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.

Recent results on achieving ferromagnetism in transition-metal-doped GaN, AlN and related materials are discussed. The field of semiconductor spintronics seeks to exploit the spin of charge carriers in new generations of transistors, lasers and integrated magnetic sensors. There is strong potential for new classes of ultra-low-power, high speed memory, logic and photonic devices based on spintronics. The utility of such devices depends on the availability of materials with practical magnetic ordering temperatures and most theories predict that the Curie temperature will be a strong function of bandgap. We discuss the current state-of-the-art in producing room temperature ferromagnetism in GaN-based materials, the origins of the magnetism and its potential applications.

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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.

AlGaN ∕ GaN high-electron-mobility transistors (HEMTs) show a strong dependence of source∕drain current on the piezoelectric-polarization-induced two-dimensional electron gas. The spontaneous and piezoelectric-polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of pressure changes. The changes in the conductance of the channel of a AlGaN∕GaN high electron mobility transistor (HEMT) membrane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in the ambient pressure. The conductivity of the channel shows a linear change of −(+)6.4×10−2mS∕bar for application of compressive (tensile) strain. The AlGaN∕GaN HEMT membrane-based sensors appear to be promising for pressure sensing applications.

The characteristics of Sc2O3/AlGaN/GaN metal–oxide–semiconductor (MOS) diodes as hydrogen gas sensors are reported. At 25 °C, a change in forward current of ∼6 mA at a bias of 2 V was obtained in response to a change in ambient from pure N2 to 10% H2/90% N2. This is approximately double the change in forward current obtained in Pt/GaN Schottky diodes measured under the same conditions. The mechanism of the change in forward gate current appears to be formation of a dipole layer at the oxide/GaN interface that screens some of the piezo-induced channel charge. The MOS-diode response time is limited by the mass transport of gas into the test chamber and not by the diffusion of atomic hydrogen through the metal/oxide stack, even at 25 °C. These devices look promising for applications requiring sensitive, long-term stable detection of combustion gases.

In this study, various surface cleaning techniques for the removal of carbon (C) and oxygen (O) from AlN and GaN were investigated. Auger electron spectroscopy (AES) and secondary mass ion spectroscopy were used to monitor the presence of surface C and O, and atomic force microscopy was used to monitor surface roughness. AES analysis showed that ex situ ultraviolet/ozone (UV/O 3 ) and wet chemical treatments based on HF and HCl were very effective in removing surface C and reducing the native oxide on both AlN and GaN. After H 2 and N 2 plasma treatments in ultrahigh vacuum at temperatures of 750 and 900ЊC, clean GaN surfaces could be achieved within the detection limits of AES. An oxygen-free AlN surface could not be obtained within the detection limits of AES. SIMS analysis showed that concentrations of surface C and O up to 3 ϫ 10 20 and 2 ϫ 10 22 cm Ϫ3 , respectively, still exists on plasma-treated GaN. The results of this study indicate that ex situ UV/O 3 followed by H 2 /N 2 plasma treatment is highly effective in reducing the C and O contamination at the GaN surface, but that further in situ methods are needed to obtain clean GaN and AlN surfaces. None of the various cleaning methods were found to affect the surface roughness.

Optical transmission spectra, microcathodoluminescence spectra, capacitance–voltage and capacitance–frequency curves, temperature dependence of resistivity and deep level spectra with both electrical and optical injection were measured on n-GaN samples implanted with high doses of Mn (3×1016 and 4×1016 cm−2) and Co (4×1016 cm−2). From optical transmission it was found that Mn forms a deep acceptor near Ev+1.8 eV while the Co acceptor is about 0.1 eV deeper. In addition, Mn and Co form complexes with native defects and these complexes are deep electron traps with a level near Ec−0.5 eV. Such complexes are most likely responsible for a strong blue luminescence band with energy near 2.9 eV. Adjacent to the implanted region a defect region about 1 μm deep is formed, most likely by out-diffusion of point defects from the implanted zone during the 700 °C annealing used to partially remove the radiation damage. This region is characterized by a high density of electron traps at Ec−0.25 eV and Ec−0.7 eV and hole traps at Ev+0.2 eV, Ev+0.35 eV and Ev+0.45 eV.