Multiphoton photoelectron spectroscopy reveals the multiple excitation of the surface plasmon in silver nanoparticles on graphite. Resonant excitation of the surface plasmon with 400 nm femtosecond radiation allows one to distinguish between photoemission from the nanoparticles and the substrate. Two different previously unobserved decay channels of the collective excitation have been identified, namely, decay into one or several single-particle excitations.
High temperature gas phase reactions between trimethylgallium ͑TMG͒ and ammonia were studied by means of in situ mass spectroscopy in an isothermal flow tube reactor. The temperature, pressure, and reaction time were chosen to emulate the gas phase environment typical of the metal-organic vapor phase epitaxy ͑MOVPE͒ of GaN. The main gas phase species is ͓(CH 3 ͒ 2 Ga:NH 2 ͔ x , where most probably xϭ3, resulting from the very fast adduct formation followed by elimination of methane. The further gas phase decomposition of this species proceeds through the stepwise elimination of methane. These studies indicate that little TMG exists within the growth ambient under most MOVPE growth conditions. The further gas phase reaction of ͓(CH 3 ͒ 2 Ga:NH 2 ͔ x may be responsible for the strong dependence of the MOVPE GaN growth rate and uniformity commonly observed. © 1996 American Institute of Physics. ͓S0003-6951͑96͒00527-X͔The metal-organic vapor phase epitaxy ͑MOVPE͒ has emerged as the current growth technique of choice for the growth of GaN for device structures.1 Recent successes in the MOVPE formation of commercially viable blue and green light emitters have resulted in an increased activity in understanding the growth process and its relationship to the physical properties of these materials. While MOVPE can produce these materials, the growth of device-quality GaN is complicated by the severe and complicated gas phase interactions, that occur between trimethylgallium ͓͑TMG or (CH 3 ͒ 3 Ga͔ and ammonia, NH 3 which are the commonly used growth precursors.2 These gas phase interactions have been suggested to lead to gas phase depletion of the growth nutrients leading to a degradation in the growth uniformity and efficiency.3 A primary gas phase reaction is the strong adduct forming reaction between NH 3 and TMG. [4][5][6][7] In this present study, we have directly monitored this gas phase reaction in order to better understand its impact on the growth process and system design under those conditions particularly important for MOVPE growth. The species responsible for growth are the direct result of these gas phase reactions.The gas phase reactions of both TMG and NH 3 , individually, are relatively understood at this point. The pyrolysis of TMG has been widely studied. [8][9][10][11][12] The initial step in the TMG decomposition is generally agreed to be the removal of the first methyl radical, resulting in methane as the main product released in the decomposition. In the stepwise first order decomposition of TMG, an apparent activation energy has been measured for the first and second methyl groups, of 58-60 kcal/mol ͑Refs. 8 and 10͒ and 35.4 kcal/ mol, respectively.8 Under similar thermal conditions ammonia is quite stable. The gas phase decomposition of NH 3 occurs only at very high temperatures. 13 The heterogeneous decomposition of NH 3 may, therefore, be essential to GaN growth.The gas phase reaction between (CH 3 ) 3 Ga and NH 3 is less understood, particularly at elevated temperatures characteristic of MOVPE ...
The excitation of the tunneling junction of a scanning tunneling microscope using ultrashort laser pulses combined with detection of a tunneling current component which depends nonlinearly on the laser intensity allows, in principle, to simultaneously obtain ultimate spatial and temporal resolution. To achieve this goal, a laser system that produces ultrashort laser pulses is combined with an ultrahigh vacuum scanning tunneling microscope. The basic technical considerations are discussed and it is shown that atomic resolution can be achieved under pulsed laser excitation of the tunneling junction. The pulsed illumination gives rise to several contributions to the measured total current. Experimental evidence for signal contributions due to thermal expansion, transient surface potentials and multiphoton photoemission are presented.
Multiphoton photoemission spectroscopy of graphite using 267 nm ͑4.65 eV͒ and 400 nm ͑3.1 eV͒ excitation wavelength reveals spectroscopic features that allow the identification of the multiphoton excitation process and that correspond to the known bulk band structure. In addition, the nϭ1 and nϭ2 image potential states on graphite are identified, with binding energies of 0.85 and 0.15 eV, respectively. They are characterized by a vanishing quantum defect and are located close to the top of the band gap in the projected bulk band structure. Accordingly, the nϭ1 image potential state and the minimum of the interlayer band are both located about 4 eV above the Fermi level. This settles the ambiguities in the interpretation of the unoccupied band structure of graphite with respect to the energetic location of the interlayer band. Time-resolved two-photon photoemission spectroscopy yields a lifetime of 40Ϯ6 fs for the nϭ1 image potential state. This rather long lifetime of an image potential state at the top of the band gap and the vanishing quantum defect are attributed to the two-dimensional structure of graphite.
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