AVS2012 Session TF-ThP: Thin Film Poster Session

The effects of deposition temperature, post-deposition annealing, and chamber gettering were investigated on the magnetocaloric properties in Ta(5nm)/Gd(30nm)/Ta(5nm) grown by magnetron sputtering. The magnetocaloric effect (MCE) in these thin films, as indicated by the magnetic entropy change around the ordering temperature, increases with both growth temperature and post-deposition annealing of samples grown under ambient conditions. The full width at half max of the entropy change peak generally decreased toward the value for bulk Gd with increasing deposition and annealing temperature. Similarly, the temperature of the maximum entropy change increased toward the bulk Gd ordering temperature. Overall the relative cooling power increased as deposition and annealing temperatures were increased. Gettering proved useful in limiting oxidation of the Gd especially for high temperature growth: ungettered samples grown at 600°C were purely diamagnetic GdO (111), while gettered samples, though still containing some GdO, were ferromagnetic.

Zinc oxide (ZnO) is a promising oxide semiconductor for optoelectronic devices because of its attractive properties such as wide direct band gap (3.3 eV), high exciton binding energy (58 meV), and material abundance. For realizing optoelectronic devices utilizing such attractive properties, a fabrication method of high quality crystalline ZnO films is essential. We have recently demonstrated a novel fabrication method of ZnO films utilizing nitrogen mediated crystallization (NMC), where the crystal nuclei density can be controlled because the nitrogen atoms suppress crystallization of ZnO films [1]. By using NMC-ZnO films as buffer layers, we have succeeded in high-quality epitaxial growth of ZnO films on sapphire substrates by RF magnetron sputtering [2]. However, for device applications such as light emitting diodes (LED), there still remains a need for improvement of properties of ZnO films. Here we apply off-axis sputtering to epitaxial growth of ZnO films on NMC-ZnO buffer layers aiming at reduction of negative-ion bombardment that causes serious damage during oxide deposition. NMC-ZnO buffer layers were fabricated by RF magnetron sputtering. The used gas was N₂-Ar and the total pressure was 0.3 Pa. ZnO ceramic targets were used. The applied RF power was 100 W and the deposition temperature was 700^oC. The thickness of the buffer layers was 10 nm. On NMC-ZnO buffer layers, ZnO films were deposited by off-axis RF magnetron sputtering at 700^oC. Ar-O₂ was used and the total pressure was 0.67 Pa. The applied RF power was 60 W. The film thickness was 1μm. After deposition, the films were annealed in a furnace at 1000^oC for 3h in air. The surface of as-deposited ZnO films fabricated by off-axis sputtering on NMC-ZnO buffer layers has subnm scale corrugation and the RMS roughness is 0.28 nm, being significantly small compared with 1.02 nm for the films fabricated by conventional on-axis sputtering. Moreover, the annealed surface has 0.26-nm-high steps corresponding to one molecular layer of ZnO. We have demonstrated fabrication of atomically flat ZnO films by using NMC-ZnO buffer layers together with the off-axis sputtering.

[1] N. Itagaki, K. Kuwahara, K. Nakahara, D. Yamashita, G. Uchida, K. Koga, and M. Shiratani, Appl. Phys. Express 4 (2011) 011101.

[2] K. Kuwahara, N. Itagaki, K. Nakahara, D. Yamashita, G. Uchida, K. Kamataki, K. Koga, and M. Shiratani, Thin Solid Films 520 (2012) 4674.

The efficiency of photovoltaic structures based on the CdS/CdTe heterojunction is far from that predicted theoretically. Within the various problems that affect negatively the efficiency of this system is the lack of a methodology to get a controlled doping of the n-type window layer of CdS. The chemical bath deposition methodollogy generally produces uncontrolled n-type CdS films. This paper proposes the use of fluoride as an electron donor to substituting the atoms of S in CdS. With this objective we deposited CdS films using different molar compositions of thiourea doped with fluorine. The molar compositions used are between 0.025 and 0.25 M with variations of 0.025 M.The samples obtained were characterized by UV-Vis transmission spectroscopy, X-ray diffraction and the electric transport by I vs V measurements. Representative samples were characterized by using Atomic Force Microscopy, Scanning Electron Microscopy, Raman, photoluminescence and photoconductivity spectroscopies. The results are discussed considering the presence of defects associated with the molar concentration of thiourea. The crystallographic quality of the samples increases with fluorine doping. There is a growing trend in the band gap values with thiourea concentrations. Raman spectrosocopy results suggest that fluorine is substitutionally incorporated in sulphur sites and photoluminescence spectroscopy indicates that chemical bath deposited samples have a low density of radiative defects. IvsV curves indicated that transport is carried out through a percolation process. Our results indicated that the samples with the best characteristics are those fluorine doped grown with thiourea concentrations between 0.075 y 0.125 M showing that fluorine substitutionally replaces to sulphur and passivates interfacial states.

* This work has been partially supported by CONACyT-Mexico and Instituto de Ciencia y Tecnologia-DF

TiO₂ has been deposited on both native oxide and etched InAs (100) surfaces by thermal atomic layer deposition (ALD) from tetrakis dimethyl amido titanium (TDMAT) and H₂O. X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) were utilized to study the interface between the TiO₂ films and the InAs substrate. For depositions at 200°C, the native oxide was thinned and part of the native oxide bubbled to the top of the TiO₂ surface. HRTEM data for a ~4 nm film of TiO₂ on InAs confirmed that the native oxide was completely removed from the interface. When TiO₂ was deposited on HF and NH₄OH etched InAs surfaces, practically sharp interfaces were maintained. To investigate the effect of temperature on the native oxide consumption, two sets of samples with film thickness of 2 and 3 nm were prepared at deposition temperatures ranging from 100 to 325°C. XPS showed that the native oxides were consumed most effectively at 250°C. Deviation of the deposition temperature from 250°C in either direction resulted in a reduction of the native oxide consumption rates.

When growing thin layers – through ALD or other processes – it is important to know how the layer is growing. Low Energy Ion Scattering (LEIS) can play a pivota role in the study of film growth. It can be used for a quantitative analysis of the outermost atomic layer. This feature is used to determine layer closure or the existence of pinholes.

At the same time, LEIS gives information about the composition below the surface (similar to RBS). This so called "static depth profiling" can be used to monitor and study the evolution of layer thickness.

Examples will be shown for ALD Ta layers on Si, demonstrating the capabilities of LEIS to determine layer closure and the development of the layer thickness in the ALD process, also at low cycle numbers.

A Diamond Like Carbon (DLC) on Si system will be shown to demonstrate the possibilities to combine static depth profiling with dynamic (sputter) depth profiling, combining the advantages of both techniques and overcoming their drawbacks.

Atomic layer deposition (ALD) is a thin film deposition technique widely used to deposit highly conformal thin films of a wide range of materials including metal oxides and nitrides, metals and more recently, polymers. Most ALD processes for the deposition of metal oxides require the use of H₂O, O₂ plasma, O₃, or H₂O₂ as the oxygen source. ALD processes for depositing metal oxides using metal halides and metal alkoxides as the oxygen source were first reported by Ritala (Science 288, 319 (2000)) to mitigate the problem of an interfacial oxide formation during deposition on semiconductor surfaces such as Si and Ge. Herein, we report an ALD process to deposit titanium dioxide using TiCl₄ and titanium tetraisopropoxide (Ti[(OC₃H₇)₄], TTIP) as the oxygen source. We have used in situ attenuated total reflection Fourier transform infrared spectroscopy to probe the corresponding surface reactions during film growth over a temperature range of 150-250 °C. Depending on the surface temperature, alkyl-transfer and β-hydride elimination have been proposed as two possible reaction pathways for TTIP on a TiCl_x-terminated surface. However, our infrared data show that alkyl-transfer is the only reaction pathway for this ALD process even at temperatures of up 250 °C, which is close to the decomposition temperature of TTIP. We also report the growth per cycle, stiochiometry, and the band gap for these TiO₂ films grown over the above temperature range. Finally, we discuss the growth of these TiO₂ films on Cl-terminated Ge nanoparticle surfaces.

Nitrogen doped zinc oxide thin films were deposited on glass and silicon substrates by reactive magnetron RF sputtering of zinc in a N₂O–Ar ambient. The deposition conditions were optimized varying the substrate temperature and the N₂O/Ar sputtering gas ratio. Representative films were studied employing structural, optical and spectroscopic techniques. A correlation between the nitrous oxide partial pressure, the chemical composition and the crystalline structure of the films was obtained. Stoichiometric and highly oriented ZnO thin films along the (0 0 2) crystallographic direction were obtained for a nitrous oxide partial pressure of ~33%. Lower temperatures produced samples with higher nitrogen content and slightly higher band-gap energy. Post-deposition annealing treatments in nitrous oxide atmosphere at 500 °C significantly improved the crystallinity of the samples as confirmed by x-ray diffraction.

^† : partially funded by CONACyT-Mexico and ICYT-DF.

In order to meet the increasing demand for high frequency electronic devices, the physical dimensions of MOSFETs have been continuously scaled down into nanoscale. However, one of the bottlenecks we encounter during the scaling-down process is the tunneling current leakage at the gates. SiO₂, the most commonly used traditional gate dielectric experiences an appreciable amount of tunneling current when the gate thickness is below 1-1.2 nm. And leakage greatly degrades the performance of nanoelectronics. Therefore, we propose using high-k dielectrics to replace SiO₂, which can effectively limit the tunneling leakage without losing the current control at gates. Our research has mainly focus on Al_xHf_yO_z deposited on silicon via Atomic Layer Deposition (ALD).The Aluminum to Hafnium ratio in the oxide is tuned to maximize the electrical and physical properties of the film. The electrical properties of each oxide will be characterized by fabricating transistors with gate oxide thicknesses of 5, 10, and 15 nm. Other than taking C-V and I-V measurements for capacitors and transistors, the films will be characterized by XPS, AFM, and spectroscopic ellipsometry. Finally, the effects of various annealing and deposition temperatures at the silicon-oxide interface will be studied using TEM.

Recently, Diamond-like carbon (DLC) films have come to the center stage of developing coatings for moisture resistant lubricant, water repellant and cathode for lithium batteries. Hydrophilic nature of DLC films played an important role in the above applications.