Impact of Surface Polarity on the Air Stability and Quantum Efficiency of Cs-Free III-Nitride Photocathodes

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.