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Jeffrey G Snyder - One of the best experts on this subject based on the ideXlab platform.
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high thermoelectric performance in bi0 25sb0 75 2te3 due to band convergence and improved by Carrier Concentration control
Materials Today, 2017Co-Authors: Hyunsik Kim, Nicholas A Heinz, Zachary M Gibbs, Yinglu Tang, Stephen Dongmin Kang, Jeffrey G SnyderAbstract:Bi_2Te_3 has been recognized as an important cooling material for thermoelectric applications. Yet its thermoelectric performance could still be improved. Here we propose a band engineering strategy by optimizing the converging valence bands of Bi_2Te_3 and Sb_2Te_3 in the (Bi_(1−x)Sb_x)_2Te_3 system when x = 0.75. Band convergence successfully explains the sharp increase in density-of-states effective mass yet relatively constant mobility and optical band gap measurement. This band convergence picture guides the Carrier Concentration tuning for optimum thermoelectric performance. To synthesize homogeneous textured and optimally doped (Bi0.25Sb0.75)2Te3, excess Te was chosen as the dopant. Uniform control of the optimized thermoelectric composition was achieved by zone-melting which utilizes separate solidus and liquidus compositions to obtain zT = 1.05 (at 300 K) without nanostructuring.
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optimum Carrier Concentration in n type pbte thermoelectrics
Advanced Energy Materials, 2014Co-Authors: Yanzhong Pei, Zachary M Gibbs, A Gloskovskii, Benjamin Balke, Wolfgang G Zeier, Jeffrey G SnyderAbstract:Taking La- and I-doped PbTe as an example, the current work shows the effects of optimizing the thermoelectric figure of merit, zT, by controlling the doping level. The high doping effectiveness allows the Carrier Concentration to be precisely designed and prepared to control the Fermi level. In addition to the Fermi energy tuning, La-doping modifies the conduction band, leading to an increase in the density of states effective mass that is confirmed by transport, infrared reflectance and hard X-ray photoelectron spectroscopy measurements. Taking such a band structure modification effect into account, the electrical transport properties can then be well-described by a self-consistent single non-parabolic Kane band model that yields an approximate (m*T)^(1.5) dependence of the optimal Carrier Concentration for a peak power factor in both doping cases. Such a simple temperature dependence also provides an effective approximation of Carrier Concentration for a peak zT and helps to explain, the effects of other strategies such as lowering the lattice thermal conductivity by nanostructuring or alloying in n-PbTe, which demonstrates a practical guide for fully optimizing thermoelectric materials in the entire temperature range. The principles used here should be equally applicable to other thermoelectric materials.
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optimization of the Carrier Concentration in phase separated half heusler compounds
Journal of Materials Chemistry, 2014Co-Authors: Julia Krez, Jeffrey G Snyder, Jennifer Schmitt, Claudia Felser, Wilfried Hermes, Markus SchwindAbstract:Inspired by the promising thermoelectric properties of phase-separated half-Heusler materials, we investigated the influence of electron doping in the n-type Ti0.3−xZr0.35Hf0.35NiSn compound. The addition of Nb to this compound led to a significant increase in its electrical conductivity, and shifted the maximum Seebeck coefficient to higher temperatures owing to the suppression of intrinsic Carriers. This resulted in an enhancement of both the power factor α2σ and figure of merit, zT. The applicability of an average effective mass model revealed the optimized electron properties for samples containing Nb. There is evidence in the literature that the average effective mass model is suitable for estimating the optimized Carrier Concentration of thermoelectric n-type half-Heusler compounds.
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stabilizing the optimal Carrier Concentration for high thermoelectric efficiency
Advanced Materials, 2011Co-Authors: Yanzhong Pei, Nicholas A Heinz, Aaron D Lalonde, Xiaoya Shi, Shiho Iwanaga, Heng Wang, Lidong Chen, Jeffrey G SnyderAbstract:The band structure of PbTe can be manipulated by alloying with MgTe to control the band degeneracy. This is used to stabilize the optimal Carrier Concentration, making it less temperature dependent, demonstrating a new strategy to improve overall thermoelectric efficiency over a broad temperature range.
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self tuning the Carrier Concentration of pbte ag2te composites with excess ag for high thermoelectric performance
Advanced Energy Materials, 2011Co-Authors: Yanzhong Pei, Andrew F May, Jeffrey G SnyderAbstract:Thermoelectric materials can be optimized by tuning the Carrier Concentration with chemical doping. However, because the optimum dopant Concentration typically increases with temperature, the optimum efficiency can not normally be achieved for a uniform material in a temperature gradient. Here, we show Ag-doped PbTe/Ag_2Te composites exhibit high thermoelectric performance (∼50% greater than La-doped composites) because of a temperature induced gradient in the doping Concentration caused by the temperature-dependent solubility of Ag in the PbTe matrix. This demonstrates a new mechanism to achieve a higher thermoelectric efficiency afforded by a given material system, and should be applicable to other thermoelectric materials.
Jianlin Liu - One of the best experts on this subject based on the ideXlab platform.
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homobuffer thickness effect on the background electron Carrier Concentration of epitaxial zno thin films
Journal of Applied Physics, 2010Co-Authors: Zheng Yang, Huimei Zhou, Winnie V Chen, Jianze Zhao, Jianlin LiuAbstract:Epitaxial ZnO thin films were grown on r-plane sapphire substrates using plasma-assisted molecular-beam epitaxy. ZnO homobuffer layers grown at a lower temperature were introduced to improve the crystallinity of the top ZnO thin films. Thicker homobuffer layers lead to better crystallinity of the subsequent epitaxial ZnO thin films due to the strain relaxation effect. Residual background electron Carrier Concentration in these undoped ZnO thin films first decreases, then increases as the buffer layer thickness increases from ∼1 to 30 nm, with a minimum electron Concentration of ∼1×1017 cm−3 occurring in ZnO homobuffer of ∼5 nm. These results demonstrate that the optimized ZnO homobuffer thickness to achieve both good ZnO crystallinity and low residual electron Concentration is determined by the relative electron Carrier Concentration ratios and mobility ratios between the buffer and epi-ZnO layers.
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electron Carrier Concentration dependent magnetization and transport properties in zno co diluted magnetic semiconductor thin films
Journal of Applied Physics, 2008Co-Authors: Zheng Yang, M Biasini, W P Beyermann, Michael B Katz, Obiefune K Ezekoye, Xiaoqing Pan, Jing Shi, Zheng Zuo, Jianlin LiuAbstract:Diluted magnetic semiconducting ZnO:Co thin films with above room-temperature TC were prepared. Transmission electron microscopy and x-ray diffraction studies indicate the ZnO:Co thin films are free of secondary phases. The magnetization of the ZnO:Co thin films shows a free electron Carrier Concentration dependence, which increases dramatically when the free electron Carrier Concentration exceeds ∼1019 cm−3, indicating a Carrier-mediated mechanism for ferromagnetism. The anomalous Hall effect is observed in the ZnO:Co thin films. The anomalous Hall coefficient and its dependence on longitudinal resistivity were analyzed. The presence of a side-jump contribution further supports an intrinsic origin for ferromagnetism in ZnO:Co thin films. These observations together with the magnetic anisotropy and magnetoresistance results support an intrinsic Carrier-mediated mechanism for ferromagnetic exchange in ZnO:Co diluted magnetic semiconductor materials.
Daniel D. Koleske - One of the best experts on this subject based on the ideXlab platform.
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method for determining the radiative efficiency of gainn quantum wells based on the width of efficiency versus Carrier Concentration curve
Applied Physics Letters, 2012Co-Authors: Qifeng Shan, Andrew J Birkel, Karl R Westlake, Mary H Crawford, Daniel D. KoleskeAbstract:We report a method to determine the radiative efficiency (RE) of a semiconductor by using room-temperature excitation-dependent photoluminescence measurements. Using the ABC model for describing the recombination of Carriers, we show that the theoretical width of the RE-versus-Carrier-Concentration (n) curve is related to the peak RE. Since the normalized external quantum efficiency, EQEnormalized, is proportional to the RE, and the square root of the light-output power, LOP, is proportional to n, the experimentally determined width of the EQEnormalized-versus-n curve can be used to determine the RE. We demonstrate a peak RE of 91% for a Ga0.85In0.15N quantum well.
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Internal quantum efficiency in light-emitting diodes based on the width of efficiency-versus-Carrier-Concentration curve
2012 Conference on Lasers and Electro-Optics (CLEO), 2012Co-Authors: Q. Shan, Andrew J Birkel, Daniel D. Koleske, E F Schubert, M. H. CrawfordAbstract:Based on the ABC model, we derive a relationship between the internal quantum efficiency (IQE) and the width of the efficiency-versus-Carrier-Concentration curve. We determine the IQE of LEDs at temperatures ranging from 100K to 300K.
Shinichi Kuroda - One of the best experts on this subject based on the ideXlab platform.
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electron spin resonance observation of charge Carrier Concentration in organic field effect transistors during device operation
Physical Review B, 2013Co-Authors: Hisaaki Tanaka, Masataka Hirate, Shun Ichiro Watanabe, Kazuaki Kaneko, Kazuhiro Marumoto, Taishi Takenobu, Yoshihiro Iwasa, Shinichi KurodaAbstract:Charge Carrier Concentration in operating organic field-effect transistors (OFETs) reflects the electric potential within the channel, acting as a key quantity to clarify the operation mechanism of the device. Here, we demonstrate a direct determination of charge Carrier Concentration in the operating devices of pentacene and poly(3-hexylthiophene) (P3HT) by field-induced electron spin resonance (FI-ESR) spectroscopy. This method sensitively detects polarons induced by applying gate voltage, giving a clear FI-ESR signal around $g=2.003$ in both devices. Upon applying drain-source voltage, Carrier Concentration decreases monotonically in the FET linear region, reaching about 70$%$ of the initial value at the pinch-off point, and stayed constant in the saturation region. The observed results are reproduced well from the theoretical potential profile based on the gradual channel model. In particular, the Carrier Concentration at the pinch-off point is calculated to be $\ensuremath{\beta}/(\ensuremath{\beta}+1)$ of the initial value, where \ensuremath{\beta} is the power exponent in the gate voltage (${V}_{\mathrm{gs}}$) dependence of the mobility (\ensuremath{\mu}), expressed as $\ensuremath{\mu}\ensuremath{\propto}{V}_{\mathrm{gs}}^{\ensuremath{\beta}\ensuremath{-}2}$, providing detailed information of charge transport. The present devices show $\ensuremath{\beta}=2.6$ for the pentacene and $\ensuremath{\beta}=2.3$ for the P3HT cases, consistent with those determined by transfer characteristics. The gate voltage dependence of the mobility, originating from the charge trapping at the device interface, is confirmed microscopically by the motional narrowing of the FI-ESR spectra.
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direct observation of the charge Carrier Concentration in organic field effect transistors by electron spin resonance
Applied Physics Letters, 2009Co-Authors: Hisaaki Tanaka, Shun Ichiro Watanabe, Kazuhiro Marumoto, Hiroshi Ito, Shinichi KurodaAbstract:Charge Carrier Concentration in operating field-effect transistor (FET) of regioregular poly(3-hexylthiophene) has been directly determined by electron spin resonance (ESR). ESR signals of field-induced polarons are observed around g=2.003 under the application of negative gate-source voltage (Vgs). Upon applying drain-source voltage (Vds), ESR intensity decreases linearly in the low Vds region, reaching to about 50% of the initial intensity at the pinch-off point (Vds≅Vgs). For larger absolute values of Vds, it becomes nearly Vds independent. These behaviors are well explained by the change in the Carrier Concentration obtained by the FET theory using gradual channel approximation.
Yanzhong Pei - One of the best experts on this subject based on the ideXlab platform.
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optimum Carrier Concentration in n type pbte thermoelectrics
Advanced Energy Materials, 2014Co-Authors: Yanzhong Pei, Zachary M Gibbs, A Gloskovskii, Benjamin Balke, Wolfgang G Zeier, Jeffrey G SnyderAbstract:Taking La- and I-doped PbTe as an example, the current work shows the effects of optimizing the thermoelectric figure of merit, zT, by controlling the doping level. The high doping effectiveness allows the Carrier Concentration to be precisely designed and prepared to control the Fermi level. In addition to the Fermi energy tuning, La-doping modifies the conduction band, leading to an increase in the density of states effective mass that is confirmed by transport, infrared reflectance and hard X-ray photoelectron spectroscopy measurements. Taking such a band structure modification effect into account, the electrical transport properties can then be well-described by a self-consistent single non-parabolic Kane band model that yields an approximate (m*T)^(1.5) dependence of the optimal Carrier Concentration for a peak power factor in both doping cases. Such a simple temperature dependence also provides an effective approximation of Carrier Concentration for a peak zT and helps to explain, the effects of other strategies such as lowering the lattice thermal conductivity by nanostructuring or alloying in n-PbTe, which demonstrates a practical guide for fully optimizing thermoelectric materials in the entire temperature range. The principles used here should be equally applicable to other thermoelectric materials.
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stabilizing the optimal Carrier Concentration for high thermoelectric efficiency
Advanced Materials, 2011Co-Authors: Yanzhong Pei, Nicholas A Heinz, Aaron D Lalonde, Xiaoya Shi, Shiho Iwanaga, Heng Wang, Lidong Chen, Jeffrey G SnyderAbstract:The band structure of PbTe can be manipulated by alloying with MgTe to control the band degeneracy. This is used to stabilize the optimal Carrier Concentration, making it less temperature dependent, demonstrating a new strategy to improve overall thermoelectric efficiency over a broad temperature range.
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self tuning the Carrier Concentration of pbte ag2te composites with excess ag for high thermoelectric performance
Advanced Energy Materials, 2011Co-Authors: Yanzhong Pei, Andrew F May, Jeffrey G SnyderAbstract:Thermoelectric materials can be optimized by tuning the Carrier Concentration with chemical doping. However, because the optimum dopant Concentration typically increases with temperature, the optimum efficiency can not normally be achieved for a uniform material in a temperature gradient. Here, we show Ag-doped PbTe/Ag_2Te composites exhibit high thermoelectric performance (∼50% greater than La-doped composites) because of a temperature induced gradient in the doping Concentration caused by the temperature-dependent solubility of Ag in the PbTe matrix. This demonstrates a new mechanism to achieve a higher thermoelectric efficiency afforded by a given material system, and should be applicable to other thermoelectric materials.