal induces strain in the top-most monolayer, resulting in a reduction of the van der Waals coupling strength of the top-most monolayer with the bulk crystal. This enables the selective peeling of the top most monolayer with high predictability and large size. The monolayers obtained, for the specific examples of TMDs were characterized using electrical device, AFM and XPS measurements, and photoluminescence and Raman spectroscopy.

The next two chapters discuss the device physics and analysis of 2D materials based devices in the context of large-area and bright light emitting monolayer devices, and lateral 2D heterostructures, using simulations and analytical modeling. A new scheme for operating light emitting devices is discussed in chapter 6. Monolayer TMD devices on insulating substrates are operated with pulsed gate voltages resulting in transient electroluminescence. The emission mechanism, generation of bipolar carrier concentration and calculation of device efficiency are discussed in detail. The light output is independent of contact barrier height and the pulsed gating technique may be useful especially for materials which are difficult to dope p, and n type, for example wide bandgap semiconductors. Finally, the monolayers absorb only ~ 10% of visible light and can be used for large area transparent displays.

Chapter 7 discusses the device physics of lateral 2D heterostructures considering the specific case of monolayer-few layer MoS2. Using device simulations, with Kelvin Force Probe microscopy and photocurrent measurements, it is shown that a type-I heterostructure exists at the interface. Lateral 2D heterostructures by thickness modulation offer a unique way to obtain atomically sharp heterostructures with potential applications in optoelectronic devices like photodetectors. Chapter 8 presents the main conclusions of the thesis, and presents an outlook into the future of two-dimensional semiconductors.