Characterization of bulk and surface transport mechanisms by means of photocurrent techniques

The transport properties of ternary alloys, as CdZnTe, affect heavily the features required in X and Gamma spectroscopic detectors. The density and the nature of bulk defects and the lacking quality of contacts, characterized by high recombination center densities near the surface, damage strongly the quality of these materials. The photocurrent technique is a powerful tool to study the bulk properties and to investigate contact and surface quality. From steady-state photocurrent spectra information about the bulk trap defects and about the kinds of surface states far and near the contacts, has been obtained. By varying the bias at fixed wavelength, instead, it can be calculated the transport parameters like the product mobility-lifetime mu*tau and the ratio s/mu, this last one related to the surface underneath the metal contacts. This paper gives some contribution in the interpretation of high energy region of the spectra. The authors correlate the response of samples illuminated with energies above the band gap to the surface states and show the different effects of this states on the electron and the hole transport case. Moreover in this work the authors compare the photocurrent measurement with the X ray spectroscopy and also discuss several phenomena like the shape difference between the spectra taken at different photon flux and bias polarities. Also the band edge shift and the modification of s/mu parameter observed on varying the bias polarity have been discussed giving new a possible interpretations.