Highly efficient InGaN/GaN LEDs grown on 4- and 8-inch silicon substrates comparable to those on sapphire
substrates have been successfully demonstrated. High crystalline quality of n-GaN templates on Si were obtained by
optimizing combination of stress compensation layers and dislocation reduction layers. The full-width at half-maximum
(FWHM) values of GaN (0002) and (10-12) ω-rocking curves of n-GaN templates on 4-inch Si substrates were 205 and
290 arcsec and those on 8-inch Si substrate were 220 and 320 arcsec, respectively. The dislocation densities were
measured about 2~3×108/cm2 by atomic force microscopy (AFM) after in-situ SiH4 and NH3 treatment. Under the unencapsulated
measurement condition of vertical InGaN/GaN LED grown on 4-inch Si substrate, the overall output power
of the 1.4×1.4 mm2 chips representing a median performance exceeded 504 mW with the forward voltage of 3.2 V at the
driving current of 350 mA. These are the best values among the reported values of blue LEDs grown on Si substrates.
The measured internal quantum efficiency was 90 % at injection current of 350 mA. The efficiency droops of vertical
LED chips on Si between the maximum efficiency and the efficiency measured at 1A (56.69 A/cm2) input current was
5%.
Experimental results on a new type of light-emitting device, the light-emitting triode (LET), are presented. The LET is a three-terminal p-n junction device that accelerates carriers in the lateral direction, i.e. parallel to the p-n junction plane, by means of an electric field between two anodes. The lateral field provides additional energy to carriers thereby allowing them to overcome barriers and increasing the carrier injection efficiency into the active region. LETs were fabricated using a ultraviolet LED structure that has an AlGaN/GaN superlattice in the p-type confinement region for high-conductivity 2 dimensional hole gas. LET mesa structures were obtained by standard photolithographic patterning followed by chemically-assisted ion-beam etching using Cl2 and Ar to expose the n-type cladding layer. The n-type contact was fabricated by electron-beam evaporation of Ti/Al/Ni/Au. Ni/Au (50/50 Å) metallization was deposited for both anodes, Anode 1 and Anode 2, and subsequently annealed at 500 oC in an O2 ambient. It is shown that both the current between Anode 1 and the cathode, and the light-output power increase with increasing negative bias to the Anode 2. This is consistent with the expectation that a negative bias to the second anode allows carriers to acquire a high kinetic energy thereby enabling them to overcome the barrier for holes, resulting in high injection efficiency into the active region that lies beyond the barrier.
Enhancement of light extraction in GaN light-emitting diodes (LEDs) employing omnidirectional reflectors (ODRs) is presented. The ODR consists of GaN, ITO nanorod low-refractive-index layer, and an Ag layer. An array of ITO nanorods is deposited by oblique-angle deposition using e-beam evaporation. The refractive index of the ITO nanorods is 1.34 at 461 nm, significantly lower that that of dense ITO, which is n = 2.06 at 461 nm. It is experimentally shown that the GaN LED with GaN/ITO nanorods/Ag ODR show much better electrical properties and higher light-extraction efficiency than LEDs with Ag contact. This is attributed to enhanced reflectivity of the ODR by using an ITO low-refractive-index layer with high transparency, high conductivity, and low refractive index.
We observed a significant enhancement in light output from GaN-based light-emitting diodes (LEDs) in which two-dimensional photonic crystal (PC) patterns were integrated. We approached two types PC LEDs. One is top loaded PC LEDs. The PC patterns were generated on the top p-GaN layer. The other is bottom loaded PC LEDs. In this LEDs, PC patterns were integrated on the sapphire substrate. Two dimensional square-lattice air-hole array patterns, whose period was varied between 300 and 700nm, were generated by laser holography. Unlike the commonly utilized electron-beam lithographic technique, the holographic method can make patterns over a large area with high throughput. The resultant PC-LED devices with a pattern period of ~500nm had more than double the output power. The experimental observations are qualitatively consistent with three-dimensional finite-difference-time-domain simulation results.
The efficiency of a conventional light emitting diode (LED) is limited by coupling of light into guided modes in the structure. Several methods to increase the extraction efficiency of nitride based LEDs are studied from the perspective of the patterned structures in LEDs. The patterned structures are made in the interface between a semiconductor and a sapphire substrate and on the surface of a semiconductor or an indium tin oxide electrode. All of these approaches show an increased light output compared to that of reference samples, which means these kinds of scattering sources are inevitable to make a highly efficient light emitter in nitride-based semiconductor system.
Conference Committee Involvement (5)
Gallium Nitride Materials and Devices VI
24 January 2011 | San Francisco, California, United States
Gallium Nitride Materials and Devices V
25 January 2010 | San Francisco, California, United States
Gallium Nitride Materials and Devices IV
26 January 2009 | San Jose, California, United States
Fourth International Conference on Solid State Lighting
3 August 2004 | Denver, Colorado, United States
Third International Conference on Solid State Lighting
5 August 2003 | San Diego, California, United States
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