Pentacenequinone (PnQ) impurities have been introduced into a pentacene source material in a controlled manner to quantify the relative effects of the impurity content on grain boundary structure and thin film nucleation. Atomic force microscopy (AFM) has been employed to directly characterize films grown using 0.0-7.5% PnQ by weight in the source material. Analysis of the distribution of capture zones areas of submonolayer islands as a function of impurity content shows that for large PnQ content the critical nucleus size for forming a Pn island is smaller than for low PnQ content. This result indicates a favorable energy for formation of Pn-PnQ complexes, which in turn suggests that the primary effect of PnQ on Pn mobility may arise from homogeneous distribution of PnQ defects.
Electrons and holes are locally injected in a single pentacene monolayer island. The two-dimensional distribution and concentration of the injected carriers are measured by electrical force microscopy. In crystalline monolayer islands, both carriers are delocalized over the whole island. On disordered monolayer, carriers stay localized at their injection point. These results provide insight into the electronic properties, at the nanometer scale, of organic monolayers governing performances of organic transistors and molecular devices.
Ultra-thin oxide (UTO) films were grown on Si(111) in ultrahigh vacuum at room temperature and characterized by scanning tunneling microscopy. The ultra-thin oxide films were then used as substrates for room temperature growth of pentacene. The apparent height of the first layer is 1.57 +/- 0.05 nm, indicating standing up pentacene grains in the thin-film phase were formed. Pentacene is molecularly resolved in the second and subsequent molecular layers. The measured in-plane unit cell for the pentacene (001) plane (ab plane) is a=0.76+/-0.01 nm, b=0.59+/-0.01 nm, and gamma=87.5+/-0.4 degrees. …
We present a model in which pit nucleation in thin films is considered to arise from a near-equilibrium nucleation process. In this model the adatom concentration plays a central role in controlling the morphological development of the surface. Although pits relieve elastic energy more efficiently than islands, pit nucleation can be prevented by a high adatom concentration. Three-dimensional islands act as adatom sinks and the lower adatom density in their vicinity promotes pit nucleation. Thermodynamic considerations predict several different growth regimes in which pits may nucleate at diffe…
We study the dislocation formation in 2D nanoscopic islands with two methods, the Molecular Static method and the Phase Field Crystal method. It is found that both methods indicate the same qualitative stages of the nucleation process. The dislocations nucleate at the film-substrate contact point and the energy decreases monotonously when the dislocations are farther away from the island-wetting film contact points than the distance of the highest energy barrier.
Pentacenequinone (PnQ) impurities have been introduced into a pentacene source material at number densities from 0.001 to 0.474 to quantify the relative effects of impurity content and grain boundary structure on transport in pentacene thin-film transistors. Atomic force microscopy (AFM) and electrical measurements of top-contact pentacene thin-film transistors have been employed to directly correlate initial structure and final film structures, with the device mobility as a function of added impurity content. The results reveal a factor four decrease in mobility without significant changes in…
Al-induced layer exchange crystallization of amorphous Ge thin films has been demonstrated recently, and provides a suitable system to characterize, model and control Ge crystal growth on non-crystalline substrates. Direct observation of Ge transfer to the surface of Al through an interposed GeOx interfacial layer allows independent measurement of the density and average area of crystalline Ge islands formed on the surface. Based on these experimental observations, the Johnson-Mehl-Avrami-Kologoromov phase transformation theory is extended to model nanoscale nucleation and growth of Ge islands…
We describe a theoretical study of the role of adsorbate interactions in island nucleation and growth, using Ag/Pt(111) heteroepitaxy as an example. From density-functional theory, we obtain the substrate-mediated Ag adatom pair interaction and we find that, past the short range, a repulsive ring is formed about the adatoms. The magnitude of the repulsion is comparable to the diffusion barrier. In kinetic Monte Carlo simulations, we find that the repulsive interactions lead to island densities over an order of magnitude larger than those predicted by nucleation theory and thus identify a sever…
The thermodynamics of the Stranski-Krastanov mode of epitaxial growth and the effect of the sign of the lattice misfit are discussed. The Stranski-Krastanov mode of growth represents a sequence of layer-by-layer or Frank-van der Merwe growth followed by the formation of three-dimensional (3D) islands or Volmer-Weber growth. The occurrence of both growth modes mentioned above is in compliance with the wettability criterion of Bauer. The positive wetting function required for the occurrence of the Volmer-Weber growth is originated by the vertical displacements of the atoms close to the edges of …
We study numerically the equilibrium shapes, shape transitions and dislocation nucleation of small strained epitaxial islands with a two-dimensional atomistic model, using simple interatomic pair potentials. We first map out the phase diagram for the equilibrium island shapes as a function of island size (up to N = 105 atoms) and lattice misfit with the substrate and show that nanoscopic islands have four generic equilibrium shapes, in contrast with predictions from the continuum theory of elasticity. For increasing substrate-adsorbate attraction, we find islands that form on top of a finite w…
This work describes morphological and crystalline properties of the InAs islands grown on templates created by focused ion beam (FIB) on indium phosphide (InP) substrates. Regular arrangements of shallow holes are created on the InP (001) surfaces, acting as preferential nucleation sites for InAs islands grown by Metal-Organic Vapor Phase Epitaxy. Ion doses ranging from $10^{15}$ to $10^{16}$ $Ga^{+}$/$cm^{2}$ were used and islands were grown for two sub-monolayer coverages. We observe the formation of clusters in the inner surfaces of the FIB produced cavities and show that for low doses temp…
We develop a theory for nucleation on top of islands in epitaxial growth based on the derivation of lifetimes and rates governing individual microscopic processes. These processes are strongly affected by the additional step edge barrier E_S for descending atoms. If the dissociation times of unstable clusters can be neglected, the theory predicts that for small critical nuclei of size i<=2 the nucleation is governed by fluctuations, during which by chance i+1 atoms are present on the island. For large critical nuclei i>=3 by contrast, the nucleation process can be described in a mean-field typ…
The formation mechanisms of evaporated Pd islands on the reconstructed Au(111) $22 /times /sqrt{3}$ herringbone surface have been here studied by Scanning Tunneling Microscopy (STM) at room temperature. Atomically resolved STM images at the very early stages of growth provide a direct observation of the mechanisms involved in preferential Pd islands nucleation at the elbows of the herringbone structure. At low Pd coverage the Au(111) herringbone structure remains substantially unperturbed and isolated Pd atoms settled in hollow sites between Au atoms are found nearby the elbows and the distort…
We present a combined experimental and theoretical study of submonolayer heteroepitaxial growth of Ag on Si(111)-7x7 at temperatures from 420 K to 550 K when Ag atoms can easily diffuse on the surface and the reconstruction 7x7 remains stable. STM measurements for coverages from 0.05 ML to 0.6 ML show that there is an excess of smallest islands (each of them fills up just one half-unit cell - HUC) in all stages of growth. Formation of 2D wetting layer proceeds by continuous nucleation of the smallest islands in the proximity of larger 2D islands (extended over several HUCs) and following coale…
We study the nucleation of GaN islands grown by plasma-assisted molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In particular, we assess the variation of their height and density as a function of GaN coverage. We show that the GaN growth passes four stages: initially, the growth is layer-by-layer; subsequently, two-dimensional precursor islands form, which transform into genuine three-dimensional islands. During the latter stage, island height and density increase with GaN coverage until the density saturates. During further GaN growth, the density remains constant and a bimo…
A seed layer-free growth of HfO$_2$ on commercially available CVD graphene from various suppliers is investigated. It is revealed that the samples of monolayer graphene transferred from Cu to SiO$_2$/Si substrates have different coverage with bi- and multi-layer graphene islands. We find that the distribution and number of such islands impact the nucleation and growth of HfO$_2$ by CVD. In particular, we show that the edges and surface of densely distributed bi-layer graphene islands provide good nucleation sites for conformal CVD HfO$_2$ layers. Dielectric constant of 16 is extracted from mea…
The observed height distribution of clusters obtained in strained epitaxy has been often interpreted in terms of electronic effects. We show that some aspects can be explained classically by the interplay of strain and edge energies. We find that soft materials can transform directly from monolayer into thicker islands by two-dimensional (2D) multilayer nucleation and growth. There is a critical thickness decreasing with the force constant. Thinner islands are thermodynamically forbidden, due to the insufficient stress relaxation upon clustering particularly under tensile stress. At sufficient…
It is known in thin-film deposition that the density of nucleated clusters $N$ varies with the deposition rate $R$ as a power law, $N \sim R^α$. The exponent $α$ is a function of the critical nucleus size $i$ in a way that changes with the aggregation-limiting process active in a given system. We extend here to generic aggregation-limiting processes the derivation of the analytical capture-zone distribution function $P_β(s) = a_βs^β\exp(-b_βs^2)$ of Pimpinelli and Einstein [Phys. Rev. Lett. 99, 226102 (2007)]. We show that the exponent $β$ is generally related to the critical nucleus size $i$ …
The ability to pattern the location of pillars in epitaxial matrix-pillar nanocomposites is a key challenge to develop future technologies using these intriguing materials. One such model system employs a ferrimagnetic CoFe$_{2}$O$_{4}$ (CFO) pillar embedded in a ferroelectric BiFeO$_{3}$ (BFO) matrix, which has been proposed as a possible memory or logic system. These composites self-assemble spontaneously with pillars forming through nucleation at a random location when grown via physical vapor deposition. Recent results have shown that if an island of the pillar material is pre-patterned on…
We have studied the monolayer-bilayer transformation in the case of the coherent Stranski-Krastanov growth. We have found that the energy of formation of a second layer nucleus is largest at the center of the first-layer island and smallest on its corners. Thus nucleation is expected to take place at the corners (or the edges) rather than at the center of the islands as in the case of homoepitaxy. The critical nuclei have one atom in addition to a compact shape, which is either a square of i*i or a rectangle of i*(i-1) atoms, with i>1 an integer. When the edge of the initial monolayer island i…
The effects of adsorbates on nucleation and growth of two-dimensional islands is investigated by kinetic Monte Carlo simulations and rate equation theory. The variation of island morphology with adsorbate parameters is discussed and the temperature-dependence of island density in the case of immobile adsorbates is studied in detail. A set of rate equations for the description of nucleation in the presence of predeposited mobile and immobile adsorbates is developed.
We develop a theory of nucleation on top of two-dimensional islands bordered by steps with an additional energy barrier $ΔE_S$ for descending atoms. The theory is based on the concept of the residence time of an adatom on the island,and yields an expression for the nucleation rate which becomes exact in the limit of strong step edge barriers. This expression differs qualitatively and quantitatively from that obtained using the conventional rate equation approach to nucleation [J. Tersoff et al., Phys. Rev. Lett.72, 266 (1994)]. We argue that rate equation theory fails because nucleation is dom…
We have demonstrated that the island nucleation in the initial stage of epitaxial thin film growth can be tuned by substrate surface charge doping. This charge effect was investigated using spin density functional theory calculation in Fe-deposition on graphene substrate as an example. It was found that hole-doping can apparently increase both Fe-adatom diffusion barrier and Fe inter-adatom repulsion energy occurring at intermediate separation, whereas electron-doping can decrease Fe-adatom diffusion barrier but only slightly modify inter-adatom repulsion energy. Further kinetic Monte Carlo si…
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 $i^\ast + 1$ 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…
The distributions of inter-island gaps and captures zones for islands nucleated on a one-dimensional substrate during submonolayer deposition are considered using a novel retrospective view. This provides an alternative perspective on why scaling occurs in this continuously evolving system. Distributional fixed point equations for the gaps are derived both with and without a mean field approximation for nearest neighbour gap size correlation. Solutions to the equations show that correct consideration of fragmentation bias justifies the mean field approach which can be extended to provide close…
In the electrochemical deposition of thin films the measurement of the current-time curve does not allow for a direct determination of the nucleus growth law, electrode surface coverage and mean film thickness. In this work we present a theoretical approach suitable to gain insight into these quantities from the knowledge of nucleation density, solution parameters and current-time behavior. The model applies to both isotropic and anisotropic growth rates of nuclei and a study on the effect of nucleus shape and aspect ratio on the kinetics is presented. Experimental results from literature are …
We report the results of x-ray scattering studies of AlN on c-plane sapphire during reactive radiofrequency magnetron sputtering. The sensitivity of in situ x-ray measurements allowed us to follow the structural evolution of strain and roughness from initial nucleation layers to fullyrelaxed AlN films. A growth rate transient was observed, consistent with the initial formation of non-coalesced islands with significant oxygen incorporation from the substrate. Following island coalescence, a steady state growth rate was seen with a continuous shift of the c and a lattice parameters towards the r…
In studies of island nucleation and growth, the distribution of capture zones, essentially proximity cells, can give more insight than island-size distributions. In contrast to the complicated expressions, ad hoc or derived from rate equations, usually used, we find the capture-zone distribution can be described by a simple expression generalizing the Wigner surmise from random matrix theory that accounts for the distribution of spacings in a host of fluctuation phenomena. Furthermore, its single adjustable parameter can be simply related to the critical nucleus of growth models and the substr…
In this work, the nucleation and growth mechanism of aluminum oxide (Al2O3) in the early stages of the direct atomic layer deposition (ALD) on monolayer epitaxial graphene (EG) on silicon carbide (4H-SiC) has been investigated by atomic force microscopy (AFM) and Raman spectroscopy. Contrary to what is typically observed for other types of graphene, a large and uniform density of nucleation sites was observed in the case of EG and ascribed to the presence of the buffer layer at EG/SiC interface. The deposition process was characterized by Al2O3 island growth in the very early stages, followed …
The nucleation and growth of point islands during submonolayer deposition on a one-dimensional substrate is simulated for critical island size $i=0,1,2,3$. The small and large size asymptotics for the gap size and capture zone distributions (GSD and CZD) are studied. Comparisons to theoretical predictions from fragmentation equation analyses are made, along with those from the recently proposed Generalised Wigner Surmise (GWS). We find that the simulation data can be fully understood in the framework provided by the fragmentation equations, whilst highlighting the theoretical areas that requir…
The transformation of monolayer islands into bilayer islands as a first step of the overall two-dimensional to three-dimensional (2D-3D) transformation in the coherent Stranski-Krastanov mode of growth is studied for the cases of expanded and compressed overlayers. Compressed overlayers display a nucleation-like behavior: the energy accompanying the transformation process displays a maximum at some critical number of atoms, which is small for large enough values of the misfit, and then decreases gradually down to the completion of the transformation, non-monotonically due to the atomistics of …
We demonstrate how first-principles calculations using density-functional theory (DFT) can be applied to gain insight into the molecular processes that rule the physics of materials processing. Specifically, we study the molecular beam epitaxy (MBE) of arsenic compound semiconductors. For homoepitaxy of GaAs on GaAs(001), a growth model is presented that builds on results of DFT calculations for molecular processes on the beta2-reconstructed GaAs(001) surface, including adsorption, desorption, surface diffusion and nucleation. Kinetic Monte Carlo simulations on the basis of the calculated ener…
The purpose of this paper is to report some new experimental and theoretical results about the analysis of in-plane lattice spacing oscillations during two-dimensional (2D) homo and hetero epitaxial growth. The physical origin of these oscillations comes from the finite size of the strained islands. The 2D islands may thus relax by their edges, leading to in-plane lattice spacing oscillations during the birth and spread of these islands. On the one hand, we formulate the problem of elastic relaxation of a coherent 2D epitaxial deposits by using the concept of point forces and demonstrate that …
We study the dynamics of island nucleation in the presence of adsorbates using kinetic Monte Carlo simulations of a two-species growth model. Adatoms (A-atoms) and impurities (B-atoms) are codeposited, diffuse and aggregate subject to attractive AA- and AB-interactions. Activated exchange of adatoms with impurities is identified as the key process to maintain decoration of island edges by impurities during growth. While the presence of impurities strongly increases the island density, a change in the scaling of island density with flux, predicted by a rate equation theory for attachment-limite…
The effect of edge and corner diffusions on the morphology and on the density of islands nucleated irreversibly on a flat substrate surface is studied. Without edge and corner diffusion, islands are fractal. As an edge diffusion constant $D_e$ increases, islands tend to take a cross shape with four needles in the $< 10 >$ direction. Additional corner diffusion with a diffusion constant $D_c$ yields square islands. When $D_e$ is small relative to the surface diffusion constant $D_s$, the square corner shows the Berg instability to produce hopper growth in the $<11>$ direction. The corner diffus…
The nucleation mechanisms during solution deposition of organic semiconductor thin films determine the grain morphology and may influence the crystalline packing in some cases. Here, in-situ optical spectromicroscopy in reflection mode is used to study the growth mechanisms and thermal stability of 6,13-bis(trisopropylsilylethynyl)-pentacene thin films. The results show that the films form in a supersaturated state before transforming to a solid film. Molecular aggregates corresponding to subcritical nuclei in the crystallization process are inferred from optical spectroscopy measurements of t…
The rate of second layer nucleation -- the formation of a stable nucleus on top of a two-dimensional island -- determines both the conditions for layer-by-layer growth, and the size of the top terrace of multilayer mounds in three-dimensional homoepitaxial growth. It was recently shown that conventional mean field nucleation theory overestimates the rate of second layer nucleation by a factor that is proportional to the number of times a given site is visited by an adatom during its residence time on the island. In the presence of strong step edge barriers this factor can be large, leading to …
Growth of atomic indium chains - 1D islands - on the Si(100)-2x1 surface was observed by scanning tunneling microscopy (STM) at room temperature and simulated by means of a kinetic Monte Carlo method. Density of indium islands and island size distribution were obtained for various deposition rates and coverage. STM observation of growth during deposition of indium provided information on growth kinetics and relaxation of grown layers. Important role of C-type defects at adsorption of metal atoms was observed. Measured growth characteristics were simulated using a microscopic model with anisotr…
We study how FeO wüstite films on Ru(0001) grow by oxygen-assisted molecular beam epitaxy at elevated temperatures (800-900 K). The nucleation and growth of FeO islands are observed in real time by low-energy electron microscopy (LEEM). When the growth is performed in an oxygen pressure of 10-6 Torr, the islands are of bilayer thickness (Fe-O-Fe-O). In contrast, under a pressure of 10-8 Torr, the islands are a single FeO layer thick. We propose that the film thickness is controlled by the concentration of oxygen adsorbed on the Ru. More specifically, when monolayer growth increases the adsorbe…
The distribution of capture zones formed during the nucleation and growth of point islands on a one-dimensional substrate during monomer deposition is considered for general critical island size $i$. A fragmentation theory approach yields the small and (for $i=0$) large size asymptotics for the capture zone distribution (CZD) under the assumption of no neighbour-neighbour gap size correlation. These CZD asymptotic forms are different to those of the Generalised Wigner Surmise which has recently been proposed for island nucleation and growth models, and we discuss the reasons for the discrepanc…
We study the morphological evolution of strained heteroepitaxial films using a kinetic Monte Carlo method in three dimensions. The elastic part of the problem uses a Green's function method. Isolated islands are observed under deposition conditions for deposition rates slow compared with intrinsic surface roughening rates. They are semi-spherical and truncated conical for high and low temperature cases respectively. Annealing of films at high temperature leads to the formation of closely packed islands consistent with an instability theory. At low temperature, pits form via a layer-by-layer nu…
Metal-organic framework (MOF) thin-films show unmatched promise as smart membranes and photo-catalytic coatings. However, their nucleation and growth resulting from intricate molecular assembly processes are not well understood, yet crucial to control the thin-film properties. Here, we directly observe the nucleation and growth behaviour of HKUST-1 thin-films by real-time in-situ AFM at different temperatures in the Cu-BTC solution. In combination with ex-situ infrared (nano-)spectroscopy, synthesis at 25°C reveals initial nucleation of rapidly growing HKUST-1 islands surrounded by a continuou…
Heteroepitaxy enables the engineering of novel properties, which do not exist in a single material. Two principle growth modes are identified for material combinations with large lattice mismatch, Volmer-Weber and Stranski-Krastanov. Both lead to the formation of three-dimensional islands, hampering the growth of flat defect-free thin films. This limits the number of viable material combinations. Here, we report a distinct growth mode found in molecular beam epitaxy of PbTe on InP initiated by pre-growth surface treatments. Early nucleation forms islands analogous to the Volmer-Weber growth mo…
The surface reconstruction of the Fe catalyst films due to high temperature processing in hydrogen prior to nanotube nucleation and its effect on the growth morphologies of partially filled carbon nanotubes (CNTs) synthesized using atmospheric pressure chemical vapor deposition (APCVD) of propane was investigated. Results show that pre-heating of the catalyst film deeply influences the particle size distribution, which governs the growth morphologies of the corresponding CNTs. The distribution of the catalyst particles over the Si substrate was analyzed before and after the heat treatment by a…
