Enhanced thermoelectric performance enabled by compositing ZrO_(2)in n-type SiGe alloy with low thermal conductivity

SiGe-based thermoelectric(TE)materials have gained increasing interests due to their low maintenance costs,environmental friendliness and long lifespan.However,the intrinsically high thermal conductivity of Si-based materials also results in poor TE properties.In this investigation,a zirconia(ZrO_(2))composite strategy was applied to an n-type SiGe alloy,tremendously elevating its TE performance.After mechanical alloying and spark plasma sintering(SPS)processes,the ZrO_(2)induced the formation of nanopores in the SiGe matrix via phosphorus adsorption.Moreover,such increase in porosity enhanced the phonon scattering and dramatically suppressed lattice thermal conductivity,from 2.83 to 1.59 W·m^(-1)·K^(-1)at 873 K.Additionally,reduced phosphorus doping led to an increase in Seebeck coefficients and a relatively minor decrease in electrical conductivity,The power factor didn't deteriorate significantly,either,as its maximum of~3.43 mW·m^(-1-)K^(-2)was achieved at 873 K with(Si_(0.8)Ge_(0.2))_(0.097)P_(0.03)(ZrO_(2))_(0.003).In short,a peak figure of merit(ZT)of~1.27 at 873 K and an average ZT~0.7 from 323 to 873 K were obtained.This study demonstrates that the electrical and thermal transportation of SiGe material can be synergistically tuned by compositing ZrO_(2),illustrating a novel strategy to optimize the TE properties of bulk materials.

Gas Source Molecular Beam Epitaxy Growth of Si_(1-x)Ge_x/Si Alloys

Strained SiGe alloys are successfully grown on Si(100) substrate in gas source molecular beam epitaxy system using disilane (Si 2H 6) and germane (GeH 4). The surface reconstructions are monitored by in situ reflection high energy electron diffraction during epitaxy. At a fixed substrate temperature, the Ge composition in the alloys increase with GeH 4/(GeH 4+2Si 2H 6) gas flow rate ratio. Surface morphologies of the grown samples show a strong dependence on the Ge composition, substrate temperature and SiGe epilayer thickness. Results indicate that high substrate temperature and large Ge composition are favorable for the growth mode transition from two dimensional to three dimensional growth.

Electronic structure of O-doped SiGe calculated by DFT+U method

To more in depth understand the doping effects of oxygen on SiGe alloys, both the micro-structure and properties of O-doped SiGe （including： bulk, （001） surface, and （110） surface） are calculated by DPT ＋ U method in the present work. The calculated results are as follows. （i） The （110） surface is the main exposing surface of SiGe, in which O impurity prefers to occupy the surface vacancy sites. （ii） For O interstitial doping on SiGe （110） surface, the existences of energy states caused by 0 doping in the band gap not only enhance the infrared light absorption, but also improve the behaviors of photo-generated carriers. （iii） The finding about decreased surface work function of O-doped SiGe （110） surface can confirm previous experimental observations. （iv） In all cases, O doing mainly induces the electronic structures near the band gap to vary, but is not directly involved in these variations. Therefore, these findings in the present work not only can provide further explanation and analysis for the corresponding underlying mechanism for some of the experimental findings reported in the literature, but also conduce to the development of μc-SiGe-based solar ceils in the future.

Thermoelectric Properties of Czochralski GeSi Crystal

In order to discuss the application possibility of SiGe crystal in thermoelectric materials, we investigated the thermoelectric properties of several silicon-germanium alloys with different content, orientation and electric conductive type. As discussed in the experiment result, the absolute value of Seebeck coefficient fluctuates from 300 to 600 μV/K in the whole temperature range. In the present paper, the relationship of Seebeck coefficient against content, orientation and electric conductive type is summarized in detail. The Seebeck coefficient of the sample with 〈111〉 orientation is smaller than that in 〈100〉 at the same temperature. Absolute value of P-type is larger than that of N-type except pure Ge. But as the temperature increases, the absolute value of pure Ge decreases many times as quickly as that of other specimens. In addition, the specimens of bulk GeSi alloy crystals for experiment were grown by the Czoehralski method through varying the pulling rate during the growing process.