(Invited) Ternary Rare Earth Based Oxides for Nitride Based Devices
Authors: Tromm Thomas Carl Ulrich , Schäfer Anna , Luysberg Martina , Wendt Fabian Andreas , Besmehn Astrid , Mikulics M. , Hardtdegen Hilde , Mantl Siegfried , Schubert Jürgen 
Published: February 2020

To proceed the miniaturization of electronic devices new materials and new device designs are needed. A promising candidate for future gate dielectrics are ternary rare earth oxides which exhibit permittivities up to 40. Such amorphous oxides (e.g. GdScO3 and LaLuO3) were introduced in silicon based MOSFETs and showed good performance according electron and hole mobility or steep subthreshold slopes down to 65 mV/dec [1-3]. They are also used in SiGe or Ge based devices with the same good performance [4].

The ternary rare earth oxides GdScO3 (GSO) and LaLuO3 (LLO) are deposited epitaxial and amorphous by PLD on α-GaN deposited by MOVPE on sapphire (Al2O3) substrates c.f. Hardtdegen et al. [5] and Y2O3 deposited on Si(111) by molecular beam epitaxy analogous to the approach of Tarnawska et al. [6] at different temperatures. Depending on the growth temperatures and the substratethe amorphous, orthorhombic, hexagonal or cubic phase is formed. On GaN GSO grows hexagonal proofed by a Θ-2Θ scan exhibiting beside the peaks of GaN and sapphire two peaks corresponding to the reflections of (0002) and (0004) hexagonal GSO (Figure 1). Furthermore the hexagonal structure was confirmed by TEM and the lattice constants determined from the diffraction pattern are a=0.36 nm and c=0.58 nm. The layers were electrical characterized by CV measurements using Al pads as top electrode. CET plots were made with different layer thicknesses and the permittivity (k) was extracted from inverses of the slope of the fitted lines. The k of hexagonal GSO on GaN is 27.

Furthermore GSO was grown on Y2O3. Depending on the thickness of Y2O3 GSO grows hexagonal on 1 nm layers and cubic on 7 nm layers. Hexagonal GSO and cubic GSO deposited on Y2O3 on Si exhibit a k of 24 and 14, respectively. LLO was grown on GaN and exhibits a hexagonal structure at a growth temperature of 600°C and cubic structure at 900 °C The transition temperature seems to be 860 °C at which the cubic and the hexagonal crystal structure both are present. The rocking curve displays a FWHM of 0.15° for the hexagonal and the cubic structure. The lattice constants measured by XRD of the hexagonal and the cubic crystal structure are a=0.375 nm, c=0.612 nm and a=1.086 nm, respectively. Also on the Y2O3 LLO grows epitaxial. At low temperatures the hexagonal structure and high temperatures the cubic structure forms, too. The lattice constants determined by XRD and TEM are a=0.375 nm and c=0.612 nm for the hexagonal structure and a=1.086 nm for the cubic structure. LLO exhibits a k of 26 and 23 for hexagonal LLO and cubic LLO on Y2O3on Si, respectively.

Finally we will present our recent results about GSO and LLO deposited by PLD especially the crystal structure and the electrical properties. Therefore we will show the dependence of the material properties to the growth conditions.

References

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Figure 1