Papers

AVS2004 Session WL-WeM: Science of Semiconductor White Light I

Wednesday, November 17, 2004 8:20 AM in Room 304B

Wednesday Morning

Start	Invited?	Item
8:20 AM		WL-WeM-1 Solid State Lighting M.R. Krames (Lumileds Lighting) Rapid progress in organometallic vapor phase epitaxy of III-phosphide and III-nitride compounds in the 1990's paved the way for the development of advanced visible-spectrum light emitting structures based on AlGaInP and InGaN active layers, respectively. These material platforms have been introduced into novel, high power, device architectures for dramatic improvements in total light generation and extraction efficiency, resulting in solid state light sources with external quantum efficiencies in the range of 30-50%. In particular, advances in the III-nitride blue and near-ultraviolet devices have enabled the efficient generation of white light through excitation of down-converting phosphors such as YAG:Ce. High power white LEDs are now commercially available with luminous efficiencies of 30-40 lm/W, exceeding that of standard incandescent (~17 lm/W) and halogen (~25 lm/W) bulbs. In addition, new classes of phosphors are being developed to further improve the emission spectral content, and thus color rendering quality, of white LEDs towards applications in indoor and retail illumination. The current state of the art for such solid state light sources will be reviewed, along with an outlook towards future applications and performance.
9:00 AM		WL-WeM-3 Growth of AlGaN-based UV LEDs Emitting at ~280 nm by Metalorganic Chemical Vapor Deposition R.D. Dupuis, U. Chowdhury, P. Li, J.-H. Ryou, T. Chung, D. Yoo, J.-B. Limb (Georgia Institute of Technology) Wide-bandgap nitride semiconductor materials in the InAlGaN system have attracted attention for deep-ultraviolet (UV) optoelectronic device applications in the spectral range lambda<300nm. In this paper, we report on the status of ternary AlGaN-based UV-LEDs emitting at ~280 nm. These devices are fabricated from epitaxial layers grown using the low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technology. The epitaxial layers are grown on dual-side polished c-plane sapphire substrates and the structure is designed for "flip-chip" operations where light is extracted from the back-side of the substrate. The device structures typically consist of an AlN buffer layer and a low-resistivity n-type Al_0.6Ga_0.4N:Si window layer in order improve light extraction while maintaining a low device series resistance and a low spreading resistance. A typical active region consists of three 10nm Al_0.48Ga_0.52N:Si barriers with three 5nm Al_0.40Ga_0.60N:Si quantum wells. The p-side cladding layer structures generally consist of a 20nm p-type Al_0.52Ga_0.48N:Mg electron barrier and a 20nm Al_0.40Ga_0.60N:Mg p-type cladding layer (or an AlGaN superlattice cladding layer) while a 25nm GaN:Mg is employed as a p-contact cap layer. We also report on the use of InGaN:Mg p-type layers to reduce the resistance of the p-contact layer in UV LED structures. Due to the lower acceptor ionization energy and lower work function, InGaN:Mg promises or offer better p-layer current spreading and lower contact resistance compared to GaN:Mg. The results of a study of the growth of InGaN:Mg along with Ohmic contact characteristics is described for application to UV-LED structures. The performance of UV LEDs fabricated from these materials will be described.
10:20 AM		WL-WeM-7 Surface Chemistry and Film Growth during ZnO Atomic Layer Deposition S.M. George (University of Colorado) Atomic layer deposition (ALD) is a thin film growth technique based on sequential, self-limiting surface reactions. ZnO ALD can be achieved using sequential exposures of Zn(CH₂CH₃)₂ [diethylzinc (DEZ)] and H₂O at 180°C. This talk will characterize the surface chemistry and film growth during ZnO ALD using in situ quartz crystal microbalance (QCM), Fourier transform infrared (FTIR) spectroscopy and 4-point probe resistivity measurements. The QCM measurements display a staircase structure that is consistent with a mass increase of 110 ng/cm² or 2.0 Å per DEZ/H₂O reaction cycle. The FTIR results show that the growing ZnO surface displays vibrational modes consistent with ZnOH* species following the H₂O exposures. DEZ exposure converts these species back to Zn(CH₂CH₃)* species. The background infrared absorbance of the ZnO film also increases progressively with number of DEZ/H₂O cycles as expected for an electrical conductor. The 4-point probe investigations reveal dramatic oscillations in the ZnO film resistivity that are dependent on both film thickness and adsorbed surface species. The resistivity is much higher with ZnCH₂CH₃* species than with ZnOH* species. This resistivity dependence on surface species may be important for an understanding of ZnO gas sensors. The ZnO ALD films are polycrystalline and have an electrical resistivity of ~10^-2 Ωcm. Excellent ultrathin and conformal ZnO ALD films are observed on ZrO₂ and BaTiO₃ particles.
11:00 AM		WL-WeM-9 Efficiency Limitations of Solid-State Sources used in General-Lighting Applications E.F. Schubert, Y.-L. Li, J. Kim, T. Gessmann (Rensselaer Polytechnic Institute) The use of highly efficient solid-state light sources for illumination applications can enable huge energy savings, a reduced generation of green-house gases, and a reduction in environmental pollution. Luminous source efficiencies exceeding 300 lm/W and color-rendering indices (CRIs) greater 90 are feasible with solid-state sources. This talk discusses critical issues in solid-state lighting including practical limits to efficacy and efficiency, scalability of chip size, and scalability of current density. Possible solutions to these issues based on new materials and structures are presented including low-refractive index materials, omni-directional reflectors, and polarization-enhanced contacts. Furthermore it is shown that solid-state sources based on light-emitting diodes (LEDs) have advantages not offered by conventional light sources, namely tunability and adaptability. Future smart light sources based on LEDs could offer full tunability of their spectral composition, color temperature, and other parameters, thereby allowing for the optimization of light sources for specific applications. Several application areas and the potential benefits of smart light sources are discussed.