The code says you need copper within 18" of the ground and the GEC is generally supposed to be unspliced with very few exceptions.
That means the wire to a rod will be copper.
The easiest solution is to use a PVC raceway and terminate the whole thing below grade. Not many thieves are going to willing to dig and rip out a raceway for a half buck's worth of copper. If your area is really that hard core, run it in RMC to a city hub below grade. In that situation the copper is really redundant anyway. The fault current will go down the pipe.
