The halo comes from anti-aliasing. These are the graduated semi-transparent pixels that are interpreted as solid by applications/engines/formats that do not support alpha. Try rendering without anti-aliasing. For a more refined interior... render twice, once with anti-aliasing, one without. use the non-anti-alias as the edge pixels and the anti-aliased as the interior. Lots `o' composite work in Photoshop to be sure. C'est la vie.