---
title: Scandium-Doped Aluminum Nitride (AlScN)
url: https://www.emergentmind.com/topics/scandium-doped-aluminum-nitride-alscn
type: topic
---

# Scandium-Doped Aluminum Nitride (AlScN)

Scandium-doped aluminum nitride, usually written \( \mathrm{Al_{1-x}Sc_xN} \) and abbreviated AlScN or ScAlN, is a wurtzite III-nitride alloy in which substitution of Al by Sc converts aluminum nitride from a conventional polar piezoelectric into a multifunctional material platform spanning ferroelectricity, enhanced piezoelectric transduction, high-\(k\) nitride heterostructures, and second-order optical nonlinearity. In the literature represented here, AlScN is used in bulk and surface acoustic resonators, guided-phonon waveguides, GaN electronic heterostructures, electro-optic and hybrid photonic circuits, ferroelectric memories, and extreme-environment transducers. Its recurring technological appeal is the combination of CMOS compatibility, scalable thin-film deposition, large remanent polarization, strong electromechanical coupling, and useful functionality in regimes ranging from ultrathin ferroelectrics to \(\sim 18\) GHz RF filters and operation at \(1000^\circ\mathrm{C}\) [1810.07968][2406.15431][2410.17037].

## 1. Crystal chemistry and the origin of functionality

AlScN is the solid solution \( \mathrm{Al_{1-x}Sc_xN} \), and its central materials significance is that Sc alloying continuously distorts the parent wurtzite AlN lattice toward a layered-hexagonal limit. In the first report of ferroelectric switching in a III-V semiconductor based material, increasing Sc content was described as increasing lattice “softness” and raising the internal parameter \(u\) toward \(1/2\), where the structure approaches the nonpolar layered-hexagonal configuration; tensile residual stress was shown to drive the same trend and to reduce the coercive field linearly in \( \mathrm{Al_{0.73}Sc_{0.27}N} \) when the residual stress was varied from about \(-0.8\) GPa to \(+0.5\) GPa [1810.07968].

Later combinatorial work formalized two complementary Sc-driven mechanisms. First, Sc has a much larger ionic radius than Al, which changes the lattice geometry, especially by reducing the \(c/a\) ratio. Second, Sc is less electronegative than Al, so Sc–N bonding is more ionic, weakening directional covalency and easing atomic displacement during switching. In the AlScN and AlScBN composition space, increasing Sc makes \(c\) decrease and \(a\) increase, lowering \(c/a\), while high-throughput XPS charge-transfer analysis showed that the energy difference \( \Delta BE_{N1s-Al2s} = BE(N\,1s)-BE(Al\,2s) \) decreases from about \(278.2\) eV at \(5\%\) Sc to about \(277.75\) eV at \(\sim 37.5\%\) Sc, correlating bond ionicity with coercive-field reduction [2606.11954].

The same alloying that enables ferroelectricity also amplifies other functional responses. Compared with AlN, AlScN is described as having enhanced second-order nonlinear and piezoelectric properties while maintaining a relatively large bandgap, and in RF-acoustic contexts it combines a substantially enhanced piezoelectric response with a relatively high dielectric constant, reduced mechanical compliance, low dielectric loss, and CMOS compatibility [2405.18717][2404.15005]. This coexistence of polar semiconductor character, strong piezoelectricity, and switchable polarization is the defining feature that distinguishes AlScN from both conventional III-nitrides and oxide ferroelectrics.

## 2. Thin-film growth, texture control, and epitaxial integration

A notable feature of AlScN research is the breadth of viable growth routes. Reactive sputtering underlies many BEOL-compatible ferroelectric and acoustic implementations, including AlScN deposited at about \(350^\circ\text{C}\) for a MoS\(_2\) ferroelectric field-effect transistor and at \(350^\circ\text{C}\) for a ferroelectric tunnel junction memristor grown directly on Si/Pt, as well as \(300^\circ\text{C}\) deposition for ultrathin AlBScN capacitors [2010.12062][2012.10019][2511.08540]. Sputtering is also used for AlScN directly on SiC wafers, an important route for Sezawa-mode SAW devices and laterally confined phononic waveguides [2311.08694][2503.18113].

For epitaxial nitride electronics, both MOCVD and MBE have established compositionally controlled AlScN/GaN integration. MOCVD growth with trimethylaluminum, \(\mathrm{NH_3}\), hydrogen carrier gas, and bis(methylcyclopentadienyl)scandiumchloride \(((\mathrm{MCp})_2\mathrm{ScCl})\) achieved Sc incorporation up to \(\sim 13\%\), with XPS identifying \(\mathrm{Al_{0.87}Sc_{0.13}N}\) and cross-sectional S/TEM confirming preservation of the wurtzite framework in both films and AlScN/AlN/GaN heterostructures [2510.12074]. Plasma-assisted MBE, meanwhile, identified a lattice-matched composition window on GaN at \(x = 0.09\text{--}0.11\), with an experimentally fitted relation \( a = 3.13 + 0.636x \) and a lattice-matched composition of \( x = 0.094 \pm 0.015 \); using that condition, pseudomorphic ten- and twenty-period AlScN/GaN multilayers were realized with excellent structural and interface properties [2410.09153].

These reports also make clear that the lattice-matched condition is not a universal scalar. One MOCVD study states that AlScN can be lattice matched to GaN in the \(\sim 9\%\text{--}14\%\) Sc range, whereas the MBE study ties lattice matching to the growth method and growth temperature and uses that dependence to explain why earlier reports ranged broadly from \(\sim 9\%\) to \(20\%\) Sc [2510.12074][2410.09153]. A common outcome across methods is strong \(c\)-axis texture or oriented wurtzite growth, which is crucial because the dominant ferroelectric, piezoelectric, and electro-optic responses are all tied to the polar axis.

## 3. Ferroelectricity, switching kinetics, and coercive-field engineering

Ferroelectric AlScN was first established through nearly box-like \(P\)-\(E\) hysteresis loops in \( \mathrm{Al_{1-x}Sc_xN} \) with \(x = 0.27\) to \(0.43\), polarity-specific wet etching after polarization inversion, stable switching polarization over \(10^5\) s, and frequency-independent switched polarization over more than two orders of magnitude in frequency. In that work, the coercive field was reported as \(> 4\) MV/cm at \(x = 0.27\) and \(< 2\) MV/cm at \(x = 0.43\), with high remanent polarization of \(110~\mu\mathrm{C/cm^2}\), an almost ideally square-like hysteresis, and a paraelectric transition temperature in excess of \(600^\circ\mathrm{C}\) [1810.07968].

Subsequent studies frame the main materials limitation as the unusually high coercive field. AlScN is described as combining a wide band gap, good thermal stability, and very large remanent polarization, with reported values around \(P_r \approx 165\,\mu\mathrm{C}/\mathrm{cm}^2\), nearly square \(P\)–\(E\) loops, and stability up to \(1100^\circ\mathrm{C}\), but with a coercive field on the order of MV/cm—two to three orders of magnitude above many oxide ferroelectrics [2509.00705]. Atomistic analysis in that study argues against a purely collective M-polar \(\rightarrow\) nonpolar hexagonal \(\rightarrow\) N-polar pathway as the dominant mechanism in realistic films: for \( \mathrm{Al_{0.75}Sc_{0.25}N} \), the concerted transition was found to induce nearly \(15\%\) \(c\)-axis strain, whereas pre-existing \(180^\circ\) domain walls provide a strain-relief route. The field dependence is correspondingly split: at low fields, switching is governed mainly by gradual domain-wall propagation and is well described by the Kolmogorov–Avrami–Ishibashi model, while at high fields additional nucleation events appear and the simultaneous non-linear nucleation and growth model gives the best description. The same work reports that the switching field \(E_s\) and the activation field \(E_a\) both decrease monotonically with increasing Sc content across roughly \(15\%\) to \(40\%\) Sc [2509.00705].

Interfacial engineering has emerged as a direct route to reduce the ultrathin-film switching penalty. In \( \mathrm{Al_{0.68}Sc_{0.32}N} \) capacitors, a Sc bottom electrode was shown to reduce the coercive field by over \(20\%\) compared to an Al bottom electrode, including a \(\sim 23.1\%\) reduction in \(E_c^-\) at \(25\) kHz and a \(21.4\%\) reduction at \(50\) kHz. The same study used the KAI relation \( E_c = E_0 \cdot f^\alpha \), finding low-frequency exponents of \(0.036\) for Sc and \(0.028\) for Al bottom electrodes, while at higher frequencies the Al-electrode device showed a much larger exponent of \(0.093\) and the Sc-electrode device remained at \(0.036\); SEND strain mapping linked this behavior to reduced lattice mismatch and lower in-plane residual strain in the Sc-template stack [2506.10169]. This suggests that AlScN coercive-field engineering is not only a compositional problem but also an interface- and strain-management problem.

## 4. Acoustic resonators, filters, and guided phonons

The most mature non-ferroelectric exploitation of AlScN is as a high-coupling piezoelectric for GHz acoustics. In overmoded bulk acoustic resonators, the material enables operation beyond the thickness-scaling limits of conventional FBARs by preserving useful coupling when the acoustic field is intentionally distributed across both the piezoelectric layer and the electrodes. A \(13\) GHz \(30\%\) Sc-doped ScAlN OBAR using a Pt/ScAlN/AlSiCu stack and a \(250\) nm ScAlN layer demonstrated second-overtone resonance at \(f_s \approx 13.2\text{--}13.3\) GHz with \(k^2 \approx 5.1\%\) to \(5.2\%\) and \(Q_s \approx 210\); the best measured fundamental tone gave \(f_s \approx 4.2\text{--}4.23\) GHz with \(k^2 \approx 15.8\%\), and the measured figure of merit was about \(10\) for both tones. The same work emphasizes the OBAR tradeoff through \( Q_m = n\,Q_{\text{piezo}} + (1-n)\,Q_{\text{metal}} \) and \( \mathrm{FOM} = k^2 \cdot Q_m \), and shows that pentagonal geometries suppress transverse spurious modes more effectively than circular ones [2404.15005].

At still higher frequency, periodically poled AlScN bulk acoustic structures use thickness-wise polarity engineering to avoid impractically thin films. A four-layer AlScN periodically poled piezoelectric film operating in the fourth thickness-extensional mode reached \( f_p = 17.9 \) GHz with \( k_t^2 = 11.8\% \), \( Q_p = 236.6 \), \( (\mathrm{FoM})_1 = 27.9 \), and \( (\mathrm{FoM})_2 = 500 \). Resonator-derived filters at \(\sim 17.4\) GHz achieved insertion losses of \(1.86\) dB for a three-element filter and \(3.25\) dB for a six-element filter, \(-3\) dB bandwidths of \(680\) MHz and \(590\) MHz, and in-band IIP3 values of \(+36\) dBm and \(+40\) dBm, respectively [2406.15431]. At lower frequency but larger fractional bandwidth, an Al\(_{0.76}\)Sc\(_{0.24}\)N two-dimensional resonant rods resonator reached \(5.31\) GHz with \( k_t^2 = 23.9\% \), \(Q_m = 101\), and \(k_t^2 Q_m = 24\), supporting projected fifth-order ladder filters with \(\sim 11.1\%\) fractional bandwidth, \(\sim 2.5\) dB insertion loss, and \(>30\) dB out-of-band rejection [2202.11284].

AlScN on SiC has also become a distinct platform for surface and guided acoustics. Sezawa-mode SAW resonators in \(\mathrm{Al_{0.58}Sc_{0.42}N/4H\text{-}SiC}\) reached \(5.9\) GHz at \(0.96~\mu\mathrm{m}\) wavelength with \(K^2 = 4.0\%\) and \(Q_{\max} = 887\), while the maximum measured \(K^2 = 5.5\%\) and \(Q_{\max} = 1048\) were reported at \(4.7\) GHz [2311.08694]. Moving beyond slab acoustics, two-dimensionally confined AlScN/SiC phononic waveguides with a \(1~\mu\)m-thick AlScN film at \(42\%\) Sc support Rayleigh-like and Sezawa-like guided modes; at \(\lambda = 1.6~\mu\mathrm{m}\), the Sezawa-like mode has velocity about \(6480\) m/s, resonance at \(4.05\) GHz, simulated \(k^2 = 4.27\%\), experimentally extracted \(k^2 = 6.08 \pm 1.2\%\), slab loss \(5.3 \pm 0.2\) dB/mm, and straight-waveguide loss \(10.7 \pm 1.7\) dB/mm [2503.18113]. A plausible implication is that AlScN’s role in acoustics is no longer limited to resonators: it now spans overmoded RF transducers, Sezawa-mode SAW, and laterally routed phononic circuitry.

## 5. GaN heterostructures, polarization-engineered transport, and memory devices

In nitride electronics, AlScN often functions as a polarization-engineered barrier rather than only as a ferroelectric. MOCVD-grown AlScN/AlN/GaN heterostructures on GaN templates showed clear 2DEG formation at the AlScN/AlN–GaN interface, with C–V-extracted sheet densities of \(1.59\times10^{12}\) cm\(^{-2}\), \(2.84\times10^{12}\) cm\(^{-2}\), and \(5.22\times10^{12}\) cm\(^{-2}\) for \(\sim 10\), \(\sim 20\), and \(\sim 30\) nm barriers, respectively. Hall measurements gave larger values of approximately \(4.0\times10^{13}\), \(4.8\times10^{13}\), and \(6.2\times10^{13}\) cm\(^{-2}\), with mobilities of \(780\), \(646\), and \(562\) cm\(^2\)/Vs; the paper attributes the Hall/C–V discrepancy to surface oxidation, buffer conduction, and mercury-contact-area uncertainty, but both methods show that thicker barriers yield higher sheet charge [2510.12074].

MBE studies extend this concept to multichannel transport. One report identified lattice-matched AlScN/GaN multilayers and measured net mobile charge densities of \( -4.63 \times 10^{14}\,\text{cm}^{-2} \) for ten periods and \( -8.24 \times 10^{14}\,\text{cm}^{-2} \) for twenty periods, with charge scaling approximately with the number of AlScN/GaN periods [2410.09153]. A later transport-focused study then introduced GaN/AlN interlayers to improve conductivity in single-channel structures and leveraged the result in multi-channel heterostructures. In the best single-channel sample, room-temperature mobility reached \(1370\ \text{cm}^2/\text{V·s}\) with \(R_{\rm sh}=181\ \Omega/\square\), while a five-channel structure reached \(n_s = 8.90\times10^{13}\ \text{cm}^{-2}\), \(\mu = 1550\ \text{cm}^2/\text{V·s}\), and \(R_{\rm sh} = 45\ \Omega/\square\) at \(300\) K, falling to \(13\ \Omega/\square\) at \(2\) K [2511.10849]. These results place AlScN/GaN alongside state-of-the-art multi-channel nitride systems for RF and high-power electronics.

Ferroelectric memory implementations use a different aspect of the same material system. A post-CMOS-compatible AlScN/MoS\(_2\) ferroelectric field-effect transistor employed a \(100\) nm \( \mathrm{Al_{0.71}Sc_{0.29}N} \) gate dielectric deposited at about \(350^\circ\text{C}\), producing an ON/OFF ratio of \(\sim 10^6\), a memory window of about \(35\) V, a normalized memory window of \(0.3~\text{V/nm}\), and stable two-state retention for up to \(10^4\) s [2010.12062]. A CMOS-back-end-of-line-compatible ferroelectric tunnel junction memristor based on \(20\) nm \( \mathrm{Al_{0.64}Sc_{0.36}N} \) deposited at \(350^\circ\text{C}\) showed diode-like polarity-dependent switching, rectification ratio \(>10^5\) at \(5\) V, On/Off ratio \(\sim 50{,}000\), and retention \(> 1{,}000\) s at \(300\) K, with transport best described by Poole–Frenkel tunneling [2012.10019]. Together these studies establish that AlScN supports both polarization-induced channel formation in GaN heterostructures and nonvolatile ferroelectric state variables in memory devices.

## 6. Photonics, electro-optics, and multifunctional transduction

AlScN has become attractive in integrated photonics because the same Sc alloying that enhances piezoelectricity also amplifies the second-order nonlinear response relevant to the Pockels effect. In silicon-integrated electro-optics, sputtered \(\mathrm{Al_{0.904}Sc_{0.096}N}\) on insulator was used as both the light-guiding medium and electro-optic medium in micro-ring resonators. The devices showed a maximum in-device effective EO coefficient of \(2.86\) pm/V at \(12\) GHz, a minimum \(V_\pi L\) of \(3.12\ \mathrm{V\cdot cm}\) at \(14\) GHz, and a \(3\)-dB modulation bandwidth of approximately \(22\) GHz; the same work notes that the \(d_{33}\) coefficient of \(\mathrm{Al_{0.9}Sc_{0.1}N}\) is about \(3\times\) that of AlN and that \(\mathrm{Al_{0.64}Sc_{0.36}N}\) reaches about \(12\times\) enhancement [2405.18717].

A central challenge for direct ScAlN photonics has been optical loss. Hybrid \(\mathrm{Si_3N_4}\)-ScAlN circuits address this by confining the optical mode in etched \(\mathrm{Si_3N_4}\) while retaining the functional ScAlN layer underneath. In a monolithic sapphire / \(90\) nm AlN / \(200\) nm \(\mathrm{Sc_{0.1}Al_{0.9}N}\) / \(500\) nm \(\mathrm{Si_3N_4}\) stack, the best Euler racetrack resonator reached \( Q_{\rm i}=3.35\times 10^5 \) at \( \lambda_0 = 1616.97\ \mathrm{nm} \), corresponding to a propagation loss of \(1.03\ \mathrm{dB/cm}\), compared with previously reported direct ScAlN photonic losses typically above \(2.4\ \mathrm{dB/cm}\) [2508.00314]. This architecture is significant because it separates low-loss waveguiding from the functional nitride layer rather than requiring the ScAlN itself to be the etched optical core.

Multifunctional transduction appears most clearly in resonant IR detection. A plasmonically enhanced flexural-mode AlScN nanoplate resonator using \(30\%\)-doped AlScN and a gold cross-shaped metasurface absorber was reported as an uncooled, ultrafast IR detector with \(f_s = 16.69\) MHz, \(Q = 759\), experimental \(k^2 = 1.101\%\), thermal time constant \(\sim 330\ \mu\mathrm{s}\), and normalized IR responsivity \(\sim 132\ \text{ppt/pW}\); the estimated thermomechanical-noise-limited NEP was \(\approx 0.57\ \text{pW}/\sqrt{\text{Hz}}\) [2506.21412]. The device is notable because it uses AlScN simultaneously as the electromechanical transducer and as the dielectric in the plasmonic absorber, reinforcing the broader view of AlScN as a platform material rather than a single-function piezoelectric.

## 7. Reliability, extreme environments, and boron-modified descendants

The main materials liabilities of AlScN are now documented as oxidation, high coercive field, and endurance limitations. Operando HAXPES on \( \mathrm{Al_{0.83}Sc_{0.17}N} \) showed that Sc doping weakens the polar AlN bonds enough to enable ferroelectric switching below dielectric breakdown, but also increases the tendency toward oxidation. The study found replacement of nitrogen by oxygen, preferential oxidation of Sc rather than Al, an oxidation-related N\(_2\) feature in the N \(1s\) spectrum, and no confirmation of self-limiting oxidation; a \(3\) nm W cap suppressed oxidation effectively, with the capped sample remaining chemically stable up to about \(-38\) V, whereas the uncapped sample oxidized further even under \(-1.5\) V bias [2410.21132]. This has direct implications for ferroelectric capacitor and transistor reliability, since top-electrode chemistry and ambient exposure become part of the functional materials problem.

At the same time, AlScN exhibits exceptional thermal robustness. In TaSi\(_2\)/\(\mathrm{Al_{0.7}Sc_{0.3}N}\)/TaSi\(_2\) capacitors, functional operation was reported up to \(1000^\circ\mathrm{C}\). Over this range, the coercive field decreased from \(4.3\text{--}4.5\) MV/cm at room temperature to \(1.2\) MV/cm at \(1000^\circ\mathrm{C}\), the effective longitudinal piezoelectric coefficient increased from \(7.6\) pm/V at \(25^\circ\mathrm{C}\) to \(75.1\) pm/V at \(800^\circ\mathrm{C}\), and the electromechanical coupling coefficient was calculated to rise from \(12.9\%\) at room temperature to \(82\%\) at \(700^\circ\mathrm{C}\); S/TEM showed no detectable electrode diffusion into the AlScN layer after thermal cycling [2410.17037]. A plausible implication is that AlScN’s reliability envelope is unusually bifurcated: chemically fragile under oxygen exposure, but structurally and functionally robust under extreme thermal load when the interfaces are properly engineered.

Much recent alloy engineering treats AlScN as the baseline ferroelectric nitride and seeks to alleviate its switching and endurance penalties by boron incorporation. A combinatorial HiPIMS study covering \(850\) unique samples and about \(400\) ferroelectric devices found that in AlScBN the remanent polarization remains \( \approx 130\text{–}150\ \mu\text{C/cm}^2 \), while the coercive field drops from \(\sim 7\) MV/cm to \(\sim 3\) MV/cm and endurance extends to \(10^7\) cycles without sample recharging; the same study concludes that B co-alloying lowers the amount of Sc needed to reduce \(E_c\) and links the endurance improvement to reduced defect density [2606.11954]. In the ultrathin limit, \(10\) nm sputtered AlBScN capacitors showed ferroelectric switching at \(2.2\) MV/cm in C–V and symmetric polarization reversal near \(4.6\) MV/cm in PUND with \(2\ \mu\)s pulses, together with about two orders of magnitude lower leakage current than \(10\) nm AlScN and a breakdown-to-coercive-field ratio \(E_{BD}/E_c \approx 2.2\) [2511.08540]. These derivative systems do not displace AlScN conceptually; rather, they clarify which of AlScN’s constraints are intrinsic to Sc-stabilized ferroelectric nitrides and which can be retuned by quaternary alloy design.

Source: https://www.emergentmind.com/topics/scandium-doped-aluminum-nitride-alscn