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71.
Yu Wu Tullio Toccoli Jian Zhang Norbert Koch Erica Iacob Alessia Pallaoro Salvatore Iannotta Petra Rudolf 《Applied Physics A: Materials Science & Processing》2009,95(1):21-27
Organic molecular beam deposition is studied systematically at thermal and hyperthermal regimes aiming at investigating the
role of molecular kinetic energy on the growth mechanism of pentacene submonolayers on SiO
x
/Si. We show that the kinetic energy of the impinging molecule (E
k
) plays a crucial role in determining island structure and shape, distribution of island sizes, the crystalline quality of
the first monolayer, and even the growth mode of subsequent layers. With increasing E
k
, the island structure changes from fractal to nonfractal, the shape becomes more anisotropic and the island size more uniform,
pointing to correlated island growth. Moreover, while 3D island growth is observed for thermal organic molecular beam deposition,
supersonic molecular beam deposition gives rise to layer-by-layer growth, at least for the first two layers. When E
k
≥5.0 eV, the first monolayer is composed of large single crystalline domains which can extend over up to 10 μm, inferred from
comparing atomic force micrographs of height and net transverse shear force. In these growth conditions both the high surface
diffusivity and energy redistribution play a major role. We propose a mechanism where the energy dissipation occurring during
the molecule–surface collision leads to the reorientation of whole islands during island coalescence, resulting in the elimination
of grain boundaries. 相似文献
72.
Chongfeng Guo Shuiting Wang Tao Chen Lin Luan Yan Xu 《Applied Physics A: Materials Science & Processing》2009,94(2):365-371
A series of double molybdates phosphors AEu(MoO4)2 (A = Li, Na, K and Ag) have been prepared by sol-gel method. Their crystal structure and luminescent properties have also
been investigated in a comparable way. The crystallization processes of the phosphor precursors were characterized by X-ray
diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA). Field emission scanning electron microscopy
(FE-SEM) was also used to characterize the shape and size distribution of the phosphors. Samples except KEu(MoO4)2 showed tetragonal scheelite structure in the range of our experiments, and no phase transition appeared. Phosphor KEu(MoO4)2 possessed two structures, and the phase transition took place at about 800°C. All samples with high purity could be obtained
at about 500°C for 5 hours, and they all showed intense red light peaked at 616 nm originated from 5D0→7F2 emission of Eu3+ under the excitation of 465 nm or 394 nm light. The excitation spectra of phosphors AEu(MoO4)2 (A = Li, Na, and K) are composed of a strong broad charge transfer (CT) band and some sharp lines, and the relative intensity
of CT band, the two strongest absorption lines at 395 nm and 465 nm are comparative, so these three phosphors are good red
phosphor candidates for violet or blue LEDs. For the excitation spectrum of phosphor AgEu(MoO4)2, intensities of CT band and the absorption line at 395 nm are much weaker than that of line at 465 nm, thus phosphor AgEu(MoO4)2 is only suit for GaN-based blue LED. 相似文献
73.
R. Imoto F. Stevens S. C. Langford J. T. Dickinson 《Applied Physics A: Materials Science & Processing》2009,94(1):35-43
Atomic force microscopy (AFM) is used to examine chemical–mechanical processes on Si(100) surfaces. The AFM tip serves as
a single asperity contact to exert tribological forces as well as an imaging tool. By scanning in chemically aggressive solutions,
material removal can be observed directly. In the silicon system, high-force scans are used to remove oxide and initiate etching
in selected locations, followed by low-force scans to image the resulting surfaces. Material removal rates were measured as
a function of applied load, number of scans, solution composition, and time. In basic solution, places where the underlying
silicon is exposed etch rapidly, producing structures 100 nm or less in size. Although the surface roughness initially increases
during etching, the final surfaces are smooth. The oxide is extremely sensitive to applied stress: even very light scanning
accelerates oxide dissolution. Once the oxide is removed, chemical etching proceeds through the underlying silicon with or
without AFM scanning; but the silicon etches more rapidly if AFM scanning is continued, due to true chemical–mechanical (tribochemical)
effects. 相似文献
74.
N. G. Semaltianos W. Perrie P. French M. Sharp G. Dearden S. Logothetidis K. G. Watkins 《Applied Physics A: Materials Science & Processing》2009,94(4):999-1009
Femtosecond laser (180 fs, 775 nm, 1 kHz) ablation characteristics of the nickel-based superalloy C263 are investigated. The
single pulse ablation threshold is measured to be 0.26±0.03 J/cm2 and the incubation parameter ξ=0.72±0.03 by also measuring the dependence of ablation threshold on the number of laser pulses.
The ablation rate exhibits two logarithmic dependencies on fluence corresponding to ablation determined by the optical penetration
depth at fluences below ∼5 J/cm2 (for single pulse) and by the electron thermal diffusion length above that fluence. The central surface morphology of ablated
craters (dimples) with laser fluence and number of laser pulses shows the development of several kinds of periodic structures
(ripples) with different periodicities as well as the formation of resolidified material and holes at the centre of the ablated
crater at high fluences. The debris produced during ablation consists of crystalline C263 oxidized nanoparticles with diameters
of ∼2–20 nm (for F=9.6 J/cm2). The mechanisms involved in femtosecond laser microprocessing of the superalloy C263 as well as in the synthesis of C263
nanoparticles are elucidated and discussed in terms of the properties of the material. 相似文献
75.
An interesting recent development in the Group III nitrides is the growth of InAlN lattice matched to GaN, with applications in distributed Bragg reflectors (DBRs), high electron mobility transistors (HEMTs) and as etch-layers. This work presents a systematic study of the effects of changing the key growth conditions of ammonia flux and growth temperature in InAlN growth by metal-organic vapour phase epitaxy (MOPVE) and describes our current optimised parameter set. We also particularly concentrate on the details of surface morphology assessed by atomic force microscopy (AFM). The nanoscale surfaces are characterised by low hillocks and dislocation pits, while at a larger scale microscopic indium droplets are also present. However, these droplets are eliminated when the layers are capped with GaN. Other trends observed are that increasing the growth temperature will lower the indium incorporation approximately linearly at a rate of approximately 0.25% per °C, and that increasing the ammonia flux from 44.6 to 178.6 mmol min−1 increased the indium incorporation, but further increases to 446 mmol min−1 did not result in any further increase. 相似文献
76.
A. Jasik A. Wnuk J. Gaca M. Wójcik A. Wójcik-Jedlińska J. Muszalski W. Strupiński 《Journal of Crystal Growth》2009,311(19):4423-4432
The influence of the growth rate and V/III ratio on the crystal quality of In0.2GaAs/GaAs quantum well structures was examined. The investigated heterostructures were grown by molecular beam epitaxy (MBE) and metalorganic chemical vapour deposition (MOCVD). Reflection high energy electron diffraction (RHEED), photoluminescence measurements (PL), high-resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) were applied for evaluation of the interfaces smoothness and the overall layer quality. Comprehensive characterisation of InGaAs/GaAs structures allowed us to establish optimal values of analysed technological parameters. Moreover, the comparison between the results obtained for samples grown by two different epitaxial techniques allowed us to find, which of the analysed growth parameters has the strongest influence on the quality of MBE and MOCVD grown structures. In contrast with the growth temperature and the interruption time, which in different manner impact on the crystal quality of QWs obtained by different method, the growth rate and the V/III ratio play similar role in both epitaxial techniques. 相似文献
77.
The effects of deposition rate on the microstructure and thermoelectric (TE) properties of Ca3Co4O9 thin films fabricated by pulsed laser deposition (PLD) technique were investigated. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) revealed that a fast deposition rate resulted in not only low crystallinity but also the existence of the CaxCoO2 secondary phase. Formation of CaxCoO2 was inevitable during the thin film growth, and this was discussed from both structural and compositional point of view. With longer deposition interval or with sufficient oxygen at a lower deposition rate, the CaxCoO2 phase was able to transit into the desired Ca3Co4O9 phase during the coalescence process. The quality of the thin films was further analyzed by electrical properties measurements. The Ca3Co4O9 thin film fabricated at a slower deposition rate was found to exhibit a low electrical resistivity of 9.4 mΩ cm and high Seebeck coefficient of 240 μV/K at about 700 °C, indicating a good quality film. 相似文献
78.
W. GuoA. Bondi C. Cornet A. LétoublonO. Durand T. RohelS. Boyer-Richard N. BertruS. Loualiche J. EvenA. Le Corre 《Applied Surface Science》2012,258(7):2808-2815
We have investigated quantitatively anti-phase domains (APD) structural properties in 20 nm GaP/Si epilayers grown by molecular beam epitaxy, using fast, robust and non-destructive analysis methods. These analyses, including atomic force microscopy and X-ray diffraction, are applied to samples grown by various molecular beam epitaxy growth modes. Roughness, lateral crystallite size of the epilayer, ratio of antiphase domains and their relationship are discussed. It is shown that both these analysis methods are useful to clarify the physical mechanisms occurring during the heterogeneous growth. Low temperature migration enhanced epitaxy is found to guarantee smoother surface than conventional molecular beam epitaxy. Effect of annealing temperature on antiphase boundaries (APBs) thermodynamics is discussed. The modification of the thermodynamic equilibrium through a thermal activation of APBs motion is expected to play an important role in the dynamic evolution of surfaces during thermal annealing and growth. 相似文献
79.
G. Kuri G. Materlik V. Hagen R. Wiesendanger 《Applied Physics A: Materials Science & Processing》2001,73(3):265-271
MgO (100) single crystals are implanted with 1.50-MeV Al+ and 3.00-MeV Al2
+ ions at a fluence of 1×1015 Al atoms cm-2 under high-vacuum conditions. The surface morphology of the substrate is measured in air using atomic force microscopy and
X-ray reflectometry followed by computer-simulated spectrum analysis. The ion-irradiated areas are found to protrude to different
heights on the nanometre scale. Small height differences are observed in the areas irradiated by Al+ and Al2
+ ions of comparable energy, dose rate and total fluence. The results indicate that protrusions are most likely caused by implantation-induced
point defects (vacancies) generated in the crystal during implantation. Other possibilities for the cause of protrusions are
discussed. Thermal treatment stimulates a partial recovery of the implantation damage and alters the topography of MgO surfaces.
Received: 22 May 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001 相似文献
80.
G. Dräger Th. Kirchner S. Bocharov C.-C. Kao 《Applied Physics A: Materials Science & Processing》2001,73(6):687-691
Resonantly excited metal K core line spectra of NiO, MnO, CuO and other compounds have been investigated at the beamlines
X21 (NSLS/BNL), BW1 and W1.1 (HASYLAB/DESY). A novel technique for quantitative resolution of NEXAFS spectra into spin-up
and spin-down components has been developed. Since the method employs spin conservation and local spin references, it needs
no circularly polarized radiation and no sample magnetization for taking both the RXS and NEXAFS spectra. Hence antiferromagnetic
and paramagnetic materials can be investigated as well.By utilizing linear dichroism with angular-dependent measurements on
single-crystal samples, additional resolution of NEXAFS spectra is possible with respect to the orbital symmetry. Application
of the method to paramagnetic MnO, for the first time, provides new and unambiguous experimental results confirming modern
(LSDA+U) calculations.
Received: 29 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001 相似文献