Stress induced leakage current in thin thermal silicon oxide
O. M. Orlov†, G. Ya. Krasnikov†, D. R Islamov‡,§, V. A. Gritsenko‡,§, T. V. Perevalov‡,§
†SC Molecular Electronics Research Institute, Zelenograd, Moscow 124460, Russian Federation
‡Rzhanov Institute of Semiconductor Physics, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russian Federation
§Novosibirsk State University, Novosibirsk 630090, Russian Federation
Abstract
This paper is devoted to research of Stress Induced Leakage Current, SILC. The current through the tunnel oxide during erase/rewrite operations by applying strong (~10MV/cm) electric field in the oxide increases the conductivity of the layer at low electric fields (~1MV/cm). This is caused by the appearance of redundant current components through the tunnel oxide compared to the current flowing in a strong electric field on the mechanism of Fowler-Nordheim. Write/erase operations of SONOS and TANOS flash memory with Si3N4-floating gates are driven by tunnel injections of electrons and holes through thin 1.8-8.0nm (tunnel) SiO2layer [Ed. by D. Brown, J. F. Brewer, IEEE Press, New York (1998); P.E. Blöchl, J.H., Stathis,Phys. Rev. Lett.83, 372 (1999)]. SILC limits reprogramming cycles number of the flash memory cells based on floating gate and the silicon nitride to 104-105. SILC also leads to accelerate the draining of charge in the flash memory elements in the data storage mode, that is, memory degradation characteristics. Despite the fact that the study of SILC is the subject of numerous articles [P.E. Blöchl, J.H., Stathis,Phys. Rev. Lett.83, 372 (1999); T. Endoh et al.,J. Appl. Phys.86, 2095 (1999); K. Komiya, Y. Omura,J. Appl. Phys.92, 2593 (2002); F. Jiménez-Molinos et al.,J. Appl. Phys.90, 3396 (2015)], the nature of this phenomenon is still a debatable issue. Recently a multiphonon model of charge transport in SILC was proposed [T. Endoh et al.,J. Appl. Phys.86, 2095 (1999); F. Jiménez-Molinos et al.,J. Appl. Phys.90, 3396 (2015)]. That model is not analytic and requires complicate numerical calculations to describe transport in SILC, and does not clarify the nature of the traps responsible for SILC. SILC phenomenon in thin thermal silicon oxide was studied experimentally. It is shown that the charge transport mechanism in SILC is phonon-assisted tunneling between traps, and oxygen vacancies act as the traps. The flow of charge through SiO2 generates oxygen vacancies, increasing the leakage currents.
SILC measurements were performed for test structures withpoly-Si top electrodes and7.5-nm-thick SiO2ononn+-Sipandp-Si substrates. Current-voltage characteristics were measured at different temperatures 25-70°C before and after stress of 0.01Q/cm2, 0.1Q/cm2, 1Q/cm2, 3Q/cm2, 10Q/cm2. The electronic structure of oxygen vacancy in SiO2was calculated within the spin polarized density functional theory using theab initiosimulation code Quantum ESPRESSO [P. Giannozzi et al.,J. Phys.: Condens. Matter21, 395502 (2009)] with B3LYP hybrid exchange-correlation functional. The oxygen vacancy was generated by the removal of an O atom, followed by relaxation of remaining atoms in 100-atom supercell. The high-field (>7MV/cm) current through the structure was limited by Fowler-Nordheim tunneling [R.H. Fowler, L. Nordheim,Proc. R. Soc. A119, 173 (1928)]. Low-field current (<6MV/cm) was limited by phonon-assisted tunneling between traps(Fig.3)[K.A. Nasyrov, V.A. Gritsenko,J. Appl. Phys.109, 093705 (2011)]. Experimental data was described quantitatively using thermal and optical trap energies ofWt=1.6eV andWopt=3.2eV. Obtained value ofWtis equal to a half of Stokes shift of photoluminescence near 4.4eV exited by 7.6-eV-phonots on oxygen vacancy in SiO2[V.A. Gritsenko, H. Wong,Crit. Rev. Solid State36, 129 (2011)] and equal to calculated trap energy for trapped electrons and holes on oxygen vacancies;Woptis close to measured electron trap energy in SiO2[K. Yamabe, Y. Miura,J. Appl. Phys.51, 6258 (1980)]. Charge trap density before stress was less than 1020cm-3. SILC stress caused arising of the trap density up to 7×1020/9.3×1020/1.05×1021/1.10×1021/1.15×1021cm-3depending on the stress.