Paper LB+EM+GR+MN+TR-WeA12
Hf-based High-k Dielectrics for Ge MOS Stacks

Wednesday, October 31, 2012, 5:40 pm, Room 14

Ge has drawn much attention recently, being a leading candidate to serve as the channel material of future metal oxide field effect transistors (MOSFETs) due to its high carrier mobility with respect to Si. The interest in Ge is mostly because of its high hole mobility. Most of Ge related researches were focused so far on the challenge of Ge surface passivation. In this research we have moved on to the next challenge - finding a suitable high-k dielectric for a Ge-MOS stack. The high-k dielectric has to be chemically and thermally stable on top of the chosen passivation layer, have sufficiently high energy barriers with respect to Ge energy band edges, and have a large dielectric constant in order to obtain the required low effective oxide thickness (EOT). We have studied the chemical, structural and electrical properties of various Hf-based high-k dielectrics: HfO₂, Hf_xZr_1-xO₂, Hf_xAl_1-xO₂ and Hf_xGd_1-xO₂. All high-k dielectrics (4 nm thick) were deposited by atomic layer deposition (ALD) on top of a constant passivation stack composed of a thin GeO₂ layer (0.7 nm thick) followed by a thin (2 nm) ALD Al₂O₃ layer. The Al₂O₃ layer, which has high band offsets to Ge and GeO₂, was added since HfO₂, as many of the leading candidates for high-k dielectrics, are unstable on top of Ge or GeO_2. A thorough and systematic electrical and chemical characterization of this complex gate stack was carried out. The interesting results show that this challenge of seeking for a superior high-k is not detached from the passivation challenge. Surprisingly, we have found that although the passivation stack was kept constant for all systems studied, the apparent D_it (density of interface states) changes when the top high-k material is modified. Another interesting phenomenon is revealed when different methods of D_it characterization are compared - each method points out a different high-k as the one with the lowest D_it. These observations imply that the C-V characteristics do not reflect only the role of Ge interface traps, but also of traps throughout the whole stack, at least to a distance of 2.7 nm (the total thickness of the passivation stack) from the Ge surface . These results emphasize even more the great challenges in integrating Ge as a new channel material. We have also analyzed the band alignment for all high-k dielectrics using XPS with respect to the underlying layers. All high-k dielectrics have similar band gaps at the range of 5.2-5.9 eV. The conductance and valence band offsets with respect to Ge are all larger than 1 eV, which make them all suitable for Ge-MOSFETs in terms of band alignment.