Male James, Agne Matthias T, Goyal Anuj, Anand Shashwat, Witting Ian T, Stevanović Vladan, Snyder G Jeffrey
Department of Materials Science & Engineering, Northwestern University, Evanston, IL, USA.
Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO, USA.
Mater Horiz. 2019 Aug 12;6(7):1444-1453. doi: 10.1039/c9mh00294d.
Semiconductor engineering relies heavily on doping efficiency and dopability. Low doping efficiency may cause low mobility and failure to reach target carrier concentrations or even the desired carrier type. Semiconducting thermoelectric materials perform best with degenerate carrier concentrations, meaning high performance in new materials might not be realized experimentally without a route to optimal doping. Doping in the classic PbTe thermoelectric system has been largely successful but reported doping efficiencies can vary, raising concerns about reproducibility. Here, we stress the importance of phase equilibria considerations during synthesis to avoid undesired intrinsic defects leading to sub-optimal doping. By saturation annealing at 973 K, we decidedly fix the composition in single crystal iodine-doped PbTe samples to be Pb-rich or Te-rich without introducing impurity phases. We show that, regardless of iodine concentration, degenerate n-type carrier concentrations with ideal doping efficiency require Pb-rich compositions. Electrons in Te-rich samples are heavily compensated by charged intrinsic Pb vacancy defects. From Hall effect measurements and a simple defect model supported by modern defect calculations, we map out the 973 K ternary Pb-Te-I phase diagram to explicitly link carrier concentration and composition. Furthermore, we discuss unintentional composition changes due to loss of volatile Te during synthesis and measurements. The methods and concepts applied here may guide doping studies on other lead chalcogenide systems as well as any doped, complex semiconductor.
半导体工程在很大程度上依赖于掺杂效率和可掺杂性。低掺杂效率可能导致迁移率降低，无法达到目标载流子浓度，甚至无法获得所需的载流子类型。半导体热电材料在简并载流子浓度下性能最佳，这意味着如果没有实现最佳掺杂的途径，新材料的高性能可能无法通过实验实现。在经典的PbTe热电系统中，掺杂在很大程度上是成功的，但报道的掺杂效率可能会有所不同，这引发了对可重复性的担忧。在这里，我们强调在合成过程中考虑相平衡的重要性，以避免导致次优掺杂的不期望的本征缺陷。通过在973 K下进行饱和退火，我们确定地将单晶碘掺杂PbTe样品中的成分固定为富Pb或富Te，而不引入杂质相。我们表明，无论碘浓度如何，具有理想掺杂效率的简并n型载流子浓度都需要富Pb成分。富Te样品中的电子被带电的本征Pb空位缺陷大量补偿。通过霍尔效应测量和现代缺陷计算支持的简单缺陷模型，我们绘制了973 K时的三元Pb-Te-I相图，以明确将载流子浓度与成分联系起来。此外，我们还讨论了在合成和测量过程中由于挥发性Te的损失而导致的意外成分变化。这里应用的方法和概念可能会指导对其他铅硫族化物系统以及任何掺杂的复杂半导体的掺杂研究。
