Ionized and neutral donor-bound excitons in ZnO

We show for ZnO a correlation between the ionized donor bound exciton $D^{+}X$ recombinations $I_0$, $I_1$, and $I_2$ and the neutral donor bound exciton $D^{0}X$ recombinations $I_{6a}$, $I_8$, and $I_9$, respectively. The experimental data on the localization energies of ionized and neutral donor bound excitons are compared to theoretical predictions and a good agreement is found. We give an estimate of the localization energies of neutral acceptor bound excitons. One can conclude that the localization energies of neutral donor and acceptor bound excitons in ZnO will be very close in energy and an assignment of transition lines to donors and/or acceptors (in the absence of decisive data from magneto-optics) will not be straightforward.

Figure 1

Photoluminescence spectrum of a homoepitaxial ZnO thin film taken at $T=4.2\mathrm{K}$ showing neutral donor bound exciton recombination $I_{6a}$, $I_8$, and $I_9$ and the corresponding ionized donor bound exciton lines $I_0$, $I_1$, and $I_2$. $A_T$ and $A_L$ are the transversal and longitudinal free $A$-exciton recombinations.

Figure 2

Photoluminescence spectra taken at $T=4.2\mathrm{K}$ of ZnO bulk crystals grown from the vapor phase and doped with (a) Na and (b) Li. A correlation between $I_9$ and $I_2$ can be derived, as well as a tentative correlation between $I_{10}$ and $I^{*}$.

Figure 3

Photoluminescence measurements at $T=1.6\mathrm{K}$ on a homoepitaxial ZnO thin film showing the behavior of the $I_0$ and $I_1$ recombinations in an applied magnetic field (Voigt configuration).

Figure 4

Localization energies of neutral $D^0$ and ionized $D^{+}$ bound excitons in ZnO as a function of the donor binding energies $E_D$.