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.

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ZnO has shown great promise for the application in optoelectronic devices. Since the modulation of conductivity is one of the key issues in device performances, we have applied the Monte Carlo method to analyze the mobility of poly-crystalline MgZnO/ZnO heterostructure thin film layer in this paper. The effects of the grain boundary scattering, ionized impurity scattering, as well as phonon scattering are considered. Our study shows that with a design of modulation doping by including the effects of spontaneous and piezoelectric polarization, the grain boundary potential can be suppressed to improve the mobility of the ZnO layer by order(s) of magnitude. Simulation results are also confirmed by our experimental works that polarization effects play an important role to attract carriers and to increase the mobility.

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