Y. B. Chen

Improvement in hard magnetic properties of FePt films by N addition

The structure and magnetic properties of (Fe56Pt44)100−xNx alloy films with x=0–15 at. % prepared by rf magnetron sputtering were investigated. Compared to an Fe56Pt44 binary film, (Fe56Pt44)100−xNx alloy films showed great enhancement in hard magnetic properties. A coercivity Hc of 1027 kA/m, a remanence Mr of 1.24 T, and a maximum energy product (BH)max of 260 kJ/m3 were obtained for an (Fe56Pt44)93N7 film annealed at 600 °C for 10 min, whereas the Fe56Pt44 film annealed at the same conditions gave Hc=682 kA/m, Mr=0.87 T, and (BH)max=132 kJ/m3. Structural analysis revealed that N addition influenced the phase structure and microstructure of FePt films. N atoms incorporated in the disordered FePt phase in as-deposited films, they released rapidly out of the FePt phase by vacuum annealing. It is suggested that the rapid release of N atoms from the FePt phase promotes the transformation to the ordered phase, results in a higher coercivity of the film. The high remanence of the (Fe56Pt44)93N7 film is attrib...

PREPARATION OF N-TYPE HIGHER MANGANESE SILICIDE FILMS BY MAGNETRON SPUTTERING

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.

PREPARATION OF ADHERENT MnSix FILMS

The adhesion of manganese silicide (MnSix) films on silicon and glass substrates is studied by using the micro-scratch method. The films were prepared by electron beam evaporation and thermal evaporation. To improve adhesion of the films, several techniques including ion bombardment, increasing substrate temperature, and insertion of a silicon intermediate layer were used. Finally, adherent MnSix(x~1.7) films were prepared through solid phase reaction as well as reactive deposition. The hardness and modulus of the MnSix(x~1.7) film were measured by a nano-indenter and the values are 8.8±1.0 GPa and 141±15 GPa, respectively.

MECHANICAL AND THERMOELECTRIC PROPERTIES OF HIGHER MANGANESE SILICIDE FILMS

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.

THERMOELECTRIC PROPERTIES OF MnSi1.7 FILMS WITH ADDITION OF ALUMINUM AND CARBON

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.

ENHANCEMENT OF THERMOELECTRIC POWER FACTOR BY A SILICON SPACER IN MODULATION-DOPED Si-HMS-Si

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.

Positron annihilation lifetime in mesoporous silica MCM-41 at different vacuum levels

Positron annihilation lifetime spectra of MCM-41 and zeolite Y were measured at different vacuum levels. When the experiments were carried out in air, a very long lifetime component (τ4 = 35-45 ns, I4 = 15-20%) was observed for MCM-41, while the longest lifetime for zeolite Y was only 2-4 ns with an intensity of 15-25%. However, when the experiments were carried out in vacuum, the very long lifetime components could be observed for both samples, although with different intensities, ~30% for MCM-41 and ~10% for zeolite Y. For MCM-41 in air, the longest lifetime (τ4) is ~42 ns, corresponding to the ortho-positronium (o-Ps) annihilation lifetime in MCM-41 cavities. This value is slightly longer but very close to the value of 39 ns, which was estimated by using a bouncing quantum particle model. These peculiar positron annihilation characteristics were explained by air quenching mechanism of o-Ps annihilation in MCM-41. It was suggested that because of the existence of the very long lifetime component even in air, positron lifetime spectroscopy could be a very useful tool for nondestructive measurement of the cavity size of mesoporous solids such as MCM-41.

CHARACTERIZATION OF p- AND n-TYPE MnSi1.7 FILMS BY AUGER ELECTRON SPECTROSCOPY

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-typ...

Structure and positron annihilation spectra of tin incorporated in mesoporous molecular sieves

Mesoporous molecular sieves (MCM-41) consist of an ordered array of silica tubules comprised of pores with uniform controllable diameters in the nanometer range. Tin was successfully incorporated into MCM-41 using wet chemical techniques. Detailed structural analysis via x-ray diffraction and high resolution transmission electron microscopy confirm this, and indicate that, after sintering samples in air, SnO2 crystal nanoclusters formed in the channels. These conclusions are further supported by a study of the positron annihilation spectrum. In particular, the insensitivity, after incorporation of tin, of the long-lived component of the positron annihilation spectrum to whether an air or a vacuum annealing atmosphere is used indicates that tin in the MCM-41 channels hinders the entry of quenching oxygen from the air. Furthermore, after sintering, the complete loss of this long-lived component indicates that SnO2 nanoclusters fill the channels.

THERMOELECTRIC EFFECT OF SILICON FILMS WITH SHALLOW- AND DEEP-LEVEL ACCEPTORS

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.