Good start, because the creak is a fit issue, and contamination can affect the fit. Next step is to check for, and remove, any burrs which are acting high spots and preventing full contact. The end of that process is lapping the mating surfaces together.
Just had to google this as wasn't sure,
Handlebar/stem interface.
I greased (very lightly) as I was getting creaking.
Going by @mdcc_tester advice, copperslip would have been the ideal treatment?