The behaviour of Na implanted into Mo thin films during annealing

Na implants have been used to study diffusion of Na in rf diode sputtered Mo thin films used as back contacts for Cu(In,Ga)Se, solar cells. The samples were analysed with secondary ion mass spectrometry before and after vacuum anneals at 420 degrees C and 550 degrees C. In addition, X-ray photoelectron spectroscopy has been used for surface studies. The diffusion of Na within the Mo grains was found to be very slow as indicated by the unchanged shape 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 rf 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 responsible for the rapid diffusion and high solubility of Na in the sputter-deposited Mo films. (C) 1999 Elsevier Science B.V. All rights reserved.