The Improvement of GeO2 Film Characteristics on Ge Substrates Using Ultrathin Hf Metal Deposition Followed by Annealing
Authors: Ueno Tomo , Niida Jun-pei , Yamaguchi Marina , Iwazaki Yoshitaka 
Published: February 2020

Combinations of high-permittivity (high-κ) dielectrics and Germanium-channel have been expected as future high-performance and low-voltage operation metal-oxide-semiconductor (MOS) devices. To realize high-k/Ge gate stacks, it is necessary to control the interfacial properties between high-k materials and Ge substrate. GeO2 has been considered one of the candidates as an interlayer to achieve high-quality High-k/Ge interface. However, GeO2 itself has many problems to overcome, such as interfacial thermal instability (GeO volatilization), water absorbency and water solubility. To fabricate a high quality GeO2/Ge system, we have to understand electrical and physical properties of the structure in detail. Especially, to clarify the degradation mechanisms and to propose improvement methods of these issues are most important challenges.

The previous our studies have shown that the GeO desorption occurs during thermal oxidation process (>450C) as well as heat treatment of GeO2/Ge structure with almost same temperature. Therefore, using such the high temperature oxidation process, the interface properties have been drastically degraded, and low temperature oxidation process such as around 400C with quite low oxidation rate would be necessary to achieve good interfacial properties of GeO2/Ge structure. Post metallization technique is one of the promising means of fabricating high quality GeO2/Ge structures even high temperature oxidation would be used.

In this paper, using Hf metal, which is component of typical high-k dielectric material, the effect of PMA process onto the thermal stability, water absorbency, water solubility of thick GeO2 (6-20 nm) as well as the interface properties of GeO2/Ge structure would be presented. The initial GeO2 film has been fabricated using thermal oxidation of p-type Ge (001) wafers with the oxidation temperature of as high as 500C. For the as oxidized sample, thermal stability and interface property from C-V characteristics was quite poor, and water contents in the as oxidized GeO2 films were quite large after exposure to air for 72h. The etching rate of the GeO2 film in pure water is quite high, so after the etching for several seconds in pure water, 6nm GeO2 film on Ge substrate disappeared.

Deposition of ultrathin Hf metal (1nm) on thick GeO2 film followed by the heat treatment of 300C, on the other hand, the characteristics of GeO2 itself have been drastically improved: high thermal stability, low water contents after 72h air exposure. It was also shown the electrical properties of GeO2/Ge interface have been drastically improved from the C-V measurements with the measurement frequency of 1000 to 10 KHz. The remarkable feature of the GeO2 films after ultrathin Hf metal deposition followed by the heat treatment was that the quite high durability from pure water rinse. 6nm-thick GeO2 films were still remaining even after the pure water rinse for several ten minutes. It is probably due to the change of electronic deviation between Ge-O bonds in whole GeO2 film with the contents of several percent of Hf atoms in the GeO2 film.