Sudha Gupta

Modeling of dark characteristics of mercury cadmium telluride n+–p junctions

Dark dynamic impedance versus applied bias voltage characteristics of HgCdTe n þ –p junctions has been modelled here using a recently proposed [Infrared Phys. Technol. 43 (6) (2002) 317–326] analytical approach. The results of the analysis on two pixels from the same array are discussed to illustrate the advantage of this approach in identifying the possible source of variation of diode impedance among the diodes in an array. It has been shown that the contribution of dark current contributing mechanisms can be separated from each other in each diode. An idea of the dominant mechanism of the two sources of the ohmic current, namely surface leakage currents and contribution of dislocations intersecting the junction can also be had from temperature dependent study of shunt impedance. 2002 Elsevier Science B.V. All rights reserved.

Effect of dislocations on the zero-bias resistance-area product, quantum efficiency, and spectral response of LWIR HgCdTe photovoltaic detectors

The effect of dislocations on the zero-bias resistance-area product, quantum efficiency, and spectral response of long wavelength infrared (LWIR) HgCdTe photodiodes has been modeled for a case in which the line dislocations are along the thickness of the wafer. The model focuses on the calculation of the impedance of individual dislocation followed by the calculation of the resultant effect by assuming the dislocations to be uniformly distributed in the sample. In the process, we have also obtained a new relation for estimating effective diffusion length of minority carriers as a function of dislocation density in the sample. The proposed model has been shown to provide an excellent fit to the experimental data.

Contribution of dislocations to the zero-bias resistance-area product of LWIR HgCdTe photodiodes at low temperatures

Dislocations in the base material are shown to significantly influence zero-bias impedance of long wavelength infrared HgCdTe photodiodes by acting as a shunt, and by influencing their minority carrier lifetime. Consequently, temperature dependence of zero-bias resistance-area product (R/sub 0/A) of these photodiodes can be described very well over a broad temperature range, down to 25 K, after taking into account the temperature and dislocation dependence of the minority carrier lifetime in addition to the shunt resistance contribution of dislocations. Further, based on the theoretical prediction that the shunt resistance contribution of a dislocation is a sensitive function of the magnitude of the charge around its core, it is proposed that the scatter of the R/sub 0/A experimental data in diodes with dislocation densities of less than 1/spl times/10/sup 7/ cm/sup -2/ could be the result of statistical variations in the charge around the core of dislocations. Interaction of dislocations among themselves may be responsible for deviations above dislocation densities of 1/spl times/10/sup 7/ cm/sup -2/.

Dependence of responsivity on the structure of a blocking contact in an intrinsic HgCdTe photoconductor

Abstract Analytical expressions relating the low frequency responsivity of an intrinsic photoconductor with the structure of the blocking contact are presented. Results of numerical calculations for an 8 to 12 μm HgCdTe photoconductive detector show that the optimum structure of a blocking contact is governed by the carrier recombination velocity at the metal-semiconductor interface.

Effect of dislocations on minority carrier lifetime in HgCdTe

This article shows that the temperature dependence of minority carrier lifetime in mercury cadmium telluride samples of differing dislocation densities can be accounted well by including the dislocation contribution in addition to the contribution of well known radiative, auger, and Shockley–Read recombination mechanisms. The dislocation contribution to the minority carrier lifetime has been calculated on the basis of a recently proposed model [V. Gopal and S. Gupta, IEEE-ED 50, 1220 (2003)] by treating dislocation as a discontinuity of the lattice.

Identification of trap-assisted tunnelling in a constant-current mode in HgCdTe photodiodes

Voltage - temperature characteristics of photovoltaic diodes fabricated on HgCdTe epitaxial layers have been studied. The diodes are operated with forward bias in a constant-current mode. Plots of forward voltage versus temperature (T) clearly reveal well resolved peaks suggestive of a trap-assisted tunnelling (TAT) mechanism. The nonlinearity of versus T plots in terms of these peaks correlates with reverse leakage currents and resistance - area products whereas the ideality factor correlates with TAT. Two such peaks are observed in our samples near 120 and 90 K. The origin of these peaks is explained by a multi-step tunnelling recombination model.

Studies on VOx thin films deposited over Si3N4 coated Si substrates

Vanadium oxide (VOx) thin films were deposited on to the silicon nitride (Si3N4) coated silicon (Si) substrate using reactive direct current magnetron sputtering at different substrate temperatures (Ts). The deposited films were characterized for their structural, morphological, optical and electrical properties. The average grain size of the deposited films was in the range of 95 to 178 nm and the strain varied from 0.071 to 0.054 %. The optical bandgap values of the films were evaluated using UV-Vis spectroscopy and lies in the range of 2.46 to 3.88 eV. The temperature coefficient of resistance (TCR) for the film deposited at 125 °C was -1.23%/°C with the sheet resistivity of 2.7 Ω.cm.

Studies on Nanostructured V2O5 Deposited by Reactive DC Magnetron Sputtering

Nanostructured V2O5 thin films were deposited on to cleaned Si (100) substrates using reactive DC magnetron sputtering technique at various substrate temperatures (Ts). The grain sizes of the films were around 140–210 nm. The field emission-scanning electron micrographs showed nanosheet like structure grown perpendicular to substrate. The optical bandgap energy of the films increased with increase in Ts. The films deposited at 100 °C exhibited a temperature coefficient of resistance (TCR) value of −2.5 %/ °C.

Isolating surface and bulk contributions in an HgCdTe junction diode

This paper reports the possibility of assessing the relative surface and bulk contributions in HgCdTe junction diodes by analyzing the Current - voltage (I - V) and dynamic impedance - voltage (Rd - V) characteristics of an individual diode. As an example, an analysis of the experimental data obtained on diodes fabricated in our laboratory on bulk grown p-type HgCdTe wafers using B+ implantation and ZnS passivating layers will be presented.

Carrier diffusion limited MTF of a back-illuminated pv detector array

Abstract Carrier diffusion limited MTF of a back-illuminated HgCdTe-PV detector array has been calculated by including the multiple reflections within a CdTe-HgCdTe structure. Results of these calculations show that there is only a marginal improvement in MTF. The gain in quantum efficiency can however become substantial if the unilluminated surface is made strongly reflecting.