Detailed study of above bandgap optical absorption in HgCdTe

Abstract

HgCdTe remains the material of choice for high-performance infrared (IR) detectors due to its tunable direct bandgap energy corresponding to the IR spectral region, and the advancement of HgCdTe materials growth and processing technologies. Accurate knowledge of the HgCdTe optical absorption coefficient is important for IR detector design, layer screening, and device analysis. The spectral response for IR detectors is dependent on optical absorption above the bandgap energy, where much of the study of absorption coefficient in HgCdTe has focused on the bandtail region. In this work, the optical absorption coefficient was studied by theoretical bandstructure calculations and experimental measurements on HgCdTe layers using techniques of IR spectroscopic ellipsometry and IR transmission. The theoretical and experimental results suggest that the absorption coefficient between 600 cm Ϫ1 and 5,000 cm Ϫ1 is related to energy relative to bandgap with a fractional exponent between 0.6 and 1, rather than the previously used expressions relating to a parabolic or hyperbolic bandstructure. The fitting parameters for Hg 1-x Cd x Te with x ϭ 0.22-0.60 are presented to develop a model for the optical absorption coefficient spectra. The calculated detector spectral response using the new and previously reported absorption coefficient models suggests that next generation IR detectors employing multilayer structures with graded compositional profiles will likely benefit from this new model.

Numerical Analysis of a Very Long-Wavelength HgCdTe Pixel Array for Infrared Detection

“…In our current simulations we use the Hougen 15 model, which provides the results shown. We are aware of recent suggested improvements 16,17 and we will test them in future developments of this work. Below 11 lm the discrepancy increases; in particular the measured external data falls lower than the simulated external data, but this is mainly a measurement artifact of the experimental set-up.…”

Performance Comparison of Long-Wavelength Infrared Type II Superlattice Devices with HgCdTe

“…The formula for a used by Pellegrino and DeWames 16 when fitting measured quantum efficiency for the T2SL gives 1620 cm À1 at 9.813 lm. In comparison, the formula for Hg 1Àx Cd x Te by Moazzami et al 17 (with x = 0.2194) gives 2017 cm À1 at 9.813 lm. Thus, in this example, the absorption coefficient at a wavelength 1 lm shorter than the cutoff is smaller in the T2SL than HgCdTe by only about 20%.…”

Absorption of Narrow-Gap HgCdTe Near the Band Edge Including Nonparabolicity and the Urbach Tail

“…(6) by assuming the dispersion relationships E c ðkÞ ¼ k 3 1þc þ E g and E lh ðkÞ ¼ Àk 2, corresponding to 0.5 £ c £ 2, exactly the same range as in our previous discussions. Moazzami et al 27 have empirically determined a c value of approximately 0.7 for a Hg 0.69 Cd 0.31 Te sample in the energy range of about 100 meV above the energy gap. Chu and co-workers 18,28 found that a logarithmic approximation works best in the energy range of 100 meV above the energy gap.…”