LED with undercut sidewalls.doc

AbstractWe used a simple and controllable method by ICP-RIE successfully fabricated with undercut sidewalls. Compaired with the conventional, we found that the output power of the with undercut sidewalls was 34.8mW under a 20-mA current injection. Under the same conditions the output power of conventional was 32.7mW. Obviously, the output intensity of thewith undercut sidewalls was 6.75% higher.Index Terms- , light-emitting diode (), undercut Galium Nitride is a direct wide bandgap semiconductor and recently, they have become promising materials for optoelectronic devices. light-emitting diodes are being used for many kinds of applications. For example, these have been used in full color displays, traffic light lamps, solid-state light source, back light for large screen displays, and so on. Although the light output of has been improved a little, it is still lower than what we expect. As we all known, the light extraction efficiency of is mainly limited to the refractive index between the material and the surrounding air. The refractive index of and the air is 2.5. So the critical angle for photons generated in the active region escape from film is about.Because of the light generated in the active region is emitted isotropically, the most will be refracted or absorbed by the materials. At last, the external quantum efficiency of conventional is often low. The brightness of can be improved either through the sample surface or through the sidewalls of the power chip. Both of the above is in order to make photons generated in the experience multiple opportunities to find the escape cone. It has been shown that one could texture the surface so that photons can escape easily and thus enhance LED output intensity.It is also possible to improve the brightness of the by fabricating oblique sidewalls. In this letter, with undercut sidewalls were fabricated. By using an controllable inductively coupled plasma reactive ion etching(ICP-RIE), we formed undercut sidewalls. The follow is the introduction of the experimental results of the with undercut sidewalls. And the structural and electrooptical properties of the fabricated devices will also be discussed in detail. Through out the experiment, we used the ray tracing method to simulate the light propagation and reflection. Thethat we used can be described as follows: the top mesa width is 300 and the depth is 2.5with vertical sidewall, and undercut sidewall. The examined samples used in this study were all grown on c-face(0001) sapphire substrates using a metal-organic chemical vapor deposition(MOCVD).The structure consist a 50nm-thick buffer layer, a 30nm-thick nucleation layer, a 4-thick Si-doped layer. The MQW consists of five pair of 3-nm-thick well layers and 7-nm-thick barrier layers. The as-grown wafers were then rapidly thermal annealed at 750 for activation of p-type dopant. Fig.1(a and b) depicts a schematic diagram of the device with undercut sidewalls and the conventional . TCL TCL (a) conventional LED (b) LED with undercut sidewall Figure.1 Schematic diagram of the LEDs A layer was than deposited on the sample surface by plasma-enhanced chemical vapor deposition. Photolithography was subsequently used to define the mesa pattern. The mesa etching was then carried out by an inductively coupled plasma reactive ion etching(ICP-RIE) system using as the etching gas. After wet etching , we removed photoresist by buffer oxide etching solution(BOE). The additional etching to form undercut sidewalls was performed after mesa etching with zero bias power. The detailed study about the etching sidewalls had been already submitted to publish. The etching depth and profile of samples were measured by the DektekII stylus profilometry measurements and. The conventional chips with the same device area() were also fabricated using the same wafer for comparison studies. At last, the metal contact layers that contained transparent contact layer and pad layer were patterned by a lift-offprocedure. We then deposited (3/5nm) and (30/50/30/150 nm) metal layers as p-type contact and n-type contact layers by using electron beam evaporation. For comparison, we used an ICP dry etcher to partially etch the conventional with vertical sidewalls until the n-type layer was exposed. And the same metal layers were deposited onto the transparent contact layer and n-type contact layer in order to get the conventional . Finally, (20/1500 nm) was deposited onto the exposed transparent and n-type contact layers to serve as bonding pads. Fig.2 EL spectra of the fabricated LEDsFig.3 L-I-V characteristics of the fabricated LEDs Current-voltage(I-V) characteristics of the both (undercut and conventional) were measured at room temperature. The forward Voltages of these two were both approximately 3.24V at a driving current of 20-mA. Such an observation indicates that the undercut sidewalls will not degrade the electrical properties of the . Electroluminescence(EL) spectra of the were also measured. Fig.2 shows measured EL spectra of the . At injection current of 20-mA, it was found that EL peak positions of the two both occurred at about 456nm with the same full-width at half-maximum of 20.75nm. From the picture, we could see that EL intensity of the undercut is larger than the conventional Intensity-current (L-I) characteristics were measured with an integrated sphere detector. Fig.3 shows measured characteristics of the two. Under 20-mA current injection, the output powers were 32.6mW and 38.4mW for the with undercut sidewalls and with vertical sidewalls respectively. In other words, we could achieve 6.75% enhancement from theMQW by the introduction of the undercut sidewalls. Compaired with conventional , the larger output power measured from the undercut LED could be attributed to the enhancement of light extraction efficiency. In summary, thewith undercut sidewalls that could enhance the output power have been fabricated. And we could use ICP-RIE with a simple method and controllable procedure to get the undercut sidewalls. Through the whole experiment, we found such undercut sidewalls can raise multiple opportunities of escaping the photons outside from the . We also found that using this simple method to form oblique sidewalls will not degrade the electrical properties of the devices.REFERENCES1T. Mukai and S. Nakamura, “Ultraviolet InGaN and GaN single-quantum-well-structure light-emitting diodes grown on epitaxially laterally overgrown GaN substrates,” Jpn. J. Appl. Phys., vol. 38, pp.57355739, 1999.2 E.S. Fred, Light-EmittingDiodes. Cambridge,U.K.: CambridgeUniv.Press, 2003.3 C. Huh, K. S. Lee, E. J. Kang, and S. J. 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