含硫宽禁带Ga2Te3基热电半导体的声电输运特性

目前对宽禁带半导体热电材料的研究开始升温, 原因是本征情况下宽禁带半导体往往具有低的热导率和高的Seebeck系数. Ga₂Te₃ 是一类带有缺陷的宽禁带半导体, 其在临界温度680± 10 K和757± 10 K处会参与共析转变和包晶反应, 因此会产生反应热. 本次工作采用少量的S元素等电子替换Ga₂Te₃中的Te元素, 观察到在临界温度附近热焓的变化, 但没有相变发生. 受热焓变化的影响这类材料在临界温度附近出现了较活跃的声电输运行为, 具体表现为热容和Seebeck系数(α)明显增大及热扩散系数(热导率)和电导率下降. 例, 对于x=0.05的材料, 其α值从596 K 时的376.3(μV·K^-1)迅速增大到695 K时的608.2(μV·K^-1), 然后又随温度升高到764 K时迅速降低到213.8(μV·K^-1). 在596 K到812 K范围, Seebeck系数和电导率几乎随温度均呈Z字形变化. 这些输运行为的变化揭示了在Ga₂Te₃基半导体中声子和载流子的临界散射特点, 这种临界散射特征对以后的继续研究具有重要的参考价值.

Acoustic charge transport behaviors of sulfur-doped wide gap Ga2Te3-based semiconductors

Wide gap semiconductors as the thermoelectric (TE) candidates have been increasingly interested because of their inherent high Seebeck coefficients and low thermal conductivities. Ga₂Te₃ is one of the typical defect compounds (E_g=1.65 eV) among the A₂^IIIB₃^VI type semiconductors, in which there are periodically self-assembled 2D vacancy planes that wrap the nanostructured domains. The vacancy planes scatter phonons highly effectively and are responsible for reducing the lattice thermal conductivity. Hence Ga₂Te₃ might be a good TE candidate. In the phase diagram of Ga-Te, Ga₂Te₃ is involved in the eutectoid and peritectic reactions at the critical temperatures (CTs) of 680± 10 K and 757± 10 K respectively. These reactions would lead to the generation of enthalpies of reactions, and induce the alteration of some thermo-physical properties. In the present work, we have not observed the phase transformations at CTs in the Ga₂Te₃-based materials with sulfur isoelectronic substitution for Te, which are prepared by powder metallurgy with the spark plasma sintering (SPS) technique, but can observe the generation of assumed enthalpies of reactions near CTs, which directly gives rise to the critical acoustic charge transport behaviors. The critical behaviors involve the remarkable increase of heat capacities and Seebeck coefficients and, at the same time, reductions of thermal diffusivities (thermal conductivities) and electrical conductivities. For example, the Seebeck coefficient (α) at x=0.05 increases rapidly from 376.3(μV·K^-1) to 608.2(μV·K^-1) when the temperature rises from 596 to 695 K, and then decreases to 213.8(μV·K^-1) at 764 K. Similarly, all the S-doped samples, which have lowest electrical conductivities (σ ) of 2.12×10² (x=0.05), 0.25×10² (x=0.1), 0.12×10² Ω ^-1·m^-1 (x=0.2) and 0.14×10² Ω ^-1·m^-1 (x=0.3) at 696–725 K, undergo dramatic changes when the temperature rises to about 750 K, and then the electrical conductivities begin to decrease, and the changes tend to slow down. It is notable that both the Seebeck coefficients and electrical conductivities exhibit a typical zigzag temperature dependence in the temperature range from 596 to 812 K. These behaviors reveal the remarkable alterations in scattering mechanism of both phonons and carriers at temperatures near the CTs. Although the materials with these critical behaviors near CTs do not have satisfactory thermoelectric performance (ZT_max=0.17 at 793 K for x=0.3) as compared with the known binary Cu₂Se, Ag₂Se(S) or ternary based AgCrSe₂ alloys, however, the findings of such critical transport behaviors have a great significance for future researches.