Optically Probing Energy Barrier Height Modulation in α-In₂Se₃ Based Ferroelectric Semiconductor Field Effect Transistors for Neuromorphic Applications

Ferroelectric materials exhibit a net electric dipole moment whose orientation can be changed by an external electric field. This useful property has been used to create non-volatile ferroelectric field effect transistors (FeFETs), which may find application in neuromorphic computing due to their efficient emulation of synaptic plasticity. Integration of 2D ferroelectric semiconducting α-In₂Se₃ with non-ferrolectric gate dielectrics enables ferroelectric <i>semiconductor</i>-field effect transistors (FeS-FETs), The FeS-FET output depends on the history of the gate bias, which modulates of the free carrier distribution and the polarization of bound charges in the semiconductor. The combined effect on the potential profile and the metal-semiconductor contact barriers creates the possibility for optical neuromorphic devices based on, for example, tunable Schottky barrier photodiodes. While several groups have reported large hysteresis in α-In₂Se₃ based FeS-FETs, device characteristics vary widely, and the microscopic connections between polarization switching and resistance changes can remain obscured. We will describe the use of functional imaging to correlate polarization switching with modulation of the dark current and photocurrent regulated by the metal-semiconductor junction, laying a foundation for design of optoelectronic neuromorphic devices. Scanning photocurrent microscopy (SPCM) was used to map the spatially dependent photoresponse and isolate the contribution of the metal-semiconductor junctions, which dominate at the reverse biased (Schottky) junction. Photocurrent spectra were acquired at the contacts to directly measure the effective barrier height by internal photoemission, and the modulation of barrier height with external field poling of the In₂Se₃. Piezoresponse force microscopy was used to establish domain structure in exposed regions of the channel, and Kelvin probe force microscopy to establish a self-consistent explanation for local ferroelectric domain switching and changes in device output. The improved microscopic understanding of the origins and magnitude of hysteresis support the design and rational optimization of FeS-FET based devices.