About HTML5 Access Keys

I heard that there is something called an access key in HTML, but
Can't I use the function key for the access key?

<button type="button" id="search" name="search" accesskey="U"/>
<Button type="Button" id="Search" name="Search" accesskey="<F3>"/>

It moves at the top, but it doesn't move at the bottom.
However, even the function (search) of F3 will not work in the first place.

Can't I assign a function key to an access key?