The InAs quantum dots (QDs) with dot-in-well (DWELL) structure are preferable than the conventional InAs QD heterostructures because of the carrier funneling mechanism in the DWELL structure. There are few reports on the InAs DWELL quantum dot infrared photodetectors (QDIPs). However, a complete study on the optimization of the well structure and thickness is still missing in the literature. Here, we report the optimization of InAs DWELL heterostructure for superior structural and optical properties. We have simulated the DWELL heterostructures by varying the thickness of In0.15Ga0.85As well in both sides of the InAs QD. The symmetric DWELLs with 2/2, 4/4, 6/6, 8/8, and 10/10 nm InGaAs well are considered. For the asymmetric DWELL, the underlying well is kept fixed at 2 nm, whereas the upper well thickness is varied as 4, 6, 8, and 10 nm. A decrease (increase) in the hydrostatic (biaxial) strain is observed as the well thickness is increased in both symmetric and asymmetric DWELL structures. There is a redshift in the absorption peak with thicker wells, but a cutoff in the absorption coefficient value is obtained as the well thickness is increased beyond 6 nm in both cases. The probability density functions of the carriers in the case of 6/6 nm symmetric DWELL are high, which attributes to higher oscillator strength. Thus, the 2/6 nm asymmetric DWELL is the optimum one and hence the corresponding QDIP is grown. The photoluminescence result has good match with the simulated result and the QDIP showed a mid-wave infrared (MWIR) photoresponse.
This study is focused on structural and optical properties of multilayer InAs/InGaAs dot-in-a-well (DWELL) heterostructure with varying capping layer thickness. Two samples A and B are considered with 2 monolayer (ML) InAs quantum dots (QDs). The top InGaAs capping layer thickness is varied from 6 to 8 nm from sample A to B, whereas the lower well thickness was kept constant at 2 nm. The ground-state peak in the photoluminescence (PL) spectra shows a blue shift with increased capping layer thickness (sample A: 1148 nm, sample B: 1140 nm). This blue shift is due to the increased well layer thickness, which leads to higher strain and shrinkage in dot size. The activation energy (Ea) is calculated from the temperature dependent PL results using the Arrhenius equation. The activation energy of sample A and B are 181 meV and 152 meV respectively. The higher activation energy leads to a reduction in dark current, which affirms that sample A would be better for the device application. Raman spectroscopy is also carried out to observe different phonon peaks in both samples. For sample B, the broadening of the GaAs transverse optic (TO) mode is higher, which may lead to increased non-radiative carrier recombination. It may also lead to reduction of carrier lifetime. Hence, sample A with lower well thickness would be useful in optoelectronic device application because of its improved optical characteristics.