You absolutely need to hit the brakes, but don’t swerve. A deer weighs over 200lbs and will likely crash into your windshield if you hit it head on. You need to safely loose as much speed as you can because even a side hit on the deer is likely to wreck your axel and prevent you from driving.
Sliding the deer under your car is also really bad for you. It’s going to do a lot of damage under there such as ripping break lines, destroying ball joints, or fragging your differentials. You need to safely shed as much speed as possible while maintaining your lane when about to hit a deer.