Don Lee

Red light emission by photoluminescence and electroluminescence from Eu-doped GaN

Visible light emission has been obtained at room temperature by photoluminescence (PL) and electroluminescence (EL) from Eu-doped GaN thin films. The GaN was grown by molecular beam epitaxy on Si substrates using solid sources (for Ga and Eu) and a plasma source for N2. X-ray diffraction shows the GaN:Eu to be a wurtzitic single crystal film. Above GaN band gap photoexcitation with a He–Cd laser at 325 nm resulted in strong red emission. Observed Eu3+ PL transitions consist of a dominant narrow red line at 621 nm and several weaker emission lines were found within the green through red (543 to 663 nm) range. Below band gap PL by Ar laser pumping at 488 nm also resulted in red emission, but with an order of magnitude lower intensity. EL was obtained through use of transparent indium–tin–oxide contacts to the GaN:Eu film. Intense red emission is observed in EL operation, with a spectrum similar to that seen in PL. The dominant red line observed in PL and EL has been identified as the Eu3+ 4f shell transitio...

Blue emission from Tm-doped GaN electroluminescent devices

Blue emission has been obtained at room temperature from Tm-doped GaN electroluminescent devices. The GaN was grown by molecular beam epitaxy on Si(111) substrates using solid sources (for Ga and Tm) and a plasma source for N2. Indium–tin–oxide was deposited on the GaN layer and patterned to provide both the bias (small area) and ground (large area) transparent electrodes. Strong blue light emission under the bias electrode was observable with the naked eye at room temperature. The visible emission spectrum consists of a main contribution in the blue region at 477 nm corresponding to the Tm transition from the 1G4 to the 3H6 ground state. A strong near-infrared peak was also observed at 802 nm. The relative blue emission efficiency was found to increase linearly with bias voltage and current beyond certain turn-on levels.

Spectral and Time-Resolved Photoluminescence Studies of Eu-Doped GaN

We report on spectral and time-resolved photoluminescence (PL) studies performed on Eu-doped GaN prepared by solid-source molecular-beam epitaxy. Using above-gap excitation, the integrated PL intensity of the main Eu3+ line at 622.3 nm (5D0→7F2 transition) decreased by nearly 90% between 14 K and room temperature. Using below-gap excitation, the integrated intensity of this line decreased by only ∼50% for the same temperature range. In addition, the Eu3+ PL spectrum and decay dynamics changed significantly compared to above-gap excitation. These results suggest the existence of different Eu3+ centers with distinct optical properties. Photoluminescence excitation measurements revealed resonant intra-4f absorption lines of Eu3+ ions, as well as a broad excitation band centered at ∼400 nm. This broad excitation band overlaps higher lying intra-4f Eu3+ energy levels, providing an efficient pathway for carrier-mediated excitation of Eu3+ ions in GaN.

Multiple color capability from rare earth-doped gallium nitride

Rare earth (RE) doping of GaN has led to a new full color thin film electroluminescent (TFEL) phosphor system. GaN films doped with Eu, Er, and Tm dopants emit pure red, green, and blue emission colors, respectively. As a host for RE luminescent centers, GaN possesses many properties which are ideal for bright multiple color TFEL. Specifically, GaN has excellent high field transport characteristics, is chemically and thermally rugged, and incorporates well the RE dopants. X-ray absorption measurements have shown that even at RE dopant levels exceeding 0.1 at.% the majority of RE dopants occupy a strongly bonded substitutional site on the Ga sublattice. According to RE crystal field theory this tetrahedrally bonded site allows optical activation and emission from RE 4f‐4f inner-shell electronic transitions. Monte Carlo calculations of GaN carrier transport have shown that at 2M V cm 1 applied field the average electron possesses 2.6 eV energy which is adequate for exciting blue emission. GaN:Er TFEL devices have exhibited a brightness of 500‐1000 cd:m 2 at 540 nm. In addition to pure colors, mixed colors can be achieved by doping with a combination of REs. For example, co-doping with Er and Tm results in an emission spectrum which is perceived by the human eye as a blue‐green (turquoise) hue. Multiple color capability in a single device has also been demonstrated by adjusting the bias voltage (in a co-doped GaN:Er,Eu layer) or by switching the bias polarity (in a stacked two layer GaN:Er:GaN:Eu structure). The combination of pure or mixed color emission, the availability of bias controlled color, and the potential for white light emission indicate that GaN:RE TFEL devices have enormous potential for display applications.

Voltage-controlled yellow or orange emission from GaN codoped with Er and Eu

Orange and yellow-colored light emission has been achieved at room temperature in the same elecroluminescent device (ELD) made on GaN thin films codoped with Er and Eu. The GaN film was grown by molecular-beam epitaxy on Si (111) substrates using solid sources for Ga, Er and Eu and a plasma source for N2. Simple Schottky devices were fabricated on the GaN films using indium–tin oxide (ITO) transparent electrodes. ELD spectra show that the yellow and orange colors result from the combination of green emission from Er (537, 558 nm) and red emission from Eu (621 nm). A color change was observed with applied bias, producing yellow at higher bias (−100 V) and orange at lower bias (−70 V). We have fabricated both relatively small (∼250 μm) and large (1.45 mm) ELDs. Parameters for the chromaticity diagram were calculated to be x=0.382, y=0.605 for the yellow emission and x=0.467, y=0.523 for the orange emission. This work shows the possibility of achieving any intermediate color in the spectrum from green to red...

Hot-pressed and hot-deformed nanocomposite (Nd,Pr,Dy)/sub 2/Fe/sub 14/B//spl alpha/-Fe-based magnets

Fully dense isotropic and anisotropic (Nd,Pr,Dy)/sub 2/Fe/sub 14/B//spl alpha/-Fe nanocomposite magnets have been successfully prepared by a hot press and hot deformation technique using melt-spun precursors. For the fully dense, hot-deformed, anisotropic nanocomposite (Nd,Pr,Dy)/sub 2/Fe/sub 14/B//spl alpha/-Fe-based magnets containing /spl sim/4 vol% and /spl sim/11 vol% /spl alpha/-Fe, the maximum energy products have reached 31 and 24 MGOe, respectively. At the time of proofreading for this paper, 42 MGOe has been reached for the nanocomposite magnets with /spl ap/4 vol% /spl alpha/-Fe.

Temperature dependence of energy transfer mechanisms in Eu-doped GaN

The temperature dependent behavior of continuous-wave and time-resolved photoluminescence of Eu-doped GaN in the visible region is measured for both the 5D0→7F2 and 5D0→7F3 transitions. The radiative decay of these transitions, following pulsed laser excitation of the GaN host, is monitored by a grating spectrometer and photomultiplier tube detector system. In addition to these two radiative energy transfer pathways within Eu3+, the data reveal two nonradiative energy transfer paths between Eu3+ and the host GaN. Decay constants for the relaxation processes are extracted from the data using a numerically solved rate equation model. Although the dominant radiative relaxation processes decayed with a temperature insensitive decay constant of 166 μs, a prominent role for nonradiative transfer between Eu3+ and impurities within the GaN host was deduced above 180 K.

Photoluminescence properties of in situ Tm-doped AlxGa1−xN

We report on the photoluminescence (PL) properties of in situ Tm-doped AlxGa1−xN films (0⩽x⩽1) grown by solid-source molecular-beam epitaxy. It was found that the blue PL properties of AlxGa1−xN:Tm greatly change as a function of Al content. Under above-gap pumping, GaN:Tm exhibited a weak blue emission at ∼478 nm from the 1G4→3H6 transition of Tm3+. Upon increasing Al content, an enhancement of the blue PL at 478 nm was observed. In addition, an intense blue PL line appeared at ∼465 nm, which is assigned to the 1D2→3F4 transition of Tm3+. The overall blue PL intensity reached a maximum for x=0.62, with the 465 nm line dominating the visible PL spectrum. Under below-gap pumping, AlN:Tm also exhibited intense blue PL at 465 and 478 nm, as well as several other PL lines ranging from the ultraviolet to near-infrared. The Tm3+ PL from AlN:Tm was most likely excited through defect-related complexes in the AlN host.

Spectroscopic and energy transfer studies of Eu3+ centers in GaN

Photoluminescence (PL), photoluminescence excitation (PLE), and time-resolved PL spectroscopies have been carried out at room temperature and 86K on transitions from D25, D15, and D05 excited states to numerous FJ7 ground states of Eu-doped GaN films grown by conventional solid-source molecular beam epitaxy (MBE) and interrupted growth epitaxy MBE. Within the visible spectral range of 1.8–2.7eV, 42 spectral features were observed and assignments were attempted for each transition. PL and PLE indicate that four Eu3+ centers exist in the GaN lattice whose relative concentration can be controlled by the duration of growth interruption. The energy levels for these four sites are self-consistently obtained, and time-resolved photoluminescence measurements reveal details about the radiative and nonradiative relaxations of excitation among these levels. The data indicate a near-resonant cross relaxation among these sites. The D25 and D15 states are observed to decay nonradiatively by filling the D05 state with c...

Bulk anisotropic composite rare earth magnets

Bulk anisotropic composite Nd13.5Fe80Ga0.5B6∕α-Fe and Nd14Fe79.5Ga0.5B6∕Fe–Co magnets with (BH)max=45–50MGOe have been synthesized by blending a Nd–Fe–Ga–B powder with an α-Fe or Fe–Co powder followed by hot compaction at 600–700 °C and hot deformation (die upsetting) at 850–950 °C with a height reduction of 71%. The composite Nd13.5Fe80Ga0.5B6∕α-Fe and Nd14Fe79.5Ga0.5B6∕Fe–Co magnets show microstructures consisting of a very large soft phase up to ∼50μm, which is more than 1000 times larger than the upper size limit of the soft phase expected from the existing models of interface exchange coupling.