Defect states in 2.0-MeV electron-irradiated phosphorus-doped silicon

Abstract

Float-zoned n-type silicon samples with phosphorus concentrations in the range 1014–1017 P atoms cm−3 have been irradiated by 2-MeV electrons at room temperature with the aim of studying the role of phosphorus in the resulting damage. Using deep-level transient spectroscopy we were able to observe a number of previously unreported deep traps. A major post-irradiation bistable defect center detected in highly P-doped (∼1017 cm−3) material has an electron trap at Ec−0.42 eV, and a hole trap at Ev+0.15 eV (Ec and Ev are the conduction- and valence-band edges, respectively), each of which is associated with one of the two configurations of the center. The center, alternating between being acceptorlike to being donorlike as it undergoes configurational transformation, is suggested to be P related. Two metastable electron traps located at Ec−0.32 eV and Ec−0.42 eV are observed immediately after irradiation in moderately P-doped samples (∼1015–1016 cm−3). Interestingly, the configurational transformation rates of these states are observed to be dependent on the magnitude of the bias applied to the diode during cooldown to LN2 temperature. This is taken to indicate that configurational transformations, often thermally activated, can in some cases be influenced by the electric field strength within the depletion region. Upon thermal annealing at 200 °C of the latter two states, a couple of new states emerge; an electron trap at Ec−0.42 eV, stable up to temperatures above 300 °C, and a hole trap at Ev+0.25 eV which anneals out at 300 °C. An electron trap situated at Ec−0.24 eV with a quite small capture cross section for electrons is seen in lightly P-doped material (∼1014 cm−3) following annealing at 300 °C. Tentative propositions for the defect assignments of the observed traps are put forward.