Temperature-dependent Photoluminescence…

In order to further understand whether the localization effect plays an important role in nonpolar InGaN MQWs as well as in polar c-plane InGaN nanostructure or not, PL spectra were measured under different temperature in the range of 20K to 300K using the CW 325 nm He–Cd laser.

Fig. 4.3.1 shows the evolution of PL spectra as a function of temperature for the a-plane InGaN/GaN MQWs with well width from 3nm to 12 nm. In these four samples, the decrease of PL intensity with increasing temperature is observed independent of quantum well width. Thermal quenching PL intensity with increasing temperature is a general phenomenon in III-V semiconductor nanostructure which is caused by carriers thermalization from the radiative recombination centers or/and localized states to the nonradiative recombination centers or/and delocalized states.

[28] This thermal quenching behavior will be discussed later.

Moreover, at low temperature, there reveals three separated peaks in the PL spectrum, the most high energy peak located around 3.35eV in all samples is suggested to the signal of bulk GaN. The middle energy peak which is obvious in the samples of 3nm and 6nm well width but is merged with the lowest-energy side signal in the samples of 9nm and 12nm well width is supposed to the signal coming

from shallow localized states. [29] The lowest energy signal comes from the deep localized states. As can be seen, only the PL emission from excitons in deep localized states dominates the luminescence from 20K to room temperature, the other two higher energy emissions suffer an apparent quickly thermal quenching when the temperature increases. Along with the increase in temperature, the nonradiative energy relaxation of excitons occurs at shallow localized states, and then the efficient radiative recombination of excitons occurs mainly at deep localized states. The exciton dynamics at shallow delocalized states is very sensitive to the lattice temperature. When the temperature goes up, it is hard for excitons to stay stable in shallow localized states, the thermal dissociation of excitons occurs at shallow localized states, and electrons and holes are thermally excited into the delocalized states, thus, more transfer and relaxation processes happen which quench the emission from high energy states. [29]

Fig. 4.3.2 shows the Arrhenius plot of the normalized integrated PL intensity for the a-plane InGaN MQWs emission from deep localized states over the temperature range under investigation. The intensity reduction is remarkable, further compare these four figures, the degree of PL intensity reduction from 20K to 300K is more severe when the well width gets thicker which indicates a higher PL efficiency in the thinner well width samples. [30] We use the equation below to get a good fit to our

experimental data of the activation energy in thermally activated processes [31]:

I(T) = I0/[1+a*exp(-Ea/KBT)+b*exp(-Eb/KBT)]

where I(T) is the temperature dependent integrated PL intensity, I0 is the integrated PL intensity at 20K, KB is the Boltzmann’s constant, a and b are the rate constants, and Ea and Eb are activation energy for two different non-radiative channels. This suggests that there are two non-radiative paths exist at the same time which one of them dominates in the low temperature region and the other dominate in the high temperature region. [32] The fitting result is listed in Table 4.3.1.

Since the activation energy we get from the four samples is much less than the bandgap energy difference between the well and the barrier, it is impossible to account the thermal quenching of InGaN MQWs emission for carriers thermalized from the InGaN wells to the GaN barriers. Therefore, the more reasonable explanation for the quenching of luminescence is that increasing temperature increases the probability of excitons to be trapped by nonradiative recombination centers within the well at the same time when carriers are activated out of the loacalization minima of potential fluctuation which may be caused by compositional、interface fluctuation and indium phase separation that always happened in InGaN quantum well layers [30].

In order to check the situation of alloy and interface fluctuations in these four

samples, we further analyze the peak shift of InGaN MQW emission over the investigation temperature range. The result is shown in figure 4.3.3. In the samples with 3nm and 6nm well width, the emission energy decreases monotonically with increasing temperature. However, in the samples with 9 nm and 12 nm well width, the emission energy decreases at temperatures below 70K with uprising trend, then increases with increasing temperature from 70K to around 140K and finally decreases with further increase of temperature up to the room temperature. This red-blue-red shift of peak energy with increasing temperature is a characteristic of the exciton localization effect [30, 33]. From 20K to 70 K, it is considered that the observed dynamical redshift of the PL spectrum is caused by radiative excitons migrating into lower localized states. At elevated temperature from 70K to 140K, nonradiative recombination processes become more pronounced, some carriers recombination before reaching deeper band-tail states, resulting in a blueshift in the PL peak position.

At higher temperature up to 300K, another redshift occurs mainly due to the temperature-dependent dilation of the lattice and electron–lattice interaction. [34, 35]

Inhomogeneity due to interface fluctuation, InN/GaN segregation, and the band tail states originating from the high density of defects are responsible for the so-called S-shaped temperature dependent behavior in the samples with thicker well width.

[31, 36]

Although the emission peak energy of samples with 3 and 6 nm well width do not show temperature induced S-curve, it is reasonable to infer that there may be still an exciton localization effect exists to a little extent for the two thinner samples. The total redshifts of the four samples over the temperature range under investigation are:

3 nm with 100 meV、6 nm with 75 meV、9 nm with 55 meV and 12 nm with 62 meV which indicates that the exciton localization effect may be much stronger in thicker well width. There is one point needed to be noted as compared with the well known shift of the band edge for nitrides of about 65meV without the exciton localization effect in polar structure, the total redshift of the emission peak energy in these four samples from 20K to 300K under the existing of localization effect showed rather big redshift variation with temperature. We couldn't give a clear explanation for such result now, a full understanding of the carrier dynamics in these nonpolar samples will probably emerge only after considerable further study. We can give two key points in this section : the first one is that in a-plane InGaN/GaN MQWs, samples with thicker well width of 9nm and 12nm reveals a more apparent localization effect which is most likely due to alloy、interface roughness fluctuations or worse crystalline quality in the MQWs. The second is that a deeper localization depth may exist in thinner 3nm and 6nm well width samples which could confine carrier tightly thus display an indistinctively visible S-shaped temperature dependent behavior.

Fig. 4.3.1 PL spectra as a function of temperature from 20K to 300K for a-plane InGaN/GaN MQWs with different well width.

360 380 400 420 440 460 480 500 520 540 Wavelength (nm)

360 380 400 420 440 460 480 500 520 540

PL Intensity (a.u.)

360 380 400 420 440 460 480 500 520 540

PL Intensity (a.u.) 360 380 400 420 440 460 480 500 520 540

20K

Fig. 4.3.2 Normalized integrated PL intensity as a function of 1/T for the a-plane InGaN/GaN MQWs with different well width.

0.00 0.01 0.02 0.03 0.04 0.05

3nm 6nm 9nm 12nm

E1(meV) 46.5 9.7 22.4 33.4

E2(meV) 166.5 86.7 106.4 144.9

Table 4.3.1 The fitting result of activation energy for the a-plane InGaN/GaN MQWs with different well width.

Fig. 4.3.3 PL peak energy position as a function of temperature for a-plane InGaN/GaN MQWs with different well width.

0 50 100 150 200 250 300