Study On Thermoelectric Properties Of Sn?Ag?-doped Ge₂Sb₂Te₅ Phase-change Memory Thin Films

The chalcogenide layered structure compound Ge₂Sb₂Te₅?GST?has been widely studied as a phase change memory material.The results show that GST has low lattice heat conduction,small band gap,and large effective mass,and thus,has potential as a thermoelectric material.Large carrier concentration limits the increase in thermoelectric efficiency.Therefore,this paper improves the thermoelectric properties of GST films by element doping.In this thesis,the element-doped GST thin films were prepared by magnetron sputtering.The element content,surface morphology,phase structure,chemical state and band gap were studied.The thermoelectric properties were measured.The main conclusions are as follows:?1?The elements in the Sn-doped film are evenly distributed and the film is intact.The crystalline film shows Ge2Sb2Te5 phase of the FCC structure,and the lattice constant increases with the increase of Sn content.There are mainly Ge-Te,Sb-Te and Sn-Te chemical bonds.It's inferred that the Sn element enters the GST film vacancy or replaces Ge by the results of chemical bonds and the percentage of elements.The band gap width of the amorphous Sn-GST film decreases with the increase of Sn content,mainly because the Sn content increases,the band structure local state increases,and the band gap decreases.The band gap width of the crystalline Sn-GST film decreases with the increase of the Sn content.This is due to the multi-body effect between hole carriers that causes the band structure to converge.The forbidden band width of the same doped power amorphous film is larger than the forbidden band width of the crystalline state,which is mainly because after the annealing,the hole concentration is increased,the impurity defect level and the valence band distance are reduced,and the forbidden band width is reduced.?2?The resistivity of the as-deposited sample shows a decreasing trend with the increase of the amount of Sn doping,mainly due to the participation of conductive Sn²⁺.The resistivity of the crystalline sample decreases as the amount of Sn doping increases,mainly because the concentration of the impurity defect increases and the hole concentration increases.The 20W sample resistivity first decreases and then increases.This is because the Sn doping content is lower,and the hole concentration increases with the increase of temperature,which is less likely to offset the decrease of mobility.The Seebeck coefficient of different Sn target power doped GST films changes with temperature,showing similar changes with resistivity.In the amorphous state,the power factor obtained by different doping powers does not exhibit a strong regularity.In the crystalline state,it is concluded that at the same doping power,the power factor increases with increasing temperature and at different doping power,the power factor increases as the doping power increases.When the doping power is 50W and the temperature is 723K,the maximum power factor 1.88mW/K²*m is obtained,which is better than the thermoelectric properties of the GST film tested in this test and the intrinsic GST bulk and the film reported in the literature.It has exceeded the minimum power factor indicator for industrial production.?3?The elements in the Ag-doped film are evenly distributed,and the film is intact.The crystalline sample shows a mixed phase of Ge₂Sb₂Te₅ and Ag₈GeTe₆ of the FCC structure.The Ge-Te,Sb-Te and Ag-Te chemical bonds are mainly present in the Ag-Ge₂Sb₂Te₅ film,indicating that the Ag element replaces the Ge₂Sb₂Te₅ film vacancy or Ge/Sb.In the amorphous state,the band gap width decreases as the Ag content increases.This is because the Ag content increases,the band structure local state increases,and the band gap decreases.In the crystalline state,the band gap width increases with the increase of Ag content,which is mainly affected by the interaction of B-M drift and multi-body effect.At the power of 12W,the regularity may be lost due to excessive precipitation of the second phase.?4?The resistivity of GST film is smaller than that of Ag-GST film,and the change trend and value of resistivity of different Ag-GST films are close to each other.This is because Ag is a deep-level impurity and does not affect its hole concentration.However,due to the increase of defects caused by impurities,the crystal structure is impaired,thus increasing the scattering effect of the lattice on carriers.This results in a decrease in mobility,and therefore,at the same temperature,the resistivity of the Ag-GST film is larger than that of the GST film.The Seebeck coefficient of GST film is smaller than the Seebeck coefficient of different Ag-GST films.The change trend is similar to the resistivity data.The Seebeck coefficient of Ag power of 6W,8W and 10W is similar.At 12W,the Seebeck coefficient is smaller than that of the other three kinds of power.Due to the small change in hole carrier concentration,there must have differences in the effective mass of hole carriers.The power factor of Ag-GST thin film in crystalline state is larger than that of GST thin film,indicating that Ag element doping can optimize the thermoelectric properties of GST thin film.The power factor of 8W in Ag power is at the maximum value,and has exceeded the lowest power factor index of industrial production.Therefore,it can be gotten that the optimum power of Ag-doped GST film is 8W,and the maximum power factor is 0.692 mW/K2*m at temperature 723K.The resistivity and Seebeck coefficient are 104.7??*m and 319.77?V/K,respectively.