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Soft X-Ray-Produced Inner-Shell Photoelectron Scanning Tunneling Micro Scope

IP.com Disclosure Number: IPCOM000099375D
Original Publication Date: 1990-Jan-01
Included in the Prior Art Database: 2005-Mar-14
Document File: 3 page(s) / 151K

Publishing Venue

IBM

Related People

Basu, S: AUTHOR [+2]

Abstract

This article describes three new features which may be added to a scanning tunneling microscope. These are elemental identification, study of insulator surfaces and, possibly, picosecond time resolution.

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Soft X-Ray-Produced Inner-Shell Photoelectron Scanning Tunneling Micro Scope

       This article describes three new features which may be
added to a scanning tunneling microscope.  These are elemental
identification, study of insulator surfaces and, possibly, picosecond
time resolution.

      The scanning tunneling microscope has been used to produce
extremely high resolution images of surfaces (1). In this method, a
small probe, the tip of which is of the order of 10 Ao in diameter,
scans the surface of an object. The tunneling current which depends
exponentially on the separation between the probe tip and the surface
is a faithful reproduction of the surface roughness in the Ao range.

      In some cases, it may be necessary to identify different atomic
species on the surface to infer about the chemical composition or the
ordering of atoms.  In biological samples, and in the insulators
where the tunneling current is low and where the application of
scanning tunneling microscopes is limited, it may be interesting to
obtain high resolution images.  In some cases, it may also be
required that surface dynamics be known with picosecond time
resolution.  In this article, a technique is described which may be
able to fulfil these requirements.

      In the proposed method, a small area of the sample is exposed
to soft X-ray radiation.  The soft X-rays may be produced by a
synchrotron, a laser plasma, a soft X-ray laser or an anode source.
The soft X-rays may be incident on the surface through an aperture,
or focusing may be carried out with a number of elements such as a
zone plate with 50 Ao resolution, a Schwarzchild objective or an
elliptical focusing element.  With a repetitive short-pulse laser
plasma source, it is possible to have a repetitive short-pulse X-ray
source for time- resolved studies.  The sample surface may be exposed
to the X-rays directly or a thin section of the sample may be exposed
on the reverse side with soft X-rays.

      During exposure to X-rays, all the atoms on the surface are
susceptible to photoionization.  At high photon energies, the atoms
undergo inner-shell photoionization. The inner-shell photoionization
cross section of a particular atom is nearly a step function of
wavelength near the bandhead and is a maximum where the incident
photon energy is just greater than the binding energy.  Let us take,
for example, a surface containing Al and As atoms. The binding
energies of Al 2p and 2s levels are 72.5 eV and 118 eV, respectively.
 For As 3s and 3p levels, the binding energies are 204.7 eV and 143
eV, respectively (2).  Thus if soft X-rays at 75 eV are used, most
electrons will be emitted from Al atoms, and for X-ray energy of 145
eV, most electrons will be available from As atoms.  Thus, depending
on the incident X-ray wavelength of choice, all the atoms of a
particular species emit electrons throughout the spot on the surface
where the X-rays are incident.

      The s...