Local Environment of Sc and Y Dopant Ions in Aluminum Nitride Thin Films

Abstract

The local environments of Sc and Y in predominantly ⟨002⟩ textured, Al_1-xDo_xN (Do = Sc, x = 0.25, 0.30 or Y, x = 0.25) sputtered thin films with wurtzite symmetry were investigated using X-ray absorption (XAS) and photoelectron (XPS) spectroscopies. We present evidence from the X-ray absorption fine structure (XAFS) spectra that, when x = 0.25, both Sc³⁺ and Y³⁺ ions are able to substitute for Al³⁺, thereby acquiring four tetrahedrally coordinated nitrogen ligands, i.e., coordination number (CN) of 4. On this basis, the crystal radius of the dopant species in the wurtzite lattice, not available heretofore, could be calculated. By modeling the scandium local environment, extended XAFS (EXAFS) analysis suggests that when x increases from 0.25 to 0.30, CN for a fraction of the Sc ions increases from 4 to 6, signaling octahedral coordination. This change occurs at a dopant concentration significantly lower than the reported maximum concentration of Sc (42 mol % Sc) in wurtzite (Al, Sc)N. XPS spectra provide support for our observation that the local environment of Sc in (Al, Sc)N may include more than one type of coordination.

Figure 1

XRD patterns of Al_0.75Sc_0.25N and Al_0.70Sc_0.30N thin films grown on ⟨100⟩ cut Si wafers with a 50 nm Ti seeding layer. The substrate is tilted 3° in order to minimize diffraction from the Si substrate.

Figure 2

XRD pattern of an Al_0.75Y_0.25N thin film grown on ⟨100⟩ cut Si with a 50 nm Ti seeding layer. A pattern with full 2θ range is included in the SI (Figure S1). The substrate has been tilted 3° in order to minimize diffraction from the Si substrate.

Figure 3

SEM images of the surface and cross-section of thin films (a diamond pen was used to prepare cross sections) of (a) Al_0.75Sc_0.25N, reproduced with permission from ref (17). (b) Al_0.70Sc_0.30N and (c) Al_0.75Y_0.25N, respectively. Pebble-like grains (84–94 nm) appear on the surface in panels (a) and (b), along with columnar growth. Grain size decreases with increasing Sc concentration. Disoriented, abnormal grains are observed on the surface of Al_0.70Sc_0.30N. We noted in our earlier report that during reactive sputtering of AlScN, the final deposition temperature influences the number of abnormally oriented grains visible in SEM images. In panel (c), individual grains and grain size on the film surface are difficult to distinguish, indicating a tendency to poor crystallinity, as has been reported in the literature. All scale bars indicate 1 μm.

Figure 4

Sc K-edge XANES spectra of Al_0.75Sc_0.25N (black trace), Al_0.7Sc_0.3N (red trace) thin films, and ScN (blue trace) powder. Note the absence of a pre-edge peak for ScN.

Figure 5

(a) k²-weighted EXAFS spectra of Al_0.75Sc_0.25N (black trace) and Al_0.7Sc_0.3N (red trace) films and ScN (blue trace) powder; (b) Fourier transform magnitude of the k²-weighted EXAFS spectra shown in panel (a). The k-range used for the Fourier transform was 2–9 Å^–1.

Figure 6

Fourier transform magnitude of k²-weighted EXAFS spectra of (a) Al_0.75Sc_0.25N; (b) Al_0.7Sc_0.3N; (c) ScN, accompanied by theoretical fits. Best fit parameters are tabulated in Table 2. The k-ranges used in the Fourier transform were 2.5–9.5 Å^–1 for Al_0.75Sc_0.25N and Al_0.7Sc_0.3N and 3–11 Å^–1 for ScN. The r-ranges used were 1.0–2.205, 1.0–2.607, and 1.0–3.229 Å for Al_0.75Sc_0.25N, Al_0.7Sc_0.3N, and ScN, respectively.

Figure 7

(a) Measured (black curve) and calculated (blue curve) XANES spectra of a Al_0.75Sc_0.25N thin film; (b) measured (black curve) and calculated (red curve) XANES spectra of ScN powder.

Figure 8

(a) X-ray photoelectron spectra of the binding energy of N 1s and Sc 2p electrons in Al_0.75Sc_0.25N (blue trace) and Al_0.7Sc_0.3N (red trace) thin films and ScN (black trace) powder samples. Profile fitting was performed for each of the three samples: (b) ScN; (c) Al_0.7Sc_0.3N; (d) Al_0.75Sc_0.25N in order to estimate the amplitude of the observed increase in Δ with increase in Sc doping in the wurtzite lattice.

Figure 9

(a) Normalized Y K-edge XANES spectrum of Al_0.75Y_0.25N. For comparison, the spectra of Y foil and Y₂O₃ are included. (b) The k²-weighted EXAFS spectra of Al_0.75Y_0.25N thin film, Y₂O₃ powder, and Y foil. (c) Fourier transform magnitude of the k² -weighted χ(k) spectra. The k range for the Fourier transformation is 2–7.5 Å^–1.

Figure 10

Measured (black curve) and fitted (red curve) Fourier transform magnitude of the k²-weighted EXAFS spectrum for (a) Y foil. The k range for the Fourier transform is 2–12 Å^–1. The r range is 2.6–4 Å; (b) Al_0.75Y_0.25N film. The k range for the Fourier transform is 2–7.5 Å^–1. The r range is 1.3–3.3 Å.