thesis, we have developed a model to elucidate the ω-term Coul and on the disk-like GaN island, respectively. By the method, we can obtain the surface work function of the tip we used here, which can in turn retrieve the surface work function of other samples using the same tip.
In the scanning EFM experiments, the results show
rm force mappings on disk-like GaN islands are well-resolved as AFM image. The intensities of electric force Fω and F2ω on the region of GaN islands were observed appearing to be lower as compared with that at the plane surface.
The higher the island is, the stronger the effect is.
According to the model we developed, we als
ESC tip,
F − ω on
the s m Si w
nt-disk mod
el or cone model is used to explain for ample at different tip-sample distances. We found that for unifor afer the ω-term Coulombic force between tip and surface ESC,
ESC tip,
F − ω(Z), can be interpreted by cone model. Furthermore we also carried out scanning EFM for disk-like GaN islands with diameters ranged from ~200 to ~500nm grown on AlGaN epilayer. Our results showed that the ω-term Coulombic force between tip and surface charge
ESC tip,
F − ω(Z) for the single GaN island can be interpreted well by the charged poi el.
Whenever charged point-disk mod
ω-term Coulombic force between tip and sample depends on radius of tip and diameter of sample. For uniform Si wafer, the dimension can be considered to be infinite compared with the radius of the tip. In this situation, cone mode can be employed to interpret the experimental results because of the infinite Si plane relative to the tip radius and the uniform electric field intensity from the ESC on the Si wafer, which cause the all part of the cone-like tip to be suffered from the electric force from the ESC on the Si wafer. For the single disk-like GaN island whose diameter of disk possesses a finite value as compared with that of the tip, the electric field is not uniform and decreases rapidly with increasing height. On the probe, because the sphere on the pointed end of the tip is the most close to the island and more charge assembles there. That is the part of sphere is the main position suffered the force Fis,ESC-tip. As a result, we must use the charged point-disk model to interpret the Fis,ESC-tip(Z) in the case of the single disk-like GaN island.
Appendix
the calculated electric field intensity vs. height (by the Eq. 4-8) in the F
We drew
ig. A.1 (marked as the black dots). The radius b is replaced with 125nm and the height Z varied from 50nm to 600nm in the calculation, which corresponds to the case of our experiments mentioned in the section 4.5. The fitted results show that the force decays rapidly and the electric field intensity can be ignored when height is larger than about 300nm.
0 100 200 300 400 500 600
y=0.99e-x/79.35+13.51e-x/305.76 y=0.96e-x/109.75
A.1. The electric field intensity vs. height calculated by Eq. (4-8).
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