Optically pumped photoconductivity and its use for assessing traps in Si:Pt

The use of double-beam photoconductivity for assessing the properties of traps between the dark Fermi level and the nearer band in semi-insulating material is described. Provided unipolar conduction (either electrons or holes but not both) can be ensured, the effect of a relatively intense pump beam is to sweep the majority-carrier quasi-Fermi level from its equilibrium position towards the band edge. By adjusting the sample temperature and the pump intensity the quasi-Fermi level can be swept in a controlled way through all the traps in the material, up to and including shallow levels. The traps can thus be detected in small-signal chopped photoconductivity due to a probe beam and their photoionisation spectra determined. The technique is applied to deep traps in Si:Pt. Shallow-level spectra show oscillatory photoconductivity. Four 'deep' levels are found in p-Si:Pt and two in n-Si:Pt.