At low temperatures bound exciton emission is the dominant radiative process, whereas at higher temperatures free exciton emission with respect to defect related states usually takes over. Several interpretation for the emission lines are reported, like, the neutral donor bound excitons (D0X) (Appl. Phys. Lett. 77, 975 (2000).), acceptor bound excitons (A0X) (Appl. Phys. Lett. 81,
1830 (2002)), nitrogen related electron-acceptor transitions (e,AN ) (Phys. Rev. B 65, 075207, 2002), transitions related to intrinsic point defects such as oxygen vacancies and zinc interstitials (VO,Zni) (J. Cryst. Growth 216, 330 (2000).). In bulk ZnO crystals the free exciton is observed at about 3.377 eV. The emission at 3.367 eV is commonly assigned to ionised donor bound excitons and the neutral donor bound excitons are positioned between 3.3628 and 3.359 eV. Bound excitons previously attributed to the Na and Li acceptors occur at 3.356 and 3.353 eV. (phys. stat. sol. (b) 241, No. 2, 231– 260 (2004))
Basically if either the pure or doped ZnO sample is of p-type and low temperature excitonic emission is seen, then we can predict it as a Acceptor-Bound-exciton emission. Based on the spectral position Kato et al. (Jpn. J. Appl. Phys.
42, 2241 (2003).) ascribed the 3.333 eV emission line to the recombination of excitons bound to neutral acceptors (A0X). However, the chemical nature of the donor and acceptor species need to be determined. Also the temperature dependent photoluminescence as well as spatially resolved kathodoluminescence might be useful to gain detail insight into the nature of these kind of recombination.
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