Session D28: Focus Session: Dopants and Defects in Semiconductors - Oxides and Interfaces

Interdiffusion, Unintentional Doping and Electronic Reconstruction at Polar/nonpolar Oxide Interfaces

The observation of conductivity at the interface of insulating polar and non-polar perovskites (general form ABO$_{3})$ has sparked considerable interest worldwide, with much of the work to date being focused on the LaAlO$_{3}$/SrTiO$_{3}$(001) heterojunction. Many attribute the interface conductivity to an electronic reconstruction alleviating the polar discontinuity via a film-to-interface charge transfer. However, the possibility of dopant- and/or defect-mediated conductivity cannot be ruled out, especially when the interfaces are not atomically abrupt. Electronic reconstruction requires an electric field within the film to facilitate the charge transfer process. However, x-ray photoemission spectroscopy studies reveal little or no electric field in the LAO film either above or below the critical thickness for conductivity, calling into question the validity of the electronic reconstruction model. In order to gain deeper insight into the electronic properties, it is worthwhile to manipulate the interface by changing the B-site cation in the polar perovskite. There are no low-lying $d$--derived bands in LaAlO$_{3}$ Therefore, if conductivity occurs, it ought to be driven by either the wholesale transfer of charge from the LaAlO$_{3}$ O2$p$-derived band into the STO (i.e. electronic reconstruction), or unintentional doping and/or defect creation. By replacing Al with a transition metal cation, we inject a new degree of freedom into the band structure -- partially occupied $d $orbitals -- and, thus, enable other mechanisms of charge redistribution. We have explored this concept by placing Cr(III) at the B-site and describe the electronic properties of epitaxial LaCrO$_{3}$/STO(001) heterojunctions. To minimize defect creation, the films were deposited using molecular beam epitaxy (MBE), in which the incoming atom energies are very low ($<$0.1 eV). Core-level and valence-band x-ray photoemission spectra measured for MBE-grown LaCrO$_{3}$/SrTiO$_{3}$(001) yield band offsets and potential gradients within the LaCrO$_{3}$ sufficient to trigger an electronic reconstruction to alleviate the polarity mismatch. Yet, the interface is insulating. Based on first principles calculations, we attribute this unexpected result to interfacial cation mixing combined with charge redistribution within CrO$_{2}$ layers, enabled by low-lying $d$ states within LaCrO$_{3}$, which suppresses an electronic reconstruction.   

Calculation of charge transition levels of oxygen vacancies in rutile TiO$_2$ using the GW method

Titanium dioxide (TiO$_2$) is a semiconductor displaying photovoltaic and photocatalytic properties and is widely used in numerous technological applications, such as solar cells, hydrolysis of water, photocatalysis, etc. Defects are important for optical properties of TiO$_2$, e.g, they play a crucial role in photocatalytic reactions. Gaining a deep understanding of the influence of intrinsic defects, such as vacancies and interstitials, on the properties of this material is highly desirable. In spite of many theoretical and experimental investigations of defects in TiO$_2$, controversies still remain. In this work, the charge transition levels of oxygen vacancies are studied in the rutile form of TiO$_2$. For this purpose the GW method is employed for the calculation of the band gap and the position of the defect levels in the gap. The effects of lattice relaxations for systems with various charge states are taken into account. This is done with the help of hybrid functionals, since LDA or PBE mean fields do not describe defect levels properly, which may result in incorrect lattice relaxations. To account for spurious effects due to periodic images of the charged defects, appropriate electrostatic correction techniques are used.   

Strain effect on diffusion properties of oxygen vacancies in bulk and subsurface of rutile TiO2

The influences of external strain on diffusion properties of the bulk and subsurface oxygen vacancy (OV) in rutile TiO$_2$ are systematically studied using first-principle calculations. For OVs in bulk, we find that tensile (compressive) strain applied in the [001] direction or isotropically applied in the equivalent [110] and [1$\bar 1$0] directions reduces (increases) the energy barriers of diffusion. Anisotropic strain applied in [110] and [1$\bar 1$0] increases the energy barriers of diffusion in the two directions. Meanwhile it results in anisotropic diffusion behaviors. Between [110] and [1$\bar 1$0], the bulk OV prefers to diffuse along the one in which more compressive or less tensile strain is applied. From subsurface to surface, the most energetically favorable OV pathway is along the [110] rows terminated with the surface bridging oxygen atoms. The diffusion barrier of the OV in the first trilayer is much lower than that of a bulk OV. External in-plane tensile strain can further reduce the energy barrier of the subsurface OV diffusion, and thus help to improve the diffusion of OVs from bulk to surface.   

Vibrational spectroscopy of O-H and O-D centers in TiO$_{2}$

While the vibrational properties of O-H centers in TiO$_{2}$ have been studied for many years, recent experiments suggest a new picture of their behavior. In the 1970s, Bates and Perkins found a single, sharp, strongly polarized O-H line in TiO$_{2}$, and similarly for O-D and O-T [1]. On the contrary, recent studies by Herklotz \textit{et al.} [2] find three closely spaced O-H (O-D) lines that were assigned to two different charge states of an O-H (O-D) shallow donor. We have in our possession the very TiO$_{2}$ samples studied many years ago by Bates and Perkins. We have introduced H and D into these samples and also into TiO$_{2}$ samples obtained recently. High-resolution vibrational spectroscopy performed as a function of temperature at Lehigh provides new insight into the different vibrational properties seen for O-H in TiO$_{2}$ in the 1970's and in recent studies by Herklotz \textit{et al.} [1] J.B. Bates and R.A. Perkins, Phys. Rev. B \textbf{16}, 3713 (1977). [2] F. Herklotz \textit{et al.}, Phys. Rev. B \textbf{83}, 235202 (2011)..   

Intrinsic Spin-Orbit Effects in Strontium Titanate

We have calculated spin relaxation times via the Elliott-Yafet mechanism for strontium titanate as a function of temperature. The approach uses a low-energy effective spin-orbit Hamiltonian constructed from a tight-binding model with atomic spin-orbit interactions. The intrinsic spin Hall conductivity for strontium titanate has also been calculated from the same low-energy Hamiltonian using a Berry's phase technique. Modifications to the spin relaxation and spin Hall conductivity from elastic strain at an interface will also be described. This work was supported by an ARO MURI.   

Prediction of a ``half semiconductor'' for spintronics in non-compensated n-p codoped TiO$_{2}$

Based on hybrid density functional calculations, we predict that by doping non-compensated Cr-N pairs a normal wide-band-gap semiconductor TiO$_{2}$ can be altered to a ``half semiconductor'', in which both the top and the valence band and the bottom of the conduction band are fully spin-polarized and exclusively contributed by the same spin component. The underlying formation mechanism of such an unusual band structure is revealed via detailed electronic structure analysis. The magnetic property of the material will also be discussed in this talk. Such a ``half semiconductor'' material may provide a new twist to generate and manipulate spin currents for spintronics.   

Charged and neutral oxygen vacancies at MgO surfaces under realistic temperature and pressure conditions

Surface O vacancies (F-centers) can strongly influence catalytic properties of MgO and metal clusters supported on MgO, but the experimental determination of their concentration at catalytic conditions is difficult. We employ density-functional theory and the \textit{ab initio} atomistic thermodynamics approach to determine concentration and charge states of F-centers at (111) and flat and stepped (100) surfaces of MgO at realistic (\textit{T, p}) conditions. Slab models and the virtual-crystal approximation [1] are used to model charged defects at surfaces. We find a strong dependence of F$^{+}$ and F$^{2+}$ formation energy on the exchange-correlation (XC) functional. Varying the amount of screening and fraction of exact exchange within the HSE functional, we find a linear correlation between defect formation energies and calculated valence-band width of the host material, in line with recent results for bulk systems [2]. Using this correlation and extrapolating to experimental band width, we conclude that only F$^{2+}$ centers can be present in significant concentrations at the (100) terraces at realistic conditions. --- [1] L. Vegard, Z. Phys. \textbf{5}, 17 (1921); M. Scheffler, Physica \textbf{146B}, 176 (1987); [2] R. Ramprasad \textit{et al.}, subm. to Phys. Rev. Lett.   

Pressure dependence of the large polaron transport in anatase $TiO_{2}$ single crystals

Anatase is a $TiO_{2}$ polymorph which is a 3.2 eV gap semiconductor interesting for several applications, including catalysis, photocatalysis, and, especially, dye-sensitized solar cells. Surprisingly, transparent single crystals of anatase grown in our laboratory show a metallic resistivity above 60 K which origin is a shallow donor level created by oxygen vacancies. The high value of the resistivity and its $T^{3} $ temperature dependence are the result of the polaronic nature of the charge carriers which is supported by the Seebeck coefficient (S). The application of hydrostatic pressure fails to close the donor level and to extend the conducting state to the entire temperature range. Instead, we have found a non-monotonic variation of the low temperature activation energy with applied pressure which is ascribed to the change of polaron's mobility. Thermo-electric power exhibits an unconventional temperature and pressure dependence shedding an additional light on the conductivity mechanism in this compound. The pressure dependence of S is governed by the transport of the large entropy associated with the polaron formation.   

Revisiting the mechanism of photocatalytic activities in N-doped TiO$_{2}$

Photocatalysis possesses a great potential for environmental remediation and fuel production [1]. Nitrogen doped TiO$_{2}$ is a well-known visible-light sensitive photocatalyst where deep impurity states associated with substitutional nitrogen at oxygen sites (N$_{O})$ are believed to be the source of the red shift in photo-absorption edge. However, such a deep level should trap hole carriers, degrading oxidation process. The contradiction between the deep N$_{O}$ level and rather a high oxidation power of N-doped TiO$_{2}$ has been an unsolved puzzle. Here, we propose a convincing mechanism which successfully solves the riddle. N$_{O}$ strongly binds with a titanium atom at an interstitial site, forming a defect-impurity band, which consists of bonding and anti-bonding states of nitrogen $p$ and titanium $d$ and narrows the band gap. Such a newly formed band, which is connected to the valence band maximum of the host TiO$_{2}$, becomes the migration path of photo-induced hole carriers, assisting carrier transfer to the surface. This clearly explains the photocatalytic activity of N-doped TiO$_{2}$ both for the visible-light absorption and the oxidation reaction. [1] Hua Tong, Shuxin Ouyang, Yingpu Bi, Naoto Umezawa, Mitsutake Oshikiri, and Jinhua Ye, Advanced Materials DOI: 10.1002/adma.20110275   

Femtosecond laser doped and nanostructured TiO$_{2}$ for photocatalysis

We present a novel method for femtosecond-laser doping of titanium dioxide (TiO$_{2})$ for above bandgap absorptance by irradiating titanium metal in the presence of oxygen and dopants. With a bandgap of 3.2 eV for the anatase crystalline phase, TiO$_{2}$ most strongly absorbs in the UV range ($\lambda <$ 387 nm). However, doping with metals and nitrogen has been shown to create intermediate states in the bandgap. Using femtosecond laser doping techniques on titanium in a gaseous environment, we produce laser-induced periodic surface structures. Altering the gas composition and pressure does not change the surface morphology, but it does impact the chemical composition of the surface. We present compositional data from x-ray photoelectron and Raman spectroscopy and structural data from scanning electron microscopy. Our research presents an innovative approach using laser scanning techniques to alter the structure of TiO$_{2}$ and generate a new material for visible-light photocatalysis that has the potential for watersplitting.   

First-principles study of interstitial hydrogen in yttria-stabilized zirconia.

Hydrogen is a common impurity in oxides and has been known to exhibit a dual behavior: either by being a dopant or alternatively an amphoteric impurity with the transition (pinning) level, E(+/-), lying inside the gap [1]. By means of calculations based on density-functional theory (DFT) and a hybrid-functional scheme (Heyd-Scuseria-Ernzerhof) we have studied the incorporation of hydrogen in yttria-stabilized zirconia. Equilibrium sites and formation energies were determined and the role of intrinsic oxygen vacancies needed to stabilize the cubic phase of the oxide was particularly examined. Whereas, in its positively-charged state, $H^{+}$, hydrogen was found exclusively to form a dative-type bond with O ions, the neutral paramagnetic $H^{0}$ displayed a coexistence with deep interstitial configurations with minimal lattice relaxation of the host lattice. A number of atomic-level mechanisms and migration paths were explored in order to understand this site interplay and the dynamics of neutral $H^{0}$ in a way that is consistent with the existing experimental data. [1]~C.G. Van de Walle and J. Neugebauer, Nature {\bf 423}, 626 (2003).   

Fundamental limits on optical transparency of transparent conducting oxides: free-carrier absorption in SnO$_2$

Transparent conducting oxides (TCOs) are a technologically important class of materials used as transparent contacts in optoelectronic devices, such as flat-panel displays, touch screens, solar cells, and light-emitting diodes. These applications are possible because the TCOs combine high electrical conductivity with transparency to visible light. However, the large concentration of free electrons introduces a source of absorption that forms a fundamental limit to the transparency. We evaluated the importance of phonon-assisted free-carrier absorption in SnO$_2$ completely from first principles. We also provide insight into the mechanisms that govern absorption in different wavelength regimes. Our results show that the absorption is weak in the visible, but it increases by as much as a factor of 5 in the UV. For the infrared region, we show that the absorption increases with the wavelength, and that this increase is proportional to the third power of the wavelength. We also explain this third power dependency.   

Self-trapping of holes in p-type oxides: Theory for small polarons in MnO

Employing the $p$-$d$ repulsion to increase the valence band dispersion and the energy of the VBM is an important design principle for p-type oxides, as manifested in prototypical $p$-type oxides like Cu$_2$O or CuAlO$_2$ which show a strong Cu-$d$/O-$p$ interaction. An alternative opportunity to realize this design principle occurs for Mn(+II) compounds, where the $p$-$d$ orbital interaction occurs dominantly in the fully occupied $d^5$ majority spin direction of Mn. However, the ability of Mn to change the oxidation state from +II to +III can lead to a small polaron mechanism for hole transport which hinders p-type conductivity. This work addresses the trends of hole self-trapping for MnO between octahedral (rock-salt structure) and tetrahedral coordination (zinc-blende structure). We employ an on-site hole-state potential so to satisfy the generalized Koopmans condition. This approach avoids the well-known difficulty of density-functional calculations to describe correctly the localization of polaronic states, and allows to quantitatively predict the self-trapping energies. We find that the tetrahedrally coordinated Mn is less susceptible to hole self-trapping than the octahedrally coordinated Mn.