Design of a gas sensitive transparent heterojunction—The system SrCu2O2–ZnO

In this study, transparent oxide p–n heterojunction diodes based on SrCu2O2 (SCO) were fabricated by RF magnetron sputtering. A Zn-terminated polar plane (Zn-face), an O-terminated polar plane (O-face), and a non-polar plane (A-face) of highly orientated polycrystalline ZnO plates were used as substrates to clarify the effect of the surface polarity of ZnO upon the p–n heterojunction characteristics. Highly transparent and very electrically conductive SCO films were obtained by the application of low RF power (Ê < 0.5 W/cm2) under high deposition pressures (8–10 Pa). Although none of the as-prepared p-SrCu2O2/n-ZnO heterojunctions showed a very good rectifying I–V characteristics, the junction between SCO and the O-terminated ZnO polar surface exhibited clear rectifying I–V characteristics after post-deposition annealing at 923 K in Ar. The origin of the variation in the I–V characteristics depending on the crystal axis orientation of the ZnO substrates is assumed to be due to the surface polarity of the ZnO surface. It would appear that not only the gas-sensing characteristics but also the diode properties depend on the crystal axis orientations of ZnO. A transparent pn junction based on zinc oxide (ZnO) is suggested to have the good gas-sensing properties.