Hafnia-alumina nanolaminates show improved smoothness and reduced crystallinity relative to pure hafnia in films formed by atomic layer deposition (ALD). However, typical nanolaminates also show reduced cross-plane thermal conductivity due to the much larger interface density relative to continuous films. We find that the interface thermal resistance in hafnia-alumina nanolaminates is very low and does not dominate the film thermal conductivity, which is 1.0 to 1.2 W(m K) at room temperature in 100 nm thin films regardless of the interface density. Measured films had a number of interfaces ranging from 2 to 40, equivalent to interface spacing varying from about 40 to 2 nm. The degree of crystallinity of these films appears to have a much larger effect on thermal conductivity than that of interface density. Cryogenic measurements show good agreement with both the minimum thermal conductivity model for disordered solids and the diffuse mismatch model of interface resistance down to about 80 K before diverging. We find that the films are quite smooth through a 400:5 ratio of hafnia to alumina in terms of ALD cycles, and the refractive index scales as expected with increasing alumina concentration.
Bibliographical noteFunding Information:
Funding support is acknowledged from the Joint Technology Office under Grant Nos. FA9550-10-1-0543 and FA9451-10-D-0224 and the Office of Naval Research under Grant No. N00014-09-1-1112. Parts of this work were carried out in the University of Minnesota (UMN) Nanofabrication Center (NFC) and the Characterization Facility (CharFac), which receive partial support from NSF through the NNIN and MRSEC programs, respectively. SPM measurements were performed in the CharFac by Greg Haugstad, and assistance with XRD measurements from Maria Torija is acknowledged.