You can use almost any
type of material for your stair treads. I suggest sticking to
construction standard lumber, plywood or oriented strand board. However,
if I had my choice, I would use 1-1/8 inch thick plywood or oriented
strand board, for interior stairs and 1-1/2" thick construction standard
for exterior stairs, with a center stringer.
If you're going to build a stairway that is only going to have one stringer on each side of the stairs, then you can probably use a 2 x 12's or 2 x 14's, for your stair treads. However, I recommend using 3 x 12 or 3 x 14, for any stairs that are going to be wider than 36 inches.
Important TipThese are only general specifications that can be applied to lightweight construction projects and it wouldn't be a bad idea to check with a structural engineer, if you are building a heavy-duty stairway. Stairways that are built with the wrong materials will create a safety hazard and serious injuries.
Oriented Strand Board Or PlywoodI'm always going to pick plywood over oriented strand board, because it's stronger. However, it's also more expensive and if price is going to be a concern, then you might not have a choice as long as the stairway you're building, doesn't create safety problems.
I dropped a 2 x 6 that was 8 foot long on a stairway that used three quarter-inch thick, oriented strand board, for their stair treads and this particular piece of lumber, almost went all the way through. It probably would have went all the way through, if it didn't hit part of the stair stringer.
That was all I needed to see, yet thousands of homes each year are built with these materials and don't seem to have any problems.