On the Suitable Choice of Metal for HgTe Nanocrystal-Based Photodiode: To Amalgam or Not to Amalgam

HgTe, thanks to its unique spectral tunability in the infrared, is the only material able to cover near-, short-, and mid-wave infrared. Current best devices rely on electrodes made from transparent conductive oxides and gold, but so far, none of these completely fit for the intended purpose. Gold is not compatible with Si foundries, and transparent conductive oxides are highly lossy in this spectral range, limiting electrode transparency. Metal-based electrodes appear as good alternative candidates but require further investigations. While obvious constraints of work function get raised, chemical stability appears equally important. Here, we screen the use of Au, Al, Ag, and Zn as possible metals and reveal that in the case of Ag, dramatic transformations of Ag and HgTe are observed. Especially, a cation exchange procedure can occur over a solid-state film without intentional heating of the sample. This process has then been studied by combining both structural and electronic probes. This work points out the importance of the careful choice of surrounding electrodes in the case of HgTe since the observed mechanism is likely not limited to Ag. On the other hand, both Au and Al appear stable toward this transformation.