Structural and paramagnetic properties of dilute ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{N}$

Systematic investigations of the structural and magnetic properties of single crystal ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{N}$ films grown by metal organic vapor phase epitaxy are presented. High-resolution transmission electron microscopy, synchrotron x-ray diffraction, and extended x-ray absorption fine structure studies do not reveal any crystallographic phase separation and indicate that Mn occupies Ga-substitutional sites in the Mn concentration range up to 1%. The magnetic properties as a function of temperature, magnetic field and its orientation with respect to the $c$ axis of the wurtzite structure can be quantitatively described by the paramagnetic theory of an ensemble of noninteracting ${\text{Mn}}^{3+}$ ions in the relevant crystal field, a conclusion consistent with the x-ray absorption near edge structure analysis. A negligible contribution of Mn in the $2+$ charge state points to a low concentration of residual donors in the studied films. Studies on modulation-doped $p$-type ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{N}/\left(\text{Ga},\text{Al}\right)\text{N}:\text{Mg}$ heterostructures do not reproduce the high-temperature robust ferromagnetism reported recently for this system.

Figure 1

(Color online) Magnetic response at 2, 15, and 200 K of a typical $\left(5\times 5\times 0.3\right){\text{mm}}^3$ sapphire substrate measured for both in-plane (darker shade squares) and out-of-plane (lighter shade circles) configurations after the application of a correction linear in the magnetic field and proportional to the magnetic susceptibility of sapphire at 200 K. The magnetic moment obtained in this way at 2 K reaches a value that would give a magnetization of $\sim 1\text{emu}/{\text{cm}}^3$ for (a typical) 200 nm thick layer. Inset: $m\left(H\right)$ at 2 K for the same sapphire sample, but without correction. The axes labels are the same as in the main panel. This ferromagneticlike signal is isotropic, decreases with temperature and vanishes above 15 K.

Figure 2

(Color online) (a) SXRD spectra for (Ga,Mn)N samples showing no presence of secondary phases over a broad range of concentration of the magnetic ions. (b) Lattice parameters vs Mn concentration. Values for an undoped GaN layer are added for reference

Figure 3

(Color online) In situ SXRD spectra upon annealing of sample No. 400A at different temperatures: no formation of secondary phases is detected up to an annealing temperature of $900°\text{C}$.

Figure 4

HRTEM images: (a) along $\left[10\overline{1}0\right]$ and (b) along the $\left[11\overline{2}0\right]$ zone axes.

Figure 5

EDS spectrum of sample 300A, with the identification of the Mn peaks [$L_{\alpha }$ (0.636 keV), $K_{\alpha }$ (5.895 keV), and $K_{\beta }$ (6.492 keV)].

Figure 6

(Color online) $k^2$-weighted EXAFS signal (a) for samples 100A (circles) and 490A (diamonds) with relative best fits (solid line) in the region $\left[R_{\text{min}}\text{−}{R}_{\text{max}}\right]$ and (b) amplitude of the Fourier transforms (FTs) carried out in the range $\left[k_{\text{min}}\text{−}{k}_{\text{max}}\right]$ by an Hanning window (slope parameter $dk=1$); the vertical lines indicate the position of the main peaks in the FT of the ${\text{Mn}}_{\text{I}}^{\text{T}}$, ${\text{Mn}}_{\text{I}}^{\text{O}}$, and ${\text{Mn}}_3\text{GaN}$ additional structures. Their possible presence would be promptly detected as they fall in a region free from other peaks. FEFF8 simulations, (c), for the tested theoretical models as described in the text.

Figure 7

(Color online) SIMS depth profiles of Mn, C, O, and H for the samples: (a) $150A$ and (b) $375A$.

Figure 8

(Color online) Normalized XANES spectra of the samples 100A and 490A (points) compared with: (a) the reference manganese oxides (MnO, ${\text{Mn}}_2{\text{O}}_3$, and ${\text{MnO}}_2$)—the chosen edge positions are highlighted by vertical lines; (b) ab initio absorption spectra (without convolution) for ${\text{Mn}}_{\text{Ga}}$ in the $3d^4$ and $3d^5$ electronic configurations. Inset (c): method used to extract the results of Table , focusing the near-edge region for sample No. 490A with the baseline (Bkg), the relative fit and its components $\left(A_1\text{−}{A}_4\right)$.

Figure 9

(Color online) Magnetization measurements at 1.85, 5, and 15 K of ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{N}$ as a function of the magnetic field applied parallel (closed circles) and perpendicular (open squares) to the GaN wurtzite $c$ axis. The solid lines show the magnetization curves calculated according to the group theoretical model for noninteracting ${\text{Mn}}^{3+}$ ions in wz-GaN.

Figure 10

(Color online) Temperature dependence of the magnetization $M$ for sample 375A (points) at $H=10\text{kOe}$. The solid lines represent the magnetization calculated within the group theoretical model of noninteracting ${\text{Mn}}^{3+}$ ions in wz-GaN.

Figure 11

(Color online) Mn concentration $n_{\text{Mn}}$ obtained from magnetization measurements (circles—series A, diamonds—series B) and SIMS (squares—series A) as a function of the Mn precursor flow rate.

Figure 12

(Color online) Magnetization curves at $T=1.85\text{K}$ of five ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{N}$ samples normalized with respect to their in plane magnetization at $H=50\text{kOe}$.

Figure 13

(Color online) SIMS depth profiles of our (Ga,Mn)N/(Al,Ga)N:Mg/GaN:Si $\left(i\text{−}p\text{−}n\right)$ structure.

Figure 14

(Color online) Room temperature magnetic signal from the (Ga,Mn)N/(Al,Ga)N:Mg/GaN:Si structure. For completeness, results of both in-plane and out-of-plane orientations are shown. Diamagnetic and paramagnetic contributions have been compensated.