Band gap characterization of Remote Plasma Enhanced Chemical Vapor Deposition-grown GeSn alloys

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 non-cryogenic detector, and provide increased sensitivity. Recent advances in non-equilibrium remote plasma-enhanced chemical vapor deposition growth enables GeSn crystalline growth with up to 10% Sn concentration, free of surface migration. Absorption spectroscopy combined with Tauc analysis results in 0.79 eV, 0.73 eV, 0.69 eV, 0.59 eV, 0.57 eV, and 0.51 eV direct band gap 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 band gap narrowing as a function of Sn content is more diverse than a numerical result. The current research evidences that the indirect-to-direct transition cross-over point occurs at a Sn content greater than 8%. From analysis of the Urbach tail, the optical band gap exhibits a potential structure disorder in the Urbach region. For example, this disorder may cause band gap narrowing by more than 50% of the intrinsic band gap 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.