Local structure of (Ga,Fe)N and (Ga,Fe)N:Si investigated by x-ray absorption fine structure spectroscopy

X-ray absorption fine-structure (XAFS) measurements supported by ab initio computations within the density functional theory (DFT) are employed to systematically characterize Fe-doped as well as Fe- and Si-codoped films grown by metalorganic vapor-phase epitaxy. The analysis of extended-XAFS data shows that depending on the growth conditions, Fe atoms either occupy Ga substitutional sites in GaN or precipitate in the form of $ϵ{\text{-Fe}}_3\text{N}$ nanocrystals, which are ferromagnetic and metallic according to the DFT results. Precipitation can be hampered by reducing the Fe content, by increasing the growth rate, or by codoping with Si. The near-edge region of the XAFS spectra provides information on the Fe charge state and shows its partial reduction from ${\text{Fe}}^{+3}$ to ${\text{Fe}}^{+2}$ upon Si codoping, in agreement with the Fe electronic configurations expected within various implementations of DFT.

Figure 1

(Color online) $k^2$-weighted EXAFS signals for (Ga,Fe)N samples at $\text{TMGa}=5\text{sccm}$ as a function of ${\text{Cp}}_2\text{Fe}$ content (sccm, labels on spectra); vertical dashed lines highlight some parts of the spectra as described in Sec. 3A; $k_{\text{min}}$ and $k_{\text{max}}$ delimit the Fourier-transformed part of the spectrum used in the fit.

Figure 2

(Color online) Amplitude of the FT for spectra shown in Fig. 1 with relative fits. $R_{\text{min}}$ and $R_{\text{max}}$ delimit the fit region; $R_1$, $R_2$, and $R_3$ (with relative dashed vertical lines) indicate the main average distances found in the fit. The $R$ scale has no phase correction (Ref. 47).

Figure 3

(Color online) $k^2$-weighted EXAFS signals for (Ga,Fe)N samples at ${\text{Cp}}_2\text{Fe}=300\text{sccm}$ as a function of TMGa flow rate (sccm, labels on spectra); vertical dashed lines highlight some parts of the spectra as in Fig. 1; $k_{\text{min}}$ and $k_{\text{max}}$ delimit the Fourier-tranformed part of the spectrum used in the fit.

Figure 4

(Color online) Amplitude of the FT for spectra shown in Fig. 3 with relative fits. $R_{\text{min}}$ and $R_{\text{max}}$ delimit the fit region; $R_1$, $R_2$, and $R_3$ (with relative dashed vertical lines) indicate the main average distances found from the fit. The $R$ scale has no phase correction (Ref. 47).

Figure 5

(Color online) $k^2$-weighted EXAFS signals for (Ga,Fe)N:Si samples as a function of TMGa and ${\text{Cp}}_2\text{Fe}$ (sccm, labels on spectra); vertical dashed lines highlight some parts of the spectra as in Figs. 1, 3; $k_{\text{min}}$ and $k_{\text{max}}$ delimit the Fourier-tranformed part of the spectrum used in the fit.

Figure 6

(Color online) Amplitude of the FT for spectra shown in Fig. 5 with relative fits. $R_{\text{min}}$ and $R_{\text{max}}$ delimit the fit region; $R_1$ and $R_3$ (with relative dashed vertical lines) indicate the main average distances as in Figs. 2, 4. The $R$ scale has no phase correction (Ref. 47).

Figure 7

(Color online) Normalized XANES spectra for (Ga,Fe)N (left) and (Ga,Fe)N:Si (right) with the prepeak reported in the inset after baseline subtraction and the fitted functions with centroids indicated by arrows.

Figure 8

(Color online) $\text{LSD}+U$ total spin DOS and DOS projected on the Fe atomic orbitals for a substitutional Fe in the neutral charge state. The DOS for Fe are magnified ten times for sake of clarity. The position of the Fermi energy is indicated by a vertical dashed line and shown in respect to the top of the valence band (TVB). The calculated absolute value of the Fermi energy $\left(E_F\right)$ is 10.52 eV.

Figure 9

(Color online) LSD total spin DOS for $ϵ{\text{-Fe}}_3\text{N}$. The position of the Fermi energy $\left(E_F\right)$ is indicated by a vertical dashed line. The calculated absolute value of $E_F$ is 17.64 eV.