Bulk GaN flip-chip violet light-emitting diodes with optimized efficiency for high-power operation

Abstract

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.

A design of a PhC-enhanced LED for electroluminescence cooling

“…The thermoelectrical pumping effects in LEDs of various wavelength were also observed and reported by experimentalists. [10][11][12][13] The first observation of η W P E > 1 is reported in 2012 when Santhanam et al used lock-in measurement tools to demonstrate an infrared GaInAsSb/GaSb LED with η W P E over 200% at 135 • C. 1 In 2013, higher-than-unity η W P E was achieved in mid-infrared LEDs at room temperature. 14 All these ultra-efficient LEDs operated at extremely low bias (qV kT ), and their output power densities were merely several hundreds nW/cm 2 , which is not enough for direct observation of temperature drop.…”

“…13 The packing processes are also mature and widely applied in commercial LEDs. However, as we will discuss in 2, due to the large surface area and thermal mass of an encapsulated LED, we need eliminate the package to maximize the ELC temperature drop.…”

“…20 Moreover, people also apply the surface texturing technology for volumetric LEDs with the dies usually shaped into special geometries and encapsulated, with the EQE exceeding 90%. 13,22 Another method to extract photons in LEDs is to etch photonic crystal (PhC) structure onto the emission surface. (Fig.…”

Irreversible Thermodynamic Bound for the Efficiency of Light-Emitting Diodes

“…For instance, a recent report demonstrates WPE exceeding 80% for a gallium-nitride-based LED [1], which inevitably leads to questions about further enhancement towards the conventional limit of unity efficiency. Nevertheless, theory and experiments suggested that for light-producing devices such as LEDs, the WPE may exceed unity in practice [2][3][4][5][6][7][8][9][10][11][12][13] since the incoherent electroluminescent radiation emitted has finite entropy.…”

Elimination of resistive losses in large-area LEDs by new diffusion-driven devices

“…[3][4][5] This limits strongly the LED efficiency at current densities above 100 A/cm 2 and therefore prevents LEDs from being used efficiently in applications requiring very high output powers. Very recently, Hurni et al demonstrated how current crowding can be reduced by using a thick n-type current-spreading layer, 6 but this still requires expensive bulk GaN substrates as well as large n-type contacts.…”