Investigating the ranges of (meta)stable phase formation in (In_xGa_1−x)₂O₃: Impact of the cation coordination

We investigate the phase diagram of the heterostructural solid solution (InxGa1−x)2O3 both computationally, by combining cluster expansion and density functional theory, and experimentally, by means of transmission electron microscopy (TEM) measurements of pulsed laser deposited (PLD) heteroepitaxial thin films. The shapes of the Gibbs free energy curves for the monoclinic, hexagonal, and cubic bixbyite alloy as a function of composition can be explained in terms of the preferred cation coordination environments of indium and gallium. We show by atomically resolved scanning TEM that the strong preference of indium for sixfold coordination results in ordered monoclinic and hexagonal lattices. This ordering impacts the configurational entropy in the solid solution and thereby the (InxGa1−x)2O3 phase diagram. The resulting phase diagram is characterized by very limited solubilities of gallium and indium in the monoclinic, hexagonal, and cubic ground state phases, respectively, but exhibits wide metastable ranges at realistic growth temperatures. On the indium rich side of the phase diagram a wide miscibility gap up to temperatures higher than 1400 K is found, which results in phase separated layers. The experimentally observed indium solubilities in the PLD samples are in the range of x=0.45 and x=0.55 for monoclinic and hexagonal single-phase films, while for phase separated films we find x=0.5 for the monoclinic phase, x=0.65–0.7 for the hexagonal phase and x≥0.9 for the cubic phase. These values are consistent with the computed metastable ranges for each phase

Experimental exploration of the amphoteric defect model by cryogenic ion irradiation of a range of wide band gap oxide materials

The evolution of electrical resistance as function of defect concentration is examined for the unipolar n-conducting oxides CdO, β-Ga 2 O 3 , In 2 O 3 , SnO 2 and ZnO in order to explore the predictions of the amphoteric defect model. Intrinsic defects are introduced by ion irradiation at cryogenic temperatures, and the resistance is measured in-situ by current–voltage sweeps as a function of irradiation dose. Temperature dependent Hall effect measurements are performed to determine the carrier concentration and mobility of the samples before and after irradiation. After the ultimate irradiation step, the Ga 2 O 3 and SnO2 samples have both turned highly resistive. In contrast, the In 2 O 3 and ZnO samples are ultimately found to be less resistive than prior to irradiation, however, they both show an increased resistance at intermediate doses. Based on thermodynamic defect charge state transitions computed by hybrid density functional theory, a model expanding on the current amphoteric defect model is proposed

Electronic properties of shallow level defects in ZnO grown by pulsed laser deposition

We have used deep level transient spectroscopy (DLTS) to characterise four defects with shallow levels in ZnO grown by pulsed laser deposition (PLD). These defects all have DLTS peaks below 100 K. From DLTS measurements and Arrhenius plots we have calculated the energy levels of these defects as 31 meV, 64 meV, 100 meV and 140 meV, respectively, below the conduction band. The 100 meV defect displayed metastable behaviour: Annealing under reverse bias at temperatures of above 130 K introduced it while annealing under zero bias above 110 K removed it. The 64 meV and 140 meV defects exhibited a strong electric field assisted emission, indicating that they may be donors

Electronic properties of shallow level defects in ZnO grown by pulsed laser deposition

We have used deep level transient spectroscopy (DLTS) to characterise four defects with shallow levels in ZnO grown by pulsed laser deposition (PLD). These defects all have DLTS peaks below 100 K. From DLTS measurements and Arrhenius plots we have calculated the energy levels of these defects as 31 meV, 64 meV, 100 meV and 140 meV, respectively, below the conduction band. The 100 meV defect displayed metastable behaviour: Annealing under reverse bias at temperatures of above 130 K introduced it while annealing under zero bias above 110 K removed it. The 64 meV and 140 meV defects exhibited a strong electric field assisted emission, indicating that they may be donors

Strain states and relaxation for Alpha AlxGa1 x 2O3 thin films on prismatic planes of alpha Al2O3 in the full composition range Fundamental difference of a and m epitaxial planes in the manifestation of shear strain and lattice tilt

Pseudomorphic and relaxed Alpha AlxGa1 x 2O3 thin films are grown by combinatorial pulsed laser deposition in the entire composition range on prismatic a and m plane alpha Al2O3 substrates. Pseudomorphic growth on m plane sapphire has been achieved for x bigger equal 0.45. A distinct difference between the a and m epitaxial plane is observed in reciprocal space map measurements being in agreement with continuum elasticity theory for rhombohedral heterostructures. While pseudomorphic layers on m plane sapphire show a pronounced shear strain e 5 along the c axis direction, relaxed layers exhibit a global lattice tilt in the same direction. Both effects are not present on the a epitaxial plane. Out of plane lattice constants as well as e 5 are modeled as function of x employing elasticity theory, confirming theoretical values of the elastic stiffness tensor for alpha Ga2O3, especially the non zero value of the C14 component. Possible pyramidal slip systems for strain relaxation in c axis direction are examined to explain and numerically model the difference in lattice tilt for the two substrate orientation

Tuning material properties of amorphous zinc oxynitride thin films by magnesium addition

The amorphous n type multi cation and multi anion compound zinc magnesium oxynitride was fabricated by reactive long throw magnetron co sputtering from a metallic zinc and a metallic magnesium target. We achieved magnesium cation compositions in the thin films between 1 at. and 7.5 at. by varying the magnesium target power variable power approach and compared this approach to a continuous composition spread. Both approaches lead to a reduction in Zn2 , an addition of Mg2 cations, and a correlated increase in the oxygen content. Both these methods have the same effect on the optoelectrical properties The increased magnesium content leads to a systematic decrease in the free charge carrier concentration regime from 1019 cm 3 to 1015 cm 3, a decrease in Hall mobility from 54 cm2 V 1 s 1 to 9 cm2 V 1 s 1, and a spectral shift of the absorption edge from 1.3 eV to 1.7 eV. The amorphous phase of zinc magnesium oxynitride is maintained until MgO crystallites form at a magnesium content bigger equal 3 at. . The electrical properties of the zinc magnesium oxynitride thin films show excellent long term stability for at least 12 month

Encapsulation, operator overloading, and error class mechanisms in OCL

Checking models for correctness or compatibility using standard formal modeling techniques such as OCL has merits in abstraction and compactness. However, it is inconvenient for developers, since there are no standard mechanisms how to handle large and complex OCL constraints. Therefore, this paper presents an approach how to split complex OCL constraints into multiple ones by defining helper functions and pack these into an OCL/P library with encapsulation mechanisms. Another drawback of using complex OCL constraints at present is the lack of descriptive and user-friendly error messages. Hence, this paper introduces an OCL extension that allows specifying error classes by synthesizing witnesses pointing directly to constraint violations. All approaches are shown on Component & Connector model examples, where OCL/P is used on the meta-level to verify backwards compatibility of interfaces