Growth Mode Changes Research Articles

High resolution electron energy loss spectroscopy study of Zinc phthalocyanine and tetrafluoro tetracyanoquinodimethane on Au (1 1 1)

Using high resolution electron energy loss spectroscopy, we have examined the vibrational and electronic information of Zinc phthalocyanine (ZnPc) and tetrafluoro tetracyanoquinodimethane (F4-TCNQ) on Au (1 1 1). The changes in the vibrational loss features reveal distinct changes in growth modes at different film thickness as well as occurrence of charge transfer between adsorbate and substrate. Up to a thickness of 10 monolayers, ZnPc film grows with its molecular plane parallel to the substrate. After the introduction of small quantities of F4-TCNQ on ZnPc film, evidence of p-doping can be seen from the red shift of C N stretching in F4-TCNQ.

Electrocrystallization of Calcium Phosphates

AbstractThe interest in electrocrystallization of calcium phosphates (CaP) in general, and of hydroxyapatite (HAp) specifically, results from the promising industrial benefits, unique microstructures, and properties of the deposits produced by this process, and the possible similarity to bone mineralization in vivo. In this paper, our work on electrocrystallization of HAp and octacalcium phosphate (OCP) on CP‐Ti and Ti‐6A1‐4V alloy, in solution containing calcium nitrate and ammonium dihydrogen phosphate, is briefly reviewed. The early stages of nucleation and growth are characterized in real time by means such as electrochemical quartz crystal microbalance (EQCM) and electrochemical atomic force microscope (EC‐AFM). The change in growth mode and the role of precursors to HAp are explained. The role of charge transfer and local increase in pH is discussed.

Heteroepitaxial growth modes with dislocations in a two-dimensional elastic lattice model

We study equilibrium shapes of adsorbate crystals by allowing a possibility of dislocations on an elastic substrate in a two-dimensional lattice model. The ground state energy is calculated numerically with the use of an elastic lattice Green’s function. From the equilibrium shapes determined for various coverages, we infer the growth mode. As the misfit parameter increases, the growth mode changes from the Frank–van der Merwe (FM) to the Stranski-Krastanov (SK), further to the FM with dislocations for a parameter range of ordinary semiconductor materials. Conceivable growth modes such as the SK with dislocations appear in a parameter range between the SK and the FM with dislocations.

Growth and Structural Properties of Indium doped SrTiO<SUB>3</SUB> Films by Pulsed Laser Deposition

Undoped and In-doped SrTiO_3(STO) films were grown on MgO/TiN/Si(100) substrates by pulsed laser deposition(PLD).The growth mechanism,crystallinity,surface morphology,and UV- Raman spectra of the films were studied.Results indicate that undoped STO films show high quality crystalline structure with highly (200) orientation.With Indium doping,the crystallinity of the STO film deteriorate,the first order Raman peaks increase indicating the breaking of crystal symmetry,and the film growth mode change from the layer-by-layer mode to island-layer mixed one,resulting in the roughening of the film surface.Furthermore,the crystallinity and (200) orientation of In-doped STO film can be enhanced significantly by introducing an undoped STO buffer layer.

Improvements in the crystalline quality of MOVPE-InN layers by facet controlling with hydrogen partial pressure

Facet control of InN was performed for the improvement of the crystalline quality of InN layers on GaAs (1 1 1)B by metalorganic vapor phase epitaxy (MOVPE). Growth mode of InN could be controlled by changing the input mole ratio of hydrogen in the carrier gas and growth temperature. When the input mole ratio of hydrogen in the carrier gas was 0 (N 2 ambient) and growth temperature was low, InN with a flat surface could be obtained. On the contrary, when a small amount of hydrogen was contained in the carrier gas (0.4% of H 2), InN islands having a (1 0 1¯ 2) facet were grown. It was also found that the crystalline quality of InN could be improved by employing a growth mode change during epitaxial growth from three dimensional (3D) to two dimensional (2D), indicating a reduction in dislocation density.

Temperature dependence of microstructure and strain evolution in strained ZnO films onAl2O3(0001)

We have studied the temperature dependence of the growth modeand microstructure evolution in highly mismatched sputter-grownZnO/Al2O3(0001) heteroepitaxial films. The growth mode was studied by real-time synchrotronx-ray scattering. We find that the growth mode changes from a two-dimensional(2D) layer to a 3D island in the early growth stage with temperature(300–600 °C), in sharp contrast to the reported transition from three dimensionsto two dimensions in metal–organic vapor phase epitaxy. At around400 °C intermediate 2D platelets nucleate in the early stage, which act as nucleation cores of 3D islandsand transform to a misaligned state during further growth. Meanwhile, at high temperature (above500 °C), the spinelstructure of ZnAl2O4 grows in the early stage, and it undergoes a transition to wurtzite-ZnO (w-ZnO) withthickness. The spinel formation is presumably driven by high temperature and largeincident energy of impacting atoms during sputtering. The results of the strain evolution asfunctions of temperature and thickness during growth suggest that the surfacediffusion is a major factor determining the microstructural properties in the strainedZnO/Al2O3(0001) heteroepitaxy.

Influence of Surface Hydroxylation on 3-Aminopropyltriethoxysilane Growth Mode during Chemical Functionalization of GaN Surfaces: An Angle-Resolved X-ray Photoelectron Spectroscopy Study

A comparative study of the chemical functionalization of undoped, n- and p-type GaN layers grown on sapphire substrates by metal-organic chemical vapor deposition was carried out. Both types of samples were chemically functionalized with 3-aminopropyltriethoxysilane (APTES) using a well-established silane-based approach for functionalizing hydroxylated surfaces. The untreated surfaces as well as those modified by hydroxylation and APTES deposition were analyzed using angle-resolved X-ray photoelectron spectroscopy. Strong differences were found between the APTES growth modes on n- and p-GaN surfaces that can be associated with the number of available hydroxyl groups on the GaN surface of each sample. Depending on the density of surface hydroxyl groups, different mechanisms of APTES attachment to the GaN surface take place in such a way that the APTES growth mode changes from a monolayer to a multilayer growth mode when the number of surface hydroxyl groups is decreased. Specifically, a monolayer growth mode with a surface coverage of approximately 78% was found on p-GaN, whereas the formation of a dense film, approximately 3 monolayers thick, was observed on n-GaN.

Carbon nanotube growth mechanism switches from tip- to base-growth with decreasing catalyst particle size

The growth of carbon nanotubes by a plasma assisted catalytic chemical vapor deposition was investigated using cobalt, nickel and iron catalyst particles of different sizes. For the three catalysts examined, it was shown that the growth mode switches from “tip-growth” for large particles (>>5 nm) to “base-growth” for smaller ones (<5 nm). While single-walled nanotubes and those with few walls (typically <7 walls) grow from their base, larger multi-walled nanotubes are fed with carbon via their tips which support the catalyst particle. A growth scenario involving two different pathways for carbon diffusion is proposed in order to explain the change in growth mode.

Insights into the growth mechanism of In xGa 1−xN epitaxial nanostructures formed using a silane predose

In x Ga 1− x N/GaN quantum dot (QD) arrays with high spatial densities have the potential to improve the performance of GaN-based optoelectronic devices. The growth of such QD arrays may be achieved in metal–organic vapour phase epitaxy by pre-dosing the GaN surface with a silicon-bearing precursor. Here, we investigate the mechanism of QD formation in this case, and suggest that for short SiH 4 predoses, the variation of island density with predose duration, and the merging of the observed islands with increasing In x Ga 1− x N growth time, indicate that the SiH 4 is roughening the GaN surface, increasing the number of available nucleation sites, and leading to a change in growth mode. For longer SiH 4 predoses, a different mechanism appears to operate. However, we find no evidence for the presence of an SiN x nanomask, but instead note that the observed islands have a distinctive and consistent shape, indicating the influence of strain and surface energy contributions on the growth mode.

Adjustment of osmotic pressure coupled with change of growth mode in Spirogyra.

Spirogyra living in running water forms a rhizoid which anchors it to the substratum. Rhizoid differentiation can be induced in the laboratory by severing algal filaments. The terminal cell changes the growth mode from diffuse growth to tip growth, and finally differentiates to be a rhizoid. We found that the intracellular osmolarity of the rhizoid was significantly lower than that of other interjacent cells which did not form rhizoids. The decrease in the intracellular osmolarity began before the start of tip growth. TEA, a K+ channel blocker, inhibited the decrease in the intracellular osmolarity of the terminal cells; increase in the external K+ also inhibited this. It was suggested that K+ efflux through K+ channel is involved in the adjustment of osmotic pressure. When the adjustment of osmotic pressure was inhibited, tip growth did not start, inevitably, no rhizoid was formed. In Spirogyra sp. which was unable to form rhizoids, the terminal cell did not show the adjustment of osmotic pressure. Thus, this adjustment seems to be intimately coupled with the rhizoid differentiation. Possible roles of the adjustment of osmotic pressure in rhizoid differentiation are discussed.

Very Low Temperature Epitaxial Growth of Silicon Films for Solar Cells

Low-temperature n-type silicon epitaxial growth has been studied for the application to n+/p homojunction solar cells. Epitaxial Si films can be grown at the substrate temperatures from room temperature (RT) to 200 °C by plasma-enhanced chemical vapor deposition. Two-step growth is effective for obtaining high-quality layers above 150 °C. In this growth, a very thin Si layer has been deposited on Si substrates at a low hydrogen-to-silane gas-flow ratio, and subsequently an epitaxial Si layer has been grown at a higher ratio. Smooth nucleation with reduced local stress has been enhanced in the initial stage of the epitaxial growth. An open-circuit voltage, Voc, of 0.610 V and a conversion efficiency, η, of 13.54% have been obtained for an n+/p cell, where the n+ layer was deposited at 200 °C. Film growth mode changes below 100 °C. The stress in the film during the epitaxial growth is relieved by introducing a small fluctuation of crystal orientation and the voids in the film at these temperatures. We have succeeded in fabricating a solar cell with a Voc of 0.608 V and an η of 13.52% having an n+ layer deposited at RT.

Evolution of strain and composition during growth and capping of Ge quantum dots withdifferent morphologies

We follow the growth of islands with different shapes by monitoring the strain relaxationby reflection high energy electron diffraction (RHEED). Comparing a bimodal ensemble ofpyramids and domes with a monomodal distribution of C-induced domes, we observedifferent relaxation pathways and a growth mode change from Stranski–Krastanow toVolmer–Weber. We also study the changes induced by the capping process with Si.Small strains in thin cap layers are revealed by spectroscopic ellipsometry. Ramanspectroscopy is employed to probe the built-in strain and silicon intermixing indifferent types of islands, evidencing that smaller islands are enriched in Si andeffectively recompressed, whereas bigger relaxed dots remain substantially unaffected.

Effects of compressive and tensile stress on the growth mode of epitaxial oxide films

Oxide films were deposited on different substrates by laser molecular beam epitaxy. Reflection high-energy electron diffraction was performed to in situ investigate the change of growth mode and the lattice relaxation during the growth. An asymmetrical phenomenon was found in the two kinds of strain states, compressive stress and tensile stress of heterostructures with different lattice mismatch. In the case of BaTiO 3/SrTiO 3 (2.2%), 2D layer-by-layer growth mode without lattice relaxation can be maintained for a longer period for BTO films on STO with compressive stress, comparing to STO films on BTO with tensile stress. When MgO films were deposited on SrTiO 3 with a large mismatch of 7.8%, compressive stress leads to rapid lattice relaxation with a very thin wet layer, and 3D strained island were observed. As a comparison, SrTiO 3 films on MgO with tensile stress were configured. No RHEED patterns can be observed duo to a large tensile stress.

The effect of cation:anion ratio in solution on the mechanism of barite growth at constant supersaturation: Role of the desolvation process on the growth kinetics

The mechanism of barite growth has been investigated in a fluid cell of an Atomic Force Microscope by passing solutions of constant supersaturation ( Ω) but variable ion activity ratio ( r = a Ba / a SO 4 ) over a barite substrate.The observed dependence of step-spreading velocity on solution stoichiometry can be explained by considering non-equivalent attachment frequency factors for the cation and anion. We show that the potential for two-dimensional nucleation changes under a constant thermodynamic driving force due to the kinetics of barium integration into the surface, and that the growth mode changes from preexisting step advancement to island spreading as the cation/anion activity ratio increases. Scanning electron microscopy studies of crystals grown in bulk solutions support our findings that matching the ion ratio in the fluid to that of the crystal lattice does not result in maximum growth and nucleation rates. Significantly more rapid rates correspond to solution stoichiometries where [Ba 2+] is in excess with respect to [ SO 4 2 - ]. Experiments performed in dilute aqueous solutions of methanol show that even 0.02 molar fraction of organic cosolvent in the growth solution significantly accelerates step growth velocity and nucleation rates (while keeping Ω the same as in the reference solution in water). Our observations suggest that the effect of methanol on barite growth results first of all from reduction of the barrier that prevents the Ba 2+ from reaching the surface and corroborate the hypothesis that desolvation of the cation and of the surface is the rate limiting kinetic process for two-dimensional nucleation and for crystal growth.

Stress dependence of growth mode change of epitaxial layered cobaltite γ-Na 0.7CoO 2

We report stress dependence of growth characteristics of epitaxial γ-Na 0.7CoO 2 films on various substrates deposited by pulsed laser deposition method. On the sapphire substrate, the γ-Na 0.7CoO 2 thin film exhibits spiral surface growth with multi-terraces and highly crystallized texture. For the γ-Na 0.7CoO 2 thin film grown on the (1 1 1) SrTiO 3 substrate, the nano-islands of ∼30 nm diameter on the hexagonal grains are observed. These islands indicate that the growth mode changes from step-flow growth mode to Stranski–Krastanow (SK) growth mode. On the (1 1 1) MgO substrate, the large grains formed by excess adatoms covering an aperture between hexagonal grains are observed. These experimental demonstrations and controllability could provide opportunities of strain effects of Na x CoO 2, physical properties of thin films, and growth dynamics of heterogeneous epitaxial thin films.

Effect of Reactor Pressure on Qualities of GaN Layers Grown byHydride Vapour Phase Epitaxy

The influence of reactor pressure on GaN layers grown by hydride vapourphase epitaxy (HVPE) is investigated. By decreasing the reactorpressure from 0.7 to 0.5 atm, the GaN layer growth mode changes fromthe island-like one to the step flow. The improvements in structuraland optical properties and surface morphology of GaN layers areobserved in the step flow growth mode. The results clearly indicatethat the reactor pressure, similarly to the growth temperature, is oneof the important parameters to influence the qualities of GaN epilayersgrown by HVPE, due to the change of growth mode.

Temperature Dependence of Electrochemical Deposition of CdSe

The temperature dependence of the growth of cadmium selenide (CdSe) films prepared by galvanostatic electrodeposition in an aqueous acidic bath was examined. We observed a growth mode change at around , below which the growth rate is reaction rate limited and above which it is transport limited. Growth below deviates from conventional laws of electrolysis and results in Se-rich “cauliflower” films. At increased temperatures, the films show irregular growth but improved stoichiometry.

Synchrotron radiation photoemission studies on Fe/ZnO(0001) interface

The growth, interfacial reaction, and electronic structure of Fe on ZnO(0001) surface were studied using synchrotron radiation photoemission spectroscopy(SRPES). It was found that Fe layer grows in a way similar to the Stranski-Krastanov mode at room temperature. After～2? Fe deposited on the surface, the growth mode changes from the layer-by-layer to a mixed mode. At low coverage, about one monolayer of iron (ca. 1.5?) is oxidized by the outer oxygen atoms on ZnO(0001) surface. Analysis of the valence band and the Fe3p photoemission spectra indicated that below 2? coverage, Fe2+ species exists at the Fe/ZnO interface, followed by the formation of a metallic iron overlayer. A sharp Fermi edge appears at 5.1? coverage, indicating the formation of the bulk metallic Fe film. Furthermore, work-function of the sample decreases to a minimum of 4.5eV at 0.2? Fe, then goes to 4.9eV with the formation of a dipole layer at the interface.

Growth mode in strained ZnO films on Al2O3(0001) during sputtering

We investigated the temperature dependence of growth mode in highly mismatched sputter-grown ZnO/Al2O3(0001) heteroepitaxial films using real-time synchrotron X-ray scattering. We find that the growth mode changes from 2 dimensional (2D) layer to 3D island in early growth stage with temperature (300∘C–500∘C). At around 400∘C, however, intermediate 2D platelets nucleate in early stage, act as nucleation cores of 3D islands and transform to misaligned state during further growth. The results of the strain evolution during growth suggest that the surface diffusion is a major factor in determining the growth mode in the strained ZnO/Al2O3(0001) heteroepitaxy.

Study on interfacial strain behavior of functional oxide heterostructures

Oxide films of BaTiO3 and MgO were deposited on SrTiO3 single crystal substrates by laser molecular beam epitaxy. Reflection high-energy electron diffraction was performed in situ to investigate the change of growth mode and the lattice relaxation. Due to the different mismatches between epitaxial oxide films and substrates, two kinds of strain relaxation behavior during the growth process can be observed. For the BaTiO3∕SrTiO3 system with the small mismatch of 2.18%, the coherent epitaxial growth can be maintained within the critical thickness. The experimental data were well in accord with the elastic strain theory in the small mismatch system. The coherent epitaxial growth is crucial to fabricate the functional oxide superlattice. However, strained island can form at the initial period to release strain energy in the case of MgO∕SrTiO3 system with the large mismatch of 7.8%. It provides a useful practical method to realize self-organized nanostructures by introducing the moderate strain at interface. Our systematic study on the interface strain of functional oxide heteroepitaxial growth could give an instructive method to realize different heterostructures, e.g., superlattice and nanostructures.