Quantum dots based devices suffer certain disadvantages like low quantum efficiency, smaller absorption co-efficient and high dark current. Dot-in-a- well heterostructures (DWELL) offers tuning of detection peak wavelength, low dark current and higher operating temperature with optimized quantum well thickness. In this report, we correlate the optoelectronics properties of 3 different DWELL structures namely samples A, B and C having SRL thickness of 4, 6 and 8 nm, respectively with concentration-dependent theoretical model. A blue shift of around 24 nm with a decrease in PL intensity is observed as the capping thickness increases above 6 nm. Full width at Half Maximum (FWHM) decreases from B to C. These are attributed to the presence of large number of defect states and the formation of InGaAs wells with dissolution of dots in C. Low temperature PL measurement at 2.54W/cm2 and Photoluminescence Excitation (PLE) spectrum validate the presence of InGaAs wells peak at 1094.4nm for C. All samples exhibited peak spectral response at 7.56 μm. A concentration-dependent theoretical model using the Schrödinger equation was developed to calculate ground-state and inter-sub band energy-levels. The developed model shows great agreement with experimentally observed peaks from PL, PLE and spectral response. The same model was used to calculate the energy levels for InGaAs well. Based on the InGaAs experimental peak at 1.134 eV, average In concentration in the well was calculated to be around 30%.
Quantum dot(QD) based devices are capped with various strain reducing layers in order to improve quality of dots by low surface diffusion and increase detection to long wavelength infrared region(LWIR). We present a model for the effect of various strain reducing layers on quantum dot heterostructures and study the corresponding variation in optoelectronic properties viz. photoluminescence (PL), photoluminescence excitation (PLE) and device characteristics of the samples. Schrödinger equation was used in the concentration dependent model in order to calculate ground state and inter-sub band energy-levels. Three InAs QD (2.7 ML) samples with different capping GaAs (Sample A), 6 nm In0.15Ga0.85As (Sample B) and 6 nm In0.15Ga0.85As DWELL (Sample C with 2 nm pseudomorphic layer of In0.15Ga0.85As) were grown. Low temperature (8 K) PL spectra exhibits ground state peak at 1112.62, 1150 and 1166.93 nm for samples A, B and C, respectively. PLE measured at 8 K exhibited first and second excited state peaks at 1046.15 nm, 991.59 nm for Sample A, 1079nm and 1003nm for Sample B and 1095.55nm and 1034.51nm for Sample C. Highest absolute area measured using temperature dependent PL (photocurrent) was observed for sample B which can be justified by increment in quantum dots formation thus resulting higher quantum yield. Single pixel detectors were fabricated and sample B yielded lowest dark current density at 80 K. A multicolor spectral response was observed from sample B with corresponding peaks at 5.13 and 7.53 μm. The calculated energy levels are in good agreement with experimental results (PL and PLE). Spectral response peaks observed from all samples were successfully matched to the energy levels calculated from the simulation.
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