Effect of Interface Regions on the Thermoelectric Properties of Alternating ZnO/ZnO:Al Stripe Structures

A series of bar-shaped samples consisting of lateral arrangements of alternating ZnO:Al and ZnO stripes was fabricated by radiofrequency (RF)-sputtering and microfabrication techniques on glass substrates. Throughout the series, the number of interfaces between ZnO and ZnO:Al was varied whilst the material fractions of ZnO:Al and ZnO within the bars were not altered. Lateral thermoelectric transport parameters, i.e., Seebeck effect and electrical resistivity, were measured as a function of temperature for all microstructured samples and two reference samples of ZnO:Al and undoped ZnO. The transport direction through the bar was perpendicular to the stripe direction, such that the electrons and phonons have to pass all interfaces. The transport coefficients of the microstructured samples show clear dependence on the number of interfaces between ZnO and ZnO:Al. Thermoelectric measurements, photoluminescence, and Raman measurements indicate that this is due to diffusion of Al donors along the grain boundaries into the undoped ZnO stripes, which takes place during the fabrication process. Modeling of the dependence of the Seebeck coefficient and the resistivity of the series of samples on the basis of a network model accounting for donor diffusion supports these findings.