Thermally stimulated discharge current studies of corona polarized pristine and swift heavy ion irradiated kapton-H polyimide

Abstract

Thermally stimulated discharge (tsd) current technique has been used to investigate the corona polarized pristine and swift heavy ion irradiated kapton-H polyimide. The samples of thickness 25 μm were irradiated with 100 MeV Fe⁺ beam (fluence: 1.8 × 10¹¹, 2.3 × 10¹² and 1.4 × 10¹³ ions/cm²). Corona charged pristine kapton-H samples show two tsd current peaks (P₁ and P₂) ∼40–70 and 150–190 °C, respectively. The peak P₁ is in the normal current direction and P₂ is in the abnormal current direction. These peaks are attributed to space charge relaxation process owing to the surface and bulk energy traps, respectively. The x¯/d value (x¯ is the mean penetration depth and d is the thickness of the sample) estimated for pristine kapton-H polyimide comes out to be 0.32 and suggests that the space charge distribution is approximated by space charge limited current (SCLC) distribution. However, the SCLC distribution in irradiated sample is not confirmed by large x¯/d value (0.86). The activation energies estimated from Bucci plot method for peaks P₁ and P₂ vary from 0.66 to 0.97 eV and from 0.92 to 1.31 eV, respectively, depending on the corona voltage. The charge trapping density for surface and bulk traps comes out to be (0.3–10.2) × 10¹⁷ and (4.9–14.6) × 10¹⁷ m⁻³, respectively. In the irradiated samples, abnormal tsd currents dominate and there is an enhancement in the bulk charge trapping density (13.2 × 10¹⁸ m⁻³). This enhancement is due to the formation of double/triple bonds and free radicals in irradiated samples. The irradiated corona polarized samples after prolonged aging (10–30 days) show a high temperature maximum with average activation energy of 0.90 eV. This current maximum has been associated with the space charge relaxation process due to bulk deep charge trapping levels.