Unlike others have suggested, it is a good idea to use kbd (or read-kbd-macro which is basically the same thing) in case you ever want to use the same configuration files in other versions of Emacs; kbd works across several versions of Emacs and XEmacs, where the internal representation of key sequences are different.
(global-set-key (kbd "<C-tab>") 'my-func)
The input format used by read-kbd-macro is documented in the docstring of edmacro-mode:
The special words RET, SPC, TAB, DEL, LFD, ESC, and NUL represent
special control characters. The words must be written in uppercase.A word in angle brackets, e.g., <return>, <down>, or <f1>, represents
a function key. (Note that in the standard configuration, the
function key <return> and the control key RET are synonymous.)
You can use angle brackets on the words RET, SPC, etc., but they
are not required there.
This is written somewhat unfortunately; the TAB referred to in the first bullet point is the ASCII character for TAB, and adding the Control modifier does something nonsensical to it. When you press Control-Tab, Emacs sees it (via your windowing system; it will not work in a text terminal) as <tab> with a Control modifier, which you can represent as C-<tab> or <C-tab>.