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Lines 14 of the luminescence spectrum shown in Fig. seem consistent with these predictions and this can be seen more clearly in Fig. 5 which shows their energies at T = 20 K. The symbols show the experimental data and the D solid curves are calculated [Eq.(5)] using Exw = 1.5551 eV, D Exn = 1.5589 eV, and = 1.33 meV. The two anticrossing regions shown circled occur at resonant voltages Vrn = 1.4 V and Vrw = 2.3 V. As noted earlier, for V > 1 V, the data are consistent with a linear field dependence F(V ) =kV with k = 7103 cm-1 at 20 K and a similar value is found at 10 K although it decreases at lower temperatures and for T = 5 K, k =3 103 cm-1. From the D data and calculated values for EB in DQWs of 8 meV [16] and for Ee of 3.32 meV we obtain a value for the binding I energy of IX of EB = 3.3 meV in good agreement with previous values for similar samples [3,4,16,17]. The voltage Vf at which F + F0 = 0 at 20 K is 1.54 V.

The intensities of lines 14 are governed by the tunnelling and by the relaxation processes of DX and IX through the emission and absorption of acoustic phonons. The results show that the exciton states DXN and IXN are in thermal equilibrium as are states DXW and IXW. However the thermal equilibrium between these two pairs is not established [19]. Detailed analysis of the luminescence Figure 6. The dependence of the total intensity I0 on applied intensities and relaxation processes will be published shortly.

voltage for T = 5 and 20 K. Since F0 and k depend on temperature, B. T h e o r i g i n o f l i n e L and changes the relative values of I0 at the two temperatures at 2 V are not i n + t he nonr adi at i ve decay of exci t ons.

comparable. The arrows show the positions Vrn and Vrw of the The experimental spectrum at 5 K in Fig. 3, b shows the DX/IX resonances for T = 20 K. The lines through the two sets appearance of a luminescence line L when V > 3 V of data are to guide the eye.

, 1997, 39, Luminescence of Excitons in Slightly Asymmetric Double Quantum Wells Further evidence of indirect exciton localization can be [8] H.W. Liu, R. Ferreira, G. Bastard, C. Delalande, J.F. Palmier, B. Etienne. Appl. Phys. Lett. 54, 21, 2082 (1989).

seen in the voltage variation of the total intensity I0 obtained [9] F. Clrot, B. Deveaud, A. Chomette, A. Regreny, B. Sermage.

by integration over the whole spectral range. Fig. 6 shows Phys. Rev. B41, 9, 5756 (1990); B. Deveaud, A. Chomette, that this variation is strongly temperature dependent and F. Clrot, P. Auvray, A. Regreny, R. Ferreira, G. Bastard. Phys.

we first discuss the data at 20 K which is too high for Rev. B42, 11, 7021 (1990).

any significant localization. There are pronounced decreases [10] T.B. Norris, N. Vodjdani, B. Vinter, E. Costard, E. Bckenhoff.

in I0 for V < Vrn and V > Vrw where the lowest energy Phys. Rev. B43, 2, 1867 (1991).

excitons are IXN and IXW (Fig. 5). These have much longer [11] R. Strobel, R. Eccleston, J. Kuhl, K. Khler. Phys. Rev. B43, radiative lifetimes than DX excitons [1] so that a much higher 15, 12564 (1991).

proportion of them decay nonradiatively accounting for the [12] Ph. Roussignol, A. Vinattieri, L. Carraresi, M. Colocci, decreases in I0 observed. Strikingly different behaviour A. Fasolino. Phys. Rev. B44, 16, 8873 (1991).

however is observed at 5 K. There are now no decreases [13] A.P. Heberle, W.W. Rhle, M.G.W. Alexander, K. Khler.

Semicond. Sci. Technol. 7, B421 (1992).

in I0 for V < Vrn and V > Vrw suggesting that the lowest [14] A.P. Heberle, X.Q. Zhou, A. Tackeuchi, W.W. Rhle, energy states are no longer the free indirect exciton states K. Khler. Semicond. Sci. Technol. 9, 519 (1994).

IXN and IXW but states which largely decay radiatively.

[15] R.G. Ispasoiu, A.M. Fox, C.T. Foxon, J.E. Cunningham, This is entirely consistent with indirect exciton localization.

W.Y. Jan. Semicond. Sci. Technol. 9, 545 (1994).

It is well-known that the probability of nonradiative decay [16] A.M. Fox, D.A.B. Miller, G. Livescu, J.E. Cunningham, from direct excitons is much reduced by localization, the W.Y. Jan. Phys. Rev. B44, 12, 6231 (1991).

greater confinement of the exciton wavefunction reducing [17] H. Schneider, J. Wagner, K. Ploog. Phys. Rev. B48, 15, the overlap with nonradiative decay centres, and it seems (1993).

very probable that a similar effect should occur for indirect [18] O. Brandt, K. Kanamoto, Y. Tokuda, Y. Abe, Y. Wada, excitons. We conclude therefore that, at 5 K, the majority N. Tsukada. J. Appl. Phys. 75, 4, 2105 (1994).

of IXN and IXW excitons become localized when V < Vrn [19] A.V. Akimov, E.S. Moskalenko, A.L. Zhmodikov, D.A. Mazurenko, A.A. Kaplyanskii, L.J. Challis, T.S. Cheng, and V > Vrw.

C.T. Foxon. Nanostructures: Physics and Technology96. Int.

The luminescence spectrum of slightly asymmetric DQWs Symp. Abstracts of Invited Lectures and Contributed Papers.

has been shown to contain four free exciton lines: two direct St.Petersburg, Russia (2428 June 1996). P. 86.

and two indirect. The indirect excitons become localized [20] D.C. Reynolds, K.R. Evans, K.G. Merkel, C.E. Stutz, P.W. Yu.

(LIX) at low temperatures, T < 10 K. One of the LIX Phys. Rev. B43, 11, 9087 (1991).

gives rise to a newline, L, while the presence of the other is inferred from the decrease in proportion of non-radiative decay that occurs on localization. Anticrossing has been observed between the direct and indirect excitons and, at resonance, the symmetric and antisymmetric eigenstates are separated by 1.3 meV.

We gratefully acknowledge financial support from the Russian Foundation for Fundamental Studies (96-0216952a) and the European Commission (INTAS-94-395).

References [1] T.B. Norris, N. Vodjdani, B. Vinter, C. Weisbush, G.A. Mourou. Phys. Rev. B40, 2, 1392 (1989).

[2] M. Nido, M.G.W. Alexander, W.W. Rhle, K. Khler. Phys.

Rev. B43, 2, 1839 (1991).

[3] R. Ferreira, C. Delalande, H.W. Liu, G. Bastard, B. Etienne, J.F. Palmier. Phys. Rev. B42, 14, 9170 (1990).

[4] R. Ferreira, P. Rolland, Ph. Roussignol. C. Delalande, A. Vinattieri, L. Carraresi, M. Colocci, N. Roy, B. Sermage, J.F. Palmer, B. Etienne. Phys. Rev. B45, 20, 11782 (1992).

[5] J.E. Golub, K. Kash, J.P. Harbison, L.T. Florez. Phys. Rev. B41, 12, 8564 (1990); M.R. Reshotko, L.D. Shvartsman, J.E. Golub.

Phys. Rev. B50, 3, 4692 (1994).

[6] K. Leo, J. Shah, E.O. Gobel, T.C. damen, S. Schmitt-Rink, W. Scfer, K. Khler. Phys. Rev. Lett. 66. 2, 201 (1991).

[7] D.Y. Oberli, J. Shah, T.C. Damen, T.Y. Chang, C.W. Tu, D.A.B. Miller, J.E. Henry, R.F. Kopf, N. Sauer, A.E. DiGiovanni. Phys. Rev. B40, 5, 3028 (1989).

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