|СЗМ Раман Нано ИК системы||Модульные СЗМ||Автоматизированные СЗМ||Специализированные СЗМ|
Латеральное разрешение при измерениях поверхностного потенциала.
Lateral resolution of surface potential measurements.
Lateral resolution of noncontact EFM versus SKM.
To improve the instrumental function in electrostatic measurements, the ratio of the parasitic capacitance of the tip side and of the flat part of the cantilever to the useful capacitance of the tip apex should be minimized. EFM signals are proportional to the second derivative of the whole capacitance, unlike the first derivative dependence of SKM signals. Therefore the contribution of the tip apex in EFM mode is more substantial, than in SKM mode . Fig.1 permits to compare lateral resolution of EFM with SKM. SKM and EFM data were taken from the same sample area and at the same bias conditions (1200 mV on p-contact) as data of Fig. 2c. The NT-MDT image processing software permits to represent data in different mode. We utilize in Fig. 1 2G images (gradient mode, that is convenient to annualize the surface electric field). The profiles of signals averaged along the direction of the interfaces are displayed over the corresponding SKM and EFM images. SKM data were acquired at the negative lift value -20 nm, set point magnitude of cantilever vibrations for topography acquisition was ~40 nm. EFM data were measured at lift value -10 nm and free magnitude of cantilever vibrations reduced to ~30 nm. Comparison of fig.1a and fig.1b shows that, in spite of larger gap between the mean tip position and the surface, EFM demonstrates notably narrower spikes of surface electric field, than SKM.
Force modulation by electrostatic excitation of the cantilever in contact mode (contact EFM).
An interesting possibility that may also grade up the instrumental function in electrostatic measurements is the application of force modulation by electrostatic excitation of the cantilever in contact mode. This mode appeared as dynamic contact electrostatic force microscopy (DC EFM) . In contact mode, by the same reasons as in noncontact mode, the electric forces act between the cantilever and the sample, and they can be measured in the same manner as in SKM. The current flow through the contact area may decrease those forces, but it is extremely small, since there are thick (a few nanometers) oxide layers on the tip and sample surface. The results of DC EFM study of another AlGaAs/GaAs based n-i-p laser are presented in Fig. 2. Along with the topography image in Fig. 2b, the simultaneously obtained first harmonic signal (phase-sensitive MagSIN) image (Fig. 2c) and the gradient of that signal in 2G image (Fig. 2d) are given. Comparison of Fig. 2d and Fig. 2b shows that measured surface electric field is mainly concentrated at the waveguide/emitters interfaces. Broadening of these spikes into emitters permits to evaluate lateral resolution on the level of 100 nm. Demonstrated high lateral resolution of electric forces is the consequence of the close proximity of the tip to the surface in contact mode, when the contribution of the tip apex capacitance is maximized. However energy resolution of DC EFM is moderate, since induced vibration of the cantilever with both ends fixed are small. To reach satisfactory signal to noise ratio, one needs to apply rather high alternative voltage to the cantilever (3-4 V r.m.s. in comparison with ordinary 0.5-1 V r.m.s in two pass techniques). It is worth mention that DC EFM mode is possible on the first generation of NT-MDT tools, e.g. P-4-SPM with resonant block [4,5].
AFM images are obtained by commercial SPM P47H with CSC12 cantilevers.