Modelling of dark current and noise dependence on capping thickness in quantum dots based infrared photodetectors

A detailed analysis of dark current and noise dependence on capping thickness in vertically coupled quaternary (InAlGaAs) capped InAs/GaAs quantum dot infrared photodetectors is presented. We are investigating the effect of varying capping thickness on device performance with theoretical proposed model. 2.7 ML InAs dots were grown with a combination capping of quaternary InAlGaAs layer (30Å) and GaAs capping thickness varying from 90-180Å (coupled Device A-C) to 500Å (Uncoupled Device D). Photoluminescence (PL) measurement 8 K exhibited multimodal ground state emission peak for device A to C whereas single emission peak was observed from device D. The measured activation energies for dominant peaks using PL were 236.63 meV, 188.92 meV, 164.88 meV and 151.25 meV respectively. The theoretical model gave activation energies of 234.57 meV, 148 meV, 131.69 meV and 144.56 meV respectively, which are on par with experimental values. The theoretical model had two components: tunneling component and the thermionic component. The thermionic component has an exponential dependence on activation energy, electric field and fitting parameter β. Similarly, tunneling component was dependent on electric field and other fitting parameters. Minimum dark current density was observed in device B. Similar trends were observed for noise spectral density and photoconductive gain. For thin capped devices (A-C), maximum photocurrent with narrow spectral response peak around 7 μm was observed. Device A measured highest responsivity of 0.85 A/W while device B measured highest detectivity of 2.48 × 10¹⁰ Jones.