AVS2010 Session TF-TuP: Thin Film Poster Session I

We have attempted improvements of thermal stability of Ag thin films which are candidates of electrodes in various electronic devices. Consequently, we have found that an introduction of very thin (about 3 nm thick) Al oxide surface and interface layers was very effective. For example, we confirmed a high thermal stability up to 600 ^oC even the Ag layer thickness was reduced to 50 nm in Al/Ag/Al films.

In the present work, we have further reduced the thickness of Ag layer in Al/Ag/Al films to obtain high transparency and investigated possibility to apply them as transparent electrodes. As a result, the Ag layer thickness could be reduced to 10 nm in Al/Ag/Al films, keeping a low electrical resistivity. On the other hand, Ag single layers thinner than 14 nm were discontinuous state having a high electrical resistance. Transmittance above 70% was obtained for the Al/Ag(10nm)/Al films. In addition, the property change of the films was found to be very little even after keeping them in air at 60 ^oC for 300 hrs, or in pure water at room temperature for 200 hrs. Consequently, it is found that the Al/Ag/Al thin films have good properties as transparent conductive electrodes.

Laser ablation technique allows depositing thin films with the same stoichiometry of the target material. The common use of solid targets is a limitation when there is a need to produce ternary alloys. In order to overcome that limitation we designed a modification of the laser ablation technique to employ powders as target.¹

The incorporation of carbon in a SiGe alloy is an alternative for achieving larger band gap and strain compensation. The main problem to control substitutional carbon concentration in SiGeC is the low solubility coefficient of carbon in silicon. Laser ablation technique allows exploring deposition parameters far from the equilibrium that could improve the content of substitutional carbon.

In this work we present the growth and characterization of thin films of Si_1-x-yGe_xC_y alloys in the compositions range 0.27 ≤ x ≤ 0.29 and 0.01 ≤ y ≤ 0.03 deposited at different temperatures using the Modified Laser Ablation technique.

The samples where characterized by scanning electron microscopy, atomic force microscopy, X ray diffraction, energy dispersive X-ray spectroscopy, Raman, photoluminescence and photoreflectance spectroscopies. Results indicate the modification of the electronic properties of the alloys depending on the carbon content.

¹ M. González-Alcudia, A. Márquez-Herrera, M. Zapata-Torres, M. Meléndez-Lira and O. Calzadilla-Amaya, Adv. in Tech. of Mat. And Mat. Proc. J. 9, 81 (2007).

In this study, Ti-doped DLC films were formed using He⁺ or Ar⁺ ion beam assistance in a naphthalene (C₁₀H₈) atmosphere. The formation conditions of DLC film were changed with ion-beam accelerating voltage and current density. Ti doping was performed by using the electron-beam deposition method with Ti evaporation rate from 0.0 to 0.2 nm/sec. The mechanical properties of hardness and friction coefficient were determined using the dynamic micro knoop hardness tester and the ball-on-disk tribotester respectively. Atomic concentration and structure of the films were investigated by X-ray photoelectron spectroscopy, X-ray diffraction and Raman spectroscopy.

The suitable mechanical property of DLC films was obtained by the condition with accelerating voltage of 5 kV at current density of 10 μA/cm². The maximum hardness was 5.37 GPa using Ar⁺ ion beam, while the minimum friction coefficient was 0.117 using He⁺ ion beam. It was clear that properties of DLC film was changed by ion species. In the case of Ar⁺ ion beam, the higher hardness film contained much sp³ state, while the film with lower friction coefficient contained much sp² state. From the other side the higher hardness film has a large crystal grain size as compared with the film with lower friction coefficient.

The fabrication and electrical and electromechanical characterization of insulated scanning probes have been demonstrated in liquid solutions. The silicon cantilevers were sequentially coated with chromium and silicon dioxide, and the silicon dioxide was selectively etched at tip apex using focused electron beam induced etching (FEBIE) with XeF₂ The chromium layer acted not only as the conductive path from the tip, but also as an etch resistant layer . The relevant nanofabrication issues relative to the metallization and the insulator deposition process have been discussed. This insulated scanning probe fabrication process is compatible with any commercial AFM tip and can be used to easily tailor the scanning probe tip properties because FEBIE does not require lithography. The suitability of the fabricated probes is demonstrated and discussed by imaging of standard grid as well as piezoresponse force microscopy (PFM ) and electrical measurements .

Polymers used on spacecraft are subjected to various threats including hyperthermal oxygen atoms, UV and VUV photons, and ions. These threats can degrade the polymer and lead to static charge accumulation. Nanometer thick inorganic films grown by atomic layer deposition (ALD) can protect polymers including Kapton, Teflon, and PMMA. We are developing multifunctional multilayer ALD films incorporating Al₂O₃ layers for preventing oxygen atom erosion, TiO₂ to minimize UV/VUV radiation damage, and ZnO to dissipate static charge. Such ALD-coated polymeric films are currently being tested in low Earth orbit on the International Space Station on MISSE-7b. In the laboratory, we are further exploring the mechanisms of polymer degradation by atomic oxygen and VUV radiation, as well as the cracking of inorganic films on polymers resulting from different thermal expansion coefficients. Field emission scanning electron microscopy images and profilometry measurements revealed that Kapton H samples coated with 25 ALD cycles (~3 nm) of Al₂O₃ completely resisted atomic oxygen erosion. Quartz crystal microbalance measurements of TiO₂ films deposited on PMMA substrates with an Al₂O₃ interfacial adhesion layer showed that 100 cycles (~6.2 nm) of TiO₂ resisted PMMA degradation upon VUV exposure. Mass losses of VUV-exposed PMMA samples coated with similar overall bilayer thicknesses but different Al₂O₃ thicknesses were compared to decouple the role of the overall Al₂O₃/TiO₂ bilayer coating acting as a physical barrier from the role of TiO₂ acting as a VUV filter.

The ZnO:Al (AZO) thin film was prepared on si and glass substrates at 200 ℃ by pulsed dc magnetron sputtering deposition. Effects of pulse frequency on the structural, electrical and optical properties of AZO films were investigated by field emission scanning electron microscopy, X-ray diffraction, Hall measurement and spectrometer. The columnar structures are observed by field emission scanning electron microscopy. X-ray diffraction analysis reveals that AZO films were polycrystalline and have preferred orientation along (002). The grain size and resistivity of AZO films were investigated as a function of pulse frequency (5-100 kHz). The maximum grain size and minimum sheet resistivity of AZO film with 20 kHz were 37.5 nm and 650 ohm/sqr, respectively. The average transmittance of AZO thin films was above 80% in the visible range. The presented results illustrate that the optimum properties of AZO films can be obtained at a pulsing frequency in the range of 5-100 kHz.