Integrated circuit transistor structures are disclosed that reduce n-type dopant diffusion, such as phosphorous or arsenic, from the source region and the drain region of a germanium n-MOS device into adjacent insulator regions during fabrication. The n-MOS transistor device may include at least 75% germanium by atomic percentage. In an example embodiment, a dopant-rich insulator cap is deposited adjacent to the source and/or drain regions, to provide dopant diffusion reduction. In some embodiments, the dopant-rich insulator cap is doped with an n-type impurity including Phosphorous in a concentration between 1 and 10% by atomic percentage. In some embodiments, the dopant-rich insulator cap may have a thickness in the range of 10 to 100 nanometers and a height in the range of 10 to 200 nanometers.