Narrow gap MCT materials: towards terahertz detection

Can MCT based photoconductors be applicable for radiation wavelengths of more than 20 microns?
http://iopscience.iop.org/0268-1242/labtalk-article/55395
It is known that effective THz sources and detectors would be of great use for a number of applications, such as security, medicine, spectroscopy, wireless communication, etc. One of the candidates for radiation detection in the FIR/THz range is a Hg_1-xCd_xTe solid solution which has been the number one material in the fabrication of photoelectric IR detectors for the spectral range of 8-18 μm (x ≥ 0.2) over the last two decades. Until now, extending the operating spectral range to longer wavelengths has been complicated by strong composition fluctuations that lead to the high dark concentration of carriers, which degrades the detector sensitivity. Recent progress in molecular-beam epitaxy (MBE) has made it possible to produce high quality Hg_1-xCd_xTe films with a molar fraction of Cd below 0.2, which corresponds to photosensitivity at wavelengths longer than 18 μm. Such structures have not been sufficiently studied before, yet they show promise as a material for photodetectors in the FIR/THz range. One of the most important parameters for the development of a photoelectric detector besides the spectral range of the photoresponse is the nonequilibrium carriers lifetime that determines the detector’s speed and responsivity.

Photoconductivity spectra of different Hg_1-xCd_xTe samples

Detector prototype assembled for the measurements

In our paper we investigated the spectra and relaxation kinetics of far-infrared photoconductors in MBE-grown Hg_1-xCd_xTe epitaxial films with x from 0.192 to 0.152 (the latter corresponds to a gapless material) and compared them with structures produced by chemical vapor deposition. Spectral studies demonstrate a complex pattern of photoresponse including optical phonon absorption and transitions into resonant impurity/defect states, resulting in sharp peaks in the photoconductivity spectra. Investigation into the kinetics of photoconductivity were performed with 1 ns time resolution and revealed that minority carrier lifetime increases with the bandgap. Lifetime values allow estimation of responsivity and NEP that reach 5 A/W and 6 × 10 ⁻¹² W/Hz^1/2, respectively, at 19 μm and T = 77 K.

Vladimir Rumyantsev

About the author

Vladimir Rumyantsev is a junior research associate at the Institute for Physics of Microstructures of the Russian Academy of Sciences, Nizhny Novgorod. This work was conducted under the guidance of Professor Vladimir Gavrilenko at the laboratory of physics of semiconductor heterostructures and superlattices. His group currently focuses on terahertz spectroscopy of narrow gap MCT related structures by means of magneto-optical, photoconductivity and photoluminescence studies. Structures under study are grown at the A.V. Rzhanov Institute of Semiconductor Physics, Siberian Branch of Russian academy of Sciences, Novosibirsk.