Thermoelectric Properties Of GeTe/Sb₂Te₃ Nano-multilayer Thin Films And Their Quantum Wells

Sb₂Te₃ and GeTe with high electrical conductivity,narrow band gap,and low lattice thermal conductivity are extremely promising thermoelectric materials.However,most studies on their thermoelectric properties have focused on elemental doping modifications,while the electrical transport and thermal conductivity of the composite thin film system（GeTe/Sb₂Te₃）have rarely been reported.In this thesis,the thermoelectric properties of GeTe films,Sb₂Te₃ films,GeTe/Sb₂Te₃ nanomultilayer films and quantum wells were systematically investigated.Sb₂Te₃ and GeTe thin films exhibit Sb and Ge rich phenomena,and excess Sb and Ge atoms replace Te atoms and form Sb _Te and Ge _Te antisite defects,resulting in P-type conductivity in their thin films.As the annealing temperature increases,the electrical resistivity of the two films decreases,mainly due to the increase of antisite defects and intrinsic excitation of charge carriers.Appropriate annealing temperature settings can reduce the defect density and lattice distortion of the thin film,which is conducive to the reduction of Femi energy levels and the improvement of seebeck coefficient.At an annealing temperature of 400℃,Sb₂Te₃ and GeTe thin films exhibit excellent thermoelectric properties,which provides favorable conditions for the subsequent preparations of GeTe/Sb₂Te₃ multilayer films and quantum wells.Based on the preparation technology of single-layer films,GeTe/Sb₂Te₃ multilayer films were prepared by magnetron sputtering in this work.Compared with intrinsic GeTe,the carrier concentration of multilayer films decreased and maintained in a relatively ideal range(10¹⁹～10²⁰cm^-3).Ge vacancies,Ge _Te antisite defects,and Sb _Te antisite defects are important sources of inducing hole carrier formation in thin films.The hole carriers move under the action of an electric field,and the multilayer film exhibits a P-type electrical conduction as a whole.Thanks to the optimization of carrier concentration,the power factor（PF）of multilayer films is superior to that of single-layer films,with PF_max up to 1081μW·m^-1·K^-2 for single-period films.The phonon scattering effect at the layered interface of multilayer films results in a much lower thermal conductivity than GeTe films with the same thickness.The reduction in layer thickness helps to enhance carrier confinement effects and suppress phonon transport,where the thermal conductivity of single-period multilayers can be as low as 1.08 W·m^-1·K^-1 at 573 K.The GeTe/Sb₂Te₃ quantum well is a nested homogeneous heterostructure material where the majority of electrons are localized in the potential well layer（Sb₂Te₃ thin layer）,and its electrical transport performance is mainly related to the layered film thickness ratio and the Sb₂Te₃ thin layer.The decrease in the thickness ratio of the layered film and the width of the potential well is beneficial to the improvement of the seebeck coefficient and conductivity of the quantum well.After measurement,the power factor of GeTe/Sb₂Te₃quantum well can reach 968.44μW·m^-1·K^-2.Compared with multilayer films,the decrease of film thickness and the increase of layered interface further decrease thermal conductivity of quantum wells,which can be as low as 0.704 W·m^-1·K^-1.The thermal conductivity of GeTe/Sb₂Te₃ quantum wells mainly depends on the transmission of phonons.The increase in test temperature promotes the increase of the lattice thermal vibration frequency and enhances phonon-phonon scattering,resulting in a decrease in the thermal conductivity of the quantum wells.By effectively controlling the thickness and ratio of the layered film,the thermoelectric properties of the multilayer thin film system have been further improved.The research in this thesis shows that the composite film structure（multilayer film/quantum well）is beneficial to improve the thermoelectric properties of GeTe and Sb₂Te₃ based films.