We report on violet-emitting III-nitride light-emitting diodes (LEDs) grown on bulk GaN substrates employing a flip-chip architecture. Device performance is optimized for operation at high current density and high temperature, by specific design consideration for the epitaxial layers, extraction efficiency, and electrical injection. The power conversion efficiency reaches a peak value of 84% at 85 °C and remains high at high current density, owing to low current-induced droop and low series resistance.

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Electrically conductive materials capable of bending and stretching are useful for a wide range of applications such as smart clothing, [1] flexible displays, [2] stretchable circuits, [3] strain gauges, [4] implants , [5] high-deformation microelectromechanical systems (MEMS), [4] and dielectric elastomer actuators (DEAs), [6] to name a few. The necessary strain varies from minimal (< 5 %) for applications such as retinal prostheses that only require conformal contact to a surface [5] to high (> 60 %) for large-strain DEAs.[7]A number of approaches have previously been used to produce stretchable electrodes. [8] Printing with carbon grease [6,9,10] or graphite powder [11,12] has been the preferred method for use with DEAs because of the low modulus of these materials, which allows large strains, but they have a relatively low conductivity and can readily be rubbed off the surface. Thin metal films deposited on elastomeric substrates, [13] such as by thermal evaporation or sputtering, can be stretched 10-20 % before failure, [14] or higher if the metal is patterned. [15] In special cases when it is possible to stretch the elastomer during deposition, so that the metal forms corrugations upon polymer relaxation, the achievable strains can be increased even further. [14,[16][17][18][19] Loading an elastomeric host polymer with a conducting material allows some degree of elasticity, but the modulus of the composite increases dramatically with loading, often before the electrical percolation threshold is reached. [20,21] Electrostatic layer-by-layer assembly has been successful in achieving highly conducting, robust, free-standing sheets capable of large strain. [22] Integrating these electrodes with other devices, however, which would include patterning the electrodes, has not yet been demonstrated. Another promising method for producing compliant electrodes is metal ion implantation, [23] which has been used in the fabrication of miniature DEAs. The advantages include patternability and an expectation of a low modulus, while the drawbacks are a rather high sheet resistance (∼ 100 kX/ square), a limited substrate size, a requirement for a low-energy ion implanter, and post-implantation bonding of the elastomer to the Si wafer. The goal of the present work was therefore to develop a new approach to achieving robust and truly elastomeric conductors that can be patterned with standard microfabrication techniques. This communication presents a method for making compliant electrodes that results in materials with a sheet resistance (R s is related to electrical conductivity r through r = 1/R s t, where t is the film thickness, so this corresponds to a conductivity of 50 S cm -1 for our 0.01 cm thick films. R s of only 2 X/& when unstretched that do not lose conductivity until they are elongated by up to 150 %. The electrodes have a Young's modulus of under 10 MPa, and they survive thousands of strain cycles. The electrodes are also photopatternable ( Fig. 1), making them compatible with microfabrication of electronic dev...