High permittivity materials for oxide gate stack in Ge-based metal oxide semiconductor capacitors

In the effort to ultimately shrink the size of logic devices towards a post-Si era, the integration of Ge as alternative channel material for high-speed p-MOSFET devices and the concomitant coupling with high permittivity dielectrics (high-k) as gate oxides is currently a key-challenge in microelectronics. However, the Ge option still suffers from a number of unresolved drawbacks and open issues mainly related to the thermodynamic and electrical compatibility of Ge substrates with high-k gate stack. Strictly speaking, two main concerns can be emphasized. On one side is the dilemma on which chemical/physical passivation is more suitable to minimize the unavoidable presence of electrically active defects at the oxide/semiconductor interface. On the other side, overcoming the SiO2 gate stack opens the route to a number of potentially outperforming high-k oxides. Two deposition approaches were here separately adopted to investigate the high-k oxide growth on Ge substrates, the molecular beam deposition (MBD) of Gd2O3 and the atomic layer deposition (ALD) of HfO2. In the MBD framework epitaxial and amorphous Gd2O3 films were grown onto GeO2-passivated Ge substrates. In this case, Ge passivation was achieved by exploiting the Ge4+ bonding state in GeO2 ultra-thin interface layers intentionally deposited in between Ge and the high-k oxide by means of atomic oxygen exposure to Ge. The composition of the interface layer has been characterized as a function of the oxidation temperature and evidence of Ge dangling bonds at the GeO2/Ge interface has been reported. Finally, the electrical response of MOS capacitors incorporating Gd2O3 and GeO2-passivated Ge substrates has been checked by capacitance–voltage measurements. On the other hand, the structural and electrical properties of HfO2 films grown by ALD on Ge by using different oxygen precursors, i.e. H2O, Hf(OtBu)2(mmp)2, and O3, were compared. Exploiting O3 as oxidizing precursor in the ALD of HfO2 is shown to play a beneficial role in efficiently improving the electrical quality of the high-k/Ge interface through the pronounced formation of a GeO2-like interface layer. In both cases, carefully engineering the chemical nature of the interface by the deliberate deposition of interface passivation layers or by the proper choice of ALD precursors turns out to be a key-step to couple high-k materials with Ge.