【Domestic Papers】Recent research progress in ultra-wide band gap Gallium Oxide heat transport by Yuan Chao's research group at Wuhan University

      Recently, in collaboration with the Filip Gucmann research group of the Slovak Academy of Sciences, The research paper entitled "Phase-Dependent Phonon Heat Transport in Nanoscale Gallium Oxide Thin Films" was published in the international authoritative journal 《Small》.

      ultra-wide band gap material Gallium Oxide (Ga₂O₃) has a variety of different phase structures: α, β, κ (ε), δ, γ. The monochromatic structure β-Ga₂O₃ has a bandgap of up to ~4.9 eV and a theoretical breakdown field strength of ~8 MV cm^-1, which makes it a strategic advanced electronic material in the field of next generation high power electronic devices. Related high-performance β-Ga₂O₃ based electronic devices have been widely reported, including Schottky Barrier Diode, solar cells, transistors, and Solar-blind ultraviolet photodetectors. The advantages of β phase Ga₂O₃ have not been fully explored, and other phase Ga₂O₃ structures (such as α-phase corundum structure and κ-phase orthogonal structure, as shown in Figure 1a) exhibit unique physical properties and are gradually receiving attention in the field of electronic devices. α-Ga₂O₃, which has the largest band-gap and the highest theoretical breakdown field strength, has broad application prospects in Solar-blind ultraviolet detectors and ultra-high voltage power electronic devices. At the same time, κ-Ga₂O₃ also has high dielectric constant, high voltage electric polarization and ferroelectric polarization, which provides a new material system for the preparation of high-frequency and high-power electronic devices and microwave RF devices of ultra-wide band gap semiconductors. However, the thermal problem of Ga₂O₃-based electronic devices has been a prominent problem, partly due to their low thermal conductivity and the large interface thermal resistance that exists between Ga₂O₃ and the heterogeneous substrate interface. Therefore, understanding the thermal transport properties of different phases of Ga₂O₃ is essential for the thermal management and reliability design of devices. However, due to the complex crystal structure and the lack of corresponding phonon thermal transport model and systematic experimental studies, it is still challenging to study the thermal transport properties of different phase nanoscale Ga₂O₃ films.

Figure 1: (a) crystal structure of α-, β- and κ-Ga₂O₃, (b) Schematic diagram of transient thermal reflection method, (c) thermal conductivity model calculation results and experimental test results of different phases of Ga₂O₃, and (d) cumulative interfacial thermal conductivity of different phases of Ga₂O₃ with variation of frequency

      The thermal conductivity and interfacial thermal conductivity (reciprocal of interfacial thermal resistance) of α-, β- and κ-Ga₂O₃ films (50-150 nm) grown by liquid-injected metal-organic chemical vapor phase deposition (LI-MOCVD) on Sapphire substrates have been systematically investigated. The effects of phase structure and thickness on the thermal transport properties of Ga₂O₃ films and Ga₂O₃/Sapphire interfaces were illustrated by using a self-developed high resolution pump-probe transient thermal reflection characterization method (TTR, as shown in FIG. 1b) and phonon thermal transport model (some of the results are excerpted in FIG. 1c and d). In addition, a special phenomenon has been discovered by the researchers: theory shows that the thermal conductivity of β-Ga₂O₃ films under bulk materials is higher than that of α-Ga₂O₃ films, while both experimental and theoretical methods prove that the thermal conductivity of β-Ga₂O₃ films under ~100 nm thickness is lower than that of α-Ga₂O₃ films. Further theoretical studies show that the mean free path of β-Ga₂O₃ phonons is larger than that of α-Ga₂O₃, and long wavelength phonons are more susceptible to boundary scattering at the nanoscale, resulting in a more obvious size effect in β-Ga₂O₃ films than α-Ga₂O₃ films. At ~100 nm film thickness, the thermal conductivity of β-Ga₂O₃ is only about 2/5 of the bulk value, while the thermal conductivity of α-Ga₂O₃ reaches about 3/5 of the bulk value.

      This study has carried out an in-depth study on the phonon transport mechanism of Ga₂O₃ thin films and interfaces, and has important guiding significance for the design and thermal management of Ga₂O₃ devices. In addition, the powerful capability of transient thermal reflection characterization method in the field of chip-level thermal physical property detection has been demonstrated. In view of the complexity and importance of heat transport in polyphase materials, this work provides a new perspective for accelerating the exploration of other important polyphase materials (such as carbon-based, Silicon Carbide based, etc.) heat transport mechanism in the future.

Paper Link:https://doi.org/10.1002/smll.202309961