August 2015: Anna Major, Florian Döring, Arne Dittrich, Sinja Pagel, Christina Klamt,
Bea Jaquet, Hans-Ulrich Krebs, and Christian Eberl
The group is working on the preparation and characterization of complex thin film systems using
pulsed laser deposition (PLD).
Our research is founded by the Sonderforschungsbereiche
und SFB1073, and by the Graduiertenkolleg
Pulsed laser deposition (PLD):
Schematics of the PLD method and dates of different materials deposited by PLD for the first time.
Because of the pioneering work of Dijkkamp et al. we also startet in 1997 with the laser deposition of HTSC and in 1993 we were the first group which systematically prepared metallic alloys and multilayers by PLD. Nowadays we are mainly concerned with the formation of complex thin films: metal/ceramics multilayers, polymers and polymer/metal composites.
PLD is a versatile technique for the preparation of all kinds of materials in the form of thin films. At this, thin films are deposited in an UHV chamber (or in inert or reactive gas envirionment) using a KrF laser (LPX110i of Lambda Physics, 248 nm wavelength, 30 ns pulse duration). The atoms and ions ablated from one or more targets are deposited on (heatable) substrates in a target-to-substrate distance of 3-8 cm. The pulse numbers on each target determine the average film concentration and the single layer thickness of multilayers, respectively.
This technique is flexible due to different reasons. First, the energy source is outside the deposition chamber, which easily allows to work in UHV as well as in inert or reactive gas atmosphere. Second, stoichiometry transfer occurs between target and substrate and high deposition rates are obtained. This allows to deposit complex perovskites structures like high-temperature superconductors (HTSC) or collossal magnetoresistance materials (CMR) and to form all kinds of oxids, carbides, nitrides, metals, and even polymers and fullerenes (for instance C60). Third, during deposition energetic particles can hit the substrate surface inducing effects like implantation, stress accumulation, defect formation, and resputtering (see Fig. 1). Normally, one gets smooth films with some droplets (see Fig. 2)
1987 we have build up in our Goettingen group thin film preparation facilities using the excimer laser to investigate the structural and superconducting properties of YBCO and BSCCO high-temperature superconductors. We especially looked on the dependence of the superconducting transition on the preparation conditions (substrate temperature and oxygen partial pressure) and studied oxygen loading, stability range, Fe-substitution and...
1993 we more or less switched over to metallic alloys and multilayers. We found that many differences exist between thin films prepared by PLD compared to conventional thin film techniques like evaporation or sputtering. The most striking feature of PLD alloys is the formation of states further away from equilibrium: in many cases, amorphous or metastable phases with higher solid solubility and unusually enlarged lattice spacings can be formed. The main reasons for these properties are the very high instantaneous deposition rate and high particle energy of up to more than 100 eV. This also leads to good adhesion and strong textures in these films. On the other hand, metallic multilayers often growth epitaxially even at room temperatures due to enhanced surface mobility. In UHV, the interfaces of these multilayers are sometimes intermixed due to the energetic particles generated during ablation, which are implanted a few monolayers below the film surface (�subsurface growth mode�). This can be suppressed by reducing the kinetic energy of the deposited particles by scattering processes in inert gas. Then, implantation and resputtering is deminished.
we additionally started the preparation and investigation of laser deposited polymers like PMMA and PC. At this, we study, how the structure and chain length depend on the preparation conditions and how the properties of these materials are influenced.
2000 we included complex materials like metal/polymer and metal/ceramic systems. While metal clusters in polymers change their optical, mechanical and electrical properties, metal/ceramic multilayers are interesting for tunneling magnetoresistance (TMR) devices and as X-ray optics for microscopes in the water window soft X-ray region.
Structure formation in layered systems:
During growth of thin films and multilayers a minimization of surface and interface roughness is of immens interest, for instance for multilayer applications as x-ray mirrors or tunneling magneto resistance systems. In optical systems, roughness leads to a strong reduction of the reflectivity and also optical resolution.
Therefore we study, how to minimize the roughness in multilayers or to avoid it. We investigate, which mechanisms are responsible for the occurrence of roughness during thin film growth in dependence on the particle energy, deposition rate and substrate temperature. Roughness, structure, texture and internal stress are systematically determined in combination of different experimental techniques: X-ray diffraction (XRD, see Fig. 3), reflectometry (XRR), Electron diffraction (TEM), in-situ electron diffraction (RHEED/THEED, see Fig. 4), scanning force microscopy (SFM) and vibrating reed.
Preparation of X-ray optics:
X-ray microscopy is of large interest in the biological science or polymer and colloid research. Our aim is the development of X-ray multilayer Laue lenses (MLL) and multilayer zone plates (MZP) consisting of multilayers for this special wavelenght region. Recently we developed the first metal/ceramics multilayer Laue lenses and zone plates (Fig. 5) by the combination of PLD and FIB (focused ion beam) and reached a focal size for hard x-rays of below 5 nm.
Laser deposition of polymers:
The properties (for instance mechanical or optical) of thin polymer films as poly(methyl-methacrylate) (PMMA) or polycarbonat (PC) are influenced by the chain length, cross-linking, variation of side groups and "alloying" with a second component. For applications, often an increase of the hardness is desired, which can be obtained for instance by cross-linking of the polymer during deposition by plasma polymerization.
With the pulsed laser deposition technique we have a flexibe method also for the preparation of thin polymer films (f.i. PMMA oder PC). In 1988, PLD was for the first time used for the deposition of polymers by Hansen and Robitaille. Also we use this method and obtain polymers with reduced chain length, a certain amount of cross-linking and an increased hardness. We investigate the properties of deposited polymer films as structure, composition, changes in side groups and thermal stability using thermogravimetry (TGA) and
infrared spectroscopy (FTIR, see Fig. 6), sice exclusion chromatography (SEC, in the group of Prof. Buback, chemistry department), X-ray reflectometry (XRR), scanning electron microscopy (SEM) and scanning force microscopy (SFM). The hardness we measure by using a nanoindenter (Fig. 7) and the mechanical properties are directly measured during film deposition using a vibrating reed (see Fig. 8).
Polymer/Metal composite materials:
Aim is to systematically study the growth of laser deposited composite materials consistng of
polymers with embedded matal nanoclusters and polymer/metal multilayers and to investigate their microstructure and electrical, mechanical and optical properties. During preparation we use the versatility of PLD to deposit such different material classes with one preparation technique. Cluster films are deposited by depositing continuous polymer films and then formation of sherical metal clusters by island growth (see Fig. 9). The characterization of the matrix and the metal clusters is performed using infrared spectroscopy (FTIR), x-ray diffraction (XRD) and elektron microscopy (REM, TEM). Especially we study the structure, form, grain size and ditribution, and lateral ordering of the clusters. In the case of polymer/metal multilayers, the stress and interface behaviors are of interest. If the stress of the metal films cannot be hold by the soft poylmer film (PMMA), wavelike structures occur (see Fig. 10a). For harder polymers like BisDMA or PC, smooth interfaces are obtained (Fig. 10b).
(complete list under
F. Döring, C. Eberl, S. Schlenkrich, F. Schlenkrich, S. Hoffmann, T. Liese, H.U. Krebs, S. Pisana, T. Santos, H. Schuhmann, M. Seibt, M. Mansurova, H. Ulrichs, V. Zbarsky, M. Münzenberg, Phonon localization in ultrathin layered structures, Appl. Phys. A. 119 (2015) 11-18.
M. Osterhoff, C. Eberl, F. Döring, R.N. Wilke, J. Wallentin, H.U. Krebs, M. Sprung, T. Salditt, Towards multi-order hard X-ray imaging with multilayer zone plates, Appl. Cryst. 48 (2015), 116–124.
C. Eberl, F. Döring, T. Liese, F. Schlenkrich, B. Roos, M. Hahn, T. Hoinkes, A. Rauschenbeutel, M. Osterhoff, T. Salditt, H.U. Krebs, Fabrication of laser deposited high-quality multilayer zone plates for hard x-ray nanofocusing, Appl. Surf. Sci. 307 (2014) 638-644.
M. Osterhoff, M. Bartels, F. Döring, C. Eberl, T. Hoinkes, S. Hoffmann, T. Liese, V. Radisch, A. Rauschenbeutel, A.L. Robisch, A. Ruhlandt, F. Schlenkrich, T. Salditt, H.U. Krebs, Two-dimensional sub-5 nm hard x-ray focusing with MZP, Proc. of SPIE 8848 (2013) 884802.
F. Döring, A.L. Robisch, C. Eberl, M. Osterhoff, A. Ruhlandt, T. Liese, F. Schlenkrich, S. Hoffmann, M. Bartels, T. Salditt, H.U. Krebs, Sub-5 nm hard x-ray point focusing by a combined Kirkpatrick-Baez mirror and multilayer zone plate, Optics Express Vol. 21 (2013) 19311–19323.
C. Eberl, T. Liese, F. Schlenkrich, F. Döring, H. Hofsäss, H.U. Krebs, Enhanced resputtering and asymmetric interface mixing in W/Si Multilayers, Appl. Phys. A (2013) DOI 10.1007/s00339-013-7587-5
A. Ruhlandt, T. Liese, V. Radisch, S. Krüger, M. Osterhoff, K. Giewekemeyer, H.U. Krebs, and T. Salditt, A combined Kirkpatrick-Baez mirror and multilayer lens for sub-10 nm x-ray focusing, AIP Advances 2 (2012) 012175.
F. Büttner, K. Zhang, S. Seyffarth, T.Liese, H.U. Krebs, C.A.F. Vaz, and H. Hofsäss, Thickness dependence of the magnetic properties of ripple-patterned Fe/MgO(001) films, J. Appl. Phys. 84 (2011) 064427.
T. Liese, V. Radisch, I. Knorr, M. Reese, P. Großmann, K. Mann, and H.U. Krebs, Development of Laser Deposited Multilayer Zone Plate Structures for Soft X-ray Radiation, Appl. Surf. Sci. 257 (2011) 5138.
F. Schlenkrich, S. Seyffarth, B. Fuchs, and H.U. Krebs, Pulsed laser deposition of polymer-metal nanocomposites, Appl. Surf. Sci. 257 (2011) 5362.
M. Reese, B. Schäfer, P. Großmann, A. Bayer, K. Mann, T. Liese, and H.U. Krebs, Sub-Micron Focusing of XUV Radiation from a Laser Plasma Source Using a Multilayer Laue Lens, Appl. Phys. A 102 (2011) 85.
A. Meschede and H.U. Krebs, Minimization of interface roughness in laser deposited Fe/MgO multilayers, Appl. Phys. A 102 (2011) 103.
T. Liese, V. Radisch, H.U. Krebs, Fabrication of multilayer Laue lenses by a combination of pulsed laser deposition and focused ion beam, Rev. Sci. Instrum. 81 (2010) 073710.
S. Seyffarth, H.U. Krebs, Epitaxial growth of �m-sized Cu pyramids on Silicon, Appl. Phys. A 99 (2010) 735.
B. Fuchs, F. Schlenkrich, S. Seyffarth, A. Meschede, R. Rotzoll, P. Vana, P. Großmann, K. Mann, H.U. Krebs, Hardening of smooth pulsed laser deposited PMMA films by heating, Appl. Phys. A 98 (2010) 711.
J. Röder, T. Liese, H.U. Krebs, Material-dependent smoothing of periodic rippled structures by pulsed laser deposition, J. Appl. Phys. 107 (2010) 103515.
A. Meschede, T. Scharf, H.U. Krebs, and K. Samwer, Mechanical spectroscopy of laser deposited polymers, Appl. Phys. A 93 (2008) 599.
J. Röder, J. Faupel, and H.U. Krebs, Growth of polymer-metal nanocomposites by pulsed laser deposition, Appl. Phys. A 93 (2008) 863.
J. Röder, H.U. Krebs, Frequency dependent smoothing of rough surfaces by laser deposition of ZrO2, Appl. Phys. A 90 (2008) 609.
B. Lösekrug, A. Meschede, and H.U. Krebs, Pulsed laser deposition of smooth poly(methyl methacrylate) films at 248 nm, Appl. Surf. Sci., 254 (2007) 1312.
H.U. Krebs, "Pulsed laser deposition of metals", chapter 16 in "Pulsed laser deposition", R. Eason, ed., (Wiley 2007), 363-382.
J. Röder and H.U. Krebs, Tuning the microstructure of pulsed laser deposited polymer-metal nanocomposites, Appl. Phys. A 85 (2006) 15.
T. Scharf and H.U. Krebs, In-situ mechanical spectroscopy of laser deposited films using plasma plume excited reed, Rev. Sci. Instrum. 77 (2006) 093901.
E. Süske, T. Scharf, T. Junkers, M. Buback, and H.U. Krebs, Mechanism of poly(methyl methacrylate) film growth by pulsed laser deposition, J. App. Phys. 100 (2006) 014906.
P. Rösner, J. Hachenberg, K. Samwer, R. Wehn, P. Lunkenheimer, A. Loidl, E. Süske, T. Scharf, and H.U. Krebs, Comparison of mechanical and dielectric relaxation processes in laser-deposited poly(methyl methacrylate) films, New J. Phys. 8 (2006) 89.
E. Süske, T. Scharf, E. Panchenko, T. Junkers, M. Egorov, M. Buback, H. Kijewski, and H.U. Krebs, Tuning of cross-linking and mechanical properties of laser-deposited poly (methyl methacrylate) films, J. Appl. Phys. 97 (2005) 063501.
C. Fuhse, H.U. Krebs, S. Vitta, and G.A. Johansson, Interface quality and thermal stability of laser deposited metal/MgO multilayers, Appl. Opt. 43 (2004) 6265.
J. Faupel, C. Fuhse, A. Meschede, C. Herweg, H.U. Krebs, and S. Vitta, Microstructure of pulsed laser deposited ceramic-metal and polymer- metal nanocomposited thin films, Appl. Phys. A 79 (2004) 1233.
E. Süske, T. Scharf, P. Schaaf, E. Panschenko, D. Nelke, M. Buback, H. Kijewski, and H.U. Krebs, Variation of mechanical properties of pulsed laser deposited PMMA films during annealing, Appl. Phys. A 79 (2004) 1295.
H.U. Krebs, M. Weisheit, J. Faupel, E. Süske, T. Scharf, C. Fuhse, M. Störmer, K. Sturm, M. Seibt, H. Kijewski, D. Nelke, E. Panchenko, and M. Buback, Pulsed laser deposition (PLD) � a versatile thin film technique, Adv. in Solid State Phys. 43 (2003) 505.
T. Scharf, J. Faupel, K. Sturm, and H.U. Krebs, Intrinsic stress evolution in laser deposited thin films, J. Appl. Phys. 94 (2003) 4273.
S. Vitta, M. Weisheit, Th. Scharf, and H.U. Krebs, Alloy - ceramic oxide multilayer mirror for water-window soft x rays, Optics Lett. 26 (2001) 1448.
K. Sturm and H.U. Krebs, Quantification of resputtering during pulsed laser deposition, J. Appl. Phys. 90 (2001) 1061.
S. Kahl and H.U. Krebs, Supersaturation of single-phase crystalline Fe(Ag)-alloys to 40 at.% Ag by pulsed laser deposition, Phys. Rev. B 63 (2001) 172103.
S. Fähler, M. Weisheit, K. Sturm, and H.U. Krebs, The interface of laser-deposited Fe/Ag multilayers: Evidence for the "subsurface growth mode" during pulsed-laser deposition and examination of the bcc-fcc transformation, Appl. Phys. A 69 (1999) 459.
M. Störmer, K. Sturm, S. Fähler, M. Weisheit, J. Winkler, S. Kahl, Ph. Kesten, A. Pundt, M. Seibt, S. Senz, and H.U. Krebs, Study of laser deposited metallic thin films by a combination of high-resolution ex-situ and time-resolved in-situ experiments, Appl. Phys. A 69 (1999) 455.
H.U. Krebs, Characteristic properties of laser deposited metallic systems, Intern. J. Non-Equilibrium Processing 10 (1997) 3, review article (invited).
S. Fähler, M. Störmer, and H.U. Krebs, Origin and avoidance of droplets during laser ablation of metals, Appl. Surf. Sci. 109/110 (1997) 433.
H. U. Krebs, O. Bremert, Y. Luo, S. Fähler, and M. Störmer, Structure of laser deposited metallic alloys and multilayers, Thin Solid Films 275 (1996) 18.
S. Fähler and H.U. Krebs, Calculations and experiments of material removal and kinetic energy during pulsed laser ablation of metals, Appl. Surf. Sci. 96-98 (1996) 61.
M. Störmer and H.U. Krebs, Structure of laser deposited metallic alloys, J. Appl. Phys. 78 (1995) 7080.
H. U. Krebs and O. Bremert, Pulsed laser deposition of thin metallic alloys, Appl. Phys. Lett. 62 (1993) 2341.