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Polycrystalline Materials
Scanning probe microscopies can be used to image ac and dc transport behavior in polycrystalline ceramic and photovoltaic materials. Resistive barriers on the interfaces in varistor ceramics, temperature behavior of grain boundaries in positive temperature coefficient of resistance (PTCR) materials and minority carrier generation in polycrystalline silicon can be imaged directly with nanometer resolution.
Grain boundaries in polycrystalline ZnO
Application of lateral bias and in-situ imaging increases the range of possibilities provided by scanning probe microscopy tenfold. Shown below is surface topography and surface potential for grounded, forward and reverse biased ZnO varistor surface. Surface topography shows a number of pores and dust particles. Grain boundaries can be detected as small grooves due to preferential grain boundary polishing. Potential image of the grounded surface exhibits a number of potential depressions associated with second phase inclusions. Application of lateral bias results in the development of potential barriers at the grain boundaries due to the lower resistivity of grain boundary region compared to the grain bulk. Upon switching the lateral bias contrast inverts. Analysis of the grain boundary potential drop dependence on external bias allows transport characteristics of grain boundary to be reconstructed.
From the potential distribution obtained by SSPM current map of the surface can be reconstructed as shown below.
Temperature dependence of grain boundary resistivity in PTCR ceramics
Positive temperature coefficient of resistance (PTCR) behavior originates from the polarization compensation of interface charge below Curie temperature. On increasing the temperature polarization disappears and interface double Schottky barrier increases, thus giving rise to higher grain boundary resistivity. As follows from this description, PTCR interfaces have rich and complex physics. As for SrTiO3 interfaces, scanning probe microscopy can be used to addresses the intrinsic grain boundary potential on the grounded surface and non-linear transport properties of biased device. Piezoresponse imaging can be used to quantify polarization behavior within the grains and near the interfaces. Combined with variable temperature studies, this provides immense field for experimental investigation. Shown below are some of the most vivid examples of SPM imaging of PTCR BaTiO3 ceramics. Surface potential image exhibits deppresions due to negatively charged grain boundaries, while piezoresponse image delineates polarization distribution within the grains.
Surface potential image of the laterally biased PTCR devices shows that at room temperature the grain boundaries are conductive, while at the increased temperatures potential drops develop at grain boundaries indicative of transformation to the resistive state.
Solar cell (grain boundaries in p-doped silicon)
Shown below is the example of simultaneous surface potential and scanning impedance imaging on polycrystalline p-doped silicon. This sample was prepared by polishing antireflection layer and n-doped layer off the solar cell. Grain structure was visualized by selective etching in 0.1M NaOH at 80C. The images were acquired from three grain junction region.
Surface topography (a) exhibits small topographic variations between the grains due to the difference in etching rate. Surface potential on grounded surface (b) exhibits positive potential regions associated with the grain boundaries consistent with positively charged interfaces in the p-doped material. Surface potential of forward (c) and reverse (d) interface show that potential drops indeed develop at the grain boundaries, i.e. grain boundary conductivity is lower than that of the surrounding regions. Finally, SIM phase image (e) shows that there is significant phase changes in the vicinity of the grain boundaries. SIM amplitude image (f) shows both amplitude decrease in the grains and at the grain boundaries. Additional information can be obtained from the potential and phase profiles across the interface shown below.
Surface potential profiles show that potential is virtually uniform in the higher-potential grain and exhibit wide tail in the lower biased grain. This behavior can be attributed to the minority carrier dynamics under illumination. Note that SIM phase profile (c) exhibits wide asymmetric feature at the grain boundary (as opposed to typical step in phase profile) consistent with SSPM data. Finally, amplitude profile (d) exhibits both ohmic decrease of amplitude in the grains and at the interface.