What is the recommended thickness for wooden steps?
At that length and with a decent thickness (>=1 1/2") you'd probably be fine just using a solid wood plank as long as the species is appropriate. Here's a chart of the modulus of elasticity for some common woods

http://www.woodworkweb.com/woodwork-topics/wood/146-wood-strengths.html

To be honest, when designing something like a staircase you should really consider having it engineered by an architect or structural engineer just for peace of mind.