A method for fabricating a semiconductor device includes forming an impurity layer over a first conductive layer; forming a first metal oxide layer over the impurity layer, wherein the first metal oxide layer includes oxygen at a lower ratio than a stoichiometric ratio; diffusing an impurity from the impurity layer into the first metal oxide layer to form a first doped metal oxide layer; forming a second metal oxide layer over the first doped metal oxide layer; and forming a second conductive layer over the second metal oxide layer.