Session B5: Surface Electronic & Lattice Properties: Interfaces, Spin Effects, Etc.

Spatially Resolved Nano-Scale Characterization of Electronic States in SrTiO$_3$(001) Surfaces by STM/STS

We have performed low temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements on TiO$_2$-terminated SrTiO$_3$(001) thin film surfaces. The conductance map exhibited electronic modulations that were completely different from the surface structure. We also found that the electronic modulations were strongly dependent on temperature and the density of atomic defects associated with oxygen vacancies. These results suggest the existence of strongly correlated two-dimensional electronic states near the SrTiO$_3$ surface, implying the importance of electron correlation at the interfaces of SrTiO$_3$-related heterostructures.   

The Structure of High Polarization Surface of the Antiferromagnet Cr$_{2}$O$_{3}$

Manipulation of magnetically ordered states by electrical means is among the most promising approaches towards novel spintronic devices. Electric control of the exchange bias can be realized when the passive antiferromagnetic pinning layer is replaced by a magneto-electric antiferromagnet, like the prototypical magneto-electric Cr$_{2}$O$_{3}$(0001), so long as there is also a finite remanent magnetization at the surface or boundary. We have demonstrated that a very unusual high polarization surface magnetic order exists at the surface of the Cr2O3 (0001) surface and is robust against surface roughness from spin polarized inverse photoemission, and X-ray magnetic circular dichroism. We have also performed LEED (low energy electron diffraction) I(V) analysis to explore the surface structure above and below Neel Temperature (308 K). Temperature dependent LEED was also carried out at several different electron kinetic energies and Debye temperature was extracted. The surface and bulk Debye temperatures were obtained by fitting Debye temperature as a function of electron kinetic energy.   

Resonance Photoemission and Spin Polarization of Fe$_{x}$Co$_{1-x}$S$_{2}$

The electronic structure in the region of the Fe/Co 3d band for Fe$_{x}$Co$_{1-x}$S$_{2}$ has been investigated by photoemission and spin polarized photoemission. The comparison between the results for different content of doped Fe was made, specifically x=0, 0.05, 0.10, 0.15 and 1. While the surface spin polarization of Fe$_{x}$Co$_{1-x}$S$_{2}$, measured by spin polarized ultraviolet photoemission, was reduced compared with the bulk value, we see that the spin polarization increases with Fe doping level for textured thin films. The resonance photoemission spectroscopy shows that sulfur bands have strong resonance at the photon energy of the Co 2p core level, indicating strong hybridization between Co and sulfur bands in Fe$_{x}$Co$_{1-x}$S$_{2}$ (small x) however, the ultraviolet photoelectron spectroscopy (UPS) of FeS$_{2}$ exhibits a slightly different d-band density of states than Fe$_{x}$Co$_{1-x}$S$_{2}$.   

Theoretical Band Offsets in c-Si/Si-XII Heterojunctions

Many different phases of silicon can be formed under pressure, with some being metastable at standard temperatures and pressures. For one such phase, Si-XII, experiments have recently suggested it to be a semiconductor, confirming theoretical predictions that it has a narrow gap in its electronic band structure. Current-voltage measurements show rectifying behavior in \mbox{c-Si/Si-XII} heterojunctions, indicative of a band discontinuity at the interface. We present computations that quantify this band discontinuity using bulk band structures obtained with Density Functional Theory within the Local Density Approximation. In particular, we demonstrate the use of a semiconductor's intrinsic charge neutrality level to determine band lineups.   

Band Alignment in Transparent Conducting Oxide Schottky Junctions

Better understanding and control of band alignment in oxide-semiconductor heterostructures is essential for improving the performance of devices such as sensitized solar cells and quantum dot based light emitting devices. We will present studies of Schottky junctions formed between Al-doped ZnO (AZO) conducting oxide thin films and lightly doped silicon. AZO films with varying oxygen content have been synthesized by control of oxygen pressure during growth. Transport measurements (I-V and C-V) on devices are used to illustrate the degree to which the oxide stoichiometry can be used to engineer the junction characteristics.   

Interface Band Structure Effects upon Hot Electron Transport Across Non-Epitaxial Metal-Semiconductor Interfaces

The interface band structure of a semiconductor is shown to affect the transport of hot electrons across a non epitaxial metal-semiconductor interface. This is observed by measuring the hot electron attenuation length of Ag utilizing ballistic electron emission microscopy (BEEM). The attenuation length is observed to increase sharply for energies approaching the Schottky barrier height when deposited upon Si(001) substrates and decrease slightly when deposited upon Si(111) substrates. A theoretical description demonstrates that this is due to conservation of parallel momentum and differences in interface band structure of the two silicon orientations. At higher tip biases the attenuation lengths converge allowing extraction of the inelastic and elastic scattering lengths in the silver.   

A numerical simulation study of inverse doped surface layer in Schottky barrier modification

The Poisson's and continuity equations are solved by iterative method to obtain the potential and electron and hole concentrations inside the semiconductor near the metal semiconductor interface for different inverse layer thickness and doping concentrations. The barrier height (BH) and ideality factor (IF) obtained by fitting of simulated current voltage data into thermionic emission diffusion current equation. The derived BH increases with increase in inverse layer thickness as well as with increase in the inverse layer doing concentration and then saturates at maximum value. The IF first rises with increase in inverse layer thickness and then attaining a maximum value at a particular thickness it decreases approaching unity value for large inverse layer thickness. It is observed that for large inverse layer thickness the BH attains a maximum value with unity IF. Thus, there are two regimes, namely, non-ideal regime corresponding to less inverse layer thickness where the BH has increased less and IF is more than unity and ideal regime corresponding to large inverse layer thickness where the BH attains maximum value with unity IF.   

Density functional calculations of the Schottky barrier height and effective work function in Ni/oxide interfaces

In high-k/metal gate stacks of complementary metal-oxide semiconductor devices, it is important to control the effective work functions of metals such that they should match to the doping levels of poly-Si gates. However, it is known that metal work functions are strongly affected by interface dipoles and defects. In this work, we perform first-principles density-functional calculations to study the Schottky barrier heights and the effective metal work functions in Ni/SiO$_2$ and Ni/HfO$_2$ interface structures. We use the advanced approaches such as hybrid density functional and quasi-particle \textit{GW} calculations for the exchange-correlation potential and discuss the limitations of GGA calculations. We also examine the effects of O-vacancy defects introduced at the interface on the Schottky barrier height and the effective work function. We find that, in the Ni/HfO$_2$ interface, the $p$-type Schottky barrier height tends to increase with increasing of the defect density due to the charge transfer at the interface, whereas it is little affected in the Ni/SiO$_2$ interface.   

Models for (001) epitaxial interfaces between CdTe and ZnO

Epitaxial interfaces between ZnO and CdTe appear difficult to achieve given their different crystal structures (CdTe is zinc blende with conventional lattice constant $a$ = 6.482 {\AA}, ZnO is hexagonal wurtzite with $a$ = 3.253 {\AA} and $c$ = 5.213 {\AA}.) However, ZnO also occurs in a metastable zinc-blende structure with an fcc primitive lattice constant close to the hexagonal $a$ value. Since this value is close to half of the CdTe conventional (simple cubic) lattice constant, (001)-oriented cubic ZnO films might grow epitaxially on a CdTe (001) surface in an R45\r{ } $\surd $2$\times \surd $2 configuration. Many alignments of the interfacial layers are possible, and we describe density-functional calculations on several of these to identify the most likely, and to predict valence-band offsets between CdTe and ZnO for each. Growth of ZnO on Te-terminated CdTe (001) is predicted to produce small or even negative (CdTe below ZnO) valence band offsets, resulting in a Type I band alignment. Growth on Cd-terminated CdTe is predicted to produce large positive offsets for a type II alignment as needed, for example, in solar cells. Calculations with the GGA + U method (with U = 7.5 eV for Zn 3d states) gave a valence band offset of +1.8 eV while a hybrid HSE06 + U calculation gave +2.6 eV. An experimental measurement on a ZnO film grown on CdTe (001) yielded a value of +2.2 eV for the valence band offset.   

The crystalline Si3N4/Si interface; the electronic structure of defects

A semiconducting beta-Si3N4(0001)/Si(111) interface model without dangling bonds is presented, and its geometric and electronic structure is compared to previous models based on calculations in a density functional theory framework. Furthermore nitrogen and phosphorus defects in the silicon layer are investigated, in particular how these defects modify the electronic structure and the electronic properties of the interface as a function of their distance to it. The local geometric structure of the nitrogen and phosphorus defects is also investigated close to the interface.   

Epitaxial n-ZnO on p-Si with native SiOx reduced by Al buffer

RF sputtering was employed to deposit n-type zinc oxide epitaxial thin films on p-type silicon substrates to form p-i-n diodes. A buffer layer of crystalline metal oxide was introduced by redox reaction between an aluminum layer and the native SiO$_{2}$. The aluminum layer was sputtered to various thicknesses and then annealed in situ for different times. The epitaxial relations follow (111) $_{Si}$//(0001) $_{ZnO}$ and [110] $_{Si}$//[11$\bar{2}$0] $_{ZnO}$, though certain degree of mosaicity was observed wavering around the [11$\bar{2}$] $_{Si}$ axis. Cross-sectional TEM observations of the interfaces, x-ray crystallography via $\omega$-2$\theta$ and rocking scans in regards to the perfection of the structures and orientations are agreeable. The current-voltage characteristics of the p-i-n diodes show promising outlooks for light emitting and photovoltaic applications.   

Emergence of orbital angular momentum due to broken inversion symmetry and its contribution to Rashba-type splitting

We demonstrate that the chiral orbital angular momentum (OAM) structure can emerge as a result of broken inversion symmetry especially at the metal surfaces. The surface-normal electric field is responsible for chiral OAM states even if spin-orbit interaction is negligible. Such chiral OAM structure can be measured by a circular dichroism (CD) in angle-resolved photoemission spectroscopy (ARPES). To confirm the existence of OAM and its detection by CD-ARPES, we perform simulation of CD-ARPES for Cu surface states by first-principles calculation and the results agree well with our CD-ARPES experiment. Addition of the spin-orbit interaction to the chiral OAM structure produces a chiral spin angular momentum (SAM) pattern and the corresponding Rashba-type band splitting. We assert that OAM polarization should be a more widespread feature than the chiral spin structure which requires strong spin-orbit coupling.   

Chiral Orbital Angular Momentum and Circular Dichroism ARPES in p- and d-orbital Bands

We derive explicit formulas relating the circular dichroism angle-resolved photoemission (CD-ARPES) signal to the existence of nonzero chiral orbital angular momentum (OAM) in the band structure. The existence of nonzero chiral OAM is a generic feature of surface states that break inversion symmetry, as pointed out in several recent articles [1-3]. We propose that CD-ARPES setup is an effective probe of the OAM of quasi-particles occupying the surface states. Explicit formulas for the $p$- and $d$-orbital bands are derived to show that the CD-ARPES signal is proportional to the OAM in the momentum space.\\[4pt] [1] S. R. Park, C. H. Kim, J. Yu, J. H. Han and C. Kim, Phys. Rev. Lett. \textbf{107}, 156803 (2011).\\[0pt] [2] S. R. Park \textit{et al}., arXiv:1103.0805 (2011).\\[0pt] [3] Choong H. Kim \textit{et al}., arXiv:1107.3285 (2011).   

A first principles study of water adsorption on $\alpha $-Pu (020) surface

Adsorptions of water in molecular (H$_{2}$O) and dissociative (OH+H, H+O+H) configurations on the $\alpha $-Pu (020) surface have been studied using \textit{ab initio }methods. The full-potential FP/LAPW+lo method has been used to calculate the adsorption energies at the scalar relativistic with no spin-orbit coupling (NSOC) and fully relativistic with spin-orbit coupling (SOC) theoretical levels. It is found that the SOC effect increases the adsorption energies by $\sim $0.30 eV for the two dissociative adsorptions. Weak physisoprtions have been observed for the molecule H$_{2}$O on the $\alpha $-Pu (020) surface with primarily a covalent bonding, while the two dissociative adsorptions are chemisorptive with ionic bonding. At the SOC level, the most stable adsorption energy is 0.58eV, the corresponding values being 5.44 eV and 5.73 eV for the partial dissociation and complete dissociation cases, respectively. Completely dissociative adsorption at a long bridge site for the dissociated O atom and two short bridge sites for the two dissociated H atoms is the most stable adsorption site. Hybridizations of O(2p)-H(1s)-Pu(5f)-Pu(6d) are observed for the two dissociative adsorptions, implying that some of the Pu-5f electrons become further delocalized and participate in chemical bonding.   

Characterization of Hydrogen Interactions with $\delta $-Pu using Electronic Structure Theory

The generalized gradient approximation to density functional theory was used to study surface, bulk, defect, and reaction states of hydrogen in $\delta $-Pu. The quasi-disordered anti-ferromagnetic arrangement gave a volume of 24.1 {\AA}$^{3}$ and a bulk modulus of 48.1 GPa for $\delta $-Pu, in reasonable agreement with the experimental values of 24.9 {\AA}$^{3}$ and 30-35 GPa. This arrangement was thus subsequently used for all calculations. We have determined that hydrogen interactions with $\delta $-Pu are exothermic in character at all levels ranging from dissociative chemisorption to interstitial absorption, the formation of hydrogen-vacancy complexes, and generation of a hydride phase. The exothermic character of these interactions appears to be the reason for the rapid hydriding reaction, which has been determined experimentally to be essentially a barrierless process. The anionic character is observed to be retained. Our studies also indicate that vacancies do not appear to be strong traps for hydrogen, since the interstitial absorption sites are exothermic in nature. We will propose a scheme by which hydrogen interacts with Pu. Results will be compared with previous studies in the literature where available.