Silicon Carbide Substrate Boosts LED Luminosity

The majority of contemporary LEDs are constructed from a combination of Indium gallium nitride (InGaN) and sapphire substrate. The architecture works well and has allowed LED manufacturers to offer products exhibiting efficacies in excess of 150 lm/W. However, the architecture does have some drawbacks which have encouraged chipmakers to seek other options.

One commercially successful alternative is silicon carbide (SiC), and LEDs based on the substrate have been on the market for two years. Now a new generation of the technology has been released that promises to double the luminosity of the current brightest single LEDs and cut lighting fixture costs by 40 percent.

This article looks at SiC technology and describes the latest chips based on the material to see how they compare with the previous generation and contemporary sapphire-substrate LEDs.

Manufacturing challenges

InGaN is the material of choice for the manufacture of today’s high-brightness white LEDs. The bandgap of the semiconductor is carefully manipulated such that the LED die emits blue photons, the majority of which are absorbed by the LED’s phosphor coating and re-emitted in the yellow part of the spectrum. The mixing of blue and yellow light produces a good approximation of white light.

Unfortunately, unlike the silicon used for most integrated circuits –– which can be cheaply produced leading to low-cost components –– InGaN is difficult to manufacture in large ingots. LED makers overcome this difficulty by using an epitaxial technique such as metalorganic-chemical-vapor deposition (MOCVD). This process overcomes the need to grow bulk InGaN, instead building up the material by depositing successive thin films on a suitable substrate.

The most common material for the substrate is sapphire (Al₂O₃). The mineral is cheap, durable, and a good insulator. Figure 1 shows a cross section of a sapphire-substrate LED.



The biggest drawback of sapphire is that there is a large mismatch between its crystal lattice structure and that of InGaN. Such a mismatch introduces microcracks (called “threading dislocations”) into the LED structure during manufacture which compromise LED efficacy because recombinations between electrons and holes that occur at such sites are primarily “nonradiative.” In other words, no visible photon is emitted. Worse yet, the microcracks multiply with temperature and age, shortening the life of the device (See the TechZone article “Material and Manufacturing Improvements Enhance LED Efficiency”).

That’s not to say that LEDs with sapphire substrates perform badly. Several major manufacturers offer proven products that are based on the material. For example, Philips Lumileds’ LUXEON T white LEDs produce 249 lm (at 2.8 V, 700 mA) with an efficacy of 127 lm/W (Figure 2). Additionally, OSRAM’s OSLON SSL 150 white is capable of producing 136 lm (at 3.1 V, 350 mA) with an efficacy of 125 lm/W.



Extreme-high-power LEDs

SiC was introduced as an alternative for LED manufacture because the lattice mismatch with InGaN is much less than sapphire. Microcracking can still occur, but defect density is dramatically reduced, improving efficacy and extending LED life. The primary disadvantages are relatively high material and manufacturing costs and patented processes that require licensing fees.

Cree –– perhaps not surprisingly considering it owns many of the process patents –– has championed the use of SiC substrates for LED fabrication. Its first commercial SiC devices, the XLamp XB-D family, were released in early 2012. The chips use the company’s “SC³” technology. The XLamp XB-D is a 97 lm/W device (at 350 mA) which can produce up to 213 lm if the current is turned up to 1 A.

The company has recently announced that its next generation of SiC LEDs is now available for sampling. This time round, the “Extreme High Power (XHP)” devices employ technology labeled “SC5.” The company claims that the new devices will enable lighting designers to save up to 40 percent on system costs compared with mid-power LEDs (for the same light output) [See the TechZone article “Mid-Power LEDs Offer Less Expensive Alternative for Lighting Applications”]. These savings are primarily realized because fewer of the brighter LEDs are required for the same light output.

Cree has yet to release the full specification for the XHP devices, but explains that a single device can produce 1800 lm at an efficacy of 112 lm/W and total dissipation of 16.1 W. All LEDs fade over time (once the device’s output reaches less than 70 percent (L₇₀) of that when new it is deemed to have failed [see the TechZone article “Determining LED Rated Life: A Tricky Challenge”]) and the XHP product is no exception. However, the company says that even after 35,000 hours the LEDs will still provide at least 90 percent of their original luminosity. Figure 3 shows an XHP package.



To produce 1800 lm using the company’s mid-range XLamp MX-3S LED –– a 104 lm (at 10.7 V, 115 mA) device with an efficacy of 85 lm/W –– would require an array of eighteen devices dissipating a total of 22.1 W. The company says that a single XHP LED will even produce double the lumens of its current high-power products, such as the XM-L2. According to the data sheet, the XM-L2 puts out 270 lm at 2.85 V, 700 mA.

A greater number of LEDs in a lighting fixture demands a larger, more complex PCB, increased assembly costs, larger heatsinks, and more sophisticated optics –– adding up to a more expensive end product. (See the TechZone article “LED Packaging and Efficacy Advances Boost Lumen Density”.)

Cree explains that the 40 percent savings made when using XHP LEDs to reduce the chip count come from simplifying both the end-product design and assembly process. That’s without taking into account the added time and expense of color matching multiple LEDs in an array to ensure uniformity of output (although manufacturers can supply LED arrays that overcome this challenge) [See the TechZone article “The Rise of Chip-on-Board LED Modules”].

Further development ahead

Sapphire-substrate LEDs have formed the mainstay of the solid-state lighting industry. There are many high-performance chips on the market based on the technology. However, in such a highly competitive industry, manufacturers are always looking for an edge and a new generation of devices based on SiC technology is said to offer higher luminosity than conventional devices –– allowing engineers to employ fewer LEDs for the same output, simplifying and shrinking lighting fixtures while reducing costs.

LED technology is far from mature, however, and while sapphire and SiC are the current leaders, other materials –– including gallium nitride (GaN) itself; and even lowly silicon – are under development (See the TechZone article “Will Silicon Substrates Push LED Lighting Into the Mainstream?”). Some manufacturers are even experimenting with LEDs that dispense with the substrate entirely.

For more information about the parts discussed in this article, use the links provided to access product pages on the DigiKey website.