Scalability of buried microreflector light-emitting diodes for high-current applications

Abstract

The combination of wafer soldering using metal layers and the introduction of buried micro-reflector structures has proven to be a promising approach to fabricate high brightness, substrate-less LEDs in the AlGaInP material system. In addition to the enhanced light output, the scalability of this approach has been predicted as a major advantage. In contrast to other approaches, larger area LEDs can be fabricated without altering the epitaxial structure and thickness of layers simply by offering a larger area for light generation. First samples of amber (λ = 615 nm) buried micro-reflector LEDs with side-length up to 1000 µm have been realized. Devices mounted in packages with improved heat sinks are capable of low voltage CW operation with currents as high as 600 mA (V fw ≤ 2,8 V) without significant thermal flattening of the light-current characteristics. The maximum luminous flux achieved at these operating conditions is 46 lumen. Already these first experiments demonstrate the potential of the concept of buried micro-reflector LEDs not only for high-brightness but also for high-current operation. The results are among the best values of high-flux LEDs in this wavelength range.

High-efficiency AlGaInP light-emitting diodes for solid-state lighting applications

“…Illek et al 39 demonstrated a TF AlGaInP LED with an array of buried coneshaped microreflectors. The LED structure is shown schematically in Fig.…”

“…In contrast to TS-LEDs, very thick window layers in order to increase sidelight extraction are not required. 37,39 For devices emitting at 650 nm, quantum efficiencies of 31% 40 and 40% 41 have been reported. Despite the presence of etched regions, the forward voltage is below 2.0 V at a forward current I f ϭ10 mA.…”

“…Despite the presence of etched regions, the forward voltage is below 2.0 V at a forward current I f ϭ10 mA. 37,39 There are disadvantages to the TF-LED structure, namely the fact that the ratio of current-injected area to chip area is small. Analysis of the schematic of Fig.…”

“…During this step, during which the wafers are kept at low temperatures ͑350°C͒ to avoid dopant redistribution, a stable metal-to-metal bond is formed. 37,39 This bond is able to sustain all further LED processing steps including the subsequent removal of the original substrate by wet-chemical etching.…”

“…A thin dielectric layer inserted between the semiconductor and the bonding metal helps to avoid thermal degradation of the metal; current flow is enabled through defined openings in the dielectric. 37,39 A distinct advantage of TF technology is the possibility to structure the AlGaInP surface before the bonding process. This results in a significant increase of the extraction efficiency since the geometrical shapes such as cones or spheres act as microreflectors ''buried'' at the bonding interface between the semiconductor and the carrier.…”