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Photoluminescence Study of the GaAs Barrier Effect on GaAs/GaInAs/GaAs Quantum Wells

A. Bardaoui, N. Ben Sedrine, J.C. Harmand*, and R. Chtourou Laboratoire de Photovoltaïque et de Semiconducteurs, Centre de Recherche et de Technologie

de l’Energie, BP. 95, Hammam-Lif 2050, Tunisia * Laboratoire de Photonique et de Nanostructures, CNRS Route de Nozay 91 460, Marcoussis, France

Tel: (00216) 21 898 230, Fax: (00216) 71 430 934, e-mail: afrah.bardaoui@crten.rnrt.tn

ABSTRACT In this work we propose a photoluminescence (PL) study of a GaAs/GaInAs/GaAs quantum well (QW) sandwiched between two GaAs0.95N0.05 layers grown on GaAs substrate by Molecular Beam Epitaxy (MBE). This structure is used as an optical switch in the telecommunication application. The effect of the GaAs barrier thickness (d) between the GaAs/GaInAs/GaAs QW and GaAsN plan was investigated for three samples with d = 25, 40 and 100 A°. We have found that at low temperature the PL spectra are essentially composed of a wide band and two sharp structures. We have attributed the wide band to the deep localized state due to the three-dimensional mode growth of the GaAsN layer at low temperature, and the two sharp structures to the fundamental states of GaInAs QW and GaAsN layers. The dependence of the energy shift with the GaAs barrier width of the two structures is explained in the frame of the coupling of the two states by the covering of their wave functions. Keywords: GaInAs, GaAsN, quantum well, photoluminescence, semiconductors, optoelectronic device.

1. INTRODUCTION The need for ultrafast all-optical communication networks is becoming more and more urgent and the communication traffic is growing faster each year. The study of III–V semiconductors alloys, especially GaAs1-xNx and Ga1-yInyAs has been increasing in the last few years due to their potential for long wavelength optoelectronics applications [1-4]. Recently, GaInAs based quantum wells have been gained so much attention based on their broad applications in devices such as ultrafast optical switches [5]. The GaInAs/GaAs material system is considered to be the most promising material among several attractive candidates for long wavelength systems and is of great interest [6, 7]. On the other hand, III-V-N alloys have been extensively studied for their well known potential applications in optoelectronic devices. The nitrogen incorporation into III-V semiconductors have numerous effects such as a reduction in the fundamental bandgap [8], an increase in the electron effective mass [9] and a significant decrease in the electron mobility [10]. Electric transport measurements have also showed that nitrogen significantly degrades the transport properties which are related to nitrogen localized states.

In this work, a photoluminescence (PL) study was performed on a set of GaAs/GaInAs/GaAs QW sandwiched between two GaAs0.95N0.05 layers grown on GaAs substrate by Molecular Beam Epitaxy (MBE). The effect of the GaAs barrier thickness between GaInAs QW and the GaAsN layers was investigated for three samples with d = 25, 40 and 100 A°. The GaAsN layers will support the depopulation of the GaInAs QW state which will involve a strong decrease of the lifetime of GaInAs QW carriers.

2. EXPERIMENT As shown in figure 1, the samples are composed of GaAs/GaInAs/GaAs QW, with 25% of indium content, sandwiched between two GaAs0.95N0.05 layers followed by two spacer layers: GaAs and AlGaAs and a 5 nm GaAs cap layer that completes the structure. The samples were grown in a solid-source MBE system on (001) GaAs oriented substrates using a Riber system equipped with solid source for Ga and As elements, and with an Addon radio-frequency (RF) plasma source for nitrogen incorporation. The growth temperature of the GaAs and GaAlAs layers was around 560 °C. For the GaAsN layers, the growth temperature was decreased to 400-450 °C to avoid phase separation. The set of the studied samples is composed of three identical structures but with different GaAs barrier thickness. Samples (a), (b), and (c) have respectively 100, 40 and 25 A° barrier thickness.

The photoluminescence measurements were performed using a variable temperature (10 K – 300 K) close-cycle cryostat under 514.5 nm line of an Argon ion (Ar+) laser as excitation source. The signal was detected through a 250 mm Jobin-Yvon monochromator and by InGaAs photodiode.

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Figure 1. (a) Sample structure and (b) band alignment of the GaAs/InGaAs/GaAs quantum well.

3. RESULTS AND DISCUSSION At low temperature, PL spectra of the three samples are composed essentially of a wide band at around 0.83 eV and two sharp structures labelled P1and P2 (Fig. 2).

Figure 2. 10K photoluminescence spectra of samples (a), (b) and (c).

We attribute the wide band to the deep localized states induced by the three dimensional GaAsN growth mode at low temperature. The same effect was observed in GaAs at low temperature growth (LTG) [11, 12]. We also note a decrease of the wide band and the P1 peak intensities with the GaAs barrier thickness. This behaviour confirms that the escape of the carriers from the GaInAs QW via the GaAsN layers is governed by the GaAs barriers.

The two peaks P1 and P2, shown in figure 2, correspond respectively to the fundamental states of GaInAs QW and that of GaAsN layers, they are in accordance with our theoretical calculations. We note that when the GaAs barrier thickness decreases, the energy shift ∆E between these two peaks increases. In table 1, we report the energy shift values for the three samples.

The observed behaviour is caused by the interaction between the two states of GaInAs QW and GaAsN layers. In fact, the coupling between the GaInAs QW and the GaAsN layers states is proportional to the over-lap integral of their wave functions. Thus by decreasing the GaAs barrier thickness, the coupling effect becomes more important and the splitting of the two structures becomes clearer.

Buffer GaAsGaAlAs

GaAs

GaAsN [N]=5%

GaAs (d)

InGaAs [In]=25% (8nm)

GaAs (d)

GaAsN [N]=5%

GaAs

GaAs Cap layer (5nm)

GaAlAs

GaAs Substrate

0.6 0.8 1.0 1.2 1.4 1.6

0.0

0.4

0.8

1.2

1.6

P2

P1

P2

P2

P1

(c)

(b)

(a)

T=10KP=10W/cm2

PL

Inte

nsity

(a. u

)

Energy (eV)

P1

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Table 1.Comparaison of the energy shift of the peaks P1 and P2 for the three samples.

Samples (a) (b) (c)

Barrier thickness d (A°) 100 40 25

∆E (meV) 23 36 331

Figure 3 shows the temperature dependence of the integrated PL intensity as a function of the reciprocal temperature from 10 to 300K range for a laser power excitation P = 10 W/cm2. The experimental data are fitted by the following equation [13]:

0( )3 3

2 21 exp1 2

II T

EtT Tk TB

α α

=⎛ ⎞⎜ ⎟+ + −⎜ ⎟⎝ ⎠

(1)

where I0 is a proportionality constant, Et the thermal activation energy, kB the Boltzmann constant and α1 and α2 are the fitting parameters associated with the temperature dependence of the capture cross-sections of the nitrogen localized states. Figure 3. Temperature dependence of the integrated PL intensity of the 0.83 eV wide band for the three samples.

The best fit using equation 1 has been achieved with the following parameters reported in table 2. Table 2. Fitting parameters obtained for the wide band around 0.83 eV for the three samples.

Samples Activation energy (meV) α1 α2

(a) 34 26 10-4 12.7 10-2

(b) 42 2.5 10-3 10 10-2

(c) 59 16 10-4 63 10-2

We notice that the activation energy increases with the GaAs barrier thickness. We have also studied the variation of the integrated PL intensity of the 0.83 eV wide band versus the excitation laser intensity at T = 10 K (Fig. 4). The experimental data can be fitted by the simple power law:

I Lβ∝ (2) where I is the PL intensity, L is the excitation laser intensity and β is a dimensionless exponent. β is generally 1 < β < 2 for the free- and bound-exciton emission, and β < 1 for free-to-bound and donor–acceptor pair recombination [14].

We have found that the exponent β decreases when the GaAs barrier thickness decreases. This effect emphasizes that the carrier transfer is easier in the case of a small GaAs barrier thickness.

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

1E-4

1E-3

PL In

tens

ity (a

. u.)

1/T (K-1)

Sample (a) Sample (b) Sample (c)

P=10W/cm2

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Figure 4. Excitation dependence of the integrated PL intensity of the 0.83 eV wide band for the three samples.

4. CONCLUSIONS Photoluminescence study of GaAs/GaInAs/GaAs quantum well sandwiched between two GaAs0.95N0.05 layers shows that the escape of the carriers from the GaInAs QW to GaAsN layers is governed by the GaAs layers thicknesses. This effect is interpreted by the coupling of GaInAs QW to GaAsN layers states which becomes more important when the GaAs barrier thickness is decreased.

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alloys, Phys. Stat. Sol. (b) vol. 228, pp. 273-277, 2001. [2] W.G. Bi, C.W. Tu: Bowing parameter of the band-gap energy of GaNxAs1 – x, Appl. Phys. Lett., vol. 70,

pp. 1608-1610, 1997. [3] J. S. Harris, Jr.: GaInNAs, a new material for long wavelength VCSELs, Journal of the Korean Physical

Society, vol. 39, pp. S306-S312, 2001. [4] R. Chtourou et al.: Effect of nitrogen and temperature on the electronic band structure of GaAs1-xNx alloys,

Appl. Phys. Lett., vol. 80, pp. 2075-2077, 2002. [5] Osamu Wada: Femtosecond all-optical devices for ultrafast communication and signal processing, New

Journal of Physics 6 183, 2004. [6] H. Q. Ni, et al.: High-indium-content InxGa1–xAs/GaAs quantum wells with emission wavelengths above

1.25 µm at room temperature, Appl. Phys. Lett,. vol. 84, pp. 5100-5102, 2004. [7] B.F. Levine : Quantum-well infrared photodetectors, J. Appl. Phys., vol. 74, pp. R1-R81, 1993. [8] K. Uesugi, N. Morooka, and I. Suemune: Reexamination of N composition dependence of coherently

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[9] C. Skierbiszewski, et al.: Large, nitrogen-induced increase of the electron effective mass in InyGa1–yNxAs1–

x, Appl. Phys. Lett., vol. 76, pp. 2409-2411, 2000. [10] S. R. Kurtz, et al.: Minority carrier diffusion, defects, and localization in InGaAsN, with 2% nitrogen,

Appl. Phys. Lett. Vol. 77. pp. 400-402, 2000 [11] R.E. Viturro, et al. : Optical emission properties of semi-insulating GaAs grown at low temperatures by

molecular beam epitaxy, Appl. Phys. Lett., vol. 60, pp. 3007-3009, 1992. [12] N. Ben Sedrine, et al. : Deep Defects Annihilation in GaAs1-xNx Layers by Si-doping, American Journal

of Applied Sciences 4 (1), pp. 19-22, 2007. [13] J. Krustok, H. Collan, K. Hjelt, Does the low-temperature Arrhenius plot of the photoluminescence

intensity in CdTe point towards an erroneous activation energy?, J. Appl. Phys., vol. 81, pp.1442, 1997. [14] N. M. Gasanly, A. Aydinli and N. S. Yuksek: Temperature- and excitation intensity-dependent

photoluminescence in TlInSeS single crystals, J. Phys.: Condens. Matter, vol. 14, pp. 13685–13692, 2002.

1E-3

Sample (a)Sample (b)Sample (c)

PL

Inte

nsity

(a. u

.)

Excitation Intensity (W/cm2)

β=0.47874

β=0.77539

β=0.77719

1

T=10K