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 Pb -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 Pb centers are neutral when the Fermi level is at mid-gap. Since the Pb centers are largely responsible for the radiation-induced interface states, one may take ΔVmg Cox/e (where ΔVmg is the ‘‘mid-gap’’ C-V shift, Cox 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 ΔVmg Cox/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 ΔVmg 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 Pb 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.