Saral K. Gupta

Theoretical Analysis of Resonant Cavity p-Type Quantum Dot Infrared Photodetector

A theoretical analysis for resonant cavity enhanced p-type quantum dot (QD) infrared photo-detector that uses intervalence subband transitions in InxGa1-xAs/GaAs QDs is presented. Multiband effective mass k.p model with the strain effect is used to calculate valance subband energy levels. Photocurrent spectra, response wavelength, and dark current density of QD infrared detector have been calculated. The calculations have been performed for a wide range of dot sizes, compositions, dot height, bias voltages, and temperatures. The effect of QD height, radius, and composition on the response of the photodetectors has been analyzed and some criteria for performance improvement have been suggested.

Analysis of Blockade in Charge Transport Across Polymeric Heterojunctions as a Function of Thermal Annealing: A Different Perspective

A blend of poly(3-hexylthiophene-2,5diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is popularly used as an active medium in polymeric solar devices. According to the most recent understanding, the blend is a three-phase system contrary to its earlier understanding of two-phase bicontinuous network. We have synthesized a P3HT–PCBM based layered heterostructure system by spin coating and thermal vacuum evaporations. Current density (J) was measured as a function of applied electric field (E) across the system bound between two metal electrodes. J–E relations were analyzed into the backdrop of space charge limited current model and Schottky model. The later was used to predict dc-dielectric constants from the linear slopes of ln (J) versus E1/2. The curves were not monotonously linear, but observe a knee-bend separating into two linear segments for each curve. Thermal annealing from 40°C to 80°C was used as an activation tool for driving changes in the internal morphology via inter-diffusion of polymers and current measurements were performed at room temperature after each annealing. At the last stage of annealing the two linear slopes were highly distinct. The presence of sharp knee-bend results in approximately 20 times jump in dielectric constant as a function of electric field. Such high jumps in dielectric constant illustrate the potential for switching applications and charge storage. The high dielectric constants can be understood in terms of space charge polarization due to isolated domains which hindrance to charge transport. The high dielectric constants were confirmed by another experiment of capacitance measurements of a different set of similar samples. A study of thermal evolution of internal morphology was also carried out using x-ray diffraction and scanning electron microscopy techniques to correlate the morphological changes with the transport properties.

Effect of Shape Anisotropy and Size on the Electronic Structure of CdSe/ZnSe Quantum Dots

The effect of shape anisotropy and size on the electronic structure of CdSe/ZnSe quantum dots is theoretically investigated. The quantum dot is modeled by assuming a parabolic confinement along xy plane and by finite well potential due to band offset along growth (z) direction. The energy eigenvalues and wave functions of holes in multivalence band are computed by numerical diagonalization of 4 × 4 k.p Luttinger Hamiltonian. The wave functions thus obtained are used for calculating dipole matrix elements to analyze the degree of linear polarization and allowed transitions between multivalence band and conduction band. The effect of variation of dot size, dot height, and shape anisotropy factor on the electronic structure is also analyzed. We observe that the size and shape anisotropy of quantum dot play a significant role in determining their electronic structure.

Study of Ionospheric precursors related to an earthquake (M=7.8) of 16April, 2013 using GPS-TEC measurements: Case study

In the present paper, we analyze the GPS-TEC data which observed at Agra station, India (27.2o N, 78o E) and also see the diurnal variations of global ionospheric maps (GIMs) TEC data over the epicenter of this earthquake (M=7.8) in Pakistan region for the month of April, 2013. We use a statistical technique for the analysis of data and identify the significant precursors using 3σ criterion. These precursors are found in form of enhancements on different days in the interval of 5-7 days prior to the earthquake. We also see the effect of geomagnetic storm and solar activity on TEC data and found no anomaly associated with them.

Anomalous Variations of Ionosphere Associated with the Strong Earthquake at Pakistan-Iran Border at a Low Latitude Station Agra, India

In this paper, we analyze the TEC data for April 2013 observed at Agra station, India (geogr. lat. 27.2° N, long. 78° E) to examine the effect of earthquake of magnitude M = 7.8 which occurred on 16 April 2013 at Pakistan–Iran border region. We process the TEC data using the s statistical criterion to find out anomalous variation in TEC data. We also study the VLF propagation signal from NPM, Hawaii (21.42° N, 158° W), which is monitored at the same station (Agra station) in the light of this earthquake as well as solar flares. The nighttime fluctuation method is used to analyze the VLF data for the period of ±5 days from the day of earthquake (11–21 April 2013). The anomalous enhancements and depletions are found in TEC data on 1–9 days before the occurrence of event.

Resonant cavity far infrared photo-detector based on self-assembled InAs/GaAs quantum dots

In this paper, a novel design for far infrared resonant cavity enhanced quantum dot photo-detector (RCE QDP) based on bound-to-bound intervalence subband transitions in self assembled InAs/GaAs quantum dots (QDs) is presented. The device structure consists of QDs active layers inserted between two distributed Bragg reflectors (DBRs). Quantum efficiency is calculated as a function of wavelength for different reflectivity of mirrors. Dark current is calculated as a function of the voltage for different value of temperature and QD radius.

Study of P3HT/ PCBM morphology using Raman spectroscopy

In the present work we have deposited PEDOT: PSS (poly3,4-ethylene dioxythiophene -poly (styrenesulfonate)) then Pristine P3HT (Poly-3 hexylthiophene-2,5-diyl), PCBM (6,6- PhenylC61 butyric acid methyl ester) and its blend composite (P3HT:PCBM) thin films on ITO substrate via spin coating technique. Pristine P3HT, pristine PCBM and blend thin film samples were annealed at different temperatures (50°C, 80°C and 110°C) for 1 hr. Raman spectra was measured for each thin film samples as prepared (at room temperature or 25°C) and annealed at different temperatures (50°C, 80°C, 110°C). Then we calculated area under Raman peaks which is representation of Raman Intensity and observed that area under Raman peak varies with annealing temperatures. The increase in peak intensity appears due to increased crystallinity in annealed thin films. The mixing of PCBM hinders the crystallization of P3HT in blend. Mixed amorphous phase plays major role in charge transportation in electronic devices.In the present work we have deposited PEDOT: PSS (poly3,4-ethylene dioxythiophene -poly (styrenesulfonate)) then Pristine P3HT (Poly-3 hexylthiophene-2,5-diyl), PCBM (6,6- PhenylC61 butyric acid methyl ester) and its blend composite (P3HT:PCBM) thin films on ITO substrate via spin coating technique. Pristine P3HT, pristine PCBM and blend thin film samples were annealed at different temperatures (50°C, 80°C and 110°C) for 1 hr. Raman spectra was measured for each thin film samples as prepared (at room temperature or 25°C) and annealed at different temperatures (50°C, 80°C, 110°C). Then we calculated area under Raman peaks which is representation of Raman Intensity and observed that area under Raman peak varies with annealing temperatures. The increase in peak intensity appears due to increased crystallinity in annealed thin films. The mixing of PCBM hinders the crystallization of P3HT in blend. Mixed amorphous phase plays major role in charge transportation in electronic devices.

Study of gain characteristics of GaAs/GaAlAs self assembled quantum dots

The effect of size and shape anisotropy on the gain characteristics of strain free GaAs/AlGaAs quantum dots is theoretically investigated. QD is modelled by anisotropic parabolic potential along x-y plane. Luttinger Hamiltonian has been numerically diagonalized over a wider range of shape anisotropy and size to obtain the eigenvalues and eigenvectors. The dipole matrix elements are obtained for interband transitions and gain spectra are calculated.

Performance investigations of multicolor, broadband quantum dot infrared photodetector

Multiband detectors have many advantages compared with the single band detectors. In this paper a multi color, broad band infrared detector is proposed and analyzed. This multilayer quantum dot infrared detector utilizes the inter-valence band transitions in the InGaAs quantum dots to achieve the multicolor broad band response. Energy eigen values and wave functions of the hole states are determined by the numerical diagonalization of the Luttinger Kohn Hamiltonian. Absorption spectra is calculated by summing over the absorption due to transition from the heavy hole ground state to the all possible states. Then self-consistent numerical calculation method is used to determine the photocurrent and spectral responsivity of multilayer QDIP.