Current status of hollow cathode gas flow sputtering for advanced TCOs

Herein, the recent findings on hollow cathode gas flow sputtering (GFS) of transparent conductive oxides (TCO) films are reviewed. The GFS is a unique deposition technique that offers extraordinary process conditions for thin-film growth. The GFS core element is a hollow cathode discharge operating in the mbar pressure range. The sputtered atoms are transported by means of forced convection. These key features allow for unique deposition conditions: i) GFS is a remote process where reactive gas does not interact with the sputtered target surface. This allows for high process stability at any reactive gas partial pressure, opening up even the pathway for combined physical vapor deposition (PVD) and plasma enhanced chemical vapor deposition (PECVD) synthesis. ii) The GFS plasma delivers high plasma density in the order of 1012 cm-3 at the substrate position. When bipolar pulsing is applied to the hollow cathode, plasma-activated growth can be obtained even for insulating substrates. iii) Doped films can be produced in an elegant way when the target is composed from ring segments to adjust the specified doping level. In this article, the focus is on the process development using direct simulation Monte Carlo modeling of the deposition process and on the optimization of bipolar pulsing. Follow on this, two application cases are introduced: i) synthesis of p-type NiOx and Cu-doped and NiOx-doped films for application as hole conductor in perovskite solar cells where improved device stability is achieved compared to surface assembled monolayers which are state of the art and ii) synthesis of ZnOxNy films with the perspective for usage as semiconductive films in thin-film transistors.