The discovery of a two-dimensional electron gas (2DEG) at the LaAlO/SrTiO interface has resulted in the observation of many properties not present in conventional semiconductor heterostructures, and so become a focal point for device applications. Its counterpart, the two-dimensional hole gas (2DHG), is expected to complement the 2DEG. However, although the 2DEG has been widely observed , the 2DHG has proved elusive. Herein we demonstrate a highly mobile 2DHG in epitaxially grown SrTiO/LaAlO/SrTiO 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.

Metal-to-insulator phase transitions in complex oxide thin films are exciting phenomena which may be useful for device applications, but in many cases the physical mechanism responsible for the transition is not fully understood. Here we demonstrate that epitaxial strain generates local disproportionation of the NiO6 octahedra, driven through changes in the oxygen stoichiometry, and that this directly modifies the metal-to-insulator phase transition in epitaxial (001) NdNiO3 thin films. Theoretically, we predict that the Ni-O-Ni bond angle decreases, while octahedral tilt and local disproportionation of the NiO6 octahedra increases resulting in a small band gap in otherwise metallic system. This is driven by an increase in oxygen vacancy concentration in the rare-earth nickelates with increasing in-plane biaxial tensile strain. Experimentally, we find an increase in pseudocubic unit-cell volume and resistivity with increasing biaxial tensile strain, corroborating our theoretical predictions. With electron energy loss spectroscopy and xray absorption, we find a reduction of the Ni valence with increasing tensile strain. These results indicate that epitaxial strain modifies the oxygen stoichiometry of rare-earth perovskite thin films and through this mechanism affect the metal-to-insulator phase transition in these compounds. PACS numbers: 68.55.Ln, 68.60.Bs, 73.50.-h, 73.20.-r 3 Metal-to-insulator phase transitions (MITs) in strongly-correlated electronic systems are fascinating phenomena which have attracted significant attention for decades [1]. Among complex oxide materials which exhibit MITs are rare-earth nickelates having the generic formula RNiO3, where the rare-earth element (R) is smaller than lanthanum, i.e. R = Pr, Nd ... [2]. The critical temperature of the MIT is dependent on the Ni-O-Ni bond angle: straightening the angle with a larger R cation stabilizes the metallic state over the insulating state and lowers the transition temperature [3-5]. For example, the MIT temperatures in bulk NdNiO3 and SmNiO3 (Ni-O-Ni bond angles of 157.1 and 153.4°, respectively) have been reported to be approximately 200 and 400 K, respectively. It should be also emphasized that a breathing order by disproportionation of the Ni-O bond length plays a crucial role in the MIT [6-10].In RNiO3 thin films, misfit strain arising from a lattice mismatch between the film layer and the underlying substrate affects the lattice volume, electrical conductivity and MIT temperature [11][12][13][14][15][16][17][18]. In particular, films under in-plane tensile strain are more insulating compared to those under in-plane compressive strain. The origin of this interesting phenomenon remains unclear, although a number of mechanisms have been proposed [19][20][21][22][23][24].We also note that the effect of oxygen non-stoichiomety on the MIT has been reported in bulk RNiO3 [25,26].It is widely accepted that epitaxial strain in transition metal oxide films can be accommodated through the formation of oxygen vacancy defects, resulting in offstoi...

III-nitrides have revolutionized lighting technology and power electronics. Expanding the nitride semiconductor family to include heterovalent ternary nitrides opens up new and exciting opportunities for device design that may help overcome some of the limitations of the binary nitrides. However, the more complex cation sublattice also gives rise to new interactions with both native point defects and defect complexes that can introduce disorder on the cation sublattice. Here, depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy measurements of defect energy levels in ZnGeN2 combined with transmission electron microscopy and x-ray diffraction reveal optical signatures of mid-gap states that can be associated with cation sublattice disorder. The energies of these characteristic optical signatures in ZnGeN2 thin films grown by metal–organic chemical vapor deposition are in good agreement with multiple, closely spaced band-like defect levels predicted by density functional theory. We correlated spatially resolved optical and atomic composition measurements using spatially resolved x-ray photoelectron spectroscopy with systematically varied growth conditions on the same ZnGeN2 films. The resultant elemental maps vs defect spectral energies and intensities suggest that cation antisite complexes (ZnGe–GeZn) form preferentially vs isolated native point defects and introduce a mid-gap band of defect levels that dominate electron–hole pair recombination. Complexing of ZnGe and GeZn antisites manifests as disorder in the cation sub-lattice and leads to the formation of wurtzitic ZnGeN2 as indicated by transmission electron microscopy diffraction patterns and x-ray diffraction reciprocal space maps. These findings emphasize the importance of growth and processing conditions to control cation place exchange.

The graphane analogues of group 14 are a unique family of 2D materials due to the necessity of a terminal ligand for stability. Here we highlight how changing the surface ligand can lead to nonobvious interactions with other chemical species. We show using XRD, FTIR, and TGA that GeCH3 reversibly absorbs water into the van der Waals space, whereas GeH does not intercalate water. Molecular dynamics and density functional theory simulations predict that water datively interacts with the Ge–C σ* pocket on the Ge framework, resulting in local structural distortions. Surprisingly, these distortions give rise to an intense above band gap luminescence state of 1.87 eV, with an average lifetime of hundreds of picoseconds. This work opens potential applications for exploiting surface functionalization chemistry of 2D materials to create membrane and separation technologies.

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