The improvement in performance of In0.5Ga0.5As Quantum Dot Infrared Photodetector (QDIP) has been investigated by introducing Al in the In0.15Ga0.85As capping layer. The QDIPs are consist of ten uncoupled In0.5Ga0.5As dot layers with 3/47 nm Inx(Aly(~x))Ga1-x-yAs/GaAs capping. The monolayer coverage for both QDIPs is 6, which accommodate an overgrowth percentage of 59%. A FWHM (46.59 nm) and higher activation energy (267 meV) has been obtained for the ground state photoluminescence emission due to the incorporation of Al in the InGaAs capping layer. This indicates better carrier confinement and homogeneous dot size distribution in the quaternary (In0.21Al0.21Ga0.58As) capped QD structure with respect to the ternary (In0.15Ga0.85As) capped QD. A six order reduction in dark current density has been obtained in the InGaAs QDIP due to the incorporation of Al in the capping layer. The narrow spectral width of 0.07 for the transition peak at 7.8 μm represents the homogeneous dot size distribution in the InAlGaAs/GaAs capped QDIP heterostructure.
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 In0.15Ga0.85As/GaAs capped conventional uncoupled QDIP has been compared with QDIP having 3/12 nm In0.15Ga0.85As/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 Vbias = -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.
Quantum dot infrared photodetectors (QDIPs) with different dot materials have been investigated in this study to analyze the optical, structural and electrical behavior. The InAs and In0.5Ga0.5As QDIPs comprise ten vertically-stacked uncoupled quantum dot (QD) layers with In0.15Ga0.85As/GaAs capping, whereas the overgrowth percentage in both the dot materials has been kept similar (~59%). The InGaAs QDIP has a red shifted photoluminescence spectra compared to the InAs QDIP along with a lower full width at half maxima (FWHM) and higher activation energy. This attributes the formation of dots with larger size and higher vertical barrier potential in the InGaAs QDIP heterostructure. The lattice mismatch between the dot and its underlying/capping layer is less in the InGaAs QDs, which has been observed from the HRXRD rocking-curve analysis. The average strain obtained in the InGaAs QD is less compared to the InAs QD. Moreover, a reduced dark current density has been obtained in the InGaAs QDIP compared to the InAs QDIP at room temperature. Both the QDIPs have their spectral response in the mid-infrared range. However, the InGaAs QDIP has peaks with lower FWHM due to minimized dot size dispersion in the structure.
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