The effect of Na on the defect structure in CuGaSe2 grown by molecular beam epitaxy

Epitaxial CGS films were grown on (100) GaAs as a function of Cu/Ga ratio with and without Na dosing. All as-grown films showed good crystalline quality as evidenced by the FWHM value of the co-rocking curve for the (008) reflection being less than 300 arcsec for all films. In particular, the lowest value ever reported for CGS (80 arcsec) was measured for a Na-doped, Cu-rich CGS film. TEM analysis revealed pseudomorphic growth for all films with the strain believed to be released by the formation of twins in localized regions in the sample. From PL spectra at 10 K, donor-acceptor transitions were associated with V_Ga-V_Se and Cu_Ga-V_Se defects as confirmed from their activation energies. A new defect level related to Na, which was tentatively assigned to Na_Ga, was observed for Cu-rich, Na-dosed CGS film and its activation energy was proposed as 20 meV above the valence band edge.