Microstructure and Thermoelectric Properties of β-Cu2Se Films Doping with Ni
Cu2Se,which possesses the structure of phonon glass-electron crystal(PGEC),is an excellent thermoelectric material,which is not only due to the excellent thermal(ultra-low thermal conductivity)and electrical(low resistivity)transport properties,but also due to its unique properties such as environmental friendliness,low cost,low toxicity,rich in rare earth elements,high power fac-tor(PF)and high figure of merit(zT)values.In this work,Ni-doped β-Cu2Se thermoelectric films were prepared on high pure intrin-sic single Si(100)substrate with a thickness of 0.5 mm using vacuum powder-sintered Cu-Se alloy target by the high vacuum magne-tron sputtering.The crystalline structure,phase composition,element content and distribution,existence form and valence state of Cu,Se and Ni element,surface and cross-sectional morphology of the deposited films were studied and observed.The room tempera-ture carrier concentration(n)and carrier mobility(μ)were calculated by Hall experiments and the resistivity and Seebeck coefficient of the deposited Cu-Se thin films were measured.Results showed that the deposited Cu-Se thin films were composed of only a single β-Cu2Se phase,and β-Cu2Se films possessed a highly(111)referred orientation and the level of the(111)referred orientation of the films increased with increasing Ni content.The doping Ni substituted Cu in β-Cu2Se lattice in deposited thin films and a β-(Cu,Ni)2Se solid solution formed instead of forming metal Ni phase or Ni compound.The X-ray photoelectron spectroscopy(XPS)of Cu 2p and Se 3d proved Cu+and Se2-valence in deposited Ni-doped β-Cu2Se films.The binding energy of 853.35 and 870.61 eV of Ni 2p3/2 and Ni 2p1/2 proved NiSe bond or NiSe2 bond valence,namely Ni2+valence.This also indicated that Ni occupied the position of Cu atoms inβ-Cu2Se lattice and β-(Cu,Ni)2Se solid solution existed with no secondary phase.All deposited films showed a columnar grain growth along the direction perpendicular to substrate due to a large number of nuclei on substrate and poor diffusion ability of deposited atoms at lower temperature(not heating substrate).The columnar crystalline size became large along growth direction due to the aggregation and annexation of growth grain.The positive Hall coefficient and Seebeck coefficient indicated that the deposited Ni-doped β-Cu2Se films possessed p-type conductivity characteristics.The carrier concentration increased and mobility decreased with the increase of Ni content in deposited Ni-doped β-Cu2Se films,respectively.Ni,which is generally Ni2+,replace Cu+in β-Cu2Se lattice,providing more electrons that annihilate some of the holes generated by Cu+vacancy in lattice.The carrier concentration of the films decreased with in-creasing Ni contents.In the studied temperature range,the resistivity of the undoped Ni films exhibited a trend of first decreasing and then increasing with increasing measured temperature,with a turning change near 60 ℃.It was attributed to the transformation of a small amount of α-Cu2Se phase to β-Cu2Se phase.The resistivity of Ni-doped β-Cu2Se films monotonically increased with increasing measured temperature due to phonon scattering.In the whole measured temperature,the resistivity of Ni-doped β-Cu2Se films was low-er than that of the undoped films and the resistivity decreased with increasing the amount of Ni doping.Seebeck coefficients of all the films increased with increasing measured temperature.In the whole measured temperature,Seebeck coefficients of Ni-doped β-Cu2Se films were lower than those of undoped films and Seebeck coefficient decreased with increasing Ni doping.The power factor of all films increased with increasing measured temperature.The power factor showed a tendency of first increased and then decreased with in-creasing Ni content.The deposited thin film with 1.80％(atom fraction)doping Ni amount possessed the highest power factor due to its appropriate resistivity and Seebeck coefficient value,which was higher than that of the undoped deposited thin films.The power factor of β-Cu2Se film could be improved by an appropriate Ni doping amount.
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