Barrier height variations and interface properties of PtSi/Si structures

To study some of the interfacial properties of PtSi/Si diodes, Schottky structures were fabricated on (100) crystalline silicon substrates by conventional thermal evaporation of Pt on Si followed by annealing at different temperatures (from 400 degrees C to 700 degrees C) to form PtSi. The PtSi/n-Si diodes, all yielded Schottky barrier (SB) heights that are remarkably temperature dependent. The temperature range (20-290 K) over which the I-V characteristics were measured in the present study is broader with a much lower limit (20 K), than what is usually reported in literature. These variations in the barrier height are adequately interpreted by introducing spatial inhomogeneity into the barrier potential with a Gaussian distribution having a mean barrier of 0.76 eV and a standard deviation of 30 meV. Multi-frequency capacitance-voltage measurements suggest that the barrier is primarily controlled by the properties of the silicide-silicon interface. The forward C-V characteristics, in particular, show small peaks at low frequencies that can be ascribed to interface states rather than to a series resistance effect.

Variable temperature, variable-gap Otto prism coupler for use in a vacuum environment

The field of surface polariton physics really took off with the prism coupling techniques developed by Kretschmann and Raether, and by Otto. This article reports on the construction and operation of a rotatable, in vacuo, variable temperature, Otto coupler with a coupling gap that can be varied by remote control. The specific design attributes of the system offer additional advantages to those of standard Otto systems of (i) temperature variation (ambient to 85 K), and (ii) the use of a valuable, additional reference point, namely the gap-independent reflectance at the Brewster angle at any given, fixed temperature. The instrument is placed firmly in a historical context of developments in the field. The efficacy of the coupler is demonstrated by sample attenuated total reflectance results on films of platinum, niobium, and yttrium barium copper oxide and on aluminum/gallium arsenide (Al/GaAs) Schottky diode structures. (C) 2000 American Institute of Physics. [S0034-6748(00)02411-4].

Interface mediated resistive switching in epitaxial NiO nanostructures

We report on the non-volatile resistive switching properties of epitaxial nickel oxide (NiO) nanostructures, 10-100 nm wide and up to 30 nm high grown on (001)-Nb:SrTiO3 substrates. Conducting-atomic force microscopy on individual nano-islands confirms prominent bipolar switching with a maximum ON/OFF ratio of similar to 10(3) at a read voltage of similar to+0.4V. This ratio is found to decrease with increasing height of the nanostructure. Linear fittings of I-V loops reveal that low and high resistance states follow Ohmic-conduction and Schottky-emission mechanism, respectively. The switching behavior (dependence on height) is attributed to the modulation of the carrier density at the nanostructure-substrate interface due to the applied electric field.

Long-Range Lateral Dopant Diffusion in Tungsten Silicide Layers

Novel diode test structures have been manufactured to characterize long-range dopant diffusion in tungsten silicide layers. A tungsten silicide to p-type silicon contact has been characterized as a Schottky barrier rectifying contact with a silicide work function of 4.8 eV. Long-range diffusion of boron for an anneal at 900 °C for 30 min has been shown to alter this contact to become ohmic. Long-range diffusion of phosphorus with a similar anneal alters the contact to become a bipolar n-p diode. Bipolar diode action is demonstrated experimentally for anneal schedules of 30 min at 900 °C, indicating long-range diffusion of phosphorus (~38 µm), SIMS analysis shows dopant redistribution is adversely affected by segregation to the silicide/oxide interface. The concept of conduit diffusion has been demonstrated experimentally for application in advanced bipolar transistor technology. © 2009 IEEE.

Characterization of optical properties of PtSi at 3.392 mu m from 300 K to 85 K and relation of morphological effects

PtSi/Si Schottky junctions, fabricated using a conventional technique of Pt deposition with a subsequent thermal anneal, are examined using X-ray diffraction, atomic force microscopy and a novel prism/gap/sample optical coupling system. With the aid of X-ray diffraction and atomic farce microscopy it is shown that a post-anneal etch in aqua regia is essential for the removal of an unreacted, rough surface layer of Pt, to leave a much smoother PtSi film. The prism/gap/sample or Otto coupling rig is mounted in a small UHV chamber and has facilities for remote variation of the gap (by virtue of a piezoactuator system) and variation of the temperature in the range of similar to 300 K - 85 K. The system is used to excite surface plasmon polaritons on the outer surface of the PtSi and thus produce sensitive optical characterisation as a function of temperature. This is performed in order to yield an understanding of the temperature dependence of phonon and interface scattering of carriers in the PtSi.

OPTIMIZATION OF THE SLOW-MODE PLASMON POLARITON IN LIGHT-EMITTING TUNNEL-JUNCTIONS

Light emitted from metal/oxide/metal tunnel junctions can originate from the slow-mode surface plasmon polariton supported in the oxide interface region. The effective radiative decay of this mode is constrained by competition with heavy intrinsic damping and by the need to scatter from very small scale surface roughness; the latter requirement arises from the mode's low phase velocity and the usual momentum conservation condition in the scattering process. Computational analysis of conventional devices shows that the desirable goals of decreased intrinsic damping and increased phase velocity are influenced, in order of priority, by the thickness and dielectric function of the oxide layer, the type of metal chosen for each conducting electrode, and temperature. Realizable devices supporting an optimized slow-mode plasmon polariton are suggested. Essentially these consist of thin metal electrodes separated by a dielectric layer which acts as a very thin (a few nm) electron tunneling barrier but a relatively thick (several 10's of nm) optically lossless region. (C) 1995 American Institute of Physics.

PHOTOSIGNAL ENHANCEMENT IN AL-GAAS DIODES DUE TO SURFACE-PLASMONS AND GUIDED-WAVE MODES

Light of wavelength 632.8 nm and p-polarization is incident on a prism-air gap (varied from 0.7 to 7 mum)-Al-GaAs arrangement. Both the photosignal generated by the Schottky diode and the reflectance are measured as a function of the internal angle of incidence in the prism. There is significant, well-defined enhancement of the photosignal, up to a factor of approximately 7.5, associated with two different types of enhanced absorption modes. For air gaps <1.5 mum there is photosignal enhancement due to an enhanced absorption feature (reflectance dip) that occurs at an angle of incidence just above critical angle in the prism; this feature corresponds to the excitation of a surface plasmon polariton at the Al-air interface. For air gaps > 1 mum there are between one and ten photoresponse peaks at input angles less than the critical angle. The corresponding enhanced absorption features are due to leaky guided wave modes set up in the air gap.

INFRARED SURFACE-PLASMONS ON PLATINUM SILICIDE

It is shown that surface plasmons (SPs) are supported on thin PtSi films. Using a prism-air gap-sample configuration, p-polarised infra-red light (3.39-mu-m) has been coupled with approximately 95% efficiency to SPs on the silicide electrode of PtSi-Si Schottky barrier structures. Stimulating SPs offers both a means of optically characterising silicide films and of enhancing optical absorption with a view to significantly increasing the Schottky barrier photoresponse.

A simple and low-cost method for the preparation of self-supported TiO2-WO3 ceramic heterojunction wafers

Robust, bilayer heterojunction photodiodes of TiO₂-WO₃ were prepared successfully by a simple, low-cost powder pressing technique followed by heat-treatment. Exclusive photoirradiation of the TiO₂ side of the photodiode resulted in a rapid colour change (dark blue) on the WO₃ surface as a result of reduction of W⁶⁺ to W⁵⁺ (confirmed by X-ray photoelectron spectroscopy). This colour was long lived and shown to be stable in a dry environment in air for several hours. A similar photoirradiation experiment in the presence of a mask showed that charge transfer across the heterojunction occurred approximately normal to the TiO₂ surface, with little smearing out of the mask image. As a result of the highly efficient vectorial charge separation, the photodiodes showed a tremendous increase in photocatalytic activity for the degradation of stearic acid, compared to wafers of the respective individual materials when tested separately.

Local conductance: A means to extract polarization and depolarizing fields near domain walls in ferroelectrics

Conducting atomic force microscopy images of bulk semiconducting BaTiO3 surfaces show clear stripe domain contrast. High local conductance correlates with strong out-of-plane polarization (mapped independently using piezoresponse force microscopy), and current- voltage characteristics are consistent with dipole-induced alterations in Schottky barriers at the metallic tip-ferroelectric interface. Indeed, analyzing current-voltage data in terms of established Schottky barrier models allows relative variations in the surface polarization, and hence the local domain structure, to be determined. Fitting also reveals the signature of surface-related depolarizing fields concentrated near domain walls. Domain information obtained from mapping local conductance appears to be more surface-sensitive than that from piezoresponse force microscopy. In the right materials systems, local current mapping could therefore represent a useful complementary technique for evaluating polarization and local electric fields with nanoscale resolution.