ICMCTF2015 Session C2: Novel Aspects in Thin Film Characterization and Data Modeling

Photonic crystals have led to numerous technological advances in areas ranging from optical communications to solid-state lighting by manipulating the flow of light through nanoscale variation in refractive index. Recently, exploration has begun to focus on a more generalized form of photonic crystal with independent variation of the real and imaginary refractive index components. Such ‘complex index modulation’ (CIM) has recently become the focus of intense theoretical interest because it forms the basis for parity-time (PT) symmetric optical potentials that enable one-way waveguides as well as more exotic effects such as unidirectional invisibility. It has nevertheless been challenging to realize CIM materials to date because of the added difficulty in controlling both refractive index components at subwavelength scale.

Here, we demonstrate PT symmetric organic thin films by co-evaporating small molecules with different complex refractive indices (n=n+ik) and continuously varying the relative proportion of each during growth. Teflon and tris-(8-hydroxyquinoline)aluminum [Alq₃] are used to vary the real index component (Δn) while 4-(dicyanomethylene)-2-methyl-6-(4-dimethyleaminostyryl)-4H-pyran [DCM] is used to vary the imaginary index component (Δk). The index variation profile is made symmetric with respect to its complex conjugate as a function of position, x, along the film growth direction, resulting in the PT symmetry condition, n(x)=n*(-x). The resulting films are characterized by variable angle ellipsometry and reflectivity measurements, where we observe PT symmetry breaking through a maximum left/right asymmetry in normal incidence reflectivity at the exceptional point Δn = Δk, consistent with numerical simulation.

We also demonstrate passive PT symmetry in the modal effective index of large area composite organic thin film waveguides achieved through shadowed oblique angle deposition of copper phthalocyanine on embedded photoresist sinusoidal gratings. PT symmetry breaking in the leaky waveguide is evident through asymmetry and the appearance of an exceptional point in the Littrow diffraction of light incident from opposite left/right directions. These results open up a new opportunity for organic materials to serve as a platform in studying PT symmetry in optics as well as an avenue for realizing integrated photonic devices such as unidirectional waveguides, coherent perfect absorbers, and all-optical switches.

The anisotropic optical properties of highly ordered, 3-dimensional nanostructured thin films change dramatically upon adsorption of analytes to the nanostructured surface. However, the dependence of these changes on the optical properties of the analyte has not been systematically studied.

Here, use of a self-assembled monolayer (SAM) composed of both HS-(CH₂)₆-OH and HS-(CH₂)₆-K-MB (methylene blue) as a model analyte. The SAM was deposited onto a 45 nm thick gold slanted columnar thin film (SCTF) grown by glancing angle deposition on a quartz crystal microbalance substrate. Alternating current voltammetry and chronoamperometry were used to reduce MB to leucomethylene blue (LMB) and thereby change the optical properties of the SAM without changing its integrity. The response of the functionalized SCTF is detected using both electrochemistry and in-situ generalized spectroscopic ellipsometry in the visible spectral range. In addition, information about the integrity of the SAM as well as the double layer capacitance was monitored by quartz crystal microbalance with dissipation (QCM-D).

We study cuprous oxide, Cu₂O, thin films prepared by RF reactive magnetron sputtering with film thickness between 50 and 400 nm. The structural and morphological properties of the as-prepared and thermally annealed films, 400 °C for 30 min under ambient conditions, are investigated by X-ray diffraction, scanning electron- and atomic force- microscopies. These measurements are correlated with optical characterization by spectroscopic ellipsometry (SE) in the wavelength range from 200 to 1700 nm. The SE data was analysed to obtain a unique solution for multi-sample, multi- angle of incidence to obtain the optical constants, thickness and porosity of the prepared films. The as-deposited films can be readily modelled by a homogeneous bulk film with a small surface roughness. In contrast, annealing is found to result in cupric oxide (CuO) films as determined by x-ray diffraction and corresponding changes in morphology (increased surface roughness and porosity) as well as refractive index ‘n’, extinction coefficient ‘k’, and band gap. In particular, the annealed films present additional structure in the form of an inhomogeneous bulk film composition characterized by a void volume fraction gradient attributed to density changes resulting from the large CuO grains after annealing. Significantly, SE reveals the non-homogeneous nature of oxygen incorporation with thickness upon annealing. The nature of such non-homogeneity and implications for the use of annealed copper oxide films for resistivity sensing of ozone gas at 573 K are discussed.

Acknowledgements: work supported by RGC-HKSAR (Project No. CityU 122812).

Thin film photovoltaics (PV) are reliant on the capability for characterizing the opto-electronic and structural properties of each layer over large areas and correlating these properties with electrical performance of the device. Growing and characterizing each layer in complete devices facilitate comparison of the substrate dependent growth process with individually grown layers. Spectroscopic ellipsometry (SE) provides not only the information about thickness and optical response in the form of complex dielectric function (ε = ε₁ + iε₂) of solar cell component layers but also band gap, grain size, defect density, disorder, and stress as well. We have applied SE to extract layer thicknesses, interface composition, and optical response in the form of ε for all doped and undoped hydrogenated silicon (Si:H) layers grown by plasma enhanced chemical vapor deposition (PECVD) onto a common sputtered back reflector (BR). In-situ real time SE (RTSE) is used during PECVD of Si:H to monitor the growth evolution and nucleation of crystallites from the amorphous phase of each layer as a function of hydrogen to reactive gas flow ratio R=[H₂]/[SiH₄]. A parameterized model has been developed for the BR structure consisting of sputtered intrinsic zinc oxide on top of silver coated glass substrates. Additionally, models generated from RTSE have been applied to study the room temperature ex-situ visible range SE simultaneously with infrared extended SE (IR-SE) measurements ranging from 0.04 to 5 eV. In conjunction with higher photon energy range measurements, the use of reflection mode IR-SE has been successfully applied as an optical metrology technique for characterization of the hydrogen bonding configuration of amorphous Si:H thin films in the solar cell device configuration. This study has yielded identification of higher energy absorption features in each material, the relative absorption strength of various hydrogen-related modes in Si:H to give some insight into the hydrogen content, relative disorder within layers, and free carrier absorption in zinc oxide and silver components of the BR.

Organic thin film electronics offer the potential to significantly impact how humans interface with their surroundings and society in general. The ability to chemically-tailor the electrical and optical properties of organic semiconductors to achieve specific electronic functionality combined with the numerous low-cost additive processing methods by which organic semiconductors can be deposited offers the potential to realize electronics on arbitrary substrates or physical structures for current, emerging, and entirely new applications. Despite tremendous improvement over the past two decades in discrete device performance and several impressive advanced technology demonstrations, the fundamental understanding and quantification of the physical properties and processes that govern device operation remains limited compared to conventional semiconductors, such as silicon. In this presentation I will discuss the development and application of combined and correlated optical-electrical measurement methods to obtain a more nuanced understanding and quantification of the critical semiconductor and device properties and fundamental processes. In particular, I will discuss the use of steady state and pulsed light bias techniques combined with modulated, DC, and magneto electrical measurements tailored to the specific operating regimes of organic solar cells, light emitting diodes, and transistors to provide greater understanding of transport, lifetime, charge density, recombination kinetics, and electronic structure.

Today’s technologies for electronic, optical, mechanical and energy devices are driven by the engineered development of advanced materials. One of the key aspects is the progress in growth technologies for the deposition of thin-layered structures with thicknesses down to the nanometer range. Typical materials that are involved are semiconductors, metal alloys, dielectrics and also polymers. The characterization and monitoring of the properties of a material are significant for its application and essential for further development and improvement.

X-ray diffraction has been appreciated in research and analytical labs for a long time due to its nondestructiveness and versatility to study structural properties of materials of any kind. In recent years other relevant X-ray techniques evolved to meet the upcoming requirements: high-resolution diffraction, grazing-incidence diffraction, reflectometry, diffuse scattering and small angle scattering.

From the scattering data, information can be extracted to identify and quantify phases, to determine composition and strain profiles, thickness, roughness, density, grain size, residual stress and preferred orientation. While in the past for each analysis method dedicated instruments were applied modern lab equipment with exchangeable optics offers all techniques on one single instrument.

In this presentation a brief overview will be given about the experimental aspects including recent hardware developments and evaluation methods of X-ray characterization techniques. Their applicability to extract depth resolved structural information of advanced layered structures is illustrated on some examples of technologically relevant materials like TiN and CrN coatings.

Variable Angle Spectroscopic Ellipsometry (VASE) data analysis using the finite-difference time-domain method (FDTD method) could provide a general method for quantitative broadband optical characterization of non-layered, structurally complex samples, including metallic nanostructures. Specifically, our results demonstrate an alternative approach to in-situ critical dimension (CD) monitoring using rigorous coupled wave analysis (RCWA) and Spectroscopic Ellipsometry (SE).

In this contribution, we show the accuracy limit of this approach by calculating the SE response of both metallic and dielectric ideal thin films. We then demonstrate a practical multi-parameter broadband optimization of the SE response from 1D periodic structure. This method requires a variable angle variable wavelength approach in acquiring SE measurements as to overcome frequency response limits in the FDTD method model. We address the challenges of this approach by comparing different strategies in the selection of angle of incidence (AoI) and wavelength for both SE measurement and FDTD method modeling.

This FDTD–SE approach inherits the distinctive advantages of the FDTD method: (i) calculation of spectral broadband results from a single simulation; (ii) sources of error are well understood, leading to the ability to simulate a large variety of electromagnetic problems; (iii) potential to simulate arbitrary general subwavelength to nano-sized structures and in particular single nanostructures; (iv) natural capacity as a time-domain technique to study complex optical phenomena such as plasmonic and non-linear effects; and (v) ability to visualize field dynamics.

Acknowledgements: work supported by RGC-HKSAR (Project No. CityU 122812). Y. Foo acknowledges support from the by RGC-HKSAR for HK PhD Fellowship Scheme 2014/15.

References

[1] K. T. Cheung, Y. Foo, C. H. To, J. A. Zapien, Towards FDTD modeling of spectroscopic ellipsometry data at large angles of incidence, Appl. Surf. Sci.281(2013) 2-7.

[2] Y. Foo, K. T. Cheung, C. H. To, J. A. Zapien, On the development of Finite-Difference Time-Domain for modeling the spectroscopic ellipsometry response of 1D periodic structures, Thin Solid Films, in-press (Feb. 2014) DOI: 10.1016/j.tsf.2014.02.017.

Strontium titanate, SrTiO₃ (STO), is one of the promising oxides for application in modern nano electronics and optics, such as integrated devices, ultrathin gate oxide insulators, capacitors, dynamic random access memories. In this work, we present a comprehensive and systematic study on RF sputtered STO films by correlating their material properties with deposition parameters. STO films were deposited on Si (100) and UV fused silica substrates by radio-frequency (RF) magnetron sputtering using RF power of 75W at 0.4 Pascal total deposition pressure at room temperature (RT). The effect of oxygen flow rate on film characteristics was investigated. The oxygen content in the films was varied by changing the flow ratio of O₂/Ar+O₂ while the total flow was kept at 30 sccm. Film microstructure, optical and electrical properties were evaluated for both as-deposited and annealed films (700°C, 1 hour). The thickness of the films ranged from 71.6 to 251.5 nm, while the oxygen flow was varied between 0 to 20%. Film structure and optical properties were evaluated by grazing-incidence X-ray diffraction, spectrophotometer, ellipsometry, and photoluminescence measurements. As-deposited and annealed films were found to be amorphous. The average optical transmission of as-deposited films was ~ 60% to 70% with the increased O₂ content in the visible and near infrared spectrum, whereas optical transmission of annealed films was ~80% in the same spectrum. The optical band gap was found to be around 4.2 eV. The refractive index ranged from 2.04 to 2.12 and absorption coefficient was approximately zero at 550 nm. After the post-deposition annealing, the film refractive indices slightly increased (2.12 to 2.15 for 10% O₂), while the band gap remained unchanged. Emission bands were investigated by photoluminescence measurements at RT, the band maxima was found in the visible region for all films. The results were corroborated by optical band gaps obtained via spectral transmission measurements. Metal-insulator-semiconductor (Ag/STO/p-Si) diodes were fabricated and characterized by capacitance-voltage, current-voltage measurements. The dielectric constants were found to be in the range of 20 to 260 within 50 to 900 kHz at RT.

Assessing absorption in optical thin films can be challenging, especially when extinction coefficient values are in the range of 10^-4 or below. Although sophisticated techniques have been developed for such purpose over the years (laser calorimetry, photoacoustic measurements), there is still a need for a simple, quick and low-cost method that would determine both the film thickness and optical properties [n(l ), k(l )] from a single measurement with a high level of accuracy. Thanks to recent advances in UV-VIS-NIR spectrophotometers, in particular the possibility to perform multi-angle Reflection/Transmission measurements with a single accessory, combined to an optimized measurement methodology, we show that it is possible to evaluate the film thickness and refractive index from R/T measurements performed at low angle of incidence, while determining the extinction coefficient down to 10^-5 from R/T measurements carried out near the Brewster angle of the coating in p polarized light. We also demonstrate how modelling errors can be significantly reduced by fitting absorption spectra (A=1-R-T) rather than R and T separately. This new methodology is exemplified for several practical cases displaying various degrees of complexity.