THE STRUCTURAL, OPTICAL, AND ELECTRICAL-PROPERTIES OF VACUUM-EVAPORATED CU-DOPED ZNTE POLYCRYSTALLINE THIN-FILMS

We have studied the structural, optical, and electrical properties of thermally evaporated, Cu-doped, ZnTe thin films as a function of Cu co ncentration and post-deposition annealing temperature. X-ray diffracti on measurements showed that the ZnTe films evaporated on room temperat ure substrates were characterized by an average grain size of 300 Angs trom with a (111) preferred orientation. Optical absorption measuremen ts yielded a bandgap of 2.21 V for undoped ZnTe. A bandgap shrinkage w as observed for the Cu-doped films. The dark resistivity of the as-dep osited ZnTe decreased by more than three orders of magnitude as the Cu concentration was increased from 4 to 8 at.% and decreased to less th an 1 ohm-cm after annealing at 260 degrees C. For films doped with 6-7 at.% Cu, an increase of resistivity was also observed during annealin g at 150-200 degrees C. The activation energy of the dark conductivity was measured as a function of Cu concentration and annealing temperat ure. Hall measurements yielded hole mobility values in the range betwe en 0.1 and 1 cm(2)/V . s for both as-deposited and annealed films. Sol ar cells with a CdS/CdTe/ZnTe/metal structure were fabricated using Cu -doped ZnTe as a back contact layer on electrodeposited CdTe. Fill fac tors approaching 0.75 and energy conversion efficiencies as high as 12 .1% were obtained.