In this chapter, we study the carrier relaxation mechanisms of ZnSe1-xOx (x = 5.3 %) and ZnSe1-yTey (y =5.0 and 10 %) using PL and TRPL measurements. The temperature of decay dynamics of ZnSe1-xOx and ZnSe1-yTey will be investigated.
Figures 3.1(a)-3.1(d) show the temperature dependence of the full width at half maximum (FWHM), PL peak position, integrated PL intensity, and PL lifetime of ZnSe0.947O0.053, respectively. The FWHM increases with increasing temperature, forming a sharp peak between 60 and 130 K, where the PL peak energy undergoes a blueshift by as much as 30 meV relative to the initial rapid redshift. The concurrence of the band broadening and the S-shaped energy shift is typical for a carrier localization effect, which is explained by thermal-activated electron transfer between the localized and free states owing to the small (~
20 meV) PL binding energy [Fig. 3.1(b)]. In Figs. 3.1(c) and 3.1(d), when the temperature exceeds 10 K, the PL intensity and lifetime decrease sharply.We note that the 10 - 80 K temperature range exhibiting the strongest thermal quenching of PL intensity and lifetime is coincident with the temperature region exhibiting the strongest PL redshift and linewidth broadening. These experimental findings are most likely due to the increased nonradiative recombination rate at higher temperatures where thermal-activated electrons hop to deeper O traps.
Fig. 3.1 Temperature-dependent (a) FWHM (b) PL peak energy (c) integrated PL intensity (d) PL Lifetime of ZnSe0.947O0.53. Solid line in (b) represents fit using BAC model.
Figure 3.2(a) shows the temperature-dependent PL spectra for ZnSe0.950Te0.050 from 10 to 300 K along with indicated peak positions. The PL peaks exhibit a V-shaped energy shift as the temperature increases. At low temperatures (< 140 K), the PL emissions shift monotonically toward lower energies, which shift is accompanied by asymmetric linewidth broadening with increasing temperature. As the temperature further increases above 140 K, a pronounced blueshift of this peak is observed and a high energy shoulder appears. Figure 3.2(b) plots the temperature-dependent PL peak energies obtained from Fig. 3.2(a). In comparison with the S-shaped energy shift of ZnSe0.947O0.053, the V-shaped energy shift herein implies a quite deep trap resulted from alloy fluctuations or Te clustering. From the PLE spectrum of ZnSe0.950Te0.050 detected at the PL peak and 10 K, it is clear that this emission is preferentially excited via band-to-band processes. Moreover, a large Stokes shift of ~ 200 meV with respect to the band edge measured by PLE is observed. The value agrees well with the PL binding energy (Eg – EPL) obtained previously using photoconductivity and reflectivity approaches [9], indicating that the emissions within the temperature range are all generated by localized excitons. Above 140 K, the peak energy initially undergoes a slow blueshift up to 260 K and then a fast shift until 300 K. The phenomena can be understood via different decay channels shown in the inset of Fig. 3.2(b), which will be discussed in details later.
Fig. 3.2 (a) Temperature-dependent PL spectra of ZnSe0.95Te0.05 and (b) corresponding PL peak energy against temperature. Dashed curve in (a) represents the PLE spectrum of ZnSe0.950Te0.050 detected at the PL peak and 10 K. Inset in (b) shows schematic diagram of carrier decay paths and only three trap states are considered for simplicity.
To gain insight into the carrier relaxation dynamics of ZnSe0.950Te0.050, Fig. 3.3(a) shows the temperature-dependent TRPL spectra. Clearly, several interesting conclusions can be drawn. (i) As the temperature increases, the PL lifetime initially increases up to 70 K and then monotonically declines. (ii) None of the PL decay profiles behave in a monoexponential decay.
(iii) All of the TRPL spectra on a double logarithmic scale exhibit straight lines [inset in Fig.
3.3(a)], which are strong evidences that the PL decay profiles of ZnSe0.950Te0.050 follow the stretched exponential law. These facts indicate a peculiar and complex carrier decay mechanism in ZnSe0.950Te0.050. Accordingly, the decay curves of ZnSe0.950Te0.050 are all fitted using the stretched exponential function, I t( )I0e( / )t , where β is the stretching exponent (0 < β ≦ 1) and τ is the PL lifetime. The β reflects the relaxation rates involved in the decay process, in which a broader rate distribution causes a smaller β. The gradient of the double natural logarithm versus the natural logarithm yields the variation of β.
Figures 3.3(b) and 3.3(c) plot the measured τ and β against temperature for ZnSe0.950Te0.050. In contrast with τ, β initially decreases to a minimum at 140 K and then monotonically increases with a further increase in temperature. It is worth mention that the turning point of β at 140 K correlates well with that of the PL peak energy shown in Fig.
3.2(b). The phenomenon can be explained by the configuration coordinate diagram shown in the inset of Fig. 3.2(b). At 10 K, holes hop among adjacent transport and trapping sites and then are tightly bound to the most numerous trap states. However, electrons are more weakly
FIG. 3.3 Temperature-dependent (a) TRPL spectra, (b) PL lifetime, (c) stretching exponent, and (d) integrated PL intensity of ZnSe0.950Te0.050. Inset in (a) shows the TRPL spectra on a double logarithmic scale.
bound by a Coulomb force, giving rise to the excitons and subsequently recombine. As the temperature increases, the holes gain additional energy to hop toward deeper traps (path I), causing a redshift of the PL emission energy and a reduction of β. Around 140 K the decay dynamics becomes complicated, part of the holes are thermally activated toward higher energy states. Consequently, PL linewidth broadens at the high energy shoulder, and the values of both PL peak energy and β reach their local dip. Above 140 K most of the holes gain sufficient energy to repopulate back to the high energy states (path II) and recombine, leading to the blueshift in the PL peaks and an increase in β. Increasing the temperature further above 260 K, a sharp energy blueshift is observed (path III). It is because holes suddenly mobilize up to higher Te traps with dissimilar energy from that in path II.
An initial increase in τ shown in Fig. 3.3(b) can be readily understood as follows. In ZnSe0.950Te0.050, holes hop among proximal Te traps and remain much tighter bound than electrons. As the temperature rises, electrons tend to achieve an equilibrium distribution between the positively charged Te traps and the conduction band. Accordingly, the weakly bound electrons are ionized and away from the strongly localized holes for an increasing fraction of their lifetimes, which in turn will lengthen the radiative PL lifetime. We note that such an explanation is valid provided that the nonradiative processes are negligible, which is confirmed by the almost constant PL intensity in the same temperature region [Fig. 3.3(d)].
The temperature dependence of τ is fitted using the following equation:
0/ [1 Cexp( e h/kT)]
, (1)
where τ0 is the decay time at T = 0 K, C is a constant, εe-h is a characteristic energy that is of the order of the electron-hole (e-h) binding energy, and k is the Boltzmann constant. In Fig.
3(b) the solid line is the fitting result within the temperature range where the PL intensity remains relatively constant, i.e., where nonradiative processes are negligible, and it yields εe-h
~ 9 meV. Such a low e-h binding energy in the temperature range correlates closely with the increasing τ due to thermal ionization of electrons.
As can be seen in Figs. 3.4(a)-4(e), similar experimental results are also found in ZnSe0.900Te0.100. Clearly, a higher Te content (y = 10 %) in ZnSe1-yTey is associated with a higher rate at which the τ increases with temperature. Fig. 3.4(a) displays the temperature-dependent TRPL spectra. In Fig. 3.4(b), a fit of the experimental data to Eq. (1) yields εe-h ~ 5 meV which is lower than that of ZnSe0.950Te0.050. This is because increasing the number of Te traps, providing alternate decay routes for holes [4], causing an increased PL peak redshift at high temperatures, and decreasing the e-h binding energy due to broader distribution of localized holes. In Fig. 3.4(c), the turning point of β at 160 K agrees excellently with that of the V-shaped PL peak energy shown in Fig. 3.4(d), which again implies complex and distinct carrier dynamics at both sides around 160 K. Figures 3.5(a) and 3.5(b) show the TRPL images of ZnSe0.900Te0.100 at 10 and 180 K, respectively. At 10 K, the emission peak dramatically shifts toward the lower energy as time elapses. However, the
FIG. 3.4 Temperature-dependent (a) TRPL spectra, (b) PL lifetime, (c) stretching exponent, (d) PL peak energy, and (e) integrated PL intensity of ZnSe0.900Te0.100. Inset in (a) shows the
TRPL spectra on a double logarithmic scale.
Fig 3.5 TRPL images of ZnSe0.900Te0.100 monitored at (a) 10 K and (b) 180 K.
emission redshift gradually disappears and starts shifting toward higher energies at 180 K. ZnSe0.947O0.053 decrease monotonically. Similar results were also found in InGaAs:N [10] and GaAs:Bi [11] HMAs. However, τ of ZnSe0.950Te0.050 initially increases with an almost constant PL intensity and then declines. Interestingly, this type of behavior was also observed for GaN:As [12] and ZnTe:O [13] HMAs. These remarkable inconsistencies can be attributed to their distinctly different localization strengths (~ 20 meV for ZnSe0.947O0.053 and ~ 200 meV for ZnSe0.950Te0.050). Based on the BAC model, the variations are associated with different depths of the isovalent O(Te) traps relative to the energy min.(max.) of the new E-(E+) subband. We point out that there is a clear systematic in the behavior of τ against temperature for the series ZnSe:O, InGaAs:N, GaAs:Bi, ZnSe:Te, GaN:As, and ZnTe:O. Considering the defect level positions as determined by the BAC model, we obtain for the previous series 0.22 and 0.30 eV above CBE, 0.40 eV below VBE, 0.10 and 0.62 eV above VBE, and 0.24 eV below CBE, respectively [1,3,5,6,14,15]. It might indicate for defect levels outside the band gap, the anticrossing interaction results in the formation of a relatively wide lower (higher) E
-(E+) subband, as a result, τ decreases monotonically with temperature owing to shallower localization depth. Otherwise, for defect levels inside the band gap, a deeper trapping depth results in an initially increased τ.