Molecular Beam Epitaxy of Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N Nanowires: Towards Group-III Nitride Piezoelectric Nanogenerators with Enhanced Response

TL;DR

This work demonstrates plasma-assisted MBE growth of self-assembled Al$_{1-x}$Sc$_x$N nanowires on TiN substrates and their integration into vertically integrated piezoelectric nanogenerators. A low-temperature growth window (<$700^\circ$C) stabilizes phase-pure wurtzite Al$_{1-x}$Sc$_x$N across $0 \\le x \\le 0.35$, while higher temperatures induce phase separation into AlN and ScN with an epitaxial alignment. The resulting nanowire–polymer composites exhibit enhanced piezoelectric response, with $d_{33, ext{eff}}$ peaking at $8.5$ pC N$^{-1}$ around $x \\approx 0.32$, aided by a reduced effective permittivity that boosts voltage and energy harvesting efficiency. An effective-medium theory reveals that device architecture is the primary performance limiter, providing concrete guidelines for optimizing nanowire-based piezoelectric energy harvesters toward higher efficiency than bulk AlN and approaching ZnO-like performance.

Abstract

We study the molecular beam epitaxy of self-assembled Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N nanowires on conductive TiN layers and demonstrate their application in piezoelectric nanogenerators. Wurtzite Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N nanowires with uniform Sc incorporation are grown across a wide composition range (0<x<0.35). At substrate temperatures below 700 $^\circ{}$C, these nanowires exhibit an inversely tapered morphology, whereas higher temperatures favor the nucleation of additional branches due to a phase separation of Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N into wurtzite AlN and rock-salt ScN. Phase-pure Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N nanowires are integrated into vertical nanogenerators, where the metallic TiN substrate serves as bottom electrode. The fabricated polymer-nanowire composite devices achieve effective piezoelectric charge coefficients of up to 8.5 pC N$^{-1}$ at x=0.32, thus exceeding the piezoelectric response of bulk AlN by nearly a factor of two. Although the charge response remains lower compared to Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N thin films, the reduced effective dielectric permittivity of the nanowire-polymer composites compensates the reduction in piezoelectric charge coefficient, eventually yielding a higher voltage response and comparable energy harvesting efficiency. Finally, effective medium modeling reveals that the device architecture is the primary factor limiting performance, providing general design principles for highly efficient nanowire-based piezoelectric energy harvesters.

Figures (6)

• Figure 1: Cross-section secondary electron micrographs of Al$_{0.80}$Sc$_{0.20}$N segments grown on self-assembled AlN nanowire stems at (a) 1150℃, (b) 830℃, and (c) 500℃. (d) Room-temperature Raman spectra of several Al$_{0.80}$Sc$_{0.20}$N nanowire ensembles grown at different temperatures (T$\mathrm{_{g}}$), excited at 473. Dashed lines indicate expected phonon positions of wurtzite Al$_{0.80}$Sc$_{0.20}$N solonenko_j.mater.sci._2020solonenko_micromachines_2022, the peak at around 680 cm$^{-1}$ is attributed to the LO(L) phonon of rock-salt ScN dinh_appl.phys.lett._2023grumbel_phys.rev.mater._2024. (e) Symmetric XRD 2$\text{$\theta$}$/$\mathrm{\omega}$ scans of the nanowire ensembles shown in (a) and (c). Dashed lines show the expected peak positions for bulk AlN(0002) and ScN(002) niewa_chem.mater._2004nilsson_j.phys.appl.phys._2016.
• Figure 2: (a) Normalized SEM-EDX spectra of Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N nanowire ensembles with varying Sc content $x$, indicated below each spectrum. The inset shows the comparison with the target Sc content expected from the cell flux calibration. (b) RHEED patterns of selected ensembles (Sc content indicated in the bottom-right corner), recorded along the [112̄0] azimuth. The AlN RHEED pattern was acquired after the growth of the nanowire stems. The RHEED patterns of Al$_{0.74}$Sc$_{0.26}$N and Al$_{0.68}$Sc$_{0.32}$N were recorded at the end of growth, whereas and the pattern of Al$_{0.65}$Sc$_{0.35}$N after half of its growth duration.
• Figure 3: Structural and chemical investigation of Al$_{0.74}$Sc$_{0.26}$N nanowires grown at 410℃ by STEM. (a) HAADF image, (b) EDX map with (c) the corresponding EDX profile. (d) Fourier filtered high-resolution micrograph, evidencing wurtzite phase.
• Figure 4: (a) Schematic illustration of Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N-based VINGs (bottom), processed from the MBE-grown Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N/AlN nanowires (top), the TiN layer serving as bottom contact. (b) Charge response of Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N VINGs with different Sc contents $x$ to a sinusoidal force excitation at 1 Hz. (c) Extracted piezoelectric charge coefficient as a function of excitation frequency (upper panel), together with the phase shift between force and charge signals (lower panel). (d) d$_{33,\mathrm{eff}}$ of all processed Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N VINGs as a function of Sc content $x$. The red data points highlight the devices with the strongest piezoelectric responses for each Sc content. The Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N VINGs outperform bulk AlN (d$_{33}$ of 5.4 pC N$^{-1}$) in the composition range $0.15 \lesssim x \lesssim 0.35$.
• Figure 5: Comparison of experimental and theoretical effective piezoelectric and dielectric properties of the Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N VINGs. (a) Shows the piezoelectric charge coefficient, (b) the relative static permittivity, (c) piezoelectric voltage coefficient, and (d) energy-harvesting figure of merit as a function of Sc content $x$. Devices exhibiting the strongest experimental performance for each composition are highlighted in red. The gray dashed line in each panel denotes the corresponding trend reported for Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N thin films ambacher_j.appl.phys._2021. The black solid line indicates the calculation of each effective parameter as a function of Sc content, while the shaded region represents the associated uncertainty.