GaN Strains to Integrate with Silicon
With applications from automotive instrument panels to efficient room lighting, high-brightness LEDs have become an important manufacturing segment over the last few years. Strategies Unlimited, a market research firm, estimated HB-LED sales of $10.8 billion in 2010, nearly double the 2009 figure. Thirteen of the 17 high volume fabs that SEMI expects to begin construction this year are LED fabs.
General lighting is an extremely cost-sensitive sector. As Rainier Beccard, VP of marketing at Aixtron (Aachen, Germany), explained, “MOCVD reactor capacity remains the key parameter with the strongest influence on [fab] operating cost.”
As in the integrated circuit industry, increasing either the wafer size or the number of wafers processed per reactor cycle can dramatically reduce the cost per device. To that end, Aixtron’s newly introduced CRIUS II-L MOCVD (photo, left) reactor can accommodate 16 4-inch or 69 2-inch substrates, which the company claims is the largest capacity available in a manufacturing-proven reactor. Moreover, the company said, the reactor design is optimized for wafers up to 8-inches in diameter, offering even more potential for productivity improvement.
Actually producing high-quality 8-inch substrates is another matter, though. The GaN material used to produce blue and green LEDs has a substantial lattice mismatch with silicon (table, below). It is usually grown on sapphire or silicon carbide substrates, but even those materials introduce lattice mismatch defects that ultimately limit the performance and efficiency of HB-LEDs. While combining GaN-based LEDs and silicon-based integrated circuits into a single device could cut packaging costs and reduce the total system footprint, such structures are not easy to realize in practice.
Table: Lattice mismatch and GaN
Substrate Lattice constant (Å) Mismatch vs. GaN
GaN 3.19 —
GaAs 5.65 77%
Al2O3 2.75 (sublattice) -14%
SiC 3.09 -3%
Si 5.43 70%
Manufacturers have several options for managing the large mismatch strains between GaN and silicon. One approach avoids the question entirely: the Soitec (Grenoble, France) SmartCut process, used to produce most commercial silicon-on-insulator (SOI) wafers, can also transfer an epitaxial GaN layer to a silicon substrate. This method could be used for 3-D integration, stacking an HB-LED array on top of a complete silicon integrated circuit. Unfortunately, the cost of the SmartCut process has limited acceptance of SOI wafers, even for high-value integrated circuits. Cost-sensitive commodity lighting suppliers are likely to be even more wary.
Alternatively, a series of buffer layers, each with a slightly different composition and lattice constant, can produce a gradual transition from the silicon to the GaN structure. This method is used for GaN deposition on sapphire or silicon carbide, but clearly a much thicker buffer structure is needed to manage the much larger mismatch between GaN and silicon.
Furthermore, according to Lattice Power (Jiangxi, China), the thermal expansion mismatch between the GaN buffer layers and silicon can still cause strain and cracking during deposition. The company’s design uses a pre-patterned substrate with trenches to keep cracks in one area of the wafer from propagating to the next. Though the silicon wafers used are still only about two inches across, Lattice Power claims to have achieved performance and yield comparable to sapphire-substrate LEDs.
While LEDs are discrete devices, GaN’s high breakdown voltage and good thermal conductivity are also of interest for advanced power devices. It can provide more efficient switching for wind turbines, electric vehicles, and other emerging energy technologies. These more complex devices would benefit from the silicon integrated circuit industry’s large manufacturing infrastructure.
The GaN-on-Si program at IMEC (Leuven, Belgium) seeks to deposit GaN-based materials on larger silicon wafers. Wafer supplier Siltronic AG (Munich, Germany) recently joined this effort, which already includes device and substrate manufacturers.