Nonvolatile optical phase shift in ferroelectric hafnium zirconium oxide.

A nonvolatile optical phase shifter is a critical component for enabling the fabrication of programmable photonic integrated circuits on a Si photonics platform, facilitating communication, computing, and sensing. Although ferroelectric materials such as BaTiO3 offer nonvolatile optical phase shift capabilities, their compatibility with complementary metal-oxide-semiconductor fabs is limited. Hf0.5Zr0.5O2 is an emerging ferroelectric material, which exhibits complementary metal-oxide-semiconductor compatibility. Although extensively studied for ferroelectric transistors and memories, its application to photonics remains relatively unexplored. Here, we show the optical phase shift induced by ferroelectric Hf0.5Zr0.5O2. We observed a negative change in refractive index at a 1.55 µm wavelength in a pristine device regardless of the direction of the applied electric field. The nonvolatile phase shift was only observed once in a pristine device. This non-reversible phase shift can be attributed to the spontaneous polarization within the Hf0.5Zr0.5O2 film along the external electric field.

Design of compact and low-loss S-bends by CMA-ES.

We employ the covariance matrix adaptation evolution strategy (CMA-ES) algorithm to design compact and low-loss S-bends on the standard silicon-on-insulator platform. In line with the CMA-ES-based approach, we present experimental results demonstrating insertion losses of 0.041 dB, 0.025 dB, and 0.011 dB for S-bends with sizes of 3.5 µm, 4.5 µm, and 5.5 µm, respectively, which are the lowest insertion losses within the footprint range smaller than approximately 30 µm2. These outcomes underscore the remarkable performance and adaptability of the CMA-ES to design Si photonics devices tailored for high-density photonic integrated circuits.

Ultrahigh-responsivity waveguide-coupled optical power monitor for Si photonic circuits operating at near-infrared wavelengths.

A phototransistor is a promising candidate as an optical power monitor in Si photonic circuits since the internal gain of photocurrent enables high responsivity. However, state-of-the-art waveguide-coupled phototransistors suffer from a responsivity of lower than 103 A/W, which is insufficient for detecting very low power light. Here, we present a waveguide-coupled phototransistor operating at a 1.3 µm wavelength, which consists 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 106 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 µs make our phototransistor promising as an effective optical power monitor in Si photonic circuits.

Low Operating Voltage, Improved Breakdown Tolerance, and High Endurance in Hf0.5Zr0.5O2 Ferroelectric Capacitors Achieved by Thickness Scaling Down to 4 nm for Embedded Ferroelectric Memory.

The comparatively high coercive field in Hf0.5Zr0.5O2 (HZO) and other HfO2-based ferroelectric thin films leads to two critical challenges for their application in embedded ferroelectric memory: high operating voltage due to a large thickness-field product and poor endurance due to the high operating field close to the breakdown field. In this study, we demonstrate that the thickness scaling of ferroelectric HZO down to 4 nm is a promising approach to overcome these challenges. As the coercive voltage scales down almost linearly with the film thickness, the operating voltage of 4 nm HZO is reduced to 0.6 V for one-shot operation and 1.2 V for stable memory operation, which is in the voltage range compatible with scaled silicon technologies. Furthermore, it is found that the breakdown field is substantially improved in thinner HZO since the breakdown mechanism is dominated by the stress voltage, not the stress field, resulting in improved cycle-to-breakdown by more than 4 orders of magnitude when thinning from 9.5 to 4 nm. We identify two concerns accompanying thickness scaling: the increase in crystallization temperature and the pinched hysteresis behavior, which can be addressed by carefully preparing temperature-thickness mapping and applying strong-field wake-up cycling, respectively. Our optimal 4 nm-thick HZO ferroelectric capacitor exhibits an operating voltage of 1.2 V with over 10 year data retention and 1012 endurance cycles at 100 kHz, which can be further improved to more than 1014 with a smaller capacitor size and higher operating frequency.

Modulation bandwidth improvement of III-V/Si hybrid MOS optical modulator by reducing parasitic capacitance.

In this work, we numerically and experimentally examined the impact of parasitic capacitance on the modulation bandwidth of a III-V/Si hybrid metal-oxide-semiconductor (MOS) optical modulator. The numerical analysis revealed that the parasitic capacitance between the III-V membrane and the Si slab should be considered to achieve high-speed modulation, particularly in the case of a thick gate oxide. We also fabricated a high-speed InGaAsP/Si hybrid MOS optical modulator with a low capacitance using a SiO2-embedded Si waveguide. The fabricated device exhibited a modulation efficiency of 0.245 Vcm and a 3 dB bandwidth of up to 10 GHz. Clear eye patterns with 25 Gbps non-return-to-zero (NRZ) modulation and 40 Gbps 4-level pulse amplitude modulation (PAM-4) were obtained without pre-emphasis.

Monolithic integration of electro-absorption modulators and photodetectors on III-V CMOS photonics platform by quantum well intermixing.

Quantum well intermixing (QWI) on a III-V-on-insulator (III-V-OI) substrate is presented for active-passive integration. Shallow implantation at a high temperature, which is essential for QWI on a III-V-OI substrate, is accomplished by phosphorus molecule ion implantation. As a result, the bandgap wavelength of multi-quantum wells (MQWs) on a III-V-OI substrate is successfully tuned by approximately 80 nm, enabling the monolithic integration of electro-absorption modulators and waveguide photodetectors using a lateral p-i-n junction formed along the InP/MQW/InP rib waveguide. Owing to the III-V-OI structure and the rib waveguide structure, the parasitic capacitance per unit length can be reduced to 0.11 fF/µm, which is suitable for high-speed and low-power modulators and photodetectors. The presented QWI can extend the possibility of a III-V complementary metal-oxide-semiconductor (CMOS) photonics platform for large-scale photonic integrated circuits.

Two-layer integrated photonic architectures with multiport photodetectors for high-fidelity and energy-efficient matrix multiplications.

Photonic integrated circuits (PICs) are emerging as a promising tool for accelerating matrix multiplications in deep learning. Previous PIC architectures, primarily focusing on the matrix-vector multiplication (MVM), have large hardware errors that increase with the device scale. In this work, we propose a novel PIC architecture for MVM, which features an intrinsically small hardware error that does not increase with the device scale. Moreover, we further develop this concept and propose a PIC architecture for the general matrix-matrix multiplication (GEMM), which allows the GEMM to be directly performed on a photonic chip with a high energy efficiency unattainable by parallel or sequential MVMs. This work provides a promising approach to realize a high fidelity and high energy efficiency optical computing platform.

A floating gate negative capacitance MoS2 phototransistor with high photosensitivity.

Monolayer MoS2 exhibits interesting optoelectronic properties that have been utilized in applications such as photodetectors and light emitting diodes. For image sensing applications, improving the light sensitivity relies on achieving a low dark current that enables the detection weak light signals. Although previous reports on improving the detectivity have been explored with heterostructures and pn junction devices, some of these approaches lack CMOS compatibility processing and sufficient low dark current suppression. Steep slope transistors that overcome the Boltzmann tyranny can further enhance the performance in photodetectors by providing efficient extraction of photogenerated charges. Here, we report a monolayer MoS2 floating gate negative capacitance phototransistor with the integration of a hafnium-zirconium oxide ferroelectric capacitor. In this study, a SSmin of 30 mV dec-1, very low dark currents of 10-13-10-14 A, and a high detectivity of 7.2 × 1015 cm Hz1/2 W-1 were achieved under weak light illumination due to an enhancement in the photogating effect. In addition, its potential as an optical memory and as an optical synapse with excellent long-term potentiation characteristics in an artificial neural network was also explored. Overall, this device structure offers high photosensitivity to weak light signals for future low-powered optoelectronic applications.

Si microring resonator optical switch based on optical phase shifter with ultrathin-InP/Si hybrid metal-oxide-semiconductor capacitor.

We propose a microring resonator (MRR) optical switch based on III-V/Si hybrid metal-oxide-semiconductor (MOS) optical phase shifter with an ultrathin InP membrane. By reducing the thickness of the InP membrane, we can reduce the insertion loss of the phase shifter, resulting in a high-quality-factor (Q-factor) MRR switch. By optimizing the device structure using numerical analysis, we successfully demonstrated a proof-of-concept MRR optical switch. The optical switch exhibits 0.3 pW power consumption for switching, applicable to power-efficient, thermal-crosstalk-free, Si programmable photonic integrated circuits (PICs) based on wavelength division multiplexing (WDM).

Taperless Si hybrid optical phase shifter based on a metal-oxide-semiconductor capacitor using an ultrathin InP membrane.

We propose a III-V/Si hybrid metal-oxide-semiconductor (MOS) optical phase shifter using an ultrathin InP membrane, which allows us to eliminate the III-V taper required for mode conversion between Si and hybrid waveguides. We numerically revealed that thinning a III-V membrane can reduce the insertion loss of the phase shifter while maintaining high modulation efficiency because the optical phase shift is induced by carrier accumulation at the MOS interface. We experimentally demonstrated the proposed optical phase shifter with an ultrathin InP membrane and achieved the modulation efficiency of 0.54 Vcm and the insertion loss of 0.055 dB. Since the taperless structure makes the hybrid integration easier and more flexible, the hybrid MOS optical phase shifter with an ultrathin III-V membrane is promising for large-scale Si programmable photonic integrated circuits.

Numerical analyses of optical loss and modulation bandwidth of an InP organic hybrid optical modulator.

We numerically analyzed the modulation characteristics of an InP organic hybrid (IOH) optical modulator consisting of an InP slot waveguide and an electro-optic (EO) polymer. Since InP has a higher electron mobility and a lower electron-induced free-carrier absorption than Si, the series resistance of an InP slot waveguide can be significantly reduced with relatively smaller optical loss than an Si slot waveguide. As a result, the trade-off between optical loss and modulation bandwidth can be remarkably improved compared with a Si organic hybrid (SOH) optical modulator. When the modulation bandwidth was designed to be 100 GHz, the optical loss of the IOH modulator was 13-fold smaller than that of the SOH one. The simulation of the eye diagram revealed that the improved optical modulation amplitude enabled the clear eye opening with a 100 Gbps non return-to-zero signal using the IOH modulator. The IOH integration is promising for a high-speed modulator with low energy consumption beyond 100 Gbps.