M. J. Reed

Room temperature ferromagnetic properties of (Ga, Mn)N

Dilute magnetic semiconductor GaN with a Curie temperature above room temperature has been achieved by manganese doping. By varying the growth and annealing conditions of Mn-doped GaN we have identified Curie temperatures in the range of 228–370 K. These Mn-doped GaN films have ferromagnetic behavior with hysteresis curves showing a coercivity of 100–500 Oe. Structure characterization by x-ray diffraction and transmission electron microscopy indicated that the ferromagnetic properties are not a result of secondary magnetic phases.

Determination of the critical layer thickness in the InGaN/GaN heterostructures

We present an approach to determine the critical layer thickness in the InxGa1−xN/GaN heterostructure based on the observed change in the photoluminescence emission as the InxGa1−xN film thickness increases. From the photoluminescence data, we identify the critical layer thickness as the thickness where a transition occurs from the strained to unstrained condition, which is accompanied by the appearance of deep level emission and a drop in band edge photoluminescence intensity. The optical data that indicate the onset of critical layer thickness, was also confirmed by the changes in InxGa1−xN surface morphology with thickness, and is consistent with x-ray diffraction measurements.

Optical band gap dependence on composition and thickness of InxGa1−xN (0<x<0.25) grown on GaN

Band gap measurements have been carried out in strained and relaxed InxGa1−xN epilayers with x<0.25. Values of x were determined from x-ray diffraction of relaxed films. The lowest energy absorption threshold, measured by transmittance, was found to occur at the same energy as the peak of the photoluminescence spectrum. Bowing parameters for both strained and relaxed films were determined to be 3.42 and 4.11 eV, respectively. The dependence of the band gap shift, ΔEg, on strain is presented.

Room temperature magnetic (Ga,Mn)N: a new material for spin electronic devices

A new dilute magnetic semiconductor (Ga,Mn)N grown by metal organic chemical vapor deposition (MOCVD) is reported. Vibrating sample magnetometer (VSM) and extraordinary Hall effect (EHE) measurements verified a ferromagnetic component at room temperature. The direction of the easy axis and the Curie temperature varies with the growth conditions, the latter ranging from 38°C to 75°C. Secondary ion mass spectroscopy (SIMS) confirms diffusion of Mn into the GaN to a depth of 380 A.

Effect of doping on the magnetic properties of GaMnN: Fermi level engineering

GaMnN dilute magnetic semiconductor samples, prepared by metalorganic chemical vapor deposition, are shown to exhibit ferromagnetism or even paramagnetism depending upon the type and concentration of extrinsic impurity present in the film. In addition, GaMnN deposited using growth parameters normally yielding a nonferromagnetic film becomes strongly ferromagnetic with the addition of magnesium, an acceptor dopant. Based upon these observations, it seems that ferromagnetism in this material system depends on the relative position of the Mn energy band and the Fermi level within the GaMnN band gap. Only when the Fermi level closely coincides with the Mn-energy level is ferromagnetism achieved. By actively engineering the Fermi energy to be within or near the Mn energy band, room temperature ferromagnetism is realized.

Critical layer thickness determination of GaN/InGaN/GaN double heterostructures

We report on the critical layer thickness of GaN/InxGa1−xN/GaN double heterostructures in the composition range 0<x<0.16. The evolution of the photoluminescence spectra and the electrical properties of the InxGa1−xN well were monitored as its thickness was increased for a given % InN. Due to compressive stress and possible quantum-size effects, the emission energy from thin InGaN wells is blueshifted relative to thicker wells of a given % InN. The transition from the blueshifted emission of strained InGaN to redshifted emission of relaxed InGaN is also accompanied by dramatic changes in film conductivity and mobility. The thickness at which the onset of relaxation occurs is deemed the critical layer thickness of the InxGa1−xN film.

Development of green, yellow, and amber light emitting diodes using InGaN multiple quantum well structures

The authors present optical and electrical data for long wavelength (573–601nm) InGaN∕GaN multiple quantum well light emitting diodes (LEDs) grown by metal organic chemical vapor deposition. These results are achieved by optimizing the active layer growth temperature and the quantum well width. Also, the p-GaN is grown at low temperature to avoid the disintegration of the InGaN quantum wells with high InN content. A redshift is observed for both the green and yellow LEDs upon decreasing the injection current at low current regime. In the case of the yellow LED, this shift is enough to push emission into the amber (601nm).

Epitaxial Y2O3 films grown on Si(111) by pulsed-laser ablation

Y2O3 has a relatively high dielectric constant (13–17) leading to several potential applications. In this work, pulsed-laser deposition was used to grow epitaxial Y2O3 films on Si(111) substrates. Structural characterization indicated two-dimensional growth without the formation of an amorphous interfacial layer. Annealing in either Ar or O2 was found to induce an O2 diffusion reaction resulting in the formation of two interfacial amorphous layers. Electrical characterization by capacitance–voltage and current–voltage indicated that the as-grown samples were poor insulating films. Annealing the samples improved the electrical performance by lowering leakage currents and exhibiting inversion during capacitance–voltage testing. This epitaxial growth points toward the possibility of the heteroepitaxial growth of silicon on insulator device structures.

Dependence of ferromagnetic properties on carrier transfer at GaMnN∕GaN:Mg interface

We report on the dependence of ferromagnetic properties of metalorganic chemical vapor deposition grown GaMnN films on carrier transfer across adjacent layers. We found that the magnetic properties of GaMnN, as a part of GaMnN∕GaN:Mg heterostructures, depend on the thickness of both the GaMnN film and the adjacent GaN:Mg layer and on the presence of a wide band gap barrier at this interface. These results are explained based on the occupancy of the Mn energy band and how the occupancy can be altered due to carrier transfer at the GaMnN∕GaN:Mg interfaces.

The Effect of Mn Concentration on Curie Temperature and Magnetic Behavior of MOCVD Grown GaMnN Films

We report on the growth and characterization of dilute magnetic semiconductor GaMnN showing ferromagnetism behavior above room temperature. GaMnN films were grown by MOCVD using (EtCp 2 )Mn as the precursor for in-situ Mn doping. Structural characterization of the GaMnN films was achieved by XRD, SIMS and TEM measurements. XRD and TEM confirmed that the films were single crystal solid solutions without the presence of secondary phases. SIMS analysis verified that Mn was incorporated homogeneously throughout the GaMnN layer which was ∼0.7μm thick. Ferromagnetic behavior for these films was observed along the c-direction (out of plane orientation) in a Mn concentration range of 0.025–2%. The saturation magnetization ranged from 0.18–1.05 emu/cc for different growth conditions. Curie temperatures of the GaMnN films were determined to be from 270 to above 400K depending on the Mn concentration. This dependence of Curie temperature on concentration of Mn in GaMnN indicates that the grown films are random solid solutions . SQUID measurements ruled out the possibility of spin-glass and superparamagnetism in these MOCVD grown GaMnN films. The grown films were electrically semi-insulating; however PL measurements showed that the films were still optically active after Mn doping. This study showed that the growth of III-Nitride films doped with Mn requires a small window of growth conditions that depend on growth temperature and (EtCp) 2 Mn flux to achieve ferromagnetism above room temperature, and the magnetic response of the film depends on the Fermi level position. We suggest that ferromagnetism occurs when the Fermi level lies within the Mn energy level which is 1.4 eV above the GaN valence band.