Uniform Islands Research Articles

Synthesis of ZnAl2O4/-Al2O3 complex substrates and growth of GaN films

With the solid phase reaction between pulsed-laser-deposited (PLD) ZnO film and α-Al2O3 substrate, ZnAl2O4/α-Al2O3 complex substrates were synthesized. X-ray diffraction (XRD) spectra show that as the reaction proceeds, ZnAl2O4 changes from the initial (111)-oriented single crystal to poly-crystal, and then to inadequate (111) orientation. Corresponding scanning electron microscope (SEM) images indicate that the surface morphology of ZnAl2O4 transforms from uniform islands to stick structures, and then to bulgy-line structures. In addition, XRD spectra present that ZnAl2O4 prepared at low temperature is unstable at the environment of higher temperature. On the as-obtained ZnAl2O4/α-Al2O3 substrates, GaN films were grown without any nitride buffer using light-radiation heating low-pressure MOCVD (LRH-LP-MOCVD). XRD spectra indicate that GaN film on this kind of complex substrate changes fromc-axis single crystal to poly-crystal as ZnAl2O4 layer is thickened. For the single crystal GaN, its full width at half maximum (FWHM) of X-ray rocking curve is 0.4°. Results indicate that islands on thin ZnAl2O4 layer can promote nucleation at initial stage of GaN growth, which leads to the (0001)-oriented GaN film.

Direct Observation of the Thermal Decomposition of Ligand-Stabilized Clusters

Ligand-stabilized Au55 clusters have been observed by scanning probe microscopy upon thermal decomposition on top of graphite and mica substrates. On highly oriented pyrolitic graphite as well as on mica the ligand shell exhibits a thermal decomposition at a temperature of about 390 K. This temperature well matches data obtained on cluster solutions and pallets. The ligand decomposition results under ultrahigh vacuum conditions in the formation of naked Au clusters. The in situ needle sensor studies show that, due to the high cluster mobility at elevated temperatures, bigger spherical Au aggregates are formed on the graphite substrate. In contrast, on the mica substrate, the naked Au clusters aggregate to form small uniform islands that are stable up to temperatures of more than 700 K. Bigger Au aggregates could only be formed upon pushing the clusters by the microscope's probe at elevated temperatures. The significant difference in the cluster decomposition and aggregation processes on graphite and mica ...

Transmission surface-plasmon resonance (T-SPR) measurements for monitoring adsorption on ultrathin gold island films.

Evaporation of ultrathin (1.3-10 nm nominal thickness) gold films onto quartz or mica leads to the formation of a layer of rather uniform gold islands on the transparent support. The morphology of ultrathin gold island films of various thicknesses was studied by using atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging. The surface plasmon (SP) absorption characteristic of such films is highly sensitive to the surrounding medium, with the plasmon band changing in intensity and wavelength upon binding of various molecules to the surface. The binding process can be monitored quantitatively by measuring the changes in the gold SP absorption, by using transmission UV/Vis spectroscopy. The method, termed transmission surface plasmon resonance (T-SPR) spectroscopy, is shown to be applicable to both chemically and physically adsorbed molecules, in liquid or gas phase, with measurements carried out either ex situ or in situ (real-time measurements) using a variety of molecular probes. Binding to a preformed molecular layer on the Au surface produces a similar response, suggesting the possible use of T-SPR for selective sensing. The sensitivity of T-SPR spectroscopy in detecting molecular binding to the gold depends strongly on the film preparation conditions, and may be comparable to that obtained in surface plasmon resonance (SPR) sensing.

Stabilization of the Pb/Si(1 1 1)-(7×7) uniform height islands to higher temperatures with oxygen adsorption

We have studied how oxygen adsorbed on top of the uniform height Pb islands, (grown on Si(1 1 1)-(7×7) at low temperatures T l<190 K) extends the temperature range of their stability. The evolution of the island height and size with temperature is monitored with SPA-LEED. The presence of oxygen suppresses Pb diffusion to higher levels and results in sharper island height distribution, when compared to the height distribution on the clean Pb/Si(1 1 1)-(7×7) system. Most likely this is because the barrier at the island edges, which controls the transfer of atoms from lower levels to the top of the islands, is increased with the adsorption of oxygen.

Stable island arrays by height-constrained Stranski–Krastanov growth

In the Stranski–Krastanov system, the lattice mismatch between the film and the substrate causes the film to break into islands. During annealing, both surface energy, and elastic energy drive the islands to coarsen: some islands enlarge and others shrink, keeping the total island volume constant. The islands produced this way are usually uneven in size and spacing. Motivated by several related studies, we suggest that stable, uniform islands should form when a stiff ceiling is placed at a small gap above the film. After contacting the ceiling, the islands are constrained to grow laterally and remain coherent with the substrate, preventing further stress relaxation. In fact, we show that the role of elasticity is reversed: with the ceiling, the total elastic energy stored in the system increases as the islands coarsen laterally. On the other hand, the total surface energy decreases as the islands coarsen. Consequently, the islands select an equilibrium size to minimize the combined elastic energy and surface energy. We estimate the equilibrium island size by analyzing an idealized model.

Uniform-height island growth of Pb onSi(111)−Pb(3×3)at low temperatures

We have studied the growth of Pb on $\mathrm{Si}(111)\ensuremath{-}\mathrm{P}\mathrm{b}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ at low temperatures, $T&lt;250\mathrm{K},$ and for coverages up to $\ensuremath{\theta}&lt;20\mathrm{ML}.$ The study is carried out with the complementary techniques of high-resolution low-energy electron diffraction and variable-temperature scanning tunneling microscopy. Uniform-height Pb islands, of flat tops and steep edges, are observed on this phase [similar to the uniform island growth observed on the $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)].$ The origin of this unusual growth is quantum size effects (i.e., the dependence of the electron energy on the island thickness because of the quantization of the electron energy levels in the island). The preferred island height is five steps on the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ and seven steps on the $(7\ifmmode\times\else\texttimes\fi{}7)$ (for the same growth conditions T, \ensuremath{\theta}) because of the different electronic structure at the two interfaces. Since different types of phases [i.e., $(1\ifmmode\times\else\texttimes\fi{}1),\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3},$ which differ in the Pb stoichiometry] can form from the initial $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase, with increasing coverage, the growth on these phases can be also used to study island-height selection. Laterally larger uniform-height islands are observed on the $(1\ifmmode\times\else\texttimes\fi{}1)$ at lower temperatures, $T&lt;170\mathrm{K};$ growth on the $\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ at higher temperatures, $T&gt;195\mathrm{K},$ shows an open film with preference of the islands to grow to larger heights with uncovered regions of the $\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase between the islands, even after deposition of 20 ML of Pb.

Formation of uniform nanoscale Ge islands on Si(1 1 1)-7 × 7 substrate

We have reported the formation of uniform nanoscale Si islands (3.8nm in diameter) on a Si(111)-7×7 substrate by using the stable reconstructed structure on the substrate as a template. It is very interesting to use the template as a method of quantum dot formation. We have tried to form the uniform nanoscale island of Ge on the Si(111)-7×7 substrate at various growth conditions (substrate temperature and annealing temperature) and observed the size distribution of Ge islands with a scanning tunneling microscope (STM). The range of substrate and annealing temperatures were chosen between 300 and 400°C. And we succeeded in the formation of uniform Ge islands showing a rounded shape with a diameter of 3.8nm, when the Ge is deposited at 320°C and annealed at 360°C.

Quantum dot and hole formation in sputter erosion

Recently, it was experimentally demonstrated that sputtering under normal incidence leads to the formation of spatially ordered uniform nanoscale islands or holes. Here, we show that these nanostructures have inherently nonlinear origin, first appearing when the nonlinear terms start to dominate the surface dynamics. Depending on the sign of the nonlinear terms, determined by the shape of the collision cascade, the surface can develop regular islands or holes with identical dynamical features, and while the size of these nanostructures is independent of flux and temperature, it can be modified by tuning the ion energy.

Self-organized quantum dot formation by ion sputtering

Key requirements for the fabrication of semiconductor quantum dots (QD) are their size, size distribution and density. Beyond conventional lithography methods self-organization processes are promising for the realization of QDs. We present a new route for the generation of quantum dots, which in contrast to epitaxial growth is a subtractive self-organized and self-ordered method. It is based on a surface instability induced by low energy and normal incidence ion bombardment of semiconductor surfaces. Uniform crystalline islands with dimensions as small as 15 nm and densities up to 10 11 cm −2 are formed by a cooperative process on the surface of semiconductors during continuous ion sputtering.

Interface formation between NPB and processed indium tin oxide

We have investigated the interface formation between ITO and N, N′-bis-(1-naphthyl)- N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), an organic materials often used as hole transport layer in organic light-emitting devices (OLED), by using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and atomic force microscopy (AFM). Acid or base treatment of indium tin oxide (ITO) surfaces can significantly alter the surface work function which, in the case of acid treatment, points to an improved energy level alignment with NPB and, therefore, enhanced hole injection efficiency. We found no significant reactions nor level bending for NPB deposited on standard ITO. In contrast, for acid-treated ITO, reaction of NPB nitrogen with the proton of the dipole layer on the ITO surface is observed. At low NPB coverages, AFM images reveal uniform island growth of NPB on ITO. Further deposition leads to a more complete covering of the ITO surface by NPB layer, corresponding to a laminar growth mode.

The self-assembly of uniform heteroepitaxial islands

We present simulation for the morphological evolution of a Stranski–Krastanow system during the annealing process. By considering the strain energy, the surface-energy anisotropy, and the film–substrate interaction, the simulation demonstrates that the morphological evolution of the system can lead to an array of nearly uniform islands that is stable against coarsening. The stability results from the suppression of strain energy relaxation by strong surface-energy anisotropy and strong film–substrate interaction.

Formation of uniform nanoscale Si islands on a Si(111)-7×7 substrate

During epitaxial growth on a substrate with a stable surface structure, rearrangement from a superlattice to a normal 1×1 lattice is essential when the substrate is covered with a growing layer. Since rearrangement needs activation energy, lateral growth is prevented at the boundary of the structure unit. In the growth of Si on a stable 7×7 structure composed of a dimer-adatom-stacking- fault layer, lateral growth of islands is prevented at the dimer row. Such a restriction on the lateral growth leads to a discontinuous size distribution of islands, whose size depends on the size of the 7×7 structure unit. In the initial stage of nucleation and growth, we have observed many rounded Si islands with uniform diameters of 3.8 nm when the substrate temperature has been held at 380 °C for 10 min after deposition of Si. From the change in the island size distribution as a function of annealing time after the deposition, we conclude that the rounded island is constructed of atoms created by the dissociation of unstable small islands (&amp;lt;40 atoms).

Formation of epitaxially strained islands by controlled annealing

A three-dimensional continuum method is developed to simulate the formation of epitaxially strained islands during controlled annealing. In the formulation, the strain energy density, surface energy and surface energy anisotropy are taken into account. Our simulations show that no surface energy anisotropy leads to spherical caps while high anisotropy leads to elongated islands. Under the present framework, a relatively uniform and regular island array can be obtained at a certain annealing time from a random surface. An almost perfectly uniform and regular island array can be obtained at a certain annealing time from a random surface with one dominant wavelength, which is in a specific range; for long-time annealing, these island arrays are undergoing ripening.

Strain modification in thin Si1−x−yGexCy alloys on (100) Si for formation of high density and uniformly sized quantum dots

The effects of alloying C with Ge and Si and varying the C/Ge ratio during the growth of very thin layers of the ternary alloy SiGeC grown on Si (100) substrates and the resulting strain modification on self-assembled and self-organized quantum dots are examined. During coherent islanded growth, where dislocations are not formed yet to relieve the strain, higher strain energy produced by greater lattice mismatch acts to reduce the island size, increase the density of islands, and significantly narrow the distribution of island sizes to nearly uniformly sized quantum dots. Strain energy can also control the critical thickness for dislocation generation within the three-dimensional islands, which then limits the maximum height which coherent islands can achieve. After the islands relax by misfit dislocations, the island sizes increase and the island size distribution becomes broader with the increase of misfit and strain. The optimal growth for a high density of uniform coherent islands occurred for the Si0.49Ge0.48C0.03 alloy composition grown on (100) Si, at a growth temperature of 600 °C, with an average thickness of 5 nm, resulting in a narrow size distribution (about 42 nm diameter) and high density (about 2×1010 dots/cm2) of quantum dots.

Influence of HCl on the chemical vapor deposition and etching of Ge islands on Si(001)

When HCl is added during the growth of Ge islands on Si(001) by chemical vapor deposition, the reduced Ge surface diffusion impedes island development. There is a shift in the relative populations of different island types even when other conditions such as temperature, coverage, and growth rate, are unchanged. The effect of HCl on the net rate of deposition is proportional to the square of the HCl partial pressure, suggesting a surface reaction with the Ge. When larger islands are etched with HCl at high enough temperature, they revert to a shape characteristic of smaller islands, confirming the reversibility of transformations from one island type to another. It has not proved possible to use etching to produce smaller and more uniform islands.

Self-organized germanium quantum dots grown by molecular beam epitaxy on Si(100)

Under proper and critical growth conditions, very uniform Ge islands could be formed on Si(100) by using molecular beam epitaxy. The atomic force microscopic and cross-sectional transmission electron microscopic observations illustrate that the size and height uniformities of the islands are about ±3%. The Auger electron spectroscopic measurements show that the wetting layer is converted into a SiGe alloy layer due to its intermixing with the substrate during the growth. A very narrow peak with the full width at half maximum (FWHM) of 1.6 meV at the energy of 0.767 eV is observed at the temperature of 16 K. We attribute this peak to the free exciton longitudinal acoustic (LA) phonon replica originated from the Ge islands. As the temperature increases, this narrow peak splits into a doublet. This `camel-like' peak is temporarily explained as the exciton-polariton luminescence.

Evolution of surface morphology in the initial stage of nitridation of the Si(111)-7×7 surface by nitrogen ions

The evolution of surface morphology in the initial stage of nitridation of Si(111)-7×7 has been investigated by using a scanning tunneling microscope (STM) and low energy electron diffraction (LEED). The STM and LEED measurements were done on the Si(111) surface nitrided under different experimental conditions including the variations in the nitrogen ion energy, nitrogen ion dose, nitridation temperature, and the postannealing temperature. A growth mechanism of the silicon nitride layer in the initial stage was proposed based upon a comparison of the surface morphology obtained under different nitridation conditions. For the growth of uniform and large silicon nitride islands, it was necessary to have proper heat treatment of the surface. In particular, the surface postannealed at 980 °C after nitridation at 950 °C produced dramatically enlarged flat silicon nitride islands compared to that postannealed at the same temperature after nitridation at room temperature, and is probably due to improved mobilities of the reacting species with elevation of the nitridation temperature.

Scanning electron microscopy of InAs aggregation on GaAs vicinal surfaces in molecular beam epitaxy

The aggregation process of InAs on GaAs(1 0 0) vicinal surfaces 0.3° inclined toward the [01¯1] direction during molecular beam epitaxy and annealing at 480°C were studied in situ by scanning electron microscopy and ex situ by atomic force microscopy. When the indium beam was supplied by 0.2 ML pulses interrupted by 90 s under constant arsenic flux, only monatomic steps were observed on the surface even up to a total supply of 2 ML InAs. After uniform and high density islands were formed by a continuous deposition of 2 ML InAs, annealing was conducted which greatly reduced the island density while the sizes of remaining islands did not increase appreciably. Obvious step advance occurred under annealing as indium atoms were supplied into the step edges from the decomposing InAs islands. It was observed in real time that as the growth of InAs was proceeded, the height to base diameter ratio of the InAs island increased before they relax or coalesce with neighboring islands.

Uniform Ag Thin Film Growth on an Sb-terminated Si(111) Surface

We report on the room-temperature-growth of highly uniform and ultrathin Ag films on Sb-terminated Si(111) surfaces, as evidenced from a scanning tunneling microscopy (STM) study in an UHV system. With predeposition of one monolayer (ML) of Sb, uniform growth of Ag islands was observed at room temperature. The Sb layer suppresses the surface diffusion of Ag atoms on Si surface and increases the Ag island density, and then the increased island density is believed to cause coalescence of Ag islands before the beginning of multilayer growth in higher coverages, resulting in the growth of atomically flat and uniform islands on the Sb surfactant layer.

Ag growth on Si(111) with an Sb surfactant investigated by scanning tunneling microscopy

We have investigated a room-temperature growth mode of ultrathin Ag films on a Si(111) surface with an Sb surfactant using STM in a UHV system. On the Sb-passivated Si surface, small sized islands were formed up to 1.1 ML. Flat Ag islands were dominant at 2.1 ML, coalescing into larger islands at 3.2 ML. Although the initial growth mode of Ag films on the Sb-terminated Si(111) surface was Volmer-Weber (island growth), the films were much more uniform than Ag growth on clean (Si(111) at the higher coverages. From the analysis of STM images of Ag films grown with and without an Sb surfactant, the uniform growth of Ag films using an Sb surfactant appears to be caused by the kinetic effects of Ag on the preadsorbed Sb layer. Our STM results indicated that Sb suppresses the surface diffusion of Ag atoms and increases the Ag-island density. The increased island density is believed to cause coalescence of Ag islands at higher coverages of Ag, resulting in the growth of atomically flat and uniform Ag islands on the Sb surfactant layer.