First principles density functional calculations for adsorption of Ti on Al 2 O 3 (0001) indicate that Ti:Al 2 O 3 (0001) interfaces become intermixed. Substitutional Ti replaces a surface Al atom rather than a subsurface Al, and the Al-terminated surface is unstable under Ti adsorption. Adsorbed Ti displaces the surface Al, resulting in a mixed Ti/Al interfacial layer instead of a sharp Ti:Al 2 O 3 interface. Our results provide a coherent picture of the structural and electronic properties of

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... this interface and are consistent with available experimental data. Metal/metal oxide interfaces are crucial to electronics devices, and aluminum (Al 2 O 3 ) and titanium (TiO 2 ) oxides play pivotal roles. 1 The Ti:sapphire interface at low Ti coverage has notable applications in high power laser 2 and optoelectronics waveguide 3 technologies. More generally, Ti/Al oxide interfaces occur in several applications. These include, for example, Ti/Al alloys for aerospace/automotive applications, when an Al 2 O 3 coat evolves into Al 2 O 3 ϩTiO 2 , 4 or nanolayered sandwich structures, such as Cu,Ag/Ti/Al 2 O 3 , where Ti prewetting improves Cu and Ag adhesion. 5, 6 While an extensive literature exists in general for clean oxide surfaces and metal-or oxide-oxide interfaces, much less effort has been devoted to understanding the specific Ti:sapphire interface. Although crucial to the technological applications mentioned above, many aspects of such interface are currently uncertain. Furthermore, the current microscopic understanding of the Ti:sapphire interface derives exclusively from experiment, 5-10 with temperatures in these studies varying from 300 K ͑Ref. 8͒ to above 1100 K. 7,9 In some cases, experimental evidence is inconclusive: for example, RHEED and ion scattering results 9 for low-dosage, shallow-implant Ti, indicate that Ti is in the surface layer atop the ͑subsurface͒ Al atoms, even though the actual site preference of the substitution is uncertain. Also, this leads to commensurate growth of crystalline Ti. 9 Moreover, XPS ͑Ref. 7͒ and Auger 8 data show clearly that at elevated temperatures the metallic Al 2p signal is present and the shape of the valence bands, which have primarily oxygen character, are broadened. Furthermore, the Ti 2p signal is shifted. All of this is consistent with the formation of an oxidized Ti layer and concomitant partial Al 2 O 3 reduction. In this paper we report the first ab initio density functional results to elucidate the nature the Ti:sapphire interface. We find that: ͑i͒ substitutional Ti prefers to replace the terminating Al rather than the subsurface Al and, additionally, little deformation of the surface occurs in the latter case; ͑ii͒ a single Ti adatom on sapphire displaces the terminating Al rather than adsorbing atop the surface; and ͑iii͒ a mixed Ti 2 Al adlayer on a Ti-terminated oxide surface is preferred over a sharp Ti 3 :Al 2 O 3 interface. These results indicate that the Al-terminated surface is unstable to Ti adsorption, and predict a mixed Al/Ti interfacial layer. The calculations were performed using density functional theory in the local density approximation, 11,12 which yields excellent structural energetics for many oxides and specifically for Al 2 O 3 . We used SEQQUEST, 13 a Gaussian-based linear combination of atomic orbitals method. Standard ͑non-local͒ norm-conserving pseudopotentials were used for O ͑Ref. 14͒ and Al. 15 For Ti, the pseudopotential 15 placed the six 3p ͑together with the 3d and 4s) electrons in the valence space, leaving the 3s electrons in the core, and invoked a nonlinear core correction. 16 The basis sets were high-quality, optimized "double zeta plus polarization" contracted Gaussian functions. 17 Our calculations involved 1ϫ1 and ͱ3ϫͱ3 surface unit cells ͑SUC͒ of sapphire ͑0001͒, aϭ4.757 Å, with slabs nine O-layers thick: 1ϫ1ϭ45, 45ϩ1, and 45ϩ3 atoms/cell, and ͱ3ϫͱ3ϭ135 atoms/cell. Several theoretical 18 -32 and experimental 33 studies of the sapphire ͑0001͒ surface are in broad agreement, showing large Al relaxations deep below the surface and predicting a Al-terminated clean sapphire ͑0001͒ surface for UHV environments. This is the surface termination used as a starting point in our calculations. Selected results for the 1ϫ1 systems were compared to and found consistent with 18 layer slabs. Ti atoms were adsorbed on only one side, and artificial interslab dipole interactions were exactly removed with the local moment counter charge method. 34 A converged Brillouin zone ͑BZ͒ sampling used a hexagonal k-grid with 32 ͑8͒ points in half of the BZ for the 1ϫ1 (ͱ3ϫͱ3) super-cell, reduced to 12 ͑4͒ special k-points in the irreducible BZ when threefold symmetry was available. The entire slab geometry was relaxed until the maximum force was Ͻ0.01 eV/Å. Figure 1 summarizes the structures investigated. Let Al t represent the bare, Al-terminated, sapphire ͑0001͒ surface, depicted in the side and top views in Figs. 1͑a͒ and 1͑b ,c͒ respectively. The three different aluminum sites are denoted by Al1-Al3, and the two oxygen sites O1-O2 ͑in order of increasing depth͒. A Ti-terminated surface in which the terminating Al is replaced by a Ti is denoted Ti t . At PHYSICAL REVIEW B 66, 125408 ͑2002͒