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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

Annealing temperature dependence of energy difference between GS and ES1 of QDs (left axis), as well as the intensity ratio of GS to ES1 at room temperature (right axis).
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Figure 6: Annealing temperature dependence of energy difference between GS and ES1 of QDs (left axis), as well as the intensity ratio of GS to ES1 at room temperature (right axis).

Mentions: To further study the electronic structure and related carrier dynamics between inter-sublevels, the RTA treatments were adopted to alter the transition energies of QDs by inter-diffusion of constituent atoms. It has been reported that it is possible to retain the three-dimensional confinement in QDs after high-temperature annealing [31,32]. Composition intermixing affects both the height and the shape of the QD confining potential, hence changing the transition energies and the inter-sublevels spacing. Figure 4 presents the room temperature PL spectra of samples annealed at different temperatures. It is clear that each PL spectrum includes two parts of peaks. The peaks at the low energy regions come from the ground and excited states of QDs, which indicate the strong quantum confinement of QDs even at high annealing temperatures. At the highenergy side, the Lorentzian-shaped peak centered at about 1.27 eV can be attributed to optical transition of e-HH energy level of the bi-QW structure, as also revealed by the PR results above. On one hand, as shown in Figure 5a, the QDs-related PL intensity decreases a little firstly and then quenches when the annealing temperature is above 800°C. Generally, the room temperature optical quality of annealed QDs samples is expected to decrease due to the diffusion of Ga atoms into InAs QDs, which lowers the potential depth and leads to weaker carrier confinement and higher quenching rate. However, the decrease is not obvious until the temperature is above 800°C, and especially, the intensity at 750°C is even a little higher than that of 600 and 650°C. Meanwhile, the intensity of QW is also enhanced after annealing at 750°C. Such phenomena can be attributed to the reduced dislocations or defects, which may result from the less lattice mismatch between InAs QDs and InGaAs capping layer after annealing. On the other hand, the RTA processes also take effects on the PL spectra of QDs sublevels, as shown in Figure 5b, c. Here we do not consider the ES2 due to its weak intensity at higher annealing temperature. Similar to that reported in [31,32], the peak of both GS and ES1 shifts to the high energy region and the FWHM becomes narrowing with increasing annealing temperatures, which is also a feature of In/Ga intermixing. Meanwhile, as shown in Figure 6, the energy difference between ES1 and GS decreases from approximately 61 to approximately 29 meV as the annealing temperature increases from 600 to 800°C. Especially, the intensity ratio at low temperature (15 K) of GS to ES1 also varies with annealing temperature. The intensity ratio decreases from 2.2 to 1.7 as the annealing temperature increases to 750°C, and then it increases to about 2.1 again for the 800°C annealed sample. It is noted that the energy difference of two ratio maximums are 29 and 61 meV, which approaches to one and two InAs bulk longitudinal optical (LO) phonon(s) energy of approximately 30 meV, respectively. Recently, Chen et al. have revealed the carrier relaxation mechanism in a typical InAs/InxGa1-xAs DWELL structure. From the selectively excited photoluminescence and photoluminescence excitation spectra, two and three LO resonant peaks have been observed, which indicate phonon-assisted carrier relaxation in the low excitation energy regime [33]. Such LO-assisted carrier relaxation from excited states to ground states has also been discussed in detail by Steer et al. [34] in the InAs/GaAs quantum dots system. In our case, the two ratio maximums are achieved when the phonon resonant conditions below are satisfied [35]:(4)


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)

Annealing temperature dependence of energy difference between GS and ES1 of QDs (left axis), as well as the intensity ratio of GS to ES1 at room temperature (right axis).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 6: Annealing temperature dependence of energy difference between GS and ES1 of QDs (left axis), as well as the intensity ratio of GS to ES1 at room temperature (right axis).
Mentions: To further study the electronic structure and related carrier dynamics between inter-sublevels, the RTA treatments were adopted to alter the transition energies of QDs by inter-diffusion of constituent atoms. It has been reported that it is possible to retain the three-dimensional confinement in QDs after high-temperature annealing [31,32]. Composition intermixing affects both the height and the shape of the QD confining potential, hence changing the transition energies and the inter-sublevels spacing. Figure 4 presents the room temperature PL spectra of samples annealed at different temperatures. It is clear that each PL spectrum includes two parts of peaks. The peaks at the low energy regions come from the ground and excited states of QDs, which indicate the strong quantum confinement of QDs even at high annealing temperatures. At the highenergy side, the Lorentzian-shaped peak centered at about 1.27 eV can be attributed to optical transition of e-HH energy level of the bi-QW structure, as also revealed by the PR results above. On one hand, as shown in Figure 5a, the QDs-related PL intensity decreases a little firstly and then quenches when the annealing temperature is above 800°C. Generally, the room temperature optical quality of annealed QDs samples is expected to decrease due to the diffusion of Ga atoms into InAs QDs, which lowers the potential depth and leads to weaker carrier confinement and higher quenching rate. However, the decrease is not obvious until the temperature is above 800°C, and especially, the intensity at 750°C is even a little higher than that of 600 and 650°C. Meanwhile, the intensity of QW is also enhanced after annealing at 750°C. Such phenomena can be attributed to the reduced dislocations or defects, which may result from the less lattice mismatch between InAs QDs and InGaAs capping layer after annealing. On the other hand, the RTA processes also take effects on the PL spectra of QDs sublevels, as shown in Figure 5b, c. Here we do not consider the ES2 due to its weak intensity at higher annealing temperature. Similar to that reported in [31,32], the peak of both GS and ES1 shifts to the high energy region and the FWHM becomes narrowing with increasing annealing temperatures, which is also a feature of In/Ga intermixing. Meanwhile, as shown in Figure 6, the energy difference between ES1 and GS decreases from approximately 61 to approximately 29 meV as the annealing temperature increases from 600 to 800°C. Especially, the intensity ratio at low temperature (15 K) of GS to ES1 also varies with annealing temperature. The intensity ratio decreases from 2.2 to 1.7 as the annealing temperature increases to 750°C, and then it increases to about 2.1 again for the 800°C annealed sample. It is noted that the energy difference of two ratio maximums are 29 and 61 meV, which approaches to one and two InAs bulk longitudinal optical (LO) phonon(s) energy of approximately 30 meV, respectively. Recently, Chen et al. have revealed the carrier relaxation mechanism in a typical InAs/InxGa1-xAs DWELL structure. From the selectively excited photoluminescence and photoluminescence excitation spectra, two and three LO resonant peaks have been observed, which indicate phonon-assisted carrier relaxation in the low excitation energy regime [33]. Such LO-assisted carrier relaxation from excited states to ground states has also been discussed in detail by Steer et al. [34] in the InAs/GaAs quantum dots system. In our case, the two ratio maximums are achieved when the phonon resonant conditions below are satisfied [35]:(4)

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