Scanning tunneling microscopy of RF oscillating surfaces

Applied Physics A: Materials Science & Processing

Chilla

Fröhlich

1998

Self Cite

Achieving atomic resolution with an STM demands a noise-free environment, where mechanical vibrations especially must be damped out. Introducing such vibrations in the form of defined ultrasound consequently leads to image distortion. In particular, the topography is smeared out. By employing surface acoustic waves, which lead to an oscillation of surface atoms on elliptically polarized trajectories, this smearing-out is directed, thereby giving a projection of the ellipse on the sample plane. However, by employing a stroboscopic heterodyne technique (mixing the highfrequency tunneling current with a slightly detuned electrical signal which is applied across the tunneling gap) a snapshot of the surface oscillation is seen. We present phase and amplitude images exhibiting atomic resolution. The atomic contrast of phase and amplitude is explained by the superposition of the surface topography and the oscillation trajectory, which can be obtained from a continuum theory model.Since its invention, scanning tunneling microscopy (STM) has been widely used to image surfaces in real space, achieving atomic resolution. However, one of the key technical problems is the damping of mechanical waves traveling through the setup to the microscope's tip area. The introduction of periodic surface oscillations by means of acoustic waves reduces the STM's contrast significantly, although it provides information about the wave's amplitude [1].In order to measure periodic high-frequency phenomena in the range from MHz to GHz, we have developed a heterodyne-type mixing STM utilizing the nonlinear dependence of the tunneling current on the tip-sample distance [2]. Here, the decisive point is the application of a high-frequency electrical signal to the tip. For the investigation of acoustic wave fields using a scanning acoustic tunneling microscope (SATM) [3] the conventional STM electronics can still be employed, since full information on the local phase and amplitude of the wave field can be transferred to a mixing frequency that can be chosen as low as a few Hz.As the heterodyne detection principle is applicable to all systems showing a nonlinear detector response, the scanning force microscope was also used for acoustic wave detection.In the scanning acoustic force microscope, acoustic waves with different amplitudes lead to different static cantilever positions allowing, for example, the mapping of surface acoustic wave (SAW) fields on conducting and nonconducting samples [4] or the qualitative investigation of elasticity [5].In this paper we present SATM measurements on Au(111) surface atoms revolving on elliptical trajectories as they are excited by propagating SAWs. We show that even though the atomic topography contrast vanishes, the phase and amplitude contrast show atomic resolution. Finally, it is demonstrated that these contrasts can be modeled, including the real surface topography and the atomic oscillation trajectory. Experimental detailsThe object under investigation was a 100 nm thick Au layer that was deposited on a Y...

Section: Methodsmentioning

confidence: 93%

Section: Methodsmentioning

confidence: 99%

See 1 more Smart Citation

Imaging of surface atoms revolving on elliptical trajectories

Applied Physics A: Materials Science & Processing

Chilla

Fröhlich

1998

Self Cite

“…50 The first part is the phase of the SAW which is changing by 2π every λ, i.e. an extremely small phase contribution on an atomic scale for frequencies around 100 MHz and λ's around 30 µm.…”

Section: Satm Principle and Contrast Formationmentioning

confidence: 99%

Nanoacoustics: probing acoustic waves on the nanoscale

2003

SPIE Proceedings

Two scanning acoustic probe microscopy (SAPM) techniques have been developed for the nanoscale analysis of the interaction of acoustic modes on the surface of a solid. The scanning acoustic force microscope (SAFM) and the scanning acoustic tunneling microscope (SATM) are extensions to the scanning probe microscopy world utilizing the non-linearity of the respective interaction-distance curves in order to detect high-frequency signals, like acoustic waves or electric fields. SAFM and SATM measurements are presented that give an overview of past and present work in this field. With SAPM, acoustic wave properties of arbitrarily polarized surface acoustic wave (SAW) modes can be measured with sub-wavelength resolution and unmatched amplitude sensitivity. SAW dispersion measurements, scattering and diffraction, SAW transducer studies, studies of the crystal anisotropy, as well as atomically resolved images of the oscillating lattice will be discussed.

“…The origin of the variation can be twofold. First, due to the polarization of the SAW, there are known gradient-induced effects on the local phase in SATM measurements [7]. Thus, in SAFM, too, the phase contrast must be modeled from the topography and must be subtracted from the measured signal.…”

Section: Influence Of Roughnessmentioning

confidence: 99%

Acoustic phase velocity measurements with nanometer resolution by scanning acoustic force microscopy

Chilla

Applied Physics A: Materials Science & Processing

Fröhlich

1998

Self Cite

With the increasing interest in nanostructures and thin films, the need for a quantitative measuring method for elastic constants on the nanometer scale has become more evident. The fundamental physical quantity characterizing the elastic constants is the acoustic phase velocity. Due to the strong localization of surface acoustic waves (SAWs) in the near-surface region, SAWs are particularly favored for such investigations. The velocity measurement is commonly performed by time delay and acoustic far-field methods. Therefore the lateral resolution of the velocity measurement is restricted by the wavelength involved to some tens of microns. Recently, we introduced the scanning acoustic force microscope (SAFM) for the measurement of SAW amplitude distributions with nanometer lateral resolution. The key to detecting high-frequency surface oscillations by the slowly responding force microscope cantilever is the nonlinear force curve. This nonlinearity can be exploited in a heterodynetype setup for high-frequency wave mixing of a probe and a reference wave, revealing the phase of the probe wave. The difference frequency can be chosen to be as low as 1 kHz. We present measurements of the phase velocity over a lateral distance of 19.9 nm. The phase velocity dispersion due to Au layers on a quartz substrate was measured over distances as small as 200 nm and compared with calculations.