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Title: Studies of n-type doping and surface modification of CVD diamond for use in thermionic applications
Author: Othman , Muhammad Zamir
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2014
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This thesis presents the investigation of potential shallow n-type donors that are candidates to be used as therm ionic emitters for converting solar energy to electrical energy. Due to the various problems associated with current n-type dopants in diamond, the work has examined the use of Li-N codoping as a possible alternative doping strategy in chemical vapour deposition (CVD) of polycrystalline diamond films. Lithium nitride (Li3N) suspension in 1 % w Iv polyoxy in chloroform and N2 or NH3 gas were used as Li and N precursors, respectively, in preparing Li-N co-doped diamond films using a hot-filament CVD system. Using this system, high dopant concentrations of Li (-5x1019 cm-3 ) and N (-3x1020 cm- 3) atoms were successfully incorporated into the diamond films. The addition of Li atoms into N-doped diamond films improved the resistance of the diamond film from >200 MO to <50 MO. A resistance as low as 85 kO was recorded for Li-N co-doped diamond grown on a SCD type Ib substrate, however, these values and the overall electrical characteristics of the films were not sufficient for electronic devices. Ab initio calculations predicted that a LiN cluster with a Li: N ratio of 1:4 should produce shallow donor characteristics with an energy level -0.1 eV below the diamond conduction band minimum. The model only favours the formation of Li in substitutional sites rather than in interstitial sites due to its lower formation energy (4.88 eV and >10 eV, respectively). This model also suggests that N atoms might act as traps to pin down Liatoms and reduced their mobility in diamond lattice. The thermionic emission characteristics of Li-N co-doped diamond films were tested in Arizona State University, Arizona, USA. The films exhibited 121 ~A cm -2 current density at 900 K with a threshold temperature at 800 K. The effective work function of the emitters varied from 2.87 eV to 3.62 eV. The presence of a negative electron affinity (NEA) surface is found to be crucial for increasing the electron emission from diamond. H terminated diamond exhibits NEA and is usually prepared by exposing the diamond films in hydrogen plasma. However, desorption of H atoms between 925-1050K limits the application for these films for thermionic-based solar panels at higher temperature. Thus, various metaloxygen- terminated diamond surfaces were examined to determine their thermionic electron yield and stability at high temperatures. Cr-"O, Al-O and Ti-O terminated diamond surfaces exhibit NEA characteristics and were thermally stable at 650°C. However, only Cr-O terminated diamond showed true NEA characteristics while Al-O and Ti-O terminated diamond showed effective NEA.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available