Synthesis, characterization and integration of ultrathin oxide for high-performance two-dimensional transistors

Abstract

Transistors based on two-dimensional (2D) materials have been considered one of the most promising techniques after the silicon-based technique. However, unlike the diversity and popularity of the study on the channel materials for 2D transistors, the research on dielectric materials received insufficient attention in the past decade. So far, the main candidates used as dielectric materials for 2D transistors are limited to native oxides such as SiO2, deposited high-κ oxides such as HfO2 by atomic layer deposition (ALD), and van der Waals crystals such as hexagonal boron nitride (h-BN). However, these materials have their respective problems. The native oxides, SiO2 as the representative, have relatively low dielectric constant and are limited by their parent materials, which can hardly be used for other semiconductors. The mechanism of deposition requires nucleation on the surface of the semiconductors, where the dangling-bond-free high-quality surface of 2D materials does not favor the uniform growth and leads to low-quality film and interface. The van der Waals crystals like h-BN are restricted by their crystal size and the dielectric constants are relatively low. Therefore, new candidates and techniques that can possibly satisfy the ultimate standards for dielectric materials in 2D transistors, i.e., scalable ultrathin high-κ materials with high quality, are urgently needed.

It was found that the native oxides of liquid metals, such as gallium oxide (Ga2O3) from liquid gallium, could be prepared as ultrathin free-standing materials for applications. These ultrathin free-standing oxides might have potentials properties that meet the requirements as dielectric materials in 2D transistors. In this thesis, the potential properties of the free-standing Ga2O3 and its integration for high-performance 2D transistors are demonstrated.

First, the morphology and dielectric properties, such as dielectric constant, breakdown field, optical band gap, etc. are characterized. Ultrathin thickness of less than 3 nm and an ultra-flat surface are measured. A high dielectric constant about 30 and high breakdown field about 11 MV/cm are extracted. A low equivalent oxide thickness (EOT) about 0.4 nm is calculated. These properties therefore indicate the free-standing Ga2O3 as an ultrathin and uniform high-κ dielectric material, which meets the requirements for high-performance 2D transistors. A surface-tension-driven method of wafer-scale preparation is introduced, providing the scalability for device fabrication.

Second, the free-standing Ga2O3 is integrated in 2D MoS2 field-effect transistors (FETs) as dielectric layer. Subthreshold swing down to 60 mV/dec is obtained, close to the thermal limit at room temperature. Low leakage current density about 4×10–7 A cm–2 and low density of interfacial state about 4×1011 eV–1 cm–2 are extracted. Immunity to the short-channel effect is revealed with short-channel FETs with channel lengths down to about 28 nm. Scalable device fabrication is demonstrated with FET arrays and logic gates, including NOT, NAND, NOR, AND, XOR. The high performance is attributed to the outstanding dielectric properties of the Ga2O3 and the van der Waals integration.

Third, the passivation ability of the ultrathin Ga2O3 as an encapsulation layer is evaluated with black phosphorus (BP) and 1T' MoTe2, which are representatives of air-sensitive 2D materials. Dramatic increases in retention time of both optical and electrical properties are obtained, due to the self-limiting oxidation that forms dense native oxide against O2, H2O, etc. in an ambient environment. Owing to the facile selective etching of the ultrathin Ga2O3, direct device fabrication without removal of the encapsulation layer. Dual-gate FETs can be directly fabricated with encapsulated BP, using the Ga2O3 simultaneously as encapsulation and dielectric material.

This dissertation, therefore, exhibits that the free-standing native oxide from liquid metal has desired properties as dielectric material and encapsulation for the fabrication of high-performance 2D transistors.