In the previous section, the optimization of RTA parameters to enhance optical proper-ties and the origin of growth-induced and RTA-induced defects have been discussed.
For accomplishing light emitting devices, it is important to understand the nature of ra-diative recombination, such as PL related to growth-induced defects versus band edge transitions or the recombination via spatially localized and delocalized excitons. Thus, this section is attributed to analyze the nature of radiative recombination in Ga(As,N).
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Figure 3.20: Cw-PL of a Ga(As,N) sample with 0.5% nitrogen (sample 1 in figure 3.13), annealed at different temperatures. The inset shows excitation density-dependent PL spectra of the sample annealed at 800◦C. All PL measurements were carried out at 10 K.
For this analysis, a single Ga(As,N) sample with 0.5% nitrogen has been investigated by means of continuous-wave photoluminescence (cw-PL),µPL, and TR-PL (sample 1 in figure 3.13). The sample has been cleaved into several pieces for subsequent anneal-ing processes. The annealanneal-ing temperature was varied in between 650◦C and 950◦C.
Prior to annealing, XRD has been performed to assure a homogeneous nitrogen con-centration of all pieces. Figure 3.20 shows cw-PL spectra of this sample annealed at different temperatures. One can clearly see a blueshift and an increase of the PL inten-sity that can be elucidated with a removal of growth-induced defects (see section 3.3).
For the sake of brevity, these growth-induced defects are henceforth called defects in this section. The inset in figure 3.20 shows PL spectra of the sample annealed at 800◦C with excitation densities ranging over 3 orders of magnitude. A blueshift occurs for higher excitation densities owing to a saturation of these defects-related states. As the position of the PL maxima remains unchanged for the samples annealed at high tem-peratures (900◦C, 950◦C), one can assume that these samples are healed out completely.
However, the PL maxima of these samples are still slightly lower in energy than the
band gap, which lies at 896 nm according to equation 2.1. This is possibly due to the existence of potential fluctuations in Ga(As,N) which will be considered in section 3.5.
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T=300 K
T=230 K
T=200 K
T=150 K T=130 K T=100 K T=70 K
T=50 K T=40 K T=30 K T=20 K T=10 K
Normalized Intensity (arb. units)
Wavelength (nm)
same spot of sample
T=10 K
Figure 3.21: Temperature-dependentµPL of the Ga(As,N) sample annealed at 800◦C. The spatial reso-lution amounts to 1µm in diameter. The inset shows two PL spectra of the same spot.
To get further insight into the nature of radiative recombination, µPL has been em-ployed. InµPL setups, the laser beam is focused onto a very small area, typically 1µm in diameter. Thus, one can investigate the microscopic structure of samples, e.g. distin-guishing between spatially localized and delocalized excitons. As discussed in section 2.1, localized excitons, such as excitons trapped in potential fluctuations or defects, are characterized by ultranarrow spikes with linewidths less than 0.1 nm[100, 101, 102].
Thus, by employingµPL, one can distinguish between band edge transitions (delocal-ized excitons) and radiative recombination via defects or potential fluctuations (local-ized excitons). Figure 3.21 shows temperature-dependentµPL of the sample annealed at 750◦C (cf. figure 3.20). A pronounced double peak feature is seen. At low temper-atures (10 K), ultranarrow spikes are prevalent in both peaks which clearly identify the localized nature of these excitons. The inset in figure 3.21 shows two subsequently
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cw PL signal
Figure 3.22: The spectral dependence of the decay time concerning the Ga(As,N) sample annealed at 800◦C (a). The transient spectra between 0 – 60 ps (solid line) and 1400 – 1600 ps (dotted line) are displayed (a). In addition, the cw-PL intensity is plotted (dashed line). The open circles show the spectral dependence of the decay time. Figure (b) depicts the decay times on the high- and low-energy side with respect to the annealing temperature. The numbers denote the wavelength for which the decay times were ascertained. The open circles stand for the cw-PL intensity (cf. figure 3.20). All measurements were accomplished at 10 K. (TR-PL measurements accomplished by V. Talalaev.)
measured PL spectra of the same spot of the sample. Apparently, these ultranarrow spikes are reproducible, thus arising from the sample. Therefore, we assume these ex-citons to be localized, either trapped in potential fluctuations or by defects. Possibly, the high-energy peak is related to potential fluctuations, whereas the low-energy peak is related to defects. At slightly higher temperatures (20 K – 40 K), there is a third peak emerging at the high energy side that does not contain any ultranarrow spikes. Appar-ently, these excitons are spatially delocalized and result from a thermal activation of excitons out of these potential fluctuations. This peak becomes dominant with higher temperatures and illustrates the transition from localized to delocalized excitons. The blueshift of this peak that continues up to 70 K mirrors the thermal activation process out of these potential fluctuations. For higher temperatures (70 K – 300 K), this peak redshifts due to the normal temperature dependence of the band gap[103]. The low-energy peak also loses its ultranarrow spikes with higher temperatures. This is likely to be caused by an energy band formed by these defects. Thus, these defects get acti-vated by thermal emission, too, leaving defect-related states and becoming freely mov-ing within this band. At ambient temperature, this peak vanishes because of a thermal activation of these excitons out of this defect-related band.
Furthermore, the existence of localized and delocalized excitons can be corroborated by means of TR-PL measurements. As depicted in section 2.1, spatially localized exci-tons are characterized by long decay times, typically in the nanosecond range. On the contrary, delocalized excitons show short decay times in the picosecond range. In ad-dition, defect-related nonradiative recombination channels expedite decay processes.
Figure 3.22 (a) shows the spectral-dependent decay time of the sample annealed at 800◦C (open circles). Apparently, there is a correlation between wavelength and de-cay time with short dede-cay times on the high-energy side and long dede-cay times on the
low-energy side. Again, this can be understood in terms of a transition from local-ized to delocallocal-ized excitons. On the low-energy side, excitons are locallocal-ized in poten-tial fluctuations, thus having long decay times. On the high energy side, excitons are delocalized showing short decay times. Figure 3.22 (b) shows the decay times of all annealed samples. The samples that are annealed at low temperatures show short de-cay times, even though radiative recombination is defect-related and therefore occurs via localized excitons. Nonetheless, there are defect-induced nonradiative recombina-tion channels that accelerate recombinarecombina-tion processes. With higher RTA temperatures, there is a removal of defects, thus reducing the impact of the nonradiative recombi-nation channels. Consequently, the decay time increases, as it is seen in the sample annealed at 800◦C. At RTA temperatures above 800◦C, there is a transition from local-ized to delocallocal-ized excitons which causes a decrease of the decay time. At very high RTA temperatures, RTA-induced defects might serve as nonradiative recombination channels that cause a further reduction of the decay time.
Summary Radiative recombination in Ga(As,N) occurs via spatially localized or de-localized excitons. Localized excitons are either trapped in potential fluctuations or defects. Even in healed out samples, excitons are still localized in potential fluctua-tions. An increase of temperature and/or excitation density leads to a transition from localized to delocalized excitons.