A Schottky diode is NOT the same as a zener diode. Do NOT install a zener diode across your motor terminals. If you do, the result will likely be a fried speed controller.

The unique characteristic of a Schottky diode is that it has a very low forward voltage drop (conducts in the forward biased mode at lower voltage) compared to common silicon diodes. This makes it more efficient.

A zener diode (which you don't want to use) has the characteristic that, in addition to conducting in the forward biased state as a normal diode, it also conducts easily in the reverse biased direction above a specific voltage threshold. They are typically used as voltage regulators.