Researchers at the University of Rochester have discovered that the pyroelectric effect in lithium niobate significantly hinders the operation of semiconductor quantum dot devices. Measurements of integrated circuits pairing gallium arsenide/aluminum arsenide and silicon/silicon germanium quantum dots with lithium niobate surface acoustic wave resonators revealed unintended consequences; GaAs/AlGaAs devices experienced unintentional carrier depletion, while Si/SiGe devices suffered electrostatic discharge. The work highlights that “either a non-capacitive coupling mechanism or a non-pyroelectric substrate should be used” for continued progress in quantum interconnects and transducers.

Lithium Niobate Pyroelectricity Inhibits Quantum Dot Operation

Measurements revealed that the pyroelectric effect within lithium niobate actively interfered with the operation of quantum dots integrated into hybrid devices, specifically manifesting as distinct failures depending on the semiconductor material used. These observations stem from experiments involving flip-chip devices designed to capacitively couple quantum dots with surface acoustic wave resonators fabricated on lithium niobate substrates. The observed carrier depletion in GaAs/AlGaAs devices is believed to arise from a combination of lithium niobate’s pyroelectric properties and the piezoelectricity inherent in the gallium arsenide material itself.

Temperature fluctuations generate charged surfaces due to the pyroelectric effect, creating an electric field that induces strain within the GaAs heterostructure. This strain, coupled with the piezoelectric response of the material, effectively depletes the two-dimensional electron gas (2DEG) within the quantum well, hindering device operation. The quantum well in these heterostructures lies 57 nanometers below the surface, making it susceptible to this induced strain.

Silicon/silicon germanium quantum dots exhibited a different failure mode; the strong electric fields generated by the lithium niobate’s pyroelectric polarization led to dielectric breakdown and subsequent electrostatic discharge. These voltage spikes were initially observed below 200 Kelvin during the cooling process, disappearing as the sample reached its base temperature of approximately 4 Kelvin. Conversely, during warming, the spikes reappeared with reversed polarity, vanishing only when room temperature was attained.

Previous research groups have also documented similar voltage spikes in lithium niobate, attributing them to its pyroelectric behavior. Blank lithium niobate chips were also tested to measure any on-plane voltage differences, confirming the presence of these effects even without the semiconductor components. The experimental setup involved fabricating double quantum dots in a stadium-style geometry using standard processes, with one plunger gate terminating in a pad designed for capacitive coupling to the surface acoustic wave chip.

Addressing the effects of pyroelectricity in semiconductor-lithium niobate structures will be important for future advancements in charge- and spin-phonon coupling within quantum-dot-based quantum systems. The team’s findings highlight the need for strategies to minimize or compensate for the pyroelectric effect to realize the full potential of these hybrid devices for quantum interconnects and transducers. The research was supported by the Army Research Office, and Manas Ranjan Sahu acknowledges discussions with Habitamu Walelign and Jake Markowski during the fabrication of the quantum dot devices.

GaAs/AlGaAs Depletion Due to Combined Pyro/Piezoelectric Effects

Devices using GaAs/AlGaAs experienced unintentional carrier depletion, while those employing Si/SiGe suffered electrostatic discharge, both linked to the pyroelectric properties of the lithium niobate substrate. This observation emphasises a critical challenge in fabricating hybrid quantum devices and highlights the need for mitigation strategies during integration. GaAs/AlGaAs heterostructures, unlike their Si/SiGe counterparts, operate in depletion mode, meaning current can flow between contacts even without applied voltages.

This inherent characteristic, however, appears to exacerbate the impact of the pyroelectric effect, leading to a reduction in available charge carriers within the quantum dots. The research suggests that the pyroelectric polarization within the lithium niobate induces stress in the GaAs/AlGaAs material, altering its electrochemical potential and ultimately depleting the two-dimensional electron gas critical for quantum dot operation. This depletion occurs over a relatively large distance, hindering device performance and limiting the potential for scalable quantum interconnects.

Si/SiGe Electrostatic Discharge Linked to Lithium Niobate

Electrostatic discharge events destroyed gate electrodes on silicon/silicon germanium quantum dot devices integrated with lithium niobate substrates during cooldown, a finding that expands understanding of pyroelectric interference in hybrid quantum systems. While earlier tests on gallium arsenide/aluminum arsenide devices revealed unintentional carrier depletion linked to lithium niobate’s pyroelectric effect, these Si/SiGe devices exhibited a distinct failure mode, demonstrating a broader challenge for semiconductor-lithium niobate integration.

The observed destruction occurred even though electronic transport within the Si/SiGe devices themselves remained unaffected prior to the thermal cycling. Optical images confirmed the physical destruction of both the gates on the Si/SiGe devices and the interdigital transducers on the lithium niobate substrates, documenting the extent of the electrostatic discharge. The observed failures in Si/SiGe devices contrast with those in GaAs/AlGaAs quantum dots in a key respect: the operational mode of the quantum dots themselves.

GaAs/AlGaAs heterostructures are doped, operating in depletion mode where current flows without applied voltages, while Si/SiGe quantum dots operate in accumulation mode, requiring a gate voltage to enable current flow. This difference in operating conditions likely contributes to the distinct failure mechanisms observed, with the pyroelectric field inducing carrier depletion in the former and dielectric breakdown in the latter. The research team observed similar discharge events across multiple Si/SiGe-lithium niobate flip-chip devices, reinforcing the consistency of the findings.

The quantum well in the Si/SiGe heterostructures used in the experiments lies 50 nanometers below the surface, a depth comparable to the 57 nanometers observed in the GaAs/AlGaAs samples, suggesting that the proximity of the quantum well to the surface does not fully mitigate the pyroelectric effects. The findings emphasize the need for careful material selection and mitigation strategies when integrating semiconductors with lithium niobate for quantum interconnects and transducers, as continued progress depends on addressing these fundamental challenges.

Surface Acoustic Wave Resonators Fabricated on Lithium Niobate

GaAs/AlGaAs quantum dots exhibited unintentional carrier depletion when integrated with lithium niobate, a consequence of the pyroelectric effect impacting device characteristics. Conversely, Si/SiGe quantum dots suffered from electrostatic discharge under the same conditions, indicating a different failure mode triggered by the same underlying pyroelectric phenomenon. This divergence in failure mechanisms highlights the material-specific responses to the pyroelectric effect, with depletion affecting GaAs/AlGaAs and discharge impacting Si/SiGe.

The design of the surface acoustic wave resonators incorporated features intended to maximize performance, including high impedance and minimal stray capacitance. These resonators, with resonance frequencies around 5 GHz, were fabricated on dual-side-polished black 128°Y-cut lithium niobate substrates, using 40-nanometer-thick aluminum for mirrors and interdigital transducers. The choice of lithium niobate was motivated by its strong piezoelectricity and its pyroelectric properties, which ultimately proved problematic.

The team employed a flip-chip assembly technique, using resist spacers to control the distance between the quantum dot chip and the surface acoustic wave resonator chip, a method previously reported by Satzinger et al. While the initial observation of the SAW resonance demonstrated successful inter-chip capacitive coupling, the researchers noted that this did not preclude the detrimental effects of pyroelectricity on the quantum dots themselves. Further analysis revealed that the pyroelectric effect, characterized by a change in polarization with temperature, generated charge accumulation that disrupted quantum dot operation.

The researchers suggest that alternative coupling strategies or the substitution of lithium niobate with a non-pyroelectric material could mitigate the detrimental effects and enable the realization of functional hybrid quantum devices. The study’s results emphasise the need for a thorough understanding of material properties and their impact on quantum device performance.

In summary, the work demonstrates that the pyroelectric polarization in lithium niobate can deplete carriers in GaAs/AlGaAs devices and destroy quantum dot gates through electrostatic discharge in Si/SiGe devices. “In both cases, we could not operate QDs as expected, likely because of effects related to pyroelectricity in LN,” the paper concludes.

👉 More information
🗞 Pyroelectric effects in semiconductor–lithium niobate hybrid quantum devices
✍️ Manas Ranjan Sahu, Suraj Thapa Magar, Yadav Prasad Kandel and John M. Nichol
🧠 DOI: http://link.aps.org/doi/10.1103/4kpk-1ywk

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