Hydrogen-related excitons and their excited-state transitions in ZnO

The role of hydrogen in the photoluminescence (PL) of ZnO was investigated using four different types of bulk ZnO single crystal, with varying concentrations of unintentional hydrogen donor and Group I acceptor impurities. Photoluminescence spectra were measured at 3 K, with emission energies determined to ±50 μeV, before and after separate annealing in ${\mathrm{O}}_2$, ${\mathrm{N}}_2$, and ${\mathrm{H}}_2$ atmospheres. Using this approach, several new hydrogen-related neutral-donor-bound excitons, and their corresponding B exciton, ionized donor, and two electron satellite (TES) excited state transitions were identified and their properties further investigated using hydrogen and deuterium ion implantation. The commonly observed $I_4$ (3.36272 eV) emission due to excitons bound to multicoordinated hydrogen inside an oxygen vacancy (${\mathrm{H}}_{\mathrm{O}}$), that is present in most ZnO material, was noticeably absent in hydrothermally grown (HT) ZnO and instead was replaced by a doublet of two closely lying recombination lines $I_{4b,c}$ (3.36219, 3.36237 eV) due to a hydrogen-related donor with a binding energy ($E_D$) of 47.7 meV. A new and usually dominant recombination line $I_{6\text{−}H}$ (3.36085 eV) due to a different hydrogen-related defect complex with an $E_D$ of 49.5 meV was also identified in HT ZnO. Here, $I_{4b,c}$ and $I_{6\text{−}H}$ were stable up to approximately 400 and 600 °C, respectively, indicating that they are likely to contribute to the unintentional $n$-type conductivity of ZnO. Another doublet $I_5$ (3.36137, 3.36148 eV) was identified in hydrogenated HT ZnO single crystals with low Li concentrations, and this was associated with a defect complex with an $E_D$ of 49.1 meV. A broad near band edge (NBE) emission centered at 3.366 eV was associated with excitons bound to subsurface hydrogen. We further demonstrate that hydrogen incorporates on different lattice sites for different annealing conditions and show that the new features $I_{4b,c}$, $I_{6\text{−}H}$, and $I_5$ most likely originate from the lithium-hydrogen defect complexes $\mathrm{L}{\mathrm{i}}_{\mathrm{Zn}}\text{−}{\mathrm{H}}_{\mathrm{O}}$, $\mathrm{A}{\mathrm{l}}_{\mathrm{Zn}}\text{−}{\mathrm{H}}_{\mathrm{O}}\text{−}\mathrm{L}{\mathrm{i}}_{\mathrm{Zn}}$, and ${\mathrm{V}}_{\mathrm{Zn}}\text{−}{\mathrm{H}}_{3,4}$, respectively.

Figure 1

(a) NBE PL spectra of four different types of ZnO single crystals at 3 K: (i) VP ZnO, (ii) melt ZnO, (iii) HT ZnO, and HT ZnO with low lithium concentration (low-Li HT ZnO); (b) neutral donor-bound exciton ($D^0{X}_A$) region for each of the four crystals. The Zn-polar (0001) face was investigated in all cases. The spectra have been vertically displaced for clarity.

Figure 2

Arrhenius plots of (a) the recombination lines $I_6$ and $I_{6\text{−}H}$ and (b) the recombination lines $I_{4b}$ and $I_{4c}$ of HT ZnO. Dots are the experimental peak intensities, and the solid lines are the fits to the data with the corresponding decay energies $E_i$ and activation energy $E_A$ [note: the intensity scaling of panel (a) is 2.5 times that of panel (b)].

Figure 3

(a) NBE 3 K PL spectra of HT ZnO single crystals after annealing at 600 °C in different atmospheres: (i) as-received, (ii) ${\mathrm{N}}_2$ (90 mins), (iii) ${\mathrm{O}}_2$ (90 mins), (iv) ${\mathrm{O}}_2$ (90 mins) followed by FG (60 mins), and (v) ${\mathrm{O}}_2$ (90 mins) followed by FG (60 mins) followed by ${\mathrm{O}}_2$ (60 mins); (b) neutral donor-bound exciton ($D^0{X}_A$) region for each of the annealing conditions; and (c) TES region for each of the annealing conditions. The Zn-polar (0001) face was investigated in all cases. The spectra have been vertically displaced for clarity.

Figure 4

3 K PL spectra of the neutral donor-bound exciton ($D^0{X}_A$) region and the TES region of (a) and (b) VP ZnO, (c) and (d) melt ZnO, (e) and (f) HT ZnO with low lithium concentration (low-Li HT ZnO) before and after annealing at 600 °C in ${\mathrm{O}}_2$ (90 mins), and ${\mathrm{O}}_2$ (90 mins) followed by FG (60 mins). The Zn-polar (0001) face was investigated in all cases. The spectra have been vertically displaced for clarity.

Figure 5

3 K PL spectra of the neutral donor-bound exciton ($D^0{X}_A$) region of HT ZnO before and after implantation (and subsequent 30 min annealing steps in ${\mathrm{O}}_2$ gas from 200 to 600 °C) with (a) hydrogen and (b) deuterium; and the same experiment repeated for low-Li HT ZnO with (c) hydrogen and (d) deuterium implantation. All spectra were taken from the Zn-polar (0001) face of samples of the same wafer. The spectra have been vertically displaced for clarity.

Figure 6

NBE 3 K PL spectra of HT ZnO before and after coverage with a thin (∼10 nm) layer of aluminum followed by 30 min annealing at $600{}^{\circ }\mathrm{C}$ in ${\mathrm{N}}_2$. The inset shows the neutral donor-bound exciton ($D^0{X}_A$) region in greater detail using a linear scale. The spectra were taken from the Zn-polar (0001) face of the same wafer.

Figure 7

Localization energy ($E_{\mathrm{loc}}$) of the neutral ($D^0{X}_A$) and ionized ($D^{+}{X}_A$) donor-bound excitons in ZnO as a function of the donor binding energy ($E_D$). The respective energy values for $I_{4b,c}$, $I_{6\text{−}H}$, $I_6$, and $I_8$ were determined from HT ZnO, while $I_4$, $I_{6a}$, and $I_9$, were determined from VP ZnO samples.

Figure 8

Likely physical structures of the hydrogen-related defect complexes associated with the neutral donor-bound excitons $I_4$, $I_{4b,c}$, $I_5$, and $I_{6\text{−}H}$, observed in ZnO PL. (Note: the $I_{4b,c}$ defect structures shown would require an additional H atom in the $\mathrm{L}{\mathrm{i}}_{\mathrm{Zn}}\text{−}\mathrm{H}\text{−}\mathrm{O}$ configuration to act as an overall donor, rather than as a neutral donor-acceptor pair).