A metal oxide alloy layer comprises a first layer including a first metal oxide and having a first thickness, and a second layer formed on the first layer, the second layer including a second metal oxide and having a second thickness, wherein a value of the first thickness is such that the first metal oxide is allowed to move into the second layer and a value of the second thickness is such that the second metal oxide is allowed to move into the first layer to form a single-layered structure in which the first and second metal oxides are mixed.