Y. J. He

Tsinghua University, Peping, Beijing, China

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Publications (36)51.52 Total impact

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    ABSTRACT: A good thermoelectric material should have a high Seebeck coefficient, a low electrical resistivity, and a low thermal conductivity. For conventional thermoelectric materials, however, increasing the Seebeck coefficient also leads to a simultaneous increase in the electrical resistivity. In this paper, a method of layer-by-layer deposition of MnSi1.7 film with high Seebeck coefficient and low electrical resistivity is developed. After deposition of the first MnSi1.7 sub-layer, the deposition process is interrupted for several minutes, and then continues for another MnSi1.7 sub-layer. Therefore, the MnSi1.7 film contains two sub-layers for one interruption, three sub-layers for two interruptions, and so on. It is found that the n-type MnSi1.7 film with two sub-layers has a higher Seebeck coefficient, −0.451 mV K−1, and a lower electrical resistivity, 19.4 mOhm-cm, at 483 K as compared to that of without deposition interruption, −0.152 mV K−1 and 44.3 mOhm-cm. The p-type MnSi1.7 film with three sub-layers also has a higher Seebeck coefficient, 0.238 mV K−1, and a lower electrical resistivity, 5.5 mOhm-cm, at 733 K in comparison with that of without deposition interruption, 0.212 mV K−1 and 10.4 mOhm-cm.
    Materials Chemistry and Physics 08/2014; 146(3):346–353. · 2.07 Impact Factor
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    ABSTRACT: It is well known that aluminum (Al) and copper (Cu) are acceptor impurities with shallow- and deep-energy levels in silicon (Si), respectively. The thermoelectric power factor of Al and Cu codoped Si film is larger than that of only Al-doped Si film. In this report, the Al and Cu codoped Si layer, Si: (Al + Cu), is used as a barrier layer, while a higher manganese silicide (HMS, MnSi1.7) layer is used as a well layer to enhance the power factor of MnSi1.7 film. It is found that the Al and Cu modulation doped MnSi1.7 film, Si: (Al + Cu)/MnSi1.7, has a power factor almost two times larger than that of only Al modulation doped MnSi1.7 film, Si: Al/MnSi1.7. It is also found that an undoped Si spacer layer between the Si: (Al + Cu) barrier layer and the MnSi1.7 well layer can enhance the power factor further. Finally, it is demonstrated that the MnSi1.7 film with double Si barrier layers, Si: (Al + Cu)/MnSi1.7/Si: (Al + Cu), has the highest power factor, 1423×10−6 W/m K2 at 783 K, which is very close to that of MnSi1.7 bulk material.
    Applied Physics A 06/2013; · 1.69 Impact Factor
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    ABSTRACT: Crystalline Si films with both shallow- and deep-level acceptors, Al and Cu, have been prepared on glass and quartz substrates by the methods of magnetron sputtering and Al-induced crystallization. Al and Cu are co-added in the Si films intermittently by regular pulse sputtering of Al and Cu targets during deposition of the Si films. By regulating the sputtering times of Al and Cu targets, the amounts of Al and Cu in the Si films can be controlled, and thus the Seebeck coefficient and electrical resistivity of the silicon films can be adjusted. It is found that the Al and Cu co-doped Si film has a larger Seebeck coefficient and a lower electrical resistivity at higher temperatures, as compared with that of only Al-doped Si film. As a result, the thermoelectric power factor of the Al and Cu co-doped Si film is greatly enhanced. The present experimental results will not only help us to understand the basic thermoelectric properties of semiconductors doubly doped with shallow- and deep-level impurities, but also open the possibility of enhancement of thermoelectric power factor by using this concept.
    International Journal of Modern Physics B 12/2012; 26(31). · 0.46 Impact Factor
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    ABSTRACT: Silicon-added and modulation-doped higher manganese silicide (HMS, MnSi1.7) films have been prepared on glass substrates by magnetron-sputtering of MnSi1.85, Si, and Al targets. Silicon-addition and modulation-doping are used to enhance the Seebeck coefficient and reduce the electrical resistivity, respectively. Raman spectra indicate that the silicon-added MnSi1.7 film consists of two phases, crystalline MnSi1.7 and crystalline silicon. It is found that the silicon-added MnSi1.7 film has a larger Seebeck coefficient (S), but a higher electrical resistivity (ρ) as well. Consequently, the thermoelectric power factor (PF = S2/ρ) is not enhanced, 0.320 × 10−3 W/m K2 at 733 K, and about the same as that of a pure MnSi1.7 film. The silicon-added MnSi1.7 layer in a modulation-doped structure Si:Al/MnSi1.7/glass, however, has a higher energy barrier height, a larger Seebeck coefficient, and a lower electrical resistivity. As a result, the thermoelectric power factor is greatly enhanced and can reach 0.573 × 10−3 W/m K2 at 733 K.
    physica status solidi (a) 07/2012; 209(7):1307-1312. · 1.21 Impact Factor
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    ABSTRACT: MnSi1.7 films with different thicknesses (16–242 nm) are prepared by magnetron sputtering and electron beam evaporation. When the MnSi1.7 film thickness is about 40 nm or above, MnSi1.7 films are p-type in the whole temperature range (300–700 K) in agreement with reports in literature. By co-sputtering of MnSi1.85 and silicon targets or deposition of Si/Mn multi-layers with a larger thickness ratio, silicon is added to the films and the Seebeck coefficients transform from positive to negative with increasing temperature. The Seebeck coefficients at room temperature and 633 K are +0.098 mV/K and -0.358 mV/K, respectively. By reducing the MnSi1.7 film thickness to 27 nm, the transition of Seebeck coefficient from positive to negative is also observed although silicon is not added intentionally. When an ultra-thin aluminum layer is deposited between MnSix(x < 1.7) and Si layers to enhance silicon diffusion, the p- to n-type transition temperature decreases about 100 K. The silicon-added MnSi1.7 films usually have higher electrical resistivity.
    International Journal of Modern Physics B 01/2012; 25(18). · 0.46 Impact Factor
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    ABSTRACT: N-type polycrystalline higher manganese silicide (MnSi1.7) films are prepared on thermally oxidized silicon substrates by magnetron sputtering. MnSi1.85, Si, and carbon targets are used in the experiments. By co-sputtering of the MnSi1.85 and Si targets, n-type MnSi1.7 films are directly obtained. By increasing the Si content to the deposited films, both the Seebeck coefficient and electrical resistivity increase to high values. A Si intermediate layer between the MnSi1.7 film and substrate plays an important role on the electrical properties of the films. Without the interlayer, the Seebeck coefficient is not stable and the electrical resistivity is higher. For preparation of MnSi1.7 films by solid phase reaction, a sandwich structure Si/MnSix/Si (x < 1.7) and thermal annealing are used. A carbon cap layer is used as a doping source. With the carbon doping, the electrical resistivity of the MnSi1.7 film decreases, while the Seebeck coefficient increases slightly. For reactive deposition, the MnSix (x < 1.7) film is directly deposited on the heated substrate with a Si intermediate layer. By using a Si cap layer, a MnSi1.7 film with a Seebeck coefficient of -292 μV/K and electrical resistivity of 23 × 10-3 Ω-cm at room temperature is obtained. The power factor reaches 1636 μW/mK2 at 483 K. With such a high power factor, the n-type MnSi1.7 material may be superior to p-type MnSi1.7 material for the development of thermoelectric generators. Several smaller (0.036 - 0.099 eV) and intermediate (0.10 - 0.28 eV) activation energies are observed from the curves of logarithm of the resistivity versus reciprocal temperature. The larger activation energies (0.35 - 1.1 eV) are consistent with the reported energy band gaps for higher manganese silicides.
    International Journal of Modern Physics B 01/2012; 23(16). · 0.46 Impact Factor
  • Q. R.hou, Y. B.chen, Y. J.he
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    ABSTRACT: Nano-scale MnSi1.7 films are prepared by thermal annealing of three-layer Si/MnSix/Si or bi-layer Si/MnSix (x < 1.7) structures at 923 K for 20–65 minutes. These layers are deposited on thermally oxidized silicon substrates at about 393 K by electron beam evaporation. It is found that the oxygen content in the MnSi1.7 film can be reduced from about 10 at.% to 6 at.% by using the bi-layer structure MnSix/Si with the MnSix layer on top. With the reduction of oxygen content in the MnSi1.7 film, the transition temperature from p-type to n-type decreases from 508 K to 463 K or less.
    Modern Physics Letters B 11/2011; 23(20n21). · 0.48 Impact Factor
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    ABSTRACT: CdZnTe crystals grown by the vertical Bridgman method were characterized by measuring their resistivity and chemical etching in the well-known Nakagawa etchant (3HF:2H2O2:2H2O, vol./vol.). It was found that the resistivity of the CdZnTe crystals was between 4.33 × 103 and 8.50 × 106 Ωcm. Defect densities were much higher around the periphery of some CdZnTe samples due to the influence of mechanical stress caused by contact with the crucible walls during the CdZnTe crystal growth. The sizes of the high defect density region ranged from 1.8 to 2.8 mm. Such high defect density region should be eliminated in order to make high-quality radiation detectors or prepare the substrates for the epitaxial growth of HgCdTe.
    Modern Physics Letters B 11/2011; 16(17). · 0.48 Impact Factor
  • Q. R.hou, Y. B.chen, Y. J.he
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    ABSTRACT: Higher manganese silicide (HMS, MnSi1.7) films have been deposited on glass, silicon and thermally oxidized silicon substrates by the methods of magnetron sputtering and thermal evaporation. Mechanical and thermo-electric properties of the films have been measured. The hardness and elastic modulus of the films are 10.0~14.5 GPa and 156~228 GPa, respectively. The sign of the Seebeck coefficient at room temperature is positive for all samples. The resistivity at room temperature is between 0.53×10-3 and 45.6×10-3 ohm-cm. The energy band gap calculated from the resistivity data for the film deposited on thermally oxidized silicon substrate is about 0.459 eV.
    Modern Physics Letters B 11/2011; 20(15). · 0.48 Impact Factor
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    ABSTRACT: Phonon-drag effect usually occurs in single crystals at very low temperatures (10–200 K). Strong phonon-drag effect is observed in ultra-thin β-FeSi2 films at around room temperature. The Seebeck coefficient of a 23 nm-thick β-FeSi2 film can reach -1.375 mV/K at 343 K. However, the thermoelectric power factor of the film is still small, only 0.42×10-3 W/m-K2, due to its large electrical resistivity. When a 27 nm-thick MnSi1.7 film with low electrical resistivity is grown on it, the thermoelectric power factor of the MnSi1.7 film can reach 1.5×10-3 W/m-K2 at around room temperature. This value is larger than that of bulk MnSi1.7 material in the same temperature range.
    Modern Physics Letters B 11/2011; 25(22). · 0.48 Impact Factor
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    ABSTRACT: P- and n-type higher manganese silicide (MnSi1.7) films are characterized by Auger electron spectroscopy (AES). The relationship between Auger chemical shift and electrical property of the film has been established. Compared with pure Mn, the peak positions of Mn [MVV] Auger spectra in p- and n-type MnSi1.7 films move to higher energy regions with +2.0 and +7.0 eV, respectively. New peaks around 50 eV in the Mn [MVV] Auger spectra, and 600, 654, and 705 eV in the Mn [LMM] Auger spectra appear in MnSi1.7 films prepared by magnetron sputtering. These new peaks are considered to arise from iron impurities which are unintentionally introduced from the Mn–Si alloy target and during the magnetron sputtering process. The intensities of these new peaks are much stronger for the n-type MnSi1.7 film. Compared with pure Si, the peak positions of Si [LVV] Auger spectra move to higher energy regions with +1.0 eV for both p- and n-type MnSi1.7 films. However, the peak positions of Si [KLL] Auger spectra in p- and n-type MnSi1.7 films move to lower energy regions with energy shifts between -1.0 and -3.0 eV.
    Modern Physics Letters B 11/2011; 21(17). · 0.48 Impact Factor
  • Q. R.hou, Z. M.wang, Y. J.he
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    ABSTRACT: The behavior of oxygen impurities during thermal annealing of manganese-silicon diffusion couples and reactive deposition of MnSi1.7 films has been studied. Samples were prepared by reactive deposition or thermal evaporation of manganese on silicon (100) substrates, which were then annealed in vacuum. The investigation techniques included depth profiling using Auger electron spectroscopy and electrical resistance measurements. The oxygen contamination originated from the preparation chamber or exposing the sample to air before thermal annealing. The oxygen diffused into the manganese film and blocked the silicide formation. For reactive deposition of the MnSi1.7 films, the competition between silicide formation and oxygen diffusion resulted in the formation of silicide films with oxygen concentration of about 5 at% or the diffusion of oxygen in the manganese film with oxygen concentration of about 35 at%. The presence of a higher concentration of oxygen in the manganese layer prevented any silicide formation.
    Modern Physics Letters B 11/2011; 16(28n29). · 0.48 Impact Factor
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    ABSTRACT: The introduction of an un-doped silicon layer (spacer) enhances significantly the thermoelectric power factor in modulation-doped Si(Al)-MnSi1.7-Si(Al) sandwich structure. This un-doped silicon layer is inserted between the MnSi1.7 (HMS) and Al-doped silicon layers. With a proper spacer thickness, the electrical resistivity decreases sharply and is weakly dependent on temperature from 300 K to 683 K. As a result, the thermoelectric power factor can reach 0.973 × 10-3 W/m-K2 at 683 K, which is about ten times larger than that of an ordinary MnSi1.7 film without modulation doping.
    Nano brief reports and reviews 11/2011; 06(05). · 1.17 Impact Factor
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    ABSTRACT: Polycrystalline higher manganese silicide (MnSi1.7, HMS) films with addition of aluminum and carbon are prepared on thermally-oxidized silicon substrates by electron beam evaporation and magnetron sputtering, respectively. An aluminum intermediate layer and a carbon cap layer are used as the doping sources. It is found that both the Seebeck coefficient and electrical resistivity are dependent on the amount of aluminum and carbon added to the films. The Seebeck coefficient changes a little in the temperature range 300 to 433 K and decreases considerably above 433 K when aluminum is added to the film. When carbon is added to the film, however, the Seebeck coefficient increases slightly. With addition of aluminum and carbon, the resistivity decreases. As a result, the thermoelectric power factor increases, especially for films with carbon addition. Several activation energies (0.022–0.20 eV) are observed from the curves of logarithm of resistivity versus reciprocal temperature. The larger activation energies of 0.35 and 0.51 eV are consistent with the energy band gaps for higher manganese silicides.
    Modern Physics Letters B 11/2011; 21(22). · 0.48 Impact Factor
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    ABSTRACT: 723 K. N-type MnSi1.7 film can be obtained by addition of iron into the film. It is found that the thermoelectric properties of n-type MnSi1. 7 are better than those of p-type MnSi 1. 7, which is consi stent with the recent thoeretical prediction. A. Preparation of bulk MnSi 1.7 materials Mixtures of Si and Mu in the mole ratio of 1.85 were mechanically ground in a mill for 60 hours. The purity of the Si and Mu powders was 99. 9%. The powders were hot-pressed using carbon die at 1373 K for one hour under 25 MPa in an argon atmosphere. The MuSi1.7 phase formation was confirmed by x-ray diffraction. The Seebeck coefficient and electrical resistivity were measured from room temperature to 783 K by the ordinary DC method in vacuum. The thermal conductivity was measured by using the laser-flash method. E. Preparation
    01/2011;
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    ABSTRACT: Nano-scale higher manganese silicide (MnSi1.7) film with thickness of about 27nm is prepared by thermal annealing of a bi-layer Si/MnSix (x
    Materials Chemistry and Physics 05/2010; 121(1):103-108. · 2.07 Impact Factor
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    ABSTRACT: Multiferroic heterostructures were fabricated by growing ferrimagnetic CoFe <sub>2</sub> O <sub>4</sub> films on ferroelectric Pb ( Mg <sub>1/3</sub> Nb <sub>2/3</sub>)<sub>0.7</sub> Ti <sub>0.3</sub> O <sub>3</sub> substrates using pulsed laser deposition. Upon applying an electric field, the in-plane magnetization of the heterostructures increases and the out-of-plane magnetization decreases. Sharp and reversible changes in magnetization under electric field were also observed for the poled sample. The relative change in magnetization-electric field hysteresis loops were obtained for both the in-plane and out-of-plane magnetizations. Analysis of the results suggests that the electric field induced change in magnetic anisotropy via strain plays an important role in the magnetoelectric coupling in the heterostructures.
    Applied Physics Letters 06/2009; · 3.52 Impact Factor
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    ABSTRACT: MnSi1.7 bulk materials and thin films are usually p-type. When the thickness of MnSi1.7 film is reduced to nanoscale, however, the carriers are p-type around room temperature and transform to n-type at high temperatures. The thermoelectric powers at room temperature and 633 K are 128 and –340 μV/K, respectively. A n-type MnSi1.7 film with a thickness of about 14 nm is prepared. The thermoelectric power reaches to –967 μV/K at 483 K. From these data, the energy band gap is estimated as 0.93 eV, which is consistent with that obtained from the temperature dependence of the electrical resistivity measurements. The large thermoelectric power may be due to the increase in the electronic density of states and energy band gap in low-dimensional semiconductors or hetero-structures being formed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Physica Status Solidi (A) Applications and Materials 09/2008; 205(11):2687 - 2694. · 1.53 Impact Factor
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    ABSTRACT: Multiferroic p-n junctions were fabricated by growing La0.1Bi0.9MnO3 films on Nb–SrTiO3 using pulsed laser deposition. The current-voltage curves of the junction show good rectifying property. Both the ferroelectric transition and ferromagnetic transition of La0.1Bi0.9MnO3 have remarkable influence on the transport properties of the junction. A large positive magnetocapacitance was also observed in this junction. Analysis suggests that the property of La0.1Bi0.9MnO3/Nb–SrTiO3 is dominated by the property of La0.1Bi0.9MnO3. This work demonstrates that multiferroic p-n junctions possess some interesting properties that may be useful for future applications.
    Applied Physics Letters 02/2008; 92(6):063513-063513-3. · 3.52 Impact Factor
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    ABSTRACT: Higher manganese silicide (MnSi1.7) films are prepared on silicon and thermally oxidized silicon substrates by reactive deposition and solid phase reaction. By co-evaporation of manganese and silicon atoms on silicon substrate at about 400 °C, a sharp film-substrate interface is obtained. For preparation of the films on thermally oxidized silicon substrates by solid phase reaction, the oxygen level in the films is found to be dependent on the deposition rate of silicon. The oxygen and manganese atoms accumulate at the silicide-Si interface and film-SiO2 interface, respectively. The sign of the Seebeck coefficient is positive in the temperature range 300 K to 773 K, indicating the p-type conductivity of the films. The Seebeck coefficient increases with increasing temperature from 294 K to 483 K and is weakly temperature dependent above 483 K. The resistivity increases with temperature near room temperature, reaches a maximum value at about 383 K, and then decreases with further increase of the temperature. The Seebeck coefficient and resistivity at room temperature are about 114 μV/K and 7.37 × 10–3 Ω cm, respectively. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Physica Status Solidi (A) Applications and Materials 09/2007; 204(10):3429 - 3437. · 1.53 Impact Factor