Bound and free excitons in ZnO. Optical selection rules in the absence and presence of time reversal symmetry

The correlation between ionized donor bound exciton recombinations and neutral donor bound exciton recombinations in ZnO has been investigated. The experimental data obtained by means of magnetophotoluminescence (MPL) concerning charge state and localization energies of ionized and neutral donor bound excitons are in good agreement with theoretical predictions. The optical selection rules in absence and presence of time reversal symmetry (TRS) are investigated. It is shown that the inclusion of extra degeneracy due to TRS reveals a number of new states of the same symmetries and essentially does not change the existing optical selection rules. & 2008 Elsevier Ltd. All rights reserved. 1. Neutral and ionized donor bound excitons Lithium-doped ZnO epilayers were grown by chemical vapor deposition on ZnO substrates. Photoluminescence (PL) and magneto-photoluminescence (MPL) measurements were performed using a 325nm HeCd laser. Fig. 1 displays the PL at 4.2K in the energetic range of the free and bound excitons. The most prominent excitonic transition lines are the I9 (3.3567eV), I8 (3.3598eV), I6a (3.3604eV), I2 (3.3674eV), I1 (3.3720 eV), and I0 (3.3726eV). Previous studies have shown that I9, I8, and I6a are neutral exciton complexes bound to an indium [1], gallium [2], and aluminum impurity, respectively [3]. These lines are accompanied by the higher energetic lines I2, I1, and I0 [4]. The neutral bound exciton line I9 is correlated to I2, I8–I1, and I6a–I0. In fact, all investigated samples exhibit only I0–I2 excitons if the related excitons I6a–I9 are present as well. Due to a similar scaling in intensity and energetic position, it is likely that these correlated pairs of transition lines are excitons bound to an impurity of the same chemical identity but in a different charge state. Consequently, we attribute I0, I1, and I2 to ionized donor bound excitons related to Al, Ga, and In impurities, respectively. Concerning the I1 complex, this correlation is in agreement with the data published by Johnston et al. [2], who reported a simultaneous decrease in the I8 and I1 intensity for ZnO crystals doped with a radioactive Ga isotope. The various bound exciton complexes with their localization energies and suggested chemical identities are summarized in Table 1. The charge states of the bound exciton complexes are investigated by MPL spectroscopy. Excitons bound to ionized impurities can be distinguished from those bound to neutral impurities by a nonlinear splitting of energy levels in the magnetic field perpendicular to the c-axis of the crystal, while excitons bound to neutral impurities exhibit a linear splitting behavior for ~ B ? c [5]. For ionized bound excitons at low magnetic fields, only a high energy Zeeman component, resulting from a G5 state is visible, whereas the low energy component, originating from G1 to G6 transition is forbidden by selection rules (SRs). However, for larger magnetic fields, the SRs can be relaxed due to a spin–spin interaction of G5 states with anti-parallel spin, mixed with G6 states with parallel spin, thus allowing the appearance of a new line associated with the G6 state. The extrapolation of the peak positions to B 1⁄4 0T reveals the presence of the zerofield splitting, ascribed to the spin–spin interaction energy. Such an interaction cannot occur in transition lines originating from excitons bound to neutral impurities, since the spin of the two equal particles are anti-parallel. Evidently, we observe a linear Zeeman splitting for the I6a–I8 lines, whereas this is not the case for the bound excitons I0 and I1 (Fig. 2). Furthermore, an additional low energy transition due to the zerofield splitting appears for I0 and I1 if a magnetic field is applied. Similar results (not shown) were obtained for I9 (linear Zeeman splitting) and I2 (zerofield splitting) in accordance with the previously published measurements [6]. Consequently, following the previous discussion, the transition lines I6a–I9 originate from recombinations of neutral bound excitons, whereas I0–I2 are related to ionized bound exciton complexes. ARTICLE IN PRESS