Zinc oxide possesses excellent electrical conductivity and has of late emerged as an alternative candidate for the prevalent transparent conductive oxide, indium tin oxide. This is possible since zinc oxide portrays interesting physical properties with visible transparency making it a prospective multifunctional electronic material. Lately, it has been observed that indium tin oxide tends to exhibit lower resistivity, credit due to a high carrier mobility. Zinc oxide-based transparent conductive oxides have also portrayed low resistivity. Moreover, the latter has exhibited excellent ferromagnetism at room temperature and high piezoelectric responses when doped with vanadium. Unfortunately, the origin of these electrical and magnetic behaviors in vanadium doped zinc oxide is yet to be understood. Researchers have thus speculated that their origin could be as a result of the defects induced by vanadium, since it has a higher affinity for oxygen than zinc. The work presented here aims at clearing such speculations.
Tohoku University researchers: Tomoyuki Kawashima, Dai Abe, and Katsuyoshi Washio looked into the source of dominant donor in vanadium doped zinc oxide films, by stoichiometry control through oxygen gas addition in inert sputtering gas for reactive RF magnetron sputtering. They thoroughly investigated the defect states using crystallographic, electronic and optical analyses. Their work is now published in the research journal, Materials Science in Semiconductor Processing.
The researchers deposited vanadium-doped zinc oxide films on a mirror polished quartz substrate by reactive RF magnetron sputtering system. The sputtering target was a ceramic zinc oxide with vanadium chips arranged on an erosion area. The researchers then performed deposition under a substrate temperature of around 170 °C, an RF power of 150W and in an argon/oxygen mixture ambient. The team then determined the film thickness and vanadium concentration. Eventually, crystallographic analyses and optical transmittance were determined.
The authors observed that by adding oxygen, resistivity of the vanadium-doped zinc oxide was increased. Additionally, the carrier density of the vanadium-doped zinc oxide was seen to decrease despite the increase in valence number of vanadium. The research team also noted that the optical transmittance of vanadium-doped zinc oxide was inferior to that of zinc oxide without oxygen addition due to the optical absorption by oxygen vacancies. However, the average optical transmittance was seen to improve with oxygen addition.
The study presented a comprehensive examination of the influences of oxygen gas addition in reactive RF magnetron sputtering on properties of vanadium-doped zinc oxide films. It has been seen that vanadium doping causes oxygen deficiency in zinc oxide due to the formation of defects such as numerous zinc interstitials, oxygen vacancies, and zinc vacancies.
Tomoyuki Kawashima, Dai Abe, Katsuyoshi Washio. Investigation on a source of dominant donor in vanadium-doped ZnO films grown by reactive RF magnetron sputtering. Materials Science in Semiconductor Processing 70 (2017) 213–218Go To Materials Science in Semiconductor Processing