Robert A. Bartynski

Band alignment issues related to HfO2∕SiO2∕p-Si gate stacks

The valence and conduction band densities of states for the HfO2∕SiO2∕Si structure are determined by soft x-ray photoemission and inverse photoemission. First principles calculations are used to help in assigning valence band maxima and conduction band minima. The energies of defect states at the band edges are estimated by comparing the theoretical and experimental results. Determinations of the local surface potentials before and after a forming gas anneal are used to help determine the possible location of the charge in the film.

Formation, dynamics, and implication of solid electrolyte interphase in high voltage reversible conversion fluoride nanocomposites

Metal fluoride nanocomposites are uniquely suited as an alternative pathway to provide very high energy density cathodes for lithium batteries. Contrasted with modern intercalation compounds, they undergo conversion upon discharge into nanodomains of lithium fluoride and highly active metal. The nanosized metal formed during the discharge process along with the dynamic nature of the crystal structure may have considerable impact on the stability of any solid state interphase formed through reaction with the electrolyte. This is in contrast to the more macrocrystalline and stable crystal structure of traditional intercalation compounds. It has been found that the cyclic carbonates are susceptible to decomposition on the nanometal surfaces at potentials as high as 2.00 V vs. Li, and the products have been identified with Field Emission Scanning Electron Microscopy (FESEM), Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and X-ray Photoelectron Spectroscopy (XPS) as lithium carbonate species. Of greater importance is the impact of these decomposition products on the reversible cycling of the metal fluoride. Through a series of potentiodynamic and galvanostatic cycling trials, a clear relationship has been developed for the bismuth fluoride nanocomposites, the decomposition of the electrolyte solvent, and the cycle life. Acyclic organic carbonate solvents have been found to have minimal interaction and exhibited better long-term cycling performance than cyclic solvents.

Epitaxial CoGa and textured CoAs contacts on Ga1−xAlxAs fabricated by molecular‐beam epitaxy

Films of CoGa and CoAs have been deposited on Ga1−xAlxAs surfaces. Control of the Ga1−xAlxAs surface prior to CoGa deposition results in the preferential formation of two epitaxial orientations, either (100)[011]CoGa∥(100)[011]Ga1−xAlxAs or (110)[001]CoGa∥(100)[011]Ga1−xAlxAs. The crystalline quality of (100)‐oriented CoGa was good as determined by Rutherford backscattering with channeling measurements, χmin∼7%, and cross‐sectional transmission electron microscopy. The (110)CoGa films did not have as good a crystalline quality, χmin∼41%. CoAs films were found to be highly textured, but not single crystal. Schottky barrier diodes fabricated from CoGa/Ga1−xAlxAs and CoAs/Ga1−xAlxAs showed good characteristics with low ideality factors, n<1.15. In general CoGa contacts had a ∼0.1 eV lower barrier height than CoAs.

Ferromagnetism in Fe-implanted a-plane ZnO films

Fe ions of dose 5×1016cm−2 were implanted at 200keV into a-plane ZnO epitaxial films. The epitaxial quality of the postannealed samples was verified by x-ray diffraction ω-rocking curves and φ scans, whereas x-ray absorption spectroscopy identified the presence of both Fe2+ and Fe3+ ions, as well as changes in their relative concentration during postannealing. Superconducting quantum interference device measurements show that the as-implanted and postannealed films are ferromagnetic at room temperature. The saturation magnetization reduces during annealing possibly due to the decrease in the number of oxygen vacancies.

Auger photoelectron coincidence spectroscopy using synchrotron radiation

The technique of Auger photoelectron coincidence spectroscopy (APECS) using synchrotron radiation is discussed. Technical considerations and experimental details are emphasized. Results from Cu(100), Ta(100), and Al(111) are presented to show the kinds of new information that APECS can provide.

The relationship between local order, long range order, and sub-band-gap defects in hafnium oxide and hafnium silicate films

We have measured x-ray absorption spectra (XAS) at the oxygen K edge for hafnium oxide (HfO2) films grown by chemical vapor deposition (CVD) and atomic layer deposition (ALD), as well as for hafnium silicate (HfSiO) films grown by CVD. The XAS results are compared to x-ray diffraction (XRD) and spectroscopic ellipsometry (SE) data from the same films. Features characteristic of crystalline HfO2 are observed in the XAS spectra from all CVD-grown HfO2 films, even for a thickness of 5 nm where XRD is not sensitive. XAS and XRD spectra from the ALD-grown HfO2 films exhibit the signature of crystallinity only for films that are 20 nm or thicker. These characteristic XAS features are absent in all HfSiO films measured, which is consistent with their being amorphous. The appearance of these peaks in XAS and XRD is correlated with sub-band-gap absorption in the SE spectra, which appears to be intrinsic to crystalline HfO2 in the monoclinic phase.

Symmetry-breaking charge transfer in a zinc chlorodipyrrin acceptor for high open circuit voltage organic photovoltaics.

Low open-circuit voltages significantly limit the power conversion efficiency of organic photovoltaic devices. Typical strategies to enhance the open-circuit voltage involve tuning the HOMO and LUMO positions of the donor (D) and acceptor (A), respectively, to increase the interfacial energy gap or to tailor the donor or acceptor structure at the D/A interface. Here, we present an alternative approach to improve the open-circuit voltage through the use of a zinc chlorodipyrrin, ZCl [bis(dodecachloro-5-mesityldipyrrinato)zinc], as an acceptor, which undergoes symmetry-breaking charge transfer (CT) at the donor/acceptor interface. DBP/ZCl cells exhibit open-circuit voltages of 1.33 V compared to 0.88 V for analogous tetraphenyldibenzoperyflanthrene (DBP)/C60-based devices. Charge transfer state energies measured by Fourier-transform photocurrent spectroscopy and electroluminescence show that C60 forms a CT state of 1.45 ± 0.05 eV in a DBP/C60-based organic photovoltaic device, while ZCl as acceptor gives a CT state energy of 1.70 ± 0.05 eV in the corresponding device structure. In the ZCl device this results in an energetic loss between E(CT) and qV(OC) of 0.37 eV, substantially less than the 0.6 eV typically observed for organic systems and equal to the recombination losses seen in high-efficiency Si and GaAs devices. The substantial increase in open-circuit voltage and reduction in recombination losses for devices utilizing ZCl demonstrate the great promise of symmetry-breaking charge transfer in organic photovoltaic devices.

The first interlayer spacing of Ta(100) determined by photoelectron diffraction

A photoelectron diffraction study of Ta(100), employing the surface shifted 4 f core levels, has been performed to determine its first interplanar spacing. Photoemission intensities of the bulk and surface shifted 4 f core levels were measured as a function of photon energy at normal emission and near the 〈111〉 symmetry direction of the crystal. The binding energy of 21.7 eV (23.6 eV) for the bulk 4 f 7/2 (5/2) core level, and the large surface core level shift of 0.75 eV, result in well‐resolved peaks in the photoemission spectra with areas that are easily measured independently. We find that the ratio of the surface to bulk core level intensities at normal emission exhibits large oscillations as a function of photon energy due to diffraction effects. Intensity modulation is also seen in the off‐normal intensities, but the magnitude is much smaller. We have performed a multiple‐scattering photoemission calculation for several first interlayer spacings ranging from the bulk value to a 15% contraction. The...

Photoluminescence and x‐ray photoelectron spectroscopy study of S‐passivated InGaAs(001)

The influence of sulfur passivation on the surface composition of In0.53Ga0.47As(001) was investigated with photoluminescence (PL) and x‐ray photoelectron spectroscopy (XPS). Films of In0.53Ga0.47As(001), epitaxially grown on InP(100) substrates, were S passivated using a dry electron cyclotron resonance (ECR)‐plasma deposition process and were either passivated as‐prepared, or exposed to a BCl3 pre‐etch prior to passivation. In the spectral range from 1450 to 1750 nm, S passivation enhances the PL yield by approximately an order of magnitude. XPS shows that S binds both to In and As, although preferably to In, and that oxidation is essentially eliminated by the passivation process. The In–S bonds are more stable upon annealing than are the As–S bonds. Furthermore, the pre‐etched+H2S treatment enhances the PL yield beyond that of the H2S passivation treatment alone and produces a higher ratio of In–S to As–S bonds at the surface. In a second set of experiments, the influence of the ECR power applied to th...

GeOx interface layer reduction upon Al-gate deposition on a HfO2∕GeOx∕Ge(001) stack

The metallization of HfO2∕Ge by Al at room temperature was studied using photoemission and inverse photoemission. Upon deposition, Al reduces the GeOx interfacial layer between Ge and HfO2, and a thin Al2O3 layer is formed at the Al∕HfO2 interface. The band alignment across the Al∕HfO2∕Ge stacks is also addressed.