Modulated resonant Raman and photoluminescence spectroscopy of Bragg confined asymmetric coupled quantum wells

Physica E 7 (2000) 245–249

www.elsevier.nl/locate/physe

Modulated resonant Raman and photoluminescence spectroscopy of Bragg con ned asymmetric coupled quantum wells M. Levya; ∗ , R. Kaponb , A. Sa’arb , R. Besermana , V. Thierry-Miegc , R. Planelc a Solid

State Institute and Physics Department, Technion, Israel Institute of Technology, Technion City, Haifa 32000, Israel of Applied Physics, The Fredi and Nadine Hermann School of Applied Science, The Hebrew University of Jerusalem, Jerusalem 91904, Israel c Laboratoire de Microstructures et Microelectronique – CNRS, 196 Avenue H. Ravera, BP107, 92225 Bagneux, France

b Department

Abstract Electronic Bragg mirrors were used to con ne carriers at energy levels above the barrier height in asymmetric coupled quantum wells. Two classes of above barrier states were resolved by using photoluminescence, photoluminescence excitation and modulated resonant Raman spectroscopy. The rst class is Bragg con ned levels that are highly localized in the asymmetric quantum wells region and are red shifted when locally excited electric eld is generated in the asymmetric coupled quantum well region. The second class of levels that extend mainly above the re ectors is not shifted when the locally excited eld is generated. This phenomenon is due to the smaller con nement of the extended states in the asymmetric quantum well region. ? 2000 Elsevier Science B.V. All rights reserved. Keywords: Bragg states; Intersubband transitions; Modulated resonant Raman scattering

The presence of Bragg re ectors on each side of a quantum well (QW) leads to a con nement of the energy levels above the barrier height. Two kinds of states exist: the rst group, called Bragg states, are highly localized in the QW region due to the Fabri– Perot eect of the re ectors while the second group forms mini-bands in the continuum that mainly extend over the re ectors. Despite several reports on above the barrier states in Bragg con ned structures none of these works provide direct evidence to the existence of two dierent classes of above the barrier states. ∗

Corresponding author. Fax: 972-4-8235107. E-mail address: [email protected] (M. Levy)

In a previous [1] work we provided experimental evidence to the existences of quasi-continuum and quasi-bound states in an asymmetric coupled QW (ACQW) structure. While quasi-continuum states are localized in the barrier region, quasi-bound states can be viewed as resonances of the QW where a signi cant fraction of the envelope wave function is localized in the ACQW region. Using the newly developed method of locally modulated resonant Raman spectroscopy (MRRS) we were able to resolve each group of states. In order to increase the con nement of the quasi-bound states and thus to increase the overlap with bound states we replaced the barriers with

1386-9477/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 9 9 ) 0 0 2 9 8 - 2

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Bragg re ectors. These quarter wavelength structures strongly reduce the envelope wave-function amplitude outside the ACQW and generate a truly bound state in the continuum, called Bragg levels [2,3]. The Bragg re ectors are composed of a nite superlattice (SL) designed so that the barriers (LB ) and wells (LW ) widths in the SL section equal the integer times the electron De-Broglie quarter-wavelength LW; B = (n × W; B )=4. Furthermore, the Bragg levels also obey the Bragg criterion where the ACQW width equals integer times the electron De-Broglie half-wavelength [4 – 6] LACQW = (n × ACQW )=2. In the present work we report a detailed investigation of the con nement of these Bragg levels in the ACQW region. By generating a localized DC electric eld in the ACQW region and using the MRRS technique we were able to resolve two classes of states. The rst class is a Bragg state con ned in the ACQW region and the second class is con ned in the re ectors region. The sample consists of 25 periods of a Bragg con ning structure (see Fig. 1) that is composed of an ACQW grown between two GaAs=Al0:34 Ga0:66 As Bragg mirrors. The ACQW is composed of a 7 nm wide GaAs QW (WQW), a 15 nm Al0:2 Ga0:8 As intermediate barrier and a 5 nm narrow GaAs QW (NQW). Each Bragg mirror is a 4 period SL where each period consists of 3 nm-GaAs QW and 9 nm-Al0:34 Ga0:66 As barrier. The sample was n-doped to the level of 2 × 1011 cm−2 in the Bragg con ning SL. The whole structure was capped with a 15 nm GaAs layer. The energy levels and the envelope wave functions were calculated by solving self-consistently the Ben Daniel–Duke Poisson equations for the conduction and the valence envelope functions [7,8] and are shown schematically in Fig. 1. The structure was designed so that the two lowest conduction states E1 and E2 are located in the WQW and NQW, respectively. The third conduction subband, E3 , extends over the entire ACQW and it’s energy is 115 meV above E1 . Fig. 1 also shows the Bragg con ned level EB and the two extended levels Eex1 and Eex2 in the re ector region. The method used to generate the local electric eld is also shown schematically in Fig. 1. Carriers located in the ground state (E1 ) of the WQW are resonantly excited by a CO2 laser to E3 . Some of these carriers loose their energy by phonon assisted relaxation processes and decay to E2 . Because of the thick barrier

Fig. 1. Schematic description of the energy levels in valence and conduction bands of Bragg con ned structure. Also shown is the intersubband excitation process and the charge carrier transfer from the WQW to the NQW. This charge transfer generates the static electric eld.

that separates these two levels the relaxation time for electrons in the second level is of the order of several hundreds of picoseconds. As a result in a steady state, there is a net charge transfer from the WQW to the NQW that induces a local electric eld across the ACQW. A detailed description of this mechanism including the relevant rate equations is given in Ref. [1]. Using infrared (IR) polarization-resolved absorption spectroscopy we measured the E1 : E3 transition energy to be 115 meV (see inset of Fig. 2) in good agreement with our calculations. Fig. 2 shows the PL spectrum (a) the PLE (b,c) of the bound states and the PL (d) and PLE (e) spectra of the continuum states. The PL in (a) shows three peaks E1 ; E2 and ESL at 1.57, 1.605 and 1.705 eV, respectively, which are identi ed as recombination of bound electrons and holes in the WQW, NQW and bound levels in the Bragg re ectors, respectively. The high-energy part of the PL (d) in Fig. 2 at 2.07 and 2.082 eV is related

M. Levy et al. / Physica E 7 (2000) 245–249

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Fig. 3. Raman spectrum of the Bragg con ned structure.

Fig. 2. PL and PLE of bound and above the barrier levels in the Bragg con ned structure. (a) PL of the three lowest bound subbands. (b) and (c) PLE of the bound levels monitored at the WQW and NQW respectively. (d) PL of above the barrier states in the re ector region. (e) PLE of above the barrier states monitored at the WQW. The inset shows the intersubband absorption spectrum.

to transitions from Eex1 and Eex2 , to valence continuum subbands. We monitored the PLE signal at 1.57 eV (WQW) (b) and 1.605 eV (NQW) (c). While the PLE is monitored at the WQW (1.57 eV), we observe a rise in the density of states at an energy of 1.67 eV that corresponds to the HH1 : E3 transition and is located 115 meV above E1 : HH1 as expected. This level around 1.67 eV is also seen when the PLE is monitored at 1.605 eV (i.e. the NQW energy). Our ndings indicate that, despite that the electron-hole pairs (HH1 : E3 ) are initially excited in the WQW, there is an ecient electron transfer to E2 in the NQW. This is due to the presence of the E3 electronic level that extend over both WQW and NQW as predicted by our model [1]. Hence, intersubband excitation at 10:6 m from our CO2 laser can generate a local dc electric eld across the ACQW. The continuum levels Eex1 and Eex2 are clearly resolved in the PLE spectrum (e) while the Bragg con ned level EB is weakly seen.

Next, the eect of locally modulated DC-electric eld on the above the barrier energy levels was investigated using MRRS. For that purpose we varied the dye laser photon energy in the 1.96 –2.16 eV range and by measuring the resonant Raman spectra at 10 K we probed the electronic levels in the continuum. Each electronic level is associated with two resonances in the RRS spectrum [9,10]. The rst resonance is achieved when the incident photon energy is close to an electronic transition EB while the second resonance is achieved when the photon energy is EL = EB + ˜!LO , where ˜!LO is the energy of the longitudinal (LO) phonon. The Raman spectrum has contributions from three dierent layers, GaAs, Al0:2 As0:8 As, and Al0:34 Ga0:66 As which constitute a single unit cell. Therefore, when recording a single Raman spectrum (Fig. 3), we observe the following phonon vibrations: 292 cm−1 (36.2 meV) from the GaAs layers, 285 cm−1 (35.3 meV) and 376 cm−1 (46.6 meV) which are the GaAs- and AlAs-like modes, of the Al0:2 As0:8 As layer, respectively, 280 cm−1 (34.7 meV) and 382 cm−1 (47.3 meV) are the GaAs- and AlAs-like modes of the Al0:34 As0:66 As layer, respectively. Fig. 4a and b show the RRS spectra from the GaAs and Al0:2 As0:8 As layers, respectively. The Bragg level at 2.52 eV denoted by EB is seen mainly

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M. Levy et al. / Physica E 7 (2000) 245–249 Table 1 The calculated values for the continuum states

Fig. 4. RRS of the above barrier levels with (solid squares) and without (open circles) IR excitation. (a) RRS from the GaAs layers, (b) RRS from the Al0:2 Ga0:8 As layers, (c) RRS from the Al0:2 Ga0:8 As layers.

in these spectra. Furthermore, under infrared excitation of 4kW=cm2 , this level is red shifted by 16 meV. The energy levels EEx1 at 2.07 eV and EEx2 at 2.082 eV were seen in the RRS spectra of all layers but had the largest amplitude in the GaAs layer (Fig. 4a) and Al0:34 As0:66 As layer (Fig. 4c). Under infrared illumination the level Eex1 at 2.07 eV has a very small shift (61 meV) which cannot be resolved in our experiments. The level Eex2 at 2.082 eV red shifts only by 4 meV. The out going beams of these two levels are found at one phonon energy distance above these levels and they are shifted exactly as the incoming beams. Near the continuum onset at 2.007 eV the RRS spectrum show a wide peak. This peak denoted Ec is strongly enhanced in the RRS spectra of the GaAs and Al0:2 As0:8 As layers, and is very weak in the RRS spectra of the Al0:34 As0:66 As layers.

Eex1

Eex2

EB

0.13

0.15

0.48

From the RRS results (Fig. 4) we see that under IR illumination the Bragg con ned EB red shifts by 16 meV while the re ector level Eex1 is shifted by less than 1 meV. These results can be explained by considering the localization of these states above the barrier levels in the ACQW region. The Bragg con ned state is highly localized in the ACQW region. Therefore, under the in uence of the local eld this level shows a relatively large shift. On the other hand, the extended levels EEx1 and EEx2 are localized mainly in the re ectors region and therefore, experience much smaller shifts than the Bragg con ned level. Our results indicate that the envelope wave function of the level EEx2 has a larger overlap with the ACQW and therefore, has a larger shift compared to EEx1 . In order to compare the degree of localization between the Bragg con ned level and the re ectors levels we de ned the con nement factor, , as follows: R | (z)|2 d z asymmetric well e R ; (1)

= | (z)|2 d z unit cell e where measures the degree of localization of the continuum levels in the ACQW region (where a unit cell is a sum of the ACQW and the SL widths). The calculated values of are summarized in Table 1.These values are in good agreement with our experimental results showing that the Bragg level EB is most localized while Eex1 is less localized state in the ACQW. The linewidths of the electronic levels: EB , Eex1 , Eex2 are found to be less than 6 meV, except for Ec which is found to have a line width of 13 meV. The linewidth of Ec suggests that this level is a mini-band formed by the interaction of the electronic wave functions with adjacent unit cells. In conclusion, by inserting Bragg re ectors in each side of an ACQW and by using modulated

M. Levy et al. / Physica E 7 (2000) 245–249

RRS as a probe we see two kinds of con ned states. The rst kind corresponds to a Bragg level that is con ned in the ACQW region. This level was only seen in the RRS pro les of the ACQW layers, and in addition it showed a relatively large red shift when a local static electric eld is generated in the ACQW region. The second kind of levels extends mainly in the re ectors region, show a very small shift under the same eld. A calculation of the probability to nd these states in the ACQW region is in agreement with our interpretation. Acknowledgements This research was supported by the Israel Science Foundation founded by the Israel Academy of Science and Humanities.

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References [1] M. Bendayan, R. Kapon, R. Beserman, A. Sa’ar, R. Planel, Phys. Rev. B 56 (1997) 9239. [2] M. Zahler, I. Brener, G. Lenz, J. Saltzman, E. Cohen, L. Pfeier, Appl. Phys. Lett. 61 (1992) 949. [3] M. Zahler, E. Cohen, J. Saltzman, E. Linder, E. Maayan, L. Pfeier, Phys. Rev. B 50 (1994) 5305. [4] F. Capasso, C. Sirtori, J. Faist, D.L. Sivco, S.G. Chu, A.Y. Cho, Nature 358 (1992) 565. [5] C. Sirtori, F. Capasso, J. Faist, D.L. Sivco, S.G. Chu, A.Y. Cho, Appl. Phys. Lett. 61 (1992) 898. [6] B. Sung, H.C. Chui, E.L. Martinet, J.S. Harris Jr., Appl. Phys. Lett. 68 (1996) 2720. [7] J. Wang, J.P. Leburton, J.E. Zucker, IEEE J. Quant. Electron 30 (1994) 989. [8] T. Worren, K.B. Ozanyan, O. Hunderi, F. Martelli, Phys. Rev. B 58 (1998) 3977. [9] J.E. Zucker, A. Pinczuk, D.S. Chemla, Phys. Rev. B 38 (1988) 4287. [10] J.E. Zucker, A. Pinczuk, D.S. Chemla, A. Gossard, W. Wiegmann, Phys. Rev. Lett. 51 (1983) 1293.