The direct growth of Ga₂O₃ thin films on highly thermally conductive substrates provides more flexibility. Various epitaxy methods such as molecular beam epitaxy (MBE), low-pressure chemical vapor deposition (LPCVD), and pulsed laser deposition (PLD) have been used to grow high-quality Ga₂O₃ thin films on these substrates. However, the hetero-epitaxially grown Ga₂O₃ thin films have a higher full-width at half-maximum (FWHM) of 558 arcsec in the X-ray diffraction (XRD) rocking curves compared to homoepitaxial Ga₂O₃ thin films^[3]. Lattice mismatch and substrate oxidation during the epitaxy process are key issues that need to be addressed.

The difficulty in splitting Ga₂O₃ thin films arises from the low utilization ratio of H ions, which is around 9%, and the large activation energy of 2.28 eV in bulk Ga₂O₃. This necessitates a much higher implantation fluence compared to other materials. Regardingwafer bonding, two different methods are employed: surface activated bonding (SAB) and hydrophilic bonding. SAB enables the achievement of a strong bonding strength at room temperature without the need for wet chemical cleaning. However, SAB involves a sputtering process that can negatively impact the crystallinity of the thin film surface and further degrade the thermal and electrical properties at the heterogeneous interface. An alternative approach is hydrophilic bonding, which occurs at an elevated temperature. This method minimizes significant damage to the wafer surfaces and helps resist thermal stress during the exfoliation process. By employing hydrophilic bonding, a 2-inch Ga₂O₃ thin film was successfully transferred onto a SiC substrate^[5].

The effective channel mobility (μ_eff) is a critical factor that influences the drive current of transistors and serves as an important measure of the quality of Ga₂O₃ films in heterogeneous integration. It has been observed that theμ_eff of heterogeneous Ga₂O₃ MOSFETs on SiC and Si substrates can be enhanced by increasing the post-annealing temperature of the Ga₂O₃ channel^[7,9]. In particular, when the GaOSiC transistor is annealed at 1200 °C, theμ_eff achieved is consistent with that of the Ga₂O₃ donor wafer used for fabricating the GaOSiC wafer, as well as reported bulk Ga₂O₃ materials and devices, as illustrated inFig. 2. This indicates that the defects induced into the Ga₂O₃ channel during the ion-cutting process can be effectively eliminated through high-temperature annealing.

Mechanical exfoliation involves splitting Ga₂O₃ thin flakes with a thickness ranging from 20 to 400 nm from the bulk Ga₂O₃ due to its large lattice constant along the [100] direction^[1]. These exfoliated thin flakes can be integrated heterogeneously onto any substrate using van der Waals interaction. The thin flakes retain good crystal quality and a smooth interface similar to bulk Ga₂O₃ due to the lossless transfer process. However, mechanical exfoliation is not suitable for wafer-scale manufacturing as it can only fabricate thin flakes of around a few micrometers in size with uncontrollable thickness and uniformity. Moreover, the weak intermolecular forces between Ga₂O₃ thin flakes and the substrate hinder the heat transport across the heterogeneous interface^[2].

Successful development of ion-cutting and bonding processes for Ga₂O₃ semiconductors has enabled theheterogeneous integration of wafer-scaleβ-Ga₂O₃ thin films onto 4H-SiC and Si (001) substrates. The utilization of these substrates with high thermal conductivity and good insulating properties has led to improved temperature-dependent ON- and OFF-state current performance in heterogeneous Ga₂O₃ MOSFETs compared to bulk devices. Experimental demonstrations have shown that the effects induced in the Ga₂O₃ channel by H⁺ implantation during the ion-cutting step can be eliminated through high-temperature annealing, resulting in comparable effective channel mobility (μ_eff) in heterogeneous Ga₂O₃ MOSFETs compared to bulk materials and transistors. Heterogeneous integration technology holds promise in overcoming the fundamental thermal limitations of Ga₂O₃ electronics for high-power applications.

The heterogeneous integration of Ga₂O₃ with a highly thermally conductive substrate leads to a significant improvement inthermal dissipation for Ga₂O₃ devices. When compared to Ga₂O₃ bulk wafers, Ga₂O₃/SiC heterostructures exhibit a much faster thermal relaxation speed. Lateral Schottky barrier diodes fabricated on Ga₂O₃-on-SiC (GaOSiC) heterogeneous wafers also demonstrate improved thermal dissipation compared to those on Ga₂O₃ bulk. The high thermal conductivity of the SiC substrate allows GaOSiC SBDs to reach a peak rising temperature that is only one quarter of that observed on Ga₂O₃ bulk when subjected to the same applied power^[6]. The heterogeneous integration of Ga₂O₃ on SiC or Si not only improves the thermal dissipation properties of the devices but also provides effective insulating substrates^[7,8]. In heterogeneous Ga₂O₃ metal−oxide−semiconductor field-effect transistors (MOSFETs), the OFF-state leakage current only increases by 1−2 orders of magnitude as the ambient temperature rises from room temperature to approximately 250 °C. In contrast, the OFF-state leakage current of Ga₂O₃ bulk devices degrades by 5−6 orders under the same conditions. It is important to note that the ideal insulating Ga₂O₃ is not yet available. Semi-insulating Ga₂O₃ can be achieved through high concentration Fe compensation doping, but this introduces n-type conductivity in the Ga₂O₃ substrate as the temperature increases.

The more severe phonon−phonon scattering in gallium oxide (Ga₂O₃) crystals leads to lower thermal conductivity compared to most other semiconductor materials. To address this issue and enhance the heat dissipation in Ga₂O₃ devices, one practical solution is to integrate Ga₂O₃ with a highly thermally conductive substrate, such as SiC and Si. Currently, there are three methods employed for the heterogeneous integration of Ga₂O₃ with highly thermally conductive substrates: mechanical exfoliation, hetero-epitaxy growth, and ion-cutting technique.

The performance ofrecessed-gate GaOSiC MOSFETs has been demonstrated to be excellent^[10]. For instance, a device with a gate-to-drain length (L_SD) of 3μm and a channel length (L_CH) of 1μm achieved a specific ON-resistance (R_ON,sp) as low as 0.72 mΩ·cm², which is the lowest value during that time. Another recessed-gate transistor with anL_SD of 11μm achieved a breakdown voltage (V_br) of 1000 V and anR_ON,sp of 100 mΩ·cm², resulting in a power figure-of-merit (PFOM) of 100 MW/cm². Notably, this PFOM value remained stable even with the ambient temperature ranging from 25 to 200 °C, as depicted inFig. 3.

The ion-cutting technique has proven to be an elegant method for heterogeneous integration. It involves splitting a high-quality wafer-scale thin film from a bulk wafer through hydrogen (H) or/and helium (He) ion implantation, creating a damage layer below the wafer surface. The sliced thin film is then transferred onto a handle wafer via a wafer bonding process, as shown inFig. 1^[4]. The exfoliated wafer-scale thin film maintains good crystallization quality similar to the donor bulk, and the remaining bulk can be recycled to lower the process cost. Ga₂O₃, being an ultra-wide bandgap semiconductor, is particularly suitable for the ion-cutting technique as it efficiently extracts heat from its active layer to the highly thermally conductive substrate. In 2019, we first realized a wafer-scale single-crystalline Ga₂O₃ thin film heterogeneously integrated onto high thermal conductivity substrates (Si and SiC) using the ion-cutting technique with H implantation and surface-activated bonding process. The resulting wafer had a size of 2 inches with a Ga₂O₃ thickness nonuniformity of less than 2%. A post-annealing process at 900 °C for 30 min resulted in an FWHM of 90 arcsec in the XRD rocking curves, which is very close to that of the donor bulk Ga₂O₃.