Surface morphology characterization of pentacene thin film and its substrate with under-layers by power spectral density using fast Fourier transform algorithms

Surface morphology of pentacene thin films and their substrates with under-layers is characterized by using atomic force microscopy (AFM). The power values of power spectral density (PSD) for the AFM digital data were determined by the fast Fourier transform (FFT) algorithms instead of the root-mean-square (rms) and peak-to-valley value. The PSD plots of pentacene films on glass substrate are successfully approximated by the k-correlation model. The pentacene film growth is interpreted the intermediation of the bulk and surface diffusion by parameter C of k-correlation model. The PSD plots of pentacene film on Au under-layer is approximated by using the linear continuum model (LCM) instead of the combination model of the k-correlation model and Gaussian function. The PSD plots of SiO₂ layer on Au under-layer as a gate insulator on a gate electrode of organic thin film transistors (OTFTs) have three power values of PSD. It is interpreted that the specific three PSD power values are caused by the planarization of the smooth SiO₂ layer to rough Au under-layer.     

Growth instabilities of vapor deposited films: atomic size versus deflection effect

Two previously suggested, physically distinct mechanisms for a growth instability of vapor deposited films, the finite atomic size effect and the particle deflection effect due to interatomic attraction, are reconsidered, further analyzed, and compared. We substantiate why the instability caused by interatomic attraction must be considered as the truly underlying instability mechanism. We demonstrate that aspects of the structure zone model of Movchan and Demchishin can also be consistently explained using the growth instability induced by particle deflection instead of the instability arising from the atomic size effect. Most significantly we show that, for vapor deposited amorphous Zr₆₅Al_7.5Cu_27.5-films, the growth instability due to the atomic size effect cannot be present. Received 12 December 2001 Published online 6 June 2002     

Description of a hybrid PECVD-PVD process: Application to Zn-Si-O and Ti-Si-O composites thin films

A radio frequency hybrid process where sputtering and plasma enhanced chemical vapour deposition (PECVD) occur simultaneously is studied to describe the specificity it gains when the two techniques are merged. A model is developed to describe how the deposition rate evolves when the flow rate of the PECVD precursor increases. First, it is shown that it is constant below a critical value of the precursor flow rate because of the wind effect due to sputtering that strongly limits the transport of the precursor. Then it increases almost linearly with the precursor flow rate when PECVD and sputtering simultaneously occur. Finally, above a certain threshold in the precursor flow rate, the surface of the target is poisoned by the precursor and composite thin films can no longer grow. The previous model is deduced from results obtained in deposition of Zn-Si-O and Ti-Si-O thin films. These composites are synthesised respectively by sputtering of zinc and titanium targets in a vapour of oxygen and hexamethyldisiloxane (HMDSO-Si₂C₆H₁₈O). Limitations of the model used are also discussed.     

Determination of cross over effects in lattice models from the local height difference distribution

Growth of interfaces during vapor deposition is analyzed on a discrete lattice. It leads to finding distribution of local heights, measurable for any lattice model. Invariance in the change of this distribution in time is used to determine the cross over effects in various models. The analysis is applied to the discrete linear growth equation and Kardar-Parisi-Zhang (KPZ) equation. A new model is devised that shows early convergence to the KPZ dynamics. Various known conservative and non conservative models are tested on a one dimensional substrate by comparing the growth results with the exact KPZ and linear growth equation results. The comparison helps in establishing the condition that determines the presence of cross over effect for the given model. The new model is used in (2+1) dimensions to predict close to the true value of roughness constant for KPZ equation.     

The importance of non-local shadowing for the topography evolution of a-C:H films grown by toluene based plasma enhanced chemical vapor deposition

The topography evolution of hydrogenated diamond-like carbon coatings deposited through toluene based capacitively coupled plasma enhanced chemical vapor deposition has been studied experimentally and with continuum growth models. The experimentally observed mound formation and surprisingly large growth exponents (β≈ 0.9±0.1) cannot be reproduced by familiar local stochastic differential equations that are successfully used for other thin film deposition techniques. Here we introduce a novel numerical approach to simulate a continuum growth model that takes into account non-local shadowing effects. We show that the major characteristics of the experimentally observed topography evolution can be accurately represented by this model.     

Roughness scaling and sensitivity to initial conditions in a symmetric restricted ballistic deposition model

In this work, we introduce a restricted ballistic deposition model with symmetric growth rules that favors the formation of local finite slopes. It is the simplest model which, even without including a diffusive relaxation mode of the interface, leads to a macroscopic groove instability. By employing a finite-size scaling of numerical simulation data, we determine the scaling behavior of the surface structure grown over a one-dimensional substrate of linear size L. We found that the surface profile develops a macroscopic groove with the asymptotic surface width scaling as , with . The early-time dynamics is governed by the scaling law , with . We further investigate the sensitivity to initial conditions of the present model by applying damage spreading techniques. We find that the early-time distance between two initially close surface configurations grows in a ballistic fashion as , but a slower Brownian-like scaling () sets up for evolution times much larger than a characteristic time scale . Received 26 May 2000     

Surface partition of ion energy during the growth of TiNx thin films

A simple evaluation of ion-deposited energy during surface displacement of adatoms has been presented for physical vapor deposition technology using an appropriate interaction model. The rf reactive magnetron sputtering deposition of titanium nitride (TiN_x) thin films was taken as evidence supporting the theoretical calculation. The evolution of crystallite morphology dependent on bias (or input power) illustrates that surface and subsurface microstructure of growing films can be optimized by increasing the mobility of adatoms through ion-assistance.     

Non-linear evolution of step meander during growth of a vicinal surface with no desorption

Step meandering due to a deterministic morphological instability on vicinal surfaces during growth is studied. We investigate nonlinear dynamics of a step model with asymmetric step kinetics, terrace and line diffusion, by means of a multiscale analysis. We give the detailed derivation of the highly nonlinear evolution equation on which a brief account has been given [6]. Decomposing the model into driving and relaxational contributions, we give a profound explanation to the origin of the unusual divergent scaling of step meander (where F is the incoming atom flux). A careful numerical analysis indicates that a cellular structure arises where plateaus form, as opposed to spike-like structures reported erroneously in reference [6]. As a robust feature, the amplitude of these cells scales as t ^1/2, regardless of the strength of the Ehrlich-Schwoebel effect, or the presence of line diffusion. A simple ansatz allows to describe analytically the asymptotic regime quantitatively. We show also how sub-dominant terms from multiscale analysis account for the loss of up-down symmetry of the cellular structure. Received 4 May 2000 and Received in final form 8 September 2000     

Layer‐by‐layer fabrication of hierarchical structures in sol–gel templated thin titania films

The fabrication of titania nanostructures with hierarchical order of different structural levels is investigated. The nanostructures are prepared with a diblock‐copolymer assisted sol–gel process. By iterative spin‐coating of the solution onto silicon substrates a thin polymer‐nanocomposite film is deposited and transformed to purely anatase titania nanostructures via calcination. In total, this procedure is repeated three times on top of the substrate. The approach is monitored with grazing incidence small angle X‐ray scattering after each fabrication step. With scanning electron microscopy the final hierarchical structure is imaged. From the characterization different structural levels are clearly identified.

     

Growth velocity and the topography of Ni-Zn binary alloy electrodeposits

We show that the electrodeposition of Ni-Zn alloys at the lowest growth velocities, v < 0.5μm/s, exclusively proceeds from an abnormal co-deposition phenomenon. The growth process in this v region greatly depends on the initial [Co²⁺] concentration of the film deposition bath. A theoretical approach of this process including the role of the saturation surface roughness of the alloy, , leads to an estimation of the transport properties of the ad-atoms involved during the deposit formation. Their surface diffusion coefficient varying between 1.76×10^-10 and 2.40×10^-8 cm^-2/s exhibits a minimal value, D _s = 2.10×10^-10 cm^-2/s located between v = 0.17 and 0.35μm/s. The spatial scaling analysis of the local roughness, σ, examined according to the power-law σ≈L ^α reveals that the resulting roughness exponents concurs with the Kardar-Parisi-Zhang dynamics including the restricted surface diffusion. Two main v regions leads to different fractal textural features of the alloy film surface. Below 0.10 μm/s, the roughness exponent obtained is α≈ 0.6, depicting a limited ad-atom mobility. Over v = 0.30μm/s, this exponent stabilises at α≈ 0.82, indicating an increase of the surface diffusion. Received 16 August 2000 and Received in final form 20 June 2001     

Influence of substrate temperature on the electrical behaviour of zinc stannate thin films deposited by electron beam evaporation technique

Summary  A series of zinc stannate (Zn₂ SnO₄) thin films were prepared at four different substrate temperatures; namely, room-temperature (25°C), 50°C, 100°C and 200°C. Direct-current resistivity measurements were performed on these samples in the temperature range from room temperature (∼290 K) up to about 500 K. A phase transition (of positive temperature coefficient (PTC) of resistance) was observed in the thin film grown at room temperature at about 385 K. Other investigated samples showed a semiconducting behaviour of three distinct conduction mechanisms extending from intrinsic to thermal freeze-out conduction. The width of the band gapE _g was found to depend on the substrate temperature and was discussed in terms of a formation of a band tailing. Thermal freeze-out was dominant at the lower temperature region. On leave from Department of Physics, Faculty of Science, Alexandria University, Alexandria, Egypt.     

A model of gravity-induced distribution of material in plasma polymerized aerosols and films

A mathematical model of the volumetric part of plasma polymerization influenced by gravity is presented. Plasma-activated adhesion of monomer molecules to a surface of a germinal particle is assumed as a basic mechanism of particulate growth. The continuity equation for the flow of matter through the discharge has been formulated and solved in two extreme asymptotic approximations --for small and major duration of the process. Several non-equilibrium distribution functions of the polymer were obtained, for instance, an amount of the particles as a function of their size or time of fall. Within the adopted model this function demonstrates a sharp downward increase inside a discharge. In addition it contains such parameters as the free fall acceleration or reaction rate coefficients, variations of which enable control of the discharge and properties of the disperse medium.     

Scaling regimes for second layer nucleation

Nucleation on top of two-dimensional islands with step edge barriers is investigated using scaling arguments. The nucleation rate is expressed in terms of three basic time scales: the time interval between deposition events, the residence time of atoms on the island, and the encounter time required for atoms forming a stable nucleus to meet. Application to the problem of second layer nucleation on growing first layer islands yields a sequence of scaling regimes with different scaling exponents relating the critical island size, at which nucleation takes place, to the diffusion and deposition rates. Second layer nucleation is fluctuation-dominated, in the sense that the typical number of atoms on the island is small compared to , when the first layer island density exponent satisfies . The upper critical nucleus size, above which the conventional mean field theory of second layer nucleation is valid, increases with decreasing dimensionality. In the related case of nucleation on top of multilayer mounds fluctuation-dominated and mean field like regimes coexist for arbitrary values of the critical nucleus size . Received 4 September 2000     

Crystallization kinetics and crystal morphology in thin poly(ethylene oxide) films

We present a detailed study of the kinetics of crystallization for thin films of poly(ethylene oxide) (PEO). Measurements of the growth rate have been carried out using optical-microscopy techniques on films of monodisperse PEO. Films with thicknesses from 13 nm to ~2 m were crystallized isothermally at temperatures ~20°C below the melting point. A remarkable non-monotonic slowing-down of the crystal growth is observed for films with thickness less than ~400 nm. The changes in the growth rate from bulk-like values is significant and corresponds to a factor of 40 decrease for the thinnest films studied. The morphologies of isothermally crystallized samples are studied using atomic-force microscopy. We find that a morphology, similar to diffusion-controlled growth (dendritic growth and densely branched growth), is observed for films with h < 150 nm. In addition, changes in the morphology occur for thicknesses consistent with changes in the growth rate as a function of film thickness.     

氧化物薄膜研制中的电子能谱技术

随着科学技术的不断发展，人们正在寻求更新的实用材料．金属氧化物，包括金属氧化物薄膜的各种实用材料，在工业界、信息产业界和能源开发等方面的应用前景，早已引起国内外学者的极大关注．例如，由于氧化物具有各种特殊的介电和光学性质，研究和开发基于氧化物薄膜的气敏材料非常热门．如何制备出有实用价值的各种薄膜材料，是科学家们一直关心和深入研究的课题．电子能谱技术在各种材料的基础研究和实际应用中起着重要的作用．本文以有序金属氧化物薄膜研制为例，简要评述了电子能谱技术（包括X射线光电子能谱（XPS），紫外光电子能谱（UPS），俄歇电子能谱（AES）和高分辨电子能量损失谱（HREELS）），以及低能电子衍射（LEED）等技术在氧化物薄膜材料制备和表征中的应用．     

氧化物薄膜研制中的电子能谱技术

随着科学技术的不断发展,人们正在寻求更新的实用材料.金属氧化物,包括金属氧化物薄膜的各种实用材料,在工业界、信息产业界和能源开发等方面的应用前景,早已引起国内外学者的极大关注.例如,由于氧化物具有各种特殊的介电和光学性质,研究和开发基于氧化物薄膜的气敏材料非常热门.如何制备出有实用价值的各种薄膜材料,是科学家们一直关心和深入研究的课题.电子能谱技术在各种材料的基础研究和实际应用中起着重要的作用.本文以有序金属氧化物薄膜研制为例,简要评述了电子能谱技术（包括X射线光电子能谱（XPS）,紫外光电子能谱（UPS）,俄歇电子能谱（AES）和高分辨电子能量损失谱（HREELS））,以及低能电子衍射（LEED）等技术在氧化物薄膜材料制备和表征中的应用.     

Effect of self assembly on the nonlinear optical characteristics of ZnO thin films

In the present work, we report the third order nonlinear optical properties of ZnO thin films deposited using self assembly, sol gel process as well as pulsed laser ablation by z scan technique. ZnO thin films clearly exhibit a negative nonlinear index of refraction at 532 nm and the observed nonlinear refraction is attributed to two photon absorption followed by free carrier absorption. Although the absolute nonlinear values for these films are comparable, there is a change in the sign of the absorptive nonlinearity of the films. The films developed by dip coating and pulsed laser ablation exhibit reverse saturable absorption whereas the self assembled film exhibits saturable absorption. These different nonlinear characteristics in the self assembled films can be mainly attributed to the saturation of linear absorption of the ZnO defect states.     

A new approach to the synthesis of functional thin films: Hierarchical synthesis of CaCO3 thin films and their transformation into patterned metal thin films

A patterned metal thin film was synthesized based on a new approach that allows the formation of thin films with complex shape, hierarchical organization, and controlled size under ambient conditions in an aqueous environment. By using a general coating system, a CaCO₃ thin film was transformed into a patterned metal thin film with functional properties, such as adhesion and hydrophobicity. Atomic force microscopy (AFM) and surface-profiling experiments were carried out to measure the adhesion between the probe tip and the substrate and to determine the film thickness. Separation of the patterned ceramic thin film occurred very easily by means of sputtering and chemical treatment with an acidic solution.     

Self-energy-functional approach to systems of correlated electrons

The grand potential of a system of interacting electrons is considered as a stationary point of a self-energy functional. It is shown that a rigorous evaluation of the functional is possible for self-energies that are representable within a certain reference system. The variational scheme allows to construct new non-perturbative and thermodynamically consistent approximations. Numerical results illustrate the practicability of the method. Received 13 January 2003 / Received in final form 5 March 2003 Published online 24 April 2003     

Deposition and stoichiometry control of Nd-doped gadolinium gallium garnet thin films by combinatorial pulsed laser deposition using two targets of Nd:Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> and Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>

We have demonstrated pulsed laser deposition of Nd-doped gadolinium gallium garnet on Y₃Al₅O₁₂ by the simultaneous ablation of two separate targets of Nd:Gd₃Ga₅O₁₂ (GGG) and Ga₂O₃. Such an approach is of interest as a method of achieving stoichiometry control over films whilst the growth parameters are kept constant and optimal for high quality crystal growth. We show here how the stoichiometry and resultant lattice parameter of a film can be controlled by changing the relative deposition rates from the two targets. Films have been grown with enough extra Ga to compensate for the deficiency that commonly occurs when depositing only from a GGG target. We have also grown crystalline GGG films with an enriched Ga concentration, and this unconventional approach to film stoichiometry control may have potential applications in the fabrication of films with advanced compositionally graded structures.