Energy Efficient Photonic Memory with Embedded III-V/Si Memristors

Introduction

Over the past few decades, processor performance has steadily increased according to Moore's Law. However, a fundamental bottleneck exists in current computer architectures known as the von Neumann bottleneck. This bottleneck limits the amount of data that can be transferred between the processor and memory. A potential solution to overcome this limitation is to implement non-volatile in-memory computing, which can surpass the constraints of traditional von Neumann designs.

Memristors, or memory resistors, are two-terminal devices that exhibit hysteretic current-voltage (I-V) behavior, enabling multi-bit non-volatile resistance states. These devices have emerged as a leading candidate for implementing analog-based neuromorphic computing systems that mimic the behavior of mammalian brains. Memristors allow for a high degree of integration density in the form of nanoscale crossbar arrays, yielding energy-efficient and parallelized in-memory computing without the need for data exchange between memory and a central processing unit.

Recently, optical memristors have been demonstrated based on three fundamental mechanisms: phase transitions, valency changes, and electrochemical metallization. These optical memristors can be classified into two groups based on their filamentary memristive opto-electronic functionality: non-volatile phase shifters and non-volatile absorbers.

In this tutorial, we will focus on non-volatile phase shifters based on embedded multi-layer HfO2/Al2O3 memristors integrated with III-V/Si photonics. These devices facilitate non-volatile optical functionality for various components, such as Mach-Zehnder Interferometers (MZIs) and (de-)interleaver filters, enabling energy-efficient and large-scale non-volatile photonic circuits for applications like neuromorphic optical networks, optical switching fabrics, optical phase arrays, quantum networks, and future optical computing architectures.

III-V/Si SISCAP Memristors

The III-V/Si semiconductor-insulator-semiconductor capacitor (SISCAP) memristor is comprised of a 300 nm thick p-type Si layer, alternating layers of HfO2/Al2O3, and a 150 nm thick n-type GaAs layer. The multi-layer HfO2/Al2O3 stack is chosen because the atomic inter-diffusion and promotion of oxygen vacancies (VO2+) at the HfO2/Al2O3 (HfAlO) interface improve resistive switching behavior.

Figure 1: (a) HRTEM image of the memristor stack, (b) "set" process involving oxygen vacancy (VO2+) formation and conductive filamentation, (c) "reset" process with filamentation break-up, (d) EDS line scan for atomic composition, (e) 3-D schematic of the test structure, and (f) I-V curves of the electro-forming, set, and reset processes.

During the "set" process, oxygen vacancies (VO2+) form in both HfO2, Al2O3, and at the HfAlO interface, initiating a conductive path for electrons to flow. This turns the high-impedance capacitor into a device exhibiting a low-resistance state. During the "reset" process, the interfacial filament is believed to rupture first due to Al2O3 having fewer VO2+ than HfO2, breaking the conductive path and restoring the high-resistance state.

III-V/Si Photonic SISCAP Memristors: Mach-Zehnder Interferometers (MZIs)

Figure 2: (a) SEM cross-section with simulated guided optical mode, (b) top view of the fabricated MZI device, and (c) device dimensions.

The optical waveguide of the III-V/Si MZI memristor has a width of 500 nm, a height of 300 nm, and an etch depth of 170 nm. The Si layer is p-type doped at 5 × 10^17 cm^-3 to ensure reasonable conductivity without significantly affecting optical loss. The multi-layer HfO2/Al2O3 stack sits on top of the silicon waveguide, followed by a 150 nm thick n-GaAs layer doped at 3 × 10^18 cm^-3.

Figure 3: Electro-optical measurements of the III-V/Si Mach-Zehnder memristor, including (a) measured I-V hysteresis, (b) corresponding resistance states, measured optical spectrum for (c) "set," (d) turning off "set," (e) "reset," (f) turning off "reset," (g) tracked resonance vs. voltage, and (h) spectral evolution vs. voltage.

The MZI memristor exhibits non-volatile optical phase shifts greater than π with ~33 dB signal extinction while consuming zero electrical power. By applying a bias from 0 to -21 V, a non-volatile wavelength shift of Δλ = 12.53 nm is observed, corresponding to a non-volatile phase shift of Δφ = 1.245π with negligible optical losses (< 0.5 dB). The group index change is calculated to be Δng,III-V/Si = 2.70 × 10^-3, which is significant.

Time duration tests were performed by biasing the MZI memristor into multiple non-volatile states, and the optical response was recorded for 24 hours. The multiple set states were stable within ±0.05 nm (8.77 GHz) over 24 hours, indicating stable non-volatile behavior and the possibility of multi-bit non-volatile weighting.

III-V/Si Photonic SISCAP Memristors: (De-)Interleavers

Figure 4: (a) Schematic of a 65 GHz 1-ring assisted Mach-Zehnder interferometer (RAMZI) (de-)interleaver with memristive phase tuning elements (green), and (b) top view of the fabricated device.

Ring-assisted asymmetric MZIs (RAMZIs) are useful for flat-top response with improved channel extinction and as linearized transfer functions for improved bit resolution in optical neural networks (ONNs) and RF photonics. In this case, a single-ring resonator assisted asymmetric Mach-Zehnder interferometer (1-RAMZI) is used, where the transmission passbands can be transformed by the III-V/Si SISCAP memristor on the ring or delay path.

Figure 5: Electro-optical measurements of the III-V/Si (de-)interleaver, including (a) measured I-V hysteresis, (b) corresponding resistance states, measured optical spectrum for (c) "set state," (d) turning off "set state," (e) "reset state," (f) turning off "reset state," and (g) overlapped spectrum for final initial, set, and reset states.

The (de-)interleaver memristor exhibits non-volatile passband transformation, transitioning from the initial state to the "set state" by applying a bias from 0 to -10 V. An attempt to reset the optical response is made by applying a bias from 0 to 5 V, resulting in a passband shape similar to the initial state with minor differences possibly associated with remaining VO2+ charge traps.

Figure 6: Time duration tests for a multi-state asymmetric Mach-Zehnder interferometer (AMZI) memristor, including (a) multiple set states for a 3rd order (de-)interleaver and (b) non-volatile stability over a 24-hour period by tracking spectra minima.

Time duration tests were performed on a separate 3rd order AMZI (de-)interleaver by achieving multiple non-volatile set states and measuring the optical response every 5 minutes for 24 hours. The filter shapes and tracked minima exhibited a maximum change of ±0.02 nm (3.51 GHz) over 24 hours, indicating non-volatile stability.

Comparison with Other Non-Volatile Photonic Technologies

Several competing technologies are capable of exhibiting optical non-volatility, including magneto-optical switches, ferro-electric materials, charge-trap devices, and phase-change materials. Table 1 from the PDF compares the performance metrics of the demonstrated III-V/Si memristor devices with other non-volatile photonic technologies.

Table 1: Experimental demonstrations of non-volatile phase shifter elements, including switching speed, switching energy, retention time, number of bits/states, footprint, extinction ratio, phase shift, insertion loss, cycles, and CMOS compatibility.

The III-V/Si memristive MZI demonstrates a π phase shift with <0.5 dB insertion loss, 33 dB extinction ratio, 6 non-volatile states, and non-volatile state retention lasting over 24 hours. While the switching energy is relatively high at 1500 nJ, the device consumes zero static power and offers a pathway for energy-efficient, large-scale non-volatile photonic circuits.

Non-Volatile Switching Dynamics

Figure 7: (a) Time measurement of non-volatile switching behavior in the memristive MZI, and (b) close-up image of electrical and optical dynamic quantities during the switching process.

By probing the device with high-speed voltage pulses, a non-volatile π phase shift was achieved with twenty 50 μs pulses at -15 V, resulting in a total non-volatile switching time of ~2 milliseconds and a switching energy of ~1500 nJ. The photodetector signal shows a clear permanent shift in optical power after the voltage pulses are turned off, indicating true zero static power consumption.

The switching dynamics reveal that a heat mechanism is required to initiate the electrical and optical non-volatility. During the first voltage pulse, a large change in optical amplitude occurs in conjunction with increased device current due to heat. After the pulse ends, no current is detected, but a noticeable residual change in the photodetector signal is observed, possibly due to charge trap defects and oxygen vacancy formation.

Conclusion

In this tutorial, we have discussed energy-efficient photonic memory based on electrically programmable embedded III-V/Si memristors. These devices combine non-volatile optical functionality with heterogeneous III-V/Si photonics, enabling various components such as Mach-Zehnder Interferometers (MZIs) and (de-)interleaver filters with non-volatile phase shifting and passband transformation.

The III-V/Si memristors exhibit non-volatile optical phase shifts greater than π with ~33 dB signal extinction and negligible optical losses while consuming zero static power. Time duration tests demonstrated stable non-volatile behavior for up to 24 hours, with the possibility of multi-bit non-volatile weighting. While the switching energy is relatively high, the devices offer a pathway for energy-efficient, large-scale non-volatile photonic circuits for applications like neuromorphic optical networks, optical switching fabrics, optical phase arrays, quantum networks, and future optical computing architectures.

By overcoming the von Neumann bottleneck and enabling in-memory optical computing, these embedded III-V/Si memristors pave the way for the next generation of high-performance, energy-efficient computing systems.

Reference

[2] Cheung, S., Tossoun, B., Yuan, Y. et al. Energy efficient photonic memory based on electrically programmable embedded III-V/Si memristors: switches and filters. Commun Eng 3, 49 (2024). https://doi.org/10.1038/s44172-024-00197-1