Origin of the Hydrogen/Deuterium (H/D) Isotope Effect of Hot-Electron Degradation of MOS Devices
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Origin of the Hydrogen/Deuterium (H/D) Isotope Effect of Hot-Electron Degradation of MOS Devices
Zhi Chen, Jun Guo, and Pangleen Ong Department of Electrical and Computer Engineering and Center for Micro-Magnetic and Electronic Devices, University of Kentucky, Lexington, KY 40506, U.S.A.
ABSTRACT In order to verify Van de Walle and Jackson’s theory on the isotope effect of the Si-H/D bonds resistant to hot-electron excitation [Appl. Phys. Lett., 69, 2441 (1996)], we measured the Si-H, Si-D, and other vibrational modes in oxidized silicon wafers annealed in hydrogen and deuterium using Fourier Transform Infrared (FTIR) spectrometry. Our FTIR data suggest that the frequency for the Si-D bending mode at the SiO2/Si interface is 490 cm-1. Our experimental data support Van de Walle and Jackson’s theory with some modification. Their theory is correct for the experiments of breaking Si-H/D bonds using scanning tunneling microscope (STM) where no oxide is involved. In the SiO2/Si case, the de-excitation of the Si-D bond may be due to the energy coupling from the Si-D bending mode to two vibrational modes; i.e., the Si-O TO mode and the Si-Si TO phonon mode. Van de Walle and Jackson only pointed out coupling to the Si-Si TO phonon mode. The strongest coupling might happen between the Si-D mode and the Si-O TO mode. Therefore, the oxide may play a crucial role in energy dissipation of the Si-D bond in metal-oxide-semiconductor (MOS) devices.
INTRODUCTION The giant hydrogen/deuterium (H/D) isotope effect was discovered in the study of the desorption of hydrogen (H) and deuterium (D) on silicon in ultra high vacuum (UHV) using Scanning Tunneling Microscopy (STM) [1,2]. This effect was later used in passivation of the SiO2/Si interface, leading to large improvement of the hot-electron-related lifetime of metal-oxidesemiconductor (MOS) transistors using deuterium process [3-9]. Van de Walle [10,11] proposed a theory to explain this remarkable isotope effect, stating that the Si-D bond is more resistant to hot-electron excitation than the Si-H bond. The Si-H/D bond-breaking at the SiO2/Si interface is caused by two competing processes. One is that the energy of the bonds is accumulated through excitation by energetic hot electrons. The other process is de-excitation where the bond energy is taken away by coupling between the Si-H/D vibrational modes and substrate phonons. Van de Walle [10] suggested that the vibrational frequency of Si-D bending mode is close to the Si-Si TO phonon mode (460 cm-1), resulting in energy coupling between the Si-D bending mode and the Si-Si TO phonon mode. This deexcitation efficiently strengthens the Si-D bond. On the other hand, because the vibrational frequency of the Si-H bond is far away from the Si-Si TO phonon mode, there is no energy coupling between the Si-H bending mode and the Si-Si TO phonon mode, leading to Si-H bonds more vulnerable to hot-electron excitation.
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However, no experimental data exist regarding the vibrational frequency of the Si-D bond at the SiO2/Si inte
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