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Impact of varying barrier thickness on the optical characteristics of multilayer InAs/GaAs QDIPs

An investigation of the optical properties of the multi stacked InAs quantum dot (QD) based photodetectors has been done by changing the capping layer composition and thickness. There is an improvement obtained in the structure and distribution of InGaAs capped QDs than the conventional GaAs capped QDs. It is due to the inhibition of In-Ga intermixing and lesser indium segregation towards the wetting layer in case of InGaAs capping. Here, the combined InGaAs/GaAs capping layer thickness has been varied to investigate the effect of the vertical strain-coupling and QD size distribution. All samples are grown by solid source molecular beam epitaxy with a V/III flux ratio of 50. A variation in InGaAs/GaAs capping layer is done by keeping the total thickness constant at 12 nm, and 18 nm. The ground state photoluminescence emission peak for the 3 nm InGaAs capped QDs have pronounced redshift than the 2 nm InGaAs capped QDs. However, the redshift is more in case of total capping layer thickness of 12 nm (i. e. 36 nm), than the 18 nm capped sample (i. e. 14 nm). It is observed due to better coupling in case of lower capping layer thickness and hence better dot size. Activation energy calculated from the temperature dependent photoluminescence study also gives incremental trend with an increase in coupling (18nm: 163.308meV, and 12nm: 215.53meV), which is attributed to lowering of QD ground state due to change in capping layer thickness. Hence the 12nm capped device with 3nm InGaAs capping gives better results probably due to better strain propagation, and hence better dot distribution.