Infrared photoconductivity of Te donor in Ge

We observed infrared photoconductivity in Ge:Te below 30 K. The photoconductivity intensity has a threshold around 100 meV and the main peak at 220 meV. The activation energy estimated from temperature dependence of photoconductivity intensity is 1.2 meV, which is much smaller than the first ionization energy 90 meV of Te donor.