Fig. 2(a) demonstrates the PL spectrum of InAs/GaAs QDs in the as-grown sample recorded at the temperatures of 16 and 300 K, respectively, under 14 mW of excitation power. As shown in the figure, the full width at half maximum (FWHM) of these two PL spectra were 133.35 meV (16 K) and 196.18 meV (300 K), respectively. As usual, the PL intensity significantly decreased with the increase of temperature from 16 to 300 K, accompanied with the peak energy shifting from 1.155 eV at 16 K to 1.068 eV at 300 K. Besides, the PL spectrum of the as-grown InAs QDs at both 16 and 300 K was apparently asymmetric, with an obvious shoulder at the high-energy side of the PL peak.
Fig. 2(b) shows a Gaussian fitting diagram of QDs PL spectrum at 16 K. As illustrated by the dash-dot lines in Fig. 2(b), the asymmetric PL spectrum of QDs at 16 K can be well fitted using two Gaussian type functions with peaks at 1.153 and 1.197 eV respectively. In particular, the spacing between both Gaussian peaks was estimated to be around 44 meV, which was significantly smaller than the typical value (60–80 meV) of the energy separation between the GS and first excited state of an InAs QD in a population of InAs QDs. Therefore, the PL spectrum of the as-grown sample could be appropriately described by the GS emission of two QDs groups with two different mean QDs sizes.
Figs. 3(a) and 3(b) present the normalized PL spectrum of as-grown and RTA-treated QDs samples with TRTA = 750, 800, 850 and 900 °C respectively at the temperatures of 16 and 300 K. As shown in both figures, all annealed QDs samples exhibited a distinct blue-shift in the PL spectrum (measured at the temperatures of 16 and 300 K) compared with as-grown QDs. Fig. 3(a) shows that in the PL spectrum at T = 16 K the main peak position of the QDs annealed at 750 °C was observed to be at 1.269 eV, which was blue shifted by 113 meV compared with that of the as-grown QDs (1.156 eV). However, the PL peak in the QDs annealed at T = 800 and 850 °C were blue shifted by 108.37 and 97.23 meV, respectively, which was slightly less than the RTA-induced blue-shift in the PL spectrum of the QDs annealed at 750 °C. Meanwhile, it can be observed that the sample annealed at 900 °C presented the largest blue-shift (169 meV). This behavior was completely different from other reports in the literature, to our knowledge, that the RTA-induced blue-shift increased significantly with the increase of annealing temperature[16, 19]. The strain-reducing effect could be enhanced since an appropriate annealing treatment could improve the crystal performance of InGaAs SRL. It was speculated that the above experimental phenomenon might be caused by the inter-diffusion of In and Ga atoms between InAs QDs and surrounding materials during the RTA process or the reduction of strain between QDs and GaAs barrier layers after RTA. The inter-diffusion of In and Ga atoms from InAs QDs to SRL dominated for as-grown QDs annealed at 750 °C and was also possible for samples annealed at higher annealing temperatures (800 and 850 °C), while the inter-diffusion rate might be relatively lower than that of the interface between QDs and GaAs barrier[20, 21]. Therefore, the strain-reducing effect of samples annealed at 800 and 850 °C might be relatively dominant, which resulted in the obvious red-shift of PL spectrum peak compared with the samples annealed at 750 °C. In addition, the blue-shift of as-grown QDs annealed above 850 °C could be attributed to the inter-diffusion of In and Ga atoms among InAs QDs, In0.15Ga0.85As SRL and GaAs barrier layer. However, RTA was relatively complicated due to the inhomogeneous size distribution of as-grown QDs, which could not be simply ascribed to one of the above mechanisms.
Furthermore, the PL peak energy of different-sized QDs was shown as a function of the RTA temperature in the insets of Figs. 3(a) and 3(b), and the similar variation trends were observed with the increase of annealing temperature.
In addition, the peak shape of the PL spectrum of the QDs sample subjected to RTA presented a slight modification. At 16 K, the asymmetry of the PL spectrum peak shape of samples annealed below 850 °C decreased with the increase of annealing temperature. The peak shape of the PL spectrum became sharper with annealing temperature increased to 900 °C. What is more, it was found that the PL spectrum of QDs annealed at 850 °C produced a new spectral peak at 1.354 eV compared with that of as-grown QDs. Moreover, it can be seen that the PL spectrum was significantly broadened when the temperature of PL measurements rose to 300 K. Therefore, the PL spectra in Fig. 3 were fitted by Gaussian fitting, and the curves of PL spectral width and integrated PL intensity versus annealing temperature were plotted as shown in Fig. 4.
Fig. 4(a) showed the PL spectral width of the RTA-treated QDs with the change of annealing temperature, measured at both T = 16 and 300 K. At 16 K, the PL spectral width of the QDs sample annealed at up to 750 °C was greater than that of the as-grown sample, which implied that RTA did not always induce the narrowing of PL spectral peaks. The PL spectral width of the samples annealed at 800 °C saw a sharp decline, which indicated that RTA treated InAs/GaAs QDs at an annealing temperature of 800 °C had relatively excellent crystal quality. With the further increase of annealing temperature, the PL spectral width increased, then decreased and obtained the minimum value at 900 °C. At 300 K, it can be observed that the PL spectral width changed from 216.4 meV to 240.86 meV with the increase of annealing temperature from 750 to 800 °C. Compared with as-grown QDs, QDs saw an increase of 44.68 meV in PL spectral width after being annealed at 800 °C. This result indicated that the electron–phonon scattering effect and thermal distribution had a significant impact on annealed samples with different QDs densities, leading to the significant broadening of PL spectrum[19, 22, 23]. Besides, it can be observed that further increasing annealing temperature gave rise to the shrinkage of the PL spectrum. When the RTA temperature rose to 900 °C, PL spectral width would be reduced to 128.09 meV. This could be explained by the fact that the inter-diffusion between the In-Ga atoms, which greatly improved the size distribution and composition fluctuation of QDs. On the other hand, it might also be caused by the QDs gradually transformed into a quantum well (QW) structure, because the partial QDs dissolved in the surrounding WL at the higher annealing temperature.
Fig. 4(b) shows the integrated PL intensity of PL peak emission with the change of annealing temperature. The integrated PL intensity of RTA-treated samples at an annealing temperature of 750 °C and above saw a slight drop at 300 K, which was caused by the weakening of carrier confinement ability and the thermal excitation of more photo-generated carriers in QDs into the InAs WL or GaAs barrier layer. At low temperature (16 K), however, the integrated PL intensity was enhanced as samples were annealed at 850 °C, which might result from the reduction of non-radiative recombination and the contribution of lower thermal energy to increasing the integrated PL intensity of annealed QDs. The integrated PL intensity of QDs annealed at 900 °C was greatly attenuated owing to the partial dissolution of QDs and the degradation of the crystal quality of samples at high temperatures.
As mentioned above, the PL spectrum of the QDs subjected to RTA treatment showed a significant inhomogeneous broadening at 300 K, especially for the sample RTA-treated at 800 °C. And that, there are new peaks on the PL high energy side of all samples as shown in Figs. 3(b). To identify the source of the PL spectrum and explain its broadening, a power-dependent PL experiment was performed on all annealed samples. The upper half of Figs. 5(a)–5(d) demonstrates the PL spectrum of four annealed samples at 300 K as a function of excitation power from 4 to 135 mW. For data analysis, the PL spectrum of all annealed samples under the excitation power of 40 mW was extracted and Gaussian fitting was performed for data analysis. As shown in the lower half of Figs. 5(a)–5(d), the PL spectrum of four annealed samples was composed of four spectral peaks, namely P1, P2, P3 and P4.
Figs. 5(a) and 5(b) show that the peak intensities of the four spectral peaks P1, P2, P3 and P4 of RTA treated samples at an annealing temperature of 750 and 800 °C almost all decreased at a constant ratio. All spectral peaks existed even under the lowest excitation power, suggesting that these peaks were not derived from the luminescence of the excited state of QDs. In the PL spectrum, the first two peaks (P1 and P2) in the low-energy region showed the size distribution of QDs corresponding to the GS luminescence of QDs with different sizes. The increase of test temperature to 300 K resulted in the thermal excitation of some carriers in QDs into the WL where radiative or non-radiative recombination occurred. Therefore, the third peak (P3) in the high-energy region was the luminescence peak of the QDs WL, which had a relatively small FWHM. Besides, it was suggested the fourth peak (P4) at the high-energy side of the spectrum might be generated from the In0.15Ga0.85As SRL. Thus, it can be seen that the broadening of the PL spectrum of QDs annealed at 750 and 800 °C was associated with the GS emission of different-sized QDs and the emission of the WL and SRL.
Figs. 5(c) and 5(d) are the PL spectrum and Gaussian fitting diagrams of InAs/GaAs QDs annealed at 850 and 900 °C, respectively. Based on Gaussian fitting, four peaks can be clearly defined on the high-energy side of PL spectrum, while the low energy side (1.165 eV) of the PL spectrum is related to the laser. For this kind of QD sample annealed at a higher temperature (850 or 900 °C), the lateral sizes of the QDs increased with increasing RTA temperature and the partial QDs dissolved into the surrounding WL, resulting in a modiﬁed WL and a gradual conversion of InAs QDs to an InGaAs QW structure. From Figs. 5(c) and 5(d), it could be found that the GS PL peak tended to be saturated and the PL intensity of the third peak (P3) was considerably enhanced with the increasing of the excitation power. In addition, the intensity ratio of QDs GS to P3 decreased slightly with further increase of excitation power, which indicated that P3 might be derived from the luminescence of QW formed by modiﬁed InAs WL and In0.15Ga0.85As SRL. In the same way, this result further illustrated that the broadening of the PL spectrum was associated with the GS emission of different-sized QDs and the emission of the modiﬁed InAs WL and In0.15Ga0.85As SRL caused by high-temperature RTA.