HfO2 as gate dielectric on Ge: Interfaces and deposition techniques

To fabricate MOS gate stacks on Ge, one can choose from a multitude of metal oxides as dielectric material which can be deposited by many chemical or physical vapor deposition techniques. As a few typical examples, we will discuss here the results from atomic layer deposition (ALD), metal organic CVD (MOCVD) and molecular beam deposition (MBD) using HfO2/Ge as materials model system. It appears that a completely interface layer free HfO2/Ge combination can be made in MBD, but this results in very bad capacitors. The same bad result we find if HfGey (Hf germanides) are formed like in the case of MOCVD on HF-dipped Ge. A GeOx interfacial layer appears to be indispensable (if no other passivating materials are applied), but the composition of this interfacial layer (as determined by XPS, TOFSIMS and MEIS) is determining for the C/V quality. On the other hand, the presence of Ge in the HfO2 layer is not the most important factor that can be responsible for poor C/V, although it can still induce bumps in C/V curves, especially in the form of germanates (Hf–O–Ge). We find that most of these interfacial GeOx layers are in fact sub-oxides, and that this could be (part of) the explanation for the high interfacial state densities. In conclusion, we find that the Ge surface preparation is determining for the gate stack quality, but it needs to be adapted to the specific deposition technique.