Publications (7)3.69 Total impact
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Article: Magnetism of dilute (Ga,Mn)N
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ABSTRACT: Systematic investigations of the structural and magnetic properties of single crystal GaxMn1−xN 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 non-interacting 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 GaxMn1−xN/(Ga,Al)N:Mg heterostructures do not reproduce the high temperature robust ferromagnetism reported recently for this system.arXiv.org. 02/2013; arXiv:0912.4216. -
Article: Manipulating Mn-Mg cation complexes to control the charge- and spin-state of Mn in GaN.
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ABSTRACT: Owing to the variety of possible charge and spin states and to the different ways of coupling to the environment, paramagnetic centres in wide band-gap semiconductors and insulators exhibit a strikingly rich spectrum of properties and functionalities, exploited in commercial light emitters and proposed for applications in quantum information. Here we demonstrate, by combining synchrotron techniques with magnetic, optical and ab initio studies, that the codoping of GaN:Mn with Mg allows to control the Mn(n) (+) charge and spin state in the range 3≤n≤5 and 2≥S≥1. According to our results, this outstanding degree of tunability arises from the formation of hitherto concealed cation complexes Mn-Mg(k), where the number of ligands k is pre-defined by fabrication conditions. The properties of these complexes allow to extend towards the infrared the already remarkable optical capabilities of nitrides, open to solotronics functionalities, and generally represent a fresh perspective for magnetic semiconductors.Scientific Reports 01/2012; 2:722. -
Article: Structural and paramagnetic properties of dilute Ga_ {1− x} Mn_ {x} N
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ABSTRACT: Systematic investigations of the structural and magnetic properties of single crystal Ga1−xMnxN 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 Mn3+ 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 Ga1−xMnxN/(Ga,Al)N:Mg heterostructures do not reproduce the high-temperature robust ferromagnetism reported recently for this system.Physical Review B 06/2010; 81(23). · 3.69 Impact Factor -
Article: Structural and paramagnetic properties of dilute Ga1-xMnxN
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ABSTRACT: Systematic investigations of the structural and magnetic properties of single crystal (Ga,Mn)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 non-interacting 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 (Ga,Mn)N/(Ga,Al)N:Mg heterostructures do not reproduce the high temperature robust ferromagnetism reported recently for this system.12/2009; -
Article: Local structure of (Ga,Fe)N and (Ga,Fe)N:Si investigated by x-ray absorption fine structure spectroscopy
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ABSTRACT: X-ray absorption fine-structure (XAFS) measurements supported by {\em ab initio} computations within the density functional theory (DFT) are employed to systematically characterize Fe-doped as well as Fe and Si-co-doped films grown by metalorganic vapour 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 $\epsilon$-Fe$_3$N nanocrystals, which are ferromagnetic and metallic according to the DFT results. Precipitation can be hampered by reducing the Fe content, or by increasing the growth rate or by co-doping with Si. The near-edge region of the XAFS spectra provides information on the Fe charge state and shows its partial reduction from Fe$^{+3}$ to Fe$^{+2}$ upon Si co-doping, in agreement with the Fe electronic configurations expected within various implementations of DFT.02/2009; -
Article: GaN:$\delta$-Mg grown by MOVPE: structural properties and their effect on the electronic and optical behaviour
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ABSTRACT: The effect of Mg $\delta$-doping on the structural, electrical and optical properties of GaN grown $\textsl{via}$ metalorganic vapor phase epitaxy has been studied using transmission electron microscopy, secondary ion mass spectroscopy, atomic force microscopy, x-ray diffraction, Hall effect measurements and photoluminescence. For an average Mg concentration above 2.14 $\times$ 10$^{19}$ cm$^{-3}$, phase segregation occurs, as indicated by the presence of Mg-rich pyramidal inversion domains in the layers. We show that $\delta$-doping promotes, in comparison to Mg continuous doping, the suppression of extended defects on the samples surface and improves significantly the morphology of the epilayers. Conversely, we can not confirm the reduction in the threading dislocation density - as a result of $\delta$-doping - reported by other authors. In the phase separation regime, the hole concentration is reduced with increasing Mg concentration, due to self-compensation mechanisms. Below the solubility limit of Mg into GaN at our growth conditions, potential fluctuations result in a red-shift of the emission energy of the free-to-bound transition. Comment: 8 pages, 11 figures09/2007; -
Article: Paramagnetic GaN:Fe and ferromagnetic (Ga,Fe)N: The relationship between structural, electronic, and magnetic properties
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ABSTRACT: We report on the metalorganic chemical vapor deposition of GaN:Fe and (Ga,Fe)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 FexN 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≲0.4%, indicating that the concentration of Fe2+ 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 Fe3+-related intraionic transition, which can be controlled by codoping with Si donors and Mg acceptors. In this regime, EPR of Fe3+ ions and Curie-like magnetic susceptibility are observed. As a result of the spin-orbit interaction, Fe2+ 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≳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 (Ga,Fe)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.Phys. Rev. B. 75(12).
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- Physical Review B (1)
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Institutions
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2010–2013
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Johannes Kepler Universität Linz
- Institut für Halbleiter- und Festkörperphysik
Linz, Upper Austria, Austria
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