Time-resolved Photoluminescence Study of ZnSe 1-x O x

In order to explore the effect of O on the decay dynamics in ZnSe1-xOx, the TRPL measurements are performed. Figure 3.7 displays the TRPL spectra of ZnSe1-xOx (x = 0.027 and 0.053) monitored at the PL peaks and 10 K. Obviously, several interesting findings can be drawn. (i) The PL decay profiles exhibit non-single-exponential decay and can be decomposed into a fast (initial) and a slow (tail) component. Moreover, the fast component is mono-exponential decay, while the slow component is stretched-exponential behavior. (ii) The PL decay rate is an order of magnitude slower than that of ZnSe (~ 5.0 ns^-1). (iii) The PL lifetime of slow component significantly increases with the O concentration. These unique optical properties are signatures of ZnSe-based isoelectronic semiconductors [18], implying complicated carrier dynamics. Quantitatively, the decay curves are fitted by using the following equation, where τ1 and τ2 is the exciton lifetime of fast and slow component respectively, and β is the stretching exponent ranging from 0 to 1. This stretching exponent is a measure of the relaxation rates involved in the PL decay process where a smaller β means a broader rate distribution. The fitted τ1 is about 850 ps for both samples; τ2 (β) is about 2.8 ns (0.96) and 4.1 ns (0.93) for x = 0.027 and 0.053, respectively. Increasing the O concentration increases τ2 and

increasing number of isoelectronic O traps provides multidecay paths for the excitons [18].

The suggestion is consistent with the hopping-transport model, in which the concentrations of transport and trapping sites determine the stretching exponent [28].

To verify the suggestions, Figure 3.8(a) and 3.8(b) show the TRPL spectra at 10 K and various monitored energies for x = 0.027 and 0.053, respectively. Decreasing the monitored energy, the decay time from the slow component significantly increases, while the decay time from the fast component is insensitive to energy. For x = 0.027, τ2 (β) is around 2.4 ns (0.97) and 4.7 ns (0.76) at 2.55 and 2.43 eV, respectively. For x = 0.053, τ2 (β) is around 2.8 ns (0.97) and 7.4 ns (0.70) at 2.44 and 2.32 eV, respectively. In accordance with the asymmetric linewidth broadening, the above results can be ascribed to exciton energy transfer among isoelectronic O traps. The TRPL image in Fig 3.9 reveals the dynamics of energy transfer in ZnSe0.973O0.027. Within the initial few nanoseconds after excitation, the emission is peaked at 2.52 eV. As time passes, an energy redshift is clearly observed, revealing that some of the LE transfer to deeper traps. Moreover, the decay profile at high energy exhibits mono-exponential decay, whereas that at low energy is a stretched-exponential function, implying that the effect of LE can be neglected at high energy region. This phenomenon becomes even more pronounced upon recording of the higher energy emissions. Therefore, the fast decay discussed herein can be ascribed to free excitons (FE).

For comparing S-shaped feature with the decay traces, Figure 3.10 and Figure 3.11

display the temperature-dependent TRPL spectra of ZnSe1-xOx (x = 0.027 and 0.053).

According to these figures, increasing the temperature significantly quenches the lifetime of slow component. As the temperature is increased above 100 K, the slow decay component is eliminated and the fast decay dominates the entire decay profile. Moreover, the bending of the logarithmic decay curve, i.e. a reduction in β, initially becomes more pronounced with increasing temperature up to 70 K and then gradually diminishes, i.e. the β starts to increase.

To get a more quantitative measure of the β, the decay curves are fitted with Eg. (3). The stretching exponent β as a function of temperature is displayed in the inset in Figure 3.10 and Figure 3.11 for x = 0.027 and 0.053. Increasing the temperature initially decreases β to a minimum at 70 K and then monotonically increases. This phenomenon can be explained by the corresponding configuration coordinate diagram schematically shown in Figure 3.12(a).

At the lowest temperature (10 K), electrons that are generated initially in the free state hop among proximal transport and trapping sites and recombine with the isoelectronic O traps (channel 1). With increasing temperature these less mobile electrons are imparted with additional energy that allow hopping to deeper trap states (channel 2), causing a rapid redshift in the LE peak and a reduction of β. Simultaneously, as the temperature approaches 70 K, portions of trapped electrons are thermally activated back to populate the higher energy states (channel 3), leading to a significant PL linewidth broadening at the high energy shoulder. At still higher temperatures, an increasing number of electrons gain sufficient energy to

delocalize into the free state (channel 4) and recombine, explaining the increase in β and the blueshift of the PL peaks. Above 100 K, the slow decay component disappears because FE dominates the recombination, resulting in a monotonic PL energy redshift with the temperature. The delocalization effect is further verified by the TRPL image of ZnSe0.947O0.053

at 80 K. In Figure 3.12(b), as time passes an energy redshift at low-energy peak is observed due to the recombination from traps. However, the high-energy peak exhibits no obvious energy shift with time, reflecting the recombination of thermally delocalized FE.

Table 3.1 Fitting parameters from the Varshni equation and the BAC model.

Fig. 3.1. Normalized PL and transmission spectra of ZnSe1-xOx at 10 K.

Fig. 3.2. Excitation power-dependent PL spectra for ZnSe0.973O0.027 at (a) 10 K and (b) 130 K.

Fig. 3.3. Temperature-dependent PL spectra of ZnSe0.973O0.027.

Fig. 3.4. Temperature-dependent PL spectra of ZnSe0.947O0.053.

Fig. 3.5. Temperature-dependent PL peak energies for (a) ZnSe0.973O0.027 and (b) ZnSe0.947O0.053. The solid and dashed curves present the fits from the Varshni equation and the BAC model, respectively. The open circles denote the PL peak energy fitted by two Gaussian functions.

Fig. 3.6. Band gap energy of ZnSe1-xOx as a function of O concentration at 300 K.

The black squares mark the PL energy in this work, the blue circles mark the value from photoreflectance in Ref. 16, and the black curve shows the best fit by BAC model.

Fig. 3.7. TRPL spectra at 10 K of ZnSe1-xOx (x = 0.027 and 0.053).

Fig. 3.8. TRPL spectra at 10 K as a function of monitored energies for (a) x = 0.027 and (b) 0.053.

Fig. 3.9. TRPL image of ZnSe0.973O0.027 at 10 K.

Fig. 3.10. Temperature-dependent TRPL spectra of ZnSe0.973O0.027 and the corresponding stretching exponent β (inset).

Fig. 3.11. Temperature-dependent TRPL spectra of ZnSe0.947O0.053 and the corresponding stretching exponent β (inset).

Fig. 3.12. (a) Schematic diagram used in the discussion, indicating complex relaxation channels. For simplicity only two trap states are considered. (b) TRPL image of ZnSe0.947O0.053 at 80 K.