Atomic layer deposition of hafnium oxide on germanium substrates

Germanium combined with high- κ dielectrics has recently been put forth by the semiconductor industry as potential replacement for planar silicon transistors, which are unlikely to accommodate the severe scaling requirements for sub- 45-nm generations. Therefore, we have studied the atomic layer deposition (ALD) of Hf O2 high- κ dielectric layers on HF-cleaned Ge substrates. In this contribution, we describe the Hf O2 growth characteristics, Hf O2 bulk properties, and Ge interface. Substrate-enhanced Hf O2 growth occurs: the growth per cycle is larger in the first reaction cycles than the steady growth per cycle of 0.04 nm. The enhanced growth goes together with island growth, indicating that more than a monolayer coverage of Hf O2 is required for a closed film. A closed Hf O2 layer is achieved after depositing 4-5 Hf O2 monolayers, corresponding to about 25 ALD reaction cycles. Cross-sectional transmission electron microscopy images show that Hf O2 layers thinner than 3 nm are amorphous as deposited, while local epitaxial crystallization has occurred in thicker Hf O2 films. Other Hf O2 bulk properties are similar for Ge and Si substrates. According to this physical characterization study, Hf O2 can be used in Ge-based devices as a gate oxide with physical thickness scaled down to 1.6 nm.