Effect of vertical coupling on the performance of mid-infrared InAs-QDIP

Strain-coupling in Quantum Dot Infrared Photodetector (QDIP) structures has been used as a strategy to achieve higher absorption efficiency and better device characteristics. Improvement in device characteristics due to the incorporation of strain-coupled QD layers has been divulged in this report. A 3/47 nm In_0.15Ga_0.85As/GaAs capped conventional uncoupled QDIP has been compared with QDIP having 3/12 nm In_0.15Ga_0.85As/GaAs capped strain-coupled QDs. The single pixel photodetector fabricated from the coupled structure has a lower dark current density (7.9×10^-4 A/cm²) compared to the uncoupled structure (12.17 A/cm²) at V_bias = -1 V and 300 K, which attributes a lower sensitivity to the thermalization effect in the former one. The strain-coupled QD heterostructure has photoluminescence peak at longer wavelength and lower full width at half maxima (24.86 nm), which indicates homogeneous dot size distribution. The surface chemical potential is less near the QDs due to the strain-relaxation. Hence, the lower lying dots forge the preferential nucleation sites for the upper QDs and it inhibits the inhomogeneous broadening occurs due to dot size fluctuation. The rocking-curve analysis from HRXRD measurement shows higher average strain in the strain-coupled QDIP (9.27×10^-4) compared to the uncoupled one (5.42×10^-4), which probably happens due to the accumulation of longitudinal strain from the lower QD layer towards the upped QD. The mid-infrared spectral response obtained from the strain-coupled QDIP has low spectral width.