Study on photovoltaic effect of quantum nanostructure grown on (100) GaAs substrate

Abstract

This thesis presents a study on the photovoltaic effect of quantum nanostructures grown on (100) GaAs substrate. The nanostructures in this work are InAs high-density-quantum-dot-molecule (HD-QDM) and InGaAs quantum ring (QR). The fabrication techniques are Stranski-Krastanov growth mode for HD-QDM and droplet epitaxy for InGaAs quantum ring (QR). A molecular beam epitaxy system is used as the fabrication tool. The stacking number of HD-QDM structure is optimized by I-V characteristic. Then, the selected structure is compared with a conventional QD structure in optical and electrical properties. Photoluminescence shows that HD-QDM structure gives a higher energy peak position than QD structure. It also gives a broader spectral response than QD structure. However, it cannot be concluded that HD-QDM has a better electrical properties than QD structure. In the case of QR structure, GaAs/AlGaAs QRs have been fabricated with various growth conditions in order to investigate the effect of fabrication parameters on the structural properties. Then, the selected growth condition is used to obtain the highest density and largest size nanostructures for InGaAs QR structure. In order to increase the density, multi-stack InGaAs QRs are used to increase the density of InGaAs QRs. The large QR size gives PL spectrum a clearer and lower energy peak than the other sample with higher density. However, the large QRs do not give an extra spectral response when compared with a non-QR reference sample, whereas the other sample does. The QR structure also gives a better I-V characteristic than the reference sample.