Excited state properties of donor bound excitons in ZnO

ZnO shows a great variety of richly structured luminescence lines in a very narrow energy range of 30 meV below the free $A$-exciton line. At very low temperatures the majority of these lines can be explained by radiative recombination of excitons bound to neutral and ionized donors. With increasing temperature even more photoluminescence (PL) lines appear with activation energies which indicate the involvement of excited states of the bound exciton complexes. Based on high-resolution temperature-dependent luminescence and luminescence excitation experiments we investigate the excited states properties of donor bound excitons in ZnO. Several possible configurations of excited states could be distinguished: (i) excitons which involve a hole from the $B$- instead of the $A$-valence band, (ii) vibrational-rotational excited states of the excitons, and (iii) electronic excited states of the excitons. Magneto-PL measurements of the ground and vibrational-rotational excited states corroborate the identification of the excited states with comparable $g$-factors for all shallow donor bound excitons. In addition to the excited states, energy transfer processes via the ionized bound excitons and free exciton polaritons are observed. The experimental results are supported by theoretical calculations and demonstrate that ZnO is a unique compound semiconductor where the basics of atomic and molecular physics can be studied and understood in a solid state matrix.

Figure 1

(Color online) Schematic of bound exciton ground and excited state recombinations observed in ZnO (energies are not to scale).

Figure 2

(Color online) Photoluminescence spectrum of a ZnO single crystal recorded at 10 K. Dashed (blue) lines mark the transitions of neutral and ionized donor bound excitons, dotted (red) lines indicate excited states of donor bound excitons.

Figure 3

(Color online) Arrhenius plots of the recombination lines $I_{0a}$ and $I_9$. Dots are experimental data, solid lines are fits to the data with the resulting activation energies.

Figure 4

(Color online) Arrhenius plots of the recombination lines $I_4^B$ and $I_b$. Dots are experimental data, solid lines are fits to the data with the resulting activation energies.

Figure 5

(Color online) Photoluminescence spectrum of a ZnO single crystal recorded at 10 K. Vertical drop lines mark recombinations of neutral donor bound excitons with holes from the $A$- and $B$-valence band.

Figure 6

(Color online) (a) Magneto-PL spectra of a homoepitaxial ZnO film recorded at 2 K at field strengths of 0, 1, and 3 T in the Voigt configuration $\left(\mathit{B}\perp \mathit{c}\right)$. Vibrational-rotational excited states are observed for the bound excitons $I_8$, $I_7$, and $I_6$. (b) Peak energies of the Zeeman split transition lines for ground and excited states as function of the magnetic field strength between 0 and 5 T.

Figure 7

(Color online) Luminescence excitation spectrum monitored at the $I_6$ recombination line. Excitation channels exist via the free ($A$, $B$, and $C$) exciton polaritons and their excited states $\left(n=2\right)$ and for the neutral donor bound exciton excited states (note the change in the energy scale). The energy spacing of the excitation channels to the $I_6$ line is displayed in the top $x$ axis.

Figure 8

Energy position as a function of donor binding energy for neutral and ionized donor bound excitons involving $A$- and $B$-valence bands at 4 K.