94 Grand Voyager starter problem

I pulled into the garage and tuned the key off and the motor stopped but the starter engaged as though it was trying to start the van. I immediately turned the key to on and the motor started running and the starter disengaged. I then turned the key off again and the same thing happened. I finally took of the positive battery cable, tuned the key to off and the motor stopped and the starter started to engage but quickly ran out of power. I replaced the ignition switch but there is no change. When I try to put the battery cable back on there is a huge spark as though something is shorting out and the starter engages. Is it the starter?