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Electron energy distribution in Si/TiN and Si/Ru hybrid floating gates with hafnium oxide based insulators for charge trapping memory devices
Author(s) -
Cerbu F.,
Andreev D. V.,
Lisoni J.,
Breuil L.,
Afanas'ev V. V.,
Stesmans A.,
Houssa M.
Publication year - 2016
Publication title -
physica status solidi (a)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.532
H-Index - 104
eISSN - 1862-6319
pISSN - 1862-6300
DOI - 10.1002/pssa.201532416
Subject(s) - tin , high κ dielectric , materials science , metal gate , analytical chemistry (journal) , trapping , electron , dielectric , optoelectronics , gate oxide , chemistry , electrical engineering , physics , transistor , ecology , chromatography , voltage , quantum mechanics , metallurgy , biology , engineering
Scaling the planar NAND flash cells to the 20 nm node and beyond mandates introduction of inter‐gate insulators with high dielectric constant ( κ ). However, because these insulators provide a smaller electron barrier at the interface with the poly‐Si floating gate, the program window and the retention properties of these scaled cells are jeopardized. To reduce the charge loss from the floating to the control gate, one may consider the introduction of a hybrid floating gate (HFG) structure comprised of poly‐Si and a high work function (WF) metal, e.g., TiN x ( x ∼ 1; WF ∼ 4.7 eV) or Ru (WF ∼ 5.3 eV). However, the very HFG concept is based on the assumption that electron trapping occurs inside the HFG stack rather than on traps present in the high‐ κ insulator. To examine this critical hypothesis, we analyzed the energy distribution of electrons trapped in flash cells with poly‐Si(2 nm)/TiN (6 nm)/Hf 0.8 Al 0.2 O x (19 nm, κ ∼ 15–19)/TiN x (10 nm) and Si(2 nm)/Ru (1 nm)/Hf 0.8 Al 0.2 O x (5 nm)/Al 2 O 3 (5 nm)/Hf 0.8 Al 0.2 O x (5 nm)/TiN x (10 nm) trapping gate stacks using the exhaustive photo‐depopulation spectroscopy. We found that trapped electron energy levels show a broad distribution (± 0.3 eV) centred at ∼3.2 eV below the oxide conduction band. The energy onset of electron de‐trapping at ∼2.8 eV matches the TiN x /HfO 2 barrier height found from internal photoemission experiments, indicating that electrons are predominantly trapped inside the HFG.