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Sapphire-free Vertical Design Boosts LED Performance

Poor current handling and thermal management are suppressing the performance of LEDs for solid-state lighting applications. These problems can be avoided, however, by switching to a low-cost vertical design and a metal alloy substrate, says Trung Doan from SemiLEDs Corporation.

GaN LEDs are widely used in handset keypads, backlighting units, camera flashes and full-color outdoor displays, but their output is, as yet, insufficient for significant penetration into the solid-state lighting market. This is primarily because the LEDs have relatively poor thermal management characteristics and cannot operate at the high injection currents required for super-bright emission. However, these issues can be overcome by producing GaN LEDs on electrically and thermally conducting substrates, and this is an approach that we have pioneered at SemiLEDs. Our vertical LEDs on metal substrates (VLEDMS), which are built using low-cost mass-production processes, use a novel vertical design and feature a metal alloy substrate. These emitters deliver many benefits over conventional and flip-chip LED designs (see figure 1) and can deliver 75 lm/W at 350 mA drive current, which is among the highest output efficacies achieved to date.

Sapphire s drawbacks

The issues affecting conventional GaN LEDs stem from the poor thermal and electrical properties of the sapphire substrates that they are grown on. Sapphire has a thermal conductivity of only 35 W/mK (see figure 2), which restricts the LEDs operating current. The material is also an insulator and so the n-contact cannot be attached to the back of the substrate, but has to be formed on top of the n-type layer. This means that the active material has to be removed from the chip, which decreases the emission intensity by 20–30%. Having both contacts on the top side of the LED also results in current transport through the n-GaN layer, which produces current crowding and a higher dynamic resistance that increases the device s temperature.

Conventional GaN LEDs also suffer from non-uniform light emission due to low current spreading in the p-GaN layer. This can be overcome with either semi-transparent contact layers or interdigitated electrode arrays that spread the current across the device. However, semi-transparent layers also absorb some of the chip emission and can reduce the output power.

These are issues that have caused leading LED manufacturers such as Lumileds to turn to designs that use a flip-chip geometry. However, this approach also requires material from p-GaN and active layers to be removed in order to form the n-type contact, which again reduces the emitting area. Current transport from anode to cathode is still routed along the n-GaN layer, which means that the current crowding and higher dynamic resistance problems remain.

One advantage of flip-chip LEDs is an improvement in the heat dissipation over conventional structures. Flip-chips also produce higher extraction efficiency than the conventional LEDs, partly because of the patterned or textured sapphire surface, but these structures are quite complicated to produce.

SemiLEDs VLEDMS overcome many issues that restrict the performance of conventional and flip-chip LEDs. For example, there is no need to remove any material to form the n-type electrode pad, which boosts emission compared with equivalently-sized GaN-on-sapphire LEDs. Current crowding is avoided because the current passes through the device in a vertical direction, while dynamic resistance is cut significantly.Sapphire-free benefits

Our chip geometry also improves the current spreading in the device. This allows the chips to be scaled to larger sizes without any loss in performance and circumvents the need for semi-transparent conductive layers that reduce the output efficiency.

In addition, our VLEDMS dissipate heat more effectively than conventional and flip-chip LEDs, thanks to the higher thermal conductivity of a copper alloy substrate. This increases their maximum operating current and output power and makes them more suitable for solid-state lighting applications.

The structure of our VLEDMS, which we have manufactured as blue, green and ultraviolet 1 mm2 LED chips, is shown in figure 3. Using our patent-pending epitaxial deposition technology, these LEDs are grown on sapphire along with an additional structure that enables removal of the sapphire. After the LED is formed on the metal alloy substrate, the n-GaN surface is patterned to reduce losses through total internal reflection.

Our VLEDMS have superior current-voltage (I-V) characteristics to conventional LEDs, including a 0.2 V reduction in the forward voltage at 350 mA drive current. These LEDs also have a dynamic resistance of 0.7 Ω, compared with 1.1 Ω for conventional GaN-on-sapphire LEDs, thanks to the switch to a vertical current path and a larger p-GaN contact area. These improvements increase the output efficiency of our VLEDMS over conventional designs.

The increased brightness of our LEDs is particularly significant at higher injection currents (see figure 4). The output from conventional emitters peaks at around 1000 mA and then falls off significantly with increasing current. This is due to poor heat dissipation that leads to device degradation. In contrast, our VLEDMS can handle currents of 3000 mA or more without light output power saturation, thanks to the superior thermal conductivity of metal alloy substrates.Performance independent of size

We have demonstrated the excellent scaling properties of our VLEDMS by manufacturing a range of chips with various dimensions and measuring their output per unit area (see figure 5). While conventional sapphire-based LEDs suffer from a significant drop in efficacy at larger chip sizes, this problem does not appear to impact on the performance of VLEDMS.

Figure 6 shows the results of our reliability tests on 1 mm2 VLEDMS chips, which were packaged using a silicone filling and mounted onto a heat sink. The measurements were made at 350 mA and 700 mA drive currents, and ambient temperatures of up to 65 °C, which led to a range of junction temperatures of up to 120 °C. Our chips, which produce an output that is equivalent to more than 75 lm/W from a white LED, showed only a small decline in light output power over time and this change can be kept below 10% even after a 2000 h burn-in test. At room temperature – the temperature at which the majority of our customers will use these devices – we observed no degradation in light output.

This proven reliability, in conjunction with the excellent heat dissipation characteristics and output efficacies of typically 75 lm/W or more, clearly illustrate the advantages of these devices over conventional LEDs. These LEDs are already being produced in large volumes at high yields and they offer a lumen/$ figure of over 100, which makes these emitters the device of choice for solid-state lighting.Further reading

Z S Luo Y 2002 et al. IEEE Photo. Tech. Lett. 14 1440.
T Fujii 2004 et al. Appl. Phys. Lett. 84 855.
T Doan et al. 2006 Proceedings of SPIE 6134 61340G-1.
C F Chu et al 2006 ISBLLED.

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