Search

The reaction probabilities of GeH4 and Ge2H6 on the Ge(100) and Ge(111) surfaces have been measured as a function of substrate temperature, incident kinetic energy, and angle of incidence employing supersonic molecular beam scattering techniques. At sufficiently large incident kinetic energies (Ei>=1 eV) both GeH4 and Ge2H6 react by direct dissociative chemisorption on both surfaces examined, with the reaction probability increasing approximately exponentially with increasing (scaled) incident kinetic energy. At moderate kinetic energies (Ei∼0.4 eV), however, Ge2H6 reacts by a precursor-mediated mechanism on Ge(100), as demonstrated by a decrease in the reaction probability with either increasing substrate temperature or incident kinetic energy. Interestingly, under similar conditions, no evidence is found for precursor-mediated adsorption of Ge2H6 on the Ge(111) surface. The reaction of Ge2H6 does not exhibit a GeH4 production channel on either Ge(100) or Ge(111) for the conditions examined here. The results obtained at high incident kinetic energies (>=1 eV) are well described by a statistical model based upon a Rice–Ramsperger–Kassel–Marcus (RRKM) framework. The moderate incident kinetic energy results for Ge2H6 on Ge(100) are well described by a model that assumes reaction via a trapping, precursor-mediated mechanism. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Homoepitaxy of SrTiO[sub 3] by pulsed-laser deposition has been studied using in situ time-resolved surface x-ray diffraction in the temperature range of 310°C to 780°C. Using a two-detector configuration, surface x-ray diffraction intensities were monitored simultaneously at the (0 0 ½) specular and the (0 1 ½) off-specular truncation rod positions. Abrupt intensity changes in both the specular and off-specular rods after laser pulses indicated prompt crystallization into SrTiO[sub 3] layers followed by slower intra- and interlayer surface rearrangements on time scales of seconds. Specular rod intensity oscillations indicated layer-by-layer growth, while off-specular rod intensity measurements suggested the presence of transient in-plane lattice distortions for depositions above 600 °C. [ABSTRACT FROM AUTHOR]

Recent studies indicated that noncompensated cation-anion codoping of wide-band-gap oxide semiconductors such as anatase TiO2 significantly reduces the optical band gap and thus strongly enhances the absorption of visible light [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)]. We used soft x-ray spectroscopy to fully determine the location and nature of the impurity levels responsible for the extraordinarily large (~1 eV) band gap reduction of noncompensated codoped rutile TiO2. It is shown that Cr/N codoping strongly enhances the substitutional N content, compared to single element doping. The band gap reduction is due to the formation of Cr 3d³ levels in the lower half of the gap while the conduction band minimum is comprised of localized Cr 3d and delocalized N 2p states. Band gap reduction and carrier delocalization are critical elements for efficient light-to-current conversion in oxide semiconductors. These findings thus raise the prospect of using codoped oxide semiconductors with specifically engineered electronic properties in a variety of photovoltaic and photocatalytic applications. [ABSTRACT FROM AUTHOR]