Photoluminescence Properties of Dynamic Quantum Dots Formed by Surface Acoustic Waves


Tetsuomi Sogawa, Haruki Sanada, and Hideki Gotoh
Optical Science Laboratory

 Surface acoustic waves (SAWs) provide a dynamic one- or two-dimensional lateral modulation of the band structure of quantum wells (QWs) [1, 2]. In this study, we investigate the dynamic optical properties of excitons in GaAs/AlAs moving dots (dynamic quantum dots, DQDs) formed by the interference of orthogonally propagating SAW beams. The SAWs induce a strain as well as a piezoelectric modulation of the materials properties. Due to the D4d symmetry of the underlying GaAs crystal, the interference of the two orthogonal SAW beams leads to the formation of two interpenetrating square arrays of DQDs [3], as shown in Fig. 1(a), where the in-plane components of the particle displacement field are shown schematically. One of the arrays (black and gray circles) consists of potential dynamic dots (p-DDs) created by the modulation of the piezoelectric potential. The second array (black and gray squares) is composed of strain dynamic dots (s-DDs), where the band gap becomes minimum or maximum due to the modulation of the hydrostatic strain.
 Spatially resolved photoluminescence (PL) measurement at 4 K was carried out by using a synchronized excitation method [1]. Figure 1(b) shows the PL mapping for a 6.3-nm QW recorded at a photon energy of 1.623 eV, which is located in the lower-energy side of the PL peak (centered at 1.630 eV in the absence of a SAW). In the PL polarization studies, we define the degree of polarization anisotropy ρ as the relative difference ρ = (PL[1-10] - PL[110]) / ( PL[1-10] + PL[110]) between the PL intensity emitted along the [1-10] (PL[1-10]) and [110] (PL[110]) propagation directions of the individual SAW beam. Figures 1 (b) and (c) clearly demonstrate the formation of the two DQD arrays. The strong and weak PL positions in Fig. 1(b) correspond to the tensile (black squares in Fig. 1(a)) and compressive (gray squares in Fig. 1(a)) s-DD positions, respectively. In contrast, the positive (negative) ρ areas in Fig. 1 (c) are located at the saddle point of the tensile s-DDs along the [1-10] ([110]) direction, denoted by a black (gray) circle in Figs. 1 (b) and (c). The positions with strong PL anisotropy correspond to the array of the p-DDs.

[1] T. Sogawa et al., Phys. Rev. B 80 (2009) 075304.
[2] T. Sogawa et al., Appl. Phys. Lett. 91 (2007) 141917.
[3] F. Alsina et al., Solid State Commun. 129 (2004) 453.

Fig. 1. (a) Schematic illustration of the in-plane components of the particle displacement field.
(b) PL mapping for the 6.3-nm QW recorded at a photon energy of 1.623 eV, which is
located in the lower-energy side of the PL peak. (c) Spatial distribution of the PL
anisotropy for 6.3-nm QW.

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