(Invited) Development of New High-Dielectric Constant Thin Film Materials for Next-Generation Nanoelectronics
Authors: Nagata Takahiro , Kumaragurubaran Somu , Takahashi Kenichiro , Ri Sung-Gi , Tsunekawa Yoshifumi , Suzuki Setsu , Chikyow Toyohiro 
Published: February 2020

The development of high-dielectric constant thin film materials is essential for future active and passive nanoelectronics devices. Recently we have developed high-dielectric constant thin film materials for thin film capacitor and gate dielectrics.

  In the case of the high-dielectric constant thin film material for the passive devices, we focused on the power device application. Compound semiconducter based power devices such as SiC require high-temperature operational passive devices such as a capacitor, resistor and inductors. 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 casse 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. The BaTiO3 based relaxor ferroelectrics wiht 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) due to its high dielectric constant and temperature stability in the bulk form. However, a thin-film process of this ceramic is not available yet. Moreover, the control of Bi composition, which affects on dielectric constant and ferroelectricity strongly, is challenging due to high volatility of Bi. In this context, high throughput combinatorial synthesis method is effective in the fast optimization of the Bi composition. By employing combinatorial method and the thermal diffusion theory, we have developed a thin-film processing technology for BT-BMN films. The dielectric constant of BT-BMN is close to 400 and the stability is <8% from room temperature to 400°C, which are promising as a high-temperature dielectric medium.

  In the presentation, we also briefly introduce the development of high-k gate materials for aGe 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.