Coupled Defect Chemistry and Surface Electrostatics Modulate Rare-Earth Upconversion Kinetics in Oxide Thin Films

Rare-earth activation in oxide thin films is often optimized empirically, although excitation-dependent upconversion (UC) is ultimately constrained by the near-surface energetic landscape and defect-mediated screening. Here, we identify a quantitative correlation that links defect chemistry and surface electrostatics to UC saturation kinetics in BaTiO3-based thin films. By combining XPS (high-binding-energy O 1s fraction and valence-band offsets), Kelvin probe force microscopy (contact potential difference and work function), Raman-Fano analysis (vibronic asymmetry), and excitation-selective confocal luminescence (405/800/980 nm), we identify a measurable near-surface energetic parameter correlating with UC kinetics, ΔECBM-F = Eg - (EF - EVBM). Across BaTiO3 (BT), BaTiO3:Er3+ (BTE), and BaTiO3:Er3+/Yb3+ (BTEY), ΔECBM-F collapses from ∼1.19 to ∼0.51 eV and ∼0.08 eV, concomitant with the suppression of defect-/OH-related oxygen signatures, stabilization of the nanoscale surface potential (Vcpd ≈ 170, 321, and 255 mV), and band-gap narrowing (3.06 → 2.30 eV). This energetic collapse constrains the transition from defect-dominated relaxation (BT) to Er-mediated UC under 800 nm (BTE, n ≈ 1.7 for green and n ≈ 1.1 for red) and to sensitized, transfer-limited UC under 980 nm (BTEY; n ≈ 1.0-1.1) with enhanced red/green branching and power-robust chromaticity. Our results identify a measurable energetic control parameter that rationalizes UC saturation beyond dopant identity, providing physically grounded design guidelines for rare-earth-active oxide thin-film emitters in integrated photonics.