Stages Of Epitaxial Growth Research Articles

First atomistic studies of epitaxial growth of Na0.5Bi0.5TiO3 on SrTiO3

AbstractThe first stages of epitaxial growth of Na0.5Bi0.5TiO3 on SrTiO3 substrates were studied by means of a newly developed kinetic Monte‐Carlo (KMC) model employing several types of particles, different types of neighbors and various energy barriers. Density functional method (DFT) has been used for computing the surface structure with adatoms and for obtaining some of the diffusion barriers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron-microscopic investigation of a SiC/Si(111) structure obtained by solid phase epitaxy

First results of the electron-microscopic investigation of thin silicon carbide (SiC) layers grown on silicon using a new method of solid phase epitaxy are presented. It is shown that, at the initial stage of epitaxial growth, a transition layer is formed which consists of various SiC polytypes. This layer occurs at the interface between the substrate and a single-crystalline SiC layer possessing predominantly a 3C polytype structure. It is established that pores with dimensions ranging from a fraction of micron to several dozen nanometers are formed in a near-surface layer of the silicon substrate, which favor the growth of epitaxial, weakly strained single-crystalline SiC layers.

First stages of epitaxial growth in the presence of an extended defect: Kinetic Monte Carlo simulations versus rate equation study on a vicinal surface

We study the influence of atom confinement during the growth process on vicinal surfaces with different terrace widths. The behavior of the mean island density $n$ and size $s$ is analyzed in a general way as a function of the flux $F$ over diffusion ${D}_{1}$ ratio and terrace width using kinetic Monte Carlo simulations. We show that the exponent in the scaling law $n\ensuremath{\propto}{(F∕{D}_{1})}^{\ensuremath{\alpha}}$ changes from $\ensuremath{\alpha}=1∕3$ for infinite terrace to $\ensuremath{\alpha}=1$ in the case of finite terrace when the flux is lowered. In the same condition, the island size is limited by confinement, leading to a critical value which depends on the terrace width, only. Very simple rate equations are shown to be able to quantitatively explain, through three parameters determined independently, the simulation results at small deposition flux, whatever the description of the islands (point, compact, or fractal). Application of these results to a physical case $\mathrm{Ag}∕\mathrm{Pt}$ leads to an excellent agreement with more complete simulations based on an atomic description of the growth mechanisms.

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.

Mechanism of eliminating basal plane dislocations in SiC thin films by epitaxy on an etched substrate

Homoepitaxial growth is performed on 8° off-axis SiC substrates that are molten KOH etched. The shape of a basal plane dislocation (BPD) etch pit at different stages of epitaxial growth is studied by atomic force microscopy. It is found that the growth steps covering a BPD etch pit are curved, and both the usual step-flow growth and lateral growth perpendicular to the step-flow direction will take place in the pit simultaneously during epitaxy. The lateral growth is usually dominant and “blocks” the path for the BPD to propagate, and thus the conversion of BPDs to threading edge dislocations is enhanced.

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.

Reflectance difference spectroscopy during CdTe/ZnTe interface formation

We have investigated the first stages of epitaxial growth of CdTe on ZnTe and ZnTe on CdTe with reflectance difference (RD) spectroscopy. Spectroscopic RD data show strong optical anisotropy responses at the critical points of the bulk dielectric function at the E 0, E 1 and E 1+ Δ 1 interband transitions of ZnTe and CdTe, respectively, which indicate that anisotropic in-plane strain occurs during epitaxial growth. Kinetic RD data taken at the E 1 transition of the respective material exhibit with an accuracy of 1 ML the onset of the formation of misfit dislocations for these material systems.

Formationof CaF2 nanostructures on Si(001)

Initial stages of epitaxial growth and formation ofCaF2 nanostructures on Si(001) were studied. A variety ofnanostructures were grown including ultrathin two-dimensional layersat 750 °C, quasi-one-dimensional stripes at 650 °C andwell-ordered dots at lower growth temperatures. Atomic forcemicroscopy and reflection high-energy electron diffraction were usedto measure the nanostructure shape and lattice orientation. Theevolution of the surface electronic structure under different growthconditions was studied by ultraviolet photoelectron spectroscopy andmetastable de-excitation spectroscopy. The leading role of thewetting layer in high-temperature formation of the fluorite-siliconinterface was established.

The simulation of epitaxy, sublimation, and annealing processes in a 3D silicon surface layer

The Monte Carlo model for simulating epitaxy, sublimation, and annealing of the (111) surface of a diamond-like crystal is developed. The model accounts for the so-called overhanging structures and allows one to consider a 3D surface layer with a complex structure and volume comprising as many as 107 atom sites. The initial stage of epitaxial growth on smooth as well as on porous surfaces and the annealing of both step-containing and porous surfaces are simulated. The effect of Schwoebel barriers on the formation of compact 3D islands during epitaxy is suggested as a possible kinetic mechanism for quantum-dot formation. The sublimation behavior of monatomic steps on the (111) surfaces of a diamond-like crystal is also studied.

In situ observation of stress relaxation in CdTe/ZnTe heterostructures by reflectance-difference spectroscopy

The first stages of epitaxial growth of CdTe on ZnTe and ZnTe on CdTe are monitored with reflectance difference spectroscopy. Spectroscopic reflectance difference data show strong optical anisotropy responses at the critical points of the bulk dielectric function at the E0, E1, and E1+Δ1 interband transitions of ZnTe, respectively, CdTe, which indicate that anisotropic in-plane strain occurs during epitaxial growth. Applying a model it is possible to determine the in-plane strain due to the disbalance of 60° dislocations along [11̄0] and [110]. Kinetic reflectance difference data taken at the E1 transition of the respective material exhibit with an accuracy of one monolayer the onset of the formation of misfit dislocations for these material systems.

Structural Transformation of Screw Dislocations via Thick 4H-SiC Epitaxial Growth

In this paper, we studied the structural transformation of screw dislocations through gas-phase 4H-SiC epitaxial growth. We confirmed based on the numbers and features of etch pits on a surface after KOH treatment that some micropipes were closed in the epitaxial layer, and were divided into several elementary screw dislocations. We discuss the magnitude of Burgers vectors of micropipes in 4H-SiC substrates in relation to the number of elementary screw dislocations generated by micropipe closing. A depth analysis further revealed that most micropipe closings took place in the initial stage of epitaxial growth.

Tracing the valence band maximum during epitaxial growth of HfS 2 on WSe 2

Applying angle resolved photoemission and scanning tunneling microscopy (STM) during different stages of epitaxial growth of HfS 2 on WSe 2 allows an evaluation of the electronic valence band spectra as a function of position z perpendicular to the interface. In combination with photon energy dependent photoemission measurements of clean WSe 2 and HfS 2 samples, mapping k ⊥ dispersions of valence bands' reliable values for the valence band maxima (VBM) have been obtained. Upon different stages of growth, the valence band maximum can thus be traced during the build up of the interface giving an accurate value for the valence band offset of this heterojunction.

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.

Magic islands and barriers to attachment: ASi/Si(111)7×7growth model

Surface reconstructions can drastically modify growth kinetics during initial stages of epitaxial growth as well as during the process of surface equilibration after termination of growth. We investigate the effect of activation barriers hindering attachment of material to existing islands on the density and size distribution of islands in a model of homoepitaxial growth on Si(111)7x7 reconstructed surface. An unusual distribution of island sizes peaked around "magic" sizes and a steep dependence of the island density on the growth rate are observed. "Magic" islands (of a different shape as compared to those obtained during growth) are observed also during surface equilibration.

Surface morphology evolutions during the first stages of epitaxial growth of Si on Ge(0 0 1): a RHEED study

Solid source molecular beam heteroepitaxial growth of thin Si films on Ge(0 0 1)-2×1 pseudo-substrates has been investigated over a large range of substrate temperatures (from room temperature to 400°C) and Si deposition rate (from 1.5 to 25 Å/min) using reflection high-energy electron diffraction (RHEED). Due to the lower surface free energy of Ge with respect to Si, a Volmer–Weber growth mode is expected at equilibrium conditions. Nevertheless, at low temperature, RHEED patterns, in-plane lattice parameter measurements ( a ‖) and specular beam intensity oscillations reveal prolonged two-dimensional growth imposed by kinetic limitations for this tensile strained system. Actually, the critical thicknesses for which island formation takes place can be significantly increased by lowering the surface diffusion length of the Si adatoms, by either temperature decrease or deposition rate increase. These thicknesses are determined by the passage from streaky to spotty patterns and the onset of concomitant a ‖ decreases. This indicates varying and delayed 2D–3D transitions with respect to the equilibrium criterion. Such variations are not observed for the Stranski–Krastanov mode of the inverse compressive Ge/Si interface, whose critical thicknesses are only weakly dependent on the kinetic conditions. These differentiated morphology stabilities as a function of the strain sign are discussed in terms of strain dependent adatom diffusivities.

Growth stage of crystalline Y2O3 film on Si(100) grown by an ionized cluster beam deposition

We investigated the initial and epitaxial growth stage of Y2O3/Si(100) grown by reactive ionized cluster beam deposition, using x-ray diffraction (XRD), atomic force microscope, and reflection high-energy electron diffraction. We also investigated the crystalline structure of the films using transmission electron microscopy and XRD. The preferred growth direction of Y2O3 grown by an ion beam changed completely from the 〈111〉 to the 〈110〉 orientation in order to minimize the overall energy of the film as the substrate temperature increased. In addition to the kinetic energy of the deposited atoms, oxygen partial pressure and the substrate surface state also bear a relationship to the change in the preferred growth direction. The crystalline growth of Y2O3 film depends on the state of the surface at the initial growth stage, whether the Si surface was first exposed to oxygen or yttrium. In particular, the silicon oxide layer which formed on the Si surface during the initial growth stage played an important role in the epitaxial growth as well as the preferred growth direction of Y2O3 film.

Atom diffusion during MBE growth on patterned substrate

MBE growth on patterned substrate has been studied by means of transmission electron microscopy (TEM). The surface morphology development during the initial stage of epitaxial growth and the lateral epilayer thickness variation have been studied on a nanometer scale. A diffusion model has been proposed to explain the observed phenomena. Based on this diffusion model, it is proved that the diffusion length of adatoms on (100) plane can be measured from the cross-sectional shadow image as well as the cross-sectional TEM images of the grown layers. Again, from the diffusion model, it is found that Ga adatoms diffusion length ratio on (115A) over that on (100) can be measured from the GaAs layer thickness variation on (100) and at the intersection between (100) and (115A). The ratio of Ga adatoms diffusion length on (115A) over that on (100) is reported for the first time.

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.

XPS study of residual oxide layers on p-GaAs surfaces

The total thickness and composition of a residual oxide layer after chemical etching of p-GaAs:Zn + In has been studied by X-ray photoelectron spectroscopy (XPS). The variation of the Ga to As oxides ratio along the depth has been determined. A concentration correlation of doping isovalent impurity and the dislocation density with the composition of residual oxides is looked for. The total thickness of the residual oxide layer on p- and semi-insulating GaAs is about 5–6Å. It is found that the Ga 2O 3 quantity in the oxide bulk is greater than the same value of As 2O 3 in highly In-doped samples. In-doping in concentrations over 1.5 × 10 19cm −3 increases the Ga 2O 3 content and the density of the residual oxide. This influence is determined by reducing the dislocation density and changing the point defect environment. The presence of As-rich precipitates on the dislocations and in the matrix decreases the sputtering time and changes the composition of the residual oxide. The correlation between the type of high temperature dislocations revealed by Abrahams-Buiocchi (AB) etching and the oxide layer composition is shown. The results obtained could be used in the first stages of epitaxial growth, metallization and other technological processes of semiconductor device and ICs fabrication.