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Studies on the thermal reaction behavior of polycrystalline cubic silicon carbide (SiC) with effusive xenon difluoride (XeF2) have been carried out over the sample temperature (Ts) range from 300to900K using molecular beam quadrupole mass spectrometry combined with a time-of-flight technique and ex situ surface analyses, i.e., x-ray photoelectron spectroscopy (XPS) and scanning Auger microscopy (SAM). Above Ts=700K, the reaction product desorbed from the SiC surface was identified as SiF4. The flux intensity of SiF4 increases monotonically as a function of Ts above 700K. The flux intensity of XeF2 desorbed from the SiC surface decreases above Ts=700K, and at Ts=900K, approximately 10% of the incident XeF2 was found to be consumed by the thermal reaction. No ions at m∕e=31 (CF+), 50 (CF2+), and 69 (CF3+) to be ascribed to carbon fluoride species were detected under the present experimental conditions, and thus C atoms in SiC were found to remain as residue. From the XPS and SAM observations of the SiC samples exposed to XeF2 vapor at 1.8×10−4Torr, we find that fluorination of a native oxide layer formed on the SiC surface takes place at Ts=300K. At Ts=520K, the native oxide layer was partially removed from the surface, and the presence of a reaction layer composed of partially fluorinated C atoms was observed. As Ts is increased above 520K, the reaction layer becomes thicker. Above Ts=700K, a thick reaction layer mainly composed of C atoms is formed, while only the near-surface reaction layer is fluorinated. The fast desorption of SiF4 products reduces the Si concentration in the SiC surface and the residual C atoms having comprised the SiC lattice are left as the reaction layer.

Thermal desorption behaviors of the AlF 3 layer formed on Al 2 O 3 in the sample temperature range from T s ¼ 300 to 930 K have been studied using molecular beam mass spectrometry combined with a time-of-flight (TOF) technique. Fluorine atoms were detected as the desorbed species at sample temperatures of T s ¼ 625 to 850 K and the intensity was found to be peaked at T s ¼ 750 K. AlF 2 species whose translational temperature T tr is approximately 100 K lower than T s were also detected as desorbed species above T s ¼ 850 K and the intensity increased exponentially as T s was raised. Based on these results, the desorption behavior of AlF 3 species is discussed.

The finite size of a fractal cluster of dimension D causes a reduction in the dimension of its projection to D* on any finite observation scale. A theoretical argument is presented which relates the apparent dimension D* to D and to the observation scale as a fraction of the outer fractal scale. The theory is tested for computer-generated fractal clusters. When applied to a sample of electron micrographs of diesel soot particles whose apparent dimension is 1.83 i-0.06, the theory gives 1.90 & 0.07 as the true fractal dimension of the soot.

Articles you may be interested inComprehensive surface analysis of GaN-capped AlGaN/GaN high electron mobility transistors: Influence of growth method J. Appl. Phys. 110, 083527 (2011); 10.1063/1.3653825 AlGaAs/InGaAs metal-oxide-semiconductor pseudomorphic high-electron-mobility transistor with low temperature liquid phase deposited Al 2 O 3 gate insulator Low coverage spontaneous etching and hyperthermal desorption of aluminum chlorides from Cl 2 / Al (111) J. Chem. Phys. 121, 9018 (2004); 10.1063/1.1805495 Selected energy epitaxial deposition of GaN and AlN on SiC(0001) using seeded supersonic free jets of NH 3 in helium J.Detailed studies on the thermal reaction behavior of polycrystalline aluminum nitride ͑AlN͒ with effusive xenon difluoride ͑XeF 2 ͒ have been carried out over the sample temperature ͑T s ͒ range from 300 to 920 K using molecular beam mass spectrometry combined with a time-of-flight technique and ex situ surface analyses, i.e., X-ray photoelectron spectroscopy, Auger electron spectroscopy, and scanning electron microscopy ͑SEM͒. The species desorbed from the AlN / XeF 2 system were monitored using molecular beam mass spectrometry, as a function of sample temperature. Above T s = 800 K, the desorbed reaction products were identified as N 2 and AlF 3 , and their flux intensities increase monotonically as the sample temperature is increased. The flux intensity of XeF 2 desorbed after physisorption to the AlN surface is found to decrease as T s is raised above T s = 800 K, and approximately one half of the incoming XeF 2 is consumed by the thermal reaction at 920 K. The results of surface analyses show that the thermal reaction of AlN with XeF 2 starts at approximately T s = 700 K, forming a reaction layer composed of AlF 3 . The AlF 3 layer becomes thick as T s is increased from T s = 700-800 K. Above T s = 800 K, however, as a result of fast desorption of AlF 3 and F atoms from the AlF 3 layer, only partially fluorinated AlF x ͑x = 1 and/or 2͒ layers are formed and the bulk AlN is revealed again. The SEM photographs indicate that the surfaces exposed above T s = 850 K are strongly etched but a slight change is observed at T s ഛ 800 K. On the basis of these results, three reaction stages are proposed for the AlN / XeF 2 reaction depending on the sample temperature range: Stage 1 ͑300ഛ T s Ͻ 700 K͒; no reaction, stage 2 ͑700ഛ T s Ͻ 800 K͒; surface fluorination, and stage 3 ͑800ഛ T s ͒; etching. At stage 3, AlF 3 formed on the surface starts to evaporate and fast etching proceeds, since the vapor pressure of AlF 3 is high enough in this temperature range.