Department of Chemistry , University of Oregon , Eugene , Oregon 97403-1253 , United States.
Department of Mechanical Engineering and Texas Materials Institute , University of Texas at Austin , Austin , Texas 78712 , United States.
Nano Lett. 2018 Nov 14;18(11):6876-6881. doi: 10.1021/acs.nanolett.8b02744. Epub 2018 Oct 29.
We report cross-plane thermoelectric measurements of SnSe and SnSe films grown by the modulated element reactant (MER) approach. These materials exhibit ultralow cross-plane thermal conductivities, which are advantageous for thermoelectric energy conversion. The initially grown SnSe films have relatively low cross-plane Seebeck coefficients (-38.6 μV/K) due to significant unintentional doping originating from Se vacancies when annealed in nitrogen, as a result of the relatively high vapor pressure of Se. By performing postgrowth annealing at a fixed Se partial pressure (300 °C for 30 min using SnSe as the Se source in a sealed tube), a transition from SnSe to SnSe is induced, which is evidenced by clear changes in the X-ray diffraction patterns of the films. This results in a 16-fold increase in the cross-plane Seebeck coefficient (from -38.6 to -631 μV/K) after Se annealing due to both the SnSe-to-SnSe transition and the mitigation of unintentional doping by Se vacancies. We also observe a corresponding 6-fold drop in the electrical conductivity (from 3 to 0.5 S/m) after Se annealing, which is consistent with both a drop in the carrier concentration and an increase in band gap. The power factor Sσ increased by 44× (from 4.5 nW/m·K to 0.2 μW/m·K) after Se annealing. We believe that these results demonstrate a robust method for mitigating unintentional doping in a promising class of materials for thermoelectric applications.
我们报告了通过调制元素反应物 (MER) 方法生长的 SnSe 和 SnSe 薄膜的面内热输运测量结果。这些材料表现出超低的面内热导率，这对于热电能量转换是有利的。最初生长的 SnSe 薄膜由于在氮气中退火时存在大量源自 Se 空位的本征掺杂，具有相对较低的面内热Seebeck 系数 (-38.6 μV/K)，这是由于 Se 的蒸气压较高所致。通过在固定的 Se 分压下进行后生长退火（在密封管中使用 SnSe 作为 Se 源，在 300°C 下退火 30 分钟），可以诱导从 SnSe 到 SnSe 的转变，这可以通过薄膜的 X 射线衍射图案的明显变化得到证实。这导致 Se 退火后面内热 Seebeck 系数增加了 16 倍（从-38.6 到-631 μV/K），这是由于 SnSe 到 SnSe 的转变以及 Se 空位对本征掺杂的缓解所致。我们还观察到 Se 退火后电导率相应地降低了 6 倍（从 3 到 0.5 S/m），这与载流子浓度的降低和带隙的增加一致。Se 退火后功率因子 Sσ 增加了 44 倍（从 4.5 nW/m·K 增加到 0.2 μW/m·K）。我们相信这些结果证明了一种在用于热电应用的有前途的材料中缓解本征掺杂的稳健方法。
