At the LpS Symposium in Bregenz, the topic of LED wafers based on silicon instead of today's widely used sapphire as the substrate material was discussed as a part of a presentation block headlined "Disruptive Technologies". Nevertheless, more or less every LED manufacturer (save Cree, who utilizes SiC materials) is preparing a switchover to silicon. The reasons are simple: Silicon wafers offer vast cost advantages over sapphire - manufacturers could then utilize mature, efficient, widely automated processes with high yield. "Silicon is cheaper than sapphire and will likely remain so", explained Pars Mukish, analyst with Yole Développement. Moreover, the processing infrastructure for silicon wafers anticipates and supports larger diameters - in silicon, manufacturers could work with 200mm wafers, while sapphire wafers do not exceed 150mm; in most cases the industry still works with 4" or even 2" wafers - and an inherently lower productivity. What's more, silicon offers better thermal conductivity and thus better temperature homogeneity whit the possible consequence of an improved binning and better yield.
All these divergent properties eventually would translate into much better price for silicon-based LEDs. Mukish expects an improvement at a factor of ten over 2" sapphire - and the processing costs make up some 55 percent of the total cost of a packaged LED. Being able to significantly reducing the processing cost thus would be something like LED manufacturer's heaven. But before the industry can go up there, they have to resolve a number of technical challenges. The list of problems connected to the use of silicon as substrate material for LEDs is long. In order to discuss these problems, it is important to remember that in both cases - sapphire substrate as well as silicon substrate - the LED functions are implemented on top of the substrate - the epitaxial LED functions are grown as a GaN stack on top of the substrate. Silicon and GaN however have rather different thermal coefficients