These new wafers can replace classical indium phosphide in a variety of applications and offer a scalable way to lower costs. The research team developed a process to deposit a thin layer of high quality InP on GaAs. After a special surface treatment, these wafers are widened for epitaxy so that customers can directly grow III-V structures and manufacture semiconductor devices based on InP.
InP-on-GaAs substrates more cost-effective than InP
"Companies can use our new InP-on-GaAs substrates to produce highly efficient devices," says Carmine Pellegrino, project manager at the Fraunhofer Institute for Solar Energy Systems (ISE). "They are significantly cheaper than InP and can be scaled to wafers with a diameter of up to 8ʺ without any restrictions."
Depositing indium phosphide on gallium arsenide is challenging because defects occur during the growth of the indium phosphide, which can impair the performance of the finished semiconductor device. The scientists were able to avoid this by incorporating a series of "metamorphic buffer layers" and subjecting the fully grown InP-on-GaAs wafer to a chemical-mechanical polishing step. Afterwards, the wafers are shiny, have a very low surface roughness and a defect density below 5×106 cm-2.
New wafer material shows the same performance
The researchers tested the material quality and performance of the new InP-on-GaAs wafers and compared them with standard InP substrates. "The results are very promising," says Frank Dimroth, Head of Department III-V Photovoltaics at Fraunhofer ISE. "Photovoltaic cells produced on our wafers achieve open-circuit voltages that are comparable to reference devices on established InP wafers. The performance is uniform across the entire 6ʺ wafer, which enables reliable production with high yields."
As part of a series of experiments, the research team has so far produced InP-on-GaAs wafers with diameters of 4 and 6ʺ, with no obstacles expected for a future transition to 8ʺ. Classic InP substrates, on the other hand, are currently available in sizes from 2 to 4ʺ, while a 6ʺ version has only recently become available. This is due to the fact that gallium arsenide substrates are more robust and formats with a diameter of up to 8ʺ are already well established in the semiconductor industry. The higher stability of gallium arsenide also enables the production of thinner wafers, which means that less material is used and additional costs can be saved.
"Our technology benefits from the fact that gallium arsenide serves as the basis," adds Pellegrino. "The production costs of the new substrates are significantly lower than those of classic indium phosphide wafers. According to initial calculations, the savings potential in mass production is up to 80 %. Our approach also allows us to avoid supply bottlenecks for indium phosphide."