M. E. Overberg

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.

Magnetic and structural properties of Mn-implanted GaN

High doses (1015–5×1016 cm−2) of Mn+ ions were implanted into p-GaN at ∼350 °C and annealed at 700–1000 °C. At the high end of this dose range, platelet structures of GaxMn1−xN were formed. The presence of these regions correlated with ferromagnetic behavior in the samples up to ∼250 K. At low doses, the implanted led to a buried band of defects at the end of the ion range.

Ferromagnetism in cobalt-implanted ZnO

The magnetic and structural properties of cobalt-implanted ZnO single crystals are reported. High-quality, (110)-oriented single-crystal Sn-doped ZnO substrates were implanted at ∼350 °C with Co to yield transition metal concentrations of 3–5 at. % in the near-surface (∼2000 A) region. After implantation, the samples were subject to a 5 min rapid thermal annealing at 700 °C. Magnetization measurements indicate ferromagnetic behavior, with hysteresis observed in the M vs H behavior at T=5 K. Coercive fields were ⩽100 Oe at this measurement temperature. Temperature-dependent magnetization measurements showed evidence for ordering temperatures of >300 K, although hysteresis in the M vs H behavior was not observed at room temperature. Four-circle x-ray diffraction results indicate the presence of (110)-oriented hexagonal phase Co in the ZnO matrix. From the 2θ full width at half maximum (FWHM) of the Co (110) peak, the nanocrystal size is estimated to be ∼3.5 nm, which is below the superparamagnetic limit at ...

Indication of ferromagnetism in molecular-beam-epitaxy-derived N-type GaMnN

Growth by molecular-beam epitaxy of the dilute magnetic alloy GaMnN is reported. The GaMnN contains 7.0% Mn as determined by Auger electron spectroscopy, and is single phase as determined by x-ray diffraction and reflection high-energy electron diffraction. Both magnetic and magnetotransport data are reported. The results show the anomalous Hall effect, negative magnetoresistance, and magnetic hysteresis at 10 K, indicating that Mn is incorporating into the GaN and forming the ferromagnetic semiconductor GaMnN. At 25 K the anomalous Hall term vanishes, indicating a Curie temperature between 10 and 25 K.

Unconventional carrier-mediated ferromagnetism above room temperature in ion-implanted (Ga, Mn)P:C.

Ion implantation of Mn ions into hole-doped GaP has been used to induce ferromagnetic behavior above room temperature for optimized Mn concentrations near 3 at. %. The magnetism is suppressed when the Mn dose is increased or decreased away from the 3 at. % value, or when n-type GaP substrates are used. At low temperatures the saturated moment is on the order of 1 Bohr magneton, and the spin wave stiffness inferred from the Bloch-law T(3/2) dependence of the magnetization provides an estimate T(c)=385 K of the Curie temperature that exceeds the experimental value, T(c)=270 K. The presence of ferromagnetic clusters and hysteresis to temperatures of at least 330 K is attributed to disorder and proximity to a metal-insulating transition.

Hydrogen incorporation and diffusivity in plasma-exposed bulk ZnO

Hydrogen incorporation depths of >25 μm were obtained in bulk, single-crystal ZnO during exposure to 2H plasmas for 0.5 h at 300 °C, producing an estimated diffusivity of ∼8×10−10 cm2/V⋅s at this temperature. The activation energy for diffusion was 0.17±0.12 eV, indicating an interstitial mechanism. Subsequent annealing at 500–600 °C was sufficient to evolve all of the hydrogen out of the ZnO, at least to the sensitivity of secondary ion mass spectrometry (<5×1015 cm−3). The thermal stability of hydrogen retention is slightly greater when the hydrogen is incorporated by direct implantation relative to plasma exposure, due to trapping at residual damage in the former case.

Thermal stability of ion-implanted hydrogen in ZnO

The evolution of implanted 2H profiles in single-crystal ZnO was examined as a function of annealing temperature (500–700 °C) by secondary ion mass spectrometry. The as-implanted profiles show a peak concentration of ∼2.7×1019 cm−3 at a depth of ∼0.96 μm for a dose of 1015 cm−2. Subsequent annealing causes outdiffusion of 2H from the ZnO, with the remaining hydrogen decorating the residual implant damage. Only 0.2% of the original dose is retained after annealing at 600 °C. Rutherford backscattering/channeling of samples implanted with 1H at a dose of 1016 cm−2 showed no change in backscattering yield near the ZnO surface, but did result in an increase near the end-of-range from 6.5% of the random level before 1H implantation to ∼7.8% after implantation. Results of both cathodoluminescence and photoluminescence studies show that even for a 1H dose of 1015 cm−2, the intensity of the near gap emission from ZnO is reduced more than 2 orders of magnitude from the values in unimplanted samples. This is due to ...

Ferromagnetism in Co- and Mn-doped ZnO

Bulk single crystals of Sn-doped ZnO were implanted with Co or Mn at doses designed to produce transition metal concentrations of 3–5 at.% in the near-surface (� 2000 A region. The implantation was performed at � 350 Ct o promote dynamic annealing of ion-induced damage. Following annealing at 700 C, temperature-dependent magnetization measurements showed ordering temperatures of � 300 K for Co- and � 250 K for Mn-implanted ZnO. Clear hysteresis loops were obtained at these temperatures. The coercive fields were 6 100 Oe for all measurement temperatures. X-ray diffraction showed no detectable second phases in the Mn-implanted material. One plausible origin for the ferromagnetism in this case is a carrier-induced mechanism. By sharp contrast, the Co-implanted material showed evidence for the presence of Co precipitates with hexagonal symmetry, which is the cause of the room temperature ferromagnetism. Our results are consistent with the stabilization of ferromagnetic states by electron doping in transition metal-doped ZnO predicted by Sato and Katayama–Yoshida [Jpn. J. Appl. Phys. 40 (2001) L334]. This work shows the excellent promise of Mn-doped ZnO for potential room temperature spintronic applications. 2003 Elsevier Ltd. All rights reserved.

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.