You can choose to do many things with regard to sending supplemental material. You could send the paper, you could send an abstract, you could send an essay talking about your research and what you learned, etc.
Any tack is fine, although Matt says
Research papers. I think research papers are best talked about in the completely optional essay ("about something that you have created") or in an extra recommendation from your research mentor. It is unlikely that if you submit a complete research paper that we will be able to have it properly evaluated during our process. We'll be most interested in your research experience: how you got interested in the field, how you acquired your research opportunity, your results, what you learned, how this experience influences your future plans, etc.
It is, however, getting pretty late to send in information, and you'll probably be best sending a short update rather than a long one. Selection committee will be starting very soon (perhaps next week), and although there is no formal deadline for sending supplemental material, there's no guarantee that all of your readers will be able to see your paper if it's sent so late.