Doping and defects are key topics in semiconductor device physics. It is possible to tune all these defects by changing external growth conditions, which is critical in the material and device properties in energy related applications. In this talk, I present my recent progresses of different defect controlling techniques using first principles density functional theory method. We found that external strains can be effective strategies to tune dopant formation, dopant site and intrinsic defects formation in various semiconductor systems. Dual surfactant (elements that always stay on top of the growth surface)enhancedp-type doping, dopant and intrinsic defects induced lattice volume change and other research highlights will also be briefly presented.