Array substrate including thin film transistor and method of fabricating the same

A method of fabricating an array substrate, comprising:

• forming a gate electrode on a substrate;
• forming a gate insulating layer on the gate electrode, the gate insulating layer including a first insulator and a second insulator on the first insulator, wherein the first insulator includes an aluminum oxide material and has a first thickness, and the second insulator includes a hafnium oxide material and has a second thickness;
• forming an oxide semiconductor layer on the gate insulating layer over the gate electrode;
• forming a source electrode and a drain electrode over the gate insulating layer, the source electrode and the drain electrode contacting the oxide semiconductor layer and spaced apart from each other;
• forming a passivation layer on the source electrode and the drain electrode, the passivation layer having a drain contact hole exposing the drain electrode;
• and
• forming a pixel electrode on the passivation layer, the pixel electrode connected to the drain electrode through the drain contact hole,
• wherein the aluminum oxide material includes a phosphate (PO43−) to be expressed as Al2(PO4)2-xO3x/2(0≦x≦1.5).