Analysis

For the structural characterization of our thin films and multilayers manufacured by the different deposition methods we employ various analytical methods.

X-Ray diffraction serves to determine the crystal structure an crystalline perfection of the prepared thin films and ceramics. With this method the lattice constants, the volume fraction of other phases and the orientiation of the crystals are determined. During the measurement a narrow, collimated X-ray beam radiates the sample and the resulting reflexes are analyzed.

To determine during growth lattice constants and growth velocity in our laser-molecular-beam-epitaxy we employ a RHEED-System. Here, an electron beam hits the surface of the substrate crystal under a very low angle of incidence. Depending on the amount of mobile atoms on the surface during deposition the intensity of the reflected electron beam changes. With very smooth, atomically flat surfaces the reflected intensity is at maximum and with half covering of the next atomic layer the intensity is at minimum. This way, the growth of the thin film can be controlled atomic layer by atomic layer. In addition, the deflection of the electron beam at the atomic lattice of the surface atoms allows to analyze the lattice constants during thin film growth.

Elektron diffraction by using thetransmission elektronen microscope TEM is also possible.

For growth characterization of our thin oxide films we employ a RHEED (Reflection High Energy Electron Diffraction) system in our laser molecular-beam-epitaxy. This setup of the RHEED-PLD-System is depicted in Fig. 1. With this RHEED-PLD-System it is possible to control the thin film growth in-situ during the pulsed laserdeposition process. To achieve this, the electron source and the beam tube are differentially pumped and kept at a pressure of 1·10^-6 mbar. Thus, up to a background pressure of 5·10^-1 mbar RHEED analysis is possible.

Fig. 2 depicts the origin of the RHEED picture. The accelerated electrons hit the sample surface at a very low angle of less than 3°. The electron beam is diffracted, since the wavelength of the electrons is of the same order of magnitude as the lattice constants of the crystalline sample. In reciprocal space the Ewald sphere can provide an explanation for the diffraction pattern. By constructing a sphere around a point in reciprocal space with the radius of the incident electron wave vector one obtains the Ewald sphere. Everytime a reciprocal lattice point coincides with the sphere surface, a reflex appears on the screen since the Bragg condition is fulfilled.

Abb. 1: Schematischer Aufbau der in-situ RHEED-PLD-Anlage — Fig. 1: Schematical set-up of the RHEED-PLD-equipment

Abb. 2: Entstehung eines RHEED-Bildes — Fig. 2: Origin of the RHEED-picture

The reciprocal lattice can be obtained from the crystal lattice in direct space. Thus, the distance of the reflexes in this diffraction experiment can be used to determine the lattice constant (at the surface) of the investigated crystal during growth. Fig. 3 shows a RHEED-picture of a SrTiO3 (001) surface. The Lauezone, sharp RHEED-reflexes and Kikuchi-Lines can be observed.

In addition, the intensity of the diffraction pattern oscillates during film growth, as depicted in Fig. 4. The roughness resulting from the evaporated atoms forming the growing thin film on the surface changes the reflexion of the electrons on the surface. If the films grows layer-by-layer, this results in intensity oscillations of the RHEED-reflexes from which the thickness and the growth velocity can be determined.

Abb. 3: RHEED-Aufnahme einer SrTiO3 (001) Oberfläche — Fig. 3: RHEED-picture of a SrTiO3 (001) surface

Abb. 4: Entstehung von RHEED-Intensitätsoszillationen — Fig. 4: Origin of RHEED-Intensityoscillations

With our RHEED-PLD-set-up we also have the opportunity to observe and characterize the growth of oxide films. We are able to follow the lattice constants of the growing film and can control the film thickness and growth velocity.

Figure 5a shows on the left hand side a RHEEDpicture of a 80 nm thick La_2/3Ca_1/3MnO₃ film. The stripe pattern results from the smooth surface of the film, so such only 2D-stripe patterns appear. On the right hand side in Fig. 5b RHEED-oscillations of the reflexes in the first Lauezone are shown. The reflexes have a phase-shift and indicate layer-by-layer growth, which is confirmed by measurements with the scanning force microscope.

Abb. 5: RHEED-Aufnahme eines 80nm dicken La2/3Ca1/3MnO3 Films — Fig.5a: RHEED-picture of a 80nm thick La2/3Ca1/3MnO3 film

Abb. 5b:Intensitätsoszillationen der RHEED-Reflexe der ersten Lauezone — Fig. 5b: Intensity oscillations of the RHEED-reflexes in the first Lauezone

We use laser-deposition, photolithography and argon-ion-etching to produce our films and contacts. The electrical properties are measured with and without high-frequency and magnetic fields by temperatures down to 4K. The roughness of layers, multilayers and ramps is analyzed by atomic force microscopy (AFM) and the epitaxial structures are determined by high resolution x-ray diffractometry (HR-XRD). The general quality is determined by symmetric Θ/2Θ- and asymmetric Δω/2Θ-scans. Furthermore, strain and orientation of thin films are examined by small angle x-ray diffractometry (SAXM) and reciprocal space mapping (RSM). We are especially interested in the behavior of thin films growing on ramps, therefore we investigate multiple layers consisting of large ramp areas.

The results will help us to understand the charge carrier transport in different barrier materials according to their epitaxial structures. The x-ray diffractometer is also used to correlate structural measurements of superlattices with the corresponding Raman spectra.

Abb. 2: Rocking-Kurve des (003)-Peaks zweier YBa2Cu3O7-Filme. Der Film mit dem niedrigeren Tc weist einen deutlich breiteren Peak auf. — Fig. 2: Rocking-curves of the (003)-peak from two YBa2Cu3O7-films. The peak representing the film with the lower Tc is clearly broader.

Abb. 3: Δω/2Θ-Bild eines YBa2Cu3O7/PrBa2Cu3O7-Supergitters. — Fig. 3: Δω/2Θ-scans of an YBa2Cu3O7/PrBa2Cu3O7-superlattice.

2014

Design of experiments for highly reproducible pulsed laser deposition
of YBa₂Cu₃O_7−δ
Meinhard Schilling, Alexander Guillaume, Jan M. Scholtyssek, Frank Ludwig
J. Phys. D: Appl. Phys. 47, 034008-1 – 10, 2014
doi: 10.1088/0022-3727/47/3/034008

2009

Laserdeposition und In-situ-RHEED-Untersuchungen von YBa₂Cu₃0₇ als Grundlage von Mehrlagenbauelementen
Matthias Karger
Dissertation. TU Braunschweig 2009. ISBN: 3 86664 599 8

2004

In-situ RHEED analysis of the epitaxial overgrowth of SrTiO₃ and YBa₂Cu₃O₇ films for multilayer devices
Matthias Karger, Erik Heim, Frank Ludwig, Meinhard Schilling
European Conference on Applied Superconductivity (EUCAS), 1556–1560, 2004

2003

In-situ RHEED analysis of the epitaxial overgrowth of SrTiO₃ and YBa₂Cu₃O₇ films for multilayer devices
M. Karger, E. Heim, F. Ludwig, M. Schilling
Institute of Physics Conference Series No. 181, edited by A. Andreone, G. P. Pepe, Chistiano and G. Masullo, Institute of Physics Publishing, Bristol (UK) and Philadelphia (USA), 1556–1560, 2004

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