Charge Transport Mechanism of Stress Induced Leakage Current in Thermal Silicon Oxide

Abstract

Write/erase operations of SONOS and TANOS flash memory with Si3N4-floating gates are driven by tunnel injections of electrons and holes through thin 1.8-8.0 nm (tunnel) SiO2 layer [Ed. by D. Brown, J. F. Brewer, IEEE Press, New York (1998); P.E. Blöchl, J.H., Stathis, Phys. Rev. Lett. 83, 372 (1999)]. The current through the tunnel oxide during erase/rewrite operations by applying strong (~10 MV/cm) electric field in the oxide increases the conductivity of the layer at low electric fields (~1 MV/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. This phenomenon is called Stress Induced Leakage Current, SILC. 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. This study describes transport in SILC with phonon-assisted tunneling between traps, shows that oxygen vacancies are the charge traps in SiO2. SILC measurements were performed for test structures with poly-Si top electrodes and 7.5-nm-thick SiO2 on on n+ -Sip and p-Si substrates. Current-voltage characteristics were measured at different temperatures 25-70 °C before and after stress of 0.01 Q/cm2, 0.1 Q/cm2, 1 Q/cm2, 3 Q/cm2, 10 Q/cm2. The electronic structure of oxygen vacancy in SiO2 was calculated within the spin polarized density functional theory using the ab initio simulation code Quantum ESPRESSO [P. Giannozzi et al., J. Phys.: Condens. Matter 21, 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 (>7 MV/cm) current through the structure was limited by Fowler-Nordheim tunneling (Fig.1) [R.H. Fowler, L. Nordheim, Proc. R. Soc. A 119, 173 (1928)]. Low-field current (<6 MV/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 of W t = 1.6 eV and W opt = 3.2 eV. Obtained value of W t is equal to a half of Stokes shift of photoluminescence near 4.4 eV exited by 7.6-eV-phonots on oxygen vacancy in SiO2 [V.A. Gritsenko, H. Wong, Crit. Rev. Solid State 36, 129 (2011)] and equal to calculated trap energy for trapped electrons and holes on oxygen vacancies; W opt is 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 1020 cm-3. SILC stress caused arising of the trap density up to 7×1020/9.3×1020/1.05×1021/1.10×1021/1.15×1021 cm-3 depending on the stress (Fig.1). Annealing at 250 °C in O2 atmosphere during 120 hours leads to decreasing of the trap density to initial values (Fig.2). The work was partly supported by Russian Science Foundation, grant #16-19-00002. Figure 1