| AVS 60th International Symposium and Exhibition | |
| Electronic Materials and Processing | Monday Sessions |
| Session EM+TF-MoM |
| Session: | High-k Gate Oxides for High Mobility Semiconductors I |
| Presenter: | S. Fadida, Technion Israel Institute of Technology, Israel |
| Authors: | S. Fadida, Technion Israel Institute of Technology, Israel F. Palumbo, Technion Israel Institute of Technology, Israel L. Nyns, IMEC, Belgium H.C. Lin, IMEC, Belgium S. Van Elshocht, IMEC, Belgium M. Caymax, IMEC, Belgium M. Eizenberg, Technion Israel Institute of Technology, Israel |
| Correspondent: | Click to Email |
One of the solutions to the constantly growing demands of the microelectronic industry is to replace Si, the channel material in metal oxide field effect transistors (MOSFETs), with a higher mobility semiconductor. Being a leading candidate, Ge has recently been the center of numerous research works. Ge surface passivation was a major challenge which was almost exclusively addressed in Ge related research. Once a few options for good passivation of Ge were found, the current challenge in Ge MOSFET research is to find a gate dielectric with the desired properties. This dielectric should have a large band gap, large band offsets with respect to Ge, chemical and thermal stability on top of Ge or on top of the passivation layer, and a high dielectric constant (high-k).
In this work the dielectric stack HfMOx/Al2O3/GeO2 with the nominal thicknesses 4/2/0.7 nm was studied on top of p-Ge. GeO2 served as the passivation layer, while Al2O3 was chosen for its wide band gap and band offsets with respect to Ge, and its excellent chemical stability on GeO2. Since the dielectric constant of Al2O3, is not sufficiently high, an additional top high-k dielectric is needed. We have studied Hf-based oxides (HfO2, HfAlOx, HfGdOx, and HfZrOx) as the top high-k dielectric.
In order to examine the suitability of the dielectric materials, current-voltage (I-V) measurements were conducted. The results show a significant difference between HfAlOx and the other high-k studied. While HfO2, HfGdOx and HfZrOx display similar I-V curves, with the same trend and the same level of leakage current, the leakage current of the HfAlOx containing sample is one order of magnitude lower.
In order to understand the root cause of this phenomenon, the band alignments of the samples were studied using X-Ray Photoemission Spectroscopy (XPS). Here, again, the HfAlOx sample stands out: only in this high-k dielectric the valence band offset (VBO) with respect to Ge is larger than the VBO of Al2O3 (3.6 and 3.1 eV respectively), while for the other dielectrics it is equal to the VBO of Al2O3 (3.0-3.1 eV). Examination of the conduction band offsets (CBO) shows no difference between the samples. The CBO of the Hf-based high-k dielectrics with respect to Ge are in the range of 1.5-1.7 eV, while the CBO of Al2O3 with respect to Ge is 3.1eV.
These results can be correlated with our I-V results leading to the conclusion that the dominant mechanism of current in these samples is hole tunneling in the Fowler-Nordheim mechanism. In order to establish this conclusion, we have also measured the leakage current of a reference sample with the structure: Al2O3/GeO2/Ge. The results confirm the model suggested.