半導體Cu3(Sb1-xMx)Se4, M= Ti, Sn, Pb, Ge的摻雜效應對熱電性質的影響

Abstract

銅銻硒礦是具有 0.3 eV狹窄能帶間隙的P型半導體，且已被發現是在中溫區下極具潛力的熱電材料。銅銻硒礦的晶體結構具有三維銅硒子框架可提供導電的電洞，而有較高的功率因子900 μW/mK2。銻硒四面體結構可藉由其他元素取代銻的位置，扭曲其鑽石結構以達到提高功率因子以及降低熱傳導的目的。理論預測可藉由 IV 族元素鍺、錫、鉛和過渡金屬鈦等元素取代銻來提供電洞載子。本研究藉由燒結與電漿放電製備樣品，探討鈦、錫、鉛、鍺取代銻的熱電效應。\r\n在上述之元素取代效應後，鈦與鉛並沒有帶來顯卓的熱電效應提升，反之錫與鍺能有效地提升電洞載子濃度，然而與摻錫的研究相似的結果已被其他團隊發表，惟鍺的取代效應則尚未被做完整的探討。2 % 鍺的取代有1200μW/mK2的功率因子，相較於母材(900μW/mK2)有 30 % 的提升，因此我們會對鍺的取代效應做完整一系列的研究。摻雜比例從 1~8 % 的結果裡，發現晶格熱傳導係數隨摻雜比例提升減少的合金效應，然而高於 6 % 的取代造成電導大幅提升，使得熱傳導的載子貢獻高於 50% 並嚴重降低載子移動率，致使功率因子大幅衰減與優質係數降低。 4% 的鍺摻雜在提高功率因子與降低熱傳導係數上皆有顯卓的表現，使得優質係數在溫度650 K達到 0.7 相對於母材 (0.54) 有30 ％的提升。

Cu3SbSe4 is a p-type semiconductor with a narrow band gap near 0.3 eV, and has been found to be a promising thermoelectric material at medium temperatures. The crystal structure of Cu3SbSe4 consists a three-dimensional [Cu3Se4] framework acting as electron hole conduction pathway which cause high power factor near 900 μW/mK2. The inserting guest atom to the Sb site of tetrahedral [SbSe4] framework cause a more distorted diamond-like structure, thus providing a relatively lower lattice thermal conductivity in relatively large electric conductivity. According to theoretical predication which are based on the defect formation energy and band structure calculations, p-type doping can be achieved by substituting Sb with group IV elements, as Ge, Sn, and Pb, and transition metals as Ti. This study is investigation of the doping effect in Cu3SbSe4 semiconductor which are prepared by melting and spark plasma sintering. \r\nHerein, we take a close look at the thermoelectric properties of Cu3SbSe4 which are mentioned in previous paragraph. No significant change in results of Ti and Pb. Carrier concentrations are dramatic increasing in results of Sn and Ge, but the results of Sn substitution were already reported by another group. Power factor of Ge substitution is 1,200μW/mK2 which is 30 % more than raw material. We did more study in germanium doping series because it have high power factor which did not be investigated in Cu3SbSe4. Alloy effects, as description of lattice thermal conductivity reducing with doping fraction increasing, are explored in Ge doping fraction from 1 % to 8 %. Although electric conductivity were largely enhanced, figure of merit were reducing by electric contribution of thermal conductivity were higher than 50 % and carrier mobility were significantly reducing when the doping fraction were higher than 4 %. Doping fraction in 4 % have relatively high power factor and relatively low thermal conductivity. Figure of merit in 4 % doping fraction is 0.7, as 30% more than 0.5 of raw material.