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Impact of Surface Polarity on the Air Stability and Quantum Efficiency of Cs-Free III-Nitride Photocathodes
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LaBella, Vincent, Ventrice, Carl, Bell, Douglas, Jennifer Hite
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Spring 2023
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2023-03
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Abstract
Photocathodes are employed as photodetectors for astronomy and defense applications,
as well as electron sources in high energy physics technologies. Photocathodes absorb incident
photons, resulting in the emission of electrons. The III-nitride material system is promising for
photocathodes due to the wide and tunable band-gap energy spanning infrared to ultraviolet
wavelengths. III-nitrides are air-stable, radiation hard, and possess internal polarization charge.
Nitrogen polar III-nitrides photocathodes have been predicted to achieve high device quantum
efficiency (QE) and effective negative electron affinity due to alignment of the polarization and
depletion charges. However, fundamental challenges of p-type doping of III-nitrides, and
experimental considerations of unintentional dopant incorporation at surfaces and interfaces
initially inhibited repeatable high QE N-polar photocathodes.
To address these challenges, a comprehensive approach was used for the development of
air-stable, high QE III-nitride photocathodes, investigating the impact of polarity on Mg-dopant
incorporation efficiency, distribution of unintentional impurities due to diffusion at interfaces,
and on surface passivation. The impact of hillock structures commonly present on the N-polar
surface on device QE was investigated, finding a 2x increase in efficiency in photocathodes
grown on high hillock density templates. Atom probe tomography (APT) measurements reveal a
decrease in Mg-dopant clustering and improved incorporation efficiency in the semi-polar facets
of the hillocks, leading to improved optical and electrical characteristics. Building upon this
finding, a selective area growth technique is used to create hexagonal pyramid structure in both
the N- and Ga-polar orientations as a model to more controllably study the mechanism of Mgincorporation
within the semi-polar planes of N-polar hillocks.
III-nitride hetero/homo-interfaces are commonly implemented in photocathode device
design. Growth of such structures may involve growth interruption and/or exposure of the
interface to ambient conditions. Incorporation and diffusion of unintentional impurities including
oxygen and carbon from these interfaces has been studied here by SIMS depth profiling and
modeling of defect mediated diffusion mechanisms, and their impact on photocathode
performance was studied. Through consideration of interface proximity to the surface, and the
temperature of processes occurring post-regrowth interface, the unintentional impurity and
electrostatic profile is controlled.
Emission of photoexcited electrons and hence QE depends greatly on the condition of the
surface/states and its impact on surface band bending. It is well known that a non-negligible
surface oxide is present on the N-polar III-nitride surface. We have observed removal of the
surface oxide and deposition of positive surface charge by HCl cleaning prior to measurement,
leading to an order of magnitude increase in QE. However, oxide regrowth occurs following airexposure.
We have studied 2D materials including graphene and h-BN as passivation layers to
prevent contamination and fully stabilize the surface charge. Sustained decrease in oxygen and
carbon coverage have been measured by XPS in h-BN/GaN photocathodes after air exposure on
the order of days.
The combined findings of the impact of material polarity on Mg-dopant incorporation
efficiency, control of the unintentional impurity profile and surface passivation was utilized to
optimize the photocathode electrostatic profile for optimal device characteristics, the result of
which is a maximum QE of 26.6% at 6 eV photon energy was achieved for an HCl cleaned Npolar
p-GaN/u-GaN cap structure grown on high hillock density GaN template without a regrowth interface between active layers. This represents the highest reported QE for a Cs-free GaN photocathode to date [1], [2].
[1] J. Marini, I. Mahaboob, E. Rocco, L. D. Bell, and F. Shahedipour-Sandvik, “Polarization
engineered N-polar Cs-free GaN photocathodes,” J. Appl. Phys., vol. 124, no. 11, p.
113101, Sep. 2018, doi: 10.1063/1.5029975.
[2] E. Rocco et al., “Overview and Progress Toward High-Efficiency, Air Stable, Cs-free IIINitride
Photocathode Detectors,” IEEE Photonics J., vol. 14, no. 2, p. 6818312, 2022.