Binita Tongbram

Effect of barrier thickness on structural, optical, and spectral behaviors of vertically strain coupled InAs/GaAs quantum dot infrared photodetectors

The optical, electrical, and spectral properties of a strain coupled InAs quantum dot detector with a fixed quaternary capping of InAlGaAs and variable GaAs barrier thickness were investigated along with an equivalent uncoupled structure. Self-assembled quantum dots with a multimodal dot size distribution were achieved owing to vertical strain coupling. Strain and electronic coupling were utilized to improve the optical and electrical performance of the fabricated quantum dot infrared photodetector. The peak spectral response was tuned by varying barrier thickness, and a blue shift (almost 1 μm) was observed by increasing the capping thickness from sample A (90 A capping) to E (500 A capping). High responsivity and detectivity (∼1010 cm Hz1/2/W) were observed for all coupled samples as compared to the uncoupled sample. All coupled samples showed high thermal stability in the photoluminescence peak with high-temperature annealing.

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.

Impact of an InxGa1–xAs Capping Layer in Impeding Indium Desorption from Vertically Coupled InAs/GaAs Quantum Dot Interfaces

This study describes the effect of a thin GaAs spacer of 4.5 nm thickness in a bilayer-coupled InAs quantum dot (QD) heterostructure. Here, we report the first demonstration of InAs/GaAs QDs capped by self-assembled InxGa1–xAs layers. Self-assembled InxGa1–xAs layers were introduced into each intermediate layer across the interface of InAs QDs and the GaAs layer in a vertical-coupled bilayer QD (VCBQD) heterostructure to prevent indium desorption from the QDs. A change in the indium content in the seed-layer InAs QDs changes the self-assembly position and modifies the InxGa1–xAs layer thickness. A theoretical approach was presented to study the formation of self-assembled InxGa1–xAs layers at each strain-free layer. We showed that the strain energy at the second intermediate (ezz2) layer is greater than that at the first intermediate (ezz1) layer; ezz2 depends on the vertical strain channel length. The impact of the InxGa1–xAs layer thickness on the strain energy was studied using high-resolution transmis...

Cross-sectional TEM (XTEM) analysis for vertically-coupled quaternary In0.21Al0.21Ga0.58As capped InAs/GaAs quantum dot infrared photodetectors

This paper presents a detailed morphological analysis of vertically strain-coupled InAs quantum dots with a fixed quaternary capping (In0.21Al0.21Ga0.58As) of 3 nm and a GaAs barrier ranging in thicknesses from 9 to 18 nm. The coupled heterostructures were studied using cross-sectional transmission electron microscopy and compared with uncoupled heterostructures with 2-nm quaternary capping and 50-nm GaAs capping thickness. Power-dependent photoluminescence spectra showed that a minimum capping of 9 nm produced a multimodal dot-size distribution. Increasing the capping from 9 to 18 nm reduced the vertical correlation, thus increasing the dot uniformity. Increasing the capping thickness reduced the coupling and increased the dot size. At a maximum capping (18nm) coupled quantum dots exhibit a bimodal dot-size distribution compared to the mono-modal distribution of the uncoupled quantum dots. The coupled samples demonstrated superior optical properties to uncoupled samples.