Faceted Islands Research Articles

Growth and characterization of Ge nanostructures on Si(111)

AbstractScanning Probe Microscopy (SPM) in situ is used to study the evolution of Ge islands grown by Physical Vapor Deposition on Si(111) 7×7 reconstructed surfaces. Large 3D islands form on the Wetting Layer (WL), with average lateral dimension in the range 200 - 500 nm. The statistical distribution of the island shapes has been analyzed, showing that three types of shapes coexist under certain conditions: strained, partially relaxed and ripened (atoll-like) islands. We measured the contact angles of the island facets, and observed the depletion of the substrate around the ripened islands. These features are attributed to the misfit strain, which is partially relieved by interdiffusion of Si into the Ge layers.

Effect of a surfactant in homoepitaxial growth of Ag (001): dendritic versus faceted island morphologies

The homoepitaxial growth of Ag(001) has been investigated in real time with X-ray diffraction by recording two-dimensional intensity distributions of the scattered intensity while the film was growing. Within a narrow range of growth parameters around T=320 K and rate=0.02 layers/s, the surface diffuse scattering, which provides information on the morphology of the surface islands, exhibits a peculiar cross-shaped pattern, which has been interpreted as being due to dendritic island morphology. The dendritic islands evolve to the faceted morphology characteristic of the equilibrium shape when the surface temperature is increased. If, prior to the growth, a small coverage of surfactant atoms is deposited on the surface, then the morphology of the growing islands at 320 K is not dendritic, as for the clean surface, but faceted. These findings provide new insights on the mechanisms underlying the effect of the surfactant.

Residual strain in Ge pyramids on Si(111) investigated by x-ray crystal truncation rod scattering

Epitaxial growth of germanium on boron-terminated Si(111) results in the formation of triangular pyramidal Ge islands which are partially relaxed. We show that the termination of the Si(111) surface with 1/3 ML of boron is essential for the formation of faceted islands. We have investigated the residual strain in the Ge islands using x-ray crystal truncation rod scattering, and developed an analytical expression for the scattered intensity from islands with a nonuniform lattice parameter. We compare the measured intensity to x-ray scattering profiles calculated on the basis of different strain models. It is found that the Ge lateral lattice parameter changes linearly from the bottom to the top of the islands.

Wetting layer thickness and early evolution of epitaxially strained thin films

We propose a physical model which explains the existence of finite thickness wetting layers in epitaxially strained films. The finite wetting layer is shown to be stable due to the variation of the nonlinear elastic free energy with film thickness. We show that anisotropic surface tension gives rise to a metastable enlarged wetting layer. The perturbation amplitude needed to destabilize this wetting layer decreases with increasing lattice mismatch. We observe the development of faceted islands in unstable films.

Strain relaxation of faceted Ge islands on Si(113)

We studied the formation and strain relaxation of Ge islands on Si(113) grown at 700 °C by molecular beam epitaxy. Atomic force microscopy reveals that they are mainly (113) top faceted and show a mesa-like shape. Initially formed Ge islands tend to elongate along [332̄] due to their anisotropic shear strain. With increasing Ge coverage, they grow mainly laterally towards particular directions, forming large V-shaped Ge clusters. This is analyzed to be caused by the formation of {111} slide plane-associated dislocations. The dislocations are shown to be localized in the islands. Considerable Si material has diffused into the Ge islands. Strain and Ge content in the islands are quantitatively analyzed by micro-Raman spectroscopy.

Synthesis and characterization of PbTiO3 powders and heteroepitaxial thin films by hydrothermal synthesis

PbTiO3 powders and heteroepitaxial thin films were produced by the hydrothermal method at 110–200 °C using different bases (Na–, K–, Rb–, Cs–, TMA–, and TBA–OH). Microstructural characterization showed that the tetragonal perovskite films were epitaxial on the SrTiO3 substrates, with a c-axis out-of-plane orientation. Sequential growth experiments showed that the growth initiates by the formation of 100 nm {100} faceted PbTiO3 islands followed by coalescence. Small cation bases (Na–, K–, Rb–OH) produced 1.5-μm {100} faceted blocky powders, whereas larger cation bases (Cs–, TMA–, and TBA–OH) formed fewer 500-nm interpenetrating platelets. Nuclear magnetic resonance results showed cation incorporation in the perovskite structure with local disorder on the Pb sites increasing cation size.

Low-energy electron microscopy of nanoscale three-dimensional SiGe islands on Si(100)

Epitaxial growth of three-dimensional SiGe islands on Si(100) has been investigated by low-energy electron microscopy. Contrast mechanisms that allow measuring the evolution in position, size and shape of individual islands during deposition have been identified, and were used to study the epitaxial embedding of faceted SiGe islands in a Si matrix. Deposition of Si onto SiGe islands drives their expansion and causes a change in their shape: the island apex decays and a (100) top facet forms. A physical interpretation of the mechanism producing this shape change is presented. The eventual smoothing of the growth front upon embedding of the islands is promoted by the strain-driven migration of adatoms away from the (100) top facet.

Self-organized nanostructures in Si 1-x Ge x films on Si(001)

In Stranski-Krastanov growth of SiGe films on Si(001) a series of stress-induced morphologies occurs including step-bunching of the pre-existing substrate steps and formation of faceted three-dimensional islands. We demonstrate how one can utilize these elastic strain-relief mechanisms to create a variety of large-scale arrays of uniform nanostructures simply by growing under appropriate conditions of substrate vicinality and misfit. By tuning substrate miscut periodic ripple structures down to a few ten nm are obtained. Self-organized growth of faceted islands can be achieved either by stress-induced self-organization in SiGe/Si multilayer films or in a single alloy film by combining step bunching and island formation mechanisms. We expect these concepts to become useful for the fabrication of large-scale arrays that may serve, for example, as quantum wire and quantum dot arrays or act as patterned substrates for deposition of various materials.

Self-Limiting Growth of Strained Faceted Islands

We consider the growth of facets associated with coherently strained semiconductor islands. Surprisingly, the island growth rate is found to rapidly self-limit, which has important consequences for island size distributions. A new explanation for the elongation of strained faceted islands is proposed as a natural consequence of facet growth kinetics.

Si growth on partially relaxed Ge islands

We use low-energy electron microscopy (LEEM) to study Si growth on {105} faceted Ge islands on Si (100). A quantitative analysis of LEEM growth sequences suggests, that for low Si coverage no growth occurs at the island apex, and that adatoms in the apex region are collected onto the {105} side facets. This “partial wetting” growth mode leads to a characteristic formation of a large (100) plane bounding the top of the islands. We find Ge surface segregation on the {105} facets to play an important role in the overgrowth process: Ge/Si intermixing leads to a stabilization of the facet structure over a large range of Si coverage and results in a shape of buried islands mapping their morphology during overgrowth: a truncated pyramid, bounded on top by a (100) plane.

Nonstoichiometry on TiO2(110) and Cu–TiO2 interfaces

A number of reconstructions have been observed on the (110) surface of TiO2 in the rutile modification that accommodates oxygen deficiency. We present atomically resolved scanning tunneling microscope images that describe a new reconstruction with (3×2) symmetry. A model for the observed reconstruction is discussed where this symmetry is achieved by removing 1/3 or 2/3 of the oxygen in the bridging oxygen rows such that a shift in position of missing oxygen by one lattice space vector along the [001] direction occurs in every second row. This structure contrasts those reported previously in which entire rows are modified or removed. Furthermore, nanometer-sized Cu islands on TiO2(110) surfaces fabricated by vapor deposition onto TiO2 at 200 °C were studied by conventional and high-resolution transmission electron microscopy (TEM). Our TEM studies show that Cu forms faceted islands (40 nm in width) on the surface with the orientation relationship given by Cu{110}∥TiO2(110) and Cu〈001〉∥TiO2[11̄0]. The orientation relationship differs from that resulting from room temperature deposition onto stoichiometric surfaces.

InGaAs island shapes and adatom migration behavior on (100), (110), (111), and (311) GaAs surfaces

The evolution of InGaAs island formation on (100), (110), (111), and (311) GaAs substrates was studied by atomic force microscopy. In addition to determining the growth mode, shape, average size and distribution of InGaAs islands on each orientation, measurement of the saturation island densities enabled an estimation of effective group III adatom surface diffusion lengths. Small lens-shaped islands in addition to larger faceted islands were formed on (100) and (311) surfaces, while trapezoidal and triangular islands were obtained on (110) and (111)B orientations, respectively. Adatom diffusion lengths on these surfaces were found to range from 0.06 μm on (311)B to 3 μm on (111)B.

Deposition of three-dimensional Ge islands on Si(001) by chemical vapor deposition at atmospheric and reduced pressures

This report summarizes observations of Ge island formation during growth on Si(001) by chemical vapor deposition from germane in the pressure range from 10 Torr to atmospheric pressure in a conventional epitaxial reactor. A four-step growth process is observed: (1) uniform pseudomorphic overlayer (“wetting’’ layer) formation; (2) three-dimensional island growth with a constant aspect ratio; (3) continued island growth with a constant diameter and increasing height; (4) rapid growth of larger, faceted islands. Ostwald ripening of the islands during continued heat treatment after terminating the deposition is slow compared to island formation and growth during deposition for the experimental conditions used.

Nonuniform Morphology and Luminescence Properties of a Molecular Beam Epitaxy GaN Film from Atomic Force Microscopy, Scanning Electron Microscopy and Cathodoluminescence

Complex faceted features of micrometer sizes and with intense luminescence rise 200-300 nm above the surface of a GaN thin film grown by molecular beam epitaxy on (0001) sapphire. Cathodoluminescence measurements at room temperature and at 8K were used to investigate the luminescence properties of these microfeatures in comparison with those of the background GaN material. The morphology of the micro-features was studied by scanning electron microscopy and by atomic force microscopy.GaN and related compounds have been considered as promising materials for light emitting devices in the short wavelength visible and UV spectral regions because of their direct, wide band gaps and high luminescence efficiency. Remarkable, rapid success has been achieved in developing devices based on III-V nitrides, which allows these material systems to rival other systems under development [1]. Blue-green LEDs fabricated from InGaN/AlGaN double-heterostructure layers on sapphire substrates by Nichia Chemical Industries in Japan are commercially available, despite the presence of high extended defect densities in the 1010 /cm2 range [2]. In December 1995, an important milestone in the development of nitride lasers was reached; Nakamura et al. in Nichia demonstrated the first nitride-based laser diode which operated at 417 nm under pulsed conditions at room temperature [3].The morphology and spatial distribution of luminescence in GaN thin films have been investigated by several research groups [4], [5], [6], although most efforts have been directed to developing appropriate growth conditions for improved film quality. From observations by scanning electron microscopy (SEM) and atomic force microscopy (AFM), Trager-Cowan et al. described an MBE-grown GaN film which contained an assembly of oriented hexagonal crystallites rising above a background of polycrystalline or amorphous material [4]. All the crystallites, oriented in a similar fashion, had roughly the same sizes, 1 or 2 μm across a hexagonal face and about 1 μm high. These crystallites were much brighter than the surrounding background material in panchromatic low-temperature cathodoluminescence (CL) images. Trager-Cowan et al. concluded that the crystallites were of better quality than the background material. They also observed a green emission band, attributed to impurities, which became weaker for higher electron beam voltages, generating luminescence from deeper in the film. From this observation, they concluded that higher quality material is located closer to the film's outer surface. Spatial variation of the luminescence efficiency from MOCVD-grown films has also been observed by Ponce et al[5]. in their room-temperature CL microscopy studies. Their results showed significant nonuniformities in both the band-edge and yellow band emissions. Although they reported no faceted island structures, one of their samples had “marked surface features” and consisted of hexagonal crystals 10 to 50 μm in diameter. The crystals gave strong band-to-band (364 nm) luminescence.In this paper, the morphology of a GaN film grown on (0001) sapphire by MBE is characterized by SEM and AFM, and CL measurements are used to investigate the luminescence properties of the film at room temperature and at 8K. Some regions of this film are similar to the one described by Trager-Cowan et al. [4] in having micron sized, brightly luminescent islands. The hexagonal facet angles and bright luminescence of islands in our film also resemble the larger scale, more regularly shaped hexagonal crystals of Ponce et al. [5]. These regions lie on the boundary of excess gallium growth conditions. Our results differ from those of Trager-Cowan in terms of the island morphologies and the spectral character of luminescence from the islands and from the background material. Possible causes of the large variations in luminescence efficiency are discussed.

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.

Scanning tunneling microscopy study of the epitaxial growth of strained In 0.82Ga 0.18As layers on InP

The 2D–3D growth mode transition of compressively strained In x Ga 1− x As layers ( x = 0.82 or 2% lattice mismatch) grown on an In 0.53Ga 0.47As buffer layer lattice matched to InP was studied using scanning tunneling microscopy. Up to 4 deposited monolayers, a layer by layer growth mode is maintained. The surface layer appears to be more compact for the strained layers than for the lattice matched buffer layer. After 5 monolayers were deposited the surface topography undergoes very significant change and threedimensional patterns, highly anisotropic in the growing plane, appear. These remarkable evolutions are attributed to the competition between surface energy necessary to form new island facets and elastic energy relaxation allowed with small sized islands.

Strain-induced formation and tuning of ordered nanostructures on crystal surfaces

The energy of an array of 3D coherent strained islands on a lattice-mismatched substrate equals: E = ΔE V EL + ΔE RENORM FACETS + ΔE EDGES EL + E EDGES + E INTER, where ΔE V EL is the volume elastic relaxation energy, ΔE RENORM FACETS is the change of the surface energy of the system due to the formation of islands, which includes the strain-induced renormalization of the surface energy of the island facets and of the planar surface, ΔE EDGES EL is the contribution of the island edges to the elastic relaxation energy, E EDGES is the short-range energy of the edges, and E INTER is the energy of the elastic interaction between islands via the substrate. The energy ΔE EDGES EL ≈ - L −2 · ln L always has a minimum as a function of the size of L, and the total energy E = E( L) may have a minimum at an optimum size L opt. E INTER is the driving force for the latera ordering of 3D islands. Among different arrays of islands on the (001) - surface of a cubic crystal, the total energy is minimum for the periodic square lattice with primitive lattice vectors along the “soft” directions [100] and [010]. Thus, a periodic square lattice of equal-shaped and equal-sized 3D islands is, under certain conditions, the stable array of islands which do not undergo ripening. The theory explains the spontaneous formation of ordered arrays of 3D islands in the InAs GaAs(001) system.

Coalescence of cylindrical islands on the crystal surface accompanied by correlated changes of their heights and radii

Coalescence of an ensemble of cylindrical faceted islands comprising N components and K chemical compounds on the crystal surface is studied in case of layer-by-layer growth of islands accompanied by correlated changes of their heights and base radii. Steps, generated on the surface of the islands by twinning dislocations, are assumed to be the sources of layers. The following correlation between the height H and the base radius R of an island is found: ( H H 0 ) = ( R R 0 ) j s where j S = 2σ S β 1 S (σ 1 S β S ) . This correlation is expressed in terms of thermodynamic and kinetic parameters of an island of the S compound, such as the specific free energies of the surfaces of the step edges σ S and σ 1 S on a lateral and the top facets respectively, and corresponding generalized specific flows β S and β 1 S. Analytical solutions of the basis system of equations are obtained. Necessary conclusions as to formation of the final structure of the continuous thin film are given. Limiting cases, when growth of cylindrical islands in the stage of coalescence is needle-like or tablet-like, are analyzed. The asymptotical dependencies obtained in this work differ from those found both in case of normal growth mechanism of the islands and in case of their needle-like or tablet-like growth proceeding layer by layer. Theoretical results are compared to experimental data on vacuum condensation through thermal evaporation of the PbTe films on the KCl substrate, and they show good agreement.

Kinetics and Mechanism of β-Brass Dealloying in Aqueous 0.5 M Sodium Chloride Solution Derived from Combined Scanning Tunneling Microscopy and Electrochemical Data

Scanning tunneling microscopy (STM) combined with conventional electrochemical techniques was used to determine the kinetics and mechanism of β-brass dealloying in aqueous neutral solutions containing NaCl. In the potential range where selective dissolution of Zn occurs, electrochemical data suggest that the corrosion process is controlled by surface diffusion of Cu atoms. STM imaging of β-brass after dealloying shows the development of an irregular surface which attains a stationary regime. The irregular surface topography consists of faceted islands with atomically smooth terraces. For a constant dealloying time, the extent of faceting increases as the potential is moved positively, and at a constant applied potential it increases with electrodissolution time. The proportionality ξ ∝ Lα was established, where ξ is the interface width related to L, the sample size. The exponent α is related to the degree of surface disorder. The value of α derived from STM is in the range 0.7 ≤ α ≤ 0.8, i.e. close to the predictions of aggregation models including surface diffusion.

Reduced effective misfit in laterally limited structures such as epitaxial islands

Numerical finite element calculations have been reported to determine a correction function Φ that describes the reduction of the misfit that occurs when laterally limited structures such as faceted islands or mesa structures are grown on a substrate. The reduction of the average strain energy density is calculated in these three-dimensional islands and compared to the constant strain energy density in a continuous layer. Ratios Φ are obtained from the calculation of different island geometries, i.e., different facet angles γ and different aspect ratios island width l to island height h. These discrete values are fitted by a function which can easily be applied to the full range of aspect ratios (l/h≳0) and facet angles (0°<γ<90°). Faceted Ge(Si) islands on Si(001) substrate, grown from the solution in the Stranski–Krastanov growth mode, serve as an example for the calculation. Experimental and theoretical values for the critical thickness of these islands agree well. This result demonstrates the drastic influence of islanding on misfit strain distribution in island and substrate as well and, consequently, on the strong increase of the critical thickness as determined by the mechanical equilibrium theory of Matthews and Blakeslee.