Influence of H2 introduction on wide-spectrum Mg and Ga co-doped ZnO transparent conductive thin films

To meet the demands of high efficient silicon thin film solar cells, transparent conductive hydrogenated Mg and Ga co-doped ZnO (HMGZO) thin films were deposited via pulsed direct current (DC) magnetron sputtering on glass substrates at a substrate temperature of 553 K. The micro-structural, morphological, electrical, and optical properties of HMGZO thin films were investigated at various H-2 flow rates. Experimental results show that all the HMGZO thin films are polycrystalline with a hexagonal wurtzite structure exhibiting a preferred (002) crystal plane orientation. Appropriate H-2 flow rate increases grain size and also enhances the RMS roughness. The deposition rate of HMGZO films decreases with the increase of H-2 flow rate due to the decrease of sputtering yield. Resistivity of HMGZO thin films decreases rapidly from 117 to 7.2 x 10(-3) Omega-cm with increasing H-2 flow rate from 0 to 4.0 sccm. With further increasing H-2 flow rate (4.0-16.0 sccm), the resistivity increases slightly due to the reduced carrier concentration and excessive H atoms as impurity. Optical transmittance of all the HMGZO thin films is higher than 87.7% in the wavelength range from 320 to 1100 nm. Burstein-Moss band-filling determined by carrier concentrations and the incorporation of Mg atoms together contribute to the band-gap (Eg) widening phenomenon. The band gap Eg varies from similar to 3.49-3.70eV and the maximum E-g of 3.70 eV is obtained at a H-2 flow rate of 16.0 sccm.