Ferroelectricity in boron-substituted aluminum nitride thin films

This manuscript demonstrates ferroelectricity in B-substituted AlN thin films and a complementary set of first-principles calculations to understand their structure-property relationships. ${\mathrm{Al}}_{1–x}{\mathrm{B}}_x\mathrm{N}$ films are grown by dual-cathode reactive magnetron sputtering on $\left(110\right)\mathrm{W}/\left(001\right){\mathrm{Al}}_2{\mathrm{O}}_3$ substrates at 300°C at compositions spanning $x=0$ to $x=0.20$. X-ray diffraction studies indicate a decrease in both the $c$ and $a$ lattice parameters with increasing B concentration, resulting in a decrease in unit cell volume and a constant $c$/$a$ axial ratio of 1.60 over this composition range. Films with $0.02\le x\le 0.15$ display ferroelectric switching with remanent polarizations exceeding $125\mu \mathrm{C}\mathrm{c}{\mathrm{m}}^{–2}$ while maintaining band gap energies of $>5.2\mathrm{eV}$. The large band gap allows low frequency hysteresis measurement (200 Hz) with modest leakage contributions. At B concentrations of $x>0.15$, $c$-axis orientation deteriorates and ferroelectric behavior is degraded. Density-functional theory calculations corroborate the structural observations and provide predictions for the wurtzite $u$ parameter, polarization reversal magnitudes, and composition-dependent coercive fields.

Figure 1

XRD symmetric θ-2θ (a) and wurtzite-002 rocking curve (b) scans of 500 nm ${\mathrm{Al}}_{0.93}{\mathrm{B}}_{0.07}\mathrm{N}$ (top) and AlN (bottom) films prepared on $\left(110\right)\mathrm{W}/\left(001\right){\mathrm{Al}}_2{\mathrm{O}}_3$ substrates. The inset drawing depicts the wurtzite crystal structure for AlN.

Figure 2

Room temperature polarization hysteresis for 500 nm ${\mathrm{Al}}_{0.93}{\mathrm{B}}_{0.07}\mathrm{N}$ (a) and AlN (b) films measured with a 200 Hz triangular wave. Nested loops are displayed at $0.1\mathrm{MV}\mathrm{c}{\mathrm{m}}^{–1}$ increments.

Figure 3

Skew symmetric φ scans of the ${\mathrm{Al}}_{0.93}{\mathrm{B}}_{0.07}\mathrm{N}$ 105 (a), AlN 105 (b), and W 310 reflections.

Figure 4

(a) XRD symmetric θ-2θ scans of the ${\mathrm{Al}}_{1–x}{\mathrm{B}}_x\mathrm{N}$ 002 reflection for $x=0.00$, 0.02, 0.07, 0.15, 0.19, 0.20. (b) Rocking curve FWHM values of the ${\mathrm{Al}}_{1–x}{\mathrm{B}}_x\mathrm{N}$ 002 reflection for the same films plotted as a function of B concentration. (c) ${\mathrm{Al}}_{1–x}{\mathrm{B}}_x\mathrm{N}$ $c$ and $a$ lattice parameters as a function of B concentration, as determined by in- and out-of-plane XRD measurements (filled markers) and first-principles calculations (open markers). The c/a axial ratio is also shown. Error in the measured lattice parameters is smaller than the size of the markers.

Figure 5

(a)–(e) Polarization hysteresis for 500 nm ${\mathrm{Al}}_{1–x}{\mathrm{B}}_x\mathrm{N}$ films with $x=0.00$, 0.02, 0.07, 0.15, 0.19, 0.20 measured with a 200 Hz triangular wave. (f) Dependence of coercive field ($E_{\mathrm{c}}$) on B concentration. Coercive field values are averages of the switching field measured under positive and negative voltage. (g) Dependence of remanent polarization ($P_{\mathrm{r}}$) on B concentration.

Figure 6

Temperature dependence of the relative dielectric permittivity and low-field dielectric loss of a 500-nm ferroelectric ${\mathrm{Al}}_{1–x}{\mathrm{B}}_x\mathrm{N}$ film with $x=0.10$. Measurements were performed at 10 kHz, 100 kHz, and 1 MHz with a small signal amplitude of 100 mV. Filled markers represent the permittivity data and open markers the dielectric loss.

Figure 7

Band gap energy dependence of AlBN films on B concentration as determined from a Tauc plot using aborption coeffiecients from ultraviolet-visible light reflectance spectroscopy. Error in the measured band gap is smaller than the size of the markers.

Figure 8

Normal incidence SEM micrographs of 500 nm ${\mathrm{Al}}_{1–x}{\mathrm{B}}_x\mathrm{N}$ films with $x=0.00$, 0.02, 0.07, 0.15, 0.19, 0.20.

Figure 9

First-principles predictions of the (a) enthalpies of formation $H_{\mathrm{f}}$ of the hexagonal and wurtzite polymorphs (relative to the wurtzite AlN and hexagonal BN reference phases), (b) wurtzite structural parameter $u$ (referenced to the Al sites, B sites, and as a global average), (c) spontaneous polarization $P_{\mathrm{s}}$, and (d) coercive field $E_{\mathrm{c}}$, plotted as a function of B content in ${\mathrm{Al}}_{1–x}{\mathrm{B}}_x\mathrm{N}$. (The dashed lines are included as a guide for the eye.)