HgCdTe/CdTe/Si infrared photodetectors grown by MBE for near-room temperature operation

Conventional HgCdTe infrared detectors need significant cooling in order to reduce noise and leakage currents resulting from thermal generation and recombination processes. Although the need for cooling has long been thought to be fundamental and inevitable, it has been recently suggested that Auger recombination and generation rates can be reduced by using the phenomena of exclusion and extraction to produce nonequilibrium carrier distributions. The devices with Auger suppressed operation requires precise control over the composition, and donor and acceptor doping. The successful development of the molecular beam epitaxy (MBE) growth technique for multi-layer HgCdTe makes it possible to grow these device structures. Theoretical calculations suggest that the p n+ layer sequence is preferable for near-room temperature operation due to longer minority carrier lifetime in lightly doped p-HgCdTe absorber layers. However, because the low doping required for absorption and nonequilibrium operation is easier to achieve in n-type materials, and because Shockley-Read centers should be minimized in order to obtain the benefits of Auger suppression, we have focused on p⁺ n structures. Planar photodiodes were formed on CdTe/Si (211) composite substrates by As implantation followed by a three step annealing sequence. Three inch diameter Si substrates were employed since they are of high quality, low cost, and available in large areas. Due to this development, large area focal plane arrays (FPAs) operated at room temperature are possible in the near future. The structures were characterized by FTIR, x-ray diffraction, temperature dependent Hall measurements, minority carrier lifetimes by photoconductive decay, and in-situ ellipsometry. To study the relative influence of bulk and surface effects, devices with active areas from 1.6 10⁻⁵ cm² to 10⁻³ cm² were fabricated. The smaller area devices show better performance in terms of reverse bias characteristics indicating that the bulk quality could be further improved. At 80 K, the zero bias leakage current for a 40 m 40 m diode with 3.2 m cutoff wavelength is 1 pA, the R₀A product is 1.1 10⁴-cm² and the breakdown voltage is in excess of 500 mV. The device shows a responsivity of 1.3 10⁷ V/W and a 80 K detectivity of 1.9 10¹¹ cm-Hz^1/2/W. At 200 K, the zero bias leakage current is 5 nA and the R₀A product 2.03-cm², while the breakdown voltage decreases to 40 mV.     

An Optimized and Hybrid Energy Aware Routing Model for Effective Detection of Flooding Attacks in a Manet Environment

In this work, the design of continuous time Fractional Order All-Pass Filter (FOAPF) is proposed. Two different design methods to realize the All-Pass Filter in the fractional domain with mathematical formulations and circuit results are investigated. The first design method is based on Fractional Order Elements (FOEs) where, FOEs of order α and β are utilized to develop a second order All-Pass Filter to the fractional domain. Whereas, in second design method, the fractional filter is approximated into higher order integer filters. The frequency response of the proposed design are validated using MATLAB (2018a) and conjointly with PSPICE (OrCAD 17.2). For first circuit design, FOEs are obtained using a single R–C parallel network. However, for the second circuit design Signal Flow Graph (SFG) approach is utilized. The evaluation of the realized FOAPF is also performed through the Lissajous pattern, AC analysis and Transient Analysis. After the simulations, the achieved results show that the second order FOAPF provides almost 180° phase shift for different values of α, β.

     

Eggshell derived hydroxyapatite microspheres for chromatographic applications by a novel dissolution - precipitation method

Hydroxyapatite ((Ca₁₀(PO₄)₆(OH)₂, HAp) based chromatography matrix has attracted great interest in the field of protein separation. However, researchers have been trying to combat the growing costs associated with the HAp matrix. In the present investigation, we utilized a cheap biological waste material, viz. eggshells, for the development of hydroxyapatite (HAp) resins and evaluated them for protein purification. Initially, the calcite of the eggshell carbonate was converted into metastable vaterite microspheres. The HAp microspheres (ECHAp) were then prepared from eggshell carbonate microspheres using a novel dissolution-precipitation process. Synthetic source calcium carbonate was also used to prepare HAp microspheres (CHAp) for comparison. The purity and morphology of the apatite microspheres were characterized using X-ray diffraction (XRD) method, Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and laser diffraction particle analysis. Although both the apatites have similar morphology, the ECHAp has a larger surface area of 33.8 m² g^?1 compared to CHAp of 17.27 m² g^?1 by surface area analysis method. A commercial HAp matrix (CHT) with similar properties was also studied for comparison. All the apatite microspheres were found to have a similar protein binding capacity for bovine serum albumin (BSA). But ECHAp showed better protein separation for BSA and lysozyme mixture compared to CHAp and CHT matrices. The ECHAp matrix was also found to be highly stable over 20 purification cycles. Hence, the eggshell waste seems to have the potential for HAp matrix by a novel carbonate route with ease of preparation and also an economical packing material for chromatographic purification of biomolecules.