AVS2004 Session WL+MS-WeA: Science of Semiconductor White Light II

Wednesday, November 17, 2004 2:00 PM in Room 304B

Wednesday Afternoon

Start	Invited?	Item
2:00 PM	Invited	WL+MS-WeA-1 Material and Device Challenges of Deep UV Emitters M.H. Crawford, A.A. Allerman, A.J. Fischer, K.H.A. Bogart, S.R. Lee, W.W. Chow (Sandia National Laboratories) One of the new frontiers of light emitting diode research is the application of wide bandgap AlGaN alloys to achieve electroluminescence at 300 nm and shorter wavelengths. While most near-UV (380-400 nm) LEDs employ InGaN quantum well structures with GaN barriers, reaching deep UV wavelengths requires the growth of AlGaN alloys with aluminum concentrations of 50% and higher. In this presentation, we will review the present status of LED technology in the deep UV range and will discuss in detail the material and device challenges for achieving high performance devices. We will present data on LEDs that are grown by metal-organic vapor-phase epitaxy and employ flip-chip device geometries. These devices have yielded > 1 mW output powers in the 275-290 nm range under DC current operation. Critical device issues that will be discussed include performance under pulsed current injection, device lifetimes and an evaluation of the origins of deep level emission in the electroluminescence spectra. Electroluminescence at wavelengths shorter than 250 nm will be presented, and limitations of short wavelength performance will be discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000. This work is supported by DARPA under the SUVOS program.
2:40 PM	Invited	WL+MS-WeA-3 Interfaces in White Light-Emitting Structures P.H. Holloway (University of Florida) Solid state devices have the promise to revolutionize the generation of white light. Solid state lighting has the potential to increase the efficiency of converting electricity to light by factors of two or more. The solid state structures are robust and hint at lifetimes of thousands of hours. Interfaces in these devices will play an important role in the fulfillment or collapse of this promise. The best performance to date has been achieved with light emitted from GaN-based LEDs which contribute a blue component to the emitted light, while simultaneously photo-pumping one or more phosphors which luminescence to provide a color spectrum with a good color rendering index. Critical interfaces in these structures range from the interfaces between the epitaxial layers, to the metal-semiconductor interfaces for ohmic contacts, to the matix-phosphor interface for light scattering and quenching luminescence. Our knowledge about these types of interfaces will be illustrated using examples of interfacial characterization and reactions, and the need for new understanding will be illustrated.
3:20 PM	Invited	WL+MS-WeA-5 Ohmic Contacts to (Al)GaN Semiconductors for Light Emitters I. Adesida (University of Illinois at Urbana-Champaign) The direct bandgap of GaN-based semiconductors have made them attractive materials for the realization of a wide range of optoelectronic devices. Examples of devices that are either commercially available or have been demonstrated include short wavelength light emitting diodes (LEDs), solar blind detectors, and laser diodes which have applications in white light illumination, bio-chemical agent sensing, solar UV detection, missile detection, flame and heat sensing, ozone monitoring, and remote sensing. Materials growth and device processing are still critical issues in terms of obtaining highly efficient GaN-based optoelectronic devices. The realization of highly reliable, thermally stable, low resistance ohmic contacts to both n-type and p-type GaN-based semiconductors is essential. To date, the formation of contacts to AlGaN with high Al concentration remain a challenge for various reasons. In this paper, we will describe our work on ohmic contact formation on both n- type and p-type AlGaN of various Al concentrations. Results on n-type contact formation using various metallization schemes will be presented. Contact formation to p-type AlGaN using Pd-based metallization schemes will be presented. Issues of thermal stability of these contacts will be discussed. The efficacy of various surface treatment schemes for GaN and AlGaN to improve the ohmic performance of the contacts will be discussed. Comprehensive studies are being performed to compare the effects of various surface treatment schemes which include both plasma and wet processes on the electrical and material characteristics of GaN and AlGaN semiconductors. Further, the mechanism of formation of ohmic contacts in these semiconductors will be discussed.
4:00 PM	Invited	WL+MS-WeA-7 Passivation and Processing-Induced Changes in GaN/Insulator Interfaces R.J. Nemanich, T.E. Cook, Jr., C.C. Fulton, W.J. Mecouch, R.F. Davis, G. Lucovsky (NC State University) Passivation of GaN and AlGaN surfaces is now a critical limitation in electronic device fabrication. The band relations of various dielectrics on III-nitrides are just being established, and some interfaces show significant process induced variations. The characteristics of clean n- and p-type GaN (0001) surfaces and the interface between this surface and SiO2, Si3N4, and HfO2 have been investigated. Layers of SiO2, Si3N4, or HfO2 were carefully deposited to limit the reaction between the dielectric and the clean GaN surfaces. After stepwise deposition, the electronic states were measured with x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). A valence band offset (VBO) of 2.0 eV with a conduction band offset (CBO) of 3.6 eV was determined for the GaN/SiO2 interface. For the GaN/Si3N4 interface, type II band alignment was observed with a VBO of 0.5 eV with a CBO of 2.4 eV, which differs substantially from prior reports. We suggest that the differences are related to the level of oxygen incorporated at the interface. A VBO of 0.4 eV with a CBO of 2.0 eV was determined for the GaN/HfO2 interface. An instability was observed in the HfO2 film, with energy bands shifting ~0.5 eV during a 650Â°C densification anneal. The deduced band alignments were compared to the predictions of the electron affinity model and deviations were attributed to a change of the interface dipole. The largest deviation was observed for the oxide layers. It was noted that the existence of Ga-O bonding at the heterojunction can significantly affect the interface dipole, and consequently the band alignment in relation to the GaN.
4:40 PM	Invited	WL+MS-WeA-9 Emissivity-Correcting Pyrometry for Group-III Nitride MOCVD J.R. Creighton, C.C. Mitchell (Sandia National Laboratories) Accurate temperature measurement during group-III nitride MOCVD is very difficult due to the broad spectral transparency of the substrates and epitaxial layers. In fact, there is no readily available method that measures the true surface temperature during deposition. We have developed a pyrometer that operates near the high-temperature bandgap of GaN, thus solving the transparency problem once a ~1 micron thick GaN epilayer has been established. At typical GaN MOCVD conditions the RMS temperature noise of the system is <0.1°C. By simultaneously measuring the reflectance, we can also correct for emissivity changes when films of differing optical properties (e.g. AlGaN) are deposited on the GaN template. By employing the virtual interface method, the reflectance measurement can also be used to monitor growth rates and compute optical properties of the thin films. Using this method we have measured the high temperature optical constants of GaN at the effective pyrometer wavelength (405 nm). Near 1000°C, the imaginary part (k) of the GaN refractive index is ~0.2, thus demonstrating that the epilayer is opaque. Small artifacts (due to stray light) in the emissivity-correction method are often observed, leading to residual oscillations in the corrected temperature. For our new nitride pyrometer the residual temperature oscillations are typically <3°C in amplitude when growing AlN/GaN heterostructures. Through proper calibration experiments, the nature of the error can be understood and quantitatively eliminated. We will also report on our recent efforts to extend this nitride pyrometer technology to multiwafer MOCVD reactors. (Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.).