Negative-U trapping centers evidenced by admittance spectroscopy at the Ge/GeO2 interface

We report on the investigation of electrically active defects at the interface between germanium and germanium oxide. Germanium is a promising channel material for high performance CMOS [1], provided the proper material is successfully implemented as gate oxide. In particular, the gate stack comprising a high-k oxide and a GeO2 insulating layer is one of the best options, in terms of scalability and density of interface traps (Dit) [2]. Still, at present the Ge/GeO2 interface is lagging behind the silicon counterpart, mainly due to traps that are not passivated by hydrogen anneal. In this regard, we previously identified the Ge dangling bond (DB) as an electron trap at the interface, by means of electrically detected magnetic resonance spectroscopy [3,4]. In this paper, we present an extensive investigation of Ge/GeO2/Al2O3 MOS capacitors by admittance spectroscopy, revealing the negative-U behavior of interface traps with transition levels about 0.1 eV above valence band edge. This result provides further insight into the reasons of the unsuccessful hydrogen passivation of the traps and the undetectability of Ge DBs by conventional electron spin resonance. [1] R. Pillarisetty, Nature 479, 324–8 (2011). [2] X. Yang et al., Appl. Phys. Lett. 105, 092101 (2014). [3] S. Baldovino, A. Molle, and M. Fanciulli, Appl. Phys. Lett. 93, 242105 (2008). [4] S. Paleari, S. Baldovino, A. Molle, and M. Fanciulli, Phys. Rev. Lett. 110, 206101 (2013).