Nitrogen-Doped Diamond

: A Thermionic Investigation on the Effects of Lithium Oxide Termination

Abstract

This project aims to investigate the emission currents produced from nitrogen-doped dia-
mond samples terminated with oxygen and lithium, to develop our understanding of such
a material for use as an emitter surface in a thermionic energy converter (TEC). Thermionic
devices offer clean, renewable energy generation from a device with no moving parts thus a
large potential for device efficiency, such a device could be revolutionary for electricity gen-
eration using solar power, or for clean hydrogen production from electrolysis. TECs could
also be used for space-based solar power or energy generation for space travel due to the
high physical and radiation hardness of diamond. For an effective TEC device, the semicon-
ductor emitter surface must exhibit a negative electron affinity (NEA) for electron emission,
as well as a low work function (WF) to reduce the operating temperatures of such a thermal
device.
Nitrogen is a deep-level electron donor when used as a diamond dopant, it easily incorpo-
rates into the diamond lattice through substitutional sites. Lithium terminations have been
studied both theoretically and experimentally to both induce an NEA and to reduce the
work function by inducing dipoles on the surface of the diamond films, altering the semi-
conductor’s band structure through electrostatic band bending.1,2,3 Oxygenation has shown
to increase the bond strength of lithium to the surface as well as that of the surface dipoles,
further reducing the WF.2,4,2
Diamond films have been grown using chemical vapour deposition (CVD) in nitrogen-rich
environments, the effects of changing nitrogen flow rates within the gaseous growth mixture
have been investigated in terms of sample quality and electronic properties of the resulting
sample. Oxygenation will be used to increase the stability of the terminated lithium surface
as well as to further reduce the resulting WF, these terminations have been carried out by
UVO-cleaning treatments and thermal evaporation respectively.
X-ray photoemission spectroscopy (XPS) has been used to identify surface contaminants as
well as to calibrate lithium deposition rates. UV photoemission spectroscopy (UPS) has been
used to obtain electron energy distributions for the samples under test to extract estimations
for the valence band maximum (VBM) and the WF of the surfaces. The lowest WF achieved
was estimated at 1.79eV. Simulated solar heating of the samples by an IR laser, employing
the use of a diamond-like-carbon (DLC) absorber surface, was used to measure emission
currents from the sample. Emission currents were obtained for the main samples under test
as well as for the common NEA-imparting hydrogen termination as a comparison for the
other samples due to the well-documented properties it bestows. The largest emission cur-
rent achieved for an oxygenated-lithiated surface was 0.016mA at 700◦C, although this value
was lower than expected, emission was very stable over all six cycles. This work demon-
strates the great potential of lithium-oxide nitrogen-doped diamond emitters.

Date of Award	23 Jan 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Martin J Cryan (Supervisor) & Neil A Fox (Supervisor)