A new white light emitting diode, the photon recycling semiconductor light emitting diode (PRS-LED) is demonstrated. The device consists of a GaInN/GaN LED emitting in the blue spectral range and an AlGaInP photon recycling semiconductor emitting at the complementary color. Thus the PRS-LED has two emission peaks, one in the blue and one in the amber wavelength range. The theoretical luminous performance of the PRS-LED exceeds 300 lm/W, higher than the performance of phosphor-based white LEDs.
Increasing optical power and electrical-to-optical conversion efficiency enable visible light-emitting diodes to advance into new applications and wider markets. InGaAlP/GaAs and InGaN/sapphire material systems cover the whole visible spectrum of saturated colors used for display, signage, and automotive use. A combination of blue InGaN LEDs with phosphor delivers a 'white' spectrum adequate for most lighting needs. Demand for high optical power requires larger chips suitable for high-current operation. Current crowding effects and their negative consequences for chip performance and reliability limit the performance of high-power chips based on both material systems. Despite the differences between InGaAlP/GaAs and InGaN/sapphire chip structures, a number of common design concepts leading to higher external efficiency and total luminous output have been proposed, including large chips operating at high drive currents. This paper highlights fundamental current spreading and reliability issues related to the chip size and operating current density, outlines a framework for quantitative analysis, proposes and compares a number of novel high-power chip designs.
As more advances are made in the performance of GaN-based devices, a trend toward the use of large scale MOCVD reactors for epitaxial growth of GaN-based device structures is clear. In this paper we describe the use of Emcore's SpectraBlueTM reactor for large-scale manufacturing of Blue and Green LEDs. The high throughput growth of GaN based LEDs is demonstrated without compromising LED uniformity or overall performance. In-situ control of key parameters critical to the production of high quality LEDs, such as buffer layer growth is now feasible using in-situ reflectance spectroscopy. Film properties as well as LED device performance are discussed.
We have used time-resolved photoluminescence (PL) to examine light-emitting diodes made of InGaN/GaN multiple quantum wells (MQWs) before the final stages of processing. The time-resolved photoluminescence from a dim MQW was quenched by nonradiative recombination centers. The PL kinetics from a bright MQW were not single exponential but stretched exponential, with the stretch parameter (beta) equals 0.59 +/- 0.05. The emission lifetime varied with energy, within error (beta) was independent of the emission energy. The stretched exponential kinetics are consistent with significant disorder in the material. Related results for an InGaN film and InGaN/GaN MQWs are also reported. We attribute the disorder to fluctuations of the local indium concentration.
In the extensive research dedicated recently to metal- organic chemical vapor deposition (MOCVD)-grown high- efficiency GaN LED device design, a significant effort has been made to increase the conductivity of p-GaN layers, while n-GaN layers received relatively little attention. We demonstrated, both experimentally and theoretically, that the resistivity of n-GaN layers has a profound effect on blue InGaN LED performance. Optimization of n-GaN epitaxial layers allows the achievement of device series resistances below 15 Ohms and forward voltages as low as 2.9 Volts at 20 mA. We have also shown that contactless measurements of sheet resistivity of the entire LED epitaxial structure closely correlate with the ohmic resistance of the GaN layer measured in the fabricated devices. This provides an excellent non-destructive characterization tool for n-GaN optimization. Insufficient n-GaN conductivity is shown to trigger a distinct degradation mechanism by initiating current crowding in a localized device area. InGaN LED lamps with optimized n-GaN layers had a high external quantum efficiency and a good long-term reliability.
AlGaN based interdigital metal-semiconductor-metal (MSM) photodetectors with 14 percent Al have been successfully grown and fabricate don sapphire substrates. The devices exhibit large gains up to 106 at high bias voltages, but with very high dark currents, > 1 mA and very long detector responses, > 60 seconds. A negative temperature coefficient for the breakdown voltage was observed indicating that tunneling is occurring. However, at high bias voltages, avalanche breakdown also appears to be present since a constant breakdown field of 105 V/cm was obtained independent of MSM geometry. Avalanche breakdown is nucleated at the non-uniform field distribution at the edge of the MSM finger.
GaN homojunction and InGaN/GaN single quantum well (SQW) light-emitting diodes (LEDs) were fabricated and characterized. The blue LED has a typical operating voltage of 3.6 V at 20 mS. Temperature dependence of the emission characteristics of the GaN-based LEDs was studied from 25 degrees C to 130 degrees C. The emission intensity of the InGaN/GaN SQW LED decays exponentially with the increase of temperature. The temperature coefficient Lc is 2.5 X 10-2/degrees C. The emission wavelength of the InGaN/GaN SQW LED was found to be relatively independent of the LED operation temperature while the UV emission of the GaN homojunction LED has a red-shift with the increase of temperature. The temperature coefficient (alpha) of the bandgap energy of Si-doped n-type GaN derived from the EL measurement is 8.5 X 10-4/K. The low temperature coefficient of emission wavelength of the InGaN/GaN SQW LED indicates that the recombination processes involves localized states. The localized states are attributed to excitons localized at the potential minima in the quantum well due to In content fluctuation.
Interdigital metal-semiconductor-metal (MSM) and p-n UV photodetectors have been successfully grown and fabricated from GaN based materials. The MSM devices were produced using two types of GaN; high-resistive GaN and Mg doped GaN. For the high-resistive GaN detector, the lowest dark current is approximately 0.1 nA and the UV responsivity of the device was about 460 A/W at a DC bias of 30 V. The Mg doped GaN exhibited larger gains, 1150 A/W at 2.0 V, but at much higher dark currents, 400 nA. The high gain in this device is not well understood but has attributed to an 'avalanche' effect and is under further investigation. The feasibility of a photovoltage detector structure based on alloys of GaN has also been proven. A GaN/GaInN structure exhibited a cut- off at 2.9 eV with a responsivity of 0.28 A/W at zero bias for an active region of only 500 angstrom thick.
The competition between bandgap and the 2.2 eV yellow luminescence of epitaxial GaN is studied for excitation densities ranging from 5 X 10-6 W/cm-2 to 50 W/cm-2. The ratio of the peak intensities of the bandgap-to-yellow luminescence changes from 4 to 1 to 3000 to 1 as the excitation density is increased by seven orders of magnitude. At room temperature, the bandgap luminescence linewidth is 2.3kT, close to the theoretical minimum of 1.8kT. A model is developed describing the intensity of the two radiative transitions as a function of the excitation density. The theoretically predicted dependences of the two different luminescence channels follow power laws with exponents of 1/2, 1 and 3/2. The theoretical dependences are in excellent agreement with experimental results. It is shown that the intensity of the yellow luminescence line is negligibly small at typical injection currents of light- emitting diodes.
The bulk of LEDs sold today are still fabricated using older epitaxial techniques such as LPE and VPE, but have relatively low brightness and a limited color range. The newer high brightness LEDs are fabricated from the InGaAlP and III-Nitride systems, with MOCVD being the preferred growth technique for manufacturing. While these new materials represent a significant increase in performance, they are also more expensive to grow. In this paper we consider the reasons for this, which include a less mature growth technology, lower production volumes, expensive starting materials, process efficiency, equipment throughput and cost, and safety and environmental concerns. Addressing each of these issues in turn, we examine what has already been accomplished, and what may be improved by further advances in equipment and process. A realistic COO model is of great utility in comparing product cost for different device structures, staffing schemes, reactor sizes, etc. We demonstrate that for a dedicated LED manufacturing facility, the lowest epitaxial cost is achieved by running around the clock with the highest throughput reactor that is fully utilized for the desired production level. When maintenance tasks such as cleaning and test or calibration runs are minimized, then materials costs will dominate the epi cost, which leads to the desirability of achieving both the best reproducibility and increasing the process efficiency. We show how in-situ control techniques are now capable of increasing preproducibility and thereby lowering product costs for manufacturing scale MOCVD reactors.
Keisuke Kojima, Marlin Focht, Joseph Freund, J. Geary, Kenneth Glogovsky, Gregory Guth, Robert Karlicek, Lars Luther, George Przybylek, C. Reynolds, D. Romero, L. Smith, Daniel Stampone, J. Stayt, Venkat Swaminathan, Frank Walters, Kevin Campbell, J. Grenko, Jean Flamand, Michael Palin
In order to meet the increasing market needs for uncooled lasers for such applications as fiber- in-the-loop, high efficiency, high power, and highly reliable 1.3 micrometer uncooled InGaAsP/InP strained multi-quantum well Fabry-Perot lasers were fabricated with 50 mm wafer processing. Slope efficiency as high as 0.39 W/A and peak power as high as 46 mW at 85 degrees Celsius was obtained by optimizing the device structure for high temperature operation. We have also demonstrated excellent uniformity and reproducibility over 6 wafers. Reliability was also shown to be very good. More than 10,000 chips sites are available on a 50 mm wafer, and the cost is expected to be low. Because of the high performance, these lasers are expected to be used for various applications.
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