IHP - Innovations for High Performance Microelectronics: Theory-9

3.6. The influence of substrate and some impurities on charge trapping



H₂O in Pr₂O₃ right after its dissociation

Moisture is definitely the factor that has to be examined in the context of fixed charge formation. As other rare earth oxides, Pr₂O₃ readily absorbs water, to the extent that it is easily converted to a hydroxide. When a water molecule is dissolved in Pr₂O₃, it dissociates into (OH)_I^- interstitial (the oxygen atom becomes fourfold coordinated: with one H connected to it, and three nearest Pr neighbors) and H⁺. The latter becomes attached to a lattice oxygen atom, forming a defect which may be termed a substitutional OH group, (OH)_O⁺; the affected oxygen atom is now fivefold coordinated, with one impurity H atom and four Pr neighbors from the lattice. Both OH groups are single negatively charged, but the substitutional group replaces in the lattice a double negatively charged oxygen atom, hence it acts as an effective positive charge. These defects do not introduce any localized electron states in the gap of Pr₂O₃ and, since they have the opposite charge, a dissolved H₂O molecule cannot act as a fixed charge.

Nevertheless, it is possible that the charge balance is affected by defect reactions in the film. For example, if the oxygen atom from dissociated H₂O is used to oxidize silicon in the substrate to (fully relaxed) SiO₂, the (OH)_I^- interstitial becomes converted to (OH)_O⁺. Two positive fixed charges are thus created by each H₂O molecule taking part in such a reaction. We calculated that this oxidation reaction is energetically favorable by about 0.3 eV per H atom in p-type Si. This means that, in principle, one cannot exclude that such processes take place in ultrathin Pr₂O₃/Si films exposed to moisture. The calculated formation energy of (OH)_I^- in Pr₂O₃ is also negative within a broad range of O potential in the oxidizing regime (we assume here that the chemical potential of H corresponds to that in H₂O remaining in thermodynamic equilibrium with H, that is, that the sum of 2µ(H) and µ(O) yields the formation energy of water), meaning that water from air would be a source of negative fixed charge in Pr₂O₃. The formation energy of (OH)_O⁺ is sufficiently small only when the chemical potential of oxygen approaches the UHV range. Although the formation energy of (OH)_O⁺ is negative in equilibrium with SiO₂, this does not seem to have a direct relevance to the fixed charge formation. Positively charged defects might be formed in this way if, for example, a water-contaminated oxide is sealed with a Si layer and then annealed.

Boron is the traditional acceptor used in MOSFET channels and in polysilicon gates. It is not a direct source of fixed charge in Pr oxides. Although boron atoms strongly segregate from Si to Pr₂O₃, they substitute Pr in the lattice. As B_Pr, B atoms are isovalent impurities. They are electrically neutral and introduce no localized states, at least not in the hazardous energy region within approximately one eV to the band gap of Si. Nevertheless, boron segregation may be responsible for fixed charge generation is by kick-out of Pr interstitials, Pr_I⁺³ by B interstitials, B_I⁺. The kick-out is energetically favorable by 0.8 eV when the Fermi level is aligned with that of intrinsic Si. This means that each B atom that makes it to the oxide produces one Pr_I⁺³ ion. If the annealing of an uncapped layer takes place in an atmosphere containing enough oxygen to oxidize these ions to Pr₂O₃, this effect is irrelevant. However, if the annealing takes place under a capping layer (as it is likely to be the case during technological processing), the Pr interstitial atoms may remain unoxidized in the film, producing a fixed charge that is particularly problematic because it may move across the dielectric in the electric field and render the device unreliable by causing a hysteresis in its CV characteristics.

Hazardous positively charged interstitial metal atoms may be also injected from such metal films as titanium. This injection becomes energetically unfavorable (by at least 2 eV) when the Ti source is oxidized to TiO₂. Nevertheless, nanosize inclusions of TiO₂ in Pr₂O₃ and in Pr₂Si₂O₇ may trap positive charge. Similarly to the charge trapped on Si_Pr, this positive charge is re-loadable and can be neutralized by electrons. This means that such inclusions act as Trap Assisted Tunneling or Poole-Frenkel centers, contributing to the leakage current flowing across the dielectric.

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