Physica E 11 (2001) 68–71
Observation of negatively charged excitons and excited states of multi-excitons in quantum dots embedded in modulation doping structures K. Ohdaira ∗ , N. Usami, K. Ota, Y. Shiraki Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan
Abstract Existence of negatively charged excitons in quantum dots (QDs) is con0rmed by the measurement of the bias dependence of luminescence from modulation doped structures with embedded QDs. Excited states of multi-particle states are also c 2001 Elsevier identi0ed from the bias as well as the excitation intensity dependence of micro-photoluminescence peaks. Science B.V. All rights reserved. Keywords: Micro-photoluminescence; Multi-exciton; Charged exciton; Modulation doping
1. Introduction Multi-excitons and their charging states in quantum dots are attracting much attention not only from the simple scienti0c interest but also from the point of view of device applications, since high density excitation is easily realized in devices with quantum structures. Although many experimental [1– 6] and theoretical [7,8] studies on the formation and properties of these multi-particle states in quantum dots have been reported, there is still ambiguity in assignment
of charged states of excitons and therefore, more detailed investigations are needed. In this report, we employ self-assembled AlInAs quantum dots embedded in modulation doping structure, which enables one to control the charging states of excitons in quantum dots. Not only the existence of negatively-charged excitons is con0rmed but also the excited multi-particle states are well identi0ed by the measurements of the bias and excitation intensity dependence of the luminescence. 2. Experiment
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The samples in this study were grown on the n+ -GaAs (1 0 0) substrate by molecular beam epitaxy.
c 2001 Elsevier Science B.V. All rights reserved. 1386-9477/01/$ - see front matter PII: S 1 3 8 6 - 9 4 7 7 ( 0 1 ) 0 0 1 7 8 - 3
K. Ohdaira et al. / Physica E 11 (2001) 68–71
Fig. 1. Bias voltage dependence of -PL spectra from AlInAs single QD in modulation doping structure.
4 ML Al0:3 In0:7 As quantum dot (QD) layer was grown between Al0:3 Ga0:7 As layers on GaAs buHer layer and Si modulation-doped Al0:3 Ga0:7 As layer was successively fabricated over this structure. After MBE growth, Al mask with sub-micron windows was fabricated on the sample surface for micro-photoluminescence (-PL) measurements. A 532 nm cw light from YAG SHG laser was focused by a microscope objective lens for excitation and emissions were gathered by the same lens and detected with some ten micro-eV resolution. Fig. 2. Excitation intensity dependence of -PL spectra from AlInAs single QD under zero bias (a) and negative bias (b).
3. Results and discussion Fig. 1 shows -PL spectra of the sample as a parameter of the bias voltage. Under positive bias, 0ve well resolved 0ne peaks, A–E, are observed. On the other hand, under negative bias, these peaks disappear and peaks F and G are seen to appear newly. The external bias changes the number of excess electrons in QDs induced by the modulation doping and the excess electrons are depleted from the dots under negative bias. Therefore, the emissions under positive bias are considered to come from negatively charged excitons, while luminescence under the negative bias is dominated by neutral excitons. The excitation intensity dependence of the spectra under negative bias, i.e., luminescence from neutral excitons, is shown in Fig. 2(a). There are only two peaks, F and G, under low excitation. They are consid-
ered as the emissions from bi-exciton and single excitons, respectively, because they are formed under very low excitation and the energy diHerence, 4.7 meV, is reasonable for the binding energy of bi-excitons. With increasing excitation intensity, the four new peaks, H– K, are seen to appear between single- and bi-exciton (X2 ) peaks. They can be assigned as the emissions from tri-excitons or excited bi-excitons. Rapid relaxation of carriers from the excited states to the ground state has been reported [9,10], though the phonon bottleneck eHect is theoretically anticipated . If so, some of the additional four peaks may be assigned as the emission from tri-excitons (X3 ), which consist of two electron–hole pairs in the ground state and one pair in the excited state. When the recombination of the ground state electron–hole pair occurs in
K. Ohdaira et al. / Physica E 11 (2001) 68–71
the tri-exciton, excited bi-excitons are left, where one electron and one hole remain in the excited states. The energy of that excited bi-exciton splits into three levels; singlet, sixfold, and ninefold states depending on spin parallel or anti-parallel and, therefore, the emission from the tri-exciton splits into three at most. The other peaks might be the emissions from X4 and so on. On the other hand, if the carrier relaxation time is not too short due to the phonon bottleneck eHect and others , several excited bi-excitons may be formed, resulting in the emission of several peaks whose energies are higher than that of the ground bi-exciton. The luminescence of the excited bi-excitons may split into seven peaks due to spin-exchange interactions. Some of them might be very weak because of the small degenerate number or have very close energies to each other and four peaks are observed as a consequence. A similar picture is proposed by Bayer et al.  to explain their excitation intensity dependence of QD luminescence. Fig. 2(b) shows excitation intensity dependence of luminescence under zero bias where charged excitons are formed. Only peak C appears under low excitation and the other four peaks, A, B, D, and E, cannot be seen until a certain excitation level. Since peak C is observed under low excitation, it is considered to be the emission from charged single excitons in which two electrons and one hole are in the ground states. The other four peaks are assigned as the emissions from charged bi-excitons and excited charged single excitons. When the recombination between carriers in the ground states occurs in charged bi-excitons, excited charged single excitons, which have one electron in the excited state, may remain. This transition gives rise to two energy levels due to spin-exchange interaction. The emission from the remaining excited charged single exciton also splits into two lines. Other charged bi-excitons, which have one hole in the excited states, can be formed and the emissions from it may also split into two due to spin-exchange interaction. The weakness or the energetic closeness of the peaks might result in four peaks observed here. Another possible explanation for these four peaks is charged multi-excitons such as tri-excitons and so on. Since both states are possible in small QDs, more detailed investigation is needed to clearly assign these emission peaks.
It should be emphasized that the emissions from the neutral exciton states were never seen under the bias condition where luminescence from the charged states was observed and vice versa, even under high excitation. This indicates that the transition between neutral and charged states of excitons is induced only by the change in the number of excess electrons in the dot. To roughly examine the above assignment of the emissions, a simple calculation was performed for single excitons, bi-excitons and charged single excitons including the eHect of interactions between carriers in the framework of eHective mass approximation . It was found that the above assignment is reasonable if the dot is assumed to be parallelpipedal with the size of 15 × 15 × 1 nm3 which approximately corresponds to the size of examined QDs.
4. Summary In summary, we performed -PL measurements on a single AlInAs QD in an AlGaAs=GaAs modulation doped structure and con0rmed the presence of negatively charged excitons including charged bi-excitons and excited charged single excitons. We also observed the emission lines assigned as excited bi-excitons.
Acknowledgements We would like to thank R.V. Dalen for providing the calculation program. This research was supported by International Priority Collaboration Program of Japan Society for the Promotion of Science (JSPS), and by a Grant-in-Aid for Scienti0c Research from the Ministry of Education, Science, Sports and Culture.
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