TWO-DIMENSIONAL LAYERED CHALCOGENIDE THIN FILM ELECTRONICS FOR FLEXIBLE, BEOL, AND 3D APPLICATIONS
Open Access
- Author:
- Lee, Sora
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 17, 2021
- Committee Members:
- Tom Jackson, Chair & Dissertation Advisor
Mauricio Terrones, Outside Unit & Field Member
Suzanne Mohney, Major Field Member
Joan Redwing, Major Field Member
John Mauro, Program Head/Chair - Keywords:
- Back end of line
Flexible electronics
Atomic layer deposition
Two-dimensional materials
Transition metal dichalcogenides
3D fabrication
Metal oxides
High-temperature-capable lift-off - Abstract:
- There has been a wide interest in two-dimensional layered chalcogenide materials, like transition metal dichalcogenides (2D-TMDs), because of their interesting electronic properties and potential device performance. With large field effect mobility (ex. ~1000 cm2/Vs for InSe) and tunability of material property with different layer thickness, 2D material-based thin film electronics is of interest for applications including flexible electronics, active devices in back-end-of-line (BEOL) processing, and 3D fabrication. Although device demonstration has been done using exfoliated flakes or small-area polymer transfer techniques, practical applications of 2D-TMD materials will require low-temperature, large area processing. This dissertation explores areas of 2D material-based thin film electronics for flexible device and BEOL-compatible device applications in three fields: device, process, and material growth. The first topic of this dissertation was 2D material-based flexible and BEOL-compatible devices with WSe2 and β-In2Se3. Different flexible device fabrication processes were investigated using mono- and multi-layer WSe2 grown by metalorganic chemical vapor deposition (MOCVD) at high (800 ˚C) and low (350 ˚C) temperatures. High-temperature and low-temperature grown WSe2 thin film transistors (TFTs) were fabricated, and device characterization with bending tests were performed. It was found that high-temperature grown WSe2 exhibited better device characteristics than low-temperature grown WSe2, but there were several limitations with a flexible device fabrication process. For low-temperature WSe2, direct film deposition was successfully made on polyimide film, and an improvement of device performance from a double-gate structure and reliability of flexible device were shown. The multilayer β-In2Se3 was also grown by MOCVD at low temperature (400 °C) and BEOL compatible TFTs were demonstrated. Since β-In2Se3 has a relatively lower melting temperature (< 900 °C) than other well-known 2D-TMD materials like MoS2 and WSe2, it provides advantages for low-temperature epitaxial growth of large-area, high quality films. The result showed that β-In2Se3 TFTs had better device characteristics than low-temperature WSe2 TFTs. Moreover, the ferroelectric property of the material and temperature dependent carrier transports were investigated. Using the polarization switching, memory device performance was present. Throughout this work, layered chalcogenides, WSe2 and In2Se3, are shown to be promising for flexible and BEOL-compatible electronic applications. The second work in the dissertation was the development of a new high-temperature-capable inorganic lift-off process that allows to study the effect of thermal oxidation on optimization of contact resistance and carrier transport of 2D material TFTs. Conventionally, polymer photoresists are used for lift-off; however, these resists cannot be used when the device fabrication requires high temperature (> 300 ˚C) with the lift-off materials in place. Here, bilayer stacks of metal oxides deposited by plasma-enhanced atomic layer deposition (PEALD) were used, and a hard mask and an undercut layer were created using selective wet etching. Nanoscale metal electrodes (~ 200 nm) were successfully patterned on MoS2 using the new lift-off technique. It was also demonstrated that there was no deformation of a metal-oxide lift-off structure under high temperature (~500 °C) annealing. Using the metal-oxide based inorganic lift-off process, thermal oxidation process was successfully introduced prior to metal contact deposition/patterning on MoS2. Likewise, this inorganic lift-off process enables high-temperature material deposition and/or annealing as a part of the fabrication process. It overcomes temperature limits of polymer resists and provides a photoresist-residue-free and high-temperature capable process. The third study in this dissertation was focused on a low-temperature material growth using plasma-enhanced atomic layer deposition (PEALD). Since PEALD is a type of ALD that has layer-by-layer deposition process and plasma-assisted surface reactions at low temperature, it could be useful for 2D material depositions for low-temperature processed applications. But so far, there have been relatively less studies on PEALD of 2D materials comparing to other deposition techniques like CVD. Particularly, there are limited studies on precursors for PEALD process of 2D materials. Using a load-lock, showerhead PEALD system, a new chalcogenide precursor candidate, carbon disulfide (CS2), was investigated. Different deposition processes were examined using CS2 with pure He gas, 5% H2 in He, and a separate hydrogen plasma exposure after the CS2 and DEZ reaction. It was found that a separate hydrogen plasma exposure gave a noticeable improvement of film growth and crystallinity, which could be related to carbon removal on the deposition surface. With the separate hydrogen plasma process, it is demonstrated that CS2 can be a useful precursor for PEALD deposition of 2D materials.