Ex-situ Atomic Force Microscopy Research Articles

Characterization of Semiconductor Surfaces during Surface Conditioning and Functionalization

Semiconductor surface cleaning, passivation and functionalization are critical steps for applications such as microelectronics, MEMS, sensors, and solar energy harvesting. Progress has relied in part on the development of surface chemical treatments. However, the ability to characterize surfaces at all steps of processing has also been essential and will be emphasized here. This work focuses on the chemical modification of oxide-free, hydrogen-passivated silicon, including wet chemical treatments, atomic layer deposition, and physical deposition. Characterization methods include in-situ infrared absorption spectroscopy, low energy ion scattering, x-ray photoelectron spectroscopy and mass spectrometry, and is complemented by ex-situ spectroscopic ellipsometry and atomic force microscopy.

Growth of single-phase Mg 0.3Zn 0.7O films suitable for solar-blind optical devices on RS-MgO substrates

Single-phase rock-salt Mg 0.3Zn 0.7O film was fabricated on MgO (100) substrate by radio-frequency plasma assisted molecular beam epitaxy. A smooth surface was observed by in-situ reflection high-energy electron diffraction and ex-situ atomic force microscopy. X-ray diffraction characterization demonstrated a high-quality single-phase structure with highly (200) orientation and cube-on-cube epitaxial relationship. Zn fraction in the single-phase rock-salt Mg 0.3Zn 0.7O film was determined by Rutherford backscattering spectrometry. Optical property of the film was investigated by reflectance spectroscopy, which indicated a solar-blind band gap of 255.5 nm. The reason why Zn solubility limit is greatly enhanced in non-polar (100) film compared with (111) polar epilayer is tentatively discussed in this work, suggesting MgO (100) is more suitable for the synthesis of single-phase rock-salt MgZnO with high Zn content towards solar-blind opto-device applications.

Dealloying Studies of Cu3Pd Single Crystal Surfaces

Nano-porous metals formed by dealloying have application potential in a variety of fields such as catalysts, actuators, biomedical sensors and fuel cells. Several aspects of formation of porous surface during dealloying have not completely been understood. The Cu-Pd system is interesting as a model system for corrosion as well as a potential catalyst material. The initial dealloying and selective dissolution of single-crystals Cu3Pd (111) and Cu3Pd (100) in 0.1 M H2SO4 in the potential range of 400-800mV has been studied. With in-situ X-ray diffraction (Synchrotron Light), we observed the epitaxial Pd layer peak to appear by increasing the potential close to the critical potential. Ex-Situ atomic force microscopy (AFM) showed the formation of nanoscale islands of Pd (20-50 nm). Our aim in this project is to study the initial steps of dealloying of well-defined Cu-Pd surfaces on the atomic scale and to compare the results to the Cu-Au system.

Surface morphology and structure of ultra-thin magnesium oxide grown on (100) silicon by atomic layer deposition oxidation

Ultra-thin magnesium oxide layers were elaborated by atomic layer deposition and oxidation process on silicon (100) starting from (2 × 1) thermally-reconstructed or hydrogen-terminated Si surfaces. Low-energy electron diffraction experiments show (2 × 3) and (3 × 3) reconstructions while depositing a magnesium monolayer on Si clean surfaces, and a 3-dimentional growth of the oxide as confirmed by ex-situ atomic force microscopy. For hydrogen-terminated or clean surfaces previously physisorbed by oxygen, uniform cobalt/magnesium-oxide/silicon stacks of layers are observed by transmission electron microscopy. Annealing above 150 °C leads to MgO dissolution and formation of an interfacial complex compound by inter-diffusion of Si and Co.

AFM phase imaging of thin films of electronically conducting polymer polybithiophene prepared by electrochemical potentiodynamic deposition

Nanoscale properties of thin films of conducting polymer polybithiophene (PBT) deposited under potentiostatic and potentiodynamic conditions were compared using ex-situ atomic force microscopy (AFM) and its extension called phase imaging (PI-AFM). While the morphologies of the films prepared using the two techniques were quite similar, the phase contrast measurements revealed a profound difference in the mechanisms of potentiostatic and potentiodynamic electropolymerization, as well as in the nanoscale crystallinity and grain structure of the resulting polymer films. The overall crystallinity and degree of order were always higher for films deposited at constant potential. The differences were especially pronounced at the early deposition stages (film thicknesses of ca. 10 nm).

Annealing-induced amorphization in a sputtered glass-forming film: In-situ transmission electron microscopy observation

The annealing-induced amorphization of a sputtered glass-forming Cu 51Zr 42Al 4Ti 3 thin film has been clarified by in-situ transmission electron microscopy in conjunction with ex-situ atomic force microscopy and X-ray diffractometry. Upon heating of the film at low temperatures, nanocrystallization and growth of metastable sputtered crystallites occur in the amorphous matrix. Heated to 438 °C, within the supercooled liquid region (ΔT), the film becomes fully amorphous. At higher temperatures above ΔT, i.e. 525 °C, crystalline Cu 10Zr 7 and AlTi 2 phases appear as a result of crystallization. The annealing-induced nanocrystallization and amorphization appear to be rather general and applicable to a large number of systems.

AFM measurement of atomic-scale Si surface etching by active oxidation

Atomic-scale etching of a clean Si surface by active oxidation with oxygen molecules was examined using ex-situ atomic force microscopy (AFM). The etch rate was directly determined by measuring the etch depth with AFM. A SiO 2 anti-etching mask was used on a H-terminated Si(001)-2 × 1:H surface prepared by low pH HF treatment followed by annealing in H 2. The etch rate under active oxidation conditions was almost proportional to the O 2 pressure, which was consistent with previous reports. The etch rate exhibited a weak temperature dependence with an apparent activation energy of 0.2 eV. A distinct transition of the reaction mode from etching to oxide formation was observed in detail as an abrupt decrease of the etch rate by lowering the temperature near the boundary condition between the etching and oxide formation conditions.

Self-assembly of well-ordered and highly uniform nanoripples induced by focused ion beam

Well-ordered and highly uniform nanoripple structures on the surface of single crystal LaAlO 3 (1 0 0), SrTiO 3 (1 0 0) and Al 2O 3 (0 0 0 1) were formed via self-assembly (not by beam writing) by focused ion-beam bombardment. The morphology and topography of nanoripple structures were characterized using in-situ focused ion-beam/scanning electron microscope, as well as ex-situ atomic force microscopy. Under off-normal bombardment without sample rotation, the characteristic wavelength of nanoripples varying from 248 to 395 nm on the LaAlO 3 (1 0 0) surface can be obtained by changing ion fluence and incident angle. When all sputtering parameters except the ion fluence are constant, the wavelength of nanoripples is increased with the enhanced ion fluence. These results demonstrate the potential application of using ion sputtering method for fabricating the well-ordered and highly uniform nanoripples which can be used in nanodevices.

In-situ Studies of High-κ Dielectrics for Graphene-Based Device

The deposition and subsequent oxidation of metallic Al and Hf on graphite has been studied as a route to the formation of a high-κ dielectric on graphene. We employ in-situ x-ray photoelectron spectroscopy (XPS), ex-situ atomic force microscopy (AFM) and Raman spectroscopy to study reactions with the substrate and dielectric film morphology. We show that Al2O3 can be formed on the freshly cleaved graphene surface by annealing the deposited metallic Al in ozone or oxygen. HfO2 is also formed in this manner, but we demonstrate that in the case of Hf, defects are created on the graphene surface unless physisorbed H2O is removed by vacuum annealing prior to deposition and the deposition performed under ultra high vacuum (UHV) conditions.

Particle Inception in a Laminar Premixed Flame of Benzene

Spectral optical techniques, including light extinction and laser induce fluorescence and incandescence measurements, are combined to characterize large-molecule soot precursors and soot in a slightly sooting flame of benzene at atmospheric pressure. Light absorption coupled to in-situ light scattering measurements and ex-situ Atomic Force Microscopy also allowed the evaluation of particle sizes. In the benzene flame high molecular mass structures with typical sizes of 3–4 nm are formed in the main oxidation region of the flame. The radical-rich flame environment in which these compounds are formed promotes their dehydrogenation increasing the level of their aromaticity. As a result, nanoparticles with typical sizes of about 5 nm, absorbing and fluorescing in the visible are formed. These compounds reach a maximum concentration just before the appearance of incandescent soot particles.

Atomically flat diamond (111) surface formation by homoepitaxial lateral growth

A process of homoepitaxial growth of diamond (111) films by microwave plasma-enhanced chemical vapor deposition has been investigated characterizing areas by ex-situ atomic force microscopy. The evolution of surface morphology during a lateral growth of (111) diamond was visualized utilizing a mesa structure as a marker. Lateral growth forms atomically flat surfaces, which show atomically flat terraces over several hundred nm widths and single bilayer steps of (111) diamond.

Brewster-angle analysis of native and photoelectrochemically grown silicon oxide nanotopographies

Brewster-angle reflectometry (BAR) is employed in-situ for the first time for investigation of the silicon oxide/silicon system during photocurrent oscillations in fluorine containing solution. Combining ex-situ Brewster-angle analysis (BAA) and atomic force microscopy (AFM) shows oxide thickness variations between 4.8 and 13.8 nm. The anodic oxide etch rate amounts to 0.1 nm s −1 in contrast to 0.01 nm s −1 for native oxide. The ambient/anodic oxide interface roughness, σ, is 2 ± 0.2 nm while σ for the oxide/silicon interface varies between 1.0 and 1.9 nm. The photocurrent onset, preceding regular current oscillations, is accompanied by roughness changes at the silicon oxide/silicon interface. A quantitative analysis of the overall system by discerning roughness and surface layer contributions is presented.

Wavelength tunability of ion-bombardment-induced ripples on sapphire

A study of ripple formation on sapphire surfaces by 300-2000 eV Ar+ ion bombardment is presented. Surface characterization by in-situ synchrotron grazing incidence small angle x-ray scattering and ex-situ atomic force microscopy is performed in order to study the wavelength of ripples formed on sapphire (0001) surfaces. We find that the wavelength can be varied over a remarkably wide range-nearly two orders of magnitude-by changing the ion incidence angle. Within the linear theory regime, the ion induced viscous flow smoothing mechanism explains the general trends of the ripple wavelength at low temperature and incidence angles larger than 30. In this model, relaxation is confined to a few-nm thick damaged surface layer. The behavior at high temperature suggests relaxation by surface diffusion. However, strong smoothing is inferred from the observed ripple wavelength near normal incidence, which is not consistent with either surface diffusion or viscous flow relaxation.

Influence of surface temperature and surface modifications on the initial layer growth of para-hexaphenyl on mica (0 0 1)

Ultra-thin films of para-hexaphenyl (6P) were prepared on muscovite mica (0 0 1) utilizing organic molecular beam deposition (OMBD) under well defined ultra high vacuum (UHV) conditions. The 6P growth characteristics were studied as a function of substrate temperature and substrate surface conditions. For the initial state of layer growth, thermal desorption spectroscopy (TDS) was used to verify the existence of a wetting layer. In this monomolecular continuous wetting layer, the molecules lie flat on the surface and are rather strongly bonded. For thicker films, in-situ X-ray photoelectron spectroscopy (XPS) in combination with (TDS) was applied to reveal the kinetics of the layer growth. Ex-situ atomic-force microscopy (AFM) was used to determine the film morphology. In particular, the influence of surface modifications (carbon contamination, sputtering) on 6P layer growth was investigated. XPS and low energy electron diffraction (LEED) were used to characterize the mica surface before the film deposition. TDS and AFM revealed a considerable change in film growth, from a needle-like island growth of flat laying molecules on top of the wetting layer (for the air cleaved mica) to terrace-like film growth of standing molecules, without a wetting layer (after surface modifications).

Nucleation of CdTe thin films deposited by close-space sublimation under a nitrogen ambient

The early stages of the close-space sublimation growth of CdTe/ITO (indium tin oxide) at 500°C under 26.7 kPa (200 Torr) of N 2 were investigated, the relatively high pressure being used to slow the growth. Film development was monitored over 60 mins by ex-situ AFM (atomic force microscopy), the growth being controlled by a shutter. The films formed by the ‘island’ or Volmer–Weber growth mechanism. Developments in the areal island growth rate, the island density and spatial distribution type were explained using growth mechanisms. Significant changes in these phenomena at t ≥ 10 mins were attributed to a change in growth mechanism from surface migration limited, to a regime in which island coalescence and direct impingement of vapour species on the islands became important. Since the islands are characterised by distinct crystalline facets, this indicates the growth mechanism to be uninterrupted step-flow like addition of material to each island. Arguments are given to relate the final grain size in the films to the density of nuclei that are stable to re-evaporation after t = 10 mins.

Growth of silicon carbide on hydrogen-terminated Si (001) 1 × 1 surfaces using dimethyl- and monomethyl silane

A problem of long standing in the high-temperature growth of 3C–SiC on Si has been the formation of pits at the SiC/Si interface. The research described in this paper addresses this problem through the use of organosilicon growth precursors and explores issues related to the mechanisms of pit formation. Specifically, this paper reports studies of 3C–SiC growth at 800 °C on hydrogen-terminated Si (001) 1 × 1 surfaces using dimethyl- and monomethyl silane under gas source molecular beam epitaxy conditions. In-situ analysis of the films by Auger electron spectroscopy and reflection high-energy electron diffraction along with ex-situ scanning electron microscopy and atomic force microscopy were used to gain insight concerning the details of the growth mechanism. The experimental variables, in addition to the growth species, included molecular flux and growth time. For growth using both dimethyl- and monomethyl silane, clear evidence for the out-diffusion, segregation, and participation of substrate Si in the growth process was found. The molecular flux at the substrate surface was found to play a key role in determining the microstructure of the initial SiC layers and the SiC/Si interface. The effective sticking coefficient for both gas species was found to be on the order of ∼ 0.13. Conditions under which interfacial pits may be eliminated are identified.

Structural and Morphological Properties of Ultrathin HfO<sub>2</sub> Dielectrics on 4H-SiC (0001)

The material properties of HfO2 thin films were studied to evaluate their potential as a high-κ gate dielectric in 4H-SiC power metal-oxide-semiconductor field effect transistors. Stoichiometric HfO2 films were deposited on n-type 4H-SiC (0001) by atomic layer deposition (ALD) at substrate temperatures of 250-450°C. No significant interfacial layer formation was observed by in-situ X-ray photoelectron spectroscopy (XPS) and an abrupt interface was confirmed by high-resolution transmission electron microscopy (HRTEM). A temperature-dependent transition from amorphous layer-by-layer growth to crystalline three-dimensional island growth was identified by in-situ reflection high-energy electron diffraction (RHEED) and ex-situ atomic force microscopy (AFM). X-ray diffraction (XRD) confirmed the presence of monoclinic HfO2 domains in crystallized films.

Probing Co/Si interface behaviour by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM)

In this work, we investigate the Co-Si reaction, the Co growth mode at room temperature, diffusion behaviour as well as morphology evolution during annealing on both H-terminated and clean Si(001) and Si(111) surfaces. From in-situ X-ray photoelectron spectroscopy (XPS) investigation, “Co-Si” reaction appears to occur on both H-terminated and clean surfaces at room temperature (RT) and the silicide crystallinity is improved upon annealing. Co growth mode on H-terminated Si surfaces occurs in a pseudo layer-by-layer manner while small close-packed island growth mode is observed on the clean Si surface. Upon annealing at different temperatures, Co atom concentration decreases versus annealing time, which in part is attributed to Co atoms inward diffusion. The diffusion behaviour on both types of surfaces demonstrates a similar trend. Morphology study using ex-situ atomic force microscopy (AFM) shows that the islands formed on Si(001) surface after annealing at 700 °C are elongated with growth directions alternate between the two perpendicular ▪ and [110] directions. Triangular islands are observed on Si (111) surface.

Surface roughening in Si 1− xGe x alloy films by 100 MeV Au: Composition dependency

Using a 100 MeV Au beam, the surface roughening kinetics of relaxed Si 1− x Ge x alloy films for x = 0.5 and 0.7 are studied by means of ex-situ atomic force microscopy (AFM). Swift heavy ion (SHI) irradiation induced surface roughening behavior is demonstrated using the trend in variation of β as a function of fluence when the data are analyzed in terms of the Edwards–Wilkinson (EW) model. By employing the EW model, the observed surface roughening is explained on the basis of the competition between SHI induced sputtering and smoothening through redeposition of the sputtered atoms. The composition dependent variation of surface morphology with increasing fluence is discussed in the light of the strain distribution along the sample surface.

Microstructure and growth mechanism of stressed complex oxide thin films in strain-modulation

A series of experiments of strain modulations in heterostructures of SrTiO3/LaAlO3 and LaAlO3/SrTiO3 perovskite thin films fabricated by laser molecular beam epitaxy (L-MBE) were performed to study the effect of the compressive stress and tensile stress on the growth and microstructure of the films. The growth process of the films was in-situ monitored by reflective high-energy electron diffraction (RHEED). The morphology of the films was studied by ex-situ atomic force microscopy (AFM). We demonstrated that the compressive stress-induced self-organized SrTiO3 films deposited on LaAlO3 (100) single crystal substrates exhibited a periodic well-ordered ripple-shaped structure, forming a unique nanopatterning tool to fabricate 1D/2D arrays of confined nanostructures (i.e., islands and wires). Small angle X-ray scattering technique was employed to investigate the superstructure. Symmetric satellite peaks were observed, which also revealed the well-aligned self-organized structures. In contrast, the similar superstructure was not observed during the growth of the tensile stress-induced LaAlO3 films on SrTiO3 substrates. Based on the experimental data, the compressive stress was estimated as the main reason of the self-organized growth. A growth model about the formation mechanisms of compressive stress-induced nanostructure was put forward and systematical kinetics elucidations about the growth processes were also discussed to illustrate the effects of different stresses on the growth and microstructures of the films.