Optimization of vertical strain coupling in InAs/GaAs p-i-p quantum dot infrared photodetectors with applied growth strategy

The authors report the optimization of vertically coupled active layers of InAs quantum dots (QDs) grown epitaxially following a unique in-situ growth strategy to achieve controlled strain propagation in subsequent layers resulting in homogenous dot-size distribution. The analysis comprises of bi, tri, penta, hepta and deca coupled layers which have been characterized for their optical properties using photoluminescence spectroscopy (PL) and photoluminescence excitation spectroscopy measurements (PLE) while high resolution X-ray diffraction (HR-XRD) measurements (in-plane and out-of-plane) have been carried out for structural characterization. A gradual shift from bi-modal to monomodal distribution is observed on increasing the coupled layers above five. Activation energies (E-a) calculated from temperature dependent PL results show least thermal quenching for hepta layer coupling which is further corroborated by the strongest room temperature luminescence obtained for the respective sample amongst others. Least out-of-plane compressive strain and also least in-plane tensile strain have been calculated from the HR-XRD results for the hepta layer coupled sample. Furthermore, the in-plane HR-XRD results show minimal amount of In-Ga intermixing for the hepta layer coupling which provides direct evidence of the superior material quality of this heterostructure. Quantum-dot infrared photodetectors (QDIPs) fabricated from the samples have been characterized for their spectral response wherein multiple sharp peaks with narrow full-width at half maxima (FWHM) are obtained in the short-wave infrared (SWIR) region for the hepta layer device, which is absent in other spectra. Also, the response survives up to room temperature (300 K) and stretches in the mid-wave infrared region (MWIR) region (similar to 5 mu m). High peak responsivity (R-p = 120.34 A/W) is obtained for the hepta layer device which is much higher than previously reported responsivity values.