Woei Wu Pai

Dislocation emission around nanoindentations on a (001) fcc metal surface studied by scanning tunneling microscopy and atomistic simulations.

We present a combined study by scanning tunneling microscopy and atomistic simulations of the emission of dissociated dislocation loops by nanoindentation on a (001) fcc surface. The latter consist of two stacking-fault ribbons bounded by Shockley partials and a stair-rod dislocation. These dissociated loops, which intersect the surface, are shown to originate from loops of interstitial character emitted along the <110> directions and are usually located at hundreds of angstroms away from the indentation point. Simulations reproduce the nucleation and glide of these dislocation loops.

The adsorption and desorption of ferrocene on Ag(100)

The molecular adsorption and desorption of ferrocene, Fe(C5H5)2, on Ag(100) have been studied by both photoemission and thermal desorption. Photoemission results indicate that the initially adsorbed surface species closely resemble that of molecular ferrocene. The molecule is adsorbed with the cyclopentadienyl (C5H5) ring ligands parallel to the surface, as determined by electron energy-loss spectroscopy. The shift in photoemission binding energies relative to the gas phase is largely independent of the molecular orbital. The ultraviolet light does lead to partial fragmentation of the ferrocene, but fragmentation occurs only in the presence of incident radiation. The energetics of molecular desorption are influenced by lateral interactions within the molecularly adsorbed film.

Magnetic nanostructures fabricated by scanning tunneling microscope-assisted chemical vapor deposition

We have successfully used scanning tunneling microscope-assisted chemical vapor deposition to fabricate magnetic nanostructures as fine as 5 nm wide and <2 nm high using ferrocene [Fe(C5H5)2] as the metal-organic source gas. The physical properties of these nanostructures were qualitatively characterized and ex situ magnetic force microscopy measurements indicate these features are strongly magnetic.

Steps as adatom sources for surface chemistry: oxygen overlayer formation on Ag(110)

Abstract Surface chemical reactions often require a ready supply of substrate atoms to occur. In principle, steps serve as an efficient source of these atoms, provided that detachment rates from the step edges are sufficiently large. In this paper, we characterize atomic detachment rates from steps on clean Ag(110) by examining step fluctuations. We show that these rates are sufficient to supply atoms to form the added-row reconstruction of oxidized Ag(110) when the oxygen partial pressure is low. For high oxygen pressures, however, we find that step detachment rates are slow compared with oxidation rates, and the step source of Ag is supplemented by vacancy-island generation on the terraces. These results are compared to those obtained for the similar O/Cu(110) and O/Ni(110) systems.

Sodium ion ordering and vacancy cluster formation in NaxCoO2 (x=0.71 and 0.84) single crystals by synchrotron X-Ray diffraction.

The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hampered by the difficulty of controlling pure phases. We discovered that certain Na concentrations are specially stable and are associated with superlattice ordering of the Na clusters. This leads naturally to a picture of co-existence of localized spins and itinerant charge carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our picture brings coherence to a variety of measurements ranging from optical to thermal transport. Our results also allow us to take the first step towards modeling the mysterious “Curie-Weiss” metal state at x = 0.71. We suggest the local moments may form a quantum spin liquid state and we propose experimental test of our hypothesis.

Comparative study of InAs quantum dots with different InGaAs capping methods

The authors have used cross-sectional scanning tunneling microscopy to examine strain relaxation profiles of InAs quantum dots with In0.33Ga0.67As layers overgrown by three distinct capping methods. A statistical analysis of strain relaxation profile allowed them to infer that the long wavelength emission (>1.3μm) of InAs quantum dots capped with sequential GaAs∕InAs binary growth is mainly due to a weaker quantum confinement effect. This particular capping method is better than the traditional molecular beam epitaxy with simultaneous In∕Ga∕As deposition, and much better than a capping method with separated Ga deposition followed by As and InAs growth.

The Roughness-Enhanced Light Emission from Metal-Oxide-Silicon Light-Emitting Diodes Using Very High Vacuum Prebake

The oxide roughness of metal-oxide-silicon diodes can be intentionally controlled by the very high vacuum pre-bake and the growth conditions during rapid thermal oxidation. Both surface and Si/oxide interface have the similar magnitude of roughness measured by atomic force microscopy, indicating the conformal growth of oxide. At accumulation bias (positive gate bias), the holes tunnel from gate electrode to n-type Si through the ultrathin oxide, and recombine with the electrons in the accumulation region radiatively if phonon scattering and roughness scattering provide the necessary momentum. The light emission intensity increases with increasing oxide roughness. Strong electroluminescence with an external quantum efficiency of ~ 2×10-6 at room temperature was observed from a rough metal-oxide-silicon tunneling diode.

Sodium trimer ordering on a NaxCoO2 surface.

Sodium ion ordering on an in situ cleaved NaxCoO2 (x=0.84) surface has been studied by ultrahigh vacuum scanning tunneling microscopy at room temperature. Three main phases, with p(3 x 3), ( radical7 x radical7), and (2 radical3 x 2 radical3) hexagonal unit cells and a surface Na concentration of 1/3, 3/7, 1/2, respectively, were identified. One surprising finding is that Na trimers act as the basic building blocks that order in long range. The stability of Na trimers is attributed to the increased Na coordination with oxygen as indicated by ab initio calculations, and possibly at finite temperature by configuration entropy.

Roughness-enhanced reliability of MOS tunneling diodes

Both electrical and optical reliabilities of PMOS and NMOS tunneling diodes are enhanced by oxide roughness, prepared by very high vacuum prebake technology. For rough PMOS devices, as compared to flat PMOS devices, the Weibull plot of T/sub BD/ shows a 2.5-fold enhancement at 63% failure rate, while both the D/sub 2/ and H/sub 2/-treated flat PMOS devices show similar inferior reliability. For rough NMOS devices, as compared to flat NMOS devices, the Weibull plot of T/sub BD/ shows a 4.9-fold enhancement at 63% failure rate. The time evolutions of the light emission from rough PMOS and NMOS diodes degrade much less than those of flat PMOS and NMOS diodes. The momentum reduction perpendicular to the Si/SiO/sub 2/ interface by roughness scattering could possibly make it difficult to form defects in the bulk oxide and at the Si/SiO/sub 2/ interface by the impact of the energetic electrons and holes.

Adsorption of Methyl (CH3) and Methylnitrene (NCH3) Radicals on Cu Surfaces Studied by Scanning Tunneling Microscopy

Formation of two-dimensional islands of methyl radical (CH3) on Cu(111) and Cu(110) surfaces, and methylnitrene radical (NCH3) islands on Cu(110) was studied with scanning tunneling microscopy. In all cases, close-packed or nearly close-packed two-dimensional islands were observed. We found that (1) chemisorbed CH3 aggregated into a (√3 ×√3)-R30° island structure that was more readily observed at T<90 K. (2) CH3 aggregated into a c(2 ×2) island structure on Cu(110), and (3) NCH3 on Cu(110) formed p(2 ×3) islands. Plausible structural models for these systems have been suggested previously. Here we further investigate their dynamic and static structural details, with focus on (1) temperature dependence of CH3 island evolution on Cu(111); (2) comparison of island structure of CH3 on Cu(110) and Cu(111); and (3) Cu adatom incorporation in NCH3 islands on Cu(110).