Debiprasad Panda

A detailed investigation of strain patterning effect on bilayer InAs/GaAs quantum dot with varying GaAs barrier thickness

In this paper, we discuss detailed strain effects on a bilayer InAs quantum dot with varying GaAs barrier thickness. The exploration of the range of GaAs barrier thickness effect on the InAs/GaAs quantum dots and detailed structure were characterized by transmission electron microscopy, atomic force microscopy, high-resolution X-Ray diffraction (HRXRD) and Raman spectroscopy to evaluate the impact of strained layer and also studied the optical properties by photoluminescence (PL) measurements. On varying the thickness of the GaAs barrier layer, the role of strain demonstrates a promising approach to tuning the quantum dot morphologies and structures and hence, optical properties. This can be easily observed from the HRXRD rocking curves which result in a shift of the zero order peak position. Both in-out-plane strain decrease as the thickness is increased. Even the Raman scattering peaks justify the decrease of strain on increasing the GaAs barrier thickness. Therefore, higher strain propagation indicates redshift in the emission wavelength and the dots are much more uniformly spread out. Structure with a range of 5.5nm-8.5nm GaAs barrier thickness interlayer reveals even high-quality crystallinity of the epilayers with the FWHM of 21.6 arcsecs for the (004) reflection. Uncoupled structure responses low crystalline quality with FWHM of 109 arcsecs. Dislocation density increases drastically with a decrease of strain which is an important aspect of lasers and other devices in increasing their efficiency. Activation energy also shows a positive correlation with coupling structure. Therefore, controlling diffusion length may be key to reducing defects in several strained structures.

Diffusion impact on thermal stability in self-assembled bilayer InAs/GaAs quantum dots (QDs)

The thermal stability of InAs/GaAs bilayer quantum dots structure has been investigated by photoluminescence (PL) measurements. The fabricated structure on thermal annealing PL shows no shift in peaks upto 650°C indicating a robustness till a certain temperature making it a suitable candidate for vertical cavity surface emitting lasers (VCSELs) and feedback lasers where ideally a fixed wavelength is required. Integrated Photoluminescence gave a high activation energy in the range of 200 meV for the ground state PL peak for all the coupled structures. Above 650°C there is a blue-shift in the PL peak. And at a very high temperature the dots start to diffuse into InAs wetting layer hence decreasing the quality of the crystal. The stability in the PL for temperatures below 650°C can be accounted by strain energy as it works against the interdiffusion of QD and the seed layer till a certain temperature hence it compensates for the temperature effect but after 650°C diffusion term becomes too strong and we observe a blue-shift in the peak. This can be justified theoretically by modifications in the Arrhenius diffusion equation. Due to this interdiffusion of In/Ga atom the dominance of the peak and the intensity of PL peak also changes as the QD composition changes [1-2]. Coupling the dots also leads to high activation energy which in-turn generates a stronger carrier confinement. But as the temperature increases, activation energy decreases weakening the carrier confinement potential because of interdiffusion between dot and seed layer.

Effect of various capping layer on the hydrostatic and biaxial strain of InAs QDs in x (100) and z (001) direction

The hydrostatic (εhy) and biaxial (εbi) strain in lateral (x) and growth (z) direction have been computed and compared for InAs quantum dot (QD) with different capping. The capping layers are: GaAs, InGaAs/GaAs, InAlGaAs/GaAs, InGaAs/InAlGaAs/GaAs, InAlGaAs/InGaAs/GaAs, and the total thickness is kept constant for all QD structures. The strain distribution is mainly confined within the dot and dies down towards the capping layer. The movement of conduction band edges is controlled by hydrostatic strain. QDs capped with InAlGaAs/InGaAs/GaAs and InAlGaAs/GaAs shows lower magnitude of εhy, which indicates better carrier confinement as compared to other capping. The electrostatic potential obtained for the InAlGaAs capped QDs is larger (~0.5 V) than other structures. The valence band splitting into the heavy hole and light hole depends on the biaxial strain. It is observed that GaAs and InAlGaAs/InGaAs/GaAs capping has the smallest and largest values of εbi respectively in the growth direction. The GaAs capped QD structure has a smaller εbi, which would increase the energy of the ground state hole, leading to blue shift in photoluminescence spectrum. However, the ground hole state has a lower energy due to larger εbi in InAlGaAs/InGaAs/GaAs capped QD, which results in a red shift in the photoluminescence spectrum (~1.35 μm at 300 K). Nonetheless, InAlGaAs capped QDs shows better results in the lateral direction also. Thus, based on the strain profile, QDs capped with InAlGaAs as the first capping layer is the optimized structure which can be useful for various optoelectronic applications.

Short wave infrared photodetector using p-i-p quantum dots (InAs/GaAs) for high temperature operation

In this study, we report high temperature operation of infrared photodetector using p-i-p InAs/GaAs quantum dots. The ground state emission peak at 18 K from photoluminescence spectroscopy was measured at 986 nm. Single pixel detectors were fabricated and device characteristics like temperature dependent dark current, blackbody and spectral response were analyzed. The measured dark current density at 220 K with applied bias of 0.2 V was 2.48×10-3 A/cm2. The spectral response peak (2 μm) was observed in short wave-infrared (SWIR) region. We report an excellent SWIR detection characteristics at 220 K with a responsivity and specific detectivity of 3.81 A/W and 2.18×1010 cmHz1/2/W, respectively. The spectral response peak was achieved till 250 K and blackbody signal was observed till 270 K.

Heterogeneously coupled InAs Stranski-Krastanov and submonolayer quantum dot infrared photodetector for next-generation IR imaging

In the present work we are introducing heterogeneously coupled InAs stranski-krastanov and submonolayer quantum dot as an active material for quantum dot based infrared photodetector. Initially, we have optimized the basic SK on SML heterostructure. The thickness of the GaAs barrier layer is varied from 2.5 to 7.5 nm to tune the vertical coupling between seed SML and top SK QDs. PL and PLE response confirms the carrier tunneling between these heterogeneous QDs. The vertical alignment of SML and SK QDs is shown in Cross sectional TEM images. The sample with 7.5 nm barrier layer is incorporated into a N-I-N based quantum dot infrared photodetector, which shows broader spectral response than standard SK QD based IR detectors.

Impact of varying barrier thickness on the optical characteristics of multilayer InAs/GaAs QDIPs

An investigation of the optical properties of the multi stacked InAs quantum dot (QD) based photodetectors has been done by changing the capping layer composition and thickness. There is an improvement obtained in the structure and distribution of InGaAs capped QDs than the conventional GaAs capped QDs. It is due to the inhibition of In-Ga intermixing and lesser indium segregation towards the wetting layer in case of InGaAs capping. Here, the combined InGaAs/GaAs capping layer thickness has been varied to investigate the effect of the vertical strain-coupling and QD size distribution. All samples are grown by solid source molecular beam epitaxy with a V/III flux ratio of 50. A variation in InGaAs/GaAs capping layer is done by keeping the total thickness constant at 12 nm, and 18 nm. The ground state photoluminescence emission peak for the 3 nm InGaAs capped QDs have pronounced redshift than the 2 nm InGaAs capped QDs. However, the redshift is more in case of total capping layer thickness of 12 nm (i. e. 36 nm), than the 18 nm capped sample (i. e. 14 nm). It is observed due to better coupling in case of lower capping layer thickness and hence better dot size. Activation energy calculated from the temperature dependent photoluminescence study also gives incremental trend with an increase in coupling (18nm: 163.308meV, and 12nm: 215.53meV), which is attributed to lowering of QD ground state due to change in capping layer thickness. Hence the 12nm capped device with 3nm InGaAs capping gives better results probably due to better strain propagation, and hence better dot distribution.

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.

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 technique and effect of post growth annealing on the optical properties of In(Ga)As/GaAs quantum dot heterostructures

In this paper, we have proposed a technique to maintain the constant overgrowth percentage of quantum dots (QDs) in all layers of a multistacked heterostructure and hence the dot size uniformity is achieved. Two samples have been grown and compared in terms of their optical properties. Post growth annealing was carried out to observe the variation in their properties. The active layer of sample A is composed of 2.7 monolayer (ML) InAs QDs and the QD deposition amount is same for all the stacks. For the proposed sample B, 8ML In(Ga)As QDs were grown as seed layer, and the subsequent QD deposition is kept constant at 5ML. The overgrowth percentage in all QD layers were constant (∼40%) for this sample. Monomodal photoluminescence (PL) emission spectra was observed for the proposed sample B, whereas sample A has multimodal spectra. The samples were subjected to post growth annealing in argon atmosphere for 650, 700, 750, 800, 850, and 900°C. A negligible shift in the PL peak was observed for sample B up to 750°C, which confirms better thermal stability. The PL activation energy variation with respect to the annealed temperature was negligible for the proposed sample B (∼ 165 meV up to 750 °C). Hence the proposed growth mode of In(Ga)As multistacked QD heterostructure has better optical characteristics than the conventional structure in terms of PL spectra, FWHM, and also activation energy.