If GF should be partnering with POET and Mubadala has decided at "first" instance to invest in Dresden/Germany, there could be one more reason why!

http://www.freiberger.com/en/products/gaas-wafers.html

FCM is a company producing top high quality GaAs Chips near to GF in Dresden (40min):

We manufacture semi-insulating and semiconducting gallium arsenide substrates with diameters from 3” to 200 mm for high-frequency and optoelectronic applications. Freiberger has established a worldwide reputation as supplier of products that meet highest quality standards. In our elaborate analysis and measuring processes all products undergo a comprehensive characterization enabling us to meet the customer requirements.

Our wafers are manufactured from LEC or VGF grown GaAs single crystals GaAs. The required electrical properties are obtained by adding dopants such as carbon, silicon, tellurium or zinc. The result is n-type or p-type high-resistance (>107 Ωcm) or low-resistance (<10-2 Ωcm) semiconductors. The wafer surfaces are generally epi-ready (extremely low contamination) i.e. their quality is suitable for direct use in epitaxy processes.