Temperature-dependent band gap, interband transitions, and exciton formation in transparent p-type delafossite CuCr1-xMgxO2 films

Temperature-dependent interband transitions and exciton excitations of sol-gel derived CuCr1-x MgxO2 (2% <= x <= 8%) films have been investigated by transmittance spectra (8-300 K) and photoluminescence (PL) spectra (77-300 K). An abnormal dependence of the optical band gap with the temperature has been found for the films with x = 0.02 and 0.04. At the low-temperature region, the gap energy shows a redshift trend with decreasing temperature. It is due to the strong Cr 3d-O 2p-Cu 3d interaction in the upper part of the valence band, which can be weaken by heavily Mg doping. The spin-orbit interactions of Cr3+ ions in an octahedral environment make the 3d states more disperse, which can contribute to the relatively high conductivity. A well-defined low-energy absorption has been assigned to the spin-allowed 3d -> 3d transition. Moreover, a strong exciton excitation around 1.8 eV has been observed due to the naturally low-dimensional structure of the delafossite, which can be modulated by temperature and hole concentration.