Photoconductivity observations on phosphorous-doped n-type CVD diamond at low temperature and varying electric field

Photocurrent measurements on phosphorous-doped n-type diamond have been performed at liquid helium temperature and varying electric field. For increasing fields the onset of the experimental photocurrent versus photon energy shifts towards smaller photon energy. At relative small hν its slope is nearly constant, although smooth structures can be observed in the spectrum. These and other observations at low photon energy are interpreted using an impurity-assisted tunnelling model. The idea behind this model is that, at finite electric fields, the impurity electron tunnels a few times towards neighbouring neutral impurity atoms, until it arrives in a non-localized conduction band state and contributes to the photocurrent. The experimental results could be explained in a semi-quantitative way. The consequences for various experimental circumstances (varying field, temperature, photon energy, impurity concentration) are discussed.