T. Hesjedal

Ti-catalyzed Si nanowires by chemical vapor deposition: Microscopy and growth mechanisms

Si nanowires grow rapidly by chemical vapor deposition on Ti-containing islands on Si surfaces when an abundant supply of Si-containing gaseous precursor is available. The density of wires is approximately the same as the density of the nucleating islands on the Si surface, although at least two different types of islands appear to correlate with very different wire growth rates. For the deposition conditions used, a minority of long, defect-free wires form, along with more numerous wires containing defects. Energy-dispersive x-ray spectroscopy shows that the Ti-containing nanoparticles remain at the tip of the growing wires. The estimated diffusion coefficient of Si in TiSi2 is consistent with the catalyzing nanoparticle remaining in the solid phase during nanowire growth.

Calculation of the magnetic stray field of a uniaxial magnetic domain

We present an analytic solution for the magnetic field of a bar-shaped permanent magnet. Assuming a constant magnetization, we derive expressions for the stray field in three dimensions. The analytic solutions can be readily applied to field calculation problems for magnetic force microscopy simulations without the need for finite element methods.

Temperature-dependent magnetic force microscopy investigation of epitaxial MnAs films on GaAs(001)

We present variable-temperature magnetic force microscopy (VT-MFM) studies of epitaxially grown MnAs films on GaAs(001). Around a critical temperature of Tc=40u200a°C, the MnAs film undergoes a first order structural phase transition. Due to the strain involved, ferromagnetic α-MnAs and paramagnetic β-MnAs phases coexist as stripes along MnAs[0001]. The dimensions of the α-phase change from isolated dots at higher temperatures to well ordered stripes at lower temperatures. VT-MFM allows a close look at the evolution of domain patterns of MnAs micromagnets.

Transverse surface acoustic wave detection by scanning acoustic force microscopy

We present a scanning acoustic force microscope (SAFM) for the study of surface acoustic wave (SAW) phenomena on the submicron lateral scale. Until now, SAWs with in-plane oscillation components could only be studied effectively via nonvanishing out-of-plane oscillation contributions. By operating the microscope in lateral force mode, where both bending and torsion of the cantilever are detected, additional amplitude-dependent signals are found, which are due to the interaction with purely in-plane polarized surface oscillations. To demonstrate the capabilities of this type of SAFM, Love waves were studied on the surface of layers deposited on ST-cut quartz with SAW propagation perpendicular to the crystal X-axis. The phase velocity of the wave as well as the amplitude of a standing wave field was measured and compared to calculated values.

Tailoring of the structural and magnetic properties of MnAs films grown on GaAs—Strain and annealing effects

MnAs films were deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. Imaging of the temperature-dependent magnetic structure by x-ray magnetic circular dichroism photoemission electron microscopy, and the comparison with magnetization measurements by superconducting quantum interference device (SQUID) magnetometry, is used to study the impact of the different strain state of MnAs/GaAs(001) and of MnAs/GaAs(111)B films on the phase transition between ferromagnetic α-MnAs and paramagnetic β-MnAs, the spatial distribution of the two structural and magnetic phases, and the transition temperature. For the isotropically strained MnAs/GaAs(111)B films, the phase coexistence range is much wider than for the anisotropically strained MnAs/GaAs(001) films. The characteristic change of the saturation magnetization with film thickness is found to be a universal property of films with different epitaxial orientation, if at least one MnAs⟨112¯0⟩ direction is in the film plane. For MnAs/GaAs(001) fil...

Chemically vapor deposited Si nanowires nucleated by self-assembled Ti islands on patterned and unpatterned Si substrates

Abstract When Ti is deposited on Si in the 600–700°C temperature range, the lattice mismatch between the Ti-containing deposit and the Si substrate causes TiSi x nanoislands to form. The nanoislands grow when annealed at temperatures above 800°C. When the nanoislands (either unannealed or annealed) are exposed to a Si-containing precursor gas, the Ti catalyzes the decomposition of the gas, allowing one-dimensional nanowires to grow. If oxide-patterned Si substrates are used, the Ti islands form selectively on the exposed Si and are preferentially positioned near the pattern edges. The subsequently grown Si nanowires are, therefore, positioned with respect to the larger lithographically formed pattern. Exposing the wires to an ion beam after deposition promotes the parallel alignment of nanowires.

The origin of ultrasound-induced friction reduction in microscopic mechanical contacts

We present a study of the origin of ultrasound-induced friction reduction in microscopic mechanical contacts. The effect of friction reduction caused by Rayleigh-type surface acoustic waves (SAWs) is demonstrated for propagating and two-dimensional, standing wave fields using lateral force microscopy (LFM). It is shown that with increasing wave amplitude, friction is completely suppressed. To detect and distinguish between the effect of lateral and vertical surface oscillation components on the cantilever movement, we employed multimode scanning acoustic force microscopy (SAFM). We found that the friction reduction effect is only due to the vertical oscillation component. Because this effect does not appear for purely in-plane polarized Love waves, we concluded that the mechanical diode effect is most probably responsible for the SAW-induced lubrication. This explanation is also supported by vertical and longitudinal SAFM measurements, which show that, in areas where friction is completely suppressed, low frequency vertical cantilever oscillations can still be observed, whereas lateral or torsional oscillations are no longer excited.

Analytical and numerical calculations of the magnetic force microscopy response: A comparison

We investigate the domain structure of submicrometer sized ferromagnetic stripes exhibiting in-plane and out-of-plane magnetized areas with magnetic force microscopy (MFM). Two simulation approaches are used to calculate the observed MFM response. The first relies on an analytical solution for the stray field of a bar magnet and the subsequent modeling of the sample as an arrangement of bar magnets. The MFM response is calculated for a realistic tip shape incorporating a distribution of magnetic dipoles. The second, numerical approach is based on a discretization scheme, breaking the tip-sample problem up into cells and then calculating the energy of the magnetic tip-sample interaction. The MFM responses obtained for the ferromagnetic stripe structure are compared. A discussion of the advantages and limitations of the two methods is given in terms of precision, computing time, and flexibility. The numerical method offers shorter computing times and greater flexibility, opening the door for realistic three...

Influence of surface acoustic waves on lateral forces in scanning force microscopies

We present a detailed study of the influence of ultrasonic surface acoustic waves (SAWs) on point-contact friction. Lateral force microscopy (LFM) and multimode scanning acoustic force microscopy (SAFM) were used to measure and to distinguish between the influence of in-plane and vertical surface oscillation components on the cantilever’s torsion and bending. The experiments show that friction can locally be suppressed by Rayleigh-type SAWs. Through the mapping of crossed standing wave fields, the wave amplitude dependence of the friction is visualized within microscopic areas without changing other experimental conditions. Above a certain wave amplitude threshold, friction vanishes completely. We found that the friction reduction effect is caused by the vertical oscillation components of the SAW. Purely in-plane polarized Love waves do not give rise to a significant friction reduction effect. Thus, we conclude that the mechanical diode effect, i.e., the effective shift of the cantilever off of the oscill...

Effect of strain on the local phase transition temperature of MnAs/GaAs(001)

We present measurements of the influence of local strain on the phase transition behavior of epitaxial MnAs films on GaAs(001). As shown previously, stripes of ferromagnetic α-MnAs and paramagnetic β-MnAs coexist around room temperature. Temperature-dependent atomic force and magnetic force microscopy reveals that the characteristic temperature T*, at which the as-grown films transform to the paramagnetic β-phase, is locally shifted up towards the value of unstrained bulk MnAs. The film areas exhibiting a higher T* were identified as regions in which the strain in the MnAs film was allowed to relax.