AVS2013 Session AS-ThP: Applied Surface Science Poster Session

X-ray Photoelectron Spectroscopy (XPS) is a well-established technique for surface-sensitive measurements of the elemental and chemical-state composition of a material. Naturally, there has been a drive to allow imaging methods with XPS, so that chemical distributions can be obtained with high spatial resolution.

One such method, parallel XPS imaging, allows for the simultaneous acquisition of the entire image at a single energy, with high spatial resolution. Conventionally, acquiring such images in small energy steps across a photoelectron peak of interest yields a spectroscopic image, where each pixel contains a spectrum.

Typically, such spectroscopic images are acquired across a single peak region, such as C1s. This minimizes acquisition time, and generates more manageable data sets that are conceptually similar to region spectra in conventional XPS. On a modern XPS instrument, it is possible to acquire a full-range spectroscopic image, where each pixel contains a survey spectrum. This is rarely done, mainly due to acquisition times, and the fact that a point survey spectrum from the imaged area will usually give the elemental composition without need for survey images. However, there are advantages to the method, namely the ability to retrospectively generate spectra from arbitrary parts of the imaged area, to identify composition at any location.

In this presentation, the extension of the parallel XPS imaging technique to wider energy ranges will be discussed, showing how in some situations the use of wide-range spectroscopic imaging can provide information that is difficult or impossible to gain using single-point spectroscopy or region spectroscopic imaging analyses.

Composition and thickness of thin films in the nanometer range are critical to the development of future electronic devices, the control of functionalized surface layers or the stability of metals and alloys in the environment (passivity). The importance of the composition of the interface beneath the thin surface film in under­standing the interaction of minerals, alloys and coatings with the environment is often neglected. However, as selective dissolution of some elements of an alloy and the enrichment of others at the interface beneath the surface film may occur the interface composition might control the dissolution process and the corrosion resistance.

The number of techniques with a depth resolution in the nanometer range is limited. Photoelectrons have an attenuation length of few nanometers, thus x-ray photoelectron spectroscopy (XPS) is a well-suited technique to study the composition of the interface beneath thin films. In this paper XPS based methods ranging from a single XPS measurement at one emission angle to angle-resolved (ARXPS) spectroscopy are presented. As state of the art XPS instrumentation allow acquiring spectra at different emission angles simultaneously, the limiting factor in thickness and composition determination of thin films and interfaces is moving from the acquisition of high-resolution XP-spectra to data processing, evaluation and interpretation.

Results are presented for different stainless steels, nickel-phosphorus coatings and sulfur-bearing minerals. It is shown that a single XPS measurement allows obtaining the average composition of the interface beneath the thin film (together with thickness and average composition of the contamination layer and the nanometer film under study). ARXPS measurements allow a further insight into the in-depth composition profile at the interface. A full in-depth composition profile of both the thin film and the interface beneath requires ARXPS data acquisition with depth profile reconstruction using a mathematical method as for example the maximum entropy method (MEM).

Beside the different mathematical algorithms and the software used to calculate the interface composition the importance of fundamental physical parameters such as the attenuation length of the photoelectrons has to be stressed.

Films fabricated by sputtering deposition are extensively used in semiconductor, optical and optoelectronic industries. Since the sputtering is the key to physically deposit the films onto substrates, its effect on the target materials is needed to investigate in order to fabricate films successfully. In this study, multiferroic bismuth ferrite, BiFeO , (BFO) with 5 mole% BaTiO₃ target was sputtered by using 3 keV argon ions continuously after various time intervals. The X-ray Photoelectron Spectroscopy (XPS) was applied to examine the cleanness and the chemical environment of the elements on the sample surface after each sputter time interval. The carbon contaminant was almost completely removed after 5120 s sputtering, but the sputtering induced metallization of the bismuth and two-component oxygen spectra were also observed. It was found that the oxidation states of bismuth from dominant 3+ state changed to equally weighted 3+ state and 0 state. The changes of the valence electrons of bismuth induced by the argon sputtering on the BFO target inevitably alter the film compositions deposited onto the substrate; however, it was found that the non-stoichiometric BFO films can be corrected by sputtering under partial oxygen pressure via the re-oxidation of bismuth metal.

Recent improvements of field emission scanning Auger instruments have led to the ability to provide elemental imaging of surfaces with a spatial resolution better than 8 nm. The PHI 710 now provides the capability to combine high energy resolution spectra with high spatial resolution chemical state imaging with a CMA analyzer. The system software also provides LLS separation of different chemical states from the Auger imaging data.

A semiconductor structure with multiple silicon chemical states will be presented. The 0.1% energy resolution spectra for silicide, silicon, and silicon oxynitride are extracted from the scanning Auger image. These basis spectra are then used to create the different chemical state images. The resulting image overlays demonstrate the ability of the CMA analyzer to image different chemical states at high energy resolution without topographical artifacts.

Laser Induced Breakdown Spectroscopy (LIBS) is a technique for elemental analysis on inorganic and organic materials. It uses a pulsed laser beam to ablate the surface of the target materials and generate plasma. By analyzing the optical emission spectra emanating from the plasma, information related the ablated material composition can be obtained. The key features for LIBS include fast analysis, simultaneous multiple element detection, analysis under ambient atmosphere, and no or very little need for sample preparation. In this presentation, LIBS analysis on thin film samples is performed with the goal of understanding its capability to detect polymer thin film materials and to perform surface mapping. The LIBS system (Insight, TSA) is equipped with a pulsed laser and a charge-coupled device (CCD). LIBS systems equipped with 266 and 1064 nm laser sources are tested under air and argon atmosphere. The polymer films tested are fluorocarbon (FC) thin films coated on silicon wafer and glass substrates. Results show that when the 1064 nm system is used, clear C- and F-emissions can be observed for FC coated substrates with a coating thickness down to 100 nm. While using the 266 nm laser, rather poor signal to noise ratio is obtained for F- and C-emissions. Substrates patterned with 500 mm wide stripes FC thin films are then tested to obtain spatially resolved information for the patterned surfaces. By using the 1064 nm laser source, spatial resolution of approximately 100 mm can be obtained. This demonstrates the capability using LIBS as the spatially-resolved surface mapping of polymer films for thin films. This work was supported by National Science Council of Taiwan, the Republic of China (101-2221-E-002-163-MY2)

Water and soil repellency confer a distinct advantage to apparel. A number of processing steps are required to produce water-repellent fabrics. The original fiber spin finish is needed for fabric weaving but then must be removed (though rarely completely) prior to water-repellent treatment. The composition of water-repellent formulations varies among different formulators and even from a single formulator over time. In addition to fluorocarbons with different polymer backbones and different fluorocarbon tail lengths/distributions, water-repellent formulations may also include so-called “extenders” or performance enhancers. Drying and curing steps are typically required.

Surface characterization plays a critical role in understanding water repellency. This presentation will focus on the surface analytical methodologies developed to help correlate formulation and processing variables to repellency performance. These include the use of multivariate statistics with ToF-SIMS mapping data to quantify fluorocarbon coverage as well as to understand the effect of “uncoated” surface character. These results will be combined with fabric surface energetics measurements to yield a more complete picture of the fabric treatments.

Five synergistic performance characteristics of the new PHI nanoTOF to improve the analysis of polymer and biological samples will be discussed. These desired characteristics are the ultimate spatial resolution, mass resolution, mass range, insensitivity to topographical artifacts and ultimate abundance sensitivity. A new LMIG has been developed which can simultaneously achieve a typical spatial resolution of 400 nm and a mass resolution greater than 10,000 m/Δm. When combined with the nanoTOF analyzer, topographical features within a depth of field of 200 µm can be imaged. To extend the molecular sensitivity beyond the traditional static SIMS limit of 1013 incident ions/cm2, the use of voxel analysis can be used. For voxel analysis, an image is acquired to the static limit, followed by the removal of the damaged surface layer with Gas Cluster Ion Beam (GCIB) source, followed by additional cycles of LMIG acquisition and GCIB damage removal. By integrating signal intensities as a function of GCIB depth of removal, three dimensional voxels of data can be acquired with much higher molecular sensitivities than data only acquired to the static SIMS limit.

Data from a mixed phase polymer sample, micron sized organic contamination features on a failure analysis sample and images of biological samples will be discussed. Retrospective quantitative line scans and spectra from these samples will demonstrate the advantages of these new capabilities for TOF-SIMS polymer and biological sample analyses.

Research on organic light-emitting diodes (OLEDs) has gained much attention due to the potential for cheap and ultrathin illumination sources with high color range. In multilayer OLEDs the carrier injection efficiency from the electrodes into the light emitting layer is improved by layers for hole or electron transport and blocking. Vacuum evaporation is the standard method to produce precisely defined interfaces, but it is an expensive process with high material usage. Alternatively, the wet chemical processing has advantages in production cost, deposition rate and area, but shows problems with the intermixing of the subsequently coated functional layers. The specific nature of small molecules used for OLEDs with their high mobility for diffusion in combination with low dry film thicknesses of only a few nanometers is challenging. For a better understanding of the physical limitations of liquid-phase processed multilayer OLED structures, the possibility to characterise the material mixing between two layers during coating and drying, and the comparison to evaporated layers is crucial.

The present study focusses on the surface analytical characterization of OLED multilayer films using X-ray photoelectron spectroscopy (XPS) and complementary time-of-flight secondary mass spectrometry (ToF-SIMS). In the case of (bio-) organic materials, both methods provide the chemical composition of the topmost layer, XPS mostly in a non-destructive manner. However, information on the in-depth distribution of the constituents in multilayer OLED systems, e.g. to prove chemistry and sharp interfaces, is unavailable via sputter depth profiling when using monoatomic Ar+ (XPS) and Cs+ (ToF-SIMS) ion sources for material erosion. Here the high fluence of even low energy monoatomic ions with a projected range in all cases greater than the XPS/ToF-SIMS sampling depth causes decomposition of the organic material. But since the recent introduction of high mass Ar cluster ion sources for both XPS and ToF-SIMS, enabling sputter depth profiling of organic materials while preserving the chemical/molecular information, this drawback can be overcome [1,2]. We present quantitative in-depth information on the desired OLED structures and, moreover, compare results achieved from monoatomic and cluster ion source depth profiling.

1. V. S. Smentkowski, G. Zorn, A. Misner, G. Parthasarathy, A. Couture, E. Tallarek and B. Hagenhoff, J. Vac. Sci. Technol. A 31 (2013) 030601.
2. P. J. Cumpson, J. F. Portoles, N. Sano, and A. J. Barlow, J. Vac. Sci. Technol. B 31 (2013) 021208.

This work was carried out with the support of the KNMF, a Helmholtz Research Infrastructure at KIT.

Cluster ion beam technologies, in particular the use of C₆₀ and Ar gas cluster ion beams (GCIB), are increasingly being used for the depth characterization of organic materials to provide information that is not available from monatomic ion beams such as Ar⁺. For surface analysis applications, these cluster sources are used for a wide range of organic materials, including multi-layer organic thin films and organic light emitting diodes (OLEDs), including metal capped devices.

Improvements in the efficiencies for OLED structures have recently focused on the incorporation of more effective organic materials and the on the development of novel structures for arranging these organic materials. Multi-layer devices, graded composition devices, and novel electrical contact layers to the organic materials are all being rapidly developed. The need for analytical techniques that allow the elucidation of the organic thin film structure as a function of device fabrication and lifetime studies is extremely important.

In combination with GCIB sputtering, XPS and TOF-SIMS are being used for quantitative chemical as well as molecular depth profiling techniques for OLED structures. The ability to accurately identify the chemistry at an interface using a co-sputtering protocol involving conventional argon ions and an argon cluster will be discussed. Quantitative compositional depth profiling of graded composition multilayer OLED films and other OPV devices will be demonstrated.

Heat-assisted magnetic recording (HAMR) uses a laser-magnetic head integrated system to rapidly heat a localized recording area of the medium above its Curie temperature in order to reduce its coercivity below that of the applied magnetic field during the recording process.^1,2 Unlike perpendicular recording, HAMR is not limited by the superparamagnetic effect associated with magnetic particle instability and low signal-to-noise ratio.² However, localized laser heating may affect the thermal stability of the protective carbon coating on the hard disk.^1,2,3 To examine the effect of heating on carbon film stability, three types of a-C films of 5nm thickness have been studied; (1) single layer a-CH film (2) single layer a-CN film and (3) double layer a-CN/CH film. This paper discusses the relationship between thermal heating and structural evolution of the carbon films. XPS was employed to investigate the sp² and sp³ carbon formation related to heating. The variation in sp²/sp³ content and the dispersion of D and G peaks in Raman spectra both prove that there is a structural change in the carbon films after heat. Differences in sp²/sp³ ratio shows that the carbon structure of a-CN/CH film is thermally more stable than a-CH and a-CN films. TOF-SIMS results reveal that the top a-CN capping layer of the double layer a-CN/CH film structure is able to preserve the C-H bonding within the a-CH under layer. This result in the carbon structure of double layer a-CN/CH film exhibiting better thermal stability compared to single layer a-CH film.

Reference:

1 N. Wang and K. Komvopoulos, IEEE Transactions on Magnetics 47, 2277 (2001)

2 S.N. Piramanayagam and K.Srinivasan, J. Magnetism and Magnetic Materials 321, 485 (2009)

3 W. Zhang, Y. Xia, J. Ju, Y. Fan, Z. Fang, L. Wang, Z. Wang, Solid State Communication 123, 97 (2002)

In recent years, the study of the synthesis and characterization of white light emitting phosphors for use in white light emitting diodes (LEDs) has generated attention worldwide. In white light LEDs, white light can be generated by combination of light of three primary colors (red, green and blue) emitted from different LED chips¹ or combination of blue LED with yellow-emitting phosphor materials^2,3. The problems with these traditional white LEDs is that the yellow YAG:Ce³⁺ phosphor has been reported to show high thermal quenching and poor colour rendition, and that the efficiency of the blue emission is often affected by re-absorption by the red or green phosphor in the three converter system. It is important to find a phosphor that can be excited under near-ultra-violet and the blue region³. In recent studies it has been established that white light can be generated by doping one or more activator(s) in one matrix. For example, in this study white photoluminescence was generated when Sr₂SiO₄ single doped with Dy³⁺ or co-doped with Tb³⁺ and Eu³⁺ was excited using a monochromatized xenon lamp. The structure, particle morphology, chemical composition and oxidation states, photoluminescence (PL) and decay properties of the phosphor were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy and PL spectroscopy, respectively. The XRD patterns showed that a monoclinic phase of Sr₂SiO₄ was crystallized. The decay characteristic data showed that the phosphor consist of single exponential decay curves. The X-ray diffraction data confirmed that.

References

(1) Yanmin, Q.; Xinbo, Z.; Xiao, Y.; Yan, C.; Hai, G.; Journal Rare Earths2009, 27(2), 323

(2) J.K. Park, M.A. Lim, C.H. Kim, H.D. Park Applied Physics Letters2003, 82(5), 683

(3) Yao, S.; Chen, D.; Central European Journal of Physics2007, 5(4), 558

The glass transition temperature (T_g) of three different polymers poly (acrylic acid) (PAA), poly (2-hydroxyethyl methacrylate) (PHEMA) and ethyl cellulose (EC) were measured using gold-coated silicon microcantilevers. The polymer-coated microcantilevers were heated to a temperature greater than the glass transition temperature and cooled down to room temperature in an inert nitrogen gas environment at a rate of 2ºC/min. The deflection of the free end of the microcantilever due to induced differential surface stress between the coated and uncoated sides were measured during the heating and cooling cycle. A gradual change in the slope was noticed around the glass transition temperature for each polymer studied.

Hydrogenated amorphous carbon (a-C:H) thin films are amongst the strongest, smoothest, and most lubricious coatings in existence. The impressive properties of these materials have resulted in their use in a wide range of applications. In particular, thin a-C:H films are employed as lynchpin materials to protect computer hard disks from corrosion and wear. Even though amorphous carbon-based materials have been studied for more than two decades, there is significant ambiguity regarding the mechanisms by which they transform in response to temperature or other energetic inputs. Quantifying the energetics and specifying the physical pathways of thermally-induced structural transformations have proven difficult. Progress has been limited by the challenges associated with the experimental investigation of the structure and bonding configuration of these materials in their thin film configurations, calling for the development of advanced analytical methods.

In this work, new insights into the thermally-induced structural evolution of a-C:H were gained by coupling experiments and molecular dynamics (MD) simulations. A new experimental methodology for quantitatively determining the bonding configuration of carbon in the near-surface region of a-C:H thin films was developed on the basis of X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES) and allowed the in situ XPS and XAES investigation of the thermally-induced structural evolution of a-C:H. Upon high vacuum annealing, three thermally-activated processes with an assumed Gaussian distribution of activation energies with mean value E and standard deviation σ occur in a-C:H: a) ordering and clustering of sp²-hybridized C (E=0.18 eV; σ=0.05 eV); b) scission of sp³ C-H bonds with formation of sp²-hybridized C (E = 1.7 eV; σ = 0.5 eV); and c) direct transformation of sp³- to sp²-hybridized C (E = 3.5 eV; σ = 0.5 eV). This first XPS-based study both demonstrates the low absolute energy barrier for clustering of the sp² phases, and indicates that hydrogen enables conversion to sp² hybridization in these films.

The experimental results were compared with the outcomes of MD simulations performed using the adaptive intermolecular reactive bond order potential. The atomic composition was chosen to match experiments, and the resulting structure was relaxed to the measured density. This enabled the direct visualization of the structure of a-C:H and its evolution as a function of temperature and time. We will discuss the comparison between the simulation and experimental results, emphasizing the insights gained from the fully atomistic picture provided by the atomistic simulations.

A novel application of x-ray photoelectron spectroscopy (XPS) is presented, measuring pyroelectricity in a non-contact mode.¹ We demonstrate, as a proof of concept, how the XPS-derived surface potential^2,3 of Lithium Tantalate crystals provides a direct and simple probe of the desired property, free of all top-contact related difficulties. An experimentally challenging feature, the increase in Lithium Tantalate spontaneous polarization under cooling, is thus evaluated, proposing insight on the roll of surface contaminants and the control over trapped surface charge at the XPS vacuum environment. Our approach can be extended to other non-contact probes, as well as to measuring additional electrical properties, such as piezoelectricity and ferroelectricity.

References:

1. Ehre and Cohen, Appl. Phys. Lett., in press.

2. Cohen, Appl. Phys. Lett., 85, 1271 (2004).

3. I. Doron Mor et al., Nature 406, 382 (2000).