R. M. Frazier

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.

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.

Role of growth conditions on magnetic properties of AlCrN grown by molecular beam epitaxy

AlCrN layers were grown by gas source molecular beam epitaxy with varying amounts of Cr (up to ∼3at.%) under a broad range of Cr cell temperatures and V/III ratio. Magnetic measurements performed in a superconducting quantum interference device magnetometer showed evidence of ferromagnetism up to 350K in single phase material. Magnetization dependence on dopant cell temperature and V/III was used to optimize the growth conditions of the AlCrN layers. The single-phase material was highly insulating (∼1010Ωcm), while the material containing second phases (predominantly Cr2N and AlxCry) was conducting with resistivity of order 1000Ωcm. High resolution x-ray diffraction rocking curves indicated high crystalline quality in the single phase material.

Properties of highly Cr-doped AlN

Cr concentrations of ∼2at.% were incorporated into AlN during growth by molecular beam epitaxy. Under optimized conditions, single-phase, insulating AlCrN is produced whose band gap shows a small (0.1–0.2eV) decrease from the value for undoped AlN (6.2eV), a decrease in a-plane lattice constant and the introduction of two absorption bands at 3 and 5eV into the band gap. This material shows ferromagnetism with a Curie temperature above 300K as judged from the difference in field-cooled and zero-field-cooled magnetization. For nonoptimized growth, second phases of Cr2N and AlxCry are produced in the AlN and the material is conducting (∼1000Ωcm) with activation energy for conduction of 0.19eV and apparent band gap of 5.8eV.

Improved oxide passivation of AlGaN∕GaN high electron mobility transistors

MgO has proven effective in the past as a surface passivation layer to minimize current collapse in AlGaN∕GaN high electron mobility transistors (HEMTs). However, MgO is not environmentally stable and more stable oxides need to be developed. MgCaO can be produced that is lattice matched to the GaN. Three samples were grown with 0%, 50% and 75% of Ca, which had respective lattice mismatches of −6.5% for MgO, −1% for Mg0.50Ca0.50O and +4% for Mg0.25Ca0.75O. Drain saturation current in HENTs had increases of 4.5% and 1%, respectively, for Mg0.5Ca0.5O and Mg0.25Ca0.75O passivated devices. However, there was a 10% decrease for the device passivated with pure MgO. This was due to strain applied on the nitride HEMT by the oxide, which is consistent with the piezoelectric effect in the nitride HEMT by the oxide, which is consistent with the piezoelectric effect in the nitride form the lattice mismatch between AlGaN and GaN. From pulsed measurements, HEMTs passivated with Mg0.5Ca0.5O and Mg0.25Ca0.75O showed highe...

Effect of growth conditions on the magnetic characteristics of GaGdN

GaGdN layers were grown by gas source molecular beam epitaxy with varying crystal quality and Gd concentrations as set by the Gd cell temperature. Magnetic measurements showed ferromagnetic behavior at room temperature, with the saturation magnetization dependent both on Gd concentration and crystalline quality. The Gd concentration was under the detection limit of secondary ion mass spectrometry, and estimated to be on the order of <1017at.∕cm3. As expected at this low dopant concentration, x-ray diffraction measurements showed the films to be single phase. Gd-doped samples codoped with Si to make them conducting with resistivity of 0.04Ωcm showed similar magnetic properties as Gd-doped films without addition of Si.