Memory Window in Ferroelectric Field-Effect Transistors: Analytical Approach

A memory window of ferroelectric field-effect transistors (FeFETs), defined as a separation of the HIGH-state and the LOW-state threshold voltages, is an important measure of the FeFET memory characteristics. In this study, we theoretically investigate the relation between the FeFET memory window and the P-E hysteresis loop of the ferroelectric gate insulator, and derive a compact model explicitly described by material parameters. It is found that the memory window is linearly proportional to the ferroelectric polarization for the small polarization regime, and converges to the limit value of 2 x coercive field x thickness when the remanent polarization is much larger than permittivity x coercive field. We discuss additional factors that possibly influence the memory window in actual devices such as the existence of interlayer (no direct impact), interface charges (invalidity of linear superposition between the ferroelectric and charge-trapping hysteresis), and minor-loop operation (behavior equivalent to the generation of interface charges)

量子井戸太陽電池のデバイス物理と設計

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 杉山 正和, 東京大学教授 中野 義昭, 東京大学教授 岡田 至崇, 東京大学教授 平川 一彦, 神戸大学教授 喜多 隆University of Tokyo(東京大学

Generalized Reciprocity Relations in Solar Cells with Voltage-Dependent Carrier Collection: Application to p-i-n Junction Devices

Two reciprocity theorems are important for fundamental understanding of the solar cell operation and applications to device evaluation: (1) the carrier-transport reciprocity connecting the dark-carrier injection with the short-circuit photocarrier collection and (2) the optoelectronic reciprocity connecting the electroluminescence with the photovoltaic quantum efficiency at short circuit. These theorems, however, fail in devices with thick depletion regions such as p-i-n junction solar cells. By properly linearizing the carrier-transport equation in such devices, we report that the dark-carrier injection is related to the photocarrier collection efficiency at the operating voltage, not at short circuit as suggested in the original theorem. This leads to the general form of the optoelectronic reciprocity relation connecting the electroluminescence with the voltage-dependent quantum efficiency, providing a correct interpretation of the optoelectronic properties of p-i-n junction devices. We also discuss the validity of the well-known relation between the open-circuit voltage and the external luminescence efficiency. The impact of illumination intensity and device parameters on the validity of the reciprocity theorems is quantitatively investigated

Invited; HfZrO-based ferroelectric capacitors and FETs for ultralow-power signal processing

Since the discovery of ferroelectricity in HfO2-based dielectric films in 2011 [1], MFM capacitors and FETs using HfO2-based thin films as dielectrics have attracted strong interest. Thus, active research and developments have been conducted for various applications including memory, logic, and AI computing with extremely low power consumption. In this paper, we introduce our recent research on a variety of HfZrO2 (HZO)-based ferroelectric devices such as FeRAM [2-3], FeFET memory [4-8], anti-ferroelectric FETs [9-10] and reservoir computing devices [11-13], for ultralow-power signal processing.The high polarization reversal voltage associated with the high coercive field of HZO films makes it difficult to achieve the low voltage operation of HZO FeRAM. Here, scaling HZO film thickness is effective in a reduction of the supply voltage of FeRAM with HZO MFM capacitors. It has been found through a systematic study on ferroelectric characteristics of Hf0.5Zr0.5O2 films with a thickness from 9.5 to 2.8 nm [2, 3] that scaling HZO film thickness to 4-5 nm can reduce operating voltage below 1 V (~0.8 V) with sufficient 2Pr by performing 106 cycles of wakeup. Also, the electric field causing dielectric breakdown can significantly increase by HZO scaling. The experimental endurance characteristic of 4-nm-thick HZO has indicated that the maximum cycle times determined by dielectric breakdown is around 1010 and 1012 times at 4 MV/cm and 3 MV/cm (1.2 V), respectively, under a pulse voltage operation of 200 kHz. Please click Download on the upper right corner to see the full abstract

Ultrahigh-sensitivity optical power monitor for Si photonic circuits

A phototransistor is a promising candidate as an optical power monitor in Si photonic circuits since the internal gain of photocurrent enables high sensitivity. However, state-of-the-art waveguide-coupled phototransistors suffer from a responsivity of lower than $10^3$ A/W, which is insufficient for detecting very low power light. Here, we present a waveguide-coupled phototransistor consisting of an InGaAs ultrathin channel on a Si waveguide working as a gate electrode to increase the responsivity. The Si waveguide gate underneath the InGaAs ultrathin channel enables the effective control of transistor current without optical absorption by the gate metal. As a result, our phototransistor achieved the highest responsivity of approximately $10^6$ A/W among the waveguide-coupled phototransistors, allowing us to detect light of 621 fW propagating in the Si waveguide. The high responsivity and the reasonable response time of approximately 100 $\mu$s make our phototransistor promising as an effective optical power monitor in Si photonics circuits

Non-volatile hybrid optical phase shifter driven by a ferroelectric transistor

Optical phase shifters are essential elements in photonic integrated circuits (PICs) and function as a direct interface to program the PIC. Non-volatile phase shifters, which can retain information without a power supply, are highly desirable for low-power static operations. Here a non-volatile optical phase shifter is demonstrated by driving a III-V/Si hybrid metal-oxide-semiconductor (MOS) phase shifter with a ferroelectric field-effect transistor (FeFET) operating in the source follower mode. Owing to the various polarization states in the FeFET, multistate non-volatile phase shifts up to 1.25{\pi} are obtained with CMOS-compatible operation voltages and low switching energy up to 3.3 nJ. Furthermore, a crossbar array architecture is proposed to simplify the control of non-volatile phase shifters in large-scale PICs and its feasibility is verified by confirming the selective write-in operation of a targeted FeFET with a negligible disturbance to the others. This work paves the way for realizing large-scale non-volatile programmable PICs for emerging computing applications such as deep learning and quantum computing

Non-volatile optical phase shift in ferroelectric hafnium zirconium oxide

A non-volatile optical phase shifter is a critical component for enabling large-scale, energy-efficient programmable photonic integrated circuits (PICs) on a silicon (Si) photonics platform. While ferroelectric materials like BaTiO3 offer non-volatile optical phase shift capabilities, their compatibility with complementary metal-oxide-semiconductor (CMOS) fabs is limited. Hence, the search for a novel CMOS-compatible ferroelectric material for non-volatile optical phase shifting in Si photonics is of utmost importance. Hafnium zirconium oxide (HZO) is an emerging ferroelectric material discovered in 2011, which exhibits CMOS compatibility due to the utilization of high-k dielectric HfO2 in CMOS transistors. Although extensively studied for ferroelectric transistors and memories, its application in photonics remains relatively unexplored. Here, we show the optical phase shift induced by ferroelectric HZO deposited on a SiN optical waveguide. We observed a negative change in refractive index at a 1.55 um wavelength in the pristine device regardless of the direction of an applied electric filed. We achieved approximately pi phase shift in a 4.5-mm-long device with negligible optical loss. The non-volatile multi-level optical phase shift was confirmed with a persistence of > 10000 s. This phase shift can be attributed to the spontaneous polarization within the HZO film along the external electric field. We anticipate that our results will stimulate further research on optical nonlinear effects, such as the Pockels effect, in ferroelectric HZO. This advancement will enable the development of various devices, including high-speed optical modulators. Consequently, HZO-based programmable PICs are poised to become indispensable in diverse applications, ranging from optical fiber communication and artificial intelligence to quantum computing and sensing