[1] M. T. Nichols, H. Sinha, C. A. Wiltbank, G. A. Antonelli, Y. Nishi, and J. L. Shohet, Appl. Phys. Lett 100, 112905 (2012)

Transition metal oxides offer an incredibly rich variety of properties which can be harnessed for countless applications. Unfortunately, this variety can be a curse as well as a blessing: the myriad oxidation states, crystal structures, and defects which may occur in the bulk and/or on the surface of any given oxide system makes it challenging to draw meaningful structure-property relationships without employing a full suite of materials characterization techniques. To keep the system as simple and well-defined as possible, and to explore materials and compositions not easily attainable by equilibrium techniques, epitaxial deposition of oxide thin films is widely utilized. However, even in these “simple” systems, thorough characterization of the crystallinity and structural defects, oxidation state, stoichiometry and dopants is critical. Unwelcome surprises are often found in nominally “good” material when one takes the time to investigate. Our laboratory has explored the electronic, magnetic, and photoactive properties of binary and complex oxides as epitaxial thin films, and several examples illustrating the importance of thorough thin film characterization will be presented. In our work on Cr-doped anatase TiO₂, a candidate dilute magnetic semiconductor (DMS), room temperature ferromagnetism was observed that appeared to depend sensitively on “preparation conditions.” We applied several characterization techniques, particularly x-ray diffraction (XRD) and transmission electron microscopy (TEM), and were able to correlate the presence of structural defects with room temperature ferromagnetic ordering. One of the most widely investigated materials as a potential DMS has been Co-doped ZnO, but the presence of intrinsic ferromagnetism in this system has been widely debated in the literature. We investigated very high quality epitaxial thin films with several x-ray absorption-based characterization techniques (XANES, EXAFS, XLD) to disprove the presence of intrinsic ferromagnetism in nominally defect-free material. In more recent work, we have explored the visible-light photoactivity of hematite Fe₂O₃ doped with Cr or V. XRD, x-ray photoelectron spectroscopy (XPS), scanning TEM (STEM), and XANES/EXAFS have been applied, as well as less widely utilized techniques such as non-Rutherford resonant elastic scattering (RES) to quantitatively measure oxygen stoichiometry non-destructively, and lab-based x-ray photoelectron diffraction (XPD) to elucidate unique surface oxidation features observed by XPS. The structural properties of doped hematite could then be correlated with the bandgap and spectroscopic photoconductivity measurements.

In addition, distinct information about sub-surface layers is obtained in a non-destructive way, giving information about the depth distribution of elements up to 10 nm. Although the mechanism for this in-depth signal is well understood, a model for the quantification of the data needs to be established.

One way of modeling the data is demonstrated using TRBS [1], a specialized version of TRIM [2] which was optimized for simulating ion scattering. Combining the TRBS data for backscattering of primary ions and an empirical model for the energy dependent reionization probability gives promising results. By fitting the simulation to the measured data conclusions about film composition, thickness and interface quality can be drawn.

This approach will be demonstrated using different thin film examples. One of the sample sets consisting of HfO₂/Al₂O₃ stacks also characterized by AR-XPS, XRR and GIXRF will be discussed in detail. We will show the possibility to determine film thickness variations in the Å range. These measurements can be performed in a few minutes without destroying the sample by sputtering. At the same time, the composition of the outer atomic layer is detected, making the approach well suited for routine analysis of films during or after deposition.

[1] A particularly fast TRIM version for ion backscattering and high energy implantation, Biersack, J.P.; Steinbauer, E.; Bauer, P.; Nucl. Instr. and Meth. in Phys. Res., B61, 1991, 77-82

[2] The Stopping and Range of Ions in Solids; Pergamon, New York, 1985

A set of 6,13-Bis((triethylsilyl)ethynyl)pentacene (TES-PEN) samples has been prepared on a silicon substrate by using a well controlled printing technique.[2] Different substrate shift speeds were used to modify the layer thickness and the crystallinity.

All X-ray based measurements were carried out at the plane grating monochromator PGM beamline for undulator radiation in the laboratory of the Physikalisch-Technische Bundesanstalt PTB at BESSY II, providing tunable radiation of both high photon flux and high spectral purity in the soft x-ray range [3]. Different kinds of X-ray spectrometry (XRS) analyses were employed to determine the chemical binding state, elemental distribution depending on the depth, and lateral mass deposition.

To analyze the chemical binding state of the molecules, the method Near-Edge X-ray

Absorption Fine Structure spectroscopy (NEXAFS) in fluorescence mode has been employed. Varying the angle of incidence in the fluorescence mode the information depth can be tuned to a pre-selectable depth of interest. In addition, the mean penetration depth at large angles of incidence is high enough to analyze even thicker layers of up to a few hundreds of nanometers as is often the case for complex organic materials. For an analysis of the molecular orientation, the angle between the electric field vector and predominant direction of the molecules has to be varied.

Initial measurement sequences exhibit the potential of this X-ray spectrometry method to significantly contribute to the quantitative analysis of organic materials in thin films. In particular, polarization dependent NEXAFS offers a clear discrimination capability for the orientation of the molecules.

[1] S. Wood, J.S. Kim, D.T. James, W.C. Tsoi, C.E. Murphy, and J. S. Kim, J. Chem. Phys. 139, 2013, 064901

[2] D.T. James, J. M. Frost, J. Wade, J. Nelson, J. S. Kim, ACS Nano 7(9), 2013, 7983.

[3] J. Lubeck, B. Beckhoff, R. Fliegauf, I. Holfelder, P. Hönicke, M. Müller, B. Pollakowski, F. Reinhardt, J.

A large number of organic molecules exhibit polymorphism and a well-known phenomenon are specific crystallographic phases which are present exclusively in thin films. Such crystallographic phases are often denoted as surface induced phases, since the presence of a surface during the crystallisation is of primary importance for their formation. In general, such thin-film polymorphs do not exist as macroscopic free standing single crystals, so that existing methods of crystal structure solution e.g. from single crystal diffraction or powder diffraction does not work. A number of surface induced crystal structures of conjugated molecules are solved during the last years, examples are in relevant molecules for organic electronic applications like pentacene or sexithophene.

Two different methods of structure solution from a thin films will be introduced. Both methods are based on grazing incidence x-ray diffraction. In a first step the crystallographic unit cells and the lattice constants are determined by indexing the diffraction pattern. The evaluation of the molecular packing is based on either rigid body refinement or molecular dynamics simulations. While rigid body refinement is based on test structures and a comparison of the calculated diffraction intensities with the experimental intensities, molecular dynamics work on energy minimisation of the molecular packing. It is found that the approach works best for fully rigid molecules like pentacene or parylene. Good results are also obtained for semi-rigid molecules like ternaphtalene or molecules with flexible side-chains like dioctyl-terthiophene. In both cases the crystallographic unit cell is filled by two molecules. However an increasing number of molecules per unit cell (e.g. four) makes the solution of the surface induced phases difficult. Similarities and differences in the molecular packing between known crystal structures (from single crystal solutions) and surface induced phases will be discussed.