Invited Paper TF-ThM3
2010 Gaede-Langmuir Award Lecture- X-ray Absorption Spectroscopy and Many Electron Theory Applied to O-vacancy State Differences between (i) Non-crystalline SiO₂ and (ii) Nano-crystalline HfO₂ Thin Films

Thursday, October 21, 2010, 8:40 am, Room Ruidoso

Correlated atomic positions of Si-atoms bonded to 2-fold coordinated O-atoms are determined by Si 3d-dervied T_2g states on these atoms [1]. These states constrain dihedral angles contributing to medium range order with correlation lengths of ~0.4 and 0.45 nm, and, a coherence length of ~1 nm, each obtained from the first sharp diffraction peak in X-ray diffraction [2]. O K edge soft X-ray SiO₂ spectra reveal conduction band edge states with singly degenerate A₁ symmetry, and stronger doubly and triply degenerate E and T₂ d-state beginning at energies ~3 eV higher. This difference is smaller in c-Si, ~1 eV. Analysis of O K and L_2,3 spectra are based on a charge transfer multiplet many electron theory [3]. Energy differences are the same for band edge features obtained from transmission/reflectivity studies in the visible/VUV where the d-state character was not previously recognized [4]. The correspondence derives from O 1s core hole localization, and a coherent process in which 1s core holes are filled by electrons from valence band O 2p p states, accounting for the one-to-one correspondence between sequenced features in the O K edge, and in the Si L_2,3 spectrum of SiO₂. L_2,3 spectra, studied by electron energy loss spectroscopy (EELS), have not detected the weaker Si A₁ features in the 100 to 104 eV regime of SiO₂, or 98 to 100 eV regime of c-Si. The O K edge X-ray spectra of transition metal elemental and complex oxides are qualitatively different with either doubly or triply degenerate d-state derived spectral features at the band edge. This results in significant quantitative differences in (i) the high-spin excited d-states of O-atom vacancy occupied d² states, and (ii) the negative ion electronically active d-state traps populated by charge injection. O-vacancy states in SiO₂ are close to mid-gap, and do not contribute to gate stack tunneling processes. This explains the order of magnitude higher interfacial trap densities, d_it, and trap-assisted tunneling in HfO₂ gate stacks. It also accounts for significant differences in radiation hardness. XAS spectra are presented for the first time for remote plasma deposited GeO₂. These deposited films display significantly different O K edge spectra than those obtained by oxidation of Ge; more importantly A₁, and E and T_2g features stronger and narrower than those of SiO₂. Electrical data for deposited GeO₂ dielectrics on Si and Ge substrates are compared for the first time with SiO₂ test devices on the same substrates.

[1] Whitten J, et al., J. Vac. Sci. Technol. B 20, 1710 (2002).

[2] Lucovsky G, et al. physica status solidi (a) 207, 631 (2010).

[3] de Grott F, Kotani A. Core level spectroscopy of solids (Boca Raton, CRC, 2008).

[4] Laughlin RB, Phys. Rev. B 22, 3021 (1980).