Effects of Environment on Tunneling Spectroscopy of Gold Films
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EFFECTS OF ENVIRONMENT ON TUNNELING SPECTROSCOPY OF GOLD FILMS G.C. WETSEL*,t, Z.M. LIU*, T.L. WENG**, M.W. GOSNEY**, and R.J. WARMACK*** Department of Physics, Southern Methodist University, Dallas, TX 75275 **Department of Electrical Engineering, Southern Methodist University, Dallas, TX 75275 ***Health and Safety Research Division, Oak Ridge National Laboratorytt, Oak Ridge, TN 37831
ABSTRACT A scanning tunneling microscope with a tunneling-current dynamic range of four orders of magnitude has been used in tunneling-spectroscopy studies of Au surfaces in moderate vacuum and in various gases at atmospheric pressure.
Rapid scans of z-motion transducer voltage (Vz)
a function of tunneling current (I t) corrected plots of log(I t)
were measured as
under computer control.
versus Vz were essentially linear.
DriftThe mean
barrier potential was determined from the slope of these plots and the measured displacement-versus-Vzcalibration
curve.
Barrier potentials so
determined are compared for different environmental conditions of the Au surface.
I.
INTRODUCTION According to one-dimensional models of metal-insulator-metal junctions
[1],
the current through the juntion in the tunneling regime varies
exponentially with the thickness (s)
of the insulator;
logarithm of the tunneling current (It) line.
The slope of the log(It) versus s curve is
potential-barrier height (4); therefore, measurements of It
thus, a plot of the
as a function of s is
a straight
related to the mean
4 can be determined from
as a function of s (tunneling spectroscopy).
We report here experimental determination of 4 for Au surfaces under different environmental (STM)
conditions using a scanning-tunneling-microscope
for localized tunneling spectroscopy.
investigation is condition.
One of the objectives of this
to evaluate the technique as a determinant of surface
Quantitative knowledge of the presence of surface substances,
whether deposited intentionally for the purposes of microfabrication [2] or unintentionally (contamination)
[3],
is
Mat. Res. Soc. Symp. Proc. Vol. 139. 01989 Materials Research Society
of great interest.
304
II.
EXPERIMENT Our STM involves a segmented-tube tip scanner [4] rigidly mounted to
the scanner base plate,
a micrometer-driven vertical positioner for coarse
approach of sample to tip, and mechanical course horizontal positioning of the sample.
The large dynamic range of the z motion (perpendicular to the
surface) of the transducer (about 5 micrometers) electronics permit a fast, stable, manual, the tip.
and efficient feedback
coarse approach without crashing
An environmental sample chamber capable of moderate vacuum (the -6
order of 10
torr) can be evacuated with a turbomolecular pump or
backfilled with various gases.
The mechanical adjustment of the sample can
be effected from outside the sample chamber; after adjustment,
the scanner
assembly can be disconnected from the external adjusting mechanism. Vibration isolation of scanner assembly is suspension.
accomplished usi
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