Study of polycrystalline CdTe films by contact and contactless pulsed photo-ionization spectroscopy

The methodology of sample preparing and combining of measurement techniques in comprehensive characterization of thin cadmium telluride (CdTe) heterostructures has been demonstrated by recording spectra of deep (bulk and surface) traps, acing coherently within photo-ionization and thermal emission processes. The CdTe heterostructures on silicon (Si) substrates have been examined by combining measurements of the microwave probed and contact photoconductivity transients varying pulsed laser excitation wavelengths. The pulsed photo-ionization spectra have been analysed using the Lucovsky model. It has been shown that microwave probed photoconductivity transients in polycrystalline CdTe films with bare surfaces are governed by diffusion limited surface recombination. The surface recombination is there mainly influenced by deep traps of 1.23 eV photo-activation energy. The bulk traps in CdTe - Si structure containing metal electrodes exhibited a spectrum with five photo-ionization steps, having the threshold photo-ionization energy of the E-b1 = 0.57 eV, E-b2 = 0.94 eV and E-b5 = 1.42 eV, E-b4 = 1.13 eV, and E-b3 = 1.05 eV which are attributed using literature data to the Te interstitials (E-b1), to Cd (E-b2 and E-b5) and Te (E-b4) vacancies, and to anti-site (Te at Cd) defect (E-b3), respectively. Shallow emission centres with thermal activation energy of 100 meV have been estimated from carrier lifetime variations with temperature.