M. Wegscheider

Controlled Aggregation of Magnetic Ions in a Semiconductor: An Experimental Demonstration

The control on the distribution of magnetic ions into a semiconducting host is crucial for the functionality of magnetically doped semiconductors. Through a structural analysis at the nanoscale, we give experimental evidence that the aggregation of Fe ions in (Ga,Fe)N and consequently the magnetic response of the material are affected by the growth rate and doping with shallow impurities.

Paramagnetic GaN : Fe and ferromagnetic ( Ga , Fe ) N : The relationship between structural, electronic, and magnetic properties

We report on the metalorganic chemical vapor deposition (MOCVD) of GaN:Fe and (Ga,Fe)N layers on c-sapphire substrates and their thorough characterization via high-resolution x-ray diffraction (HRXRD), 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. In layers with iron content x<0.4% the presence of ferromagnetic signatures, such as magnetization hysteresis and spontaneous magnetization, have been detected. 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.

Experimental probing of exchange interactions between localized spins in the dilute magnetic insulator (Ga,Mn)N

The sign, magnitude, and range of the exchange couplings between pairs of Mn ions is determined for (Ga,Mn)N and (Ga,Mn)N:Si with x . 3%. The samples have been grown by metalorganic vapor phase epitaxy and characterized by secondary-ion mass spectroscopy; high-resolution transmission electron microscopy with capabilities allowing for chemical analysis, including the annular dark-field mode and electron energy loss spectroscopy; high-resolution and synchrotron x-ray diffraction; synchrotron extended x-ray absorption fine-structure; synchrotron x-ray absorption near-edge structure; infra-red optics and electron spin resonance. The results of high resolution magnetic measurements and their quantitative interpretation have allowed to verify a series of ab initio predictions on the possibility of ferromagnetism in dilute magnetic insulators and to demonstrate that the interaction changes from ferromagnetic to antiferromagnetic when the charge state of the Mn ions is reduced from 3+ to 2+.

Observation of strong-coupling effects in a diluted magnetic semiconductor Ga1-xFexN.

The giant Zeeman splitting of free excitons is measured in Ga(1-x)Fe(x)N. Magneto-optical and magnetization data imply the ferromagnetic sign and a reduced magnitude of the effective p-d exchange energy governing the interaction between Fe(3+) ions and holes in GaN, N_{0}beta(app)=+0.5+/-0.2 eV. This finding corroborates the recent suggestion that the strong p-d hybridization specific to nitrides and oxides leads to significant renormalization of the valence band exchange splitting.

On the effect of periodic Mg distribution in GaN:δ-Mg

p-type doping of group-III nitrides represents a well recognized crucial challenge in the realization of optical and magneto-optical devices based on wide band gap semiconductors. The insertion of Mg in a δ-doping fashion into a GaN matrix is expected to be very promising for the enhanced incorporation of acceptors. The authors present a comprehensive study of the GaN:δ-Mg material system, starting from the optimization of the metal organic chemical vapor deposition process monitored in situ via spectroscopic ellipsometry and reflectometry and giving then evidence of the periodic distribution of the Mg ions through high-resolution secondary ion mass spectroscopy measurements. Furthermore, the effect of the acceptor distribution in the layers on the optical and transport properties is highlighted.

GaN:δ-Mg grown by MOVPE: Structural properties and their effect on the electronic and optical behavior

Abstract The effect of Mg δ -doping on the structural, electrical and optical properties of GaN grown 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 × 10 19 cm - 3 , phase segregation occurs, as indicated by the presence of Mg-rich pyramidal inversion domains in the layers. We show that δ -doping promotes, in comparison to Mg continuous doping, the suppression of extended defects on the sample surface and improves significantly the morphology of the epilayers. Conversely, we cannot confirm the reduction in the threading dislocation density—as a result of δ -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.

Stress and interdiffusion during molecular beam epitaxy of Fe on As-rich GaAs(001).

Interdiffusion during growth of Fe on As-rich GaAs(001) substrates has been investigated by real time stress measurements. Compared to Ga-rich GaAs(001), interdiffusion processes are decisively reduced. The optimum growth temperature (characterized by abrupt interfaces, pseudomorphic growth and negligible intermixing) is found to lie below 50 °C. At higher growth temperatures interdiffusion effects increase and eventually lead to the formation of a compact crystalline alloy layer of presumably Fe(2 + x)Ga(1 - x), as evidenced by transmission electron microscopy and x-ray diffraction.

Effects of magnetic ions on optical properties: the case of (Ga, Fe)N

Because of strong exchange interactions between localized spins and effective mass carriers, transition metal impurities in semiconductors lead to giant magneto-optical effects. Furthermore, band-gap levels derived from open d shells of magnetic impurities act as efficient recombination centers for photo-carriers. This paper reviews studies of excitonic magneto-reflectivity performed on (Ga, Fe)N epilayers, and shows how hybridization between d levels and band states, particularly strong in nitrides and oxides, renormalizes the exchange splitting of the valence band states in these systems. Photoluminescence measurements on the same structures demonstrate an increase of infrared Fe-related emission at the expense of ultraviolet near band-gap luminescence. This sensitivity of luminescence to the presence of Fe impurities is exploited to monitor the aggregation of FexN nanocrystals that account for the room temperature ferromagnetism of (Ga, Fe)N, but do not act as inhibitors of excitonic luminescence.

Microstructure of (Ga,Fe)N Films Grown by Metal-Organic Chemical Vapour Deposition

We report on the structural and chemical analysis of (Ga,Fe)N films grown by metal-organic chemical vapour deposition using transmission electron microscopy and energy dispersive X-ray spectroscopy. In homogeneously Fe-doped GaN, we have found Fe-rich nanoclusters to be assigned to α-Fe and e-Fe3N. In GaN δ-doped with Fe and co-doped with 5Mg, we have observed a significant reduction of the threading dislocations to be associated with the growth interruption, while the Fe ions tend to float to the surface.