Early Stages Of Epitaxial Growth Research Articles

Kinetic Monte Carlo simulation study of the early stages of epitaxial SiC (0001) growth

A kinetic Monte Carlo (kMC) simulation with a cluster-multiple labeling technique was carried out to study the dynamic behavior of Si-C clusters during the early stage of epitaxial growth. In the model, a crystal lattice was established to fix the physical location of atoms and interatomic bonding. Events, such as adatoms adsorption, diffusion on the planar surface, and attachment to and detachment from clusters of adatoms were considered in the model. The Si and C atoms were treated individually to achieve more elaborate information for dynamic behavior of the clusters in the crystal surface. Moreover, a cluster multiple labeling technique, also known as the Hoshen - Kopelman algorithm was used to identify Si-C clusters. Then the properties of the clusters were calculated and analyzed by the algorithm. The simulation results showed that the number of Si-C clusters gradually decreases with increasing substrate temperature and decreasing deposition rate, and that the average size of Si-C clusters at high temperatures is much larger than that at low temperatures. Furthermore, the Si:C ratio on the growing surface has a significant effect on the surface morphology. At Si-rich conditions, the cluster density is relatively high, and the cluster shape is not homogeneously distributed. While with the increase of C mole fraction, the cluster density gradually decreases and the cluster shape becomes more uniform. Therefore, a higher concentration of C in deposition flux is beneficial to the early stage of nucleation. These findings are consistent with predictions for the case of the epitaxial growth of SiC.

Surface morphology evolution and underlying defects in homoepitaxial growth of GaAs (110)

The evolution of the surface morphology and underlying pyramidal defects in homoepitaxial GaAs (110) layers was investigated with respect to the layer thickness up to 1 µm. Ga and As atoms coexisting in a one-to-one atomic ratio on the (110)-oriented GaAs surface exhibit different incorporation rates and adatom migration rates, giving rise to local As atomic disordering in the epitaxial layer. Accordingly, this disordering triggers distinctive shapes of three-dimensional surface islands composed of specific facets and underlying microtwins. Single triangular-shaped islands initially appear at an early stage of epitaxial growth. With increasing layer thickness, all of the single triangular islands suddenly become paired triangular islands by forming secondary single triangular islands nearby, like decalcomania. A further increase of the layer thickness induces a complete transformation into four-legged starfish shapes containing directional side edges. At this stage, the underlying pyramidal defects of the surface islands are filled with {111} microtwins. Surprisingly, the evolutions of island shapes and underlying twin formation are closely associated with the strain relaxation in the thick GaAs (110) layers, which is unexpected in any homoepitaxial system. We found that a higher growth temperature retards the shape evolution of surface islands, leading to suppression of compressive strain.

Scanning tunneling microscopy study of the early stages of epitaxial growth of CoSi2 and CoSi films on Si(111) substrate: Surface and interface analysis

We report scanning tunneling microscopy observations of solid-phase epitaxial growth of ultra-thin CoSi2 and CoSi films at heating temperatures in the range of 350–600°C using a disordered ‘(1×1)’ phase of Co/Si(111) as a pre-existing template. It is found that depending on the growth conditions two types of epitaxial cobalt silicide films can be formed. At coverages below ~2 ML (1 ML=7.83×1014atoms/cm2) of Co, CoSi2 phase grows in the form of flat two-layer patches spreading over the surface at elevation of the temperature. It consists of precisely two the completely filled Co layers and four Si layers above the interface. At coverages higher than ~2 ML of Co, the layer-by-layer growth of CoSi phase with the CsCl-type crystal structure also takes place. The CoSi phase consists of one or more Co-Si double layers, which are located on top of the second Si-Co-Si triple layer of CoSi2, i.e., it has а coherent double interface CoSi/CoSi2/Si(111). According to experimental findings and ab initio total-energy calculations, the structures with the eightfold coordination of interfacial Co atoms and lateral unit cells of the silicide layer and substrate mutually rotated by 180° prove to be the most stable ones.

Kinetic Monte Carlo Simulation of Nucleation and Growth Behavior in Surfactant-Mediated Epitaxy

The behavior of island shape transition and nucleus density evolution in the early stage of epitaxial growth mediated by a monolayer of surfactant is studied by using a kinetic Monte Carlo simulations. The main kinetic processes included in the model are deposition, diffusion of atoms, exchange of adatoms with their underneath surfactant atoms, and reexchange in which an exchanged adatom resurfaces to the top of the surfactant layer. The simulation produces pattern transitions from small compact islands at low temperatures, to fractal-like islands at intermediate temperatures where the deposited adatoms can be easily incorporated into the surfactant layer, and then to regular compact islands at high temperatures where the reexchange becomes active. The island density as a function of temperature exhibits a complex N-shape. These results provide a reasonable explanation for the observed temperature dependence of the growth mode in existing experiments.

Thin film epitaxy with or without slope selection

Two nonlinear diffusion equations for thin film epitaxy, with or without slope selection, are studied in this work. The nonlinearity models the Ehrlich–Schwoebel effect – the kinetic asymmetry in attachment and detachment of adatoms to and from terrace boundaries. Both perturbation analysis and numerical simulation are presented to show that such an atomistic effect is the origin of a nonlinear morphological instability, in a rough-smooth-rough pattern, that has been experimentally observed as transient in an early stage of epitaxial growth on rough surfaces. Initial-boundary-value problems for both equations are proven to be well-posed, and the solution regularity is also obtained. Galerkin spectral approximations are studied to provide both a priori bounds for proving the well-posedness and numerical schemes for simulation. Numerical results are presented to confirm part of the analysis and to explore the difference between the two models on coarsening dynamics.

Morphological and structural investigation of the early stages of epitaxial growth of α-Fe 2O 3 (0001) on α-Al 2O 3 (0001) by oxygen-plasma-assisted MBE

We have investigated the early stages of α-Fe 2O 3 (0001) film growth on α-Al 2O 3 (0001) using oxygen-plasma-enhanced molecular beam epitaxy along with reflection high-energy electron diffraction, non-contact atomic force microscopy, and X-ray photoelectron spectroscopy and diffraction. A compressionally strained, fully stoichiometric α-Fe 2O 3 film three monolayers thick forms prior to the onset of three-dimensional island formation and lattice relaxation. The surface of this film appears to buckle along 〈112̄0〉, giving rise to a new set of inwardly contracted diffraction spots, which, if not resolved from the substrate spots, could be interpreted as a 12% in-plane lattice parameter expansion. Such an interpretation has led prior investigators to conclude that the interfacial layer consists of a disordered cation layer with an in-plane lattice parameter ∼6% larger than that of α-Fe 2O 3 [T. Fujii, D. Alders, F.C. Voogt, T. Hibma, B.T. Thole, G.A. Sawatzky, Surf. Sci. 366 (1996) 579]. Our interpretation of the diffraction data suggests that the interfacial layer is badly distorted but commensurate with the substrate.

Preparation and surface morphology of YBa 2Cu 3O 7− y films by metalorganic chemical vapor deposition block by block deposition using liquid sources

We have recently studied the preparation and the characterization of the films deposited by metalorganic chemical vapor deposition (MOCVD) using liquid state sources [Y. Yoshida, Y. Ito, I. Hirabayashi, H. Nagai, Y. Takai, Appl. Phys. Lett. 69 (1996) 845; Y. Ito, Y. Yoshida, Y. Mizushima, I. Hirabayashi, H. Nagai, Y. Takai, Jpn. J. Appl. Phys. 35 (1996) L825.]. In this paper, we describe the CVD process for the ultra-flat YBCO films block-by-block deposition using liquid sources. The high stability and good controllability of the sources enable us to study the early stage of epitaxial growth by depositing c-axis-oriented films with different number of sequence and different kind of substrates. Investigating the films deposited at 800°C, we confirmed that the initial growth of YBCO films on MgO is multinuclear growth, and in the early stage ( t≤200 Å), spiral growth appears, whereas that on SrTiO 3 is two-dimensional growth and spiral growth appears after coalescence of initially grown crystallites having a flat-faceted surface.

Microscopic view of epitaxial metal growth: nucleation and aggregation

Thin films are often grown away from thermodynamic equilibrium and their morphology becomes determined by kinetics. The final structure of the epitaxial film is decided in the very early stage of submonolayer nucleation and island growth. Recent experiments with scanning tunneling microscopy opened up an unprecedented view of this early stage of epitaxial growth. Variable sample temperatures enabled quantification of the rates of the most important atomic diffusion events and tracing back their interplay yielding the final submonolayer morphology. The present understanding of nucleation and aggregation in light of these new experimental results is reviewed for the case of metals. Examples are given how the growth kinetics can be employed to create well-defined island morphologies and island arrays in a self-organization process.

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.

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.

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.

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.

Correlation Length Scaling During the Early Stages of Epitaxial Growth

ABSTRACTWe have calculated the height-height correlation function by means of Monte Carlo simulations of a solid-on-solid model. These simulations reveal that the position and sizes of the islands formed during the first half-monolayer are highly correlated, a feature which has also been noted in recent x-ray studies of growth by OMCVD. This morphology results in the emergence of a characteristic length,ℓ, in the correlation function, which is found to obey the scaling lawℓ = (D)/J)γ, whereDis the diffusion length, andJis the deposition flux. The value of the exponent γ varies with coverage and depends on the growth conditions.

Dislocation density of lattice mismatched epitaxial layers

Heteroepitaxial InAs, GaAs and Si layers were grown by vapor phase deposition. Having determined that the dislocation density of InAs layers on GaAs substrates is about 1×10 4 cm -2, that of Si on GaAs is less than 10 4 cm -2, and that of GaAs on InAs substrates is about 1×10 8 cm -2, we classified these growth systems into two categories, low and high dislocation density. We ascribed that the dislocation density was related to the formation of islands in the early stages of epitaxial growth.

Formation of planar defects in the epitaxial growth of GaP on Si substrate by metal organic chemical-vapor deposition

The very small lattice mismatch between GaP and Si implies negligible coherency strains when GaP is grown epitaxially on Si. Moreover, GaP is expected to show little plasticity below ∼700 °C. It is thus unlikely that either the small coherency strains or the thermal mismatch stresses would be sufficient to generate the partial dislocations responsible for planar defects in an epitaxial film grown at temperatures below ∼700 °C. However, high-resolution electron microscopy micrographs of the early stages of epitaxial growth of GaP on Si at 500 °C show a high density of stacking faults and microtwins in the film. It is argued that in epitaxial growth processes, planar faults are not formed as a result of interfacial stresses; instead, a mechanism for the formation of stacking faults and twins is proposed which is based on errors in the stacking of close-packed planes in the early stages of nucleation and growth.

Selective growth of InP buried structure by chloride vapor phase epitaxy

Selective growth of an InP buried layer by In/PCl3/H2 vapor phase epitaxy was developed for buried layer GaInAsP/InP long wavelength laser diodes. For the first time, a completely flat-surface buried layer was grown into grooves with good morphology on a (100) exactly oriented InP substrate, but not on a (100) 2° off oriented substrate. We found that the side of the groove was covered with a buried InP layer in the early stage of epitaxial growth. Therefore, the present selective growth would be effective for the protection of the interface between the active and buried layers from thermal degradation. The resistivity of InP, measured by using an n-i-n structure, was found to be higher than 103 Ω cm at room temperature, which is sufficient for the buried layer of an usual laser diode.

Atomic Structure of HgTe and CdTe Epitaxial Layers Grown by MBE on GaAs Substrates

ABSTRACTUsing high resolution transmission electron microscopy (HRTEM) it is possible to directly image the projected structure of semiconductors with point resolutions at the atomic level. This technique has been applied to the study of interface and defect structures associated with molecular beam epitaxial (MBE) growth of HgCdTe layers on GaAs substrates. The structure of the CdTe/GaAs interfaces is described for (100) and (111) epitaxy. From the atomic structure, a model for the early stages of epitaxial growth is presented. The structure of HgTe-CdTe superlattices is discussed from the HRTEM point of view.