Na implants have been used to study diffusion of Na in rf diode sputtered M
o thin films used as back contacts for Cu(In,Ga)Se, solar cells. The sample
s were analysed with secondary ion mass spectrometry before and after vacuu
m anneals at 420 degrees C and 550 degrees C. In addition, X-ray photoelect
ron spectroscopy has been used for surface studies. The diffusion of Na wit
hin the Mo grains was found to be very slow as indicated by the unchanged s
hape and position of the implant peak after the anneal. An increased level
of Na in the bulk of the Mo layer strongly suggests diffusion of Na out of
the soda lime glass substrate into the Mo film. The oxygen content of the r
f diode sputtered Mo films was 8 at% as found by Rutherford backscattering
spectroscopy. It is suggested that Mo oxide phases are present in the grain
boundaries and that these oxides, being intercalation hosts for Na, are re
sponsible for the rapid diffusion and high solubility of Na in the sputter-
deposited Mo films. (C) 1999 Elsevier Science B.V. All rights reserved.