Thermal behaviour of the Cu-related 2.177 eV bound exciton in GaP

Abstract

New experimental results from photoluminescence (PL) data are presented for the Cu-related 'characteristic-orange-luminescence' (COL) emission in GaP. The PL spectrum shows a sharp electronic line at 2.1774 eV at 2K, together with a very complicated phonon wing from interaction with continuum modes as well a large number of rather sharp inband resonance modes. The nature of these discrete low-energy phonon modes is discussed in the paper, in relation to a plausible model for the identity of the centre. The phonon coupling causes the electronic line in the PL spectrum to disappear completely above 45K, while the entire COL emission is thermally quenched by an electronic process above 140K. The thermal activation energy deduced from this electronic quenching is consistent with the thermal emission of the exciton as a single entity. A qualitative estimate of the contributions of discrete and continuum phonon modes to the quenching of the zero-phonon line is also presented. The presence of a low-energy discrete mode of 2.6 meV makes a quantitative evaluation difficult, since coupling to this phonon mode is observed only in the excited vibronic state, when it is thermally excited. Comparison between the discrete phonon energies on the anti-Stokes wing and those obtained from absorption measurements reported previously shows that these agree within experimental accuracy. The phonon energies in the excited vibronic state are reduced compared with the values for the ground state, illustrating anharmonic effects in the electron-phonon coupling.