It is well known that in a perfect crystal, the continuous free-electron states are quantized into many Bloch bands separated by energy gaps. These Bloch electrons move freely inside the crystal with an effective mass different from the free-electron mass. In the presence of defects, however, the field-driven current flow of Bloch electrons in the perfect crystal will be scattered locally by these defects, leading to a reduced electron mobility. Also, the photo-excited electron lifetime, due to non-radiative recombination with defects, has been proven to be a key factor affecting the sensitivity or the performance of optoelectronic devices (e.g., photo-detectors and light-emitting diodes).

The dangling bonds attached to the point defects may capture extra electrons to form charged defects. In this case, the positively charged holes in the system will be trapped to produce a strong space-charge field, while the negatively charged electrons may generate the so-called 1/f−current noise in their bumpy motions due to the presence of many potential minima and maxima from randomly distributed charged defects. Therefore, understanding of defect production, stabilization, clustering, migration and interaction with microstructural features is crucial for developing a multi-scale theory to explore radiation degradation in electronic and optoelectronic devices.

At the most fundamental level, molecular dynamics can be used to study defect production, migration and interaction, while kinetic Monte Carlo methods can be employed to simulate defect evolution and their spatial distribution.

One particular semiconductor, gallium arsenide (GaAs), has received considerable attention due to its potential electronic applications, such as GaAs-based metal semiconductor field effect transistors and logic gates, near-infrared imaging devices and photovoltaic nodules. As a means for fabricating compound semiconductors, the interest in using ion implantation continuously increases, which inevitably introduces defects. The diffusion and accumulation of these defects are critical factors controlling the dynamics of ion implantation processes and device degradation, as well as affecting charge compensation, minority-carrier lifetimes and luminescence efficiencies.

Recently, use of GaAs in high-power space-energy systems and special space-probe applications has been proposed. However, space radiation damage to the GaAs may be a limiting factor on interplanetary missions unless sufficient shielding is provided to keep damage levels under acceptable limits. Consequently, radiation damage studies have been made experimentally on the effects of electron, and proton and neutron irradiation, including defect production and annealing, as well as effects on the performance of GaAs devices.

On the other hand, understanding the basics of ion-solid interaction and irradiation damage has led to significant developments in state-of-the-art atomic-level, kinetic Monte Carlo and meso-scale simulations, and these simulations have dramatically advanced the knowledge of defects and defect processes in a number of materials, ranging from metals to semiconductors to ceramics. Recently, large-scale ab initio and classical molecular dynamics (MD) methods have been developed for the study of radiation damage in semiconductors, and these methods are used to explore the number of displacements produced, defect clustering and disordering, as well as the effects of charge transfer and charge-density redistribution on the dynamics and ultimate charge-state of defect formation. These simulations have demonstrated some nonlinear effects taking place at low and high energies, which can greatly modify the number of displacements as predicted by the simplified Kinchin-Pease model.

It has been long realized that the observed radiation damage to electronic components made from semiconductors or semiconducting compounds is proportional to the non-ionizing energy loss (NIEL). Consequently, the experimental use of NIEL for correlating proton-induced displacement damage in semiconductor devices has been widely applied over the past decade, which has proven useful in the study of both Si and GaAs. Moreover, radiation damage in other semiconductors such as diamond and SiC, and many III-V compounds, such as InP, has been investigated using the NIEL concept. Previously, several models have been developed to estimate the NIEL in GaAs using a Monte Carlo charged particle transport code or based on an empirically determined damage efficiency.

Although these models provide significant insights into the correlation between the NIEL and displacement damage, there remain discrepancies between proton NIEL calculations and experimental measurements in GaAs devices. Determining accurate damage levels and improving the NIEL model have been an ongoing focus of the Space Radiation Effects Community (SREC).

This work was done by Fei Gao of the University of Michigan for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) below. AFRL-0300

This Brief includes a Technical Support Package (TSP).
Atomistic- and Meso-Scale Computational Simulations for Developing Multi-Timescale Theory for Radiation Degradation in Electronic and Optoelectronic Devices

(reference AFRL-0300) is currently available for download from the TSP library.

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This article first appeared in the February, 2021 issue of Aerospace & Defense Technology Magazine.

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