Cerium dioxide (CeO₂), with its wide bandgap of approximately 3.4 eV, has emerged as a critical material for epitaxial growth of ultra-wide bandgap semiconductors like gallium oxide (Ga₂O₃). Its primary function lies in serving as a lattice-matching buffer layer when Ga₂O₃ is deposited on sapphire (Al₂O₃) substrates. The large lattice mismatch between Ga₂O₃ and sapphire, typically around 4.7%, introduces significant strain and defects, degrading device performance. CeO₂ mitigates these issues through two key mechanisms: suppression of oxygen diffusion and strain relaxation. Additionally, the optimization of the RF sputtering process for CeO₂ deposition plays a crucial role in achieving high-quality films.
The lattice parameters of CeO₂ (fluorite structure, a = 5.41 Å) provide a closer match to Ga₂O₃ (β-phase, a = 12.23 Å, c = 5.80 Å) compared to sapphire (a = 4.76 Å, c = 12.99 Å). This intermediate buffer layer reduces the direct strain between Ga₂O₃ and the substrate. The epitaxial relationship typically observed is (111) CeO₂ || (0001) Al₂O₃ and (100) Ga₂O₃ || (110) CeO₂. The CeO₂ layer accommodates the lattice mismatch through the formation of misfit dislocations at the interface, preventing their propagation into the Ga₂O₃ film. High-resolution X-ray diffraction (HRXRD) studies confirm that CeO₂ buffers reduce the full width at half maximum (FWHM) of Ga₂O₃ rocking curves by up to 30%, indicating improved crystallinity.
Oxygen diffusion suppression is another critical role of CeO₂. Ga₂O₃ is highly sensitive to oxygen vacancies, which can alter its electronic properties. Sapphire substrates, when exposed to high temperatures during growth, facilitate oxygen migration into the Ga₂O₃ layer, leading to non-stoichiometric regions. CeO₂ acts as a barrier due to its high oxygen storage capacity and stable fluorite structure. Oxygen ions in CeO₂ exhibit lower mobility compared to sapphire, reducing unwanted diffusion. Secondary ion mass spectrometry (SIMS) depth profiles demonstrate that CeO₂ interlayers decrease oxygen intermixing by a factor of two compared to direct growth on sapphire.
Strain relaxation in CeO₂-buffered Ga₂O₃ occurs through a combination of elastic and plastic deformation mechanisms. The initial growth of CeO₂ on sapphire involves partial strain accommodation via tetragonal distortion, followed by dislocation nucleation at critical thicknesses (~20 nm). Beyond this thickness, the CeO₂ layer relaxes progressively, creating a more compliant substrate for Ga₂O₃. Transmission electron microscopy (TEM) reveals that threading dislocation densities in Ga₂O₃ are reduced by an order of magnitude when grown on CeO₂/sapphire compared to bare sapphire. The strain state of Ga₂O₃ can be further tuned by adjusting the CeO₂ thickness, with optimal values ranging between 50-100 nm for full relaxation without excessive roughening.
RF sputtering is the preferred deposition method for CeO₂ buffer layers due to its scalability and precise control over stoichiometry. The process involves a cerium oxide target sputtered in an argon-oxygen plasma. Key parameters affecting film quality include RF power density, substrate temperature, oxygen partial pressure, and deposition rate. A power density of 3-5 W/cm² balances between sufficient adatom mobility and minimal ion bombardment damage. Substrate temperatures between 500-700°C promote crystalline growth while avoiding excessive interfacial reactions. Oxygen partial pressures of 10-20% ensure stoichiometric CeO₂ formation without inducing oxygen vacancies.
The deposition rate must be carefully optimized to avoid kinetic limitations. Rates below 0.5 Å/s result in highly ordered films but are impractical for thick buffers, while rates above 2 Å/s introduce defects. A compromise of 1-1.5 Å/s yields the best structural and morphological properties. Post-deposition annealing at 800-900°C in oxygen ambient further improves crystallinity and reduces point defects, as evidenced by photoluminescence spectroscopy showing suppressed defect-related emission peaks.
The interfacial chemistry between CeO₂ and sapphire also influences buffer performance. X-ray photoelectron spectroscopy (XPS) studies indicate the formation of a thin Al₂CeO₅ interlayer (~2 nm) during high-temperature growth, which enhances adhesion and reduces interfacial strain. This interlayer forms spontaneously when the substrate temperature exceeds 600°C and does not adversely affect the epitaxial relationship.
For Ga₂O₃ deposition on CeO₂-buffered sapphire, metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) are commonly employed. The CeO₂ surface pretreatment is critical; exposure to oxygen plasma removes carbon contamination and creates a uniform termination for Ga₂O₃ nucleation. The initial Ga₂O₃ growth conditions must be carefully controlled, with low V/III ratios preferred to prevent island formation. A two-step growth process—starting with a low-temperature nucleation layer (~500°C) followed by high-temperature bulk growth (~800°C)—yields the best results.
The electrical properties of Ga₂O₃ films benefit significantly from CeO₂ buffering. Hall effect measurements show electron mobilities increasing by 40-50% compared to films grown directly on sapphire, with typical values reaching 150 cm²/V·s for moderately doped layers. The breakdown voltage of Ga₂O₃ devices also improves due to reduced defect-assisted leakage paths. For Schottky diodes, the ideality factor decreases from 1.3-1.5 to 1.1-1.2 when using CeO₂ buffers, indicating better interface quality.
Thermal stability is another advantage of CeO₂-buffered structures. During high-temperature device processing (e.g., ohmic contact annealing at 900°C), the CeO₂ layer prevents Ga out-diffusion into the sapphire, maintaining stoichiometry. Rutherford backscattering spectrometry (RBS) confirms that CeO₂ reduces Ga segregation at the interface by over 60% compared to unbuffered samples.
Despite these advantages, challenges remain in optimizing CeO₂ buffers for industrial adoption. The primary issue is controlling the oxidation state of cerium, as non-stoichiometric CeO₂₋ₓ can introduce parasitic conduction paths. This is mitigated by strict oxygen partial pressure control during sputtering and annealing. Another challenge is the thermal expansion coefficient mismatch between CeO₂ (11 × 10⁻⁶ K⁻¹) and Ga₂O₃ (4-5 × 10⁻⁶ K⁻¹), which can cause cracking in thick films. Graded buffer layers or superlattices help alleviate this issue.
Future developments may explore doped CeO₂ buffers (e.g., with Gd or Sm) to further tune lattice parameters and oxygen mobility. Additionally, alternative deposition techniques like pulsed laser deposition (PLD) could offer better stoichiometric control for specialized applications. The integration of CeO₂ buffers with other ultra-wide bandgap semiconductors like AlN or diamond is another promising research direction.
In summary, CeO₂ serves as an effective buffer layer for Ga₂O₃ epitaxy on sapphire by suppressing oxygen diffusion and relaxing strain. RF sputtering process optimization enables high-quality CeO₂ films with controlled stoichiometry and microstructure. These advances contribute to improved performance of Ga₂O₃-based power electronics, RF devices, and solar-blind photodetectors.