Enhanced Phonon-Energy Coupling: Dramatic Reduction of Leakage Current of Silicon Oxide
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0917-E05-25
Enhanced Phonon-Energy Coupling: Dramatic Reduction of Leakage Current of Silicon Oxide Zhi Chen, Jun Guo, and Chandan B Samantaray Dept. of Electrical & Computer Engineering, University of Kentucky, Lexington, KY, 40506
ABSTRACT We study in detail a newly discovered effect, phonon-energy-coupling enhancement (PECE) effect, produced by rapid thermal processing (RTP). It includes two aspects: (1) Strengthening Si-D bonds and Si-O bonds and (2) Change of energy band structure and effective mass due to thermal shock. It is shown that not only Si-D bonds but also Si-O bonds have been strengthened dramatically, leading to enhancement of robustness of the oxide structure and the oxide/Si interface. For thick oxides (>3 nm), the gate leakage current has been reduced by two orders of magnitude and the breakdown voltage has been improved by ~30% due to phonon-energy coupling. For ultrathin oxides (2.2 nm), the direct tunneling current has been reduced by five orders of magnitude, equivalent to that of HfO2, probably due to the increased effective mass and barrier height. INTRODUCTION One of the fundamental limitations for scaling MOS transistors is the exponential increase in gate leakage current as oxide is scaled down to below 3 nm [1]. High-k gate oxides have been considered as candidates to replace silicon dioxide or oxynitride [2-4]. However, there are numerous challenging issues facing high-k gate oxides, e.g. threshold and flat-band voltage shifts, low mobility, and Fermi-level pining at the metal-gate/oxide interface [2-4]. The most difficult issue for high-k gate oxides is the low-k interfacial oxide layer, which dominates the dielectric constant. Therefore, this layer makes high-k oxide very difficult to be scaled down to less than 0.5 nm. If the leakage current of the oxide or oxynitride can be reduced by several orders of magnitude through the structure modification, the oxide/oxynitride may be further scaled down to the ultimate limit. In 1996, Lyding et al [5] discovered the hydrogen/deuterium (H/D) isotope effect of hotelectron degradation of MOS transistors. However, the H/D isotope effect has no effect on improvement of gate oxide [6,7]. The Si-H/D bond-breaking at the SiO2/Si interface is determined by two competing processes. One is that the energy of the bonds is accumulated through excitation by energetic hot electrons [8]. The other process is de-excitation where the bond energy is taken away by coupling [9]. It was suggested theoretically that the isotope effect is originated from the energy coupling from the Si-D bond to the Si-Si TO phonon mode, which significantly strengthens the Si-D bonds [9]. Wei et al [10] confirmed the above theory based on the study of vibrational modes of hydrogenated and deuteriated amorphous silicon. In 2000, one of us (Chen) noticed that there is large mismatch between the Si-D vibrational mode (510 cm-1) and the Si-Si TO phonon mode (495 cm-1) based on the data of amorphous silicon and proposed a bold hypothesis that if the similar mismatch exists in the SiO
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