Matthew Charles

In contrast to the previous work which solved the problem of gallium pollution using hardware modifications, herein, the changes are examined to process conditions to reduce gallium pollution in InAlN layer. Using a model of GaN decomposition followed by gallium desorption and diffusion to the showerhead, different process conditions are used to limit either the desorption or the diffusion. Reducing the GaN growth temperature gives some improvement by reducing desorption, but affects channel mobility, likely due to increased carbon incorporation. Increasing the wafer–showerhead distance also reduces the gallium pollution, this time by a factor of around 2. Finally, using AlGaN layers instead of GaN as the channel completely removes gallium pollution, even for composition as low as 5%. It is suggested that in addition to reducing desorption due to alloying, the aluminum precursor may be acting as a getter for this gallium, and so stops it getting to the showerhead. A combination of these different approaches suggests that process conditions can significantly reduce gallium pollution in close‐coupled showerhead reactors.