Are you positive it's coming from the handlebar to stem interface? Creaking sounds are notoriously hard to diagnose. It's highly possible that the noise could be from the stem to steer tube interface or your headset or the spacers on your steer tube or topcap or another source, while it may sound like it's coming from your handlebars.
Also, be cautious with making things "as tight as I can make it" as you may risk over-tightening the part and risking more damage to your bike and/or components. If you are unfamiliar with how hard to tighten bolts, you should consult your owner's manual, the part manufacturers technical documents or simply take your bike into a reputable mechanic to properly tighten everything.