【International Papers】Modulation of thermal conductivity of iron-doped β-Ga₂O₃ by helium-ion irradiation

      Researchers from the Utah State University have published a dissertation titled "Modulation of thermal conductivity of iron-doped β-Ga₂O₃ by helium-ion irradiation" in Journal of Alloys and Compounds.

Background

      β-Ga₂O₃ emerges as a versatile semiconductor, distinguished by its exceptional properties that make it a prominent candidate for various applications. In particular, it exhibits a wide band gap (≈4.5 – 4.8 eV), an elevated breakdown electric field strength (≈8 MV/cm), and remarkable thermal stability. Among the five different polymorphs of Ga₂O₃ – namely α, β, γ, δ and ϵ – β-Ga₂O₃ stands out due to its excellent thermodynamic stability from the ambient temperature to the melting point, 2068.15 K. However, the remaining polymorphs show instability at higher temperatures, transforming to β form in the temperature range of 1000–1200 K. β-Ga₂O₃ crystallizes the monoclinic crystal structure centered on the base with the space group C2/m where the O^2 − ions are placed in a distorted cubic packing arrangement and the Ga³⁺ ions are placed in distorted tetrahedral and octahedral vacancy sites. The lower packing fraction of β-Ga₂O₃ makes it host to many impurity elements in interstitial spaces. The common interstitial impurities reported in β-Ga₂O₃ are hydrogen and fluorine with higher diffusion coefficients which have been well studied to optimize near surface electrical conductivity. Furthermore, doping with impurities such as carbon, silicon, and germanium, known for their donor-like behavior, was proven to contribute to the n-type semiconducting characteristics in β-Ga₂O₃. These exceptional characteristics, along with the tunable properties of β-Ga₂O₃, make it a promising material for applications in power electronics, optoelectronics and, particularly, in radiation sensors.

Abstract

      β-Ga₂O₃ is an important ultra-wide bandgap oxide semiconductor that has recently gained significant attention in radiation detection. This study examined the impact of helium-ion irradiation on the anisotropic thermal conductivity of iron-doped β-Ga₂O₃. A laser-based spatial domain thermoreflectance technique was used to measure the thermal conductivity map, which was validated against simulation results derived from density functional theory-based phonon transport simulations. Our experimental results revealed that the irradiation damage led to 40 % reduction in the thermal conductivity along [001] direction and 25 % along [100] directions, which significantly reduced the anisotropy of thermal conductivity. Phonon transport simulations indicated that the thermal conductivity decreases when helium atom is at interstitial sites or vacancy sites, and such reduction is particularly evident when helium atom occupies vacancy sites. This work underscores the role of irradiation-induced microstructural changes in the thermal transport properties of β-Ga₂O₃, which is crucial for its applications in sensor devices for extreme environments.

Highlights

      ● Thermal conductivity of Fe-doped β-Ga₂O₃ is reduced under helium ion irradiation.

      ● Helium ion irradiation leads to the reduction of anisotropy in thermal conductivity.

      ● Both helium and its accumulation sites determine the thermal conductivity.

Conclusions

      This study examined the microstructural changes and lattice thermal conductivity of pristine and helium-ion irradiated Fe-doped β-Ga₂O₃. TEM-based microstructural characterization confirmed that the β-Ga₂O₃ phase remains stable after helium irradiation. The micrographs further revealed a high density of helium gas bubbles, with an average diameter of around 1 nm, embedded within the crystallographic matrix of the irradiated samples. Thermal conductivity measurements on helium-ion irradiated samples at room temperature indicate a reduction in the anisotropy in the thermal conductivity, with the thermal conductivity reducing around 40 % along the [001] direction and 25 % along the [100] direction. DFT-based phonon-mediated heat transport studies indicated that the thermal conductivity decreases when helium atom occupies interstitial or vacancy sites, and such reduction is particularly evident when helium atom occupies vacancy sites. Our study suggests that both the presence of helium and its specific accumulation sites within the lattice play a critical role in determining the extent of thermal conductivity reduction and the variation in anisotropy of heat transport in β-Ga₂O₃ under helium ion irradiation.