A fully automated multi-target reactive magnetron sputtering (MS) process is presented in which real-time modeling and in situ standard or Mueller matrix ellipsometry is combined demonstrating growth of nanoscale multi-layer optical thin films having desired properties such as thickness, while observing properties such as index of refraction (n), extinction coefficient (k), and complex permittivity throughout growth. For each material layer isotropic or anisotropic properties as required can be modeled automatically in real-time, allowing for the development of hyperbolic metamaterials. In situ use of an RC2 ellipsometer from JA Woollam is presented, having a spectral range of 210nm - 2500nm. TEM measurements of the thin films are presented.
We experimentally analyze the simultaneous processes of difference frequency generation (DFG) and second harmonic (SH) generation in multilayer structures exhibiting an effective epsilon-near-zero (ENZ) response and ENZ properties in one of their layers. The structures consist of subwavelength-thin tri-layer periods of 75% ITO, 12.5% Al2O3, and 12.5% BaTiO3, with the total thickness kept near 120 nm. The number of periods and ITO layer thickness (3-30 nm) vary between samples, allowing to tune the effective ENZ wavelength over 1000 nm. We demonstrate that the level of DFG and SH enhancement can be increased by over two orders of magnitude with multilayer composition, with the highest enhancement in samples having 12-15 nm thick ITO layers. The peak enhancement wavelength follows the effective ENZ wavelength, while the relative enhancement levels of DFG and SH depend on sample composition. Our findings are supported by COMSOL simulations, TEM analysis, and ellipsometry data.
The future of photonic devices involves harnessing non-linear effects, for applications such as frequency upconversion and down-conversion, optical switching, and emission control. To effectively do this, the optical properties of designed material systems are needed. Hyperbolic metamaterials that use both conductors and dielectrics have been shown to have enhanced non-linear properties near the topological transition point. Creating that topological transition point in a layered hyperbolic metamaterial offers a way to control the non-linear properties without a complicated 3D design. Layered 1D metamaterials still have a large enough design space to achieve various non-linear effects across a large frequency range and have a relative ease of fabrication. For this research, ITO was chosen as the conductor, which has advantages due to its ready availability and CMOS compatibility. The chosen dielectric, SiO2, is also easily available. The non-linear properties of the hyperbolic metamaterials were modeled with an efficient Matlab code, and the results show the capability of controlling the non-linear properties and optimizing for many different possible applications.
The future of photonic devices involves harnessing non-linear effects, for applications such as frequency upconversion and down-conversion, optical switching, and emission control. To effectively do this, the optical properties of designed material systems are needed. Metamaterials can be fabricated in a layered form to operate in many wavelength bands, and they exhibit strong non-linear effects. To make the layered metamaterial, alternating layers of metal and dielectric were used. Samples were fabricated using physical vapor deposition for the material system ITO-SiO2, with varying layer thicknesses for each sample. First, the linear properties of the samples were measured using variable angle spectral ellipsometry, and then the non-linear properties were measured using the Z-scan technique. The linear results show a good agreement with effective medium theory, which signifies that the metamaterials are suited for computer-aided design. Also, the non-linear results show strong non-linear properties, of n2 = 1 ∗ 1014 cm2/W, and β = 2 ∗ 1010 cm/W, which is larger than many natural materials. This demonstrates the potential for use in non-linear applications.
Layered oxide materials having alternating repeated layer thicknesses of 10 nm or less are difficult to make, especially with sharp interfaces. Nanostructured thin films having repeated layers of two different oxide materials were obtained by using pulsed laser deposition and two independent stationary targets consisting of Al2O3 and BaTiO3. Desired thicknesses were achieved by using a specific number of pulses from a 248-nm KrF excimer laser, at an energy of 450 mJ/pulse, a galvanometer mirror system, and a background pressure of oxygen. Trends in material properties were identified by systematically varying the number of pulses for multiple nanostructured thin films and comparing the resulting properties measured using in-situ spectroscopic ellipsometry and ex-situ capacitance measurements, including relative permittivity and loss. Four films were deposited with a goal of having 0.25-, 1-, 4-, and 10-nm thick layers, and each ∼220 nm thick. Ellipsometry data were modeled in situ to calculate thickness, n and k. A representative transmission electron microscopy measurement was also collected for the 10-nm sample with corresponding x-ray photoelectron spectroscopy and energy disperive x-ray spectroscopy. Ellipsometry and capacitance measurements were all performed on each of the samples, with one sample having calculated impedance greater than 30 GOhm at 0.001 Hz.
The co-sputtered Cu-Si-O and Cu-Ge-O thin films were prepared using reactive DC, pulse DC and modulated pulse power magnetron sputtering (MPPMS) on two separate Cu and Si or Cu and Ge targets simultaneously. The powers on each target and Oxygen/Argon flow ratio f(O2) were varied to have different stoichiometies determined by XPS. The film thickness, refractive index n and extinction coefficient k were extracted from in situ ellipsometry and the reactive plasma discharge was monitored by optical emission spectroscopy in real time during film growth. The grazing incident x-ray diffraction measurements reveal that the films deposited at low f(O2) have the nanocrystalline structure of cuprous Cu2O with diffraction peaks of (111) and (200). The films deposited at high f(O2) (≥ 1) have cupric oxide CuO phase. The optical constant n and k, film density and band gap of the co-sputtered film were investigated and determined by in situ ellipsometry, X-ray reflectivity and UV-Vis-NIR spectroscopy. Their structural, chemical and optical properties are able to be tuned by incorporating Cu2O, CuO and the mixtures of them into Silicon oxide or Germanium oxide matrix with varying target powers and oxygen/Argon ratio for applications in optical coatings and optical filters.
John Jones, Jonathan Goldstein, Steven Smith, Gerald Landis, Lawrence Grazulis, Neil Murphy, Gregory Kozlowski, Rachel Jakubiak, Charles Stutz, Lirong Sun
Oxide materials of desired stoichiometry are challenging to make in small quantities. Nanostructured thin films of multiple oxide materials were obtained by using pulsed laser deposition and multiple independent targets consisting of Si, BaTiO3, and B. Programmable stoichiometry of nanostructured thin films was achieved by synchronizing a 248-nm krypton fluoride excimer laser at an energy of 300 mJ/pulse, a galvanometer mirror system, and the three independent target materials with a background pressure of oxygen. Island growth occurred on a per pulse basis; some 500 pulses are required to deposit 1 nm of material. The number of pulses on each target was programmed with a high degree of precision. Trends in material properties were systematically identified by varying the stoichiometry of multiple nanostructured thin films and comparing the resulting properties measured using in situ spectroscopic ellipsometry, capacitance measurements including relative permittivity and loss, and energy dispersive spectroscopy (EDS). Films were deposited ∼150 to 907 nm thickness, and in situ ellipsometry data were modeled to calculate thickness n and k. A representative atomic force microscopy measurement was also collected. EDS, ellipsometry, and capacitance measurements were all performed on each of the samples, with one sample having a calculated permittivity greater than 20,000 at 1 kHz.
World-wide interest in germanium-on-silicon photonics has grown enormously during the past few years. We report on our study of germanium deposition for which we found that there is potential to engineer films with significant increases in hole mobility. In addition, we report on our development of wet-etch techniques to pattern thin films and to form tapered regions of Ge, both important for the fabrication of Ge photonic devices.
This article [J. Nanophoton.. 8, (1 ), 083890 ( Feb 5 , 2014)] mistakenly appeared in the Special Section on Metamaterials and Photonic Nanostructures. It was republished in the Special Section on Nanostructured Thin Films VI with a corrected CID on 10 February 2014. The updated citation is shown below:
Pulsed laser deposition is an energetic deposition technique in which thin films are deposited when a laser pulse at 248-nm wavelength strikes a target and material is subsequently deposited onto a substrate with ideally the same stoichiometry. By synchronizing a high-speed mirror system with the pulsing of the laser, and using two separate targets, thin films having tunable stoichiometry have been deposited. Depositions were performed in a high vacuum environment to obtain as much kinetic energy as possible during growth. Typically, some 150 pulses at 300 mJ/pulse were required to deposit 1 nm. Island growth must occur on a per pulse basis since over 100 pulses are required to deposit a 1 nm film thickness. Films were deposited to ∼100-nm thickness, and in situ ellipsometry data were modeled to calculate thickness, n and k . X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and atomic force microscopy (AFM) were all performed on each of the films. XPS demonstrated change in film composition with change in laser pulse ratio; ellipsometry displayed thickness from the model generated as well as the optical properties from 370 to 1690 nm. AFM thickness measurements were in agreement with independently modeled ellipsometry thickness values.
Electronic circuits alone cannot fully meet future requirements for speed, size, and weight of many sensor systems, such as digital radar technology and as a result, interest in integrated photonic circuits (IPCs) and the hybridization of electronics with photonics is growing. However, many IPC components such as photodetectors are not presently ideal, but germanium has many advantages to enable higher performance designs that can be better incorporated into an IPC. For example, Ge photodetectors offer an enormous responsivity to laser wavelengths near 1.55μm at high frequencies to 40GHz, and they can be easily fabricated as part of a planar silicon processing schedule. At the same time, germanium has enormous potential for enabling 1.55 micron lasers on silicon and for enhancing the performance of silicon modulators. Our new effort has begun by studying the deposition of germanium on silicon and beginning to develop methods for processing these films. In initial experiments comparing several common chemical solutions for selective etching under patterned positive photoresist, it was found that hydrogen peroxide (H2O2) at or below room temperature (20 C) produced the sharpest patterns in the Ge films; H2O2 at a higher temperature (50 C) resulted in the greatest lateral etching.
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