Sodium Doping Effects on Layered Cobaltate Bi2 Sr2 Co2 O y Thin Films.

Highly c-axis oriented Bi2− x Na x Sr2 Co2 O y ( x = 0, 0.1, 0.2, 0.3) thin films were successfully deposited onto LaAlO3 (0 0 1) single crystal substrates by the chemical solution deposition method. The Na-doping effects on microstructures as well as transport and thermoelectric properties were investigated. The crystallite size normal to the thin film surface was decreased with Na doping, and the carrier concentration and mobility was enhanced and decreased, respectively. As for the transport properties, it is suggested that Na-doping-induced disorders play a more important role at low temperatures, while at the medium temperature the doping play a trivial role, at higher temperatures Na-doping weakens the electron correlation. Combined with the transport properties and Seebeck coefficient results, it is suggested that the thermoelectric properties are mainly controlled by carrier concentration due to the hole doping in blocking layers. Power factor at 300 K was enhanced about 22% for the Bi1.7 Na0.3 Sr2 Co2 O y thin film as compared with that of the undoped thin films, suggesting that Na doping at blocking layer in Bi2 Sr2 Co2 O y was an effective route to enhance the thermoelectric power factor. [ABSTRACT FROM AUTHOR]