Growth Of Pentacene Films Research Articles

Low temperature growth of epitaxial pentacene films on the Si(1 1 1)-(√3 × √3) R30°-Ag surface

Pentacene thin films were grown on the Si(1 1 1)-(√3 × √3) R30°-Ag surface at low substrate temperature (120 K). The growth process was investigated by scanning tunneling microscopy and low energy electron diffraction. In contrast to the growth at room temperature when 3D pentacene agglomerates form on an initial self-assembled pentacene monolayer surface, we observed an epitaxial pentacene film growth at low temperature. The pentacene molecules adopt a planar, π-stacked geometry, aligned head-to-head to form one-dimensional structures within each layer. The structural evolution as a function of film thickness was analyzed in detail and the mechanism of low temperature growth was discussed.

Structure versus electron effects in the growth mode of pentacene on metal-induced Si(111)-3×3 surfaces

The growth of pentacene films on different metal (Ga, Pb, Bi, Ag) induced Si(111)-(square root(3) x square root(3))R30 degrees surfaces is investigated by scanning tunneling microscopy. On surfaces with high atomic surface roughness, such as GaSi-square root(3), beta-PbSi-square root(3), and alpha-BiSi-square root(3), pentacene forms an initial disordered wetting layer followed by the growth of crystalline thin films. The growth behavior is independent of the metallicity of the substrate surface in this regime. On the other hand, on surfaces with low adatom surface roughness, pentacene molecules form self-organized structures without forming a wetting layer. Moreover, the molecular orientation is critically dependent on the surface metallicity. This work reveals that the growth mode of pentacene on solid surfaces is determined by the combined effects of structural and electronic properties of the substrate.

Effect of Polymer Gate Dielectric Surface Viscoelasticity on Pentacene Thin-Film Transistor Performance

AbstractPentacene is one of the most studied semiconductor for organic thin-film transistors (OTFTs), and enhanced understanding of pentacene-based TFTs has significantly advanced the organic electronics. We report here the crucial effect of the polymer gate dielectric glass transition temperature (Tg) on pentacene film growth mode, microstructure, and the resulting TFT performance. Nanoscopically-confined thin polymer films are known to exhibit reduced glass-transition temperatures versus the corresponding bulk values, and we demonstrate here that pentacene films grown on polymer gate dielectrics at temperatures well below their bulk Tg exhibit morphological/microstructural transitions and OTFT performance discontinuities at well-defined growth temperatures [defined as the surface Tg, or Tg(s)] characteristic of the underlying polymer structure and independent of the film thickness. The results argue that realistic OTFT response must take into account this fundamental polymer property, and that TFT measurements represent a new probe of polymer surface thermal properties.

Growth of pentacene on clean and modified gold surfaces

The growth and evolution of pentacene films on gold substrates have been studied. By combining complementary techniques including scanning tunneling microscopy, atomic force microscopy, scanning electron microscopy, near-edge x-ray-absorption fine structure, and x-ray diffraction, the molecular orientation, crystalline structure, and morphology of the organic films were characterized as a function of film thickness and growth parameters (temperature and rate) for different gold substrates ranging from Au(111) single crystals to polycrystalline gold. Moreover, the influence of precoating the various gold substrates with self-assembled monolayers (SAM's) of organothiols with different chemical terminations has been studied. On bare gold the growth of pentacene films is characterized by a pronounced dewetting while the molecular orientation within the resulting crystalline three-dimensional islands depends distinctly on the roughness and cleanliness of the substrate surface. After completion of the first wetting layer where molecules adopt a planar orientation parallel to the surface the molecules continue to grow in a tilted fashion: on Au(111) the long molecular axis is oriented parallel to the surface while on polycrystalline gold it is upstanding oriented and thus parallels the crystalline orientation of pentacene films grown on $\mathrm{Si}{\mathrm{O}}_{2}$. On SAM pretreated gold substrates the formation of a wetting layer is effectively suppressed and pentacene grows in a quasi-layer-by-layer fashion with an upstanding orientation leading to rather smooth films. The latter growth mode is observed independently of the chemical termination of the SAM's and the roughness of the gold substrate. Possible reasons for the different growth mechanism as well as consequences for the assignment of spectroscopic data of thin pentacene film are discussed.

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 SiO 2 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 SiO 2 layer to rough Au under-layer.

半導体表面とペンタセン薄膜間の界面形成過程の微視的構造観察

Interface formation process of pentacene thin films on semiconductor surfaces has been studied using low energy electron microscopy (LEEM) and scanning tunneling microscopy. Interface interaction between clean Si surface and pentacene film is optimized by inserting bismuth thin film at the interface, resulting in the formation of monolayer islands larger than 0.2 mm in diameter. Systematic surface reactivity control on Si substrate reveals that the surface states nearby the Fermi level are the origin of the interface interaction between pentacene molecules and semiconductor surfaces. Effect of kinetic growth process on the pentacene film growth is also examined by detailed LEEM observations. The anisotropic growth speed originating from the anisotropic structure of pentacene accompanied with the kinetic growth condition could result in the formation of textured structure inside the single grain grown from a single nucleus.

Correlation of growth of pentacene films at various gas ambience conditions to organic field-effect transistor characteristics

Abstract Pentacene based thin-film transistors (TFTs) have been fabricated using pentacene films grown under various ambiences, such as N 2 , H 2 , Ar, He, and high vacuum, to define correlation of the device performance, in particular mobility properties, to molecular ordering in pentacene films. The field-effect mobility of 0.24 cm 2 /Vs was obtained from TFTs fabricated under 2 × 10 −5 Torr nitrogen ambience, however, the pentacene TFTs fabricated in hydrogen ambience under the same pressure yielded very poor mobility of 0.008 cm 2 /Vs. Pentacene films deposited by thermal evaporation at increased pressure in nitrogen ambience have a high degree of molecular ordering with larger dendritic grains without any surface modification on silicon oxide dielectric. A clean relation between field-effect mobility and XRD estimated crystallites size was obtained.

Growth dynamics of pentacene thin films: Real-time synchrotron x-ray scattering study

Real-time synchrotron x-ray scattering in the anti-Bragg configuration was used to monitor the dynamics of pentacene film growth on inert substrates. A distributed-growth model, according to which pentacene molecules adsorbed on the nth layer can either nucleate and contribute to the growth of the (n+1)th layer or transfer downward and contribute to the growth of the nth layer, gave a good description of the data. For molecules adsorbed on the first and second layers, the probability of downward transfer was found to be dependent on the substrate, and independent of temperature within the range from 25 to 60 deg. C. For films grown on SiO{sub 2}, an Ehrlich-Schwoebel barrier of the order of 70 meV dominated downward transfer of pentacene molecules in layers away from the substrate. For films grown on an alkylated self-assembled monolayer, significant desorption of pentacene molecules from the substrate at elevated temperatures forced the growth mode toward the three-dimensional limit.

Pentacene films grown on surface treated SiO 2 substrates

As a potential candidate for inorganic FETs in various applications, organic FETs have received much attention during recent years. In order to remove surface contamination and to improve the device performance, surface treatment is generally adopted before deposition of organic films. However, the microstructure of the organic films is very sensitive to growth conditions. Surface treatment and heating can influence the film morphology and the device performance greatly. In order to study the effect of surface treatment on the growth of pentacene film, we deposited and contrasted pentacene films at room temperature and elevated temperature on SiO 2 substrate treated by several typical cleaning procedures. The results indicated that surface treatment could influence the film morphology greatly even for a thickness far beyond the first monolayer. We also found that there was a low critical temperature of about 55 °C at a deposition rate of 0.5 nm/min, above which substantial desorption of pentacene molecule occurred regardless of the cleaning procedure. Deposition on heated substrates is widely reported for increased grain size and improved device performance, while the possible desorption seems to be seldom noticed. Therefore, an implication of the low desorption temperature is that the film thickness may be frequently overestimated for deposition on heated substrates. Consequently, errors for any data that are dependent on film thickness may be caused without taking into account the possible desorption.

Atomic-scale mechanisms of selective adsorption and dimerization of pentacene on Si surfaces

We report results of first-principles calculations in terms of which we elucidate the mechanisms for nucleation and initial growth of pentacene films on Si. Pentacene molecules bond in flat, distorted configurations on bare surfaces. On H-passivated surfaces, direct bonding or H replacement are not energetically favored. However, molecules bond in an upright configuration at isolated depassivated Si dangling bonds and film growth continues over the passivated area. The results elucidate generic adsorption issues on inert surfaces and suggest procedures for controlling film growth.

Inorganic Oxide Core, Polymer Shell Nanocomposite as a High K Gate Dielectric for Flexible Electronics Applications

Organic/inorganic core shell nanoparticles have been synthesized using high K TiO(2) as the core nanoparticle, and polystyrene as the shell. This material is easy to process and forms transparent continuous thin films, which exhibit a dielectric constant enhancement of over 3 times that of bulk polystyrene. This new dielectric material has been incorporated into capacitors and thin film transistors (TFTs). Mobilities approaching 0.2 cm(2)/V.s have been measured for pentacene TFTs incorporating the new TiO(2) polystyrene nanostructured gate dielectric, indicating good surface properties for pentacene film growth. This novel strategy for generating high K flexible gate dielectrics will be of value in improving organic and flexible electronic device performance.

Pentacene Thin Film Growth

Pentacene stands out as a model molecule among organic semiconductors due to its ability to form well-ordered films that show a high field effect mobility. We discuss the processes involved in pentacene film growth, emphasizing differences with respect to inorganic films. The influence of growth parameters such as the substrate nature and temperature, the deposition rate, and the kinetic energy of the molecular beam on the structure and morphology of pentacene films are discussed. Finally, we overview recent attempts to model pentacene film nucleation and growth, and draw attention to the role of dislocations.

Dislocation arrangements in pentacene thin films

We have studied the growth of pentacene films (2--8 monolayers) on modified Si-wafer surfaces by means of synchrotron x-ray diffraction. The diffraction data reveal a nonthermal damping of the (coherent) Bragg reflection intensities according to an exponential dependence on the $3∕2$ power of the momentum transfer. The simultaneous presence of strong diffuse scattering centered around the Bragg positions indicates the presence of local defects. A quantitative analysis of the Bragg and diffuse scattering allows us to identify screw and edge dislocations as the main defects on the molecular scale. We quantify dislocation densities as a function of substrate termination.

Epitaxial growth of pentacene films on Cu110.

The molecular structure of thin pentacene (C(22)H(14)) films grown on a Cu(110) surface has been studied by means of He atom scattering, low energy electron diffraction, thermal desorption spectroscopy, x-ray photoelectron spectroscopy, and x-ray absorption spectroscopy. Depending on the actual film thickness three different crystalline phases have been identified which reveal a characteristic reorientation of the molecular plane relative to the substrate surface. In the monolayer regime the molecules form a highly ordered commensurate (6.5x2) structure with a planar adsorption geometry. For thin multilayers (thickness <2 nm) a second phase is observed which is characterized by a lateral ((-0.65 5.69) ( 1.90 1.37)) structure and a tilting of the molecular plane of about 28 degrees around their long axis which remains parallel to the surface. Finally, when exceeding a thickness of about 2 nm subsequent growth proceeds with an upright molecular orientation and leads to the formation of crystalline films which are epitaxially oriented with respect to the substrate. The present study thus demonstrates that also on metal substrates highly ordered pentacene films with an upright orientation of the molecular planes can be grown. Photoelectron spectroscopy data indicate further that thick films do not grow in a layer-by-layer mode but reveal a significant degree of roughness.

Early stages of pentacene film growth on silicon oxide

Abstract Among the various materials suitable for organic thin-film transistors (OTFTs), pentacene stands out as a model molecule, exhibiting one of the highest field effect mobilities reported so far. Understanding the growth mechanism of pentacene on dielectrics is essential for controlling film morphology and for fabricating high quality, large-grain, defect-free films. Such films will help us gain insight into the fundamentals of transport in organic films as well as enable the ultimate OTFT performance. In situ synchrotron X-ray scattering was used to probe the early stages of pentacene growth on SiO 2 in real time and under conditions relevant to the fabrication of OTFTs. Reflectivity measurements reveal that a thin layer of water, initially present on the substrate, is trapped at the interface when the pentacene film is deposited. Therefore, the interface relevant for OTFTs is that between pentacene and water. Anti-Bragg oscillations, observed for the first time during organic film growth, reveal that the first monolayer of pentacene completes fully before the second one nucleates. Subsequent layers nucleate before the underlying layers complete, giving rise to the characteristic island structure observed in pentacene films used in OTFTs. A simple distributed growth model was found to adequately describe the growth mechanism.