Basics: High-K dielectrics for nanoelectronics applications

The replacement of SiO2 gate dielectrics and poly-Si gate electrodes with high-K gate dielectrics and metal gate electrodes are two of the most pressing upgrades that the MOSFET must undergo in order for scaling to continue at its current pace. The band offsets of the metal gate/high-K dielectric stacks are critical to the success of these devices, as they are the barrier heights which resist the transfer of electrons and holes across the gate dielectric, i.e. gate leakage current. We use a combination of UV photoemission and inverse photoemission spectroscopies (UPS, IPS) in the same vacuum chamber to measure the occupied and unoccupied densities of states of thin film oxide/semiconductor samples, from which measurements we determine valence and conduction band offsets (VBO, CBO) between the dielectric and the semiconductor. By sequentially depositing metal in situ, and performing UPS and IPS between depositions, we also can determine the VBO and CBO between the metal and dielectric layers. In addition, we have performed similar photoemission measurements at higher photon energies at beamline U5 of the National Synchrotron Light Source to observe the core levels of these materials as well as the valence bands.

       Recent Highlights: High-K dielectrics

Room temperature reduction of HfO2/Ge interfacial oxide upon deposition of Al
     With the trend towards the use of these alternative gate dielectric materials, and the departure from reliance on the Si/SiO2 interface, Ge has emerged as an attractive candidate semiconductor material for next-generation MOS devices, but material stability and band at the high-K interface are critical issues for successful implementation of this technology.  In this work, we study the stability of a HfO2/GeOx/Ge stacks upon Al deposition at 300 K, studied using synchrotron X-ray photoemission spectroscopy (XPS), ultra-violet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPS). We have determined that the high-K/Ge VBO = 3.0 eV, the CBO = 2.0 eV, and the oxide bandgap is 5.7 eV in good agreement with earlier photoemission and internal photoemission measurements. Surprisingly, we find that the Al oxidizes immediately upon deposition, but not at the expense of reducing the HfO2 film. Rather, the interfacial GeOx is reduced upon metallization. This process occurs even at room temperature. The deposition of additional Al results in elimination of the interface oxide, metallic overlayer growth, and establishing an electric field across the oxide, but no apparent reduction of the HfO2 film.
     The figure at the left shows SXPS spectra as Al is sequentially deposited on the HfO2/GeOx/Ge sample.  With only 3Ang Al, the Ge core levels show that the interfacial GeOx is almost completely reduced.  Moreover, there is no appreciable change in the Hf core levels indicating that HfO2 is not reduced in this process.  Spectra of the Al 2p core level (not shown) indicate only oxidized Al is present.   Upon deposition of additional Al, metallic emission is seen at the Fermi level, the HfO2 features are all shifted by 0.7 eV away from the Fermi level (creating a field across the oxide), and further reduction of the HfO2/Ge interface occurs.

Read more: GeOx interface layer reduction upon Al-gate deposition on a HfO2/GeOx/Ge(001) stack S.Rangan, E.J. Bersch, R.A. Bartynski, and E. Garfunkel, Applied Physics Letters Appl. Phys. Lett.92, 172906 (2008)

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