Positive temperature variation of the bandgap energy in AgGaSe 2

Optical‐absorption, photoreflectance (PR), and photoluminescence spectra have been measured on the ternary chalcopyrite semiconductor AgGaSe 2 at T = 15–300 K. The direct‐bandgap energy E 0A of AgGaSe 2 determined from the optical absorption measurements shows unusual temperature dependence at low temperatures. The resultant coefficient ∂ E 0A /∂ T is positive at T < 70 K and negative above 70 K. The PR spectra also indicate positive temperature dependence of the bandgap energies, E 0A and E 0B , at T < 70 K. These results have been successfully explained by taking into account the negative lattice thermal expansion at low temperatures. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)