Comparison of the influence of boron and aluminium doping on the material properties of electrochemically deposited ZnO films

The effect of varying the boron and aluminium content of the starting electrolyte for extrinsically doped ZnO films grown on SnO2:F substrates by electrochemical deposition was investigated. The ZnO:B film surface was characterized by grains with mainly hexagonal faces exposed while the exposed faces of the ZnO:Al grains were rectangular. Whereas a B3+/Zn2+ ratio of up to 10at.% in the electrolyte had no significant effect on the crystalline structure of the ZnO films, an Al3+/Zn2+ ratio above 0.25at.% increased the disorder in the crystalline structure. All the boron doped films exhibit a strong E2-high Raman mode related to wurtzite ZnO structure but this peak was much weaker for ZnO:Al and diminished with increasing Al incorporation in the films. Exposing the films to ultra-violet light reduced their effective sheet resistance from values beyond measurement range to values between 40 and 5000kΩ/sq for film thicknesses of 200–550nm. Inspection of the optical spectra near the bandgap edge and the plasma edge in the mid infrared range, showed that the Al-doping resulted in a higher carrier concentration ~1020cm−3 than B-doping. X-ray electron spectroscopy showed that the dopant efficiency was limited by the absence of dopant atoms near the surface of all the ZnO:B films and of the lightly doped ZnO:Al and, by the formation of aluminium oxide at the surface of the more highly doped ZnO:Al films.