Effect of varying capping composition and number of strain-coupled stacks on In0.5Ga0.5As quantum dot infrared photodetectors

In this paper, we have reported the optical and electrical properties of strain coupled multi-stack quantum dot infrared photodetectors (QDIPs) of In0.5Ga0.5As dots with different capping compositions. Bilayer, trilayer, pentalayer and heptalayer coupled QDIPs are grown by solid source molecular beam epitaxy with one set of samples containing conventional GaAs capping (12nm) and second set containing a combinational capping of In0.15Ga0.85As (3nm) and GaAs (9nm) layers with same total thickness. The entire set of strain coupled quantum dots (QDs) shows a red shift in ground state photoluminescence peak in comparison to the uncoupled structures. Due to the reduction in indium interdiffusion from In0.5Ga0.5As dots in the combinational capped structures, a higher redshift is observed compared to the GaAs capped structures, which attributes larger dot size in the former ones. Full width half maximum value (FWHM) of In0.15Ga0.85As/GaAs capped QDs are lower, showing uniform distribution of dot size compared to the corresponding GaAs capped QDs. Trilayer sample with In0.15Ga0.85As/GaAs capping shows the best result in terms of the peak emission wavelength of 1177nm, FWHM of 15.67nm and activation energy of 339meV compared to all the structures. Trilayer sample seems to be the optimum stacking having the best confinement resulting lower dark current density of 6.5E-8 A/cm(2) measured at 100K. The sample also shows a multicolor response at similar to 4.89 mu m and at similar to 7.08 mu m in the mid infrared range. Further optimization of the spacer thickness and dot layer deposition can improve the response towards the long infrared range.