Vol. 84 (1993)

ACTA PHYSICA POLONICA A

No. 3

Proceedings of the XXII International School of Semiconducting Compounds, Jaszowiec 1993

RECENT DEVELOPMENTS
IN ;CANNING TUNNELING MICROSCOPY
R. WIESENDANGER

Institute of Applied Physics and Centre for Microstructural Research
University of Hamburg
Jungiusstrasse 11, 2000 Hamburg 36, Germany

Scanning tunneling microscopy and related local probe methods have
led to a novel perception of nanometer- and atomic-scale structures andprocesses. Since the structural information is obtained directly in real space,
the scanning probe techniques ocher considerable advantages compared withdiffraction techniques for the investigation of non-periodic structures at solid
surfaces. In addition, the local probe methods allow to study almost any
kind of physical property of microstructures with submicron down to atomicresolution.
PACS numbers: 61.16.Bg, 68.35.Bs, 75.25.+z

1. Introduction
Scanning tunneling microscopy (STM)/spectroscopy (STS) has become themost powerful tool to study the atomic-scale surface stucture of electrically conducting solid surfaces [1-4]. Combined STM/STS studies provide valuable information about the relationship between surface atomic and local electronic structure, particularly for semiconductor surfaces [5, 6]. The surfaces of poorly conducting substrates or bulk insulators can be investigated by force microscopy/spectroscopy [7, 3, 4] yielding information about the atomic-scale topography as well as
local mechanical properties. By using a magnetic tip in force microscopy, spatialvariations of the stray field at surfaces of magnetic substrates can be probed. Morerecent developments in STM/STS aimed at the demonstration of chemical [8] aswell as magnetic contrast [9] at the atomic level. These additional capabilities ofSTM/STS are important for the investigation of multicomponent and magnetic
materials, respectively, thereby extending the useful applicability of STM/STS tomore complex systems.
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R. Wiesendanger
2. Atomic-scale structure

Atomic-resolution topographs can easily be obtained by STM applied tosemiconductor surfaces with dangling bonds. STS even allows to map out the spatial variation of different types of dangling bonds showing up at different energies.
Combined with other surface analytical techniques, STM/STS has allowed to solve
numerous stuctures of reconstructed surfaces during the past ten years [2, 5, 6]. In
addition, non-periodic surface structures such as point defects, steps, dislocations,and grain boundaries can be characterized directly in real space [10]. This is evenpossible in the presence of relatively thick native oxide layers if force microscopy
is used to measure the surface topography [11].
3. Chemical contrast

The distinction between different chemical species b y STM/STS has beenachieved in a few favorable cases only. For compound semiconductors, such asGaAs, for which the direction of charge transfer between the two types of atomic
sites is known, bias-polarity dependent STM studies can offer atom-selective images of the two different species [12]. Besides spectroscopic STM/STS studies,
simultaneous laser excitation has recently been proposed to aclieve chemical contrast down to the atomic level [8]. In particular, the use of coordinated electronic
and photon biasing has been demonstrated to provide speciflc atomic imaging forCuInSe2 , a covalently bonded semiconductor with applications for thin-film photovoltaic technologies. It is assumed that the photon bias at a particular wavelength
directly excites the atom, with the wavelength corresponding to the atomic transition probability. This method has been applied to study the local chemical orderin the vicinity of defects such as grain boundaries.
4. Magnetic contrast

Magnetic contrast is achieved, for instance, by magnetic force microscopy(MFM) probing the surface magnetic stray fleld distribution [13, 14]. The long-range nature of the magnetic dipole interaction limits the spatial resolution achieved in MFM images to about 10-20 nm. By exploiting the spin dependence of thetunneling current flowing between two magnetic electrodes, a spin-polarized STM
has been developed having the potential of providing magnetic contrast down tothe atomic level. Vacuum tunneling of spin-polarized electrons in STM was first
demonstrated by probing the alternately magnetized terraces of a Cr(001) surface
by means of a ferromagnetic CrO2 tip [15]. Subsequent improvements in the in situ
preparation of atomically sharp and chemically well-defined ferromagnetic STM
probe tips. [16, 17] finally led to the first observation of magnetic contrast on theatomic scale [9, 18, 19]. This was demonstrated by using a Fe3O4(001) surface
for which the two different magnetic ions, Fee+ and Fe 3 +, on the octahedrally
coordinated iron sites of magnetite were distinguished by a ferromagnetic iron tip.
The contrast mechanism is most likely due to spin-polarized tunneling, whereas
long-range magnetic interactions cannot lead to an atomic-scale contrast.

Recent Developments in Scanning Tunneling Microscopy

421

5. Summary
STM and related local probe methods allow to study atomic-scale surface
stuctures and their physical properties directly in real space. Recent developments
in STM/STS have led to first demonstrations of chemical as well as magnetic
contrast at the atomic level which further extends the applicability of STM/STS.
In particular, the investigation of the relationship between chemical, magnetic,mechanical properties and the underlying atomic-scale structure, as one of themajor challenges in the research on microstructures, can now be performed in aneven more powerful way.
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