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Strong localization effect and carrier relaxation dynamics in self-assembled InGaN quantum dots emitting in the green.

Weng GE, Zhao WR, Chen SQ, Akiyama H, Li ZC, Liu JP, Zhang BP - Nanoscale Res Lett (2015)

Bottom Line: Strong localization effect in self-assembled InGaN quantum dots (QDs) grown by metalorganic chemical vapor deposition has been evidenced by temperature-dependent photoluminescence (PL) at different excitation power.The integrated emission intensity increases gradually in the range from 30 to 160 K and then decreases with a further increase in temperature at high excitation intensity, while this phenomenon disappeared at low excitation intensity.Using this model, the evolution of excitation-power-dependent emission intensity, shift of peak energy, and linewidth variation with elevating temperature is well explained.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Semiconductor Photonics Research Center, Xiamen University, 422 South Siming Road, Xiamen, 361005 P. R. China.

ABSTRACT
Strong localization effect in self-assembled InGaN quantum dots (QDs) grown by metalorganic chemical vapor deposition has been evidenced by temperature-dependent photoluminescence (PL) at different excitation power. The integrated emission intensity increases gradually in the range from 30 to 160 K and then decreases with a further increase in temperature at high excitation intensity, while this phenomenon disappeared at low excitation intensity. Under high excitation, about 40% emission enhancement at 160 K compared to that at low temperature, as well as a higher internal quantum efficiency (IQE) of 41.1%, was observed. A strong localization model is proposed to describe the possible processes of carrier transport, relaxation, and recombination. Using this model, the evolution of excitation-power-dependent emission intensity, shift of peak energy, and linewidth variation with elevating temperature is well explained. Finally, two-component decays of time-resolved PL (TRPL) with various excitation intensities are observed and analyzed with the biexponential model, which enables us to further understand the carrier relaxation dynamics in the InGaN QDs.

No MeSH data available.


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Schematic diagram of potential distribution of the non-isolated QDs describing possible processes of carrier transport, relaxation and recombination.
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Fig4: Schematic diagram of potential distribution of the non-isolated QDs describing possible processes of carrier transport, relaxation and recombination.

Mentions: To explain the observed different variation under different excitation power in 1) the S-shaped temperature-dependent emission energy, 2) the evolution of linewidth as well as 3) the variation of emission intensity, a strong localization model describing possible processes of carrier transport, relaxation, and recombination is proposed, as shown in Figure 4. In comparison with the localization model in blue InGaN QWs, the localized luminescence centers in green InGaN QDs are much more deeper for the higher indium content and have stronger quantum confinement effect (QCE). Using this model, the distinct phenomena observed in our experiments can be well explained as follows: At low temperature of 5 K, carriers are randomly distributed among both deep and shallow potential minima caused by potential fluctuations and the radiative recombination process is dominant. For high excitation intensity (P = 18.5 mW), the carrier concentration is much higher and, as a result, a majority of photo-generated carriers are trapped in deep localization states, resulting in a smaller Ep [Figure 5b]. As the temperature increases from 5 to 60 K (100 K), shallow localized carriers are thermally activated and relax down into other deep localization via hopping, leading to the initial redshift of Ep as large as 16 meV. This is also true for low excitation case, but the redshift is much larger, 48 meV. The relatively smaller decrement in Ep for the high excitation is ascribed to the more markedly band filling effect in the deep localization centers. In case of low excitation where the carrier density is much lower, on the other hand, most of the carriers can relax into the lowest energy level of the deep localization center with rising in temperature up to 100 K, and thus, the carrier distribution narrows [Figure 5c], accompanied by a remarkable redshift of Ep and a slightly decrease of the PL linewidth [Figure 2]. When further increase of the temperature above 60 K (100 K), band-filling in the deep localization is dramatically enhanced due to the temperature-dependent intra-dot relaxation behavior of the carriers. In addition, the regular thermalization of the carriers becomes more and more significant. These temperature-dependent thermal broadening effects of the carriers lead to a continuous increase of the spectra linewidths for both excitation intensities, as shown in Figure 2b. The anomalous enhanced emission over the temperature range of 30 to 160 K at high excitation intensity observed in Figure 3 can be understood as the following. As is well known, with the temperature increases, the defect-related non-radiative recombination would be more and more serious and then results in the quenching of PL. In fact, however, the emission intensity increases monotonously in this temperature range despite of the aggravated non-radiative recombination, which is also observed by Ma et al. [20] and Masumoto and Takagahara [23]. Due to the stronger QCE and better crystal quality in deep localization, it is convincing to consider that the radiation efficiency of carriers in deep localization is higher and thus resulting in a faster increase in emission intensity. With elevating the temperature, the carriers escape from the shallow localization and then converge to fill in the deep localization center by relaxation and recapture processes. Hence, the carriers consumed by radiative recombination in deep localization can be compensated rapidly. As a consequence, a majority of carriers would radiate in the deep localization with a higher radiation efficiency and eventually lead to an anomalous enhanced emission. The maximum emission intensity at 160 K is enhanced by 40% in comparison to that at low temperature, and a high IQE of approximately 41% is obtained at room temperature. For weak excitation, however, the carrier density is much lower. The defect-assisted non-radiative recombination should not be disregarded anymore with rising temperature. During the relaxation and recapture processes, a considerable proportion of carriers would be captured by the non-radiative centers, which suppresses the compensation of carriers that consumed by radiative recombination and results in a monotonous decreasing of the emission intensity. It is rather complicated but reasonable for the emission intensity that decreases slower with a small “uplift” in this particular temperature range, as balanced with the improved radiation efficiency in the deep localization and the aggravated non-radiative recombination during the relaxation or recapture processes. For T > 160 K, the carriers may have sufficient energy to repopulate the shallow localization. The non-radiative recombination then gradually dominates the recombination process, leading to a rapid quenching of PL. On the other hand, the thermal-induced band-filling and carrier redistribution contribute to a conspicuous blueshift of the Ep [Figure 2a]. At even higher temperature, most carriers start to escape from the localized states and become free carriers. A redshift of the Ep is then observed due to the temperature-induced bandgap shrinkage. It should be noted that the temperature of the turning point from blueshift to redshift is as high as 260 K. Such high temperature denotes that the carriers need large energy to escape from the deep localization, indicating a strong confinement effect in the localization potentials. Such temperature-dependent emission energy at localized states can be described as [24]:Figure 4


Strong localization effect and carrier relaxation dynamics in self-assembled InGaN quantum dots emitting in the green.

Weng GE, Zhao WR, Chen SQ, Akiyama H, Li ZC, Liu JP, Zhang BP - Nanoscale Res Lett (2015)

Schematic diagram of potential distribution of the non-isolated QDs describing possible processes of carrier transport, relaxation and recombination.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4384949&req=5

Fig4: Schematic diagram of potential distribution of the non-isolated QDs describing possible processes of carrier transport, relaxation and recombination.
Mentions: To explain the observed different variation under different excitation power in 1) the S-shaped temperature-dependent emission energy, 2) the evolution of linewidth as well as 3) the variation of emission intensity, a strong localization model describing possible processes of carrier transport, relaxation, and recombination is proposed, as shown in Figure 4. In comparison with the localization model in blue InGaN QWs, the localized luminescence centers in green InGaN QDs are much more deeper for the higher indium content and have stronger quantum confinement effect (QCE). Using this model, the distinct phenomena observed in our experiments can be well explained as follows: At low temperature of 5 K, carriers are randomly distributed among both deep and shallow potential minima caused by potential fluctuations and the radiative recombination process is dominant. For high excitation intensity (P = 18.5 mW), the carrier concentration is much higher and, as a result, a majority of photo-generated carriers are trapped in deep localization states, resulting in a smaller Ep [Figure 5b]. As the temperature increases from 5 to 60 K (100 K), shallow localized carriers are thermally activated and relax down into other deep localization via hopping, leading to the initial redshift of Ep as large as 16 meV. This is also true for low excitation case, but the redshift is much larger, 48 meV. The relatively smaller decrement in Ep for the high excitation is ascribed to the more markedly band filling effect in the deep localization centers. In case of low excitation where the carrier density is much lower, on the other hand, most of the carriers can relax into the lowest energy level of the deep localization center with rising in temperature up to 100 K, and thus, the carrier distribution narrows [Figure 5c], accompanied by a remarkable redshift of Ep and a slightly decrease of the PL linewidth [Figure 2]. When further increase of the temperature above 60 K (100 K), band-filling in the deep localization is dramatically enhanced due to the temperature-dependent intra-dot relaxation behavior of the carriers. In addition, the regular thermalization of the carriers becomes more and more significant. These temperature-dependent thermal broadening effects of the carriers lead to a continuous increase of the spectra linewidths for both excitation intensities, as shown in Figure 2b. The anomalous enhanced emission over the temperature range of 30 to 160 K at high excitation intensity observed in Figure 3 can be understood as the following. As is well known, with the temperature increases, the defect-related non-radiative recombination would be more and more serious and then results in the quenching of PL. In fact, however, the emission intensity increases monotonously in this temperature range despite of the aggravated non-radiative recombination, which is also observed by Ma et al. [20] and Masumoto and Takagahara [23]. Due to the stronger QCE and better crystal quality in deep localization, it is convincing to consider that the radiation efficiency of carriers in deep localization is higher and thus resulting in a faster increase in emission intensity. With elevating the temperature, the carriers escape from the shallow localization and then converge to fill in the deep localization center by relaxation and recapture processes. Hence, the carriers consumed by radiative recombination in deep localization can be compensated rapidly. As a consequence, a majority of carriers would radiate in the deep localization with a higher radiation efficiency and eventually lead to an anomalous enhanced emission. The maximum emission intensity at 160 K is enhanced by 40% in comparison to that at low temperature, and a high IQE of approximately 41% is obtained at room temperature. For weak excitation, however, the carrier density is much lower. The defect-assisted non-radiative recombination should not be disregarded anymore with rising temperature. During the relaxation and recapture processes, a considerable proportion of carriers would be captured by the non-radiative centers, which suppresses the compensation of carriers that consumed by radiative recombination and results in a monotonous decreasing of the emission intensity. It is rather complicated but reasonable for the emission intensity that decreases slower with a small “uplift” in this particular temperature range, as balanced with the improved radiation efficiency in the deep localization and the aggravated non-radiative recombination during the relaxation or recapture processes. For T > 160 K, the carriers may have sufficient energy to repopulate the shallow localization. The non-radiative recombination then gradually dominates the recombination process, leading to a rapid quenching of PL. On the other hand, the thermal-induced band-filling and carrier redistribution contribute to a conspicuous blueshift of the Ep [Figure 2a]. At even higher temperature, most carriers start to escape from the localized states and become free carriers. A redshift of the Ep is then observed due to the temperature-induced bandgap shrinkage. It should be noted that the temperature of the turning point from blueshift to redshift is as high as 260 K. Such high temperature denotes that the carriers need large energy to escape from the deep localization, indicating a strong confinement effect in the localization potentials. Such temperature-dependent emission energy at localized states can be described as [24]:Figure 4

Bottom Line: Strong localization effect in self-assembled InGaN quantum dots (QDs) grown by metalorganic chemical vapor deposition has been evidenced by temperature-dependent photoluminescence (PL) at different excitation power.The integrated emission intensity increases gradually in the range from 30 to 160 K and then decreases with a further increase in temperature at high excitation intensity, while this phenomenon disappeared at low excitation intensity.Using this model, the evolution of excitation-power-dependent emission intensity, shift of peak energy, and linewidth variation with elevating temperature is well explained.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Semiconductor Photonics Research Center, Xiamen University, 422 South Siming Road, Xiamen, 361005 P. R. China.

ABSTRACT
Strong localization effect in self-assembled InGaN quantum dots (QDs) grown by metalorganic chemical vapor deposition has been evidenced by temperature-dependent photoluminescence (PL) at different excitation power. The integrated emission intensity increases gradually in the range from 30 to 160 K and then decreases with a further increase in temperature at high excitation intensity, while this phenomenon disappeared at low excitation intensity. Under high excitation, about 40% emission enhancement at 160 K compared to that at low temperature, as well as a higher internal quantum efficiency (IQE) of 41.1%, was observed. A strong localization model is proposed to describe the possible processes of carrier transport, relaxation, and recombination. Using this model, the evolution of excitation-power-dependent emission intensity, shift of peak energy, and linewidth variation with elevating temperature is well explained. Finally, two-component decays of time-resolved PL (TRPL) with various excitation intensities are observed and analyzed with the biexponential model, which enables us to further understand the carrier relaxation dynamics in the InGaN QDs.

No MeSH data available.


Related in: MedlinePlus