Akshay Balgarkashi

A low temperature investigation of the optical properties of coupled InAs quantum dots with GaAsN/GaAs spacers

Epitaxially-grown 10-layer coupled InAs quantum dots with GaAsN/GaAs barrier layers have been investigated. The PL spectra was seen to be a complex convolution of bimodal distribution of QDs along with an asymmetric signature introduced by incorporation of nitrogen into the structures. Reducing the GaAsN/GaAs barrier thickness (from 2/16nm to 2/8nm) resulted in an improvement of PL linewidth as low as 20meV of the dominant PL peak for the sample with thinnest barrier layer. A blueshift in emission was observed due to higher indium intermixing as a result of an increase in overall strain in the multilayer structure. The highly asymmetric exponential tail signature evident from the PL spectra of as-grown samples indicated a higher presence of localized N-induced excitonic states near the conduction band edge. Samples with thicker barriers showed relatively lower asymmetry compared to samples with thinner barriers. Also, samples with thinner barriers showed an arrest in blueshift in the PL spectra with annealing temperature indicating thermal stability.

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.

Low-temperature photoluminescence studies in epitaxially-grown GaAsN/InAs/GaAsN quantum-dot-in-well structures emitting at 1.31 μm

We report a single layer GaAsN/InAs/GaAsN quantum-dot-in-well (DWELL) structure with PL emission at 1.31μm important for applications in communication lasers. This extension has been achieved with a nitrogen composition of only 1.8% and QDs embedded within 1/6nm GaAsN which is higher compared to single layer QDs with GaAs and GaAsN capping layers as a result of confinement reduction on both sides of the QD energy levels. The structures remain as QDs till 800°C of annealing temperature alongwith a drastic enhancement in PL intensity as a result of annihilation of N-induced crystal defects which provide non-radiative recombination centers for carriers in the as-grown sample which is responsible for degraded luminescence. A typical highly asymmetric PL signature observed in dilute nitride structures is seen with a sharp cut-off at lower wavelengths and a large exponential tail at higher wavelengths in the as-grown and 650°C annealed samples. This is due to the presence of localized excitonic states extending into the bandgap close to the band edges. For higher annealing temperatures, this asymmetry disappears indicating an improvement in uniformity of nitrogen distribution and absence of localized states; which is also confirmed from a smaller blueshift in excitation intensity-dependent PL spectra of these samples. Well-resolved ground and first excited states in the PL spectrum of 700°C annealed sample indicates an improvement in QD confinement.

A low-temperature photoluminescence study of GaAs1-xNx/GaAs multiple quantum-wells

Five-period GaAs1−xNx/GaAs multiple quantum wells (MQWs) were grown on GaAs(001) substrates under different nitrogen background pressures through solid-source molecular beam epitaxy and the structural and optical properties at low temperature were investigated. High resolution x-ray diffraction revealed sharper satellite peaks observed for GaAs0.978N0.022/GaAs MQWs as compared to GaAs0.982N0.018/GaAs MQWs, indicating better interfaces. The MQWs with higher nitrogen content exhibited high photoluminescence (PL) intensity, whereas a degraded PL intensity was observed for the latter, attributed to reduction in surface recombination with high nitrogen incorporation. Moreover, the spectrum for the MQWs with higher nitrogen content was observed to be consisted of several Gaussian spectra, indicating thickness variation of QWs caused by randomness in distribution of N atoms. In the low energy regime of PL, a long asymmetric tail was observed because of nitrogen introduced potential fluctuations. Rapid thermal annealing enhanced PL intensity by multi-fold and substantially reduced the full width at maximum because of homogenization of MQWs. This investigation could enhance understandings of the MQWs-based optoelectronic devices.

Comparison of InAs/GaAs and InGaAs/GaAs quantum dot solar cells and effect of post-growth annealing on their optical properties

We have reported InAs/GaAs and InGaAs/GaAs quantum dot solar cells (QDSCs) and the effect of rapid thermal annealing (RTA) on their optical properties. A thermal stability was observed up to 700°C, and further increase in temperature results in a blue shift in photoluminescence (PL) emission peak. A maximum open circuit voltage (Voc) of 0.33V was obtained for the InGaAs/GaAs solar cell (SC), whereas a maximum short circuit current (Jsc) of 20 mA/cm2 was obtained for the InAs/GaAs SC. A fill factor (FF) of 0.31, and 3.0758% efficiency was obtained for the InGaAs/GaAs SC which is higher than the InAs/GaAs QDSC.

Impact of rapid thermal annealing on dilute nitride (GaAsN)-capped InAs/GaAs quantum dots exhibiting optical emission beyond ~1.5 μm

We report here self-assembled 2.6 ML InAs QDs capped with GaAsN0.021 on GaAs (001) substrate grown under high arsenic overpressure and high power by solid source molecular beam epitaxy. With variation in GaAsN0.021 layer thickness, InAs/GaAs QDs were studied by photoluminescence (PL) spectroscopy. It was found that with InAs dot density of 3 ×1010 cm-2 and 4 nm GaAsN capping layer, emission wavelength was possible to extend beyond 1.5 μm at 300K. Rapid thermal annealing was carried out in nitrogen ambient for 30 sec at temperatures ranging from 700°C to 800°C and a continuous blue-shift for the nitride-capped QDs was observed at 19 K PL spectra, and the sample annealed at 800°C exhibited highest intensity with narrowest full width at half maximum (FWHM). Both the as-grown and annealed samples exhibited asymmetric PL behavior in low energy region at low temperature, associated to the N-related states or cluster of N atoms. The peak emission wavelength at the annealing temperature domain of 750-800°C was remained constant, attributed to no In/Ga diffusion at the interface between the dot and the barrier. Hence, the InAs/GaAs dots capped with 4-nm GaAsN0.021 layer could be implemented in lasers in the temporal range of 750-800°C.

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.