Optical absorption band edge in single-crystal GeS

The absorption band edge of single-crystal orthorhombic GeS has been measured in all three polarizations at 4.2, 77 and 300°K. Strong excitonic absorption is observed for E ∥ a, and the transitions in this direction are identified as direct-allowed. Transitions in the E ∥ b and E ∥ c polarizations are tentatively identified as direct forbidden and direct allowed, respectively, with values for Eg similar to those found for E ∥ a. Evidence for the partially two-dimenśional character of GeS is discussed.