This paper reports the formation of two-dimensional electron gas ͑2DEG͒ in rf-sputtered defective polycrystalline MgZnO/ZnO heterostructure via the screening of grain boundary potential by polarization-induced charges. As the MgZnO thickness increases, the sheet resistance reduces rapidly and then saturates. The enhancement of the interfacial polarization effect becomes stronger, corresponding to a larger amount of resistance reduction, when the Mg content in the cap layer increases. Monte Carlo method by including grain boundary scattering effect as well as 2D finite-element-method Poisson and drift-diffusion solver is applied to analyze the polycrystalline heterostructure. The experimental and Monte Carlo simulation results show good agreement. From low temperature Hall measurement, the carrier density and mobility are both independent of temperature, indicating the formation of 2DEG with roughness scattering at the MgZnO/ZnO interface.

No abstract

Modulation doping effect is studied in large-area rf-sputtered polycrystalline MgZnO/ZnO heterostructures. Both polarization effect at the MgZnO/ZnO interface and carrier transferring from the modulation doping layer contribute to the improvement of electrical conductivity of the heterostructure. Modulation doping provides greater enhancement in electrical properties when Mg content in the barrier layer is lower. Temperature-independent carrier concentration is observed in low-temperature Hall measurement, indicating the existence of two-dimensional electron gas in the modulation-doped polycrystalline MgZnO/ZnO structure. The slight drop in mobility at low temperatures is caused mainly by the roughness scattering and impurity scattering.

No abstract

The electrical responses of materials and devices subjected to thermal inputs, such as the Seebeck effect and pyroelectricity, are of great interest in thermal-electric energy conversion devices. Of particular interest are phenomena which exploit heterogeneities in the mechanics of heterostructured materials and systems for novel and unexplored thermal-electric responses. Here we introduce a new mechanism for converting thermal stimuli into electricity via structural heterogeneities, which we term "pyro-paraelectricity." Specifically, when a paraelectric material is grown on a substrate with a different lattice constant, the paraelectric layer experiences an inhomogeneous strain due to the lattice mismatch, establishing a strain gradient along the axis of the layer thickness. This strain gradient, induced via the lattice mismatch, can be multiple orders of magnitude higher than strain gradients in bulk materials imparted by mechanical bending (0.1 m -1 ). Consequently, charge separation is induced in the paraelectric layer via flexoelectricity, leading to a polarization in proportion to the dielectric constant. The dielectric constant, and thus the polarization, in turn changes with temperature. Therefore, when a strained metal-insulator-metal (MIM) heterostructure is subjected to a thermal input, changes in the permittivity generate an electrical response. We demonstrate this concept of "pyro-paraelectricity" by employing a MIM heterostructure with a high permittivity sputtered barium strontium titanate (BST) film as the insulating layer in a platinum sandwich. The resulting strain gradient of more than 10 4 m -1 due to the structural heterogeneity was verified by an X-ray diffraction scan. To demonstrate "pyro-paraelectricity," the MIM heterostructure was subjected to a thermal input, thereby generating current which was highly correlated to the thermal input. A theoretical model was found to be consistent with the experimental data. These results prove the existence of "pyro-paraelectricity."