| Author: | Lin, Ziyuan |
| Title: | Elemental two-dimensional materials for nanoelectronics applications |
| Advisors: | Chai, Yang (AP) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Two-dimensional materials Nanoelectronics Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xviii, 139 pages : color illustrations |
| Language: | English |
| Abstract: | Elemental two-dimensional (2D) materials stand out for their simple compositions and excellent electrical properties, showing the great potential for nanoelectronics applications. In this thesis, we study the carrier transport of emerging elemental 2D materials: black phosphorous (BP) and tellurene (Te), which develops the understanding on transport behaviours and benefits the improvement of electronics devices based on 2D BP and Te. In addition, we develop a thinning method for few-layer Te, which allows local thinning with thickness control. This thinning process is compatible with device fabrication process, facilitating the further applications of few-layer Te in nanoelectronics. We first demonstrate the n-type field-effect transistors (FETs) based on few-layer BP achieved by contact engineering. The copper (Cu) is chosen as contact metals for theoretical Ohmic contact and n-type doping effect. With detailed investigation on the interface between BP and Cu, it is found that highly diffusive Cu atoms migrate into BP and intercalate between BP layers without changing the crystal structure of BP. The BP with interstitial Cu is theoretically predicted to have smaller band gap than pristine BP. The Fermi level is greatly shifted towards conduction band after the Cu penetration. Through this interstitial Cu-doped edge contact, n-type dominant BP transistors are achieved with high electron mobility of ~ 138 cm²V-1s-1 and on/off ratio of ~ 100 at room temperature. The current density can reach 58 µA/µm. The Schottky barrier height for electron is negligibly low. This n-type transport property is attributed to n-doping induced by the penetration of highly-mobile Cu atoms at the contact region. The penetrated Cu in BP also changes the electrical property of BP to metallic-like behavior, bringing low contact resistance. Afterwards, we study the transport property of few-layer Te and fabricate high-performance p-type FETs based on 2D Te with high-work-function contact metals, platinum (Pt) and palladium (Pd). The work functions of Pt and Pd are lying far below the valence band maximum (VBM) of 2D Te, facilitating the hole transport in Te. The devices exhibit unipolar p-type transport property with hole mobility reaching 700 cm²V-1s-1 and on/off ratio up to 10⁴. The current density can reach 0.17 mA/µm. The hole concentration is measured to be 3 × 1018 cm-3, which is comparable with 2D BP. The contact resistance can be as low as 400 Ω•µm, indicating the excellent contact between 2D Te and contact metals. The remarkable p-type transport property results from the well-match band alignment between few-layer Te and high-work-function contact metals, which prompts the hole transport in Te. The high density of states of Te near Fermi level also facilitates the current injection from contact metals, bringing large sheet conductivity at the contact region. Lastly, a local thinning process for few-layer Te involving Pt metal is developed. Under light illumination, few-layer Te near Pt contacts experiences thinning process in water. The thickness can be reduced by ~ 3 nm with 10 min process. Due to the small bandgap of few-layer Te, the electron-hole pairs can be easily generated under white light. The photogenerated electrons will transfer to Pt metal and react with H+ in water producing H2 with Pt catalyst. The left OH- change the pH value near Pt and etch the few-layer Te, resulting thickness reduction of Te near Pt. This local thinning process is compatible with device fabrication process, providing feasible approach to few-layer Te with desirable thickness for various kinds of electronic devices. In conclusion, the transport properties in 2D BP and Te receive detailed study to implement high-performance FET devices. Contact engineering is the focus of this study for the improvement of device performance. A local thinning method is also developed for few-layer Te to extend the applications of emerging 2D Te. |
| Rights: | All rights reserved |
| Access: | open access |
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