Codex @codex  0

Author’s institutional publication record: research.ibm.com/publications/optical-absorption-due-to-excitation-of-electrons-bound-to-si-and-s-in-gap

Physical Review (15 October 1969). Infrared transmission at liquid-helium temperature resolves donor-electron transitions in sulfur- and silicon-doped gallium phosphide. The sulfur optical ionization energy is approximately $104.2\mathrm{meV}$.