AVS2011 Session TF-MoM: Thin Films: Growth and Characterization I

There has been growing interest in thin bi-metallic multilayer films for the usage under extreme radiation conditions because of their radiation healing properties. Recent discovery and research indicate that materials can be hardened against radiation damage by building nanolayered structures with an optimized layer thickness to increase point defect recombination relative to a non-layered structure and that can self-heal. In this study, we investigate whether the internal interfaces can be manipulated at the nanoscale to enhance dynamic recombination of radiation-produced defects, or self-healing, so as to dramatically reduce radiation damage without compromising other properties using Ti/Al multilayer films.

Ti/Al multilayer films were fabricated on Si (100) and epi polished MgO (100) substrates using DC magnetron sputtering and Molecular beam epitaxy (MBE) The growth parameters for each method, for sputtering – pressure, power and substrate temperature deposition rate; and for MBE- deposition rate and substrate temperature were optimized to achieve high-quality thin films. The films were characterized using x-ray diffraction (XRD), x-ray reflectivity (XRR), Rutherford backscattering spectrometry (RBS), and x-ray photoelectron spectroscopy (XPS) measurements. The films show mostly polycrystalline structure with no elemental interdiffusion at the interfaces. Detailed structural and compositional analysis was also performed using high resolution TEM/STEM and atom probe tomography (APT). The films were irradiated using 1-8 MeV Au ions to understand the radiation effects. The damage peak, stopping range and ion distribution were simulated using binary collision approximation based Monte Carlo method (SRIM software program). Au ion energies, estimated from the simulation, were used to position the damage peak at the interested interfaces and away from the interfaces to obtain the complete picture at the interfaces and in the bulk of the films. The interface and crystal lattice damage, amorphization, and defect density were studied by RBS, HAADF-STEM and APT, and compared with those results from the pristine samples. The relationships between film properties and radiation healing characteristics will be presented and discussed.

The state-of-the art approach in the encapsulation of high-end devices such as flexible (polymer) solar cells and organic light emitting diodes against water vapour permeation is an organic/inorganic multi-layer system. Although this approach allows increasing the lifetime of the encapsulated device, the optimization of a multi-layer is rather empirical as the mechanisms behind the improvement of the barrier performance are not yet unraveled. In particular, the role of the organic interlayer is rather controversial since effectively it does not act as a moisture vapor barrier, yet its application appears to be fundamental in the multi-layer solution. In this contribution, the role of the organic interlayer is investigated by selecting a system in which the barrier layer, a 100 nm-thick SiO₂ film, is plasma- deposited while the organic interlayer, a 200 nm- thick organosilicon film, is synthesized by means of initiated- chemical vapor deposition, i.e. via thermal decomposition of an initiator molecule promoting the polymerization of 1,3,5-trimethyl-1,3,5-trivinyl-cyclotrisiloxane (V₃D₃) at the substrate.

The implementation of in situ (real time) spectroscopic ellipsometry allows following the different growth stages in the V₃D₃ polymerization process. In particular, when applied to the polymer bulk growth, the determination of the growth rate allows monitoring the transition from a kinetic-limited (with activation energy of 65 ± 4 kJ/mol) to a mass transfer-limited regime. Furthermore, the deposition process is found to be monomer adsorption- limited with an activation energy of -39 ± 4 kJ/mol. When spectroscopic ellipsometry is applied to the initial monomer adsorption steps, isothermal adsorption/desorption studies provide insight into the microstructure of the underlaying SiO₂ barrier layer, characterized by a residual open porosity in the micro/meso transition region (pore radius ≤ 2nm). The microstructure characterization by means of the above-mentioned studies implicitly points out the role of the i-CVD organic interlayer in multi-layer barrier structures, i.e. the filling of the open micro-meso porosity of the inorganic barrier layer, therefore, improving the intrinsic barrier quality of the underlaying SiO₂ film. This outcome nicely correlates with the superior water vapor barrier performances (a barrier improvement factor of 2400 is reported with respect to the pristine polymer) of multi-layers based on the application of i-CVD organic interlayers with respect to fully-PECVD developed multi-layers.

Barium strontium titanate (BST) is a very promising material for tunable microwave applications like phase-shifters and tuneable filters. Due to this the influence of e.g. annealing conditions and processes on thin film properties and their dielectric performance were largely investigated. But very few researchers have tried to change the sputtered BST thin film properties by using different dopants at the same time. Such iron and fluorine co-doped thin films can be achieved by RF magnetron sputtering, with a co-sputter target and a two step annealing process after deposition. The first annealing process provides the crystallinity of the films. In the second annealing process the fluorine co-dopant is introduced into the BST thin films by a diffusion controlled process.

The present contribution focuses on the processing and characterization of the iron doped BST thin films with various amounts of fluorine co-dopant. The characterization of the thin films by X-ray photoelectron spectroscopy (XPS) provides chemical binding states and film composition. XPS and time of flight secondary ion mass spectrometry (ToF SIMS) sputter depth profiles prove the chemical homogeneity and the film thickness. Grazing incident X-ray diffraction (XRD) and Raman spectroscopy validate the crystallinity and the identification of chemical phases. Furthermore film morphology is determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Dielectric measurements, to investigate the influence of the donor and acceptor co-doping on the dielectric performance, were carried out in metal insulator metal (MIM) structures with ground signal ground probes.

Integrating a high-quality layer of epitaxial Ge on Si has been a longstanding engineering challenge, despite its technological importance. The applications of Ge-on-Si include ‘virtual substrates’ for III-V multijunction solar cells, high-mobility field-effect transistors, and optical interconnects monolithically integrated with Si-based circuitry. The primary difficulties in achieving Ge films of sufficient quality stem from the lattice mismatch that leads to a large density (> 10⁹ cm^-2) of threading dislocations (TDs) and the thermal expansion coefficient mismatch between Ge and Si that leads to microcracks or delamination of Ge film upon cooling from growth to room temperature. Herein, we present a new method to reduce the TD density, using a minimal number of standard microfabrication steps. The method begins with growing a 500-nm-thick epitaxial Ge layer on Si. A post-growth anneal step leads to a TD density of approximately 5x10⁷ cm^-2, as revealed by plan-view transmission electron microscopy (TEM) and etch pit density (EPD) measurements. The close agreement between EPD measurements and TEM shows that the EPD measurements reliably decorate all TDs. Etch pits are created around the dislocation cores in the Ge film. A 15-nm-thick layer of SiO₂ is subsequently deposited on the etch-pit-decorated Ge film. A thin layer of polymethyl methacrylate is then spin-coated onto the sample, which fills the etch pits and planarizes the Ge surface. Next, a reactive ion etching step is used to remove the polymer and SiO₂ from the planar regions of the sample surface surrounding the etch pits. An O₂ plasma is then used to selectively remove the remaining polymer, so that SiO₂ remains only within the etch pits. Lastly, a second layer of Ge is selectively grown on the exposed Ge surface and laterally over the SiO₂-lined etch pits until a fully coalesced Ge film is created. A final polishing step produces an atomically flat continuous Ge film. Ensuing EPD measurements reveal that the density of twin defects and TDs in the upper Ge layer is approximately 1.7x10⁶ cm^-2, such that the overall defect density is reduced by a factor greater than 30 compared to that in the initial Ge layer. Both theoretical and experimental evidence suggest that the defect density in GaAs films on Ge/Si must be less than 2x10⁶ cm^-2 to have a minority carrier lifetime comparable to GaAs films grown on Ge and GaAs substrates. Therefore, our new method of using SiO₂-lined etch pits to block the propagation of TDs in Ge may finally lead to device quality III-V materials integrated on Si substrates.

Hybrid organic-inorganic nanolaminates combine the functionality of an inorganic material with the flexibility and mechanical integrity provided by the organic polymer layer. They are integral components in various applications serving as advanced dielectrics, flexible barrier coatings, and as optical components. This work focuses on the low temperature synthesis of alumina/silicone nanolaminates by plasma-enhanced chemical vapor deposition (PECVD) in a single chamber for dielectric applications.

Self-limiting synthesis of alumina was accomplished via pulsed PECVD at the synthesis temperature of ~ 100 °C using trimethyl aluminum (TMA) and oxygen as precursors. The deposition kinetics and film quality were evaluated as a function of precursor exposure, plasma power, substrate temperature, and pulse parameters. Film composition was assessed by using spectroscopic ellipsometry and Fourier transform infrared spectroscopy (FTIR). The deposition rate per pulse scaled with the degree of precursor exposure during the plasma off step. Through appropriate control of the TMA concentration and pulse duration, the depositing rate could be adjusted over a narrow range (1.6 – 2.8 Å/pulse). Alumina films deposited at 105 °C contained a very small concentration of hydroxyl impurities. Polymeric silicone-like coatings were deposited using hexamethyldisiloxane (HMDSO) and oxygen as precursors. A wide range of coatings, from inorganic SiO₂-like films to flexible polymeric films could be deposited by appropriate control of parameters including the rf power, substrate temperature and working pressure. Growth rates as high as 100 nm/min were obtained for polymeric silicone films.

Alumina/silicone nanolaminates were constructed as a function of nanolaminate composition and dyad thickness. Precise control of nanolaminate construction was confirmed through field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The dielectric performance of these structures was examined by using capacitance-voltage and current-voltage measurements. The effective dielectric constant could be controlled by changing the alumina content of the nanolaminates, and modeling these structures as capacitors in series accurately described the observed variations in κ.