Vikram Dhar

Carrier density approximation for non-parabolic and highly degenerate HgCdTe semiconductors

We propose simple, analytical, approximations of carrier density for HgCdTe semiconductors that have non-parabolic energy bands and are highly degenerate. The proposed expressions are in the form similar to the classical Boltzmanns approximation without any adjustable parameter. These relations can be applied to HgCdTe for the case where electron densities are very high and the material is highly degenerate, e.g. highly accumulated surface due to passivant induced negative fixed charge density in n-HgCdTe photoconductor device. The expressions are valid in the LWIR and MWIR bands, and can be tuned further to apply to other bands.

Engineering Interface Composition for Passivation of HgCdTe Photodiodes

A compositionally graded surface layer has been created for the passivation of Hg1-xCdxTe photodiodes. The graded CdTe-Hg1-xCdxTe interface was created by deposition of CdTe and subsequent annealing. It was found that the composition gradient and width of the graded region could be tailored by adopting a suitable annealing procedure. The effect of grading on the interface electrical properties and photoelectrical properties was studied by X-ray photoelectron spectroscopy (XPS), photoconductive decay, and C-V measurements. Insulator fixed-charge density and interface-trap density could be reduced to 3times1010 cm -2 and 2times1010 cm-2middoteV-1, respectively, by creating a graded interfacial composition. The interface conditions so engineered led to a low surface recombination velocity ~3000 cm/s. A direct correlation has been established between the process conditions, interfacial composition, and the electrical/photoelectrical properties of the CdTe-Hg1-xCdxTe heterostructures. The passivation layer formed by this method is shown to be suitable for the fabrication of high-performance infrared detectors

Zener current contribution to the resistance-area product of 8- to 14-µm Hg 1-x Cd x Te photodiodes

The effect of Zener current, due to the internal field emission in the space-charge region of an n+-p junction, on the zero-bias resistance-area product RoA of narrow-bandgap HgCdTe photodiodes has been investigated theoretically. It is shown that the contribution of this mechanism in these photodiodes could appreciably lower the RoA product in the temperature range of 10 to 100 K.

Effect of built-in electric field on crosstalk in focal plane arrays using HgCdTe epilayers

A simple two-dimensional model, for calculating the effect of the built-in electric field arising in HgCdTe epilayers due to the composition gradient that occurs in LPE growth, on the crosstalk is developed for the case of a back illuminated focal plane array. The calculations predict that the crosstalk does not decrease monotonically with increasing electric field (i.e., weak peaks occur), unlike in the simple one-dimensional model of Kamins and Fong. The zero-field crosstalk varies between 1% and 10%, depending upon the material parameters. For electric fields in the range of 1–10 V cm−1, the crosstalk is reduced to negligible values (<0.01%). The model is approximate to the extent that the effect of the electric field in reducing the crosstalk is somewhat overestimated. The crosstalk depends strongly on the diffusion length and on the absorption coefficient, but very weakly on the diffusion velocity of carriers. The effects of diode size and dead space are similar to those of other models. The effect of the CdTe–HgCdTe interface recombination velocity has been taken into account, whereas that of the top surface recombination velocity has been neglected. The crosstalk to the second nearest neighbour diode is an order of magnitude smaller than that to the first neighbour diode. It goes to zero at a much smaller electric field. The crosstalk is shown to be slightly higher in medium wavelength IR arrays (MWIR) than in long-wavelength IR (LWIR) arrays.

Dependence of zero-bias resistance-area product and quantum efficiency on perimeter-to-area ratio in a variable-area diode array

The dependence of the zero-bias resistance-area (RA) product and the quantum efficiency (η) of variable-area diode arrays is numerically calculated by solving the diffusion equation in a cylindrical, three-dimensional geometry in the thick base approximation. The calculation is done for long-wavelength IR HgCdTe n+-on-p diffusion-limited photodiodes at 77 K. The inverse resistance-area product 1/(RA) and the square root of the quantum efficiency, η1/2, are plotted against the perimeter-to-area (P/A) ratio. The 1/RA results are fitted to a quadratic dependence on the P/A ratio. The dependence of the 1/RA on the minority carrier diffusion length, the junction depth and the surface recombination velocity (SRV) is evaluated. An empirical expression is proposed that largely accounts for the dependence of the coefficients of the quadratic on these parameters and is more general than those used in previous studies. The results are also in reasonable agreement with the results of Briggs, expressed in terms of the parameter f3D, that are valid for zero junction depth and zero SRV. The slope of the quantum efficiency versus P/A plot, which is approximately a straight line, is related to an effective length Lopt, that also depends on the diffusion length, junction depth, SRV and the absorption coefficient α. The parameter Lopt varies as α1/2.

Optimum diode geometry in a two-dimensional photovoltaic array

The quantum efficiency and the crosstalk of a photodiode in a long wavelength IR (LWIR) 2-D diode array are studied by numerically solving the 2-D diffusion equation of photocarriers in an array environ- ment. The quantum efficiency depends strongly on both the diode size and the junction depth, and can be expressed, to good accuracy, as a quadratic function of the two variables. The results, corresponding to HgCdTe n 1 -on-p backside illuminated photovoltaic (PV) diodes, are compared with published Monte Carlo results and with analytical (1-D) special cases. For a given pitch, smaller diodes have lower crosstalk, but also lower quantum efficiency. This implies an optimal diode size. The highest quantum efficiency is obtained for low junction depths (1 mm), and the optimum diode size, maximizing the quantum efficiency and minimizing the crosstalk, lies in the range of 15 to 30 mm for a pitch of 50 mm over a wide range of diffusion lengths. The quantum efficiency de- pends weakly on the pitch, for a given ratio of diode size to the pitch.

Effect of 300 K ambient background on C–V characteristics of HgCdTe MIS structures

Abstract We propose a simple model that describes the effect of 300 K ambient background on low-frequency (LF) capacitance–voltage characteristics of metal–insulator–semiconductor (MIS) structures based on HgCdTe materials. The materials used for both photoconductive (PC) and photovoltaic (PV) infrared detectors are investigated. We modify the solution of the one-dimensional Poisson’s equation and calculate LF capacitance including the effect of incident photon flux corresponding to the 300 K background. We identify the situations where this effect cannot be neglected. Based on our calculations, we find that in n-type materials used for medium-wave infrared (MWIR) PC devices, the minimum capacitance ( C min ) can increase by as much as 4.4%, whereas this increase is about 9.6% for materials used for the long-wavelength infrared (LWIR) devices, for experimentally achievable minority carrier lifetimes and carrier concentrations. Similarly, in p-type materials used for LWIR PV devices, there is a very small increase of 1.5% in C min , whereas there is no change in the MWIR region. The predictions of our model agree with experimental results and imply that the effect of 300 K ambient background radiation is very small compared to that of traps in p-type materials used for LWIR PV devices.

Quasi-2D analysis of the effect of passivant on the performance of long-wavelength infrared HgCdTe photodiodes

The results of a quasi-two-dimensional model for calculating passivant-induced surface leakage currents due to band-to-band tunnelling in mercury cadmium telluride (MCT) photovoltaic (PV) diodes are presented. The object is to assess the effect of a fixed surface state charge density due to a passivant on the zero-bias resistance - area product for the technologically important case when surface state charges accumulate the MCT surface. Calculations are carried out to estimate the tolerable value of beyond which the of the MCT diode degrades. To the best of our knowledge, this is the first time that such a detailed calculation involving the acceptor concentration profile near the surface has been reported for long-wavelength IR (LWIR) MCT photodiodes. This calculation has been done numerically, and hence the depletion width (pinched near the surface), the electric field and the band-to-band tunnelling are calculated as a function of depth, layer by layer, from the passivant - semiconductor interface. The currents - diffusion, generation - recombination, band-to-band and trap-assisted tunnelling - have been calculated in each layer, and the zero-bias resistance - area product is determined for each mechanism. Hence, the resultant is calculated. The results are compared with the earlier step model of Bhan and Gopal (Semicond. Sci. Technol. 9 (1994) 289), which assumed a surface layer of constant concentration . The present model indicates that for photodiodes with a cut-off wavelength of and an acceptor concentration , operating at 77 K, a would degrade significantly. This value is insensitive to composition (in the LWIR). For an diode, the tolerable value of is found to depend on the concentration in the layer. Further, the tolerable value of for an diode is much higher than for an diode, for donor concentrations .

Cold shield shading effects in 2-D arrays

Abstract Cold shield shading effects in a 2-D array have been calculated for square- and circular-aperture geometries of the cold radiation shield. It is concluded that a circular-aperture geometry leads to a minimum cold shield shading effect for a square mosaic array.

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

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.