Direct observation of a two-dimensional hole gas at oxide interfaces

The discovery of a two-dimensional electron gas (2DEG) at the LaAlO 3 /SrTiO 3 interface 1 has resulted in the observation of many properties 2 – 5 not present in conventional semiconductor heterostructures, and so become a focal point for device applications 6 – 8 . Its counterpart, the two-dimensional hole gas (2DHG), is expected to complement the 2DEG. However, although the 2DEG has been widely observed 9 , the 2DHG has proved elusive. Herein we demonstrate a highly mobile 2DHG in epitaxially grown SrTiO 3 /LaAlO 3 /SrTiO 3 heterostructures. Using electrical transport measurements and in-line electron holography, we provide direct evidence of a 2DHG that coexists with a 2DEG at complementary heterointerfaces in the same structure. First-principles calculations, coherent Bragg rod analysis and depth-resolved cathodoluminescence spectroscopy consistently support our finding that to eliminate ionic point defects is key to realizing a 2DHG. The coexistence of a 2DEG and a 2DHG in a single oxide heterostructure provides a platform for the exciting physics of confined electron–hole systems and for developing applications. A SrTiO 3 /LaAlO 3 /SrTiO 3 heterostructure is fabricated in high oxygen partial pressure to prevent oxygen vacancy formation. Electrical transport and electron holography directly observes a highly mobile two dimensional hole gas at the top interface.