Current mechanisms in VLWIR Hg1−xCdxTe photodiodes

Abstract

VLWIR (c∼15 m to 17 m at 78 K) detectors have been characterized as a function of temperature to determine the dominant current mechanisms impacting detector performance. Id−Vd curves indicate that VLWIR detectors are diffusion limited in reverse and near zero bias voltages down to temperatures in the 40 K range. At 30 K the detectors are limited by tunneling currents in reverse bias. Since the detectors are diffusion limited near zero bias down to 40 K, the R0Aimp versus temperature data represents the diffusion current performance of the detector as a function of temperature. The detector spectral response measurement and active layer thickness are utilized to calculate the HgCdTe layer x value and the optical activation energy Ea optical. The activation energy, Ea electrical, obtained from the measured diffusion limited R0Aimp versus temperature data is not equal to the activation energy, Ea optical, obtained from the spectral response measurement for all x values measured. Ea electrical=*Ea optical, where ranges between 0.64 and 1.0 For cutoff wavelengths in the 9 m at 78 K, Ea electrical=Ea optical. Ea electrical=0.65* Ea optical have been measured forc=17 m at 78 K detectors. As the band gap energy decreases to values in the range of 70 meV and lower, it is reasonable to expect a more dominant role of band tailing effects on the transport properties of the material system. In such a picture, one would expect the optical band gap to be unmodified, whereas the intrinsic concentration could be enhanced from its value for the ideal semiconductor. Such a picture could explain the observed behavior. Further probing experiments and modeling efforts will help clarify the physics of this behavior.