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The influence of well-defined steps on the dynamics of the dissociative chemisorption of methane on Pt(533) has been investigated using molecular beam techniques. The initial dissociative chemisorption probability SO has been determined as a function of incident energy E[sub i], angle of incidence θ[sub i], and surface temperature T[sub S]. For incident kinetic energies in the range 26 < E[sub i](meV) < 1450, the initial dissociation probability of CH[sub 4] on the Pt(533) surface is higher than on Pt(111), for all surface temperatures investigated. This enhancement in dissociation is associated with the additional direct sticking mediated by the step sites, with no evidence for any additional indirect dynamical channel to dissociation induced by the step sites in the range of energies studied. The E[sub i] dependence can be separated into the contributions of the (111) terraces and the (100) steps. The latter exhibits an effective activation barrier for dissociation ≈300 meV lower than the (111) terraces. The angular dependence can also be interpreted as having two contributions, one associated with the (111) terraces, and the second associated with the steps. The angular dependence associated with the step sites is broader than the dependence expected for the (111) terraces, and has a maximum for incident trajectories with an angle between the angles corresponding to the normal directions of the (111) and (100) facets. An enhanced T[sub S] dependence is also observed on the Pt(533) surface over Pt(111). This is also associated with the influence of the step sites, and results either from the lower barrier to dissociation, or more likely a more effective coupling of the energy from the surface into the reaction coordinate. [ABSTRACT FROM AUTHOR]

The dissociative adsorption of oxygen on the stepped Pt(533) surface has been investigated using supersonic molecular beams by measuring the initial dissociative sticking probability S[sub 0] as a function of incident kinetic energy E[sub i], angle Φ, surface temperature T[sub S], and S as a function of coverage Θ. By comparison with dynamical data available on the Pt(111) surface we have been able to establish that step sites dominate the dissociative adsorption process. S[sub 0](E[sub i]) for oxygen on Pt(533) at T[sub S]=200 in the energy range 52 meV-1.4 eV shows a similar functional dependence to results on Pt(111), however, the magnitude of S[sub 0] on Pt(533) is significantly greater at all energies. The measurement of S[sub 0](Φ), scattering in a plane perpendicular to the step direction at E[sub i]=1.18 eV at T[sub S]=350 K, reveals a strong and asymmetric angular dependence which contains contributions associated with activated adsorption and dissociation of the chemisorbed precursor on the (111) terraces, and a second contribution associated with activated dissociation through a similar channel at the step sites. The latter exhibits a maximum in S[sub 0](Φ) at 35°, near the angle corresponding to the normal of the (100) step plane. S[sub 0](T[sub S]) at E[sub i]=1.18 eV and Φ=0° reveals a much smaller temperature dependence in the range 150>T[sub S](K)>800 on Pt(533) than on Pt(111). At E[sub i]=1.18 eV and Φ=0° ca. 15%-25% of dissociation takes place through molecules impinging directly at step sites. The remaining fraction dissociate through activated adsorption of the chemisorbed species on the (111) terrace and subsequent partition between desorption, and dissociation at step sites. Dissociation of the chemisorbed precursor on the (111) terrace appears highly activated, a result which is consistent with theory. The rapid decrease in S[sub 0](E[sub i]) observed below 0.15 eV on Pt(533), observed also on P... [ABSTRACT FROM AUTHOR]

We develop ultraviolet (UV) light emitting diodes (LEDs) that operate in the sub-240 nm (far UVC) regime. The traditional limitations of high Al content AlGaN—the low light extraction efficiency due to transverse magnetic polarization and the poor doping efficiency—were overcome by using AlN:GaN short period superlattices (SPSLs) of varying compositions throughout almost the entire device structure. The quality of our molecular beam epitaxy process for growing SPSL devices was verified via transmission electron microscopy and x-ray diffraction—together, these demonstrate our capacity for growing high quality interfaces with a great deal of control over the thicknesses of each individual layer. Resistivity measurements of our SPSL were three times smaller than bulk AlGaN of similar composition due to the improved doping efficiency. Devices fabricated on 6 in. sapphire substrates showed uniform performance, leading to over 10 000 functional devices per wafer. Packaged diodes exhibited a peak wall plug efficiency of 0.55% and an output power of 1.7 mW when operated at peak efficiency. The devices had an operating life (70% initial power) of 2800 h (at 20 mA continuous wave). Additionally, they could be driven as high as 1 A, which resulted in a record for radiant flux of a far UVC LED grown on sapphire (17.4 mW). [ABSTRACT FROM AUTHOR]

Polycrystalline thin-film libraries of Bi2O3-TiO2-Na2O system have been synthesised over a wide composition region using elemental molecular beam epitaxy sources combined with an oxygen plasma source. Structural characterisation and the screening of both dielectric and ferroelectric properties of fabricated compounds have been carried out. The bismuth sodium titanate perovskite-type phases exhibited a high relative permittivity (up to 375), a loss tangent lower than 0.02 and ferroelectric properties superior to thin films synthesised using other deposition methods: a remnant polarization, Pr, approximately 22 μC/cm2 was observed in the optimal composition range. The high throughput approach allowed us to establish clearly the compositional boundaries of the ferroelectric perovskite phase in the system. [ABSTRACT FROM AUTHOR]

The development of gate systems suitable for high κ dielectrics is critical to the advancement of complementary metal-oxide-semiconductor (CMOS) devices. Both the effective work function and material stability are key parameters to these systems. A systematic study of metal gates of the composition HfxSi1-x (0.25 ≤ x ≤ 1) is demonstrated here, including XPS, XRD and four point probe measurements. The effective work function of each material is evaluated and it is shown that it can be tuned from 4.5 to less than 4.0 eV. Suitable work functions for n-channel metal-oxide-semiconductor applications (4.05 ± 0.2 eV) were achieved using hafnium rich compositions; however, XPS and diffraction measurements confirmed that these materials demonstrated a high propensity to oxidise, causing the reduction of the underlying oxides, making them unsuitable for commercial application. [ABSTRACT FROM AUTHOR]