Hole traps and trivalent silicon centers in metal/oxide/silicon devices

We report electron spin resonance (ESR) measurements of E′-center (a "trivalent silicon" center in SiO₂) density as well as capacitance versus voltage (C-V) measurements on γ-irradiated metal/oxide/silicon (MOS) structures. We also report a considerable refinement of earlier ESR measurements of the dependence of radiation-induced P_b -center (a "trivalent silicon" center at the Si/SiO₂ interface) occupation as a function of the Fermi level at the Si/SiO₂ interface. These measurements indicate that the P_b centers are neutral when the Fermi level is at mid-gap. Since the P_b centers are largely responsible for the radiation-induced interface states, one may take ΔV_mg C_ox/e (where ΔV_mg is the "mid-gap" C-V shift, C_ox is the oxide capacitance, and e is the electronic charge) as the density of holes trapped in the oxide. We find that radiation-induced E′ density equals ΔV_mg C _ox/e in oxides grown in both stream and dry oxygen. Etch-back experiments demonstrate that the E′ centers are concentrated very near the Si/SiO₂ interface (as are the trapped holes). Furthermore, we have subjected irradiated oxide structures to a sequence of isochronal anneals and find that the E′ density and ΔV_mg annealing characteristics are virtually identical. We conclude that the E′ centers are largely responsible for the deep hole traps in thermal SiO₂ on silicon. This observation coupled with observations regarding the P_b center indicates that two intrinsic centers, both involving silicon atoms lacking one bond to an oxygen atom, are largely responsible for the two electrically significant aspects of radiation damage in MOS devices: charge buildup in the oxide and interface-state creation at the Si/SiO₂ interface.