Surface modification of AlInAs/InGaAs high electron mobility transistor (HEMT) wafers has been investigated to suppress the diffusion of fluorine (F) atoms into an n-AlInAs layer. The surface of the HEMT wafer is phosphidized using remote phosphine (PH3) plasma. Secondary ion mass spectroscopy analysis reveals that F diffusion is suppressed even after annealing at 400degreesC for 15 min for the phosphidized sample. It is also demonstrated by Hall measurements that no significant reduction in the two-dimensional electron density occurs due to annealing. Formation of the modification layer containing phosphorus (P) atoms is shown by X-ray photoelectron spectroscopy analysis. The PH3 plasma treatment is effective in suppressing the diffusion of F atoms into the n-AlInAs layer.