If I were you, I would schedule a 1/1 meeting with

your superviosr, don't say "I don't have enough work to do..." instead, say something like:" I've been with this group for 6 months now and feel like that I'm much more up to speed and ready to take on additional responsibilities/projects...".

Try to work out a detailed plan with him (picking up slacks from other team members is not a good solution.)