Fe-Mg interplay and the effect of deposition mode in (Ga,Fe)N doped with Mg

The effect of Mg codoping and Mg deposition mode on the Fe distribution in (Ga,Fe)N layers grown by metal-organic vapor phase epitaxy is investigated. Both homogeneously and digitally Mg-codoped samples are considered and contrasted to the case of (Ga,Fe)N layers obtained without any codoping by shallow impurities. The structural analysis of the layers by high-resolution transmission electron microscopy and by high-resolution and synchrotron x-ray diffraction gives evidence of the fact that, in the case of homogeneous Mg doping, Mg and Fe competitively occupy the Ga-substitutional cation sites, reducing the efficiency of Fe incorporation. Accordingly, the character of the magnetization is modified from ferromagneticlike in the non-codoped films to paramagnetic in the case of homogeneous Mg codoping. The findings are discussed vis-à-vis theoretical results obtained by $\mathit{\text{ab}}\mathit{\text{initio}}$ computations, showing only a weak effect of codoping on the pairing energy of two Fe cations in bulk GaN. However, according to these computations, codoping reverses the sign of the pairing energy of Fe cations at the Ga-rich surface, substantiating the view that the Fe aggregation occurs at the growth surface. In contrast to the homogeneous deposition mode, the digital one is found to remarkably promote the aggregation of the magnetic ions. The Fe-rich nanocrystals formed in this way are distributed nonuniformly, giving reason for the observed deviation from a standard superparamagnetic behavior.

Figure 1

Fe concentration averaged over the layer thickness, as evaluated from SIMS measurements as a function of the Fe source flow rate for the reference (Ga,Fe)N and for the correspondent (Ga,Fe)N:Mg ($x_{\text{Mg}}$ $=$ 0.2$\%$) and (Ga,Fe)N:$\delta$Mg ($x_{\text{Mg}}$ $=$ 0.02$\%$).

Figure 2

Mg concentration as determined by SIMS vs Cp${}_2$Fe source flow for (Ga,Fe)N:Mg (squares) and (Ga,Fe)N:$\delta$Mg (triangles). GaN:Mg reference samples [homogeneous ($◃$) and digital ($◃$)] are included. From XRD and TEM analysis, all layers with $x_{\text{Mg}}$ $<$ 0.03$\%$ show Fe crystallographic phase separation for Cp${}_2$Fe source flows as low as 50 sccm.

Figure 3

HRXRD curves for a reference (Ga,Fe)N grown at Fe precursor flow rate of 300 sccm with $\varepsilon$-Fe${}_3$N embedded nanocrystals and for the corresponding dilute (Ga,Fe)N:Mg layer with $x_{\mathrm{Mg}}$ = 0.2$\%$. Insets: HRTEM images: (a) $\varepsilon$-Fe${}_3$N embedded in the (Ga,Fe)N reference sample; (b) no precipitates in the corresponding (Ga,Fe)N:Mg layer.

Figure 4

HRXRD spectra for a reference dilute (Ga,Fe)N sample and for corresponding layers homogeneously and digitally codoped by Mg, respectively. All films were grown at the Fe precursor flux rate of 100 sccm.

Figure 5

SXRD spectra of highly Fe-doped (Ga,Fe)N (*) and (Ga,Fe)N:$\delta$Mg samples with different Fe concentrations ($0.15\%\le x\le 0.38\%$) giving evidence of the presence of secondary Fe-rich phases in the layers. Inset: detail of diffraction peaks from the secondary phases (Gaussian fit of the peaks). The phases corresponding to the diffraction peaks labeled 1 to 8 are listed in Table .

Figure 6

(a) SIMS profiles of (Ga,Fe)N:$\delta$Mg proving the inhomogeneous distribution of Fe in the layers. (b), (c): TEM bright-field images of nanocrystals in the GaN:Fe:$\delta$Mg sample S993 ($x_{\mathrm{Fe}}$ =  0.38$\%$); (b) close to the sample surface; (c) at the interface GaN buffer/(Ga,Fe)N:$\delta$Mg layer.

Figure 7

Mean concentrations of Fe ions contributing to the ferromagnetic response as a function of the Fe flux rate, as measured by SQUID for (Ga,Fe)N (circles); (Ga,Fe)N:Mg (squares); and (Ga,Fe)N:$\delta$Mg (triangles). Dashed lines are guides for the eye.

Figure 8

Mean concentrations of Fe ions contributing to the paramagnetic response as a function of the Fe flow rate, as measured by SQUID for (Ga,Fe)N (circles); (Ga,Fe)N:Mg (squares); and (Ga,Fe)N:$\delta$Mg (triangles). Results obtained earlier for (Ga,Fe)N:Si (Ref. 3) are shown for comparison (diamonds). Dashed lines are guides for the eye.

Figure 9

Total concentrations of Fe ions determined by SIMS (full symbols) and obtained as a sum of the ferromagnetic and paramagnetic contributions determined by SQUID magnetometry (open symbols) for (Ga,Fe)N (circles); (Ga,Fe)N:Mg (squares); and (Ga,Fe)N:$\delta$Mg (triangles). Dashed and dotted lines are guides for the eye.

Figure 10

Normalized magnetization at various temperatures of sample S995 [(Ga,Fe)N:$\delta$Mg, $x_{\mathrm{Fe}}$ = 0.2$\%$] with embedded nanocrystals. Points: experimental data; solid line: fit a Langevin-type function with $H_0$ = 800 Oe; arrow: sweep direction of the magnetic field.

Figure 11

ESR for (Ga,Fe)N:Mg (squares) and reference (Ga,Fe)N (circles). Upper panel: one of the selected ${\mathrm{Fe}}^{3+}$ transitions and double integrals for the samples grown with 200-sccm ${\mathrm{Cp}}_2\mathrm{Fe}$ flow rate: (Ga,Fe)N (circles, solid line) and a (Ga,Fe)N:Mg (squares, dashed line), respectively. Lower panel: ${\mathrm{Fe}}^{3+}$ concentration, as determined by ESR (filled symbols) and SQUID (open symbols).

Figure 12

ESR for (Ga,Fe)N:$\delta$Mg (triangles) and reference (Ga,Fe)N (circles). Upper panel: selected ${\mathrm{Fe}}^{3+}$ transition and double integral for a digitally Mg codoped (triangles, dashed line) and for a reference (Ga,Fe)N (circles, solid line) grown with 300-sccm ${\mathrm{Cp}}_2\mathrm{Fe}$ flow rate. Lower panel: ${\mathrm{Fe}}^{3+}$ concentration as determined by ESR (filled symbols) and SQUID (open symbols).

Figure 13

Heat of reaction (pairing energy $E_{\text{d}}$) for the formation (at negative $E_{\text{d}}$) of successive Fe${}_n$Mg${}_m$ (upper panel) and Fe${}_n$Si${}_m$ (lower panel) nearest-neighbor cation clusters ($n,m\le 2$) in bulk zb-GaN (squares) and on the Ga-terminated (0001) wz-GaN surface (triangles). The solid lines are guides to the eye. The data points not connected by the solid lines (reversed triangles for surfaces, and diamonds for bulk) denote the heat of reaction of two nearest-neighbor Fe cations in the presence of Mg or Si at the distance of 0.6 nm.

Figure 14

Energy difference between ferromagnetic and antiferromagnetic configurations for several Fe${}_2$$X_m$ ($m\le 2$, and $X$ denotes Mg or Si) clusters in zb-GaN and at the Ga-terminated (0001) wz-GaN surface. The solid lines are guides to the eye.