Thermal Conductivity of β-Phase Ga₂O₃ and (Al_xGa_1–x)₂O₃ Heteroepitaxial Thin Films

Abstract

Heteroepitaxy of β-phase gallium oxide (β-Ga 2 O 3 ) thin films on foreign substrates shows promise for the development of next-generation deep ultraviolet solar blind photodetectors and power electronic devices. In this work, the influences of the film thickness and crystallinity on the thermal conductivity of (2̅ 01)-oriented β-Ga 2 O 3 heteroepitaxial thin films were investigated. Unintentionally doped β-Ga 2 O 3 thin films were grown on c-plane sapphire substrates with off-axis angles of 0°and 6°toward ⟨112̅ 0⟩ via metal−organic vapor phase epitaxy (MOVPE) and low-pressure chemical vapor deposition. The surface morphology and crystal quality of the β-Ga 2 O 3 thin films were characterized using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The thermal conductivities of the β-Ga 2 O 3 films were measured via time-domain thermoreflectance. The interface quality was studied using scanning transmission electron microscopy. The measured thermal conductivities of the submicron-thick β-Ga 2 O 3 thin films were relatively low as compared to the intrinsic bulk value. The measured thin film thermal conductivities were compared with the Debye−Callaway model incorporating phononic parameters derived from first-principles calculations. The comparison suggests that the reduction in the thin film thermal conductivity can be partially attributed to the enhanced phonon-boundary scattering when the film thickness decreases. They were found to be a strong function of not only the layer thickness but also the film quality, resulting from growth on substrates with different offcut angles. Growth of β-Ga 2 O 3 films on 6°offcut sapphire substrates was found to result in higher crystallinity and thermal conductivity than films grown on on-axis c-plane sapphire. However, the β-Ga 2 O 3 films grown on 6°offcut sapphire exhibit a lower thermal boundary conductance at the β-Ga 2 O 3 / sapphire heterointerface. In addition, the thermal conductivity of MOVPE-grown (2̅ 01)-oriented β-(Al x Ga 1−x ) 2 O 3 thin films with Al compositions ranging from 2% to 43% was characterized. Because of phonon-alloy disorder scattering, the β-(Al x Ga 1−x ) 2 O 3 films exhibit lower thermal conductivities (2.8−4.7 W/m•K) than the β-Ga 2 O 3 thin films. The dominance of the alloy disorder scattering in β-(Al x Ga 1−x ) 2 O 3 is further evidenced by the weak temperature dependence of the thermal conductivity. This work provides fundamental insight into the physical interactions that govern phonon transport within heteroepitaxially grown β-phase Ga 2 O 3 and (Al x Ga 1−x ) 2 O 3 thin films and lays the groundwork for the thermal modeling and design of β-Ga 2 O 3 electronic and optoelectronic devices.

Phase Stabilized MOCVD Growth of β‐Ga2O3 Using SiOx on c‐Plane Sapphire and AlN/Sapphire Template

“…[ 45 ] We did not observe any Raman signature related to high silicon doping, further substantiating the presence of SiO x phase stabilizer in β‐Ga 2 O 3 . The phonon mode, the intense Raman peak reported in the literature for the bulk β‐Ga 2 O 3 substrate, [ 17,44 ] is sensitive to residual strain, and the peak position can be anywhere between 195 and 205 cm −1 . [ 44,46 ] Figure 6b shows the phonon lines associated with the peak being positioned at 198.70 and 198.30 cm −1 in sample A2 and B1, respectively.…”

“…The ω 0 value is obtained from literature to be ≈199.70 cm −1 for β‐Ga 2 O 3 substrate. [ 17 ] Because is a constant, the biaxial strain is directly proportional to frequency shift, i.e., . A red shift corresponds to tensile strain ( > 0), whereas a blue shift corresponds to compressive strain (< 0).…”

“…The 2θ (01) peak at 18.97° and Raman mode peak at 199.7 cm −1 are reported for (01) β‐Ga 2 O 3 substrate. [ 17 ] The tensile strain in sample A2 is found to be ≈0.87% from XRD with a relative Raman wave number shift of 0.50%, and in sample B1 is found to be ≈0.60% from XRD with a relative Raman wave number shift of 0.73% compared to (01) β‐Ga 2 O 3 substrate.…”

“…[ 1,11,14–16 ] However, β‐Ga 2 O 3 substrates have low thermal conductivity (≈0.27 W cm −1 K), which is not suitable for power semiconductor devices. [ 17 ] Developing β‐Ga 2 O 3 heteroepitaxy on thermally conductive materials like AlN/sapphire template, bulk AlN or 4H‐SiC or 6H‐SiC substrates can improve heat dissipation in power devices. The β‐Ga 2 O 3 grows in the (01) direction as plane of β‐Ga 2 O 3 is parallel to the (001) plane of sapphire or AlN or 4H‐SiC.…”

Monolithic β -Ga2O3 NMOS IC based on heteroepitaxial E-mode MOSFETs

“…On the contrary, heteroepitaxial devices on more commercially viable substrates, such as sapphire and silicon, could largely address these issues. [11][12][13][14] Those substrates have proven to be instrumental in the commercial success of wide bandgap compound semiconductor devices. For instance, most commercial GaN-based light-emitting diodes and high-electron-mobility transistors (HEMTs) are heteroepitaxially grown on sapphire and silicon substrates, whereas similarly the bulk GaN substrates offer superior material quality but suffer from high cost and low scalability.…”