Saikalash Shetty

Enhancement in Peak Detectivity and Operating Temperature of Strain-Coupled InAs/GaAs Quantum Dot Infrared Photodetectors by Rapid Thermal Annealing

We report the effects of rapid thermal annealing on the optical, structural, and device properties of 30 layer strain-coupled InAs/GaAs quantum dot infrared photodetectors. Stability in the photoluminescence peak is observed for annealing up to 800°C, which has not been previously reported. Cross-sectional transmission electron microscopy images show preservation of quantum dots is observed up to 800°C. Device with total capping thickness of 150 nm annealed at 750°C exhibit a fivefold enhancement in spectral intensity compared to as-grown devices and increase in the temperature of detector operation is observed from 100 to 140 K from the same device. The annealed devices exhibited a single-order enhancement in peak detectivity compared to as-grown quantum dot infrared photodetector.

A detailed investigation of the impact of varying number of dot layers in strain-coupled multistacked InAs/GaAs quantum dot heterostructures

Strain-coupled InAs quantum dot (QD) heterostructures has been compared in terms of their optical properties, with varying the number of stacks. Each structure consists of seed layer dots (2.5 monolayer of InAs) with a capping layer of 6.5nm GaAs followed by active layer dots (2.1 monolayer of InAs). The active layer QD with the capping layer is repeated by one, two, four, and six times in bilayer, trilayer, pentalayer, and heptalayer samples, respectively. Thickness of the GaAs spacer layer in between active layer QD stacks is different for each structure. A red shift in photoluminescence (PL) emission was obtained for the strain-coupled multi-stack samples compared to the conventional uncoupled one. This is due to the formation of larger dot size in coupled structures. We also observed a monomodal dot distribution till the pentalayer sample, but after that a bimodal distribution was found, which may be due to the enhancement of strain as we further increase the stacks. Compared to an uncoupled sample, all coupled samples exhibited lower full width at half maximum (FWHM) values (uncoupled-35.89nm, bilayer-32.83nm, trilayer-30.17nm, pentalayer-68.91nm, and heptalayer-67.55nm) which attributes to homogeneous dot size distribution. Higher activation energies were measured in coupled samples compared to the conventional uncoupled one. Trilayer sample claimed the highest PL activation energy of 303.42meV, whereas the uncoupled sample has only 243.89meV. This increased activation energy in the coupled structures will be helpful for lower dark current in the devices.

Effect of varying capping composition and number of strain-coupled stacks on In0.5Ga0.5As quantum dot infrared photodetectors

In this paper, we have reported the optical and electrical properties of strain coupled multi-stack quantum dot infrared photodetectors (QDIPs) of In0.5Ga0.5As dots with different capping compositions. Bilayer, trilayer, pentalayer and heptalayer coupled QDIPs are grown by solid source molecular beam epitaxy with one set of samples containing conventional GaAs capping (12nm) and second set containing a combinational capping of In0.15Ga0.85As (3nm) and GaAs (9nm) layers with same total thickness. The entire set of strain coupled quantum dots (QDs) shows a red shift in ground state photoluminescence peak in comparison to the uncoupled structures. Due to the reduction in indium interdiffusion from In0.5Ga0.5As dots in the combinational capped structures, a higher redshift is observed compared to the GaAs capped structures, which attributes larger dot size in the former ones. Full width half maximum value (FWHM) of In0.15Ga0.85As/GaAs capped QDs are lower, showing uniform distribution of dot size compared to the corresponding GaAs capped QDs. Trilayer sample with In0.15Ga0.85As/GaAs capping shows the best result in terms of the peak emission wavelength of 1177nm, FWHM of 15.67nm and activation energy of 339meV compared to all the structures. Trilayer sample seems to be the optimum stacking having the best confinement resulting lower dark current density of 6.5E-8 A/cm2 measured at 100K. The sample also shows a multicolor response at ~4.89μm and at ~7.08μm in the mid infrared range. Further optimization of the spacer thickness and dot layer deposition can improve the response towards the long infrared range.

Growth strategy to achieve mono-modal quantum dot size distribution in InAs/GaAs multi stack coupled heterostructures

In multi-stack structures, the strain field existing in the seed layer induces the nucleation of subsequent dots on the preexisting dots, and as the InAs quantum dot (QD) coverage is fixed we eventually get a dissimilar overgrowth percentage between subsequent layers. Therefore, such structures are prompt to defects and dislocations and also produce a multimodal distribution of dots. In this paper, a detailed investigation has been done on the growth strategy of strain-coupled multi-stack InAs/GaAs heterostructures, to achieve mono-modal distribution of InAs QDs. The heterostructures discussed in this paper are grown with fixed seed layer coverage of 2.5 monolayer (ML) InAs in order to maintain the constant overgrowth percentage between the subsequent QD layers. The subsequent QD layer coverage has been varied from 2.5ML to 2.1ML and the GaAs spacer thickness in between them varied from 10nm to 12nm. Power dependent photoluminescence (PL) spectra at 18K revealed the transition from multimodal to monomodal as the growth parameters varied. We have also optimized the spacer thickness between the seed layer and immediate dot layer to 6.5nm, by keeping other parameters constant. It results in a red shift in PL emission peak and lowers the full width half maximum by 12nm, which seems to be improving in dot size and homogeneity. The highest activation energy has been obtained from the optimized structure, which attributes to a better QD confinement and hence lowers dark current value. An enhancement in the optical properties may happen with further optimization.

A detail investigation on quaternary and ternary capped strain coupled quantum dots based infrared photodetectors and effect of rapid thermal annealing temperature

In this report, we are comparing two different QDIP architectures capped with quaternary and ternary layer of different barrier thicknesses and effect of rapid thermal annealing on the device performances. Low temperature power dependent PL spectra exhibit a multimodal distribution of the QDs in all the heterostructures which has been confirmed by XTEM. High thermal stability up to 800°C i.e. minimum PL peak shift in terms of wavelength was observed in all quaternary coupled devices with annealing compared to ternary (it was up to 700°C) capped QDIP. The vertical strain propagating from underlying QDs prevents the inter-diffusion by maintaining a strain relaxed state. Minimum Dark current density was observed in quaternary capped QDIP with total capping thickness of 15nm and one order enhancement in detectivity compared to ternary. Quaternary capped QDIP with 12 nm total capping thickness was most red shifted and a peak spectral response was observed at 7.3μm. Compared to ternary all quaternary devices showed narrow spectrum with less than 20% spectral line-width. Quaternary capped QDIP with 15 nm total capping thickness was annealed and devices were fabricated using 2 step lithography process. For 750°C annealed QDIP a maximum operating temperature of 140K with 5-fold increase in photocurrent compared to other was observed.

Stability in peak emission wavelength in strain-coupled multilayer InAs/GaAs quantum dot heterostructures when subjected to high-temperature rapid thermal annealing

In quantum dot laser and solar cell structures, high temperature is required for the growth of cladding or window layer. Therefore to check the thermal stability in emission peak in high temperature strain coupled InAs/GaAs QDs are grown by MBE capped with two different type of spacer layer- InGaAs and InAlGaAs. Photoluminescence spectra shows multimodal distribution of QDs. Till 700oC annealing temperature, no shift in peak emission wavelength is observed for InGaAs capped sample. The vertical strain prevents the inter-diffusion by maintaining a strain relaxed state due to coupling. For quaternary InAlGaAs capped QDs this stability is observed till 800oC.

Comparison of Three Design Architectures for Quantum Dot Infrared Photodetectors: InGaAs-Capped Dots, Dots-in-a-Well, and Submonolayer Quantum Dots

In this letter, we compare three design architectures for quantum dot infrared photodetectors-InGaAs-capped InAs dots, InAs dot-in-a-well (DWELL), and InAs submonolayer (SML) heterostructures-in terms of optical and spectral behavior. The photoluminescence (PL) intensity measured of the SML sample at 8 K was 20 times stronger than that of the other samples, and its full-width at half-maximum value broader than the rest. Activation energy was calculated using temperature-dependent PL and dark current measurements, which showed the same trend. Peak spectral responses for the InGaAs-capped InAs dot and DWELL samples were observed at 4.1 and 7.3 μm and at 4.1 and 8.5 μm, respectively; however, only a single transition was observed for the SML sample because of the absence of a wetting layer. Spectral response of DWELL sample exhibited bias tunability at 87 K, and the SML sample exhibited high temperature of operation till 110 K. One order increment in responsivity was observed in the SML sample compared to others. The peak detectivity of InGaAs-capped InAs dot,DWELL,andSMLsampleswas 4.1 × 109, 4.99 × 109, and 3.89 × 109 Jones, respectively, at 87 K.