Molecular beam epitaxial growth of III-V semiconductor ... - KOBRA

Experimental Growth and Characterization Techniques
mode, and usually results in a fairly faithful topographical (hence the alternative
name, height mode). During contact with the sample, the probe predominately
experiences repulsive Van der Waals forces (contact mode). This leads to the tip
deection described previously. As the tip moves further away from the surface
attractive Van der Waals forces are dominant (non-contact mode) as illustrated
in Fig. 4.5. Generally, the AFM operation modes can be classied to mainly three
modes [? ]:
• Contact mode: (repulsive VdW force regime), when the spring force of
the cantilever is less than the surface repulsion force, the cantilever bends.
The force on the tip is repulsive. By maintaining a constant cantilever
deection (using the feedback loops) the force between the probe and the
sample remains constant and an image of the surface is obtained. The
probe-surface separation in this mode is less than 0.5 nm.
• Intermediate contact or (tapping) mode: The imaging is similar to contact
with probe-surface separation less than 2 nm. However, in this mode the
cantilever is oscillated at its resonant frequency. The probe lightly taps on
the sample surface during scanning, contacting the surface at the bottom
of its swing. By maintaining a constant oscillation amplitude a constant
tip-sample interaction is maintained and an image of the surface is obtained.
• Non-contact mode: Non-contact mode belongs to a family of AC modes,
which refers to the use of an oscillating cantilever. A sti cantilever is
oscillated in the attractive regime, meaning that the tip is quite close to
the sample, but not touching it (hence, non-contact). The forces between
the tip and sample are quite low, on the order of pico Newton (pN) (10 −12
N). The detection scheme is based on measuring changes to the resonant
frequency or amplitude of the cantilever with a probe-surface separation
between 0.5 - 10 nm.
However, most of the AFM measurements reported in this thesis, are conducted
via the last mode (non-contact mode) using CP II AFM system, due to the
advantages of very low force exerted on the sample (pN range) and the large
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