Optoelectronically Active GaAs/GeSn‐MQW/Ge Heterojunctions Created via Semiconductor Grafting

ABSTRACT Semiconductor devices have traditionally relied on lattice‐matched heterojunctions formed via epitaxy. However, expanding device functionalities increasingly demands integration across extreme lattice mismatches. Here, a lattice‐mismatched, single‐crystalline GaAs/GeSn‐multi‐quantum well (MQW)/Ge n‐i‐p heterojunction photodiode is realized through advanced “semiconductor grafting” technology. This method enables simultaneous engineering of band alignment and optical field distribution. Comprehensive STEM characterization reveals an atomically clean GaAs/GeSn‐MQW interface with no observable interdiffusion. The resulting photodiode exhibits outstanding performance: a record‐low dark current density of 1.22 × 10 ⁻⁷ A/cm ² , extended spectral response from ∼0.5 to 2 µm, and high photo responsivities of 0.85 A/W and 0.40 A/W at 520 nm (VIS) and 1570 nm (NIR), respectively. Compared to epitaxial reference device, this represents a reduction in dark current by over five orders of magnitude, and responsivity enhancements of ∼7× in the VIS and ∼3× in the NIR ranges. This work establishes a new strategy for constructing lattice‐mismatched optoelectronic heterojunctions. Beyond the GaAs/GeSn‐MQW/Ge system, the approach offers a broadly applicable platform for high‐performance device integration free from conventional strict lattice‐matching requirement.