Stages Of Epitaxial Growth Research Articles

Growth and surface alloying of Al on Au(111) at room temperature

The initial stages of epitaxial growth of Al on Au(111) at room temperature have been investigated by scanning tunneling microscopy. In the submonolayer regime, large dendritic islands form. Their size distribution is found to be bimodal with the coexistence of a large area density of small and few large islands. The nucleation of second layer islands starts in advance of monolayer completion. Thicker films are rough. High-resolution data reveal that surface strain and intermixing of the two metals play a decisive role in the growth behavior: small quantities of adsorbed Al atoms strongly perturb the chevron reconstruction of clean Au(111) and a reconstruction pattern with hexagonal symmetry evolves for Al coverages exceeding ≈0.1 monolayers. This hexagonal domain wall phase consists of an intermixed AlAu surface alloy layer with reduced atomic Al radii.

Atomic structure of the reactive Fe/Si(111)7×7 interface

The early stages of Fe/Si(111) interface formation have been investigated using x-ray photoelectron diffraction. Deposition of Fe in the range of one monolayer on Si(111)7\ifmmode\times\else\texttimes\fi{}7 at room temperature results in the destruction of the 7\ifmmode\times\else\texttimes\fi{}7 reconstruction. After a slight anneal, a well ordered interface is formed. This reacted layer is terminated by a top Si bilayer, which is rotated 180\ifmmode^\circ\else\textdegree\fi{} with respect to the substrate stacking sequence. Fe atoms occupy substitutional Si lattice sites in the next bilayer underneath, keeping an interface coordination similar to the eightfold type. Thus, this initial interface already exhibits several of the most important features of thicker epitaxial silicide films. This result is evidence of the importance of the earliest stages of epitaxial growth to understand the properties of thicker layers.

Self-consistent rate theory of submonolayer homoepitaxy with attachment/detachment kinetics

The reversible nucleation and growth of two-dimensional islands during the submonolayer stage of epitaxial growth is studied with self-consistent rate equations and kinetic Monte Carlo simulations. In contrast to most previous work, we take account of the effects of both a finite energy barrier to the detachment of atoms from islands and a finite barrier to the incorporation of adatoms into islands. A correct boundary condition for the diffusion field at island edges is derived that takes account of these processes. For small detachment rates, quantitative agreement is obtained between the solutions to the rate theory and the simulations for the average monomer and island densities as a function of coverage. {copyright} {ital 1997} {ital The American Physical Society}

Early stages of epitaxial growth of vanadium oxide at the TiO2(110) surface studied by photoelectron diffraction.

This paper reports experimental evidence for an incipient epitaxial growth of vanadium oxide at the ${\mathrm{TiO}}_{2}$(110) surface. This finding is based on a x-ray photoelectron diffraction and low-energy electron diffraction study of a submonolayer of vanadium deposited at the ${\mathrm{TiO}}_{2}$(110) surface, performed after annealing at 473 K in ultrahigh vacuum conditions. The data unambiguously show that vanadium atoms occupy selected sixfold-coordinated titanium sites which underbridge oxygen atoms of the topmost layer and that the ${\mathrm{TiO}}_{2}$(110) long-range surface structural order is preserved. \textcopyright{} 1996 The American Physical Society.

Competing desorption pathways during epitaxial growth: LEEM investigation of Cu/W(110) heteroepitaxy.

Competing desorption during the initial stages of epitaxial growth of Cu on the W(110) surface has been studied with low-energy electron microscopy (LEEM). LEEM observations of a strain-induced (1\ifmmode\times\else\texttimes\fi{}1)--(15\ifmmode\times\else\texttimes\fi{}1) transformation of the Cu overlayer which occurs at a critical coverage of ${\mathrm{\ensuremath{\theta}}}_{\mathit{c}}$=2.13 ML have been used as a very accurate, local probe of coverage during deposition. It is found that the growth rate vanishes at high temperature, Tg950 K, due to competing desorption. A mean-field growth model which includes desorption and parametrization of interlayer diffusion flux quantitatively accounts for the experimentally measured dependence of the growth rate upon temperature and incident flux. Our results indicate that there is little or no Cu interlayer diffusion during growth. The desorption energy E=3.67 eV and attempt frequency \ensuremath{\nu}=2.4\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ are determined by examining the balance of incident and desorption fluxes. A step-flow-like growth morphology of the (15\ifmmode\times\else\texttimes\fi{}1) phase occurs when the supersaturation is significantly reduced by competing desorption at high temperature. Island nucleation and coalescence is prominent in the absence of competing desorption at low temperature. \textcopyright{} 1996 The American Physical Society.

Adatom migrations and nucleations on reconstructed (001) surfaces I. Si

Single Si adatom migration and the initial stage of epitaxial growth on (2 × 1) reconstructed Si(001) surfaces were investigated by an ab initio method using density functional theory. The obtained results are as follows: (a) the most stable binding site of a single Si adatom is different from the position that is required by epitaxial growth; (b) two Si adatoms interacting in order to produce a dimer take just the positions required for epitaxial growth. We conclude that the extreme stability of the Si dimer compared to other structures generated from two Si adatoms interacting on Si(001) surfaces is the origin of the formation of the perpendicular dimer rows alternately layer by layer on the reconstructed Si(001) surface in the process of epitaxial growth.

Microstructural evolution during epitaxial growth of Ag on vicinal InP(100) surfaces

The initial stages of epitaxial growth of Ag on InP(100) have been studied using in situ and ex situ electron microscopy. Vicinal InP substrates were cleaned by heating to about 400 °C in ultrahigh vacuum. Silver was deposited at substrate temperatures between 350 and 500 °C. The microstructural evolution was monitored for coverage between 0.5 and 8 monolayers using in situ high spatial resolution secondary electron microscopy and ex situ plan-view transmission electron microscopy (TEM). At sub-monolayer coverage, uniform, faceted islands of Ag are formed with edges aligned along InP 〈110〉 directions. Continued deposition leads to growth of preexisting islands followed by coalescence. During coalescence, a reduction in the island height, an increase in island-substrate interfacial area, and a disappearance of faceting are observed. At high coverage, during high temperature (450 °C) growth, the formation of faceted pits are observed. For higher temperatures (500 °C) and at low coverage, islands transform from square to rectangular shapes. Plan-view TEM indicates that the smallest Ag islands (∼5 nm) are not coherent with the substrate and that there are two distinct epitaxial relationships between the Ag islands and the InP substrate. For high-temperature growth, the (110)Ag plane is parallel to InP(100) substrate and there is evidence for chemical reaction and presence of In on the surface. For lower temperature depositions (350–400 °C), the (100)Ag is parallel to (100) InP.

Crossover scaling during submonolayer epitaxy onvicinal substrates

The nucleation and growth of two-dimensional islands on a vicinal substrate during the early stages of epitaxial growth is studied with kinetic Monte Carlo simulations and self-consistent rate equations. The island size distribution and island number density are shown to satisfy a dynamic crossover scaling form. A generalization of the self-consistent mean-field rate theory to vicinal substrates provides quantitative agreement with simulations.

LEEM studies of the early stages of epitaxial growth

The early stages of epitaxy which determine to a large extent the later growth and the physical properties of epitaxial layers can be studied with a resolution of the order of 10 nm using slow electrons in a variety of structural, magnetic and chemical contrast modes. These imaging modes are described and their application illustrated by several case studies including Si homoepitaxy, metals on Si and metals on metals.

Detecting stacking faults during epitaxial growth by low energy electron diffraction

We study the sensitivity of LEED to stacking faults in the early stages of epitaxial growth. By theoretical simulation of stacking faults in different locations, in particular for Cu/Cu(111), Co/Cu(111), and p(4×4)Pb/Cu/Cu(111), we show that quantitative LEED can detect stacking faults down to three or four layers deep into the substrate. This is also true in the presence of a The modification of the intensity spectra by stacking faults is large enough to be identified by visual comparison with defect-free surfaces. In the cases where different stacking sequences coexist on the surface, a minimum coverage of 30%–50% imperfect stacking sequences is needed in order to be clearly detected in the LEED I- V curves. Therefore, our results provide guidance to conduct experiments where early detection of stacking faults is important.

Microstructure and epitaxial characteristics of PbTiO3 ferroelectric thin film on NaCI by mocvd

Abstract Ferroelectric PbTiO3 thin films have been epitaxially grown on (100) NaCl cleavage surface by MOCVD using tetraethyllead and titanium isopropoxide as precursors. The microstructures and epitaxial characteristics of as-grown thin films were investigated by means of transmission electron microscopy. TEM studies have shown that the epitaxial thin film was highly (001) oriented and even near single-crystal epitaxy if without a small volume of 45° rotated grains, the average grain size is around 1000 A. Lowangle grain boundary formed universally when islands coalesced during the early stage of epitaxial growth. A possible growth mechanism has been given to explain this epitaxial phenomenon by consideration of two-dimensional coincidence superlattice. 90° ferroelectric domains were also observed both in the large matrix grains and small 45° rotated grains.

Scanning Tunneling Microscopic Studies of Epitaxial Films of Polyurethane, Polyester, and Polysiloxane Formed during Step Polymerization

Crystalline films of poly(hexamethylene-hexamethylenediurethane) were found to grow on the basal plane of graphite during polyaddition of diol and diisocyanate in a homogeneous solution containing a graphite substrate. Direct observations by scanning tunneling microscopy indicate that the film has been grown epitaxially. Also, an epitaxial film of poly(hexamethylene sebacate), an aliphatic polyester having the same number of bonds per monomeric unit as the polyurethane, was grown on the graphite surface during polycondensation of diol and dichloride. These results demonstrate that step polymerization induces epitaxy of polymers during polymerization. Then, the same polycondensation procedures were applied to study the films of an aromatic polyester, poly(hexamethylene terephthalate), and a polyester containing a triple bond, poly(butynyl sebacate) on graphite. The aromatic ring and the triple bond can be considered as the structural «kinks» on the aliphatic backbones that cause a misfit with a polyethylene-like repeat of the graphite hexagonal lattice. Formation of epitaxial films of these polyesters indicates the importance of having molecular structures capable of forming a close packed state in two dimensions for this epitaxial growth. Lastly, ring-opening polymerizations of cyclic dimethylsiloxanes were performed with the presence of a graphite substrate either in a good solvent at room temperature or in melt at 160 o C. Formation of crystalline films even at the temperatures much higher than the melting point of bulk polymer (-42 o C) and entropically driven polymerization of siloxanes suggest the two-dimensional nature of the crystallization process. On the basis of the present results and the previous studies, a mechanism of the late stage of epitaxial growth induced by polymerization is proposed

Crossover from Dimer Nucleation to Adatom Exchange During Submonolayer Epitaxy

ABSTRACTThe nucleation and growth of islands in the early stages of epitaxial growth is studied with kinetic Monte Carlo Simulations and self-consistent mean field rate equations. Specifically, adatom exchange and irreversible dimer formation are allowed to compete equally as the origin of two-dimensional islands. The island size distribution and number density are found to satisfy a dynamic crossover scaling form. The critical island size evolves from one to zero with increasing temperature, decreasing flux, and increasing coverage.

CVD Growth of SiC on Ultrathin SOi2: A Step Towards Development of a Compliant Substrate for SiC and III-V Compounds

ABSTRACTThis paper addresses the initial stage of epitaxial growth of SiC on thin (about 300A) and thick (2000Å) Si films. Our results as obtained by Rutherford backscattering spectroscopy (RBS), Auger spectroscopy, and plan-view/cross-sectional TEM, demonstrate epitaxial growth of 3C-SiC structures on ultrathin Si films (even under non-optimized growth conditions). These preliminary results indicate that the crystalline quality of SiC on thin SIMOX is better than that grown on thick SIMOX or bulk Si substrates. Growth of SiC epi on thin Si will pave the way for growth of SiC directly on SiO2 (a compliant substrate) by carbonization of the entire thin Si top layer of SIMOX.

The mobility of Pt atoms and small Pt clusters on Pt(111) and its implications for the early stages of epitaxial growth

We use the effective medium approximation to calculate the activation energy for the site-to-site motion of Pt atoms and Pt clusters with two to seven atoms, on the Pt(111) surface. We find that clusters are highly mobile and will move as a whole rather than by breaking into fragments that reunite in a different place. The results are used to examine the properties of the islands formed in the earliest stages of Pt deposition on the surface. We show that island formation at temperatures below 500 K occurs through the formation of stable clusters. There is no critical nucleus and the density of the supersaturated gas of single atoms on the surface is very close to zero; the islands do not originate through density fluctuations. We also rationalize the existence of three distinct regimes in the temperature dependence of island sizes at low coverage. They are due to absence of atom “evaporation” from islands onto the surface, at low temperature; to evaporation from open shell islands but not from the closed shell ones, at intermediate temperature; and to evaporation from all islands, at higher temperature.

Dynamics of irreversible island growth during submonolayer epitaxy.

The nucleation and growth of two-dimensional islands during the submonolayer stage of epitaxial growth is studied with kinetic Monte Carlo simulations and mean-field rate equations. Previous work on irreversible growth is extended to include relaxation of island shapes by edge diffusion. Island morphologies range from ramified structures at low temperatures to compact, polygonal shapes at higher temperatures. Using a self-consistent calculation of the rate coefficients, [ital quantitative] agreement is obtained between the solution to coupled, mean-field rate equations and the simulation results for average quantities. The island size distribution function is described by a single universal scaling function.'' The average island size is the only important scale for determining island densities. It is shown that the general form of the scaling [ital ansatz] applies to wider range of coverages than anticipated previously. This scaling form is combined with the solution to the rate equations to explore explicitly the [ital D]/[ital F] dependence of the number density of islands ([ital D] is the surface diffusion coefficient and [ital F] is the deposition flux).

Structural Correlation among Different Phases in the Initial Stage of Epitaxial Growth of Au on Si(111)

Structures created on the Si(111) substrate in the presence of Au atoms corresponding to coverages (θ; Au) of ∼0.7–1.4 monolayers (ML) are discussed on the basis of the observed scanning tunneling microscopy images. At small θ; Au's ( ∼0.7–1.0 ML), a √3×√3 phase and a less-ordered phase coexist, the fractional area of the latter being increased with increasing θ; Au, and at larger θ; Au's (\\stackrel>∼ 1.1 ML), a 6×6 phase appears as a single phase, in agreement with previous reports. It is shown that the less-ordered phase coexisting with the √3×√3 phase already has a close relation to the 6×6 phase in structure. The most fundamental structure of the 6×6 phase is seen at a large θ; Au of ∼1.4 ML, in which a layer of Au atoms sits on the Si(111) substrate incommensurately. As θ; Au decreases, some Au atoms appears to be removed with a periodicity of 6×6. The Au coverage of the √3×√3 phase seems to be ∼2/3 ML.

Nucleation and growth of gallium arsenide on silicon (111)

The nucleation and growth of undoped gallium arsenide (GaAs) epitaxial layers, grown by metalorganic vapour phase epitaxy (MOVPE) on Si (111) substrates were investigated by transmission electron microscopy (TEM). The initial stages of epitaxial growth are considered at high and low growth temperatures. The influence of growth time and thermal annealing on the initial stages of growth are also studied and reported.

Initial stages of epitaxial growth of Y-stabilized ZrO2 thin films on a-SiOx/Si(001) substrates

The initial stages of crystallization and growth of epitaxial Y-stabilized ZrO2 (YSZ) thin films on Si (001) substrates covered with amorphous native oxide were investigated by transmission electron microscopy (TEM) and reflection high-energy electron diffraction (RHEED). The YSZ crystallizes by solid-phase epitaxy with an initial thickness of 1 nm. The crystallization is initially incomplete, yielding both epitaxial and disordered regions of the YSZ film. In situ RHEED measurements showed the lattice parameter of the YSZ gradually decreased during growth, reaching the bulk value at a film thickness of 5–7 nm. For a larger thickness, the film is fully elastically relaxed. TEM revealed misfit dislocations at the YSZ/Si interface. The average spacing between misfit dislocations with Burgers vector b=1/2 〈110〉 was measured as 8.3 nm, showing epitaxial misfit strain to be fully accommodated by misfit dislocations. Thermal strain introduced during cooldown from the deposition temperature is fully elastically accommodated. For YSZ thicknesses below 7 nm, the thickness of the regrown layer of silicon oxide at the YSZ/Si interface is 0–0.5 nm.

Microstructural Evolution During the Epitaxial Growth of Ag/InP (100)

The initial stages of epitaxial growth of Ag on InP(100) has been studied using in-situ and ex-situ electron microscopy techniques. Vicinal InP substrates were cleaned by heating to about 450°C in ultra-high vacuum. Silver was subsequently deposited at substrate temperatures between 350–500°C. The microstructural evolution was monitored for coverages between 0.5–8ML using high spatial resolution secondary electron microscopy and transmission electron microscopy. At sub-monolayer coverages, square islands of Ag are formed with edges aligned along InP directions. Subsequent deposition leads to secondary nucleation or growth of pre-existing islands, depending on the substrate temperature. Surfaces with a uniform distribution of steps have a higher island density than those without steps but the steps are not preferred sites for island nucleation. Coalescence of islands begins at a smaller coverage for lower temperatures and for stepped surfaces. At the early stages of coalescence, island shapes with low symmetry are formed, with diminished island heights. These subsequently transform into shapes of higher symmetry. Except at the highest temperatures studied (500°C), island shape transformations (e.g. from square to elongated) are not very significant. Plan view transmission electron microscopy shows that the islands are epitaxially aligned and that even the smallest islands are generally incoherent. Island size distributions are used to analyze the growth mechanisms.