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We have studied the effects of AlGaAs energy barriers surrounding self-assembled InAs quantum dots in a GaAs matrix on the properties of solar cells made with multiple quantum dot layers in the active region of a photodiode. We have compared the fenced dot samples with conventional InAs/GaAs quantum dot solar cells and with GaAs reference cells. We have found that, contrary to theoretical predictions, the AlGaAs fence layers do not enhance the transport properties of photogenerated carriers but instead suppress the extraction of the carriers excited in the dots by light with wavelengths longer than the cutoff wavelength of the GaAs matrix material. Both the standard quantum dots and the fenced dots were found to give solar cell performance comparable to the GaAs reference cells for certain active region thicknesses but neither showed enhancement due to the longer wavelength absorption or improved carrier transport.

The formation and evolution process of self-assembled InGaAs quantum dot molecules (QDMs) are studied in terms of configuration, volume, and types of QDMs. QDMs are formed around self-assembled GaAs nanoscale island induced by adapting a hybrid growth approach combining droplet homoepitaxy and Stranski–Krastanov mode. In distinction from our previous results [Lee et al., Appl. Phys. Lett. 89, 202101 (2006)], hexa-QDMs are fabricated without the formation of background QDs, which can be due to a combinational effects of enhanced intermixing of Ga and In atoms, enhanced surface diffusion (high mobility) of adatoms, and higher In desorption rate due to the higher thermal energy provided during the fabrication of QDMs. In addition, a detailed evolution mechanism from bi-QDMs (two QDs per each GaAs island) to hexa-QDMs (six QDs per island) is proposed based on atom diffusion, material transfer, and equilibrium dimension (saturation) of QDs. Under a fixed InAs coverage, depending on postannealing process after ...