Stranski-Krastanov Growth Mode Research Articles

Properties of ultrathin Pb layers on the Ni3Al(1 1 1) face

The atomic structure and morphology of ultrathin Pb layers deposited on the Ni3Al(111) face in ultrahigh vacuum at the substrate temperature, ranging from 200K to 950K, were investigated with the use of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and directional elastic peak electron spectroscopy (DEPES). The analysis of AES measurements indicates that two-dimensional growth of the first Pb monolayer ‘wetting layer’ takes place for substrate temperature 200K≤T≤650K. For T=200K, lead on the Ni3Al(111) grows layer-by-layer, while for T=300K flat three atomic-layer-high islands seem to grow after the completion of the first lead monolayer. Above 350K, the Stranski–Krastanov growth mode is observed. The ordered LEED patterns corresponding to p(4×4) and p(√3×√3)R30° structures are observed. AES and LEED data indicate that lead atoms and the Ni3Al(111) face form a Pb-Ni3Al(111) surface alloy. The DEPES results show that the stacking fault abcabcBACBAC or abcabcACBACB is formed in the Ni3Al(111)–Pb interface region.

In-rich molecular beam epitaxy of InAs on Sb-terminated GaAs(001) surface

The effect of various Sb-terminated GaAs(001) templates on the growth of InAs was investigated by varying the As flux. The growth of InAs under In-rich growth conditions on Sb-terminated GaAs(001)-(2×3), (2×4) and (2×8) templates was demonstrated to occur in the Stranski-Krastanov (SK) growth mode, in contrast to the Sb-free case, where the growth process follows the Franck-van der Merwe (FM) growth mechanism. The observed antimony dependent change of growth mode from FM to SK is qualitatively explained by the decrease of the surface energy of the growing InAs layer caused by the segregation of Sb on the surface of the InAs(001).

Tunnel-injection GaN quantum dot ultraviolet light-emitting diodes

We demonstrate a GaN quantum dot ultraviolet light-emitting diode that uses tunnel injection of carriers through AlN barriers into the active region. The quantum dot heterostructure is grown by molecular beam epitaxy on AlN templates. The large lattice mismatch between GaN and AlN favors the formation of GaN quantum dots in the Stranski-Krastanov growth mode. Carrier injection by tunneling can mitigate losses incurred in hot-carrier injection in light emitting heterostructures. To achieve tunnel injection, relatively low composition AlGaN is used for n- and p-type layers to simultaneously take advantage of effective band alignment and efficient doping. The small height of the quantum dots results in short-wavelength emission and are simultaneously an effective tool to fight the reduction of oscillator strength from quantum-confined Stark effect due to polarization fields. The strong quantum confinement results in room-temperature electroluminescence peaks at 261 and 340 nm, well above the 365 nm bandgap of bulk GaN. The demonstration opens the doorway to exploit many varied features of quantum dot physics to realize high-efficiency short-wavelength light sources.

Nanoscale Assembly of Paramagnetic Organic Radicals on Au(111) Single Crystals

The successful thin-film deposition of a pyrene-substituted nitronyl nitroxide radical under controlled conditions has been demonstrated. The electronic properties, chemical environment at the interface, and morphology of the thin films have been investigated by a multitechnique approach. Spectroscopic and morphological analyses indicate a Stranski-Krastanov growth mode and weak physisorption of molecules onto the metallic surface. Electron spin resonance (ESR) spectroscopy shows that evaporation processes and deposition do not affect the paramagnetic character of the molecules. Useful concepts for the engineering of new, purely organic-based magnets, which may open the way to fruitful exploitation of organic molecular-beam deposition for assembly on solid surfaces in view of future technological applications, are presented.

Structural and Optical Investigation of GaInP Quantum Dots According to the Growth Thickness for the 700 nm Light Emitters

We investigate Ga0.33In0.67P quantum dot structures appropriate for special lighting applications in terms of structural and optical behaviors. The Ga0.33In0.67P materials form from 2-dimentional to 3-dimensional dots as the nominal growth thickness increases from 0.5 nm to 6.0 nm, indicating a Stranski-Krastanov growth mode. As the ambient temperature is increased to 300 K, the PL spectrum of the B-type dots is annihilated quickly because the large dot size induces a defect-related nonradiative recombination process. In contrast, the PL spectrum of the A-type dots is well maintained to 300 K. These data indicate that the Ga0.33In0.67P material is appropriate for an active layer of 700 nm light emitters.

Increased normal incidence photocurrent in quantum dot infrared photodetectors

We have increased the ratio of s-polarization (normal incidence) to p-polarization photocurrent to 50% in a quantum dot-in-a-well based infrared photodetector form the typical s-p polarization ratio about 20%. This improvement was achieved by engineering the dot geometry and the quantum confinement via post growth capping materials of the Stranski Krastanov growth mode quantum dots (QDs). The TEM images show that the height to base ratio of shape engineered QDs was increased to 8 nm/12 nm from the control sample's ratio 4 nm/17 nm. The dot geometry correlates with the polarized photocurrent measurements of the detector.

Reverse mass transport during capping of In0.5Ga0.5As/GaAs quantum dots

The rates of indium mass transport between the wetting layer, the quantum dots, and the capping layer are derived from the indium distributions probed by cross-sectional scanning tunneling microscopy of the In0.5Ga0.5As/GaAs quantum dot system. During capping, a lateral back-segregation from the quantum dots toward the wetting layer is found, reversing the Stranski-Krastanov growth mode during quantum dot formation. This lateral back-segregation critically affects the resulting indium distribution in the wetting layer, the apparent segregation coefficients as well as the quantum dot shape. Furthermore, the strain effect on the segregation coefficient is quantified.

Influence of group III and group V partial pressures on the size and density of InGaN quantum dots in MOVPE

AbstractInGaN quantum dots (QDs) were grown on GaN templates in the Stranski–Krastanov growth mode at 675 °C via metalorganic vapor phase epitaxy (MOVPE). Typically, QDs with overall density of 109 cm−2, an average diameter 50 nm and average height of 7 nm were obtained. A systemic investigation on the influence of group III and group V partial pressure on the size and distribution of the InGaN QDs is reported. A higher growth rate and/or higher V/III ratio increases the mean height and the density of QDs. The distribution of the dots is found to scale with growth rate and the V/III ratio. This is attributed to a reduction in the surface diffusion of adatoms as the results of increase in partial pressure of the precursor in the reactor.

Layer-by-Layer Deposition of Pd on Pt(111) Electrode: an Electron Spectroscopy–Electrochemistry Study

Ultrathin palladium films, at coverages that ranged from one half to eight monolayers, were prepared, one layer at a time, on a well-defined Pt(111) single-crystal electrode surface by surface-limited redox replacement reaction (galvanic exchange) of Cu initially coated via underpotential deposition. Adlayers produced by the galvanic-exchange method were completely free of any residual Cu after the displacement-by-Pd step. Analysis of the current-potential profiles in the hydrogen-adsorption region indicated that at one monolayer coverage, the film was essentially pure terrace, devoid of steps, which did not exhibit hydrogen absorption. At higher coverages, a Stranski–Krastanov growth mode was manifested by the emergence of a voltammetric peak characteristic of hydrogen adsorption–desorption at step sites. Peaks diagnostic of hydrogen adsorption on both terrace and steps persisted even at a Pd-film thickness of eight monolayers; the intensities were considerably diminished, however, which indicated that hydrogen absorption into the now-bulk-like film had become dominant.

New Experimental Evidences of Pt–Pd Bimetallic Nanoparticles with Core–Shell Configuration and Highly Fine-Ordered Nanostructures by High-Resolution Electron Transmission Microscopy

In our facile synthesis method, poly(vinylpyrrolidone)-protected Pt and Pt–Pd bimetallic nanoparticles with controllable polyhedral core–shell morphologies are precisely synthesized by the reduction of Pt and Pd precursors at a certain temperature in ethylene glycol with silver nitrate as structure-controlling agent. The Pt nanoparticles exhibited well-shaped polyhedral morphology with highly fine and specific nanostructures in the size range of 20 nm. Important evidences of core–shell configurations of the Pt–Pd core–shell nanoparticles were clearly characterized by high-resolution transmission electron microscopy (HRTEM) measurements. The results of HRTEM images showed that the core–shell Pt–Pd nanoparticles in the size range of 25 nm with polyhedral morphology were synthesized with the thin Pd shells of ∼3 nm in thickness as the atomic Pd layers grown on the Pt cores. Very interesting characteristics of surface structure of Pt nanostructures and Pt–Pd core–shell nanostructures with surface defects were observed. The high-resolution TEM images of Pt–Pd bimetallic nanoparticles showed that the Frank–van der Merwe and Stranski–Krastanov growth modes coexist in the nucleation and growth of the Pd shells on the as-prepared Pt cores. It is predicted that the FM growth becomes the main favorable growth compared with the SK growth in the formation of the thin Pd shells of Pt–Pd core–shell nanoparticles. The experimental evidence of the deformations of lattice fringes and lattice-fringe patterns was found in Pt and Pt–Pd core–shell nanoparticles. The interesting renucleation and recrystallization at the attachments between the nanoparticles are revealed to form a good lattice match. In addition, our novel ideas of the largest surface-area superlattices and promising utilization of them are proposed for next generations of various fuel cells with low cost. Finally, the products of Pt–Pd core–shell nanoparticles can be potentially utilized as highly efficient catalysts in the realization of polymer electrolyte membrane fuel cell and direct methanol fuel cell using the very low Pt loading with better cost-effective design.

Investigation of Wetting Layers in InAs/GaAs Self-Assembled Nanostructures with Reflectance Difference Spectroscopy

As a system with both profound physics and promising application potentials, the strain-induced self-assembled semiconductor nanostructures have been investigated for decades of years. The optical and electrical properties of this system are mainly determined by the nanostructures, but can be greatly affected by the generally existent two dimensional structures from which the nanostructures evolve, the so called wetting layer (WL). The WL configurations can be varied with different growth conditions, and further influence the nanostructure morphology and properties. This is a reviewing article introducing some recent progresses in investigating the evolution of WLs in InAs/GaAs system with reflectance difference spectroscopy. Different kinds of WL evolution processes in Stranski-Krastanov growth mode are introduced. The segregation and desorption of indium atoms and the effect of growth interruption are mentioned. The existence and the evolution process of wetting layers in droplet epitaxy method are also discussed.

Surface evolution of NaCl-type cubic AlN films on MgO (100) substrates deposited by laser molecular beam epitaxy

Highly oriented NaCl-type cubic AlN (c-AlN) films were successfully deposited on MgO (100) substrates by laser molecular beam epitaxy (LMBE) technique, and the time-dependent evolution of film surface was characterized. Based on the topography images and roughness analysis, c-AlN surfaces display the Stranski–Krastanov growth mode. In the initial stage, AlN adatoms prefer to settle along the substrate steps and the surface appears to be slightly rough. The following deposition causes an undulate surface, and the nucleation and early growth of 3D islands are observed. During the islands growth, a rapid increase of roughness occurs, and large and deep gaps on the top of islands are striking. A smooth surface with a roughness of 1.06nm is obtained at the growth time of 2h owing to the spread and coalescence of islands.

An Explanation for Bends of 1-Dimensional Nanorods

Abstract Growth of artificial pinning centers (APCs) in YBa2Cu3O7−J (YBCO) films are discussed. The APCs used in this research are BaZrO3 and BaSnO3 nanorods which are reported by Mele et al. TEM images show these nanorods graduallybend accordinglywith approachingtoasurfaceof films. This featureshowedagrowth patternlikeafirework. Weexplainthe featureofthe nanorodsas follows;Atanearlygrowth stage, filmsgrowinalayerbylayergrowth mode. The surface of the films is flat and very smooth. After the early growth stage, the film surface gradually becomes rough, indicating the film grows in a Stranski-Krastanov growth mode. This roughness was caused by a spiral growth of films with manysteps. At the step of YBCO films, nanorod materials such as BaZrO3 are supplied from one direction. Then, the center of nanorods sifts to the same direction of the step flow. Then, the nanorods bend to the edge of the grains in the films. As a grain in spiral growth had a convex surface, nanorods bent to the direction perpendicular to the grain surface. Finally, nanorods in rough grains form firework structures.

Influences of lattice mismatches on equilibrium morphologies and strain distributions of quantum dots

The morphologies of quantum dots and distributions of stresses in and around quantum dots structures have a significant effect on photoelectric properties and electronic structures. Optical and electronic devices of different efficiencies based on quantum dots can be manufactured by choosing self-assembly of different materials to control epitaxial growth. In this article we investigate the equilibrium morphologies and the strain distributions of self-assembled pyramidal semiconductor quantum dots in Stranski-Krastanov growth mode based on the finite element method of the anisotropic theory of elasticity. We also give the equilibrium morphologies and the distribution of the stress and the strain, the hydrostatic strain and the biaxial strain for different lattice mismatched quantum dots. The results can serve as a basis for interpretation of experiments.

Biaxial stress‐induced giant bandgap shift in BiFeO3 epitaxial films

AbstractThickness‐dependent stress relaxation and its influence on the bandgap of BiFeO3 have been investigated. BiFeO3 films with 15 at% Bi excess allow the control of lattice mismatch‐induced biaxial stress up to 10.8 GPa on SrTiO3(001) substrates. Heteroepitaxy of such films follows the Stranski–Krastanov growth mode, as suggested by island formation on the wetting layer accompanied with stress relaxation. The bandgap is linearly decreased with the in‐plane compressive stress, and a large stress coefficient of 67 meV/GPa was extracted. A 0.7 eV red‐shift of the bandgap was observed in the fully epitaxially strained film. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of Ge quantum dots doped TiO 2 films with high optical absorption properties via layer-by-layer ion-beam sputtering

Ge quantum dots (QDs)-doped TiO 2 films were prepared by ion-beam sputtering. TEM results showed Ge QDs were in a uniform size distribution and high density. XPS indicated that Ge incorporated in the TiO 2 films was in elemental form. The Stranski–Krastanov growth mode was accounted for the growth of Ge QDs in TiO 2 films by AFM analysis. Optical absorption spectra exhibited the optical response of the Ge QDs-doped TiO 2 films was shifted from UV to the near infrared region, suggesting a good optical absorption property.

Epitaxial growth of atomically flat gadolinia-doped ceria thin films by pulsed laser deposition

Epitaxial growth of Ce0.8Gd0.2O2(CGO) films on (001) TiO2-terminated SrTiO3 substrates by pulsed laser deposition was investigated using in situ reflective high energy electron diffraction. The initial film growth shows a Stransky–Krastanov growth mode. However, this three-dimensional island formation is replaced by a two-dimensional island nucleation during further deposition, which results in atomically smooth CGO films. The obtained high-quality CGO films may be attractive for the electrolyte of solid-oxide fuel cells operating at low temperature.

Growth mechanism of InGaN quantum dots during metalorganic vapor phase epitaxy

Growth mechanism of InGaN quantum dots on GaN was studied during metalorganic vapor phase epitaxy in a temperature range of 600–750°C in a horizontal reactor. Atomic force microscopy and high resolution X-ray diffraction show that InGaN quantum dots on GaN are formed according to the Stranski–Krastanov growth mode. When growth temperature was varied between 600 and 750°C, a 2–3-dimensional growth transition was observed below 650°C due to the increase in indium content. While the layer thickness was varied, similar transitions were observed with critical thicknesses of ∼2.7nm and ∼4nm at growth temperatures of 625°C (In0.31Ga0.69N) and 675°C (In0.22Ga0.78N), respectively. InGaN quantum dots with a density of ∼1010cm−2, an average diameter of ∼40nm and average height of ∼4nm were obtained. Reciprocal space maps from the asymmetric (10–15) reflection show that all the InGaN samples are fully strained. Strong green emission at 534nm was observed from InGaN grown at 675°C.

Structural analysis of site-controlled InAs/InP quantum dots

We present atomic-scale characterization of site-controlled InAs/InP(001) quantum dots grown by metal-organic chemical vapor deposition using nano-area selective area growth. We have developed for this purpose a process combining e-beam lithography, inductively coupled-plasma etching and focused ion beam etching to isolate a few quantum dots. The size, the shape and the composition of the quantum dots are investigated by Scanning Transmission Electron Microscopy. A comparison with the well-known single self-assembled quantum dots highlights the specificities of our growth mode compared to the Stranski–Krastanov growth mode.

The effect of hard nitridation on Al 2O 3 using a radio frequency operated plasma cell

We report on an atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) investigation of hard nitridation of sapphire (α-Al 2O 3) substrate, using an Epi UNI-Bulb RF plasma cell at substrate temperatures ranging from 250 to 600 °C. Our results show that an AlN 1- xO x layer forms on sapphire after extended nitridation at all temperatures, following a Stranski–Krastanov growth mode, with less islands forming at higher temperatures. We also observe a layer-dependent charging shift in XPS, separating smooth AlN 1- xO x layers from rough AlN 1- xO x islands due to their different electronic coupling to the substrate. Although the island growth is suppressed at higher temperatures, the surface roughness increases at higher temperatures as seen by AFM. We also observe sputtering effects with protrusions and pits.