Noncontact Atomic Force Microscopy - Yale School of Engineering ...

From non-contact to atomic scale contact between a Si tip and a Si
surface analyzed using an nc-AFM and nc-AFS based instrument
P.I-03
Toyoko Arai 1 , Kosei Kiyohara 1 , Taiki Sato 1 , Shugaku Kushida 2 and Masahiko Tomitori 2
1 Graduate School of Natural science & Technology, Kanazawa University, Kanazawa, Ishikawa, Japan
2 School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa Japan
The scanning probe microscopy (SPM) is a powerful tool to observe a sample surface
with atomic resolution as well as spectroscopic measurements and atom/molecule
manipulation. Since the advent of SPM, we are able to bring an atomically sharpened tip
very close to a sample in a well-controlled manner, in particular, by non-contact atomic
force microscopy (nc-AFM); the force interaction detected by nc-AFM changes so
drastically at atomically close separations that the precise control of separation is
performed. The quantum mechanical properties become prominent at those distances
between a tip and a sample; chemical covalent bonding is formed at less than 1 nm, the
tunneling barrier between them collapses, and at closer distances, an atomically necking
can be shaped and quantized conductance comes up through atomically confined
channels or intermediate electronic states in between. The quantized phenomena at the
point contact have attracted much interest, including its formation process from noncontact
through pseudo-contact to contact. Not only the force interaction, but also the
electric conductance properties can be evaluated using an instrument based on nc-AFM
combined with spectroscopic methods at those separations, i.e., bias-voltage non-contact
atomic force spectroscopy (nc-AFS) with the ability to measure electric current with
respect to bias voltage between a tip and a sample [1]. From a viewpoint of application,
contact formation between two pieces of condensed matter is crucial to fabricate novel
nanoscale devices. Here we focus on Si-Si contact and non-contact states analyzed by an
nc-AFM and nc-AFS based instrument. Although silicon-silicon contacts sounds very
important in industries and various types of Si nanowires have been synthesized, there are
few reports on their nanoscale electromechanical properties.
Experiments were conducted using a home-made UHV-AFM/AFS instrument with a
B-doped Si piezoresistive cantilever having a [001]-oriented Si for samples of n- and ptype
Si(111). After cleaning the tip and the sample by heating in UHV, we took nc-AFM
images simultaneously with current, averaged over a cantilever oscillation cycle, and
damping with changing bias voltage, while taking spectroscopic curves of frequency shift
current and damping versus bias voltage or tip-sample separation. By slowly approaching
and retracting with compensation of z-direction thermal drift, the current-separation
curves with a good S/N ratio exhibited features from tunneling regime to saturation
toward tunneling barrier collapse, leading to chemical bond formation. The damping also
exhibited curious features possibly due to charge change around the tip and the sample.
For Si point contacts current-voltage curves showed p-p or p-n junction characteristics
with staircase behaviors. The details and discussion will be presented.
1. [1] T. Arai and M. Tomitori: Phys.
Rev. B 73 (2006) 073307.
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