Ferroelectric materials such as hafnium zirconium oxide (HZO) are likely to form the basis of a next-generation memory technology. For instance, replacing the conventional gate dielectric in a field-effect transistor (FET) with HZO can form a ferroelectric FET, or FeFET. Chris Regan’s group at UCLA has been working with Suman Datta’s group at Georgia Tech to speed development of this exciting technology. Standard scanning transmission electron microscopy (STEM) (see top left figure), shows the basic layout of a device fabricated at Georgia Tech, but it reveals little about its function. STEM electron-beam induced current (EBIC) imaging at UCLA, on the other hand, shows exactly how the FeFET works. With two EBIC transimpedance amplifiers (Fig. X, top right), the STROBE team can form two linear combinations. The STEM EBIC sum image (see lower left figure), gives secondary-electron EBIC (SEEBIC) contrast. It maps the conductors, correctly showing that the electrode in the upper left corner is not connected to the gate, source, or drain. The STEM EBIC difference image (see lower right figure) gives electron-hole-pair separation contrast, and it is even more informative. It reveals the electric fields that control the FeFET’s function: the depolarization fields in the ferroelectric HZO where the gate electrode overlaps the semiconducting indium tungsten oxide (IWO). Moreover, it reveals these fields just as well whether it has to image through 40 nm of palladium or not. Higher magnification images of the FeFET in both polarization states (not shown) show exactly how much of the ferroelectric switches, providing a structural explanation of the FeFET threshold voltages and memory window. This functional imaging technique – STEM EBIC imaging – is a unique and uniquely powerful development enabled by STROBE.
