ZnO grains
A practical example of AFM impedance imaging in the electronic domain is provided by a study of grain/grain boundary transport in commercial polycrystalline ZnO varistors. The impedance of a cross-sectioned commercial ZnO varistor was probed laterally between the AFM tip and a bulk top electrode. Thus, in addition to the local impedance response at the AFM tip, nonlocal impedance contributions from any intervening grain boundaries between the tip and the bulk electrode are also probed.
Coupled scanning electron microscopy (SEM), AFM topography deflection, and AFM Z0 images from a 50 um region of the ZnO varistor are shown in Figure 1. The Z0 image was acquired with a 100 mV excitation signal under 15 V dc bias at 1 kHz. Several distinct ZnO grains are visible in the images. The ZnO grains at the upper left of the image show purely ohmic behavior at a 15 V dc bias. These grains are closest to the bulk top electrode, which was positioned approximately 30 um above and to the left of the image field of view.
The highly nonlinear IV properties of ZnO varistors arise from double-Schottky like barriers formed at the grain boundaries of the material. Below a critical grain-boundary breakdown voltage (typically 3-4 volts), transport across the boundary is almost purely capacitive and the boundary is highly insulating. Above the critical voltage, however, transport across the grain boundary becomes ohmic.
Figure 2 demonstrates the submicron resolution capabilities of the AFM impedance imaging technique with a series of ‘‘zoom-in’’ magnifications on a ZnO triple junction. The small triangular shaped region at the junction between the three ZnO grains (clearly visible in the 6 um image) is a Bi2O3 second-phase inclusion confirmed by energy dispersive x-ray analysis (EDAX). Bi2O3 is added to ZnO varistors to control the grain-boundary properties of the material. Excess Bi2O3 typically phase segregates to the ZnO triple junctions. It is nonconductive.
