There is no explicit “deposit” field in state channels, though the
responder_amount are to some extent being deposited into the channel.
deposit in a contract create is exactly this, an amount that is deposited (i.e. locked until it is later released). There are (though not in the near future!) plans to allow contracts to be (safely!) deactivated, the details are not yet worked out. If this is implemented this change will (most likely) make a sizeable deposit mandatory - in order to incentivize contract deactivation. But at the moment it can safely be put to
0, and at the moment there is no way to get back this deposit…