In an embodiment, an apparatus includes: a susceptor including substrate pockets; a gas injector disposed over the susceptor, the gas injector having first process regions, the gas injector including a first gas mixing hub and first distribution valves connecting the first gas mixing hub to the first process regions; and a controller connected to the gas injector and the susceptor, the controller being configured to: connect a first precursor material and a carrier gas to the first gas mixing hub; mix the first precursor material and the carrier gas in the first gas mixing hub to produce a first precursor gas; rotate the susceptor to rotate a first substrate disposed in one of the substrate pockets; and while rotating the susceptor, control the first distribution valves to sequentially introduce the first precursor gas at each of the first process regions as the first substrate enters each first process region.