Beyond Silicon: Ferroelectric Semiconductors for Next-generation Devices
Ferroelectric materials are highly versatile functional materials characterized by their pronounced responsiveness to a wide range of external stimuli. These stimuli—collectively referred to as external fields—include electric fields, magnetic fields, mechanical stress, optical irradiation, and thermal gradients. Due to their intrinsic coupling mechanisms such as piezoelectricity, pyroelectricity, electro-optic effects, and magnetoelectric coupling, ferroelectrics exhibit multifaceted responses to these external perturbations. Consequently, they have been widely implemented in advanced technologies including non-volatile ferroelectric random-access memory (FeRAM), microelectromechanical systems (MEMS) actuators, and various types of sensors, making them indispensable in modern electronic and electromechanical systems. Although ferroelectrics are fundamentally wide-bandgap insulators, our research focuses on their semiconducting behavior, particularly in doped or defect-engineered systems. By harnessing the semiconducting properties of ferroelectric materials—such as their internal polarization-induced band bending and switchable built-in electric fields—we aim to develop novel device architectures for photovoltaic energy conversion (ferroelectric photovoltaics), optically driven actuators, and neuromorphic computing elements for artificial intelligence applications.
Materials such as gallium oxide (Ga₂O₃), silicon carbide (SiC), and diamond—traditionally regarded as electrical insulators—are now being actively explored as wide-bandgap semiconductors for power electronics applications. This paradigm shift stems from the fact that insulators and semiconductors inherently share similar energy band structures, differing primarily in bandgap width and carrier concentration. The growing interest in utilizing these materials as semiconductors highlights the blurring boundary between insulating and semiconducting classifications.
Among insulators, our focus is on ferroelectric materials—characterized by spontaneous polarization, piezoelectricity, and pyroelectricity—and their potential as functional semiconductors. We investigate ferroelectric semiconductors, a class of materials that couples the rich physical phenomena of ferroelectrics (such as switchable polarization, domain dynamics, and field-induced strain) with semiconducting properties like carrier transport, band modulation, and defect engineering. This coupling enables the emergence of novel functionalities, paving the way for next-generation devices such as polarization-driven transistors, multifunctional sensors, and energy-harvesting systems. By integrating ferroelectricity with semiconductor physics, we aim to unlock new device concepts that transcend conventional limitations in electronics and optoelectronics.
Conventional silicon (Si) solar cells are fundamentally based on a p–n junction architecture, formed by the interface between p-type and n-type semiconductors. In contrast, ferroelectric semiconductors possess a non-centrosymmetric crystal structure that inherently breaks spatial inversion symmetry, enabling photovoltaic activity solely upon light illumination—without the need for a p–n junction. This phenomenon is known as the bulk photovoltaic effect (BPVE), a non-linear optical response absent in conventional Si-based semiconductors. We have demonstrated that by introducing trace amounts of manganese (Mn) into BiFeO₃ thin films—a prototypical ferroelectric semiconductor—an exceptionally large photovoltage of 852 V can be generated at 80 K under light illumination alone. This result highlights the potential of ferroelectric semiconductors for high-efficiency, junction-free photovoltaic devices. Moreover, ferroelectric materials exhibit the inverse piezoelectric effect, where mechanical strain is induced upon application of an electric field. By coupling the bulk photovoltaic effect with the inverse piezoelectric response, we propose a novel class of light-driven actuators. In our experiments, we observed that illuminating a stripe-shaped BiFeO₃/SrTiO₃ heterostructure induces a measurable displacement at the tip of the stripe, confirming the feasibility of opto-mechanical actuation. Additionally, we are exploring the ability to modulate the electrical conductivity of BiFeO₃ via its spontaneous polarization orientation, enabling non-volatile, polarization-controlled resistive switching. This functionality opens pathways toward the development of neuromorphic and AI-integrated devices, where ferroelectric domain states can serve as programmable logic or memory elements.
Specifically, our research is expected to contribute to the development of the following advanced technologies:
- Next-generation photovoltaic devices that surpass the performance limits of conventional p–n junction-based solar cells.
- Light-driven actuators that operate without external electrical input.
- Neuromorphic computing elements and AI-integrated devices.
| Research | |
|---|---|
| Journal | Scientifiv Reports |
| Title | Enhancement of photovoltage by electronic structure evolution in multiferroic Mn-doped BiFeO3 thin films |
| Author | Seiji Nakashima, Tohru Higuchi, Akira Yasui, Toyohiko Kinoshita, Masaru Shimizu, Hironori Fujisawa |
| Member | Seiji Nakashima, Masaru Shimizu, Hironori Fujisawa |
| URL | https://doi.org/10.1038/s41598-020-71928-5 |
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