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

Abstract

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 H2 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 H2 flow rate increases grain size and also enhances the RMS roughness. The deposition rate of HMGZO films decreases with the increase of H2 flow rate due to the decrease of sputtering yield. Resistivity of HMGZO thin films decreases rapidly from 117 to 7.2×10-3 Ω·cm with increasing H2 flow rate from 0 to 4.0 sccm. With further increasing H2 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 ～ 3.49–3.70 eV and the maximum Eg of 3.70 eV is obtained at a H2 flow rate of 16.0 sccm.