Publications by Thorsten Hesjedal

Elementary surface acoustic wave effects studied by scanning acoustic force microscopy

Proceedings of the IEEE Ultrasonics Symposium 1 (2000) 223-226

T Hesjedal, G Behme

Scanning Acoustic Force Microscopy (SAFM) has been used to study elementary surface acoustic wave phenomena with nanoscale spatial resolution. The SAFM technique is capable of detecting acoustic wave properties of arbitrarily polarized modes with sub-wavelength resolution and unmatched sensitivity. Elementary model systems like symmetric single finger wave sources and circular wave sources are studied for the first time in detail.

Simultaneous phase velocity measurement of non-collinear SAWs by scanning acoustic force microscopy

Proceedings of the IEEE Ultrasonics Symposium 1 (2000) 611-614

G Behme, T Hesjedal

We used multimode Scanning Acoustic Force Microscopy (SAFM) for the simultaneous phase velocity measurement of differently polarized SAWs with non-collinear propagation directions at a submicron lateral scale. By analyzing the torsion of the cantilever in addition to its bending (standard SAFM), normal and in-plane oscillation components due to acoustic waves can be measured. Investigating the mixing of complementary oscillation components of crossed Rayleigh and Love waves on the layered system SiO2ST-cut quartz, both phase velocities were obtained simultaneously at the same sample spot.

Investigation of single surface acoustic wave sources

Electronics Letters 36 (2000) 1903-1904

T Hesjedal, G Behme

The first experimental investigation of a single gap surface acoustic wave (SAW) source on GaAs is reported. Using scanning acoustic force microscopy, SAWs are measured with sub-wavelength resolution and an unmatched sensitivity. Phase and amplitude images reveal the radiation characteristics of a single gap source, allowing for a deeper insight into SAW device operation and future design improvements.

Simultaneous bimodal surface acoustic-wave velocity measurement by scanning acoustic force microscopy

Applied Physics Letters 77 (2000) 759-761

G Behme, T Hesjedal

We present scanning acoustic force microscopy (SAFM) mixing experiments of differently polarized surface acoustic waves (SAW) with noncollinear propagation directions. The phase velocities of the SAWs are measured at a submicron lateral scale, employing a multimode SAFM that is capable of detecting the wave's normal and in-plane oscillation components. Hereby, the down conversion of the surface oscillations into cantilever vibrations due to the nonlinearity of the tip-sample interaction is utilized. The simultaneous determination of the phase velocities within a microscopic sample area is demonstrated for the mixing of Rayleigh and Love waves on the layered system SiO2/ST-cut quartz. © 2000 American Institute of Physics.

Influence of ultrasonic surface acoustic waves on local friction studied by lateral force microscopy

Applied Physics A: Materials Science and Processing 70 (2000) 361-363

G Behme, T Hesjedal

We studied dynamic friction phenomena introduced by ultrasonic surface acoustic waves using a scanning force microscope in the lateral force mode and a scanning acoustic force microscope. An effect of friction reduction was found when applying surface acoustic waves to the micromechanical tip-sample contact. Employing standing acoustic wave fields, the wave amplitude dependent friction variation can be visualized within a microscopic area. At higher wave amplitudes, a regime was found where friction vanishes completely. This behavior is explained by the mechanical diode effect, where the tip's rest position is shifted away from the surface in response to ultrasonic waves.

Si in-diffusion during the 3D islanding of Ge/Si(001) at high temperatures

Applied Physics A: Materials Science and Processing 69 (1999) 467-470

J Walz, T Hesjedal, E Chilla, R Koch

The 3D islands of the Stranski-Krastanow system Ge/Si(001) that form either during the annealing of previously flat and nearly strain-relieved Ge films at 1020 K or directly at the Ge deposition at 1020 K are found to be composed of a mixture of Ge and Si, thus pointing to considerable interdiffusion at 1020 K. Direct measurement of the elastic energy unambiguously reveals that neither the 3D islanding nor the Si in-diffusion are driven by the reduction of misfit strain; this strain being the result of increasing configurational entropy.

Spatially resolved measurement of transverse surface acoustic waves for the investigation of elastic properties

Surface and Interface Analysis 27 (1999) 558-561

G Behme, T Hesjedal, E Chilla, HJ Fröhlich

This paper reports new developments in the field of spatially resolved surface acoustic wave (SAW) analysis. With scanning acoustic force microscopy (SAFM) the investigation of SAW phenomena with lateral resolution of the scanning force microscope became possible. This technique was limited to SAW modes with an out-of-plane oscillation component. Recently, we demonstrated that purely in-plane polarized SAWs can also be investigated by using a non-linear coupling to the cantilever's torsional movement. Now it is possible to measure the SAW phase velocity dispersion for any given SAW polarization. We used SAFM for investigation of the layered system SiO2 on ST-cut quartz.

Phase velocity measurement of in-plane polarized surface acoustic waves with high spatial resolution

Proceedings of the IEEE Ultrasonics Symposium 1 (1998) 127-130

G Behme, T Hesjedal, E Chilla, HJ Froehlich

In this paper we present a new method that allows the measurement of the phase velocity of in-plane polarized SAWs with high spatial resolution. The capabilities of the scanning acoustic force microscope had to be extended by the analysis of torsional cantilever motion. A nonlinear coupling mechanism between in-plane oscillations and this movement could be found, that allows an mechanical mixing of in-plane SAWs. Phase velocity measurements of Love waves on the system Au/SiO2/ST-quartz are presented. A good agreement with theoretical predictions for the velocities could be found.

High resolution acoustic field imaging applied to surface acoustic wave devices

Proceedings of the IEEE Ultrasonics Symposium 1 (1998) 265-268

G Behme, M Bloecker, E Bigler, T Hesjedal, HJ Froehlich

This paper reports measurements of acoustic wave amplitude distributions within SAW devices with high spatial resolution. A modified scanning force microscope transfers the high frequency surface oscillations of the SAW into detectable cantilever vibrations by exploiting a nonlinear coupling mechanism. The capabilities of our technique are demonstrated on conventional Rayleigh wave devices up to 3 GHz and on surface transverse wave resonator devices, where the amplitude in the reflector section was mapped. The demonstrated spatial resolution of the imaged SAW amplitude patterns considerably exceeds the results obtained by conventional techniques.

Acoustic phase velocity measurements with nanometer resolution by scanning acoustic forcemicroscopy

Applied Physics A: Materials Science and Processing 66 (1998)

E Chilla, T Hesjedal, HJ Fröhlich

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. © 1998 Springer-Verlag.

Forcemicroscopy for the investigation of high-frequency surface acoustic wave devices

Applied Physics A: Materials Science and Processing 66 (1998)

T Hesjedal, HJ Fröhlich, E Chilla

Surface acoustic wave (SAW) devices are of great importance in mobile communication and signal processing applications. For their optimization second-order effects, like diffraction or mass loading, have to be studied. However, meeting today's demands of GHz operation new ways of wave field mapping have to be developed, since common methods, like laser optical or electron microscope probing, are resolution limited to the micron range. Scanning acoustic force microscopy (SAFM) allows the detection of the high-frequency surface oscillations having sub-Å amplitudes with the force microscope's typical lateral resolution. The key for measuring the high mechanical frequencies is the nonlinear force curve. Another approach is the force microscope mapping of rearranged fine particles, revealing the nodes and antinodes of the standing wave field. We present measurements in the near field of, and within, acoustic devices fabricated on piezoelectric substrates, such as LiNbO3 and quartz, and being operated at frequencies around 600MHz. By employing SAFM, the local influence of the electrodes on the wave field, leading to undesired performance losses, was investigated. © 1998 Springer-Verlag.

Imaging of surface atoms revolving on elliptical trajectories

Applied Physics A: Materials Science and Processing 66 (1998)

T Hesjedal, E Chilla, HJ Fröhlich

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. © 1998 Springer-Verlag.

Stress and relief of misfit strain of Ge/Si(111)

Applied Physics Letters 73 (1998) 2579-2581

J Walz, A Greuer, G Wedler, T Hesjedal, E Chilla, R Koch

The intrinsic stress and morphology of the Stranski-Krastanow system Ge/Si(111) have been investigated at deposition temperatures of 700-950 K. In a broad range of intermediate temperatures, only one distinct decline of stress is observed at the onset of three-dimensional islanding. Supported by a recent transmission electron microscopy study, the results demonstrate that the strain of Ge/Si(111), where the substrate surface in contrast to Ge/Si(001) is the glide plane for dislocations, is relieved by incorporation and continuous rearrangement of dislocations during the island stage. © 1998 American Institute of Physics.

Transverse surface acoustic wave detection by scanning acoustic force microscopy

Applied Physics Letters 73 (1998) 882-884

G Behme, T Hesjedal, E Chilla, HJ Fröhlich

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. © 1998 American Institute of Physics.

Intrinsic stress upon Stranski-Krastanov growth of Ge on Si(001)

Surface Science 402-404 (1998) 290-294

G Wedler, J Walz, T Hesjedal, E Chilla, R Koch

It is well established that the growth of Ge on Si(001) proceeds by Stranski-Krastanov mode, i.e. 3D islands ("hut" and macroscopic clusters) nucleate on top of a 3-4 ML thick pseudomorphic layer. Here, we present in-situ intrinsic stress measurements of Ge/Si(001) up to the film thicknesses at which the 3D islands percolate. From the film stress - and supported by AFM investigations - three stages of film growth characterised by different reliefs of the misfit strain can be discriminated: (1) the pseudomorphic layer-by-layer stage, (2) nucleation and growth and (3) coalescence of 3D islands. © 1998 Elsevier Science B.V. All rights reserved.

Stress and relief of misfit strain of Ge/Si(001)

Physical Review Letters 80 (1998) 2382-2385

G Wedler, J Walz, T Hesjedal, E Chilla, R Koch

The intrinsic stress of the Stranski-Krastanov system Ge/Si(001) was investigated in the range 700 1050 K. Characteristic stress features indicate that the relief of the misfit strain proceeds mainly in two steps: (i) by the formation of 3D islands on top of the Ge wetting layer and (ii) via misfit dislocations in larger 3D islands and upon their percolation. The temperature dependence of strain relief by 3D islands as well as their nucleation and growth behavior support a kinetic pathway for 3D islanding. © 1998 The American Physical Society.

Intrinsic Stress and Misfit Relaxation Ge/Si(001)

Proceedings of the 1998 SSDM (1998)

G Wedler, J Walz, T Hesjedal, E Chilla, R Koch

Surfing the SAW: Visualizing the oscillation of Au(111) surface atoms

Proceedings of the IEEE Ultrasonics Symposium 1 (1997) 511-514

T Hesjedal, E Chilla, HJ Froehlich

In this paper we report the observation of surface acoustic waves using a scanning tunneling microscope (STM). As the STM's control electronics has a bandwidth limit in the kHz range, SAWs at typical frequencies of MHz to GHz cause a loss of contrast which can be clearly seen on an atomic scale. In order to access the amplitude and phase of a SAW, we introduced a heterodyning type STM, the scanning acoustic tunneling microscope (SATM). Contrary to the STM technique, the SATM measures snapshots of the state of oscillation. On the nanometer scale, two contributions to the phase and amplitude contrast are discussed. First, the SAWs phase delay gives a mainly linear dependence on the distance of the source. Second, the atomic oscillation trajectories within the SAW lead to a signal contribution that is made up of the shape of the oscillation trajectory and the local topography. On an atomic scale where the influence of the phase delay on the contrast can be neglected the oscillation trajectories of single surface atoms are studied. Finally, the atomically resolved phase and amplitude images are compared to simulated data.

Towards the determination of elastic constants on a submicron scale using scanning acoustic force microscopy

Proceedings of the IEEE Ultrasonics Symposium 1 (1997) 549-552

T Hesjedal, E Chilla, HJ Froehlich

This paper reports first steps towards the determination of elastic constants with submicron lateral resolution. The experimental phase velocity dispersion data were obtained on a micron scale using scanning acoustic force microscopy. The minimum of the corresponding error field is only weakly localized, thus giving a large error for the elastic constants. The localization can not be increased by using more of data points. In order to decrease the elastic constant's error, a Love mode is additionally regarded. However, the error field crossing is in this case not leading to a significant increase of the accuracy. We propose the inclusion of further surface guided modes.

Scanning acoustic tunneling microscopy and spectroscopy: A probing tool for acoustic surface oscillations

Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures 15 (1997) 1569-1572

T Hesjedal, E Chilla, HJ Fröhlich

A method is presented for the universal probing of surface acoustic waves (SAWs). For measuring high frequency SAWs by scanning tunneling microscopy (STM) a stroboscopic snapshot technique was employed, named scanning acoustic tunneling microscopy. The amplitude and the phase of a state of oscillation within a SAW field are a superposition of the surface topography and the oscillation trajectory. Measuring with atomic resolution the observed contrast can be understood by a spherical tunneling model. A STM based system is proposed that reaches submicron resolution for the quantitative evaluation of elastic constants. With this system the velocity dispersion is obtained from the detection of laser generated SAW pulses by a broadband STM. Scanning acoustic tunneling spectroscopy using different acoustic modes opens a door for quantitative studies of nanoscale structures. © 1997 American Vacuum Society.