With the help of DOE funding, Philips Lumileds is exploring the use of nitride epitaxy on 150mm silicon substrates to produce low-cost, warm-white, high-performance general-illumination LEDs. Most LEDs are made with C-plane sapphire substrates, but silicon—at roughly half a penny per square millimeter—is much cheaper, and it's also easier to obtain. Philips Lumileds is attempting to adapt the use of silicon to the manufacture of LEDs, drawing upon the knowledge base and depreciated equipment of the computer industry, which has been using silicon substrates for decades.
Sapphire is well-matched with gallium nitride (GaN), the standard epitaxy material used to make LEDs. As a result, cracking is not much of a problem with sapphire. However, silicon is not well-matched with GaN and tends to crack during post-growth wafer cool-down. Philips Lumileds found that growing the GaN compressively strained can remedy that; thin aluminum nitride control layers minimize cracking and maximize the thickness of the epitaxial layers, with good wafer uniformity.
The project shows that there are no major barriers to switching to silicon substrates, although the cost of converting to silicon has to be factored into the equation. Philips Lumileds will have a better idea of the relative cost of that conversion by the project's end.
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