Dynamics of trapping on donors and relaxation of the B-exciton in GaN

There are three exciton branches in wurzite-GaN: A, B and C. Here, time-resolved photoluminescence and magneto-optic spectroscopy of the B-exciton are presented, both free (FX<INF>B</INF>) and bound to donor (D⁰X<INF>B</INF>) in homoepitaxial GaN. The free B-exciton has an energy of about 3.483 eV and a very short lifetime of about 20 ps. The B-exciton bound to donor (D⁰X<INF>B</INF>) has an energy of 3.475 eV and a lifetime longer than the free excitons but shorter than the lifetime of the A-exciton bound to donor. It is shown that a simple model based on an invariant expression describes properly the behavior of D⁰X<INF>A</INF> and D⁰X<INF>B</INF> in a magnetic field. Results of resonance excitation experiments suggest that in many cases the process of binding a free exciton to a donor conserves the exciton type and that the relaxation of B- to A-exciton takes place later. Moreover, thermal excitation of D⁰X<INF>A</INF> to D⁰X<INF>B</INF> has been observed.