Enhancement of UV Response and Suppression of Visible Response of p-Si/n-ZnO Heterojunction Photodiodes via NiO and MgO Insertion Layers

Nickel oxide (NiO) and magnesium oxide (MgO) thin films were inserted into p-Si/n-ZnO heterojunction photodiodes (HPDs), and their mechanisms in enhancing ultraviolet (UV) response and suppressing visible response were studied. The insertion of the MgO layer greatly reduced the leakage current of the HPDs due to the passivation of surface states in ZnO and hence enhanced the rectification ratio from 258.7 to 3769.2 by 14.6-times. HPDs with the NiO layer exhibited the highest UV response and photocurrent for all HPDs because NiO absorbs UV light. HPDs with the MgO layer demonstrated the least visible response and dark current. Among all of the HPDs, those with the NiO layer exhibited the largest UV-to-visible rejection ratio, while those with the MgO layer had the highest photo-to-dark current ratio and detectivity. The UV-to-visible rejection ratio was greatly enhanced from 101.1 to 652.3 by 6.4-times after the insertion of the NiO layer, and the photo-to-dark current ratio was enhanced from 3.3 × 10 2 to 4.5 × 10 4, by 136.4-times, after the insertion of the MgO layer. The HPDs with NiO insertion layer presented the fastest transient response, and those with MgO insertion revealed the slowest transient response due to the carrier trapping. Related mechanism for the improvements was elucidated by the band diagram.