InN films with free electron concentrations ranging from $\mathrm{mid}{\text{-10}}^{17}$ to $\mathrm{mid}{\text{-10}}^{20}\hspace{0.5em}{\mathrm{cm}}^{\text{−3}}$ have been studied using optical absorption, Hall effect, and secondary ion mass spectrometry. The optical absorption edge covers a wide energy range from the intrinsic band gap of InN of about 0.7 to about 1.7 eV which is close to the previously accepted band gap of InN. The electron concentration dependence of the optical absorption edge energy is fully accounted for by the Burstein–Moss shift. Results of secondary ion mass spectrometry measurements indicate that O and H impurities cannot fully account for the free electron concentration in the films.

We report on the metalorganic chemical vapor deposition of $\mathrm{GaN}:\mathrm{Fe}$ and $(\mathrm{Ga},\mathrm{Fe})\mathrm{N}$ layers on $c$-sapphire substrates and their thorough characterization via high-resolution x-ray diffraction, transmission electron microscopy (TEM), spatially resolved energy dispersive x-ray spectroscopy (EDS), secondary-ion mass spectroscopy (SIMS), photoluminescence (PL), Hall-effect, electron-paramagnetic resonance (EPR), and magnetometry employing a superconducting quantum interference device (SQUID). A combination of TEM and EDS reveals the presence of coherent nanocrystals presumably ${\mathrm{Fe}}_x\mathrm{N}$ with the composition and lattice parameter imposed by the host. From both TEM and SIMS studies, it is stated that the density of nanocrystals and, thus the Fe concentration increases towards the surface. According to Hall effect measurements, electrons from residual donors are trapped by midgap Fe acceptor states in the limit of low iron content $x\lesssim 0.4\%$, indicating that the concentration of ${\mathrm{Fe}}^{2+}$ ions increases at the expense of Fe ions in the $3+$ charge state. This effect is witnessed by PL measurements as changes in the intensity of the ${\mathrm{Fe}}^{3+}$-related intraionic transition, which can be controlled by codoping with Si donors and Mg acceptors. In this regime, EPR of ${\mathrm{Fe}}^{3+}$ ions and Curie-like magnetic susceptibility are observed. As a result of the spin-orbit interaction, ${\mathrm{Fe}}^{2+}$ does not produce any EPR response. However, the presence of Fe ions in the $2+$ charge state may account for a temperature-independent Van Vleck–type paramagnetic signal that we observe by SQUID magnetometry. Surprisingly, at higher Fe concentrations, the electron density is found to increase substantially with the Fe content. The coexistence of electrons in the conduction band and Fe in the $3+$ charge state is linked to the gradient in the Fe concentration. In layers with iron content $x\gtrsim 0.4\%$ the presence of ferromagnetic signatures, such as magnetization hysteresis and spontaneous magnetization, have been detected. A set of precautions has been undertaken in order to rule out possible sources of spurious ferromagnetic contributions. Under these conditions, a ferromagneticlike response is shown to arise from the $(\mathrm{Ga},\mathrm{Fe})\mathrm{N}$ epilayers, it increases with the iron concentration, it persists up to room temperature, and it is anisotropic—i.e., the saturation value of the magnetization is higher for in-plane magnetic field. We link the presence of ferromagnetic signatures to the formation of Fe-rich nanocrystals, as evidenced by TEM and EDS studies. This interpretation is supported by magnetization measurements after cooling in and without an external magnetic field, pointing to superparamagnetic properties of the system. It is argued that the high temperature ferromagnetic response due to spinodal decomposition into regions with small and large concentration of the magnetic component is a generic property of diluted magnetic semiconductors and diluted magnetic oxides showing high apparent Curie temperature.

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. DOI: http://dx.doi.org/10.1103/PhysRevB.81.235210 © 2010 The American Physical Society

Molecular beam epitaxy has been employed to obtain Ga${}_{1$-${}x}$Mn${}_{x}$N films with $x$ up to 10% and Curie temperatures ${T}_{\text{C}}$ up to 13 K. The magnitudes of ${T}_{\text{C}}$ and their dependence on $x$, ${T}_{\text{C}}(x)$\propto${}{x}^{m}$, where $m=2.2\ifmmode\pm\else\textpm\fi{}0.2$, are quantitatively described by a tight-binding model of superexchange interactions and Monte Carlo simulations of ${T}_{\text{C}}$. The critical behavior of this dilute magnetic insulator shows strong deviations from the magnetically clean case ($x=1$), in particular, (i) an apparent breakdown of the Harris criterion, (ii) a nonmonotonic crossover in the values of the susceptibility critical exponent ${$\gamma${}}_{\text{eff}}$ between the high temperature and critical regimes, and (iii) a smearing of the critical region, which can be explained either by the Griffiths effects or by macroscopic inhomogeneities in the spin distribution with a variance $$\Delta${}x=(0.2\ifmmode\pm\else\textpm\fi{}0.1)$%.

We report on the fabrication of pseudomorphic wurtzite Ga_1−xMn_xN grown on GaN with Mn concentrations up to 10% using molecular beam epitaxy. According to Rutherford backscattering, the Mn ions are mainly at the Ga-substitutional positions, and they are homogeneously distributed according to depth-resolved Auger-electron spectroscopy and secondary-ion mass-spectroscopy measurements. A random Mn distribution is indicated by transmission electron microscopy, and no Mn-rich clusters are present for optimized growth conditions. A linear increase of the c-lattice parameter with increasing Mn concentration is found using x-ray diffraction. The ferromagnetic behavior is confirmed by superconducting quantum-interference measurements showing saturation magnetizations of up to 150 emu/cm³.