Sputtering-driven formation of interstitial oxygen for intrinsic NIR detection in IGZO phototransistor

Amorphous indium gallium zinc oxide (a-IGZO) is a promising wide-bandgap semiconductor for large-area optoelectronics; however, its intrinsic insensitivity to sub-bandgap photons typically necessitates extrinsic dopants or heterostructures for near-infrared (NIR) photodetection. Here, we report a heterostructure-free and dopant-free broadband phototransistor that achieves intrinsic NIR sensitivity through geometry-driven defect engineering during sputter deposition. Amorphous IGZO thin films with a nominal In:Ga:Zn atomic ratio of ≈ 1:2:1 were deposited using on-axis (vertical) and off-axis (horizontal) sputtering configurations. While on-axis IGZO only exhibited visible-light photosensitivity, off-axis IGZO displayed a pronounced NIR response, enabled by the formation of interstitial oxygen (Oᵢ) shallow states. X-ray photoelectron spectroscopy (XPS) and composition-matched density functional theory (DFT) calculations confirmed that these Oᵢ-induced defect states lie 0.1–0.5 eV above the valence band maximum (VBM), effectively narrowing the optical bandgap and enabling sub-bandgap absorption and photogating under 850 nm illumination. The optimized a-IGZO phototransistor achieves a responsivity of 42.5 A W⁻¹, an external quantum efficiency of 6.2 × 10³ %, and a specific detectivity of 8.3 × 10¹¹ Jones, all without plasmonic, hybrid, or quantum-dot sensitizers. Moreover, the off-axis process exhibits <10 % device-to-device variation across 5 samples, confirming its robustness and compatibility with large-area fabrication. To validate its practical utility, the off-axis IGZO device was further employed to quantify the sugar content (Brix) of coffee samples under NIR illumination, showing a clear correlation between photocurrent and concentration. This work demonstrates a simple, scalable, and CMOS-compatible approach to extending the spectral response of oxide semiconductors, opening new opportunities for cost-effective broadband photodetectors and integrated photonic systems.