ICMCTF2016 Session CP: Symposium C Poster Session

In the last few years, there has been an important interest on the transition metal nitrides for their potential for thermoelectric applications. Indeed a recent theoretical study predicted the potential of the AMN₂ (A = Ca, Ti, Sr and M = Ti, Zr, Hf) layered nitride materials to exhibit a high Seebeck coefficient and good electrical conductivities[1]. Another study on a first principles calculations of the phase stability of the Sc based nitride materials, which already showed its potentialities for thermoelectric applications[2], demonstrated a very low mixing enthalpy of the ScMN₂ phase (M = Nb, Ta)[3]. Furthermore, nitride alloy thin films is one of the strategy for lattice thermal conductivity reducing. Indeed, ScN exhibited a low thermoelectric figure of merit (ZT) about 0.2-0.3 at 800°C mainly due to the high total thermal conductivity[4]. Insertion of heavy elements by alloying may introduce phonon scattering therefore reduce the thermal conductivity and increase the ZT.

With the same perspective, our experimental study is focused on the deposition of the thin films in the Sc-Nb-N system by co-magnetron sputtering on sapphire and MgO substrates. The different parameters of depositions (power, pressure, Ratio Ar/N₂ and temperature) played an important role on the phase formation. The system Sc-Nb-N can be found in different phases from the pure corresponding nitrides, the disordered solid solution and the AMN₂ phases. The morphology and structural properties of the Sc-Nb-N thin films were investigated and their effects on the electrical and thermoelectric properties were evaluated.

[1] R. Al Rahal Al Orabi et al, Journal of Materials Chemistry A 3 (2015) 9945.

[2] S. Kerdsongpanya et al, Applied Physics Letters 99 (2011) 232113.

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

[4] P.V. Burmistrova et al, Journal of Applied Physics 113 (2013) 153704.

Transparent conducting electrodes (TCEs) have attracted great interest because of their wide applications in solar cells, liquid crystal displays, organic light-emitting diodes, and touch screens. As an integral part of the flexible versions of these electronic devices, the flexibility of the TCEs is essential. Despite the exceptional optoelectronic properties of the conventional indium–tin-oxide (ITO) thin films as TCEs, they suffer from considerable drawbacks basically due to their brittle nature. This has motivated research for alternative materials such as carbon nanotubes (CNTs), graphene, conducting polymers, silver nanowires, and metal meshes. Among these, metal meshes are considered as the most promising replacement for ITO becase of their excellent optical and electrical properies. However, metal meshes have visibility issues because of high reflectance of metals. Also, few studies on the improvement of reflectance properties of metal meshes have been reported in literature. Furthermore, the sheet resistance of metal may increase greatly when exposed to air for a long time due to the oxidation of metals. To address these problems, several methods, such as embedment of metal meshes in a polymer matrix or additional coating of protective layers on metal meshes, have been investigated. However, the embedment of metal meshes and coating of metal oxide protective layers need to be processed at high temperatures. Also, the polymer-based protective layers are typically very thick and electrically insulating materials.

Recently, zinc-oxide based thin-film transistors (ZnO-TFTs) have shown specific characteristics for transparent displays. M ost of the successful ZnO-based TFTs incorporate indium (In) such as indium zinc oxide (IZO) and indium gallium zinc oxide (IGZO). However, In is expensive and relatively rare on Earth. Thus, some researchers have recently reported experimental results which indicate In-free oxide semiconductors, such as silicon-zinc-oxide (SZO), aluminum-zinc-oxide (AZO), and gallium-zinc-oxide (GZO). It is believed that the element aluminum (Al, group III) acts as an effective donor in the ZnO lattice because they may be substitutionally placed on Zn sites. Thus, they will then enhance the carrier concentration and conductivity. Also, hafnium (Hf, group IV) is stable at high temperature stress and bias stress in the ZnO lattice. Until now, however, there has been a dearth of comprehensive studies on the hafnium-aluminum-zinc-oxide (HAZO) films and their applications for TFTs.

In this study, HAZO films were deposited on glass and Si substrates via room-temperature co-sputtering where the electric power applied to the aluminum-zinc-oxide target was fixed to 100 W and the electric power applied to the hafnium-oxide target varied from 0 W to 60 W. Other deposition conditions were a working pressure of 10 mTorr and a gas flow rate of 60 sccm for Ar. The structural, optical, and electrical properties of HAZO films were evaluated using various methods, such as X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), energy dispersive x-ray spectroscopy (EDS), UV/visible spectrophotometry, and Hall measurement. All the properties measured were characterized in terms of the deposition conditions of HAZO films. Moreover, TFTs using the HAZO films as the channel layer were fabricated and their device characteristics were analyzed. The experimental results obtained in this study have confirmed that HAZO thin films have the potential for the channel layer of transparent TFTs.

Diluted magnetic semiconductors (DMSs) based on ZnO have attracted a great deal of attention due to their potential application in magnetic semiconductor devices. In particular, they are predicted to display ferromagnetic properties at Curie temperature above room temperature and have large magnetization. (Ga, Co)-ZnO films for use as DMSs have been fabricated using various techniques, including molecular beam epitaxy (MBE) [1], pulsed laser deposition (PLD) [2] and inductively coupled plasma enhanced physical vapor deposition (ICP-PVD) [3], etc. However, there are very few reports about (Ga, Co)-ZnO thin films deposited using radio frequency (rf) magnetron sputtering .

In this study, (Ga, Co)-ZnO [Co_xGa_yZn_(1-x-y)O] films with different Ga contents are co-sputtered on glass substrates by rf magnetron sputtering. The x content [Co/(Ga+Co+Zn)] in the films is ~ 0.05. The content of y [Ga/(Ga+Co+Zn)] varies from 0 to 0.032. Using Hall effect analysis, the resistivity (ρ) of the film is 42.9 Ω-cm when y content is 0. When the content of y increases to 0.032, the ρ value drops greatly to 4.93 × 10^-3 Ω-cm. In photoluminescence analysis, all the oxygen-rich (Ga, Co)-ZnO films contain acceptor-like defects. In magnetic properties analysis, when low Ga contents are doped and with low carrier concentration range (< 2.48 × 10¹⁷/ cm³), saturation magnetization (Ms) is established by Bound Magnetic Polaron (BMP) due to defects and magnetic atoms in the films. However, Ms is created by the free electrons and Co atoms that produce the exchange coupling effect when high Ga concentration is present in the films and in high carrier concentration range (> 7.34 × 10¹⁸/ cm³). In the transition zone, since the two mechanisms mentioned above are overlapped and exchanged, Ms value drops when the carrier concentration in the films increases from 2.48 × 10¹⁷/ cm³ to 7.34 × 10¹⁸/ cm³.

Keywords：rf magnetron co-sputtering, diluted magnetic semiconductor s, (Ga, Co) co-doped ZnO films, optical properties, electrical properties, magnetic properties.

[1] Zhonglin Lu, Hua-Shu Hsu, Yonhua Tzeng, Fengming Zhang, Youwei Du and Jung-Chun-Andrew Huang , Appl. Phys. Lett. 95 (2009) 062509.

[2] Liping Zhu, Zhigao Ye, Xuetao Wang, Zhizhen Ye and Binghui Zhao, Thin solid films 518 (2010) 1879-1882.

[3] Xue-Chao Liu, Zhi-Zhan Chen, Bo-Yuan Chen, Er-Wei Shi and Da-Qian Liao, J. Cryst. Growth 312 (2010) 2871-2875

Co-sputtering simulation (CO-SS) of the sputtering yield amplification of Al, C and Si

Sputtering yield amplification is a phenomenon that was discovered by S. Berg et al. in 1991. This phenomenon can occur during the preparation of thin films by co-sputtering of a target composed of two different materials. If the difference in the atomic weight of the elements of the two parts of the target is large, then it has been observed that the sputtering yield of the lightest element can be greater than the normal value. However, at present there is no adequate phenomenological or theoretical explanation of this phenomenon.

The co-sputtering simulation freeware, CO-SS, was developed by the authors to simulate the spatial variation and composition of the deposition by magnetron sputtering and co-sputtering. In this study thin films of Al/Ti, C/W and Si/W were produced to study the modification of the sputtering yield both experimentally and by modelling using CO-SS. The 4” x ¼” sputtering targets were of high purity Al, C and Si and the sputtering conditions for all deposits was maintained at 20 sccm of argon, a gas pressure of 30 mTorr (4 Pa) and a plasma power of 40 watts. A series of films was prepared of the pure metal or with 1, 2 or 3 small pieces (0.7 X 2.5 cm) of the second material (Ti or W) placed on the racetrack directly above the 3” x 1” glass substrates. The substrates were placed such that they received material from both the uncovered and covered parts of the racetrack. The spatial variation of the thickness and composition of the films was determined by profilometer and the ⁴He RBS technique, respectively. The results showed the existence of the previously mentioned sputtering yield amplification of Al, C and Si.

* Work partially supported by DGAPA IN102015 project

Ophthalmic lenses are made of thermoplastic polymeric substrates usually coated with functional treatments composed of 5 to 15 layers, ranging from micrometers to nanometers. The first treatment consists of a primer, conferring impact resistance properties to the lens. A hardcoat with nanoparticles, is then deposited on top of this primer, bringing anti-scratch properties to the system. Both primer and hardcoat are within the micrometer scale and are deposited by wet chemical methods. Nanometric anti-reflective stacks are then evaporated onto the hardcoat, to enhance wearers’ comfort.

Each of these interfaces may lead to delamination due to poor adhesion, and therefore affect the vision and comfort of wearers. The interface between the anti-reflective stack and the hardcoat is particularly sensitive because of chemical and mechanical contrast of its materials.

The ultimate goal is to better understand mechanisms that lead to loss of adhesion between the SiO₂ layer deposited on the anti-scratch hardcoat. Emphasis is first placed on characterizing mechanical properties of materials composing the interface. In parallel, an applicable and effective method to quantify adhesion needs to be determined and tested. The use of nano-indentation and AFM; not only to obtain information on surface’s topography, but also to access mechanical and adhesion properties, is under study. Micro-tensile tests to acquire interfacial cracking energies, are considered and will be performed as well.

Heat mirrors are defined as transparent multilayer structures that transmit visible light and reflect infrared radiation coming from the solar spectrum. In addition, heat mirrors can generate significant energy savings in buildings by reducing the amount of heat absorbed by windows. The present work focused on the heat mirrors based on three layer structures composed of dielectric/metal/dielectric (D/M/D) materials. Studies were made with HfO₂ dielectric material and Mo for the metal. HfO₂/Mo/HfO₂ multilayered devices have been deposited using electron beam evaporation. The effect of variable Mo thickness on the efficiency of HfO₂/Mo/HfO₂ multilayered devices is evaluated. Unheated substrates were employed with HfO₂ layer thickness 45 nm and 25 nm while varying the Mo metal interlayer thickness in the range of 9-20 nm. The optical properties of the films reveal a higher transmittance (%) of the films in the infrared (IR) to the near infrared (NIR) region, whereas it reduces in the visible region suggesting a more feasible application for these devices in cold environments for heat collection. A detailed analysis of the optical constants i.e. refractive index, band gap energy, and absorption coefficient, are evaluated for HfO₂/Mo/HfO₂ multilayered devices. Comparison of the data will be presented and discussed to determine the optimum Mo interlayer thickness and feasibility of the heat mirror for warm or cold environments.

We report the synthesis of MnO₂ nanoparticles (NPs) on carbon nanofiber (CNF) for use as electrode material in supercapacitor. The CNF was functionalized using acid treatment in solutions containing different H₂SO₄/HNO₃ ratios. A microwave-assisted hydrothermal method was then used to grow MnO₂ NPs on the surface modified CNF in a very short time (5 min). X-ray photoelectron spectroscopy (XPS) was employed to analysis the surface functionalized CNF. The morphology and microstructure of the resulting MnO₂/CNF were observed using x-ray diffraction, field emission scanning electron microscopy, and high resolution analytical electron microscope. The electrochemical performance of the MnO₂/CNF electrode was studied using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in 1 M Na₂SO₄ solution.

Double-stacked ITO/Ag thin films were fabricated on polyethersulfone flexible substrates by using dual-facing target sputtering at room temperature. Precise control for film thickness in ultrathin Ag films were successfully confirmed to obtain high conductivity and transparency in the visible ranges. Ag stacked ITO films showed higher transparency than only Ag film. It could be attributed to reduce the surface scattering of Ag film through the low-emissivity ITO film.

Optimized ITO (50 nm)/Ag (12 nm) films had the sheet resistance of 8 ohm/sq. and the transmittance of over 80 % at the visible range. Additionally, the mechanical durability of ITO/Ag films had low electrical degradation with below 10% compared with the pristine films.

The V₂O₅ nanostructures have been known for the potential application of gas sensors, electrochromatic devices, critical temperature resistors, optical switch devices, etc., which is attributed to the reduction behavior and the reversible phase transitions. In this study, the V₂O₅/Pt core-shell nanowires were fabricated by the thermal evaporation via vapor-solid mechanism. Pt nanoparticles were subsequently coated on the V₂O₅ nanowires using the atomic layer deposition (ALD) process with 50, 100, and 150 cycles.

The FESEM images showed that the nanowire diameters were of 50-200nm with an aspect ratio of 10²-10³. The XRD pattern indicated the dominant peaks at (200), (001), and (002) belonging to the α-V₂O₅ phase of the orthorhombic crystal structure, the higher intensity and narrower FWHM, implying that the V₂O₅ nanowires posses a good crystallinity. The HRTEM image on a specific V₂O₅ nanowire of 108nm had the growth direction along the wire axis of [110] and the Pt coated V₂O₅ by ALD (150 cycles) showed a thickness of 4.5nm Pt-shell with the (111) plane. XPS spectra gave the result of the binding energy (eV) of the respective orbitals: 517(V2p_3/2), 524(V2p_1/2), and 530(O1s). The ΔE (13eV) between V2p_3/2 and O1s indicated that the V₂O₅ phase was formed and also agreed with the XRD results. The binding energies of Pt4f_7/2 at the ALD 50 cycles, 100 cycles, and 150 cycles were of 73, 72.8, and 71.6eV, which meant that the ratio of Pt/PtO increased with the increasing ALD cycles. The intensity of Pt4f_7/2 increased with increasing ALD cycles confirmed that the Pt successfully coated on the V₂O₅ nanowires and thus the content of metallic Pt increased. The photoluminescence (PL) results showed that the peak indicated the original light emission of the V₂O₅ at 550nm without any shift by the effect of Pt. However, the intensity decreased due to the presence of the Pt contents. On the other hand, the emission peak that appeared at 466nm was caused by the Pt and its intensity increased with the increasing ALD cycles, also consistent with the result of the XPS.

Vanadium oxides have been widely studied as “smart materials” because of their capability of going through a metal-insulator-transition. They are of considerable technological interests for applications in optoelectronics, ultrafast optical switches, electrochomic devices, and lithium microbatteries. However, vanadium-oxygen system is very complex due to the multivalency of vanadium, which makes preparation of vanadium oxide with single stoichiometry difficult. Structural characterization of vanadium oxides of different stoichiometry is highly desirable and would provide helpful guideline to both materials preparation and their structural characterization. Vanadium oxide thin films with different stoichiometry under various bonding states are successfully prepared by reactive sputtering with and without post oxidation or reduction and characterized using Raman spectroscopy. Characteristic Raman spectra of single phase and multiphases of V_xO_y including V₂O₃, VO₂, V₆O₁₃, and V₂O₅ are presented and discussed. The results have demonstrated that high purity single phase oxides can be obtained under well controlled conditions.

CdS thin film is a common material used as an n-type buffer layer in Cu(InGa)Se₂ (CIGS), Cu₂ZnSnS₄ (CZTS) and CdTe thin film solar cells. In this study, pure CdS and Ga-doped CdS (Ga:CdS) were deposited onto soda lime glass, FTO, Mo-coated glass and CIGS absorber substrates using chemical bath deposition (CBD) technique. We used Gallium nitrate Ga(NO₃)₃ as gallium dopant source. The Ga doping concentration varied from 0.0005 to 0.002 M and the effect of gallium doping on CdS properties has been investigated. X-ray diffraction results confirmed that all CdS and Ga:CdS films have a cubic crystal structure with (111) plane preferred orientation. Also the lattice strain increased with increasing Ga doping concentration while the crystal size decreased with increased Ga contents. The UV-visible spectroscopy results showed that the transmittance was improved as Ga contents increase. Current-voltage characteristics of Cd and Ga:CdS measured by photoelectrochemical measurement demonstrated that Ga doping enhanced current but decreased impedance. Finally, the effect of Ga:CdS on CIGS solar cell performance will be discussed.

Implementation of RF-sputtered lithium phosphorus oxynitride (LiPON) films on lithium aluminum titanium phosphate (LATP) solid electrolytes as protective coatings to minimize the reaction between metallic Li electrodes and LATP solid electrolytes was explored. In addition, the effects of substrate temperature and N₂ content in the working gas on the characteristics of the LiPON films and the performance and the chemical stability of the passivated LATP solid electrolytes were also investigated. It was found that regardless of the substrate temperature and the N₂ content in the working gas employed in the work, LiPON films deposited by RF magnetron sputtering using lithium phosphate (Li₃PO₄) target under working gases with different N₂ contents were amorphous. It was also found that the deposition rate of LiPON film increased when the N₂ content in the working gas decreased from 100% to 25% while the deposition rate remained roughly the same when the substrate temperature increased from 50℃ to 250℃. Finally, LATP solid electrolytes passivated with LiPON protective layers indeed were more stable than the un-passivated ones when put in contact with metallic Li.

In Cu(InGa)Se2(CIGS)-based thin film solar cells, it is well known that the formation of MoSe₂ at the Mo/CIGS interface is generally preferred because it gives electrically preferred ohmic contact between Mo and CIGS. During the deposition of CIGS light absorber, the thickness of MoSe2 should be controlled to maintain below 100 nm because it is reported that too thick MoSe2 layer has insulating property and thus lead to the reduction in cell performance. It is widely reported that the MoSe2 layer easily forms during the selenization of metallic Cu-Ga-In precursor in a H2Se gas or Se vapor ambient. However, it was hardly reported that the MoSe2 layer formed after the CIGS formation by conventional three-stage co-evaporation process. Indeed, it is difficult to maintain sufficiently high Se overpressure within a evaporation chamber to form MoSe2 layer at Mo/CIGS interface after completing CIGS formation. Therefore, even though three-stage co-evaporation process has been employed to produce the best CIGS cell efficiency for last three decades, further improvement in cell efficiency has been limited by the absence of MoSe2 layer.

Mechanical and tribological properties of thin films from the mixed system CdO-TiO₂ are reported. The films were deposited on glass substrates by sol–gel using a mixture of CdO and TiO₂ solution precursors. Depending of the Ti/Cd ratio (0.20, 0.49, 0.65, 0.70 and 1), the films were constituted of CdO+CdTiO₃ and CdTiO₃ crystals. Films with Ti/Cd ratio of 0.65 and 0.70 have CdO+CdTiO₃ phases and show wear rate of 740 and 1000 ×10^-5 mm³ N^-1m^-1 that are higher than TiO₂ films (30×10^-5 mm³ N^-1m^-1). However, when the Ti/Cd=1 and the CdTiO₃ phase is fully reached, the wear rate falls at 300×10^-5 mm³ N^-1m^-1 which is much lower than the ones obtained for CdO films (5000×10^-5 mm³ N^-1m^-1). Friction coefficient values are in the range 0.09-0.17 for all films. The reduced elastic modulus and the local plastic deformation, obtained by nanoindentation, are improved with the TiO₂ addition . Photocatalytic activity tests in benzene gas degradation reported for these films showed that the films with Ti/Cd=0.65, 0.70 and 1 have a degradation rate that is 4-9 times higher than the values for TiO₂ films. Because of their high photocatalytic activity and wear resistance, these films are good candidates for being applied as a photocatalytic material.