Activation Energy

The most significant size-dependent property of NRs can be found in the temperature-dependent quenching of PL intensity. Fig. 3-16 shows the temperature dependence of the normalized PL intensity (Arrhenius plot) for sample A D, and the InN film. For the InN film, the PL drops by at least 2 orders of magnitude from T=5K to room temperature. In contrast, the low temperature/room temperature PL is significantly smaller than NRs. In contrast, the low temperature/room temperature PL is significantly smaller than NRs. PL quenching due to the activation of nonradiative recombination process can be described by (65), PL quenching of the InN epilayers is governed by the thermal activated process with = 4.5 meV and = 23 meV , constants with the previously report result^[35].

The thermal quenching process for NRs is different from that for the film. The most interesting feature of the PL quenching in NRs is that the onset temperature of the PL quenching locates at much higher temperature than that of the epilayer.

Especially for sample C and D, the PL signal keeps the high PL intensity up to 100 K and 270 K and dramatically drops at higher temperature. However, we still using one peak gauss function to fit. The corresponding a activation energies for sample A ( = 3 meV and = 25.3 meV) and B ( = 5.1 meV and = 30 meV) are much higher than those of sample C ( = 10 meV and = 52 meV) and D ( = 11.9 meV and = 62 meV). Larger activation energy implies large impurity binding energies or carrier/exciton localization energies.^[35] The activation energy is related to delocalization of carrier/excitons and to the quenching from the extended state.

Meanwhile, the enhancement of the activation energies with the shrinkage of NRs size may be associated with the carrier redistribution on electronic states within the narrow NRs.^[19] Due to Fermi-level pinning at the surface, InN has the downward

72

band bending near surface and electrons tend to move to the surface, while holes are left in the inner core of NRs. The surface electron accumulation layer of InN has typical extension of from the surface. For NRs with , recombination of carriers would occur within the narrow core area and the PL intensity from NRs depends on the effective volume of this core area. Further shrinkage of rod diameter up to leads to the complete depletion of charge within the rod and holes in the inner core of NRs can recombine with electrons in the surface accumulation only when carriers surpass the band bending energy band –tail states for nonequilibrium carriers. Thermally activated electrons in NRs can easily fill up the low-density band-tail states and the remaining carriers. Then for sample C and D, the major carrier recombination can occur when carriers surpass the energy barrier of surface layer, and it can explain much large of sample C and D.

Remarkably, the activation energy estimated for sample C and estimated for sample D are related to the band bending energy of 600 meV known for InN.^[44] Meanwhile, as the temperature increases, recombination of non-equilibrium carriers is reduced or is maybe even impossible if non-radiative recombination via surface traps in the prevailing recombination mechanism. The delayed fast quenching observed for sample C and D in Figure 3-13 clearly shows the dominant carrier recombination by surface defects at high temperature.

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Figure 3-16. Intensity variation of the PL intensity as a function of reciprocal temperature for the InN epilayer and the nanorods samples. The activation energies obtained by Eq. (65) are listed in Table 3-2. Schematics in the figure illustrate the downward surface band bending in sample B and C in an exaggerated scale.

Table 3-2. Physical and optical parameter of InN film and nonorods

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3.6.5 FWHM

There is the plot of the temperature dependence of the FWHMs as shown in Figure 3-17. The PL spectra of NRs with large diameters (samples A and B) and epilayer get broader linearly with the increase of temperature.However, the broadening of PL band is much slower for NRs with the critical diameter (samples C and D). The PL In NRs, holes tend to populate in different sample regions at different T, leading to the broadening of the PL band.^[27] Since carrier transportation is limited within the narrow core bulk region of NRs in samples C and D, the PL band broadening may be insignificant at low temperature. The thermalization of carriers at higher temperature may cause the slow broadening of the PL band through the interaction with impurity or defects in the surface layer.

Figure 3-17. Temperature-dependent PL band widths of the InN epilayer and the nanorod arrays. The temperature dependent broadening is not significant for sample C and D.

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Chapter 4 Conclusion

In summary, we have introduced the basic concept of sum frequency generation and how it can be applied for measuring time-resolved luminescence spectra with the time resolution near the ultrafast laser pulse. The experimental setup, photon counting technique and data acquisition by software have been demonstrated in detail. The system test result compared with the theoretical value to estimate the possible TRPL signal level so that the parameter of our system of SFG of 326nm could get theoretically. In the change of laser light of 400nm to the PL of InGaN of 550nm, the interference in the optical path makes the overlapping spot light unstable. Because of the PL of 550 is weak, it need a more sensitive CCD camera to ensure that the gate light of 800nm and the PL light of 550nm were still overlapping. To reduce the laser background, it needs to select a PMT power supply with low ripple. However, the system construction process is very close to the goal of getting the time-resolved PL data of InGaN.

In the study of second part, we have measured the temperature-dependent near-infrared PL from nanorods. As the nanorod size reduces to a critical value (the thickness of surface electron accumulation layer), the nonradiative carrier recombination needs a higher activation energy. For InN nanorods with a critical diameter, the measured activation energy consists with the band bending energy near the surface. We also found that the PL band broadening is according to holes move in the central core region of the Rds depends on temperature.

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