AlGaAs coupled double quantum wells

Journal of Luminescence 130 (2010) 2437–2441

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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Optically induced level anticrossing in undoped GaAs/AlGaAs coupled double quantum wells Y.H. Shin a, Y.H. Park a, Yongmin Kim a,b,, Y. Shon c a

Department of Applied Physics, Dankook University, Yongin 448-701, Republic of Korea Quantum Dot Research Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0003, Japan c Quantum Functional Semiconductor Research Center, Dongguk University, Seoul 100-715, Republic of Korea b

a r t i c l e in fo

abstract

Article history: Received 18 March 2010 Received in revised form 2 July 2010 Accepted 4 August 2010 Available online 7 August 2010

We report a photoluminescence detected anticrossing of the energy levels in an undoped asymmetric coupled-double-quantum-well buried in a p-i-n structure. Due to the built-in electric field, the quantum wells are tilted in such a way that the symmetric energy level is higher than that of the antisymmetric one in the conduction band. Keeping the laser excitation energy below the barrier, with increasing laser power, the level anticrossing and the quantum confined Stark effect were observed due to decreasing built-in electric field by the photogenerated electron and hole pairs. & 2010 Elsevier B.V. All rights reserved.

Keywords: GaAs Quantum well Photoluminescence

In recent years, III–V compound semiconductor based quantum dots are a great deal of interest due to their quantum device applications. Applying electric fields across the quantum dots (QD) alter the electronic states and hence researches on the quantum confined Stark effect (QCSE) is increasingly important for the applications of QD based devices [1–5]. Coupled double quantum well (CDQW) systems are also ideal for a Bose–Einstein condensation of excitons [6,7] and metal to insulator transitions [8,9]. In a bulk semiconductor, with sufficiently high electric fields, excitons can be ionized by driving the electrons and holes in opposite directions. However, unlike a bulk semiconductor, application of electric fields across a quantum well cannot break apart excitons because the barrier prevent such field induced exciton ionization. These excitons confined in a quantum well which interact with electric field, diminish their binding energies similar to the atomic Stark effect [10]. CDQWs are ideal system to study QCSE. Applying electric field across a CDQW not only increases QCSE but also causes to tilt the entire quantum wells which provides interactions between the tunneling energy levels [11–17]. It can be expected that an indirect exciton (IX) which is consisted by a hole in one quantum well and an electron in the other well has stronger QCSE than a direct exciton (DX) formed in

 Corresponding author at: Department of Applied Physics, Dankook University, Yongin 448-701, Republic of Korea. Tel.: + 82 2 31 8005 3209; fax: + 82 2 31 8005 3208. E-mail address: [email protected] (Y. Kim).

0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.08.008

the same well because the IX undergoes larger spatial separation between electron and hole (e–h) pairs. The variation of external electric fields tune the energy levels which is strongly related with the QCSE in optical spectra. A CDQW has energetically close symmetric (ground state) and antisymmetric (excited) levels. Applying electric field tilts the CDQW as well as the energy levels in such a way that the ground level tend to push upward while the excited level moves to downward. Therefore, with sufficiently strong electric field, the antisymmetric level can be located below the symmetric level and becomes the ground state (see Fig. 1) [16,17]. Such electric field induced level anticrossing was observed from undoped asymmetric coupled double quantum wells (ACDQW) [15,16]. Due to the different e–h distances of the IX and the DX which provide the different strength of QCSE, it is easy to identify the level anticrossing by monitoring the transition energy of IX which varies rapidly with varying external electric field before and after the level anticrossing. Unlike an undoped quantum structure, for the case of n-type doped GaAs–AlGaAs single heterojunctions, when the photoexcitation energy exceeds the AlGaAs barrier that decreases the two-dimensional electron density in the quantum well due to ‘‘compensation of charge of the ionized donors in the AlGaAs layer by the photocreated electron–hole plasma’’ [18]. In the present work, we report excitation energy and power dependence of photoluminescence spectra of an undoped ACDQW sandwiched between a p-type capping and an n-type substrate wherein the quantum wells are tilted by built-in electric field. In such a structure, when it is illuminated with a laser, the photogenerated electrons and holes tend to move in opposite

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-

_ hν1

_ _ _ Ebg

IX

DX +

+

hν2 _ _

_ _

_

DX

IX

+ + + + +

Fig. 1. (a) Potential profiles of an ACDQW buried between p-type capping and n-type substrate. When the excitation energy exceeds the barrier, electrons and holes tend to move opposite direction. (b) Achieved flat-band due to opposite movement of electrons and holes. (c) Below the barrier excitation. (d) Charge separation occurs such a way that the conduction band electrons tend to stay in the narrow well whereas the valence holes stay in the wide well.

directions. The opposite movement of the photogenerated carriers cancels the built-in electric field and the flat-band structure is achieved immediately. Therefore, application of external electric field as well as illuminating external excitations are necessary in order to investigate the QCSE effects. In this work, we tried to investigate the QCSE by tuning the optical excitation energy and power without applying external bias voltage. The basic idea of the present work is that electric field can be generated and tuned by spatially separated e–h pairs within the wells which are generated by optical excitation below the barrier energy. We also calculated energy levels of the undoped ACDQW by using single particle calculations with varying electric fields across the quantum wells that matches quantitatively well with the experimental data. The sample used for this study is grown on a Si-doped n-type ˚ GaAs substrate that has two quantum wells, 150 and 100 A, separated by 30 A˚ Al0.3Ga0.7 As barrier. The sample was capped with a Be-doped p- type GaAs layer. Due to the p- and the n- type doping on the top and the bottom side of the wells, it has a builtin electrical field with an estimated value of 17.0 kV/cm. Fig. 1(a) depicts the electronic structures of the sample used for this study. To measure the excitation energy dependent PL, we used Ar + laser

(514.5 nm) and a Ti:S laser (708 nm). PL measurements were performed at 2 K base temperature using 4He exchange system. The data were recorded using a f/4, 0.3 m spectrograph equipped with an 1200 line/mm grating and a liquid nitrogen cooled charge coupled device (CCD) detector. We also calculated the transition energy with respect to electric fields across the quantum wells to compare with experimental results. In a symmetric CDQW, the symmetric and the antisymmetric levels have equal electron distribution in each well. However, due to an asymmetricity, an ACDQW has asymmetric charge distribution in each well such a way that electrons tend to stay mostly in wide well in the ground state (symmetric level) and stay mostly in narrow well in the excited state (asymmetric level). For a tilted ACDQW as described in Fig. 1(a), when the built-in electric field is strong enough, the asymmetric level can be a ground state. When the excitation photon energy exceeds the AlGaAs energy barrier ðhn 4 Ebg Þ, photogenerated electrons and holes tend to move to opposite direction which lead to cancel the built-in electric fields. Consequently, the tilted band (Fig. 1(a)) becomes the flat-band structure as depicted in Fig. 1(b). To generate electron–hole pairs in the Al0.3Ga0.7As barrier (1.893 eV) region, a 514.5 nm (2.406 eV) Ar + laser line was

Y.H. Shin et al. / Journal of Luminescence 130 (2010) 2437–2441

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3.0x105 L

U

2.5 Areal Intensity

DX

300x103 top 1238.0 mW/cm2

250

L 2.0 1.5 1.0

955.0 0.5

601.3 530.6

0.0 0

339.6

400

800

1200

Power Density (mW/cm2)

214.0

PL Intensity (a.u.)

200

158.1 107.5

U

42.1

IX

L U

1200

6.7

150

1400

bottom

1000 800 600

100 400 1528 1530 1532 1534 1536

50

0 1529

1531

1533

1535

Energy (meV) Fig. 2. (a) PL spectra with respect to the excitation power density with the 514.5 nm Ar + laser. (b) Even at very low excitation power density (4.5 mW/cm2), The transition energy of IX is located above the DX. (c) Integrated intensity vs power density plot. The DX transition intensities are bigger than the IX.

illuminated as photoexcitation. Experimental results in Fig. 2(a) and (b) clearly show doublet emission labeled as U and L which correspond to energetically upper and lower, respectively. These two peaks emerge even with a very low laser power density (4.5 mW/cm2 in (b)) and show monotonic increase with increasing excitation power density (Fig. 2(a)). Power density with respect to integrated intensity is plotted in Fig. 2(c) wherein the L peak intensity grows faster than the U peak with increasing power density. It indicates that within the experimental power density limit ð Z 4:5 mW=cm2 Þ, the tilted band immediately turns to the flat-band and as a consequence, the symmetric DX state becomes the ground state (L). Exciting the electrons below the barrier (or within the wells) gives quite different experimental results from the excitation energy above the barrier. The PL transitions using the 708 nm (1.744 eV) Ti:Sa laser line of which its energy lies below the barrier (1.893 eV) are displayed in Fig. 3. With the lowest laser power density (3.4 mW/cm2), two peaks (Uu and Lu) are resolved (Fig. 3(a)). With increasing power density, these Uu and Lu peaks energetically get close and Lu disappears at  6:4 mW=cm2 . With

further increasing power density in Fig. 3(b), a new peak labeled as L emerges. The dotted and dashed lines which guide for the eye, show developments of Lu (L) and Uu (U) peaks, respectively. By summarizing these results, Lu and Uu peak energies merge whereas L and U peak energies move away with increasing power density. Between 6.4 and 29.0 mW/cm2 range, Lu and Uu peaks are barely resolved and undergo energetic anticrossing. Therefore, it is concluded that before the anticrossing Lu was a transition from the IX which becomes L (DX) after the anticrossing and the same way, Uu (DX) becomes U (IX) peak. For the transition intensity behaviors in Fig. 3 inset, U peak (IX) has bigger intensity and grows faster than L peak. This is due to the fact that the photogenerated conduction electrons tend to stay in the narrow well while the valence holes stay in the wide well which increases the formation of IX than DX. For the clear examination of the development of U and L peaks, transition energy with respect to the excitation power density is plotted in Fig. 4 wherein the excitation power dependence of the level anticrossing is clearly seen. In the low power density region below 10 mW/cm2 in the box region of Fig. 4(a),

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6 1.0x10

U

2.5x105

U

0.8

2.0

Areal Intensity

L

L 0.6

0.4

1.5

PL Intensity (arb. unit)

0.2

1.0

400

800 2 Power Density (mW/cm )

1200

top

0.5 565.9 (mW/cm2) 275.9

0.0 4

2.0x10

180.4 127.3 108.6

1.5

1.0

0.5

77.5

6.4

41.4

4.9

29.0

3.7

11.7

3.4

6.8

0.0

bottom

L'

1527

1528

U'

1529

1530

1531

1532

1533

Energy (eV) Fig. 3. (a) Low power PL spectra with the 708 nm excitation which lies below the barrier. Two peaks, Uu and Lu are detected. (b) Anticrossing of Uu and Lu peaks turn into U and L with high power density. (c) Areal intensity vs power density plot. Above the anticrossing, the IX peak intensity is bigger than the DX.

the IX and the DX transitions labeled as Lu and Uu, respectively, in Fig. 3 anticross with increasing power density around 5.0 mW/ cm2. With further increasing power density, above the anticrossing range, the DX becomes the IX and vice versa. One thing of note is that the IX transition energy changes faster than that of the DX transition before and after the anticrossing. This is mainly due to the consequence of quantum confined stark effect (QCSE). Little et al. reported that photocreated e–h pairs significantly diminish its energy due to the QCSE in ACDQW [13]. In a ACDQW, since the IX undergoes more spatial distance separation than the DX, the QCSE induced energy change has to be more significant for the IX transition than for the DX transition. Fig. 4(b) exhibits the expand view for the box region in Fig. 4(a) along with calculated results (circular markers) relative to the mid-point of anticrossing energy. The anticrossing occurs at 4.8 mW/cm2 which corresponds to the electric field of 8.2 kV/ cm across the quantum wells. Increasing the excitation power density (top axis) increases the spatially separated IX density

that is the same effect as decreasing the built-in electric field across the quantum wells (bottom axis). Increasing power density within 3:0 mW=cm2  6:5 mW=cm2 range quantitatively well matches with decreasing bias voltage from 8.9 kV/cm to 7.5 kV/cm. In conclusion, we have measured excitation energy and power density dependence of the photoluminescence transitions of an undoped ACDQW sandwiched between p-type capping and n-type substrate. Above the barrier excitation (514 nm) within our experimental limit (laser power density Z4:5 mW=cm2 ), only the flat band behavior was observed because photogenerated electrons and holes push to opposite direction which canceled the built-in field. On the other hand, below the barrier excitation (708 nm), electrons and holes are generated only within the wells. In this case, the effect of canceling the built-in field is much weaker and as a consequence the energy level anticrossing was detected by tuning the excitation power density. We also found that the quantum confined Stark effect (QCSE) is enhanced with decreasing excitation power density for the IX transition before

Y.H. Shin et al. / Journal of Luminescence 130 (2010) 2437–2441

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Acknowledgments Excitation 708 nm

This work was supported by Mid-career Researcher Program through NRF Grant funded by the MEST (2009-0058372).

Energy (eV)

1532 IX (U)

1531

References

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DX (L)

DX (U')

1529 IX (L')

1528 2

3

4 5 6 78

2

3

4 5 6 78

2

10 100 Laser Power Density (mW/cm2) 3.5

4.0

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Relative Energy (meV)

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 8.8

8.6

8.4 8.2 8.0 Applied Voltage (kV/cm)

7.8

7.6

Fig. 4. (a) Excitation power density vs peak energy position plot (below the barrier excitation, 708 nm). At low power density within the box, a level anticrossing occurs (thick arrow) where IX turns to DX and vice versa. (b) Expand view for the box region in (a). y-axis is relative energy of the crossing point. Circular markers with dashed lines indicate the single particle calculation for the DX and the IX transitions.

and after the anticrossing. We confirm that these experimental results quantitatively in good agreement with the theoretical calculations.

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