Darren, your deck can be built right over top of the slab, but you'll likely need to lose the step. The step is sometimes a separate pour, and with a little initial leverage, the step will separate and can be removed. Look closely at the base of the step where it meets the slab, chpping some away with a chisel if needed, to see if you can spot a cold joint between the two. If so, and you can get a prybar between it and the concrete foundation/basement wall below the siding, you can usually get them started moving. If the step was poured at the same time, the step will need to be broken out.
If the slab is failing, and is sinking towards the home, where it would direct water towards the foundation, it should all be broken out & the area carefully refilled & tamped to ensure drainage away from the home.