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Optical identification of electronic state levels of an asymmetric InAs/InGaAs/GaAs dot-in-well structure.

Zhou X, Chen Y, Xu B - Nanoscale Res Lett (2011)

Bottom Line: It is shown that the carrier transfer via wetting layer (WL) is impeded according to the results of temperature dependent peak energy and line width variation of both the ground states (GS) and excited states (ES) of QDs.Additionally, as the RTA temperature increases, the peak of PL blue shifts and the full width at half maximum shrinks.Especially, the intensity ratio of GS to ES reaches the maximum when the energy difference approaches the energy of one or two LO phonon(s) of InAs bulk material, which could be explained by phonon-enhanced inter-sublevels carrier relaxation in such asymmetric dot-in-well structure.PACS: 73.63.Kv; 73.61.Ey; 78.67.Hc; 81.16.Dn.

View Article: PubMed Central - HTML - PubMed

Affiliation: Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P,O, Box 912, Beijing 100083, People's Republic of China. zhouxl06@semi.ac.cn.

ABSTRACT
We have studied the electronic state levels of an asymmetric InAs/InGaAs/GaAs dot-in-well structure, i.e., with an In0.15Ga0.85As quantum well (QW) as capping layer above InAs quantum dots (QDs), via temperature-dependent photoluminescence, photo-modulated reflectance, and rapid thermal annealing (RTA) treatments. It is shown that the carrier transfer via wetting layer (WL) is impeded according to the results of temperature dependent peak energy and line width variation of both the ground states (GS) and excited states (ES) of QDs. The quenching of integrated intensity is ascribed to the thermal escape of electron from the dots to the complex In0.15Ga0.85As QW + InAs WL structure. Additionally, as the RTA temperature increases, the peak of PL blue shifts and the full width at half maximum shrinks. Especially, the intensity ratio of GS to ES reaches the maximum when the energy difference approaches the energy of one or two LO phonon(s) of InAs bulk material, which could be explained by phonon-enhanced inter-sublevels carrier relaxation in such asymmetric dot-in-well structure.PACS: 73.63.Kv; 73.61.Ey; 78.67.Hc; 81.16.Dn.

No MeSH data available.


Related in: MedlinePlus

Temperature dependence of PL peak energy (a), FWHM (b), normalized intensity (c) of the as-grown sample. Solid lines in (a) are the fitting by varshni relation for bulk material and dashed lines in (c) are the fitting of quenching of intensity according to the Arrhenius relation.
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Figure 2: Temperature dependence of PL peak energy (a), FWHM (b), normalized intensity (c) of the as-grown sample. Solid lines in (a) are the fitting by varshni relation for bulk material and dashed lines in (c) are the fitting of quenching of intensity according to the Arrhenius relation.

Mentions: To elucidate the thermally activated processes, including the carrier thermal escape and transfer, temperature-dependent PL of all the three energy levels were measured, as displayed in Figure 2a, b, c for the peak energy, FWHM, and integrated intensity, respectively. It is generally accepted that carriers can transfer between different QDs assemblies via the wetting layer with increasing temperature [16-20]. The net carrier transfer from small QDs to large QDs can explain the abnormal temperature dependence (ATD) of PL spectra, i.e., rapid red-shift of peak energy compared to the bulk material and S-shaped variation of FWHM at the medium temperature interval (e.g., 100-200 K). The states with higher energy are often expected to present more obvious ATD effects due to their less activation energy needed. However, as shown in Figure 2a, not only the GS peak, but also the ES1 and ES2 peaks show similar variation as that of InAs bulk material. Such variation can be fitted using the typical varshni law: E(T) = E0 - αT2/(T + β), where E0 is the peak energy at low temperature (15 K in our case), α and β are the fitting parameters of InAs bulk. Meanwhile, the FWHM also varies slightly with temperature for all three peaks, as shown in Figure 2b. The absence of ATD of all states could then be ascribed to the impeded carrier transfer process via wetting layer (WL). In an asymmetric DWELL structure, the InAs WL is coupled with the InGaAs QW both spatially and energetically, which means that carrier transfer via WL is strongly influenced by the InGaAs capping layer. For example, Torchynska et al. [21] have found there are lots of nonradiative recombination centers in the capping In0.15Ga0.85As layer when the growth temperature is low enough. So, in our case, it is assumed that the thermally excited carriers from QDs are mostly lost non-radiatively in the QW + WL structure before carrier redistribution happens. To further demonstrate this viewpoint, the temperature dependence of PL intensity is presented in Figure 2c. In some previous reports, especially for some DWELL structures, the temperature dependence of PL intensity has been fitted by Arrhenius relation using two exponential terms, i.e., two activation energy Eα [22-24]. For two Eα fitting, the higher Eα often represents the carrier escape from QDs to QW and the lower Eα represents the carrier escape from QW to GaAs barrier or other barrier-related non-radiative processes. However, in our case, the two Eα fitting is not suitable due to the first decrease and then increase trend of both GS and ES intensity, which would give rise to nontrivial fitting error for the value of the lower Eα. So, in our case, only the process of carrier escape from QDs to QW is fitted at high temperature regime, i.e., intensity quenching. The activation energy could be extracted using one exponential term, as has expressed by lots of existing studies [20,25,26]:(1)


Optical identification of electronic state levels of an asymmetric InAs/InGaAs/GaAs dot-in-well structure.

Zhou X, Chen Y, Xu B - Nanoscale Res Lett (2011)

Temperature dependence of PL peak energy (a), FWHM (b), normalized intensity (c) of the as-grown sample. Solid lines in (a) are the fitting by varshni relation for bulk material and dashed lines in (c) are the fitting of quenching of intensity according to the Arrhenius relation.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211404&req=5

Figure 2: Temperature dependence of PL peak energy (a), FWHM (b), normalized intensity (c) of the as-grown sample. Solid lines in (a) are the fitting by varshni relation for bulk material and dashed lines in (c) are the fitting of quenching of intensity according to the Arrhenius relation.
Mentions: To elucidate the thermally activated processes, including the carrier thermal escape and transfer, temperature-dependent PL of all the three energy levels were measured, as displayed in Figure 2a, b, c for the peak energy, FWHM, and integrated intensity, respectively. It is generally accepted that carriers can transfer between different QDs assemblies via the wetting layer with increasing temperature [16-20]. The net carrier transfer from small QDs to large QDs can explain the abnormal temperature dependence (ATD) of PL spectra, i.e., rapid red-shift of peak energy compared to the bulk material and S-shaped variation of FWHM at the medium temperature interval (e.g., 100-200 K). The states with higher energy are often expected to present more obvious ATD effects due to their less activation energy needed. However, as shown in Figure 2a, not only the GS peak, but also the ES1 and ES2 peaks show similar variation as that of InAs bulk material. Such variation can be fitted using the typical varshni law: E(T) = E0 - αT2/(T + β), where E0 is the peak energy at low temperature (15 K in our case), α and β are the fitting parameters of InAs bulk. Meanwhile, the FWHM also varies slightly with temperature for all three peaks, as shown in Figure 2b. The absence of ATD of all states could then be ascribed to the impeded carrier transfer process via wetting layer (WL). In an asymmetric DWELL structure, the InAs WL is coupled with the InGaAs QW both spatially and energetically, which means that carrier transfer via WL is strongly influenced by the InGaAs capping layer. For example, Torchynska et al. [21] have found there are lots of nonradiative recombination centers in the capping In0.15Ga0.85As layer when the growth temperature is low enough. So, in our case, it is assumed that the thermally excited carriers from QDs are mostly lost non-radiatively in the QW + WL structure before carrier redistribution happens. To further demonstrate this viewpoint, the temperature dependence of PL intensity is presented in Figure 2c. In some previous reports, especially for some DWELL structures, the temperature dependence of PL intensity has been fitted by Arrhenius relation using two exponential terms, i.e., two activation energy Eα [22-24]. For two Eα fitting, the higher Eα often represents the carrier escape from QDs to QW and the lower Eα represents the carrier escape from QW to GaAs barrier or other barrier-related non-radiative processes. However, in our case, the two Eα fitting is not suitable due to the first decrease and then increase trend of both GS and ES intensity, which would give rise to nontrivial fitting error for the value of the lower Eα. So, in our case, only the process of carrier escape from QDs to QW is fitted at high temperature regime, i.e., intensity quenching. The activation energy could be extracted using one exponential term, as has expressed by lots of existing studies [20,25,26]:(1)

Bottom Line: It is shown that the carrier transfer via wetting layer (WL) is impeded according to the results of temperature dependent peak energy and line width variation of both the ground states (GS) and excited states (ES) of QDs.Additionally, as the RTA temperature increases, the peak of PL blue shifts and the full width at half maximum shrinks.Especially, the intensity ratio of GS to ES reaches the maximum when the energy difference approaches the energy of one or two LO phonon(s) of InAs bulk material, which could be explained by phonon-enhanced inter-sublevels carrier relaxation in such asymmetric dot-in-well structure.PACS: 73.63.Kv; 73.61.Ey; 78.67.Hc; 81.16.Dn.

View Article: PubMed Central - HTML - PubMed

Affiliation: Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P,O, Box 912, Beijing 100083, People's Republic of China. zhouxl06@semi.ac.cn.

ABSTRACT
We have studied the electronic state levels of an asymmetric InAs/InGaAs/GaAs dot-in-well structure, i.e., with an In0.15Ga0.85As quantum well (QW) as capping layer above InAs quantum dots (QDs), via temperature-dependent photoluminescence, photo-modulated reflectance, and rapid thermal annealing (RTA) treatments. It is shown that the carrier transfer via wetting layer (WL) is impeded according to the results of temperature dependent peak energy and line width variation of both the ground states (GS) and excited states (ES) of QDs. The quenching of integrated intensity is ascribed to the thermal escape of electron from the dots to the complex In0.15Ga0.85As QW + InAs WL structure. Additionally, as the RTA temperature increases, the peak of PL blue shifts and the full width at half maximum shrinks. Especially, the intensity ratio of GS to ES reaches the maximum when the energy difference approaches the energy of one or two LO phonon(s) of InAs bulk material, which could be explained by phonon-enhanced inter-sublevels carrier relaxation in such asymmetric dot-in-well structure.PACS: 73.63.Kv; 73.61.Ey; 78.67.Hc; 81.16.Dn.

No MeSH data available.


Related in: MedlinePlus