Scanning tunnelling microscopy studies of growth morphology in highly epitaxial c-axis oriented Pt thin film on (0 0 1) SrTiO3

A noble metal Pt thin film was successfully grown on (0 0 1) SrTiO₃ substrate by using a DC-sputtering technique. The surface morphology and growth features of the as-grown Pt films were investigated by scanning tunnelling microscopy. Growth conditions, such as pre-sputtering, deposition ambience, and oxygen ratio are found to greatly affect the orientation, the crystallinity, and the epitaxial behavior of Pt films on (0 0 1) SrTiO₃. Single-crystalline Pt films have been achieved by introducing a few percentage oxygen into the sputtering ambient. The in-plane-relationship of the c-axis oriented Pt thin films on (0 0 1) SrTiO₃ was determined to be (0 0 1)_Pt∥(0 0 1)_SrTiO3 and [0 0 1]_Pt∥[0 0 1]_SrTiO3. Oxygen in the sputtering ambient was found to be a key factor to achieve the epitaxial Pt films.     

Epitaxial structures of self-organized, standing-up pentacene thin films studied by LEEM and STM

Growth of pentacene (Pn) thin films has been studied in situ by means of low-energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). A very low nucleation density of Pn grains has been observed on Bi(0 0 0 1)/Si(1 1 1) template, resulting in formation of large, monolayer-high Pn grains with diameter exceeding several hundreds of micrometers. We determined that formation of self-organized, standing-up Pn epitaxial layers was stabilized by a weak interaction between the substrate and Pn molecules and by the presence of the commensurate structure between the oblique Pn lattice and trigonal substrate surface lattice. The ‘point-on-line’ commensurability has been found along a-axis of Pn and one of the primitive vectors of substrate surface lattice. Strong ‘point-on-line’ commensurability in Pn/Bi(0 0 0 1)/Si(1 1 1) system resulted in a bulk-like epitaxial thin film growth, starting from the first layer. The presence of twins, often having a mirror line parallel to the direction of the ‘point-on-line’ matching, has been also detected using an asymmetric dark-field imaging mode in LEEM experiments, which, we believe, is the first LEEM demonstration of molecular tilt imaging.     

Morphological anisotropy during migration enhanced epitaxy of GaAs(0 0 1)

The GaAs(0 0 1) surface morphology and structure during growth by migration enhanced epitaxy (MEE) has been studied by reflection high energy electron diffraction and scanning tunneling microscopy. Changes induced by varying the incident As/Ga flux ratio, growth temperature and the total amount of material deposited in each cycle have been studied and the results compared with GaAs(0 0 1) growth by conventional molecular beam epitaxy (MBE). Comparison of the surface morphology at the end of the Ga and As cycles indicates no clear evidence for any enhancement in the Ga adatom diffusion length during the Ga cycle. However, the morphological anisotropy of the growth front does change significantly and it is proposed that this changing anisotropy during MEE enables Ga adatom diffusion along both azimuths. The surface anisotropy during MEE growth is found to increase with the Ga/As ratio. Although there is a clear correlation between composition and morphology, we have also found that highly ordered and flat surfaces are not necessarily an indication of stoichiometric material. We also attempt to clarify a recent controversy on the structure of the c(4 × 4) reconstruction by studying the surface structure at the end of the As cycle as a function of the As/Ga ratio.     

Computer simulations of the Stranski-Krastanov growth of heteroepitaxial films with elastic anisotropy

A three-dimensional finite element method is developed to simulate the surface morphological evolution during the Stranski-Krastanov heteroepitaxial growth. In the formulation, the surface evolves through surface diffusion driven by the gradient of the surface chemical potential, which includes the elastic strain energy, elastic anisotropy and surface energy. Surface condensation rate is assumed to depend on the difference between the surface chemical potential and the chemical potential of the vapor phase. Our simulations reveal that the self-assembly of quantum dots are strongly dependent on the variation of growth rate and elastic anisotropy strength. With appropriate choice of growth rate and elastic anisotropy strength, a relatively more uniform and regular quantum dot array can be obtained.     

The role of antiphase boundaries during ion sputtering and solid phase epitaxy of Si(0 0 1)

The Si(0 0 1) surface morphology during ion sputtering at elevated temperatures and solid phase epitaxy (SPE) following ion sputtering at room temperature has been investigated using scanning tunneling microscopy. Two types of antiphase boundaries form on Si(0 0 1) surfaces during ion sputtering and SPE. One type of antiphase boundary, the AP2 antiphase boundary, contributes to the surface roughening. AP2 antiphase boundaries are stable up to 700 °C, and ion sputtering and SPE performed at 700 °C result in atomically flat Si(0 0 1) surfaces.     

Epitaxial C60 thin films on Bi(0 0 0 1)

We have used the Bi(0 0 0 1)/Si(1 1 1) template to grow highly ordered C₆₀ epitaxial thin films and analyzed them using scanning tunneling microscopy and low-energy electron microscopy. The in situ low-energy electron microscope investigations show that the initial nucleation of the C₆₀ islands on the surface takes place at surface defects, such as domain boundaries and multiple steps. The in-plane lattice parameters of this C₆₀ film turns out to be the same as that of the bulk fcc(1 1 1) C₆₀. The line-on-line epitaxial structure is realized in spite of a weak interaction between the C₆₀ molecules and Bi(0 0 0 1) surface, while scanning tunneling spectroscopy indicates that there is a negligible charge transfer between the molecules and the surface.     

Epitaxial growth of ultra-thin NiO films on Cu(1 1 1)

The very first stages of the growth of NiO on Cu(1 1 1) is examined on a microscopic scale. The paper focuses on the morphological and structural characterization of nanostructures formed in the 0-1 Å thickness range. Ultra-thin NiO films, obtained through evaporation of a Ni rod under an oxygen atmosphere were grown at 550 K. In the early stages of the growth the oxide film morphology shows 10-30 nm large, monolayer high, islands with a partial incorporation of metallic Ni in the first Cu(1 1 1) surface plane. The first layer is formed by an epitaxial atomic layer exhibiting a STM contrast similar to the one observed on adsorbed oxygen on Cu(1 1 0). A NiO cluster nucleation and coalescence mechanism is proposed in order to explain the formation of the second NiO layer. A α-Ni₂O₃ hexagonal phase, or a structural distortion of the NiO(1 1 1)()R30° structure could both explain the complex LEED patterns.     

The dense α-√3 × √3Pb/Si(1 1 1) phase: A comprehensive STM and SPA-LEED study of ordering, phase transitions and interactions

The T-θ phase diagram for the system Pb/Si(1 1 1) was determined in the coverage range 6/5 ML < θ < 4/3 ML from complementary STM and SPA-LEED experiments. This coverage is within the range where a “Devil’s Staircase” (DS) has been realized. The numerous DS phases answer conflicting information in the Pb/Si(1 1 1) literature and update the previously published phase diagram. The measurements reveal the thermal stability of the different linear DS phases with the transition temperature found to be a function of phase period. Because of additional complexity in the experimental system (i.e. two-dimensionality and 3-fold symmetry) the linear DS phases transform at higher temperature into commensurate phases of 3-fold symmetry HIC (historically named “hexagonal incommensurate phase”). Different types of HIC phases have been discovered differing in the size of the supercell built out of √3 × √3 domains separated by domain walls of the √7 × √3 phase. The detailed structures of these HIC phases (coverage, binding site, twist angle, etc.) have been deduced from the comparison of STM images and diffraction patterns. After heating the system to even higher temperature the HIC phase transforms into the disordered phase. For sufficiently high coverage a SIC (“striped incommensurate phase” which is also built from √3 × √3 domains but meandering √7 × √3 domain walls) is observed which also disorders at high temperatures.     

Characterization of ZrB2(0 0 0 1) surface prepared by ex situ HF solution treatment toward applications as a substrate for GaN growth

ZrB₂(0 0 0 1) crystals grown by the rf-floating zone technique were characterized by X-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and atomic force microscopy. These characteristics were investigated as a function of thermal cleaning temperature up to 1000 °C in vacuum for as-received substrates as well as substrates treated ex situ in HF aqueous solution. The HF treatment process removed the ZrO₂ native oxide layer present on as-received substrates and resulted in ZrB₂(0 0 0 1) surfaces exhibiting long-range order. Upon annealing the HF-treated surface in high vacuum, two types of reconstructions were observed: an incommensurate reconstruction from 650 to 900 °C related to residual H₂ gas, and n × n reconstructions at 1000 °C, possibly related to oxygen.     

Electrodeposited quantum dots IV. Epitaxial short-range order in amorphous semiconductor nanostructures

The structure of diffraction-amorphous CdSe (a-CdSe) quantum dots (QDs) electrodeposited on evaporated Pd substrate was studied by high resolution transmission electron microscopy (HRTEM), and compared with epitaxial (crystalline) QDs obtained by the same procedure on Au, as well as with a simulated image of random a-CdSe. Digital analysis of HRTEM images established the existence of repeating ordered motifs in a-CdSe QDs on Pd substrates, in the form of epitaxial sub-nanometre to nanometre size clusters. The QDs are shown to be intermediate between crystalline and random amorphous material. Digital Fourier analysis indicated epitaxial relationship with the 111inPd substrate, rotated 30° relative to the orientational relationship on 111_Au.