Insulating concrete forms are an excellent choice for foundations for passive solar, net zero energy homes. They create a highly insulated, air-tight foundation, so essential for extremely high energy performance.
Be sure to install under-the-footing conduit to run electrical and water pipes, including sewer. I like to run pipes and wire under the foot to prevent penetrating the band joist or the foundation to create a more airtight, water tight home.
Insulation under the slab -- and lots of it -- is vital for the performance of a net zero energy home. So is the footprint. You can make the most of passive solar by creating a longer, narrower house in which each room is heated by the sun.