NER: Enhancement of the Efficiency of Thermoelectric Devices via Engineering of the Electron-Phonon Interaction in Quantum Dot Superlattices

该提案是根据美国国家科学与工程倡议（NSF 01-157）的NER类项目招标收到的。提出了一种通过调整半导体量子点多阵列中的电子-声子相互作用来提高半导体材料热电优值ZT的新方法。量子点阵列是热电“电子传输-声子阻挡”材料的潜在候选材料。同时，为了实现显著的ZT改进并与传统热电相竞争，必须设计和制造量子点超晶格，其中通过形成载流子微带促进载流子输运。事实上，随机量子点阵列中横向载流子传输的典型跳变电导率具有非常低的载流子迁移率，而微带传输可能导致非常高的迁移率值，特别是如果通过智能微带工程设法至少部分地抑制载流子散射。本文提出的重要任务是在量子点超晶格中实现小带输运和载流子散射抑制的理论概念验证研究。由于空间约束和边界散射导致量子点阵列中声子模式的改变，导致声子态密度的变化，声子群速度的降低，相应的面内晶格导热系数下降。与载流子约束效应相比，声子约束效应的研究较少，如纳米结构边界对声子色散的影响。同时，它可以作为降低晶格导热系数值和提高热电器件效率的附加工具。因此，本项目的另一项任务是研究所需的结构参数，例如点大小，形状，声学失配等，以实现理想的声子传输特征，如沿感兴趣方向的低群速度；共振声子散射条件；声子浴与电子的解耦；等。基于纳米结构材料的新型高效热电器件由于其高可靠性、轻量化、紧凑性、易于操作和环保等固有优势，将对广泛的能源需求产生巨大影响。

This proposal was received in response to the National Science and Engineering Initiative, Program Solicitation NSF 01-157, in the NER category. The proposal focuses on a new approache to enhancement of the thermoelectric figure of merit ZT of semiconductor materials via tuning of electron-phonon interaction in multiple arrays of semiconductor quantum dots. Quantum dot arrays are potential candidates for the thermoelectric "electron transmitting - phonon blocking" material. At the same time, in order to achieve a significant ZT improvement and compete with conventional thermoelectrics one has to design and fabricate quantum dot superlattice where carrier transport is facilitated by the formation of carrier mini-bands. Indeed, hopping conductivity typical for lateral carrier transport in random quantum dot arrays has very low carrier mobility while mini-band transport may lead to very high mobility values, particularly if one manages to suppress at least partially the carrier scattering via smart mini-band engineering. The important task addressed in this proposal is theoretical proof-of-concept investigation of requirements for achieving mini-band transport regime and carrier scattering suppression in quantum dot superlattices.Modification of acoustic phonon modes in quantum dot arrays due to spatial confinement and boundary scattering leads to a change in the phonon density of states, a decrease of the phonon group velocity, and corresponding drop of the in-plane lattice thermal conductivity. Acoustic phonon confinement, e.g. modification of phonon dispersion due to nanostructure boundaries, is much less researched phenomenon than carrier confinement effect. At the same time, it can serve as an additional tool to decrease the lattice thermal conductivity value and increase the effectiveness of the thermoelectric device. Thus, another task of this project is a study of the required structure parameters, e.g. dot size, shape, acoustic mismatch, etc., for achieving desirable acoustic phonon transport features such as low group velocity along direction of interest; resonant phonon scattering conditions; decoupling of phonon bath from electrons; etc. New efficient thermoelectric devices based on nanostructured materials may have a tremendous impact on a wide range of energy needs due to their inherent advantages such as high reliability, light weight, compactness, quit operation, and environmental safety.