Optical Properties of GaAs Quantum Dots Formed in (Al,Ga)As Nanowires


Haruki Sanada, Hideki Gotoh, Kouta Tateno, Tetsuomi Sogawa, and Hidetoshi Nakano
Optical Science Laboratory

 Semiconductor nanowires (NWs) have been extensively investigated as fascinating candidates for nanoscale photonics and electronics applications. The vapor-liquid-solid (VLS) method is a technique for fabricating freestanding NWs with a broad range of semiconductor materials. The method not only produces one-dimensional structures, but also offers additional flexibility as regards band structure engineering by employing a conventional heterostructure technology. Here we report the results of an experimental photoluminescence (PL) measurement undertaken to clarify the optical properties of single GaAs quantum dots (QDs) formed in (Al,Ga)As NWs grown by the VLS method [1].
 We fabricated GaAs QDs in (Al,Ga)As NW by the following two steps: (1) VLS growth of the NWs that have GaAs/(Al,Ga)As heterojunctions; and (2) normal MOVPE for covering the NWs with (Al,Ga)As cap layers, which suppress the non-radiative surface recombination of photoexcited carriers [2]. Figure 1 shows a cross-sectional SEM image of the similar structure composed of a GaAs/AlAs system instead of a GaAs/(Al,Ga)As system. The image shows that several QDs have formed inside the NW.
 In Fig. 2, we compare the PL spectra with different excitation powers (Pexc) to examine the characteristics of the two peaks labeled by A and B. For Pexc < 100 W/cm2, the integrated intensities of peak A and B have linear dependence with Pexc and Pexc2, respectively, which is a typical behavior of exciton and biexciton emissions. In addition, the biexciton peak (peak B) energy exhibits a red shift and its linewidth broadens as Pexc increases. This might be a characteristic of QDs in NW structures grown by the VLS method because similar features have been reported in VLS-based InAs/GaAs and Ga(As,P)/GaP QDs systems. We also found that the PL depends on the optical polarization axis, indicating that the nanostructures have a highly asymmetrical shape. Although a more explicit consideration of their structural configurations is required to clarify the mechanism, our method is a promising way of engineering the positions and optical properties of GaAs/(Al,Ga)As nanostructures.

[1] H. Sanada, H. Gotoh, K. Tateno, and H. Nakano, Jpn. J. Appl. Phys. 46 (2007) 2578.
[2] K. Tateno, H. Gotoh, and Y. Watanabe, Appl. Phys. Lett. 85 (2004) 1808.

Fig.1. Cross-sectional SEM image of dot-in-wire structure composed of a GaAs/AlAs system.
Fig.2. Excitation power dependence of the PL spectra measured at 4 K.

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