Practice details

Measurements of contact potential differences and external voltage drop profiles.

    Preparation steps for the SKM study of the heterostructure on the cleavages.

a) A 50 mkm scanner, an optical viewing system, and x-y translation stage may be necessary to have comfortable conditions of measurements. 

b) The samples with freshly prepared cleavages may be griped in a miniature vice or fixed by a super glue on the vertical side of the small block. A double-side scotch can be used instead of glue, but when the linear sizes of the sample are less than 1 mm, the scotch is often not effective means for good fixation.

c) Long side of the cantilever beam should be perpendicular to the cleavage edge to facilitate the search of the edge in the scan area. This space arrangement of cantilever and sample will be also optimal to decrease the role of the parasitic capacitance, since it permits to have minimal area of overlapping between the surfaces of the cantilever and the sample. 

d) Before approach, the tip should be placed in the center of the scanning area, and approximately 15-20 mkm away of the edge of the cleavage (for the reference, when using the optical viewing system, the width of the cantilever NSG11 is about 50 m). 

e) When the edge has been found, first we move the tip several microns away of the edge, then withdraw the tip from the sample (switch out a feedback loop), close a microscope by the protective cover, suspend the whole system, and finally switch on a feedback loop again.

f) The sequence of actions to adjust SKM mode (it can be found in the manual "SKM setting"). 

g) To optimize the feedback action for contact potential difference (CPD) measurements we also may recommend to find the phase domain (180 degree) allowing negative feedback loop gain, and to adjust the phase shift of the input signal in the middle of that domain.

Specific parameters of the scan can be found in f8.opt, f9.opt and f8.mdt, f9.mdt. Lift values, applied for data acquisition in SKM mode, are also given in scan descriptions.

 

To extract the surface voltage drops profiles, two set of data were used: topography height profile and CPD profile (SKM data) taken under equilibrium, and those measured under some bias applied across the studied device.

To improve resolution the profiles may be averaged along the direction parallel to interfaces of the heterostructure. Using topography data we mark the heterostructure interfaces on the CPD profiles, then conform and align the equilibrium CPD profile to the CPD profile under some bias, and finally subtract the first profile from the second one to obtain the surface voltage drop profile.