Germanium selenide (GeSe) is a promising IV–VI layered semiconductor with a direct bandgap of ~1.1–1.2 eV and a high absorption coefficient exceeding 10⁴ cm^–1 in the near-infrared region, making it an excellent candidate for photodetection applications. However, the high volatility of selenium during thermal processing inevitably induces severe stoichiometric deviation, selenium vacancy formation, and void defects, which significantly deteriorate film crystallinity and compromise device performance and stability. Conventional preparation methods, such as magnetron sputtering or thermal evaporation followed by open-face annealing, suffer from uncontrollable selenium loss, significant composition fluctuations, and poor reproducibility that fail to meet the requirements for high-performance optoelectronic devices.

To address these critical challenges, we present a two-step strategy that integrates microgap sublimation with inverted self-sealed annealing for the synthesis of high-quality GeSe thin films and fabricatingf GeSe/n-Si heterojunction photodetectors. Firstly, compact and uniform amorphous GeSe precursor films are deposited via microgap sublimation at 550 ℃ with a precisely controlled source–substrate spacing of 0.5 mm. This configuration creates a locally concentrated vapor-phase environment that minimizes the escape of selenium during deposition, ensuring a high-density, stoichiometric precursor. Subsequently, an inverted self-sealed annealing process is conducted at 400 ℃, where the film is placed face-down against a supporting spacer to form a confined microcavity. This unique geometry establishes a self-balanced selenium vapor circulation within the sealed space. It effectively suppresses selenium resublimation while promoting atomic diffusion and grain recrystallization through the localized vapor-pressure equilibrium.

The optimized films exhibit a pure orthorhombic phase with high crystallinity and no secondary impurities such as GeSe₂. The resulting GeSe/n-Si heterojunction photodetector demonstrates exceptional performance under 940 nm illumination, achieving a peak responsivity of 77.2 mA/W and a specific detectivity of 1.12 × 10¹² Jones at –2 V bias. Notably, the photocurrent exhibits a 41-fold enhancement compared with unannealed devices and a 5-fold improvement over devices subjected to conventional open-face annealing, underscoring the effectiveness of the self-sealed environment in defect reduction. Furthermore, the device exhibits millisecond-scale response times (rise/fall: 38.9 ms/6.2 ms) and maintains excellent operational stability over thousands of light on/off cycles without performance degradation.

This study demonstrates that the synergistic combination of microgap sublimation and self-sealed annealing provides a facile yet powerful route to simultaneously control composition and crystallization kinetics. By physically confining the volatile selenium species, this method effectively resolves the long-standing trade-off between crystallization quality and stoichiometric preservation in GeSe thin films, offering a reproducible and scalable platform for developing high-stability, high-performance near-infrared optoelectronic devices.