Optical characterization of ZnO whiskers grown without catalyst by hot wall epitaxy method

We investigated the photoluminescence (PL) characteristics of ZnO whiskers grown without a catalyst by the hot wall expitaxy method. In the low-temperature PL measurement, free exciton (n=1, 2) emissions appeared as a shoulder. Exciton emissions bound to the neutral donor (D0X) and acceptor (A0X) dominated the edge emission region while phonon replicas of D0X and donor-acceptor pair (DAP) emission dominated the lower energy region. Furthermore, no defect-related band was observed, indicating high crystal quality. With increased temperature, the A0X (I9) peak decreased abruptly and was then suppressed above 130 K. Finally, the room temperature PL spectrum was dominated by DAP emission and free exciton emission appeared faintly around 3.308 eV. Based on these results, the room temperature bandgap energy was estimated to be 3.368 eV, which takes the exciton binding energy 60 meV.