European Union tackles wide-bandgap material challenges
A key part of the IST program is research into wide-bandgap materials. One such project called DENIS was set up in April of this year, and aims to develop low dislocation density gallium nitride substrates. Headed by Acreo of Sweden, the research team also comprises Osram Opto Semiconductors of Germany, Okmetic Oy of Finland and the Polish Academy of Sciences High Pressure Research Center, as well as the University of Bremen in Germany and Linköping University in Sweden.
Focusing on GaN
By using hydride chemical-vapor deposition (HCVD), the DENIS team aims to fabricate 2 inch GaN wafers with initial dislocation densities that are less than 1 x 107/cm2. Their final goal is to reduce these defect densities to less than 1 x 106/cm2. "We have been growing GaN with HCVD for a few years and at present we can grow thick layers of GaN on 2 inch sapphire or SiC substrates," said Bo Monemar of Linköping University. "Our first step is to produce free-standing 2 inch GaN of decent quality, and we are using different techniques, including laser lift-off, to remove the substrate."
The team will also attempt to grow 2 inch GaN boules later on in the project, something Monemar says is the biggest challenge he and his colleagues will face. They will also carry out device trials to evaluate material quality throughout the project. "We aim to demonstrate a dramatically improved [device] performance that will motivate a rapid uptake of nitride technology in Europe," Monemar added.
Another project called EURONIM (European sources of nitride materials) is also planning to do just this. Researchers from Thales of France, UK-based IQE, Osram Opto Semiconductors and various other European research institutes have teamed up to create a European industrial source of nitride epitaxial wafers for the electronics and optoelectronics industry.
The group intends to develop epi-ready SiC wafers, SiC wafers for the regrowth of active structures, GaN-on-sapphire and high-quality GaN wafers. Devices such as HEMTs and laser diodes will also be fabricated to test the material quality.
Preparations for an increased uptake of GaN-based materials will be good news for a research consortium working on a project called ISCE-MOCVD. Researchers from Aixtron of Germany, Thales, IQE and Philips Analytical of the Netherlands, as well as scientists from several European universities, have joined forces to develop advanced in situ monitoring and control of MOCVD production. As well as looking at GaN-based materials, the research team is also focusing on ternary and quaternary (Al, Ga, In)/(As, P, Sb) semiconductors.
"MOCVD [already] offers excellent control [for materials growth], especially compared with MBE," said Rainer Beccard, head of sales and marketing at Aixtron. "But the target of this project is to develop more advanced monitoring tools that offer information which has not been available before."
The team is designing a new reactor and plans to combine real-time monitoring techniques, including ellipsometry, reflectance anisotropy spectroscopy and X-ray diffraction, with closed-loop feedback control of epitaxial growth. Beccard says that the team expects their work to drive down development costs and increase process yields.
"The classical research and development process uses iteration loops, for example MOCVD growth followed by ex situ characterization and then a change of parameters, which is very time consuming," Beccard explained. "in situ control means that small process deviations will be detected during growth and easily compensated for. The impact of different substrate qualities [on epitaxial growth] will also be drastically reduced."
Adding that the team s work will be applicable across all material systems, Beccard expects that in situ technologies in MOCVD will become widely used across the III-V industry. "Today, MOCVD development focuses on highly sophisticated structures, novel and complex material systems, and the improvement of productivity; our work will address these issues," he said. "For the production of GaN materials, in situ monitoring [for MOCVD or HVPE] will increase production yield and quality, and hopefully reduce the price."
