Working with local wafer supplier Okmetic and ITME in Poland, the researchers compared the characteristics of gallium nitride (GaN) layers grown on SOI wafers to those grown on silicon substrates more commonly used for the process. The objective of the SOI technology, where a layer of silicon dioxide insulator is sandwiched between two silicon layers, is to improve the capacitive and insulating characteristics of the wafer.
"We used a standardised manufacturing process for comparing the wafer characteristics," said Jori Lemettinen, a doctoral candidate from the Department of Electronics and Nanoengineering. at Aalto. "GaN growth on SOI wafers produced a higher crystalline quality layer than on silicon wafers. In addition, the insulating layer in the SOI wafer improves breakdown characteristics, enabling the use of clearly higher voltages in power electronics. Similarly, in high frequency applications, the losses and crosstalk can be reduced."
Growth of GaN on a silicon substrate is challenging. GaN layers and devices can be grown on substrate material using metalorganic vapor phase epitaxy (MOVPE). When using silicon as a substrate the grown compound semiconductor materials have different coefficients of thermal expansion and lattice constants than a silicon wafer. These differences in their characteristics limit the crystalline quality that can be achieved and the maximum possible thickness of the produced layer.
"The research showed that the layered structure of an SOI wafer can act as a compliant substrate during gallium nitride layer growth and thus reduce defects and strain in the grown layers," said Lemettinen.