Optical and electronic properties of Ge1−<i>x</i>Sn<i>x</i>/Si alloys grown by remote plasma-enhanced chemical vapor deposition

Solid state detectors composed of GeSn (germanium-tin) alloys offer improved capabilities compared with mercury cadmium telluride detectors. GeSn detectors may be smaller in size and weight, capable of operating with a noncryogenic detector, and provide increased sensitivity. Recent advances in nonequilibrium remote plasma-enhanced chemical vapor deposition growth enable GeSn crystalline growth with up to 10% Sn concentration, free of surface migration. Absorption spectroscopy combined with Tauc analysis results in 0.79, 0.73, 0.69, 0.59, 0.57, and 0.51 eV direct bandgap energies for GeSn samples with 0%, 2.7%, 4.6%, 6.6%, 7.1%, and 8.0% Sn, respectively. These absorption bandgap energies closely agree with density functional theory energies within ±0.05 eV. However, the rate of change of indirect bandgap narrowing as a function of Sn content is more diverse than a numerical result. The current research evidences that the indirect-to-direct transition crossover point occurs at a Sn content greater than 8%. From the analysis of the Urbach tail, the optical bandgap exhibits a potential structure disorder in the Urbach region. For example, this disorder may cause bandgap narrowing by more than 50% of the intrinsic bandgap energy in the highest Sn content (e.g., 8% Sn) sample. The surface Fermi level approximation validates p-type Fermi level pinning very close to the valence band maximum, often seen in highly doped semiconductors.