Observation of bandgap narrowing in <i>p</i> -type doped GeSn alloys

A series of p-type doped GeSn samples has been epitaxially grown by the ultra-high vacuum chemical vapor deposition method, and direct bandgap narrowing (BGN) is observed through photoluminescence and infrared spectroscopic ellipsometry measurements, demonstrating a positive correspondence with increasing carrier concentration. The experimental results could be fitted by a square-root correlation of ΔE0 (meV) = −13.69 N/1018 cm−3. The theoretical expression for the BGN of p-type Ge and GeSn has been revisited with updated carrier effective masses, and satisfactory agreement with the experimental data has been achieved. The results show that BGN in p-GeSn is more pronounced as compared to those in n-type counterparts, suggesting that special attention should be paid in future scientific research and device designing processes that involve such materials.