Systematic investigation into the influence of growth conditions on InAs/GaAs quantum dot densities and sizes for intermediate band solar cell application

Abstract

The influence of the conditions during growth of InAs/GaAs quantum-dot structureson GaAs(001) by molecular-beam epitaxy was investigated systematically with respect to achieving high quantum dot densities, homogeneity and narrow sizes distribution. These are requirements for obtaining the necessary intermediate band in intermediate band solar cells. The growth temperature, InAs deposit, As flux, growth rate and III/V flux ratio were varied. Atomic force microscopy and a modular program for SPM data analysis were used to study the morphological properties of the QDs. The effect of several operations during image processing was analysed and a suitable routine was defined. The optimal temperature for the growth was found to be 480◦C although 420◦C was sufficient to induce 2D-3D transition. Varying the InAs thickness, a self-limiting size effect was detected at 2.2 ML, while increasing As-flux to 6x10−6 Torr led to big clasters formation. Finally, low growth rate (0.1-0.5 ML/s) was necessary to keep good homogeneity and the optimal III/V flux ratio was found to be in the range 1/10 - 1/5.