ICMCTF2014 Session C4-2: Thin Films for Energy Related Application

When studying the physical properties of thin-film structures, it is often necessary to determine both the elemental and chemical composition through out the entire thickness of the structure (depth profiles). Herein, methods of obtaining such information using secondary ion mass spectroscopy and Raman scattering spectroscopy are described. As an example, hydrogen diffusion from nanocrystalline titanium into Al_xO_1-x films deposited onto the flat surface of specimens by magnetron sputtering was studied.

As previously shown [1], VT-6 (Rus) titanium in the nanocrystalline (NC) state absorbs hydrogen; thus, nanocrystalline titanium may be used as a hydrogen accumulator. The development of such accumulation materials remains a significant challenge in hydrogen energetic. At high concentrations, hydrogen has a tendency to escape from the metal and disperse into the environment, which is unacceptable for efficient hydrogen storage. One way to limit the emission of hydrogen is to create a barrier on the surface of a saturated material. Thus, the aim of the present study was to investigate the penetration of hydrogen from nanocrystalline titanium through an aluminium oxide coating, which is a good barrier for hydrogen, according to previous reports [2].

In this study, thin films were produced by magnetron sputtering nanocrystalline specimens of titanium saturated in hydrogen and were evaluated by layer-by-layer secondary ion mass spectrometry (SIMS) and Raman spectroscopy. Due to magnetron sputtering, the chemical composition of the films was nonhomogeneous and was variable among layers. Moreover, in the deposition of specimens saturated with hydrogen, hydrogen diffused throughout the depth of the film; diffusion, however, was restricted to the area near the film-substrate interface, affecting less than 50% of the thickness of the film.

1. N.N. Nikitenkov, Yu.I. Tyurin, T.I. Sigfusson // Bulletin of the Russian Academy of Sciences. Physics, 2012, Vol. 76, No. 6, pp. 803–806.

2. M.P. Larin, V.V. Bystrov. Vacuum equipment and technology. 2003, 13(4). P. 221. (Rus).

The purpose of this study is to investigate the difference between coatings on AISI 1045 steel plates with machining flow-channel along with low-temperature pack chromization for preparing bipolar plates of PEM fuel cells. Within the scope of machining feed rate of 50~200 mm/min, those pack chromization specimens that flow-channel was milled at 100 mm/min are propose in this work. The results show that the specimens have a corrosion current (I_corr) of 6.93×10^-7 Acm^-2 and an interfacial contact resistance of 8.0 mΩcm². It is expected to be applied to manufacturing of PEM fuel cell bipolar plates.

ZnO is a bio-compatible, earth-abundant semiconductor that has long been viewed as an attractive candidate for optoelectronic devices in the ultraviolet spectrum due to its wide direct band gap (3.35 eV at RT), large exciton binding energy (60 meV), excellent radiation hardness, and the availability of free-standing bulk single crystal substrates. ZnO is an intense UV emitter with its photoluminescence spectra containing a vast array of spectroscopic features [1]. It is also a member of a small class of metal oxide semiconductor with unusually active surfaces, characterized by persistent 2-D electron gases, that can hinder device stability while at the same time providing opportunities for the production of transparent environmental sensors [2].

We have recently developed a simple methodology for the reliable fabrication of high quality ZnO-based Schottky contacts with high rectifying barriers and ideality factors approaching the image force controlled limit [2]. This has opened the door to the low cost production of devices such as UV photodetectors and transparent metal semiconductor field effect transistors [3]. In this paper, we report on the high UV sensitivity of ZnO Schottky photodiodes and MSM photodetectors. We examine the issue of persistent photoconductivity on detector performance, explore passivation techniques to mitigate the environmental sensitivity of the ZnO surface, and provide examples of applications in public health research.

[1] Mendelsberg et al., Phys. Rev. B 83, 205202 (2011)

[2] Allen et al., Phys. Rev. B 81, 075211 (2010)

[3] Allen et al., Appl. Phys. Lett. 94, 103508 (2009)

[4] Elzwawi et al., Appl. Phys. Lett. 101, 243508 (2012)

[1] T. Minami, H. Sato, S. Takata, N. Ogawa and T. Mouri, Jpn. J. Appl. Phys., 31 (1992) L1106.

The development of clean and sustainable energy sources is one of the most critical objective for today’s society. Thermoelectric devices have the potential to contribute to solve this problem since they can directly convert heat into electricity or vice versa. The heat source can be solar, geothermal or a waste that comes from automobiles exhaust or industrial processes. However, the conversion efficiency of thermoelectric devices of today is limited and further materials improvements are needed. The critical material-dependent parameter is the figure of merit (ZT = S²T/ρκ, where ρ is the electrical resistivity, S is the Seebeck coefficient and κ is the total thermal conductivity). Since, ZT value is scaled by temperature, thus good thermoelectric materials also need high thermal stability. Therefore we have chosen to study transition metal nitrides due to their high thermal stability. Here we present recent results from our experimental and theoretical investigations of the ScN thin film system. We have found that ScN thin films is a promising thermoelectric material because of its high power factor (S²/ρ) of 2.5×10^-3 W/mK² at 800 K [1]. We have suggested that this result can be explained by nitrogen vacancies generating an asymmetric sharp feature in the ScN electronic density of states which allows low electrical resistivity with retained relatively large Seebeck coefficient [2]. Unfortunately, ScN has high thermal conductivity, thus its figure of merit of is low, about 0.2 at 800 K. In order to reduce lattice thermal conductivity nanostructuring, alloying or nanoinclusion formation can be considered. To understand which alloying elements that could be of interest at elevated temperature were diffusion can be activated, we have investigated the trends in mixing thermodynamics of ScN-based solid solutions in the cubic B1 structure by first-principle calculations. 13 different Sc_1-xM_xN (M = Y, La, Ti, Zr, Hf, V, Nb, Ta, Gd, Lu, Al, Ga, In) and three different ScN_1-xA_x (A = P, As, Sb) solid solutions are investigated and their trends for forming disordered or ordered solid solutions or to phase separate are revealed [3]. Moreover, experimental studies have been carried out in ScN-based solid solution thin films which were prepared by reactive magnetron sputtering. The results are used to discuss suitable candidate materials for different strategies to reduce the high thermal conductivity in ScN-based systems.

[1] Sit Kerdsongpanya et al. Applied Physics Letters 99, 232113 (2011).

[2] Sit Kerdsongpanya et al. Physical Review B 86, 195140 (2012).

[3] Sit Kerdsongpanya et al. Journal of Applied Physics 114, 073512 (2013).

Transparent conducting oxide (TCO) films are extensively applied as electrodes i n the fields of solar cells and displays, due to their high transparency and excellent conductivity. Multicomponent oxides such as Zn-Sn-O (ZTO) have attracted much attention resulting from no expensive elements, i.e. indium (In), involved. In addition, thermal stability and mechanical strength of ZTO could be tailored as well by varying its stoichiometry. Howev er, making different ratios of Zn/Sn compounds systematically is not trivial.

Combinatorial methodology has been proven its validity in such an application. This approach allows Zn/Sn continuously changing across the single sample area and a feasible intimate mix of Zn and Sn. Therefore, a single ZTO composition spread sample essentially includes a full spectrum of properties to be investigated.

The delta-cubic Bi₂O₃ phase posses one of the highest ionic conductivities reported, however, as bulk material it is stable in a reduced temperature range from 730 to 825 ^oC. On the other hand, as a thin film produced by atomic aggregation methods, the delta-phase can be produced and kept at room temperature for long periods. Nevertheless, the application of these films as electrolytes for micro-solid oxide fuel cells is limited due to the fact that the films transform into the metastable beta-phase between 250-350 ^oC, temperature too low for the operation of the cell. In this work, we report the stabilization of the delta-phase of Bi₂O₃ thin films from RT to about 600 ^oC by the addition of Tantalum (Ta) as a dopant. The films were produced by reactive magnetron sputtering and the Ta addition was achieved by attaching a pure Ta wire to the 4 inches Bi₂O₃ target. The film structure was studied by X-ray diffraction and Raman spectroscopy.

In this study, we compared the electrical, optical and structural properties of aluminum-doped ZnO (30 nm)/Aurum (5-20 nm)/ aluminum-doped ZnO(30 nm) multilayer thin films on glass substrate deposited by RF magnetron sputtering and ion sputtering for transparent electrode. Optimization of the thin films resulted with low resistivity of 1.01 × 10^-5 Ω-cm, mobility of 27.665 cm²/V-s and carrier concentration of 4.563 × 10²² cm^-3 were obtained at a Au layer thickness of 20 nm. Maximum transmittance of 86.18 % for wavelengths above 650 nm and Haacke figure of merit (FOM) are 9.69 × 10^-3 Ω^-1 of AZO/Au/AZO films with the Au layer thickness of 8 nm. These results indicate that AZO/Au/AZO multilayer thin films are a promising high conductivity transparent electrode scheme for solar cells and various displays applications.

Keywords: Aluminum-doped zinc oxide, AZO, multilayer, RF magnetron sputtering, AZO/Au/AZO, thin film, transparent conductive oxides.