**1. Introduction**

In the last several decades, dominated by silicon (Si) and gallium arsenide (GaAs), semiconductors have shaped the new technological era with diodes, transistors, and integrated circuits [1]. Gradually, semiconductor technology has entered all industry areas, including nuclear power production. While the previous generation of nuclear power plants restricted the use of electronic devices to an inevitable minimum, recent nuclear plants rely on the electronics used not just in the digital computers and process control systems in a mild environment, but also in harsh radiation conditions, whereas the use of different electronic systems is not limited to detector technology only.

The application of semiconductors in harsh radiation environments is significantly increasing, not just by nuclear power plants, but also in medical diagnostics, nuclear science, technology, research, and space applications. In all of these fields, high-energy charged particles interact with essential safety and other components, modifying their microstructure and affecting their lifetime. Therefore, the need for safe long-term operation of the semiconductors is crucial for the reliability of electronic instruments, and any failure in critical components leads to substantial economic and human safety hazards in all of these applications.

Despite their susceptibility to permanent degradation and catastrophic failure due to heavy-ion exposure [2], numerous research publications have already pointed out that future semiconductor technologies, including those for space, detectors, medicine, and

**Citation:** Neuhold, I.; Noga, P.; Sojak, S.; Petriska, M.; Degmova, J.; Slugen, V.; Krsjak, V. Application of Proton Irradiation in the Study of Accelerated Radiation Ageing in a GaAs Semiconductor. *Materials* **2023**, *10*, 1089. https://doi.org/10.3390/ ma16031089

Academic Editor: Scott M. Thompson

Received: 21 December 2022 Revised: 17 January 2023 Accepted: 21 January 2023 Published: 27 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

nuclear applications, consider the application of wide band gap (WBG) semiconductors such as GaN and SiC. In these crystals, the gap between the valence and conduction bands is an essential parameter that defines not only the electrical properties, but also the susceptibility to radiation [3]. The advantage of WBG compared with classical semiconductors such as silicon and gallium arsenide is in the improved electrical properties, such as a higher efficiency, switching frequency, operating temperature, and higher operating voltage [4,5]. This leads to faster, dimension-wise, smaller, more powerful, and more efficient components. These capabilities will be reflected in smaller sizes and weights and will have less power demand due to limited power losses [3,6].

While natural radiation environments, such as the ionosphere, trapped radiation belts, solar particle events, and galactic cosmic rays dominate in outer space, on the ground, various man-made applications lead to the exposure of semiconductor materials to ionizing radiation. Although the understanding of their radiation tolerance is far from complete, silicon carbide (SiC) and gallium nitride (GaN) semiconductors are expected to have superior electrical properties, and their susceptibility to harsh radiation environments compared with the more conventional semiconductors needs to be understood in more detail. It could be expected that because of the improved electrical and radiation properties, GaN has excellent potential to improve a safe long-term operation, decreasing the life-cycle cost and lowering the occurrence of failure, which could lead to personal safety risks. [7,8].

Radiation effects in semiconductor-based electronics due to harsh radiation environments can be divided into two categories, namely the short-term temporary effects and the long-term permanent degradation. The short-term temporary effect comes mainly from the effects of ionising energy loss in the semiconductor by the energetic particle, causing single event effects (SEEs) such as single event upset (SEU), single event transient (SET), single event latch-up (SEL), single event gate rupture (SEGR), or single event burnout (SEB). On the other hand, the long-term effects are dominantly created by the non-ionising energy losses in the material by the displacement damage, where elastic collisions with the material can eject atoms from their standard position in the lattice or when primary recoil atoms collide with other atoms in the lattice [9,10].

While "realistic" low-dose long-term irradiation experiments provide reliable data for assessing the electronic components and circuits resistant to damage or malfunction caused by high levels of ionizing radiation, their potential is significantly decreased for fast technological development due to the time-consuming nature of the approach, which also represents a significant cost in the radiation experiment. To guarantee reliable long-term operation in harsh radiation environments for a reasonable duration of the experiment, semiconductors must undergo suitable accelerated ageing tests. A proper accelerating radiation ageing mechanism is necessary among other ageing mechanisms such as thermal and mechanical vibration, contributing to the successful assessment of the lifetime of electronic devices. However, there has not been an engineering consensus ye<sup>t</sup> on how the results of accelerated ageing experiments can be extrapolated to the engineering and design of technologies for long-term applications. A deep understanding of the evolution of the microstructure exposed to accelerated radiation tests inevitably requires employing both theoretical modelling and suitable experimental characterisation methods sensitive to the atomic-scale lattice defects. This is a very complicated and challenging task due to the limited size sensitivity of the experimental techniques on the one hand, and the limited size of the theoretical calculation models on the other.

For the characterisation of the material damage, a positron annihilation spectroscopy using a 22Na positron source was used. Positron annihilation spectroscopy (PAS) has been used as a microstructural characterisation tool that is sensitive to vacancy-type defects. This technique has been widely used in characterising various types of defects in semiconductors since the 1970s. Positron annihilation experiments were successfully used in the characterisation of radiation effects in (not only) semiconductors modified in numerous types of radiation experiments [11,12], including gamma radiation [13], electron irradiation, and neutron irradiation [14], as well as proton irradiation [15]. In this

paper, we used this technique to obtain a quantitative characterisation of the radiationinduced vacancy-type defects, which were investigated as a function of the proton flux (displacement damage rate).

This work aims to explore the feasibility of using proton implantation as a mechanism for the radiation ageing of semiconductors and to improve the understanding of the process of creating displacement damage in bulk GaAs semiconductor material. The particular goal of the study is to describe the role of the "flux effect" on the evolution of the microstructure. In other words, the work was aimed at achieving a better understanding of how to optimise accelerated-ageing irradiation experiments in order to make them a physically meaningful representation of the long-term permanent degradation of the material exposed in the radiation field, so as to establish a comparison between the produced defects and surviving defects in the irradiated materials. The "flux effect" on the WBG semiconductors will be investigated in our forthcoming study, and will be compared with present work.
