Vishnu Gopal

Analysis of dark current contributions in mercury cadmium telluride junction diodes

Abstract An analytical approach to analyze the dark current–voltage (I–V) and dynamic impedance vs reverse bias voltage (Rd–V) characteristics of an HgCdTe junction diode is presented in this paper. Application to the experimental data is discussed to illustrate the approach. It is shown that the relative contributions of the various dark current contributing mechanisms viz. diffusion, generation–recombination, thermal trap assisted tunneling, band-to-band tunneling, avalanche multiplication and ohmic current component can all be isolated, if present.

Pulsed laser deposited vanadium oxide thin films for uncooled infrared detectors

Vanadium oxide thin films were deposited by pulsed laser deposition (PLD) technique using V 2O5 as target. A new deposition parameter has been extracted to deposit vanadium oxide thin films at room temperature for uncooled microbolometers. Temperature coefficient of resistance (TCR) is one of the vital bolometric parameters, which influences the performance of the uncooled microbolometer infrared detectors was determined. The TCR values of vanadium oxide films deposited by PLD at room temperature are coinciding with the reported TCR values of successful vanadium oxide thin films deposited at elevated temperatures by other techniques for bolometric applications. We further investigated the influence of laser fluence on the electrical property of the vanadium oxide films.

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.

Study of a pulsed laser deposited vanadium oxide based microbolometer array

A 2D 10-element test microbolometer array was fabricated without an air-gap thermal isolation structure. The microbolometer uses vanadium oxide film deposited by pulsed laser deposition at room temperature as the infrared (IR) sensitive layer. The IR response of the uncooled microbolometer was evaluated in the spectral region of 8–15 μm. The detectivity and the responsivity were determined as ~ 6 × 105 cm Hz1/2 W−1 and 36 V W−1 respectively at a 10 Hz chopper frequency with 50 μA bias current for a thermal conductance G ~ 10−3 W K−1 between the thermal sensing layer and the substrate. The preliminary results for the test microbolometer array are discussed and compared with those for microbolometers fabricated on micromachined thermally isolated structures.

Room temperature deposited vanadium oxide thin films for uncooled infrared detectors

We demonstrate the room temperature deposition of vanadium oxide thin films by pulsed laser deposition (PLD) technique for application as the thermal sensing layer in uncooled infrared (IR) detectors. The films exhibit temperature coefficient of resistance (TCR) of 2.8%/K implies promising application in uncooled IR detectors. A 2-D array of 10-element test microbolometer is fabricated without thermal isolation structure. The IR response of the microbolometer is measured in the spectral range 8-13 {mu}m. The detectivity and the responsivity are determined as {approx}6x10{sup 5} cm Hz{sup 1/2}/W and 36 V/W, respectively, at 10 Hz of the chopper frequency with 50 {mu}A bias current for a thermal conductance G{approx}10-3 W/K between the thermal sensing layer and the substrate. By extrapolating with the data of a typical thermally isolated microbolometer (G{approx}10{sup -7} W/K), the projected responsivity is found to be around 10{sup 4} V/W, which well compares with the reported values.

Analysis of the infrared plasma reflectivity spectra of semiconductors

Abstract A combination of the following techniques is proposed to analyse the infrared plasma reflectivity spectra in semiconductors: (i) Slope Technique, to determine ϵ x . (ii) Reflectivity Minimum Technique, to determine N/m c and τ. (iii) Curve Fitting Technique, to include frequency dependence of τ, which seems to be due to two carrier effects in multiple energy band structures. Experimental data of Dionne & Woolley (3) on Pb-Sn-Te have been used to demonstrate the application of the present method.

Energy gap-refractive index interrelation

Abstract The interrelations between the energy band gap and high frequency refractive index are summarized. The Penn model for the dielectric constant of semiconductors is used to construct a general interrelation, which avoids the shortcomings of the presently used relations. It is shown that the new relation with a characteristic set of constants for each alloy systems, Pb x Sn 1− x TE, Pb x Sn 1− x Se and Cd x Hg 1− x Te, describes the variation of high frequency refractive index with the energy band gap satisfactorily over the entire range of alloy compositions.

An analysis of the dynamic resistance variation as a function of reverse bias voltage in a HgCdTe diode

This paper reports an analytical approach to interpret the variation of dynamic resistance as a function of the reverse bias voltage in HgCdTe diodes. The method consists of estimating the trap density contributing to the trap assisted tunnelling in the given diode from the observed position of maxima in a dynamic resistance versus reverse bias voltage curve. Good agreement between theory and experiment is reported by using the estimated trap density in this way.

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/.