Type of presentation: Poster

MS-8-P-2794 Epitaxial high-k dielectrics: ternary rare-earth based oxides

Wendt F.^{1, 2}, Schäfer A.^{1, 2}, Mantl S.^{1, 2}, Hardtdegen H.^{1, 2}, Mikulics M.^{1, 2}, Lenk S.^{1, 2}, Barthel J.^{2, 3}, Schubert J.^{1, 2}, Luysberg M.^{3, 4}

¹Peter Grünberg Institute 9, Forschungszentrum Jülich GmbH, Jülich, Germany, ²JARA, Fundamentals of Future Information Technology, Germany, ³Ernst Ruska Center, Forschungszentrum Jülich GmbH, Jülich, Germany, ⁴Peter Grünberg Institute 5, Forschungszentrum Jülich GmbH, Jülich, Germany

f.wendt@fz-juelich.de

Ternary rare-earth based oxides are promising candidates for gate insulators in e.g. high electron mobility transistors due to their wide band gap (> 5 eV) and large permittivity (>24). Here we report on oxides, such as GdScO₃ or LaLuO₃, for which epitaxial growth could be achieved on GaN (0001) [1]. Such crystalline, high-k epitaxial layers potentially enable epitaxial overgrowth and hence offer new pathways towards 3D integration. Therefore a careful determination of the layers’ structural properties including interface abruptness and dielectric properties is essential.

The ternary rare-earth based oxides were deposited by pulsed laser deposition (PLD) using a KrF excimer laser (wavelength 248 nm, pulse width 20 ns, fluence 2.5 J/cm²) at between 620 °C and 740 °C at which crystalline growth occurs. X-Ray diffraction (XRD) and electron diffraction (ED) experiments were carried out to determine the structure of the crystalline phase. For high-resolution analyses of the interface, wedge shaped TEM specimens with a wedge of 3° were prepared by mechanical grinding with the MultiPrep^TM polishing system from Allied High Tech Products. Subsequently Ar ion milling at 4.5 keV and 6° angle was employed for 5 min. High resolution images were obtained with an aberration corrected FEI Titan 80-300 TEM. The dielectric properties of the films are determined by capacitance-voltage measurements.

The studies revealed a novel hexagonal structure opposed to the well-known orthorhombic phase. This is demonstrated in the diffraction pattern of GdScO₃/GaN(0001) shown in Figure 1. Clearly, pseudomorphic growth has taken place. The lattice constants deduced from XRD, a=0.360 nm and c=0.595 nm, agree within the margin of errors with the values measured by ED. Because of the large mismatch between GdScO₃ and GaN of 12 % structural defects appear within the layers (not shown). Figure 2 displays the interface between GdScO₃ and GaN in atomic resolution. The interface is atomically sharp. Local variations in contrast arise due to changes in crystal tilt, which typically arise from structural defects. Complete results of GdScO₃ and LaLuO₃ will be presented. The dielectric properties of the films are determined by capacitance-voltage measurements. In the case of GdScO₃ a permittivity of 27 is measured for a 16 nm thick film, making this novel crystalline material promising for high k dielectrics.

References

[1] A. Schäfer et al, submitted to Semiconductor Science and Technology (2014)

Thanks to P. Bayle-Guillemaud and N. Mollard from the CEA Grenoble for the introduction to the MultiPrep.

Fig. 1: Diffraction pattern of GdScO₃/GaN. The encircled spots are indexed with respect to the hexagonal lattices of GaN (red) and GdScO₃ (blue).

Fig. 2: High resolution TEM image of GdScO₃ grown onto GaN. The growth direction [0001] is marked by arrows.