Showing papers by "Rangarajan Muralidharan published in 2011"

Virtual Ground Technique for Crosstalk Suppression in Networked Resistive Sensors

Abstract: Two-dimensional resistive sensor arrays that utilize shared row and column connections to simplify the interconnect complexity suffer from the crosstalk problem among its elements introduced due to the interconnection overloading. In this letter, we present a method of overcoming the problem of crosstalk by putting all of the row nodes at virtually equal potential using virtual ground of high-gain operational amplifiers (opamps) in negative feedback. The circuit, though it requires a large number of opamps, solves the crosstalk problem to a large extent and provides faster scanning. We verified the circuit functionality with PSPICE simulations. We have also derived the expressions of crosstalk rejection and sensitivity to show that, by using high-gain, low-noise opamps, we may get excellent performance.

Experimental Study of the Influence of Process Pressure and Gas Composition on GaAs Etching Characteristics in Cl2/BCl3-Based Inductively Coupled Plasma

Abstract: A study of Cl2/BCl3-based inductively coupled plasma (ICP) was conducted using thick photoresist mask for anisotropic etching of 50 μm diameter holes in a GaAs wafer at a relatively high average etching rate for etching depths of more than 150 μm. Plasma etch characteristics with ICP process pressure and the percentage of BCl3 were studied in greater detail at a constant ICP coil/bias power. The measured peak-to-peak voltage as a function of pressure was used to estimate the minimum energy of the ions bombarding the substrate. The process pressure was found to have a substantial influence on the energy of heavy ions. Various ion species in plasma showed minimum energy variation from 1.85 eV to 7.5 eV in the pressure range of 20 mTorr to 50 mTorr. The effect of pressure and the percentage of BCl3 on the etching rate and surface smoothness of the bottom surface of the etched hole were studied for a fixed total flow rate. The etching rate was found to decrease with the percentage of BCl3, whereas the addition of BCl3 resulted in anisotropic holes with a smooth veil free bottom surface at a pressure of 30 mTorr and 42% BCl3. In addition, variation of the etching yield with pressure and etching depth were also investigated.

Origin of Berreman effect in GaN layers on sapphire substrates

Abstract: Oblique incidence polarized IR reflectivity measurements of Metal Organic Chemical Vapor Deposition (MOCVD) grown GaN epitaxial layers on sapphire are discussed in the context of recent literature on Berreman effect. The dependence of the p-polarized reflectivity spectrum on incidence angle and thickness of the GaN films is analyzed theoretically and the results are compared with experiment. The “Berreman minimum” that is the reflectivity minimum near the longitudinal-optical (LO) phonon frequency is shown to be due to optical interference. Our calculations show that in GaN layers with thickness greater than 0.1 μm and for high incidence angles, the frequency of “Berreman minimum” does not correspond to the A1 LO phonon frequency.

On the role of dislocations in influencing the electrical properties of HgCdTe photodiodes

Abstract: The influence of dislocations on the electrical and photo-electric characteristics of HgCdTe has been widely discussed in published literature. However, an unexplored aspect of the dislocations that has not yet attracted the attention of any of the investigators, is the band gap narrowing/widening induced by the intense stress field around dislocation core. Preliminary estimations show that the band gap narrowing due to the tensile region of the stress field along the dislocations in HgCdTe is high enough to cause significant band gap narrowing in low band gap HgCdTe. An enhanced Zener like band-to-band tunneling is proposed in the vicinity of dislocation cores. The calculations presented here qualitatively explain the observed influence of dislocations on HgCdTe photodiode characteristics.

Comparison of the performance of LWIR and MWIR thermal imagers for varying ambient temperature and humidity conditions

Abstract: Hodgkin (SPIE 6207(2006)) extended NVThermIP to be applicable to cold weather conditions. We also (IRPhys&Technol.51 (2008)520) later published an analysis of the effect of varying ambient temperature (Tamb) by modifying the inputs to NVTherm2002, and by using spectrally-weighted atmospheric transmission calculated from MODTRAN at different ambient temperatures and relative humidities (RH). We took into account the effects on the integration time and NETD, and we now account for the variation of uT with varying Tamb, as Hodgkin has done. The overall trends are similar, but we have NVTherm, not NVThermIP. We vary the parameters associated with Johnson's criteria to obtain similar results. Note that diurnal, seasonal, climatic and microclimatic variations of relative humidity (RH) significantly impact the performance of thermal imagers, especially LWIR ones. We compare the performance of thermal imagers a horizontal mean-sea-level path in clear weather conditions for terrestrial imagers and ground targets/scenes in both LWIR and MWIR bands, as a function of the ambient temperature from -40°C to +40°C and also as a function of RH (30%, 50% and 70%). To understand the differences in the results reported by Hodgkin and our paper, we do a sensitivity analysis as a function of system and environmental parameters (f/#, RH, detection probability, spectral width etc). For one set of parameters, we observe that the range curves RLW and RMW intersect at more than one value of Tamb and suggest an analogy to a 're-entrant phase'. We also analyze how motion blur affects the two bands, at different Tamb.

Fermi-edge singularity in photoluminescence spectra of modulation-doped AlGaAs/InGaAs/GaAs quantum wells

Abstract: The photoluminescence study of Fermi-edge singularity (FES) in modulation-doped pseudomorphic AlxGa1−xAs/InyGa1−yAs/GaAs quantum well (QW) heterostructures is presented. In the above QW structures the optical transitions between n = 1 and n = 2 electronic subband to the n = 1 heavy hole subband (E11 and E21 transitions, respectively) are observed with FES appearing as a lower energy shoulder to the E21 transition. The observed FES is attributed to the Fermi wave vector in the first electronic subband under the conditions of population of the second electronic subband. The FES appears at about 10 meV below E21 transition around 4·2 K. Initially it gets stronger with increasing temperature and becomes a distinct peak at about 20 K. Further increase in temperature quenches FES and reaches the base line at around 40 K.