[Paper Review] Ferroelectricity in Polar ScAlN/GaN Epitaxial Semiconductor Heterostructures

Joseph Casamento, Ved Gund+12 more|arXiv (Cornell University)|May 21, 2021

Acoustic Wave Resonator Technologies4 citations

TL;DR

This study demonstrates room-temperature ferroelectricity in lattice-matched, epitaxial ScAlN/GaN heterostructures grown by molecular beam epitaxy on GaN substrates. With a low coercive field of ~0.7 MV/cm and remnant polarization of ~10 μC/cm² in ~100 nm thick layers, the MBE-grown ScAlN exhibits stable, repeatable polarization switching, enabling low-voltage ferroelectric operation compatible with GaN’s inherent polarization and electronic properties.

ABSTRACT

Room temperature ferroelectricity is observed in lattice-matched ~18% ScAlN/GaN heterostructures grown by molecular beam epitaxy on single-crystal GaN substrates. The epitaxial films have smooth surface morphologies and high crystallinity. Pulsed current-voltage measurements confirm stable and repeatable polarization switching in such ferroelectric/semiconductor structures at several measurement conditions, and in multiple samples. The measured coercive field values are Ec~0.7 MV/cm at room temperature, with remnant polarization Pr~10 μC/cm2 for ~100 nm thick ScAlN layers. These values are substantially lower than comparable ScAlN control layers deposited by sputtering. Importantly, the coercive field of MBE ScAlN is smaller than the critical breakdown field of GaN, offering the potential for low voltage ferroelectric switching. The low coercive field ferroelectricity of ScAlN on GaN heralds the possibility of new forms of electronic and photonic devices with epitaxially integrated ferroelectric/semiconductor heterostructures that take advantage of the GaN electronic and photonic semiconductor platform, where the underlying semiconductors themselves exhibit spontaneous and piezoelectric polarization.

Motivation & Objective

• To achieve stable, room-temperature ferroelectricity in ScAlN/GaN heterostructures for integration with III-nitride semiconductor platforms.
• To demonstrate epitaxial growth of high-quality ScAlN on GaN substrates using molecular beam epitaxy (MBE).
• To achieve low coercive field values in ScAlN to enable low-voltage ferroelectric switching compatible with GaN's breakdown field.
• To compare ferroelectric properties of MBE-grown ScAlN with sputtered control layers, highlighting growth method impact.
• To enable new ferroelectric/semiconductor heterostructures for electronic and photonic devices leveraging GaN's intrinsic polarization.

Proposed method

• Molecular beam epitaxy (MBE) was used to grow lattice-matched, ~18% ScAlN layers on single-crystal GaN substrates.
• Epitaxial films were characterized for surface morphology and crystallinity using high-resolution techniques.
• Pulsed current-voltage measurements were performed to assess polarization switching stability and reversibility.
• Coercive field (Ec) and remnant polarization (Pr) were extracted from hysteresis loops under varying conditions and in multiple samples.
• The ferroelectric properties of MBE-grown ScAlN were compared with those of sputtered ScAlN control layers.
• The critical breakdown field of GaN was used as a benchmark to evaluate the feasibility of low-voltage operation.

Experimental results

Research questions

• RQ1 Can room-temperature ferroelectricity be achieved in epitaxial ScAlN/GaN heterostructures grown by molecular beam epitaxy?
• RQ2 What are the coercive field and remnant polarization values in MBE-grown ScAlN on GaN, and how do they compare to sputtered layers?
• RQ3 Is the coercive field of MBE-grown ScAlN sufficiently low to enable low-voltage ferroelectric switching without exceeding GaN's breakdown field?
• RQ4 Can stable and repeatable polarization switching be demonstrated across multiple samples and measurement conditions?
• RQ5 To what extent do the intrinsic spontaneous and piezoelectric polarizations of GaN influence the functionality of the heterostructure?

Key findings

• Room-temperature ferroelectricity was successfully demonstrated in lattice-matched, ~18% ScAlN/GaN heterostructures grown by molecular beam epitaxy.
• The MBE-grown ScAlN films exhibited smooth surface morphology and high crystallinity, indicating high-quality epitaxial growth.
• Pulsed current-voltage measurements confirmed stable and repeatable polarization switching across multiple samples and conditions.
• The coercive field was measured at ~0.7 MV/cm at room temperature, significantly lower than in sputtered ScAlN control layers.
• Remnant polarization reached ~10 μC/cm² for ~100 nm thick ScAlN layers, indicating strong ferroelectric response.
• The coercive field of MBE-grown ScAlN is below GaN’s critical breakdown field, enabling low-voltage ferroelectric operation.