Effects of processing on the stability of molybdenum oxide ultra-thin films

The effects of wet chemical processing conventionally employed in device fabrication standards are systematically studied on molybdenum oxide (MoOx) ultra-thin films. We have combined x-ray photoelectron spectroscopy (XPS), angle resolved XPS and x-ray reflectivity techniques to provide deep insights into the changes in composition, structure and electronic states upon treatment of films with different initial stoichiometry prepared by reactive sputtering. Our results show significant reduction effects associated with the development of gap states in MoOx, as well as changes in the composition, density and structure of the films, systematically correlated with the initial oxidation state of Mo.Comment: 16 pages, 5 figures, Appendix include

Functional Mn--Mgk_k cation complexes in GaN featured by Raman spectroscopy

The evolution of the optical branch in the Raman spectra of (Ga,Mn)N:Mg epitaxial layers as a function of the Mn and Mg concentrations, reveals the interplay between the two dopants. We demonstrate that the various Mn-Mg-induced vibrational modes can be understood in the picture of functional Mn--Mg$_k$ complexes formed when substitutional Mn cations are bound to $k$ substitutional Mg through nitrogen atoms, the number of ligands $k$ being driven by the ratio between the Mg and the Mn concentrations.Comment: Accepted for publication in Applied Physics Lette

Compensation-dependence of magnetic and electrical properties in Ga1-xMnxP

We demonstrate the control of the hole concentration in Ga1-xMnxP over a wide range by introducing compensating vacancies. The resulting evolution of the Curie temperature from 51 K to 7.5 K is remarkably similar to that observed in Ga1-xMnxAs despite the dramatically different character of hole transport between the two material systems. The highly localized nature of holes in Ga1-xMnxP is reflected in the accompanying increase in resistivity by many orders of magnitude. Based on variable-temperature resistivity data we present a general picture for hole conduction in which variable-range hopping is the dominant transport mechanism in the presence of compensation.Comment: 3 pages, 3 figures; accepted for publication in Applied Physics Letter

Paramagnetic GaN:Fe and ferromagnetic (Ga,Fe)N - relation 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.Comment: 21 pages, 30 figures, submitted to Phys. Rev.