(Invited) Combinatorial Synthesis and Interface Analysis for Development of High Dielectric Constant Thin Films
Authors: Nagata Takahiro , Kumaragurubaran Somu , Takahashi Kenichiro , Ri Sung-Gi , Chikyow Toyohiro 
Published: July 2022

Compound semiconductor-based power devices such as SiC and GaN require high-temperature operational passive devices such as a capacitor. SiC based active devices are capable of operating over 250°C. For module and system level advancement, the high-temperature operational passive should be developed urgently. In the case of capacitors, current available capacitors can efficiently function up to 175°C only. Therefore, a thin film capacitor that can be operated at high-temperatures and integrated monolithically in the proximity of active device should be developed. In this talk, I will introduce our achievements of development of high temperature operational dielectric film materials and its interface control of metal/oxide stacking structure mainly.

By employing combinatorial method and the thermal diffusion theory, we found some candidates for the dielectric materials based on ferroelectric materials. The BaTiO3 based relaxor ferroelectrics with a high dielectric constant (> 200) and free of hazardous elements are promising candidates. Among the BaTiO3 based relaxor ferroelectrics, we have chosen x[BaTiO3]-(1-x)[Bi(Mg2/3Nb1/3)O3] (BT-BMN). The permittivity of 400 and the stability <8% from room temperature to 400°C has been obtained, which are promising as a high-temperature dielectric medium. For the practical device application, a thermal stable electrode on the dielectrics is also required. To control the interface stability of the electrode/BT-BMN interfaces, several electrodes were investigated by photoelectron spectroscopy (PES). From the viewpoints of the oxidization energy, cost, and Bi thermal diffusion energy, RuO2, TiC, and TaC were selected as the electrode candidates to compare with the Pt electrode. To probe the interface structure and the band alignment non-destructively, we employed a combination of conventional x-ray PES using Al Kα light source (SXPES: hν= 1486.6 eV) and hard x-ray PES (HAXPES, hν= 5.95 keV). HAXPES is a powerful tool for investigating heterostructure interfaces due to its longer inelastic mean free path (IMFP) than that of SXPES. The IMFP of Pt 4f7/2, which was the heaviest element with the shortest IMFP in our measurement, was 4.68 nm, and HAXPES probed approximately 15 nm (probing depth: 3×IMFP) below the surface. For the Pt electrode, which is usually used as an electrode for Bi-contained oxides, the Bi diffusion into the electrode layer and the change of band alignment were clearly observed after annealing at 400°C. In contrast, the TiC electrode inhibited the Bi diffusion and did not show any change of the band alignment after annealing, meaning that the TiC electrode is a potential candidate for the electrode of Bi-contained oxide for high temperature operational devices.

In the presentation, we also introduce the development of new high-k gate materials for GaN and Ge channels by combining the combinatorial method and HAXPES analysis. For a Ge channel, which has been attracting a lot of attention as a replacement for the Si channel used in current Si-based metal-oxide-semiconductor (CMOS) devices. This is because the Ge channel has high electron and hole mobility, which lead to a higher drive current, and Ge has a narrower band gap than Si thus allowing supply voltage scaling. However, Ge has the same issue as Si, namely an unintentionally oxidized layer with a low dielectric constant (~5.6) can form at an oxide/Ge interface. Furthermore, in contrast with SiO2, which is a good insulating layer for MOS devices, GeOx is thermodynamically unstable and water soluble. These properties cause high defect densities at the interface between high-k and Ge and a large hysteresis in the capacitance-voltage characteristic. To overcome these issues, we have proposed the direct growth of (110) rutile TiO2 or non-oxide materials on (100) Ge substrates.