Figure 2.3-1 shows the intensity-dependent PL spectra of the AlGaInAs MQWs from 23 to 384 kW/cm2 at 253 K. At the lowest excitation intensity of 23 kW/cm2 only the emission line of fundamental n=1 transition exists. The peak wavelength of this emission line is located at around 1500 nm and with linewidth of 58 nm. However, above the threshold excitation intensity of 80 kW/cm2, a new and relatively sharp luminescence spectral feature appears at the low energy side of the n=1 transition with the full width at half maximum (FWHM) around 5.6-8.4 nm. This emanation mode is attributed to the many-body state luminescence of the renormalized band-edge induced by the high density EHP under high intensity photo-excitation. With increasing excitation intensity, the many-body state emission line grows superlinearly and, in contrast, the n=1 transition varies slowly with linear tendency. The integrated PL intensities of many-body state emission line as the functions of excitation intensity are shown in Fig. 2.3-2 at various temperatures of 253, 193 and 123 K. In this plot the integrated intensities of three curves are all increased rapidly at low excitation intensity and become saturated at relatively high excitation intensity. This means that when the excitation intensity is high enough the many-body states will be all filled and the luminescence intensity will be held unchanged with increasing pump intensity.
In addition, this plot also indicates that the saturated luminescence intensity could be raised via the reduction of temperature which will be mentioned in the following.
Fig. 2.3-1 Photo-luminescence spectra of the AlGaInAs MQWs chip with variance excitation intensity from 23 to 384 kW/cm2 at 253 K.
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Fig. 2.3-2 Integrated PL intensity of the many-body state emission line as the function of excitation intensity at 253, 193 and 123 K.
To further investigate the temperature dependence of the many-body state luminescence, Fig. 2.3-3 shows the PL spectra of the AlGaInAs MQWs with the setting temperatures of 123, 193, 253 and 313 K at a fixed excitation intensity of 407 kW/cm2. The peak of fundamental n=1 transition line is red-shifted from 1422 to 1510 nm due to the thermally induced band-gap shrinkage. When the temperature is decreased, the bandwidths of the n=1 transition line are narrowed significantly as the peak intensities hold unchanged. Because the excitation intensity of 407 kW/cm2 is far from the threshold at temperature range up to 253 K which has been shown in Fig.
2.2-2, the emission line of many-body state could be observed clearly at 123, 193 and 253 K. The tendency of decreasing luminescence intensity with arising temperature is also corresponding to the results demonstrated in Fig. 2.3-2. Besides, the many-body state emission line still exists even at the temperature which is scaled up to 313 K as shown in Fig. 2.3-3. To our knowledge, this is the first demonstration of high density EHP many-body state luminescence of AlGaInAs MQWs under room temperature operation. The peak wavelength of many-body state emission line follows the reduction of band-edge of the fundamental ground state in MQWs and shifts to the low energy side as the temperature arising. Figure 2.3-4 depicts the variance of many-body state peak wavelength as the function of temperature from 93 to 313 K.
The peak wavelength shift rate of 0.59 nm/K with diamond heat spreader is nearly the same with respect to the framework without diamond heat spreader. Therefore, the peak wavelength offset of 10.7 nm between these two schemes at the identical setting temperatures indicates that the sample temperature in use of the diamond heat spreader was 18 K lower than the case without diamond heat spreader. This suggests that the use of diamond heat spreader is not to promote the heat dissipation but to lessen the difference between the sample temperature and setting
Fig. 2.3-3 Photo-luminescence spectra of the AlGaInAs MQWs chip with the excitation intensity of 407 kW/cm2 measured at different temperatures from 123 to 313 K.
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Fig. 2.3-4 Peak wavelength shift of the many-body state luminescence versus temperature at the excitation intensity of 407 kW/cm2.
temperature. Besides, the peak wavelength shift rate of 0.59 nm/K also elucidates that this specific emission line is not resulted from the particular reflection band depicted in Fig. 2.2-3 which has thermal induced shift rates about 0.1 nm/K.
In Fig. 2.3-2 we have revealed that the luminescence intensity of the many-body state is strongly related to the temperature. To obtain more information about this topic, the integrated PL intensity of many-body state emission line is depicted as a function of temperature from 93 to 313 K as in Fig. 2.3-5. It could be seen that the integrated PL intensity of the many-body state emission line exhibits three variant trends in the different portion of temperature regimes. At low temperature, the integrated PL intensity held unchanged up to 173 K. As the temperature operated above 253 K, a strong luminescence quenching was appeared with exponentially decay of the integrated intensity. In addition, the transition stage of the integrated intensity dropped slowly with increasing temperature between 173 to 253 K. This PL quenching phenomenon could be attributed to the presence of two nonradiative recombination mechanisms which are characterized by two different activation energies in variant temperature regions. To determine the activation energies a simple model using the three-level Boltzmann distribution is developed by Bimberg et al. [20]
as follows,
1 0[1 1exp(- 1/ B ) 2exp(- 2 / B )]
I I C E k T C E k T , (1)
where I is the integrated PL intensity at particular temperature, I0 is the saturated integrated PL intensity at low temperature limit, kB is the Boltzmann constant, T is the sample temperature, E1 and E2 are the specific activation energies for different nonradiative recombination centers, and C1 and C2 are the corresponding constants in connection with the density of states of E1 and E2. The best fit to the integrated
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Fig. 2.3-5 Plots of the integrated PL intensity of the many-body state luminescence versus temperature at the excitation intensity of 407 kW/cm2.
intensity of the many-body state emission line is shown in Fig. 2.3-5 as the solid curve. By fitting the experimental data the value of activation energies E1 and E2 are yielded to be 0.19 and 0.52 eV, corresponding to the integrated PL intensity drop at intermediate and high temperature region, respectively. Similar behaviors are reported with exciton state or electron-hole pair luminescence in various MQWs samples [21,22]. Because E2 is much larger than the possible inter-particle interaction energies, the luminescence quenching above 253 K is inferred to be resulted from the strengthened nonradiative recombination processes due to thermally induced carrier leakage from the renormalized band to barrier region [23]. The activation energy of the thermal emission of excited carriers under high excitation intensity has been reported to be equal to half of the confined energy of the electron-hole pairs theoretically and experimentally for the single quantum well [23,24]. However, one half of the energy gap between the barrier layer and renormalized band in our experiment was found to be 0.33 eV which is two-thirds of the fitted activation energy.
This result bears that the periodically aligned gain structure is contributive to the existence of many-body state luminescence under high temperature. Although the fitted value of C1 is six-order smaller than C2, the insertion of E1 is necessary in matching the fit curve to experimental data at the intermediate temperature regime from 173 to 253 K. The origin of activation energy E1 is still not clear at this time but we believe that it may be connected to the dissociation of the many-body state interaction. At the end, the integrated luminescence intensity of the n=1 transition line stayed nearly invariant in comparison to the many-body state luminescence in the entire temperature range. This suggests that the carriers tend to be trapped by the renormalized band due to the reabsorption and reemission processes and the relatively low radiative recombination lifetime.
In Fig. 2.3-2 and 2.3-5 we have examined that the saturation luminescence intensity is a strong function of temperature. However, the threshold excitation intensity is also depending on the temperature as observed in Fig. 2.3-2. To further understand this phenomenon, the threshold excitation intensity versus temperature from 123 to 293 K is shown in Fig. 2.3-6 quantitatively. The solid curve is the exponential fit of the experimental results. The excitation threshold intensity grew exponentially with the increasing temperature. It is because the higher excitation intensity produces more heat which will result in the more serious carrier leakage.
However, the enhanced luminescence by using the periodically-aligned gain structure compensates the thermally induced emanation gain reduction as the temperature raised. Consequently, the observation of the spontaneous EHP renormalized state emission is obtained even at the sample temperature as high as 313 K. The relatively low excitation threshold intensity of 198 kW/cm2 is achieved under room temperature of 293 K.