A. J. Steckl

Enhanced emission efficiency in organic light-emitting diodes using deoxyribonucleic acid complex as an electron blocking layer

Enhanced electroluminescent efficiency using a deoxyribonucleic acid (DNA) complex as an electron blocking (EB) material has been demonstrated in both green- and blue-emitting organic light-emitting diodes (OLEDs). The resulting so-called BioLEDs showed a maximum luminous efficiency of 8.2 and 0.8cd∕A, respectively. The DNA-based BioLEDs were as much as 10× more efficient and 30× brighter than their OLED counterparts.

A nearly ideal phosphor-converted white light-emitting diode

A phosphor-converted light-emitting diode was obtained with nearly ideal blue-to-white conversion loss of only 1%. This is achieved using internal reflection to steer phosphor emission away from lossy surfaces, a reflector material with high reflectivity, and a remotely located organic phosphor having (1) unity quantum efficiency (ηq), (2) homogeneous refractive index to minimize scattering, and (3) refractive index-matched to the encapsulation to eliminate total internal reflection. An inorganic composite phosphor is also reported with a nearly homogeneous refractive index to minimize diffuse scattering of emitted light, thereby maximizing the effective phosphor ηq and light extraction.

Rare-earth-doped GaN: growth, properties, and fabrication of electroluminescent devices

A review is presented of the fabrication, operation, and applications of rare-earth-doped GaN electroluminescent devices (ELDs). GaN:RE ELDs emit light due to impact excitation of the rare earth (RE) ions by hot carriers followed by radiative RE relaxation. By appropriately choosing the RE dopant, narrow linewidth emission can be obtained at selected wavelengths from the ultraviolet to the infrared. The deposition of GaN:RE layers is carried out by solid-source molecular beam epitaxy, and a plasma N/sub 2/ source. Growth mechanisms and optimization of the GaN layers for RE emission are discussed based on RE concentration, growth temperature, and V/III ratio. The fabrication processes and electrical models for both dc- and ac-biased devices are discussed, along with techniques for multicolor integration. Visible emission at red, green, and blue wavelengths from GaN doped with Eu, Er, and Tm has led to the development of flat-panel display (FPD) devices. The brightness characteristics of thick dielectric EL (TDEL) display devices are reviewed as a function of bias, frequency, and time. High contrast TDEL devices using a black dielectric are presented. The fabrication and operation of FPD prototypes are described. Infrared emission at 1.5 /spl mu/m from GaN:Er ELDs has been applied to optical telecommunications devices. The fabrication of GaN channel waveguides by inductively coupled plasma etching is also reviewed, along with waveguide optical characterization.

Optoelectronic Properties and Applications of Rare-Earth-Doped GaN

As discussed in the accompanying articles in this issue of MRS Bulletin , the optical properties of rare-earth (RE) elements have led to many important photonic applications, including solid-state lasers, components for telecommunications (optical-fiber amplifiers, fiber lasers), optical storage devices, and displays. In most of these applications, the host materials for the RE elements are various forms of oxide and nonoxide glasses. The emission can occur at visible or infrared (IR) wavelengths, depending on the electronic transitions of the selected RE element and the excitation mechanism. Until recently, the study of semiconductors doped with RE elements such as Pr and Er has concentrated primarily on the lowest excited state as an optically active transition. The presence of transitions at IR wavelengths (1.3 and 1.54 μm) that are coincident with minima in the optical dispersion and the loss of silica-based glass fibers utilized in telecommunications, combined with the prospect of integration with semiconductor device technology, has sparked considerable interest. The status and prospects of obtaining stimulated emission in Si:Er are reviewed by Gregorkiewicz and Langer in this issue and by Coffa et al. in a previous MRS Bulletin issue. While great progress is being made in enhancing the emission intensity of Er-doped Si, it still experiences significant loss in luminescence efficiency at room temperature, as compared with low temperatures. This thermal quenching was shown by Favennec et al. to de crease with the bandgap energy of the semiconductor. Hence wide-bandgap semiconductors (WBGSs) are attractive candidates for investigation as hosts for RE doping.

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...

Superhydrophobic and Oleophobic Fibers by Coaxial Electrospinning

Control of surface wetting properties to produce strongly hydrophobic or hydrophilic effects is at the heart of many macro- and microfluidic applications. In this work, we have investigated coaxial electrospinning to produce core-sheath-structured nano/microfibers that combine different properties from individual core and sheath materials. Teflon AF is an amorphous fluoropolymer that is widely utilized as a hydrophobic material. Hydrophobic fluoropolymers are normally not electrospinnable because their low dielectric constant prevents sufficient charging for a solution to be electrospun. The first Teflon electrospun fibers are reported using coaxial electrospinning with Teflon AF sheath and poly(epsilon-caprolactone) (PCL) core materials. Using these core/sheath fibers, superhydrophobic and oleophobic membranes have been successfully produced. These coaxial fibers also preserve the core material properties as demonstrated with mechanical tensile tests. The fact that a normally nonelectrospinnable material such as Teflon AF has been successfully electrospun when combined with an electrospinnable core material indicates the potential of coaxial electrospinning to provide a new degree of freedom in terms of material combinations for many applications.

Visible emission from Er-doped GaN grown by solid source molecular beam epitaxy

Visible light emission has been obtained from Er-doped GaN thin films. The GaN was grown by molecular beam epitaxy on sapphire substrates using solid sources (for Ga, Al, and Er) and a plasma gas source for N2. Above GaN band-gap photoexcitation resulted in strong green emission. The emission spectrum consists of two narrow green lines at 537 and 558 nm and a broad peak at light blue wavelengths (480–510 nm). The narrow lines have been identified as Er transitions from the 2H11/2 and 4S3/2 levels to the 4I15/2 ground state. The intensity of the 558 nm emission decreases with increasing temperature, while the intensity of the 537 nm line actually peaks at ∼300 K. This effect is explained based on the thermalization of electrons between the two closely spaced energy levels.

High-voltage Ni- and Pt-SiC Schottky diodes utilizing metal field plate termination

We have fabricated 1 kV 4H and 6H SiC Schottky diodes utilizing a metal-oxide overlap structure for electric field termination. This simple structure when used with a high barrier height metal such as Ni has consistently given us good yield of Schottky diodes with breakdown voltages in excess of 60% of the theoretically calculated value. This paper presents the design considerations, the fabrication procedure, and characterization results for these 1 kV Ni-SiC Schottky diodes. Comparison to similarly fabricated Pt-SiC Schottky diodes is reported. The Ni-SiC ohmic contact formation has been studied using Auger electron spectroscopy and X-ray diffraction. The characterization study includes measurements of current-voltage (I-V) temperature and capacitance-voltage (C-V) temperature characteristics. The high-temperature performance of these diodes has also been investigated. The diodes show good rectifying behavior with ON/OFF current ratios, ranging from 10/sup 6/ to 10 at 27/spl deg/C and in excess of 10/sup 6/ up to 300/spl deg/C.

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.

Red light emission by photoluminescence and electroluminescence from Pr-doped GaN on Si substrates

Visible light emission has been obtained at room temperature by photoluminescence (PL) and electroluminescence (EL) from Pr-doped GaN thin films grown on Si(111). The GaN was grown by molecular beam epitaxy using solid sources (for Ga and Pr) and a plasma gas source for N2. Photoexcitation with a He–Cd laser results in strong red emission at 648 and 650 nm, corresponding to the transition between 3P0 and 3F2 states in Pr3+. The full width at half maximum (FWHM) of the PL lines is ∼1.2 nm, which corresponds to ∼3.6 meV. Emission is also measured at near-infrared wavelengths, corresponding to lower energy transitions. Ar laser pumping at 488 nm also resulted in red emission, but with much lower intensity. Indium-tin-oxide Schottky contacts were used to demonstrate visible red EL from the GaN:Pr. The FWHM of the EL emission line is ∼7 nm.