InAs/GaAs Quantum Dots have piqued the interest of researchers owing to the advantages they offer in the fabrication of highly efficient optoelectronic devices. In this study, we aim to examine the consequence varying V-III ratio on optical and structural behavior of self-assembled InAs/GaAs Stranski-Krastanov (SK) Quantum Dots grown on GaAs substrate using Molecular Beam Epitaxy (MBE). Three samples consisting of three layers of vertically stacked Quantum Dots with three different V-III ratios (48, 60 and 80 respectively) grown at a substrate temperature of 490°C have been thoroughly examined using PL spectroscopy and HR-XRD. The best optical response is seen in the sample with 80 as VIII ratio. A higher As vapor pressure during growth seems to suppress the surface migration of Indium atoms leading to bigger dot size, increased PL intensity and more uniform distribution rendering better optical response. The absence of satellite peaks in HR-XRD measurements of sample with lower V-III ratio indicates significant density of point-defects. HRXRD analysis reveals an increase in perpendicular strain with greater V-III ratio. Reduced FWHM in sample with higher V-III ratio is in accordance with suppressed Indium diffusion and strain propagation across multi-layered nanostructure contributing to greater uniformity in dot-size. PL spectrum of sample with least V-III ratio shows sharp peaks around 900 nm indicating incomplete dot-formation at such low ratios leaving significant part of wetting layer exposed. Our investigation provides interesting insights into kinetics of nanostructure growth which will prove to be helpful in fabrication of optimized nanostructures.
In spite of numerous advantages offered by Quantum Dot (QD) based imaging systems in infrared photo-detection, the physical realization of such systems has always been a challenging task. In this study, we aim to analyze the effects of growth rate variation on the structural and optical properties of self-assembled InAs/GaAs Stranski-Krastanov (SK) QDs grown on semi-insulating GaAs substrate using MBE (Molecular Beam Epitaxy). Five samples grown at a substrate temperature of 490°C with varying growth rates (0.025ML/s, 0.05ML/s, 0.075ML/s, 0.1ML/s, 0.15ML/s) were investigated using PL spectroscopy, and AFM measurements. PL spectroscopy showed a blue shift in the ground state peak wavelength with an increase in growth rate which was further corroborated by AFM measurements, showing reduced dot-size with an increased growth rate. AFM measurements showed an increase in dot density with an increased growth rate suggesting increased tendency towards nucleation. Integrated PL intensity witnessed an initial increase with an increased growth rate before achieving its maxima for sample grown at 0.075ML/s, rendering the sample grown at 0.075ML/s best in terms of optical activity. These observations provided key insights into the growth kinetics operating during dot-formation through SK growth mode by evaluating the competition between the forces due to surface diffusion and nucleation.
Uncapped In(Ga)As quantum dots (QDs) have got very little attention in comparison with its enfolded counterpart. The existence of surface states makes it less attractive to the research community. On the other hand, colloidal QDs have immense recognition in the field of bio-sensing and bio-imaging. Various surface passivated stable colloidal QDs are now commercially available, but only in solution form. Stable solid-state QDs are still a virtue. So, there is a huge demand for stable solid-state surface QDs which can be easily coupled with an electronic device for sensing application. Simply we need to change the ligand, corresponding to a particular target molecule, and we can detect various chemical and biological elements from low molar solution (Nano bio-sensor regime). With this motivation, we have epitaxially grown a simple vertically coupled InAs QD structure, where both seed and top dot layers are of 2.7 ML InAs. In addition, the top QDs are left uncapped to form a surface quantum dot layer. The as-grown sample is acid-etched (to remove the native oxides) and passivated in 0.5 M Thiourea solution for one hour. Significant enhancement of ground state photoluminescence peak has been observed after the passivation for both surface and buried QDs. Atomic force microscopic (AFM) images confirm the modulation of the surface before and after the ex-situ treatment.
The work on Au-Ge nanoparticles (Nps) carried out so far by us has been successfully applied to devices like Bilayer, Trilayer, Hepta-layer Quantum dot infrared photodetectors (QDIP). An improved photonic response is achieved for the devices in terms of responsivity, photocurrent, responsivity, absorption and scattering. The dedicated standard recipe to get Nano-particles of different materials (Metals, Semiconductor) on distinguished substrate are revealed. It has been observed that the processes are repeated multiple time at the condition where desirable plasmonic condition does not match. Here the process has been optimized with multiple repeated annealing of Gold (Au) and Gold-Germanium (Au88Ge12) that shows the consistent pattern of reflectance where each anneal modifies the refractive index in same order with variable thickness of annealed film. This technique dilutes the constraints of fresh sample preparation whenever the nanoparticle response is dull, then the induced variation in size and volume of particle along with tuned distribution will become suitable.
Epitaxially grown III-As nanostructures, like quantum well (QW)/ quantum dot (QD) have already been scrutinized rigorously and incorporated into various devices, like light-emitting diode (LED), laser, Photodetector, solar cell, etc. Most of them use buried QW/QD heterostructure, where the as-grown nanostructures are capped with various combination of thick (In/Ga/Al)As matrix. In contrary, near-surface nanostructures have very less attention owing to additional surface states. However, these near-surface nanostructures have the potential to communicate with the external world. Therefore, it might act as a confined channel, which can be probed externally. Assertively, these near -surface nanostructures have immense potential to act as sensors. Before that, we need to passivate the surface states to hold the best communication with the outer environment. In the present study, we have used Thiourea as a source of sulfur and show the effect of passivation in terms of improved luminescence behavior. Near-surface GaAs/In0.15Ga0.85As/GaAs quantum well and self-assembled GaAs/InAs/GaAs SK quantum dots are grown on GaAs wafer through molecular beam epitaxy. The effective thickness of the top GaAs capping layer has been kept around eight nanometers and twelve nanometers to keep the nanostructure (QW/QD) very close to the surface. As grown samples have shown very poor photoluminescence peaks and increased by few orders after passivation. Pre-etching followed by sulfur passivation has shown the best enhancement of luminescence intensity.
A futuristic thin-film transistor based on a double-well heterostructure exploiting the band-gap tailoring property of Zinc Oxide (ZnO) has been proposed. Effects of carrier confinement in the MgZnO/CdZnO heterojunction have been studied by employing an additional MgZnO barrier in the CdZnO channel to create two potential wells. Carrier transport and device operation have been explained with the help of energy band diagrams extracted at different operating voltages. The optimised double-well structure yields an unprecedented ION/IOFF=1015, simultaneously achieving a sub-threshold swing=74mV/decade, thereby indicating high switching speed. A high value of field-effect mobility, (μFE, max=32cm2 /V-s) over a wide range of gate bias, manifests its ability to overcome the carrier scattering problem due to confinement, making it a promising candidate for high resolution and fast response optical display applications.
In this paper, the authors have propounded a pragmatic solution to circumvent the problem of inherent n-type conductivity of Zinc Oxide (ZnO), which remains the major obstacle to superior device performance. The proposed method employs the concept of carrier confinement in heterojunction thin film transistors that offers a prospective alternative to intruding into lesser known materials and their associated complexities. Carrier confinement is achieved in the low band-gap Cadmium Zinc Oxide (CdZnO) channel shielded by high band-gap Magnesium Zinc Oxide (MgZnO) on one side and gate dielectric SiO2 on the other, which has been further corroborated by Energy Band diagram of the confined region that manifests the formation of two dimensional electron gas (2-DEG) at the CdZnO-SiO2 interface. The device exhibits almost ideal transfer characteristics with a very narrow transition region between the ON and OFF states. The sub-threshold region is characterized by a high ION/IOFF ratio (1011) and near ideal sub-threshold swing (74mV/decade). Although a slight compromise in field effect mobility is incurred owing to the carrier transport mechanisms in confined regions, the benefits of carrier confinement in the low potential well far outweigh its detriments to emerge as a promising method for improving device performance.