A study of capping layers for sulfur monolayer doping on III-V junctions

Recently, high dosage doping on Si multi-gate field effect transistors and III-V planar structures using a self-limiting monolayer doping technique was reported to overcome challenges in scaling nano-sized transistors. The stoichiometry or composition of the capping layer was found to affect the diffusion efficiency of this process. In this work, we study the effect of a capping layer in sulfur monolayer doping on III-V junctions. Various capping temperatures and growth methods were compared. Based on the theoretical and experimental results, we suggest an optimized scheme consisting of a bi-layer capping structure. From Hall measurements and secondary ion mass spectrometry, a SiN_x/BeO bi-layer capping, compared to single layer cap, exhibited the best results with a surface sheet resistance of 232 ω/sq, junction depth of 11 nm, dopant profile abruptness of 3.5 nm/dec, electrically active S concentration of 4.9 × 10¹⁹/cm³ (=1.34 × 10¹³/cm²), and 3 times higher activation efficiency without significant transient-enhanced dopant diffusion.