Effect of 14 MeV Neutrons on Space-Charge-Limited Current of Electrons in High Purity Silicon

Alloyed and symmetrical n+υn+ devices made of nominally 75k Ωcm υ-type Si are analyzed before and after irradiation with 14MeV neutrons at room temperature and doses of 1.2×1011, 5.5×1011 and 4.0×1012n/cm2. Immediately after the application of a large turn-on voltage step at t ≈ O, the flow of electrons through these devices is by pure, trap free, space-charge-limited current (sclc). From an analysis of this sclc, it is established that the drift velocity-field relationship of electrons in Si is affected by the radiation only at low temperatures in the low field range. For t>0, the current after irradiation decays below its initial trap free sclc value, thus revealing the presence of traps. Two categories are identified: fast traps with energy levels at about ΔEtf ≈ 0.13eV below the conduction band and slow traps with energy levels at roughly ΔEts≈005eV. Cross sections are also obtained. Introduction rates are about equal for both (llcm-l for fast traps and 8cm-1 for slow traps). These results demonstrate the sensitivity of sclc as a tool to detect traps and changes in the drift velocityfield relationship caused by radiation. Some implications of these results are discussed and additional experiments are suggested.