Improvement of HgCdTe LWIR photovoltaic detector characteristics by hydrogenation

Abstract

In this thesis, hydrogenation is applied to reduce the diffusion current which is the fundamentally limiting current mechanism of LWIR HgCdTe diodes on Hg-vacancy doped HgCdTe wafer and the mechanism is revealed. Post-implantation annealing has been introduced as the method of improving the RoA value of HgCdTe diode. However, post-implantation annealing reduces generation-recombination current, TAT, and BTB current except diffusion current that is related to the minority carrier lifetime of the substrate. It is found that post-implantation annealing has limits on the improvement of LWIR diode performance because the RoA value of LWIR diode is limited by the diffusion current. The hydrogenation is, initially, introduced to increase the minority carrier lifetime of Hg-vacancy doped substrate. It is found that the hydrogenation increases the RoA value and photo-current response, which can be explained by the increased minority carrier lifetime. However, the LBIC measurement that is performed to measure the steady state diffusion length of minority carrier showed that the minority carrier diffusion length was not affected by the hydrogenation. The important clue that explains the hydrogenation mechanism was found by Hall measurement. It was found that p-type substrate was converted to n-type at the surface and gradually increasing p-type concentration appeared. These can be explained by the passivation of Hg-vacancy by the hydrogen. N- type is supposed to appear when the whole Hg-vacancies are passivated by the hydrogen. The doping concentration from the end of n-type will become p-type and the concentration of p-type would gradually increase because the amount of passivated Hg-vacancy would gradually decrease. The gradually doped p-type concentration can explain both the decreased diffusion current and increased quantum efficiency. The internal field within the doping grading region prevents the piling of minority carrier and the diffusion current decreases. T...