G. T. Thaler

Wide band gap ferromagnetic semiconductors and oxides

Recent advances in the theory and experimental realization of ferromagnetic semiconductors give hope that a new generation of microelectronic devices based on the spin degree of freedom of the electron can be developed. This review focuses primarily on promising candidate materials (such as GaN, GaP and ZnO) in which there is already a technology base and a fairly good understanding of the basic electrical and optical properties. The introduction of Mn into these and other materials under the right conditions is found to produce ferromagnetism near or above room temperature. There are a number of other potential dopant ions that could be employed (such as Fe, Ni, Co, Cr) as suggested by theory [see, for example, Sato and Katayama-Yoshida, Jpn. J. Appl. Phys., Part 2 39, L555 (2000)]. Growth of these ferromagnetic materials by thin film techniques, such as molecular beam epitaxy or pulsed laser deposition, provides excellent control of the dopant concentration and the ability to grow single-phase layers. T...

Magnetic properties of n-GaMnN thin films

GaMnN thin films were synthesized using gas-source molecular-beam epitaxy. Mn concentrations between 3 and 12 at. % were investigated. No evidence of second-phase formation was observed by powder x-ray diffraction or high-resolution cross section transmission electron microscopy in films with 9% or less Mn. The films were n type as determined by capacitance–voltage or Hall analysis. Magnetic characterization performed using a squid magnetometer showed evidence of ferromagnetic ordering at room temperature for all samples. In agreement with theoretical predictions, material with 3% Mn showed the highest degree of ordering per Mn atom. At 320 K, the samples show a nonzero magnetization indicating a TC above room temperature.

Wide bandgap GaN-based semiconductors for spintronics

Recent results on achieving ferromagnetism in transition-metal-doped GaN, A1N and related materials are discussed. The field of semiconductor spintronics seeks to exploit the spin of charge carrier ...

Indication of hysteresis in AlMnN

AlN films grown by gas-source molecular beam epitaxy were doped with different levels of Mn during growth. High resolution x-ray diffraction characterization revealed good crystallinity in single phase material, with lattice constant decreasing with increasing Mn concentration. Single phase AlMnN was found to be p type while AlMnN/AlMn mixed phase material was found to be highly conductive n type. Magnetization measurements performed with a superconducting quantum interference device magnetometer indicated ferromagnetism in single phase material persisting to 300 K and showed no evidence of room temperature magnetization in multiphase material. In particular, it was shown that Mn4N second phases are not contributing to the magnetization in the AlMnN under optimized growth conditions.

Effect of Gd implantation on the structural and magnetic properties of GaN and AlN

Gd+ ions were implanted at total doses of 3–6×1014cm2 into single-crystal GaN or AlN epilayers grown on sapphire substrates and annealed at 700–1000°C. The implanted Gd showed no detectable diffusion in either material after annealing, as measured by secondary ion mass spectrometry, corresponding to a diffusion coefficient <8×10−12cm2s−1. Under all annealing conditions, x-ray diffraction shows the formation of second phases. In the case of GaN, these include Gd3Ga2, GdN, and Gd, while for AlN only Gd peaks are observed. Both the GaN and AlN show high saturation magnetization after annealing at 900°C (∼15emucm−3 for GaN and ∼35emucm−3 for AlN). The magnetization versus temperature characteristics of the Gd-implanted GaN show a blocking behavior consistent with the presence of precipitates, whereas the AlN shows a clear difference in field-cooled and zero-field-cooled magnetization to above room temperature which may also be due to Gd inclusions.

Optical and magnetic properties of Eu-doped GaN

GaN films were doped with Eu to a concentration of ∼0.12at.% during growth at 800°C by molecular beam epitaxy, with the Eu cell temperature held constant at 470°C. All samples were postannealed at 675°C. The films exhibited strong photoluminescence (PL) in the red (622nm) whose absolute intensity was a function of the Ga flux during growth, which ranged from 3.0×10−7to5.4×10−7Torr. The maximum PL intensity was obtained at a Ga flux of 3.6×10−7Torr. The samples showed room temperature ferromagnetism with saturation magnetization of ∼0.1–0.45emu∕cm3, consistent with past reports where the Eu was found to be predominantly occupying substitutional Ga sites. There was an inverse correlation between the PL intensity and the saturation magnetization in the films. X-ray diffraction showed the presence of EuGa phases under all the growth conditions but these cannot account for the observed magnetic properties.

Room temperature magnetism in GaMnP produced by both ion implantation and molecular-beam epitaxy

The magnetization of the dilute magnetic alloy GaMnP:C prepared by the implantation of Mn into p-GaP:C or by direct molecular-beam epitaxy is reported. The material implanted to produce a Mn level of 3% produces ferromagnetic behavior that persists up to a temperature of 330 K, while the epitaxially derived material shows evidence of ferromagnetism at a temperature of 300 K. In both cases, no second phases were observed by x-ray diffraction, transmission electron microscopy, or selected area diffraction pattern analysis. A phase diagram of the GaMnP:C system, determined by epitaxial growth, is also reported.

Optical and electrical properties of GaMnN films grown by molecular-beam epitaxy

Optical absorption spectra, microcathodoluminescence (MCL) spectra, and electrical properties of GaMnN films grown by molecular-beam epitaxy with Mn concentration in the range of 3 to 10 at. % were studied. Optical absorption and MCL spectra show the presence of strong bands corresponding to the transition from the Mn acceptors near Ec−2 eV to the conduction band. The other strong band observed in MCL measurements was the blue band peaked near 2.9 eV and associated with the transition from the valence band to deep donors with a level near Ec−0.5 eV. All GaMnN samples were shown to be lightly n-type which suggests close self-compensation of the Mn acceptors by some native defect donors. A plausible scenario is that such compensating donors could be due to nitrogen vacancies and that the Ec−0.5 eV donor defects are complexes between the Mn acceptors and the nitrogen vacancy donors.

Properties of Co-, Cr-, or Mn-implanted AlN

AlN layers grown on Al2O3 substrates by metalorganic chemical vapor desposition were implanted with high doses (3×1016 cm−2, 250 keV) of Co+, Cr+, or Mn+. Band-edge photoluminescence intensity at ∼6 eV was significantly reduced by the implant process and was not restored by 950 °C annealing. A peak was observed at 5.89 eV in all the implanted samples. Impurity transitions at 3.0 and 4.3 eV were observed both in implanted and unimplanted AlN. X-ray diffraction showed good crystal quality for the 950 °C annealed implanted samples, with no ferromagnetic second phases detected. The Cr- and Co-implanted AlN showed hysteresis present at 300 K from magnetometry measurements, while the Mn-implanted samples showed clear loops up to ∼100 K. The coercive field was <250 Oe in all cases.

Effect of Mn concentration on the structural, optical, and magnetic properties of GaMnN

The room temperature magnetization of GaMnN films grown by molecular beam epitaxy on (0001) sapphire substrates with Mn concentrations varying from 0 to 9 at. % was found to depend on Mn concentration, with a maximum magnetization found at ∼3 at. % Mn. High-resolution x-ray diffraction measurements show that the c-plane lattice constant initially decreases with increasing Mn concentration, then increases when the Mn content increases above ∼3 at. %. This increase is accompanied by a decrease in the full width at half maximum of the rocking curves. Extended x-ray absorption fine structure results indicate that the nonsubstitutional Mn is not present in the form of GaxMny clusters and thus is most likely present in the form of an interstitial. Optical absorption measurements show only a slight increase in the band gap for material with 3 at. % Mn, relative to undoped GaN.