The question itself is confusing. DO you want to receive a signal using the uC or do you want to modulate. If you want to transmit, you can always make use of some readily available ICs for modulating RC-5, NEC, sony.. formats if you really want to do it with a uC (which is an overkill i say, you can always use some kind of carrier generator (some random thoughts: use an NE 555 in a stable to generate 38 kHz and feed the serial data that you wanna send , to the reset (4th pin) of the 555.)